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For over fifty years, a number of nations have been involved in the exploration of outer space. This research is very costly. Has this money been well-spent or wasted?

Some people believe that most space research should be eliminated because of its expense. These people point out the fact that it costs billions of dollars to send astronauts to the moon, but all they bring are some worthless rocks. These people say that the money wasted in oouter space could be spent on more important projects on earth, such as providing housing for homeless people, improving the education system, saving the environment, finding cures for diseases. However, other people believe that space research has provided many benefits to mankind. They point out that hundreds of useful products, from personal computers to foods, are the direct or indirect results of space research. They say that weather are communication satellites have benefited people. Supporters of the space program point tto the scientific knowledge, acquired about the sun, the moon, the planets.

I agree with those people who support space research and want it to continue. Space research will bring even more benefits in the future. Moreover, just as individual people nneed challenges to make their lives more interesting, I believe the human race itself needs a challenge. I think that the peaceful exploration of outer space provides just such a challenge.

Mars or ,,The Red Planet“ is looked upon as the next frontier to space researches. Mars has several enormous canyons. Mars has one mountain which is twice as tall as Everest. It could cost as much as 700 billion dollars to put an astronaut on Mars. That makes it the most expensive journey in history. Even so, it’s a journey which scientists are planning, and they hope that it will happen in the next 25 years in five stages.

There are long-term prospects for space travel, too. After we have explored oour own solar system we predict to travel to other galaxies in huge ,,star ships“. These will wander through space for thousands of years. Each will contain a large human population. When the ship discovers a suitable new planet, some of these people will colonise it. This way, the human race will gradually colonise the whole universe.

In fact, one day, life on Earth could be just distant memory. The TV series ,,Star Trek“ could hold the answer.


The planets divide nneatly into two broad categories: terrestrial and jovian. The terrestrial are basically small, rocky worlds and include Mercury, Venus, Earth, Mars; the jovian planets are gas giants and consist of Jupiter, Saturn, Uranus, Pluto, the outermost and smallest planet (although some scientists argue that it shouldn’t be considered in this privileged class), is an oddball that doesn’t fit easily into either category.

The largest planet is Jupiter. It is followed by Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury, and finally, tiny Pluto. Jupiter is so big that all the other planets could fit inside it.

In general, planetary scientists have scrutinized the terrestrial planets in far greater detail than the jovian planets. Earth, of course, has been an object of scientific inquiry ever since people first started to ponder their place in the universe. Sophisticated spacecraft have examined both Venus and Mars from orbit and from the surface. (NASA’s most recent success placed two rovers on the martian surface during 2004.)

Mercury remains the most enigmatic of the terrestrial worlds. It lies so close to the Sun that observations from Earth reveal preciuos little. In the mid-1970s, NASA sent the Mariner 10 spacecraft on three separate flybys of the innermost planet. TThe spacecraft revealed a startling fact: Mercury has such I high density that more than half of it must be made out of iron and nickel. The planet’s surface shows lots of craters, most dating from the age of heavy bombardment that characterized the solar system about 4 billion years ago. During this period, errant comets and debris left over the solar system’s formation pummeled most planets and moons.

Of all the planets, Venus most resembles Earth. The two have nearly the same size and density, yet the moniker ,,Earth’s twin“ fails miserably for Venus. Earth has a relatively benign climate conducive to the presence of liquid water (and thus life). The surface of Venus, however, bakes at a temperature of 8000 Fahrenheit. Its massive atmosphere of carbon dioxide traps solar radiation and creates a runaway greenhouse effect. Atmospheric pressure at the planet’s surface is nearly 100 times greater than that on Earth’s. Craters on the surface of Venus show that volcanic activity resurfaced the entire planet about 600 million years ago. This contrasts with Earth, where steady volcanism and erosion gradually covered up signs of ancient impacts.

Mars has long fascinated humans, in no small part because its surface is tthe only one that can be seen clearly from Earth. Changes in its appearance led some imaginative scientists to believe that a dying civilization was tapping into dwindling water supplies. Those hopes were dashed when the first spacecraft images revealed a crater and apparently barren surface. Yet subsequent missions revealed a more nuanced world, where craters share space with massive (albeit extinct) volcanoes, giant canyons, and dry channels. The most recent rovers have left little doubt that liquid water once existed on the martian surface. So the question remains: Could life

ever have started on so Red Planet? A famous meteorite from Mars known as ALH84001 contains tantalizing evidence of possible microfossils. And, if water once flowed on the surface, life might have followed.

The jovian planets seem to have less diversity than terrestrial counterparts because all we see are the tops of their cloud layers. Jupiter, Saturn, Uranus and Neptune all have thick atmospheres consisting largely of hydrogen and helium. Various minor constituents create the subtle colors that cause them to look different through a telescope. The term ,,gas giant“ fits these planets perfectly – even the smallest, Uranus, weights in at 15 times the mass of Earth. All the

jovian planets have ring systems as well, although only Saturn’s shines bright enough to be seen easily from Earth.

Prior to Charon’s discovery, astronomers believed that Pluto was much larger. Because Pluto is so distant, the images of Charon and Pluto were blurred together making the planet appear much larger. Pluto stands apart from the other planets because it is much smaller and less massive than others and has the most elongated orbit. It also consists of a mixture of iice and rock, which puts it more in line with some of the moons of the outer planets. Most scientists now consider it to be the largest Kuiper Belt object, a group of objects now numbering more than 700 that orbit beyond Neptune. Even so, it also remains officially a planet.

Moons in the solar system run the gamut from small objects that likely were captured by their parent planets – think of the martian satellites Phobos and Deimos, as well aas most of the dozens of small, irregular satellites orbiting the gas-giant planets – to big objects that rival the planets themselves in size. Jupiter tows its own miniature solar system with it as it orbits is the Sun. Its ffour large moons – Io, Europa, Ganymede, and Callisto (in order from Jupiter)- were discovered by Galileo when he first pointed this telescope at Jupiter in 1610. The largest, Ganymede, has a diameter of 3.270 miles, making it larger than Mercury. Tidal forces from Jupiter heat Io’s interior so intensely that this moon is the most volcanically active body in the solar system. The same tidal forces heat Europa’s interior, melting the moon’s subsurface ice and creating perhaps the largest ocean of liquid water in the solar system.

Saturn also hosts several moons, including the mysterious Titan. This moon, second in size to Ganymede, possesses a hazy, nitrogen-rich atmosphere thicker than Earth’s atmosphere that hides its surface from view. The equally eenigmastic Iapetus features one hemisphere that appears ten times brighter than the opposite one. Both will be prime targets for NASA’s Cassini spacecraft, which went into orbit around Saturn in early July 2004.

Of all the moons in the solar system, none has been studied more thoroughly than Earth’s. Even from Earth, the Moon appears big enough to show detail through a telescope. Its highly crater highlands stand in stark contrast to the darker, lightly crater Maria, crater by giant impacts tthat took place during the era of heavy bombardment and subsequently filled with lava. The Moon ranks as the fifth largest satellite in the solar system and was born in what seems to be a unique process. Most of the large moons in the solar system were created in protoplanetary disks, dusty disks that surrounded the planets during their formation. The moons condensed out of these disks in much the same way as the planets condensed out of the solar nebula. But our Moon appears to have formed when an object the size of Mars gave a glancing blow to the protoEarth, ejecting debris into orbit that eventually coalesced into the Moon.

The Inner Planets vs. the Outer Planets

The inner planets (those planets that orbit close to the Sun) are quite different from the outer planets (those planets that orbit far from the Sun).

The inner planets are: Mercury, Venus, Earth, Mars. They are relatively small, composed mostly of rock, and have few or no moons.

The outer planets include: Jupiter, Saturn, Uranus, Neptune and Pluto. They are mostly huge, mostly gaseous, ringed and have many moons (again, the exception is Pluto, which is small, rocky and has only one moon).

Temperatures on the PPlanets

Generally, the farther from the Sun, the cooler planet. Differences occur when the greenhouse effect warms a planet (like Venus) surrounded by a thick atmosphere.

Density of the Planets

The outer, gaseous planets are much less dense than the inner, rocky planets.

The Earth is the densest planet. Saturn is the least dense planet; it would float on water.

The Mass of the Planets

Jupiter is by far the most massive planet; Saturn trails it. Uranus, Neptune, Earth, Venus, Mars, Pluto are orders of magnitude less massive.

Gravitational Forces on the Planets

The planet with the strongest gravitational attraction at its surface is Jupiter. Although Saturn, Uranus, and Neptune are also very massive planets, their gravitational forces are about the same as Earth. This is because the gravitational force a planet exerts upon an object at the planet’s surface is proportional to its mass and to the inverse of the planet’s radius squared.

A Day on Earth of the Planets

A day is the length of times that it takes a planet to rotate on its axis (3600 ). A day on Earth takes almost 24 hours.

The planet with the longest day is Venus; a day on Venus takes 243 Earth days. (A day on Venus is longer than iits year; a year on Venus takes only 224.7 Earth days).

The planet with the shortest day is Jupiter; a day on Jupiter only takes 9.8 Earth hours. When you observe Jupiter from Earth, you can see some of its features change.

The Average Orbital Speed of the Planets

As the planets orbit the Sun, they travel at different speeds. Each planet speeds up when it is nearer the Sun and travels more slowly when it is far from the Sun.

A Tenth Planet?

No tenth planet beyond Pluto has been directly observed. A few astronomers think that there might be a tenth planet (or companion star) orbiting the Sun far beyond the orbit of Pluto. This distant planet/companion star may or may not exist. The hypothesized origin of this hypothetical object is that a celestial object, perhaps a hard-to-detect cool, brown dwarf star (called Nemesis), was captured by the Sun’s gravitational field. This tenth planet is hypothesized to exist because of the unexplained clumping of some long-period comet’s orbits. The orbits of these far-reaching comets seem to be affected by the gravitational pull of a distant.


Mercury is the closest planet to the Sun and the eighth largest. Mercury is slightly

smaller in diameter than the moons Ganymede and Titan but more than twice as massive:

orbit: 57.910.000 km (0.38 AU) from the Sun

diameter: 4.880 km

mass: 3.30e23kg

An account of the non-discovery of a planet inside Mercury’s orbit. A much more interesting tale than you might imagine. In Roman mythology Mercury is the god of commerce, travel and thievery, the Roman counterpart of the Greek god Hermes, the messenger of the Gods. The planet probably received this name because it mmoves so quickly across the sky.

Mercury has been known since at least the time of the Sumerians (3rd millennium BC). It was given two names by the Greeks: Apollo for its apparition as a morning star and Hermes as an evening star. Greek astronomers knew, however, that the two names referred to the same body. Heraclitus even believed that Mercury and Venus orbit is the Sun, not the Earth.

Mercury has been visited by only one spacecraft, Mariner 10. It flew bby three times in 1974 and 1975. Only 45% of the surface was mapped (and, unfortunately, it is too close to the Sun to be safety imaged by HST). A few discovery-class mission to Mercury, MESSENGER was launched in 2004 aand will orbit Mercury starting in 2011 after several flybys.

Mercury’s orbit is highly eccentric; at perihelion it is only 46 million km from the Sun but at aphelion it is 70 million. The perihelion of its orbit processes around the Sun at a very slow rate. 19th century astronomers made very careful observations of Mercury’s orbital parameters but could not adequately explain them using Newtonian mechanics. The tiny differences between the observed and predicted values were a minor but nagging problem for many decades. It was thought that another planet (sometimes called Vulcan) might exist in an orbit near Mercury’s to account for the discrepancy. But despite much effort, no such planet was found. The

real answer turned out to bbe much more dramatic: Einstein’s General Theory of Relativity. Its correct prediction of the motions of Mercury was an important factor in the early acceptance of the theory. Until 1962 it was thought that Mercury’s ,,day“ was the same length as its ,,year“ so as to keep that same face to the Sun much as the Moon does to the Earth. But this way shown to be false in 1965 by doppler radar observations. It is now known that Mercury rrotates three times in two of its years. Mercury is the only body in the solar system known to have an orbital/rotational resonance with a ratio other than 1:1 (though many have no resonance at all).

This fact and the high eccentricity of Mercury’s orbit would produce very strange effects for an observer on Mercury’s surface. At some longitudes the observer would see the Sun rise and then gradually increase in apparent size as it slowly moved toward the zenith. At that point the Sun would stop, briefly reverse course, and stop again before resuming its path toward the horizon and decreasing in apparent size. All the while the stars would be moving three times faster across the sky. Observers at other points on Mercury’s surface would see different but equally bizarre motions.

Temperature variations on Mercury are the most extreme in the solar system ranging from 90 K to 700 K. The temperature on Venus is slightly hotter but very stable.

Mercury is in many ways similar to the Moon: its surface is heavily cratered and very old; it has no plate tectonics. On the other hand, Mercury is much denser than the Moon (5.43 gm/cm3 vs 3.34). Mercury is the second ddensest major body in the solar system, after Earth. Actually Earth’s density is due in part to gravitational compression; if not for this, Mercury would be denser than Earth. This indicates that Mercury’s dense iron core is relatively larger than Earth’s, probably comprising the majority of the planet. Mercury therefore has only a relatively thin silicate mantle and crust.

Mercury’s interior is dominated by a large iron core whose radius is 1800 to 1900 km. The silicate outer shell (analogous to Earth’s mantle and crust) is only 500 to 600 km thick. At least some of the core is probably molten.

Mercury actually has a very thin atmosphere consisting of atoms blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly escape into space. Thus in contrast to the Earth and Venus whose atmospheres are stable, Mercury’s atmosphere is constantly being replenished.

The surface of Mercury exhibits enormous escarpments, some up to hundreds of kilometers in length and as much as three kilometers high. Some cut through the rings of craters and other features in such a way as to indicate that they were formed by compression. It is estimated that the surface area of Mercury shrank bby about 0.1% (or a decrease of about 1 km in the planet’s radius).

One of the largest features on Mercury’s surface is the Caloris Basin; it is about 1300 km in diameter. It is thought to be similar it the large basins (maria) on the Moon. Like the lunar basins, it was probably caused by a very large impact early in the history of the solar system. The impact was probably also responsible for the odd terrain on the exact opposite side of the planet.

In addition to the heavily cratered terrain, Mercury also has regions of relatively smooth plains. Some may be the result of ancient volcanic activity but some may be the result of the deposition of ejecta from cratering impacts.

A reanalysis of the Mariner data provides some preliminary evidence of recent volcanism on Mercury. But more data will be needed for confirmation.

Amazingly, radar observations of Mercury’s north pole (a region not mapped by Mariner 10) show evidence of water ice in the protected shadows of some craters.

Mercury has a small magnetic field whose strength is about 1% of Earth’s.

Mercury has no known satellites.

Mercury is often visible with binoculars or even the unaided eye, but it is always very

near the Sun and difficult to see in the twilight sky. There are several Web sites that show the current position of Mercury (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

Mercury is so close to the Sun that you can see it near sunrise or sunset.

The gravity on Mercury is 38% of the gravity on Earth. A 100 pound person on Mercury would weight 38 pounds. To calculate your wweight on Mercury, just multiply your weight by 0.38 (or go to the planetary weight calculator).

Mercury’s thin atmosphere consist of trace amounts of hydrogen and helium. The atmospheric pressure is only about 1×10-9 millibars; this is a tiny fraction (about 2 trillionths) of the atmospheric pressure on Earth.

Since the atmosphere is so slight, the sky would appear pitch black (except for the sun, stars, and other planets, when visible), even during the day. Also, there is no ,,greenhouse effect“ on MMercury. When the Sun sets, the temperature drops very quickly since the atmosphere does not help retain the heat.

Mercury is just over a third as far from the Sun as the Earth is; it is 0.387 A.U. from the Sun ((on average). Mercury’s orbit is very eccentric; at aphelion (the point in the orbit farthest from the Sun) Mercury is 70 million km from the Sun, at perihelion Mercury is 46 million km from the Sun. There are no seasons on Mercury. Seasons are caused by the tilt of the axis relative to the planet’s orbit. Since Mercury’s axis is directly perpendicular to its motion (not tilted), it has no seasons.

If you were on the surface of Mercury, the Sun would look almost three times as big as it does from Earth.

Mercury has no moons.

So, Mercury was named after Mercury, the mythical Roman winged messenger and escort of dead souls to the underworld. It was named for the speedy Mercury bbecause it is the fastest-moving planet.


Venus is the second planet from the Sun and the sixth largest. Venus’ orbit is the most nearly circular of that of any planet, with an eccentricity of less than 1%.

Orbit: 108.200.000 km (0.72 AU) from the Sun

Diameter: 12.103.6 km

Mass: 4.869e24 kg

The latest results from Magellan in an accessible and easygoing book. Covers mythology and history of our ,,sister planet“ as well as up to date science and a hhistory of the Magellan project.

Venus (Greek: Aphrodite; Babylonian: Ishtar) is the goddess of love and beauty. The planet is so named probably because it is the brightest of the planets known to the ancients. (With a few exceptions, the surface features on Venus are named for female figures).

Venus has been known since prehistoric times. It is the brightest object in the sky except for the Sun and the Moon. Like Mercury, it was probably thought to be two separate bodies: Eosphorus as the morning star and Hesperus as the evening star, but the Greek astronomers knew better.

Since Venus is an inferior planet, it shows phases when viewed with a telescope from the perspective of Earth. Galileo’s observation of this phenomenon was important evidence in favor of Copernicus’s heliocentric theory of the solar system.

The first spacecraft to visit Venus was Mariner 2 in 1962. It was subsequently visited by many others (more than 20 in all so far), including Pioneer Venus and the Soviet Venera 7 the first spacecraft to land on another planet, and Venera 9 which returned the first photographs of the surface. Most recently, the orbiting US spacecraft Magellan produced detailed maps of Venus’ surface using radar.

Venus’ rotation iis somewhat unusual in that it is both very slow (243 Earth days per Venus day, slightly longer than Venus’ year) and retrograde. In addition, the periods of Venus’ rotation and of its orbit are synchronized such that it always presents the same face toward Earth when the two planets are at their closest approach. Whether this is a resonance effect or merely a coincidence is not known. Venus is sometimes regarded as Earth’s sister planet. In some ways they are very similar:

• Venus is only slightly smaller than Earth (95% of Earth’s diameter, 80% of Earth’s mass).

• Both have few craters indicating relatively young surfaces.

• Their densities and chemical compositions are similar.

Because of these similarities, it was thought that below its dense clouds Venus might be very Earthlike and might even have life. But, unfortunately, more detailed study of Venus reveals that in many important ways it is radically different from Earth.

The pressure of Venus’ atmosphere at the surface is 90 atmospheres (about the same as the pressure at a depth of 1 km in Earth’s oceans). It is composed mostly of carbon dioxide. There are several layers of clouds many kilometers thick composed of sulfuric acid. These clouds completely obscure our vview of the surface. This dense atmosphere produces a run-away greenhouse effect that raises Venus’s surface temperature by about 400 degrees to over 740 K (hot enough to melt lead). Venus’ surface is actually hotter than Mercury’s despite being nearly twice as far from the Sun.

There are strong (350kph) winds at the cloud tops but winds at the surface are very slow, no more than a few kilometers per hour. Venus probably once had large amounts of water like Earth but it all boiled away. Venus is now quite dry. Earth would have suffered the same fate had it been just a little closer to the Sun. We may learn a lot about Earth by learning why the basically similar Venus turned out so differently.

Most of Venus’ surface consists og gently rolling plains with little relief. There are also several broad depressions: Atalanta Planitia, Guinevere Planitia, Lavinia Planitia. There two large highland areas: Ishtar Terra in the northern hemisphere (about the size of Australia) and Aphrodite Terra along the equator (about the size of South America). The interior of Ishtar consists mainly of a high plateau, Lakshmi Planum, which is surrounded by the highest mountains on Venus including the enormous

Maxwell Montes.

Data from Magellan’s imaging radar shows that much of the surface of Venus is covered by lava flows. There are several large shield volcanoes (similar to Hawaii or Olympus Mons) such as Sif Mons. Recently

announced findings indicate that Venus is still volcanically active, but only in a few hot spots; for the most part it has been geologically rather quiet for the past few hundred million years.

There are no small craters on Venus. It seems that small meteoroids bburn up in Venus’ dense atmosphere before reaching the surface. Craters on Venus seem to come in bunches indicating that large meteoroids that do reach the surface usually break up in the atmosphere.

The oldest terrains on Venus seem to be about 800 million years old. Extensive volcanism at that time wiped out the earliest surface including any large craters from early in Venus’ history.

Magellan’s images show a wide variety of interesting and unique features including pancake volcanoes which seem tto be eruptions of very thick lava and coronae which seem to be collapsed domes over large magma chambers. The interior of Venus is probably very similar to that of Earth: an iron core about 3000km in radius, a molten rrocky mantle comprising the majority of the planet. Recent results from the Magellan gravity data indicate that Venus’ crust is stronger and thicker than had previously been assumed. Like Earth, convection in the mantle produces stress on the surface which is relieved in many relatively small regions instead of being concentrated at plate boundaries as is the case on Earth.

Venus has no magnetic field, perhaps because of its slow rotation.

Venus has no satellites, and thereby hangs a tale.

Venus is usually visible with the unaided eye. Sometimes (inaccurately) referred to as the „morning star“ or the „evening star“, it is by far the brightest „star“ in the sky. There are several Web sites that show the current position of Venus (and tthe other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

On June 8 2004, Venus will pass directly between the Earth and the Sun, appearing as a large black dot travelling across the Sun’s disk. This event is known as a „transit of Venus“ and is very rare: the last one was in 1882, the next one in 2012 but after than you’ll have to wait until 2117. While no longer of great sscientific importance as it was in the past, this event will be the impetus for a major journey for many amateur astronomers. For all the details see Fred Espenak’s site.

This is a planet on which a person would asphyxiate in the poisonous atmosphere, be cooked in the extremely high heat, and be crushed by the enormous atmospheric pressure.

Venus’ mass is about 4,87 x 1024 kg. The gravity on Venus is 91% of the gravity on Earth. A 100 pound person would weight 91 pounds on Venus.

The density of Venus is 5.240 kg/m 3 , slightly less dense than the Earth and the third densest planet in our Solar System (after the Earth and Mercury).

Venus is 67.230.000 miles (108.200.000 km) from the Sun. Venus has an almost circular orbit. On average, Venus is 0.72 AU, 67.230.000 miles = 108.200.000 km from the Sun.

Venera 3 (from the U.S.S.R.) was the first manmade abject to reach Venus. This Soviet spacecraft was launched on November 16, 1965. On March 1. 1966, the spacecraft arrived at Venus and the capsule parachuted down to the planet, but contact was lost just before entry into the atmosphere.

Venus was named after the Roman goddess of love.


Earth is the third planet from the Sun and the fifth largest:

Orbit: 149.600.000 km (1.00 AU) from the Sun

Diameter: 12.756.3 km

Mass: 5.972e24 kg

Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful.

The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km):

0 – 40 Crust

40 – 400 Upper mantle

400 – 650 Transition region

650 – 2700 Lower mantle

2700 – 2890 D ‘ ‘ layer

2890 – 5150 Outer core

5150 – 6378 Inner core

The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The

inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle.

Most of the mass of the Earth iis in mantle, most of the rest in the core; the part we inhabit is a tiny fraction of the whole (values below x10^24 kilograms);

Atmosphere = 0.0000051

Oceans = 0.0014

Crust = 0.026

Mantle = 4.043

Outer core = 1.835

Inner core = 0.09675

The core is probably composed mostly of iron (or nickel/iron) though it is possible that some lighter elements may be present, too. Temperatures at the center of the core may be as high as 7500K, hotter than the surface of the Sun. the lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium and aluminum. We know most of this only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes but the majority of the Earth in inaccessible. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth’s chemical composition (by mass) is:

34.6% Iron

29.5% Oxygen

15.2% Silicon

12.7% Magnesium

2.4% Nickel

1.9% Sulfur

0.05% Titanium

The Earth is the densest major body in the solar system.

The other terrestrial planets probably have similar

structures and compositions with some differences: the Moon has at most a small core; Mercury has an extra large core (relative to its diameter); the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth may be only one with distinct inner and outer cores. Note, however, that our knowledge of planetary interiors is mostly theoretical even for the Earth.

Unlike the other terrestrial planets, Earth’s crust is divided into several sseparate solid plates which float around independently on top of the hot mantle below. The theory that describes this is known as plate tectonics. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below. Subduction occurs when two plates collide and edge of one dives beneath the other and ends up being destroyed in the mantle. There is also transverse mmotion at some plate boundaries (i.e. the San Andreas Fault in California) and collisions between continental plates (i.e. India/Eurasia). There are (at present) eight major plates:

• North American Plate – North America, western North Atlantic and Greenland

• South American Plate – South AAmerica and western South Atlantic

• Antarctic Plate – Antarctica and the ,,Southern Ocean“

• Eurasian Plate – eastern North Atlantic, Europe and Asia except for India

• African Plate – Africa, eastern South Atlantic and western India Ocean

• Indian – Australian Plate – India, Australia, New Zealand and most of Indian Ocean

• Nazca Plate – eastern Pacific Ocean adjacent to South America

• Pacific Plate – most of the Pacific Ocean (and the southern coast of California)

There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates. Earthquakes are much more common at the plate boundaries. Plotting their locations makes it easy to see the plate boundaries.

The Earth’s surface is very young. In the relatively short (by astronomical standards) period of 500.000.000 years or sso erosion and tectonic processes destroy and recreate most of the Earth’s surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters). Thus the very early history of the Earth has mostly been erased. The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years are rare. The oldest fossils of living organisms are less than 3.9 billion yyears old. There is no record of the critical period when life was first getting started.

71% of the Earth’s surface is covered with water. Earth is the only planet on which water can exist in liquid from on the surface (though there may be liquid ethane or methane on Titan’s surface and liquid water beneath the surface of Europa). Liquid water is, of course, essential for life as we know it. The heat capacity of the oceans is also very important in keeping the Earth’s temperatures relatively stable. Liquid water is also responsible for most of the erosion and weathering of the Earth’s continents, a process unique in the solar system today (though it may have occurred on Mars in the past).

The Earth’s atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. There was probably a very much larger amount of carbon dioxide in the Earth’s atmosphere when the Earth was first formed, but it has since been almost all incorporated into carbonate rocks and to a lesser extent dissolved into the oceans and consumed by living plants. Plate tectonics and biological processes now maintain a continual flow of carbon dioxide from the atmosphere to tthese various ,,sinks“ and back again. The tiny amount of carbon dioxide resident in the atmosphere at any time is extremely important to the maintenance of the Earth’s surface temperature via the greenhouse effect. The greenhouse effect raises the average surface temperature about 35 degrees C above what it would otherwise be (from a frigid – 21 C to a comfortable +14C); without it the oceans would freeze and life as we know it would be impossible.

The presence or free oxygen is quite remarkable from a chemical point of view. Oxygen is a very reactive gas and under ,,normal“ circumstances would quickly combine with other elements. The oxygen in Earth’s atmosphere is produced and maintained by biological processes. Without life there would be no free oxygen.

The interaction of the Earth and the Moon slows the Earth’s rotation by about 2 milliseconds per century. Current research indicates that about 900 million years ago there were 481 18-hour days in a year.

Earth has a modest magnetic field produced by electric currents in the outer core. The interaction of the solar wind, the Earth’s magnetic field and the Earth’s upper atmosphere causes the auroras. Irregularities in these factors cause the magnetic poles tto move and even reverse relative to the surface; the geomagnetic north pole is currently located in northern Canada. (The ,,geomagnetic north pole“ is the position on the Earth’s surface directly above the south pole of the Earth’s field).

The Earth’s magnetic field and its interaction with the solar wind also produce the Van Allen radiation belts, a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around the Earth. the outer belt stretches from 19,000 km in altitude to 41,000 km; the inner belt lies between 13,000 km and 7,600 km in altitude.

Earth has only one satellite, the Moon. But:

• Thousands of small artificial satellites have also been placed in orbit around the Earth.

• Asteroids 3753 Cruithne and 2002 AA29 have complicated orbital relationships with the Earth; they are not really moons, the term ,,companion“ is being used. It is somewhat similar to the situation with Saturn’s moons Janus and Epimetheus.

• Lilith doesn’t exist but it’s an interesting story.

The Moon is the only natural satellite of Earth:

Orbit: 384,400 km from Earth

Diameter: 3476 km

Mass: 7,35e22 kg

The Moon, of course, has been known since prehistoric times. It is the second brightest object in the sky after the

Sun. as the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this as the cycle of the Moon’s phases. The time between successive new moons is 29,5 days (709 hours), slightly different from the Moon’s orbital period (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.

Due to its size and composition, the Moon is sometimes classified aas a terrestrial ,,planet“ along with Mercury, Venus, Earth and Mars.

The Moon was first visited by the Soviet spacecraft Luna2 in 1959. It is the only extraterrestrial body to have been visited by humans. The first landing was on July 20, 1969; the last was in December 1972. The Moon is also the only body from which samples have been returned to Earth. In the summer of 1994, the Moon was very extensively mapped by the little spacecraft Clementine and aagain in 1999 by Lunar Prospector. The gravitational forces between the Earth and the Moon cause some interesting effects. The most obvious is the tides. The Moon’s gravitational attraction is stronger on the side of the Earth nearest to the MMoon and weaker on the opposite side. Since the Earth, and particularly the oceans, is not perfectly rigid it is stretched out along the line toward the Moon. From our perspective on the Earth’s surface we see two small bulges, one in the direction of the Moon and one directly opposite. The effect is much stronger in the ocean water than in the solid crust so

the water bulges are higher. And because the Earth rotates much faster than the Moon moves in its orbit, the bulges move around the Earth about once a day giving two high tides per day.

But the Earth is not completely fluid, either. The Earth’s rotation carries the Earth’s bulges slightly ahead of the point directly bbeneath the Moon. this means that the force between the Earth and the Moon is not exactly along the line between their centers producing a torque on the Earth and an accelerating force on the Moon. This causes a net transfer of traditional energy from the Earth and the Moon, slowing down the Earth’s rotation by about 1,5 milliseconds/century and rising the Moon into a higher orbit by about 3,8 centimeters per year. (the opposite effect happens to satellites with uunusual orbits such as Phobos and Triton). The asymmetric nature of this gravitational interaction is also responsible for the fact that the Moon rotates synchronously, i.e. it is locked in phase with its orbit so that the same side is always facing toward the Earth. Just as the Earth’s rotation is now being slowed by the Moon’s influence so in the distant past the Moon’s rotation was slowed by the action of the Earth, but in that case the effect was much stronger. When the Moon’s rotation rate was slowed to match its orbital period (such that the bulge always faced toward the Earth) there was no longer an off-center torque on the Moon and a stable situation was achieved. The same thing has happened to most of the other satellites in the solar system. Eventually, the Earth’s rotation will be slowed to match the Moon’s period, too, as is the case with Pluto and Charon.

Actually, the Moon appears to wobble a bit (due to its slightly non-circular orbit) so that a few degrees of the far side can be seen from time to time, but the majority of the far side was completely unknown until the Soviet spacecraft Luna3 pphotographed it in 1959.

The Moon has no atmosphere. But evidence from Clementine suggested that there may be water ice in some deep craters near the Moon’s south pole which are permanently shaded. This has now been confirmed by Lunar Prospector. There is apparently ice at the north pole as well. The cost of future lunar exploration just got a lot cheaper.

The Moon’s crust averages 68 km thick and varies from essentially 0 under Mare Crisium to 107 km north of the crater Korolev on the lunar far side. Below the crust is a mantle and probably a small core (roughly 340 km radius and 2% of the Moon’s mass). Unlike the Earth, however, the Moon’s interior is no longer active. Curiously, the Moon’s center of mass is offset from its geometric center by about 2 km in the direction toward the Earth. Also, the crust is thinner on the near side.

There are two primary types of terrain on the Moon: the heavily crater and very old highlands and the relatively smooth and younger maria. The maria (which comprise about 16% of the Moon’s surface) are huge impact craters that were later flooded by molten lava. Most of the surface is ccovered with regolith, a mixture of fine dust and rocky debris produced by meteor impacts. For some unknown reason, the maria are concentrated on the near side.

Most of the craters on the near side are named for famous figures in the history og science such as Tycho, Copernicus and Ptolemaeus. Features on the far side have more modern references such as Apollo, Gagarin and Korolev (with a distinctly Russian bias since the first images were obtained by Luna3.

In addition to the familiar features on the near side, the Moon also has the huge craters South Pole-Aitken on the far side which is 2250 km in diameter and 12 km deep making it the largest impact basin in the solar system and Orientale on the western limb (as seem from Earth; in the center of the image at left) which is a splendid example of a multi-ring crater.

A total of 382 kg of rock samples were returned to the Earth by the Apollo and Luna programs. These provide most of our detailed knowledge of the Moon. They are particularly valuable in that they can be dated. Even today, more than 30 years after the last Moon landing, scientists still study these

precious samples.

Most rocks on the surface of the Moon seem to be between 4.6 and 3 billion years old. This is a fortuitous match with the oldest terrestrial rocks which are rarely more than 3 billion years old. Thus the Moon provides evidence about the early history of the solar system not available on the Earth.

Prior to the study of the Apollo samples, there was no consensus about the origin of the Moon. There were three principal theories: co-accretion wwhich asserted that the Moon and the Earth formed at the same time from the Solar Nebula; fission which asserted that the Moon split off of the Earth; and capture which held that the Moon formed elsewhere and was subsequently captured by the Earth. None of these work very well. But the new and detailed information from the Moon rocks led to the impact theory: that the Earth collided with a very large object (as big as Mars or more) aand that the Moon formed from the ejected material. There are still details to be worked out, but the impact theory is now widely accepted.

The Moon has no global magnetic field. But some of its surface rocks exhibit remanent magnetism iindicating that there may have been a global magnetic field early in the Moon’s history.

With no atmosphere and no magnetic field, the Moon’s surface is exposed directly to the solar wind. Over its 4 billion year lifetime many irons from the solar wind have become embedded in the Moon’s regolith.

Thus samples of regolith returned by the Apollo missions proved valuable in studies of the solar wind.

The Earth’s mass is about 5.98 x 1024 kg.

The Earth’s axis is tilted from perpendicular to the plane of the ecliptic by 23.450 . this tilting is what gives us the four seasons of the year: Summer, Spring, Winter and Autumn. Since the axis is tilted, different parts of the globe are oriented ttowards the Sun at different times of the year. This affects the amount of sunlight each receives.


American astronaut Neil Armstrong became the first man to set foot on the Moon in July 1969.

Human footprints on the lunar surface won’t disappear for millions of years. That’s because there’s no rain or wind to erode them.

On average, Earth’s nearest neighbour is 384.00 miles away. A train travelling at 161 kilometers per hour would take just under 100 days tto travel that distance.

So far, astronauts have brought back 382 kilos of rock and dust from the Moon.

The lunar surface area is 25% larger than Africa. It takes the Moon 27.3 days to travel around the Earth. As it does so, we see different amounts of its sunlit side. That’s why it seems to get larger and then smaller. It’s important to build lunar bases as a starting point for longer journeys into the solar system. The first base should be completed by 2010. Only 20 or 30 scientists will live in it. The base will have its own oxygen and water under a large roof or ,,dome“. This will make it possible for the astronauts to live and work without spacesuits. It also means that they’ll be able to grow food.

If bases like this first one are a success, lunar cities will quickly follow. These will have schools, cinemas, roads, offices and universities. Thousands of people will travel from Earth and live on them. Some 21st century citizens may even be born, live and die on the Moon.


Mars is the fourth planet from the Sun and the seventh largest:

Orbit: 227,940,000 km (1,52 AU) ffrom the Sun

Diameter: 6,794 km

Mass: 6,4219e23 kg

Mars (Greek: Ares) is the god of War. The planet probably got this name due to its red color, Mars is sometimes referred to as the Red Planet. (An interesting side note: the Roman god Mars was a god of

agriculture before becoming associated with the Greek Ares, those in favor of colonizing and tcrraforming

Mars may prefer this symbolism.) The name of the month March derives from Mars.

Mars has been known since prehistoric times. It is still a favorite of science Fiction writers as the most favorable place in the Solar System (other than Earth!) for human habitation. But the famous „canals“ „seen“ by Lowell and others were, unfortunately, just as imaginary as Barsoomian princesses.

The first spacecraft to visit Mars was Mariner 4 in 1965. Several others followed including Maps 2. The

first spacecraft to land on Mars and the two Viking landers in 1976. Ending a long 20 year hiatus, Mars

Pathfinder landed successfully on Mars on 1997 July 4. In 2004 the Mars Expedition Rovers „Spirit“ and“Opportunity“ landed on Mars sending back geologic data and many pictures. Mars’ orbit is significantly elliptical. One result of this is a temperature variation of about 330 C at the subsolar point between aphelion and perihelion. This has a major influence on Mars’ climate. While the

average temperature on Mars is about 21 8 K (-55 C, -67 F), Martian surface temperatures range widely

from as little as 140 K (-133 C, -207 F) at the winter pole to almost 300 K (27 C, 80 F) on the day side during summer. Though Mars is much smaller than Earth, its surface area is about the same as the land

surface area of Earth. Except for Earth, Mars has the most highly varied and interesting terrain of any of the terrestrial planets, some of it quite spectacular:

• Olympus Mons: the largest mountain in the Solar System rising 24 km (78,000 ft.) above the surrounding plain. Its base is more than500 km in diameter and is rimmed by a cliff 6 km (20,000 ft) high.

• Tharsis: a huge bulge on the Martian surface that is about 4000 km across and 1 0 km high.

• Valles Marineris: a system of canyons 4000 km long and from 2 to 7 km deep;

• Hellas Plaitia: an impact crater in the southern hemisphere over 6 km deep and 2000 km in diameter.

Much of the Martian surface is very old

and cratered, but there are also much younger rift valleys, ridges, hills and plains.

The southern hemisphere of Mars is predominantly ancient cratered highlands somewhat similar to the Moon. In contrast, most of the northern hemisphere consists of plains which are much younger, lower in elevation and have a much more complex history. An abrupt elevation change of several kilometers seems to occur at the boundary. The reasons for this global dichotomy and abrupt boundary are unknown (some speculate that tthey are due to a very large impact shortly after Mars’ accretion). Mars Global Surveyor.has produced a nice 3D map ot’Mars that clearly shows these features.

The interior of Mars is known only by inference from data about the surface and the bulk statistics of the planet. The most likely scenario is a dense core about 1700 km in radius, a molten rocky mantle somewhat denser than the Earth’s and a thin crust. Data from Mars Global Surveyor indicates that MMars’ crust is about 80 km thick in the southern hemisphere but only about 35 km thick in the north. Mars’ relatively low density compared to the other terrestrial planets indicates that its core probably contains a relatively large fraction oof sulfur in addition to iron (iron and iron sulfide).

Like Mercury and the Moon, Mars appears to lack active plate tectonics at present; there is no evidence of recent horizontal motion of the surface such as the folded mountains so common on Earth. With no lateral plate motion, hot-spots under the crust stay in a fixed position relative to the surface. This, along with the lower surface gravity, may account for the Tharis bulge and its enormous volcanoes. There is no evidence of current volcanic activity, however.

There is very clear evidence of erosion in many places on Mars including large floods and small river systems. At some time in the past there was clearly some sort of fluid oon the surface. Liquid water is the obvious fluid but other possibilities exist. There may have been large lakes or even oceans; the evidence for which was strenghtened by some very nice images of layered terrain taken by Mars Global Surveyor and the mineralology results from MER Opportunity. But it seems that this occurred only briefly and very long ago; the age of the erosion channels is estimated at about nearly 4 billion years. (Valles Marineris was NOT created by rrunning water. It was formed by the stretching and cracking of the crust associated with die creation of the Tharsis bulge.)

Early in its history, Mars was much more like Earth. As with Earth almost aii of its carbon dioxide was used up to form carbonate rocks. But lacking the Earth’s plate tectonics, Mars is unable to recycle any of this carbon dioxide back into its atmosphere and so cannot sustain a significant greenhouse effect. The surface of Mars is therefore much colder than the Earth would be at that distance from the Sun. Mars has a very thin atmosphere composed mostly of the tiny amount of remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars is only about 7 millibars (less than 1% of Earth’s) but it varies greatly with altitude from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. But it is thick enough to support very strong winds and vast dust storms that on occasion engulf the entire planet for-months. Mars’ thin atmosphere produces a greenhouse effect but it is only enough tto raise the surface temperature by 5 degrees (K); much less than what we see on Venus and Earth.

Mars has permanent ice caps at both poles composed of water ice and solid carbon dioxide („dry ice“). The ice caps exhibit a layered structure with alternating layers of ice with varying concentrations of dark dust. In the norther summer the carbon dioxide completely sublimes, leaving a residual layer of water ice. ESA’s Mars Express has shown that a similar layer of water ice exists below the southern cap as well. The mechanism responsible for the layering is unknown but may be due to climatic change related to long-term changes in the inclination of Mars’ equator to the plane of its orbit. There may also be water ice hidden below the surface at lower latitudes. The seasonal changes in the extent of the polar caps changes the global atmospheric pressure by about 25% (as measured at the Viking lander sites).

Rccent observations with the Hubble Space Telescope have revealed that the conditions during the Viking missions may not have been typical. Mars’ atmosphere now seems to be both colder and dryer than measured by the Viking landers.

The Viking landers pperformed experiments to determine the existence of life in Mars. The results were somewhat ambiguous but most scientists now believe that they show no evidence for life on Mars (there is still some controversy, however). Optimists point out that only two tiny samples were measured and not from the

most favorable locations. More experiments will be done by future missions to Mars. A small number of meteorites the SNC meteorites) are believed to have originated on Mars.

(On 1996 Aug 6, David McKay et al announced the first identification of organic compounds in a Martian meteorite. The authors further suggest that these compounds, in conjunction with a number of other mineralogical features observed in the rock, may be evidence of ancient Martian microorganisms.

Exciting as this is, it is important to note while this evidence is strong it by no means establishes the fact of extraterrestrial life. There have also been several contradictory studies published since the McKay paper. Remember, „extraordinary claims require extraordinary evidence.“ Much work remains to be done before we can be confident of this most extraordinary claim.

Large, but not global, weak magnetic fields exist in various regions of Mars. This unexpected finding made by Mars

Global Surveyor just days after it entered Mars orbit. They are probably remnants of an earlier global field that has since disappeared. This may have important implications for the structure of Mars’ interior and for the past history of its atmosphere and hence for the possibility of ancient life.

When it is in the nighttime sky, Mars is easily visible with the unaided eye. Its apparent brightness varies greatly according to its relative position to the Earth. There are several WWeb sites that show the current position of Mars (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

Mars’ Satellites:

Mars has two tiny satellites which orbit very close to the martian surface:

Satellite Distance (000 km) Mass (kg) Radius (km)

Phobos 9 11 1,08e16

Hall 1877

Deimos 23 6 1,80e15

Phobos („FOH bus“) is tlie larger and innermost of Mars’ two moons. Phobos is closer to its primary than any other moon in the solar system, less than 6000 km above the surface of Mars. IIt is also one of the smallest moons in the solar system.

orbit: 9378 km from the center of Mars

diameter: 22.2 km (27 x 21.6 x 18.8)

mass: I,08el6 kg

In Greek mythology, Phobos is one of the sons of AAres (Mars) and Aphrodite (Venus), „phobos“ is Greek for „fear“ (the root of „phobia“). Discovered J 877 August 18 by Hall; photographed by Mariner 9 in 1971, Viking I in 1977. and Phobos in 1988.

Phobos orbits Mars below the synchronous orbit radius. Thus it rises in the west, moves very rapidly across the sky and sets in the east, usually twice a day. It is so close to the surface that it cannot be seen above the horizon from all points on the surface of Mars.

And Phobos is doomed: because its orbit is below synchronous altitude tidal forces are lowering its orbit (current rate: about 1.8 meters per century). In about 50 million years it will either crash oonto the surface of Mars or (more likely) break up into a ring. (This is the opposite effect to that operating to raise the orbit of the Moon.)

Phobos and Deimos may be composed of carbon-rich rock like C-type asteroids. But their densities are so low that they cannot be pure rock. They are more likely composed of a mixture of rock and ice. Both are heavily cratered. New images from Mars Global Surveyor indicate that Phobos is covered with aa layer of fine dust about a meter thick, similar to the regolith on the Earth’s Moon.

The Soviet spacecraft Phobos2 detected a faint but steady outgassing from Phobos. Unfortunately, Phobos 2 died before it could determine the nature of the material; water is the best bet. Phobos 2 also returned a few images.

The most prominent feature on Phobos is the large crater named Stickney, the maiden name of Hall’s wife. Like Mimas’ crater Herschel (on a smaller scale) the impact that created Stickney must have almost shattered Phobos. The grooves and streaks on me surface were probably also caused by the Stickney impact.

Phobos and Deimos are widely believed to be captured asteroids. There is some speculation that they originated in the outer solar system rather than in the main asteroid belt.

Phobos and Deimos may someday be useful as „space stations“ from which to study Mars or as intermediate slops to and from the Martian surface; especially if the presence of ice is confirmed.

Deimos („DEE mos“) is the smaller and outermost of Mars’ two moons. It is one of the smallest known moons in the solar system

orbit: 23,459 km from Mars

Diameter: 12,6 km (15 x 12.2 x 11)

mass: 1.8el5 kg

In Greek mythology, Deimos is one of the sons of Ares (Mars) and Aphrodite (Venus); „deimos“ is Greek for „panic“. Discovered 1877 August 12 by Hall, photographed by Viking1 in 1977.

Deimos and Phobos are composed of carbon-rich rock like C-type asteroids and ice. Both are heavily cratered. Deimos and Phobias are probably asteroids perturbed by Jupiter into orbits that allowed them to be captured by Mars.


„The Red Planet“

Mars, the red planet, is the fourth planet from the sun and the most Earth-like planet in our solar system. It is about half the size of Earth and has a dry, rocky surface and a very thin atmosphere.


The surface of Mars is dry, rocky, and mostly covered with iron-rich dust. There are low-lying plains in the northern hemisphere, but the southern hemisphere is dotted with impact craters. The ground is frozen; this permafrost extends for several kilometers. The north and south poles of Mars are covered by ice caps composed of frozen carbon dioxide and water.

Scientists have long thought that there is no liquid water on the surface of Mars now, but recent photos from Mars indicate that there might be ssome liquid water near the surface. The surface of Mars shows much evidence of the effects to ancient waterways upon the landscape; there are ancient, dry rivers and lakes complete with huge inflow and outflow channels. These channels were probably caused by catastrophic flooding that quickly eroded the landscape.

Scientists think that most of the water on Mars is frozen in the land and frozen in the polar ice caps.

Mantle: Silicate rock, probably hotter than the Earth’s mantle at corresponding depths.

Core: The core is probably iron and sulphides and may have a radius of 800-1,500 miles (1,300-2,400 km). More will be known when data from future Mars missions arrives and is analyzed.


Mars’ mass is about 6.42 x IOA23 kg. This is I/9th of the mass of the Earth. A 100-pound person on Mars would weigh 38 pounds.


Each day on Mars takes 1.03 Earth days (24.6 hours). A year on Mars takes 687 Earth days; it takes this long for Mars to orbit the sun once.


Mars is 1.524 times farther from than the sun than the Earth is. It averages 141.6 million miles (227.9 million km) from the sun. Its orbit

is very

elliptical; Mars has the highest orbital eccentricity of any planet in our Solar System except Pluto.


Mars has a very thin atmosphere. It consists of 95% carbon dioxide (CO2), 3% nitrogen, and 1.6% argon (there is no oxygen). The atmospheric pressure is only a fraction of that on Earth (about 1% of Earth’s atmospheric pressure at sea ievel), and it varies greatly throughout the year. There are large stores of frozen carbon dioxide at the north and south poles. DDuring the warm season in each hemisphere, the polar cap partly melts, releasing carbon dioxide. During the cold season in each hemisphere, the polar cap partly freezes, capturing atmospheric carbon dioxide. The atmospheric pressure varies widely from season to season; the global atmospheric pressure on Mars is 25% different (there is less air, mostly carbon dioxide) during the (northern hemisphere) winter than during the summer. This is mostly due to Mars’ highly eccentric orbit; Mars is about 20% closer to tthe Sun during the winter than during the summer. Because of this, the northern polar cap absorbs more carbon dioxide than the southern polar cap absorbs half a Martian year later.

Occasionally, there are clouds in Mars’ atmosphere. Most of tthese clouds are composed of carbon dioxide ice crystals or, less frequently, of frozen water crystals.

There are a lot of fine dust particles suspended in Mars’ atmosphere. These particles (which contain a lot of iron oxide) absorb blue light, so the sky appears to have little blue in it and is pink/yellow to butterscotch in color.


Mars’ surface temperature averages -81 °F (-63 °C). The temperature ranges from a high of 68° F(20° C) to a low of-220° F(-140° C). Mars is much colder than the Earth.


Mars has 2 tiny moons, Phobos and Deimos. They were probably asteroids that were pulled into orbit around Mars.


Mariner 4 was the first spacecraft to visit Mars (in 1965). Two Viking sspacecraft landed in 1976. Mars Pathfinder landed on Mars on July 4, 1997, broadcasting photos. For more on the Mars missions, click here.


This photograph of the Cydonia Mense region of Mars was taken by NASA’s Mars Global Surveyor in 1998. It is a coincidental alignment of rocks and other geologic formations that happens to look like a human face from this angle.


Mars has been known since ancient times.


Mars was named after the RRoman god of war. Jupiter is die fifth and largest planet in our solar system. This gas giant has a thick atmosphere, 39 known moons, and a dark, barely-visible ring. Its most prominent features are bands across its latitudes and a great red spot (which is a storm).


Jupiter is the fifth planet from the Sun and by far the largest. Jupiter is more than twice as massive as all the other planets combined (318 times Earth):

orbit: 778,330,000 km {5.20 AU) from Sun

diameter: 142,984 km (equatorial)

Jupiter is the fourth brightest objcct in the sky (after the Sun, the Moon and Venus). It has been known since prehistoric times as a bright „wandering star“. But in 1610 when Galileo first pointed a telescope at the sky he discovered Jupiter’s four large moons Io, Europa. Ganymede and Callisto (now known as the Galilean moons) and recorded their motions back and forth around Jupiter. This was the first discovery of a center

of motion not apparently centered on the Earth. lt was a major point in favor of Copgniicus’s heliocentric thcory of the motions of the planets (along wilh olher new evidence from his telescope: Ihe phases of Venus aand the mounlains on the Moon). Galileo’s outspoken support of the Copernican theory got him in trouble with the Inguisition. Today anyone can repcal Galileo’s observations (vvithout fear of retribution :-)
using binoculars or an inexpensive telescope.

Jupiter was first visited by Pioneer J O in 1973 and later by Pioneer 11. Voyager l. Voyager 2 and Ulysses. The spacecraft Galileo orbited Jupiler for eighl years. It iš štili regularly observed by the Hubbie Space Telescope.

The gas planets do not have solid surfaces, their gaseous material simply gets denser

with depth (Ihe radii and diameters quoted for the planets are for levels corresponding to a pressure of l atmosphere). What we see when looking at these planets is the tops of clouds high in their atmospheres (slightly above the I atmosphere level).

Jupiter is about 90% hydrogen and 10% helium (by numbers of atoms, 75/25% by mass) with traces of methane, water, ammonia and „rock“. This is very close to the composition of the primordial Solar Nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neplune have much

less hydrogen and helium.

Our knowledge of the interior of Jupiter (and the other gas pllanets) is highly indirect and likely to remain so for some time. (The data from Galileo’s atmospheric probe goes down only about 150 km below the cloud tops.)

Jupiter probably has a core of rocky material amounting to somelhing like 10 to 15 Earth-masses.

Above the core lies the main bulk of the planet in the form of liquid metallic hydrogen. This exotic form of the most common of elements is possible only at pressures exceeding 4 million bars, as is the case in the interior of Jupiter (and Saturn). Liquid metallic hydrogen consists of ionized protons and electrons (like the interior of the Sun but at a far lower temperature). At the temperature and pressure of Jupiter’s interior hydrogen is a liquid, not a gas. It is an electrical conductor and the source of Jupiter’s magnetic field. This

layer probably also contains some helium and traces of various „ices“.

The outermost layer is composed primarily of ordinary molecular hydrogen and helium which is liquid in the interior and gaseous further out. The atmosphere we see is just the very top of this deep layer. Water, carbon dioxide, methane and other simple molecules are also present in tiny amounts.

Recent experiments

have shown that hydrogen does not change phase suddenly. Therefore the interiors of the jovian planets probably have indistinct boundaries between their various interior layers.

Three distinct layers of clouds are believed to exist consisting of ammonia ice, ammonium hydrosulfide

and a mixture of ice and water. However, the preliminary results from the Galileo probe show only faint indications of clouds (one instrument seems to have detected tlie topmost layer while another may have seen the second). But the probe’s entry ppoint was unusual – Earth-based telescopic observations and more recent observations by the Galileo orbiter suggest that the probe cntry site may well have been one of the warmest and least cloudy areas on Jupiter at that time.

Data from the Galileo atmospheric probe also indicate that there is much less water than expected. The

expectation was that Jupiter’s almosphere would contain about twice the amount of oxygen (combined with the abundant hydrogen to make water) as the Sun. But it nnow appears that the

actual concentration much less than the Sun’s. Also surprising was the high temperature and density of the uppermost parts of the almosphere.

Jupiter and the other gas planets have high velocity winds which are confined in wide bbands of latitude.

The winds blow in opposite directions in adjacent bands. Slight chemical and temperature differences

between these bands are responsible for the colored bands that dominate the planel’s appearance. The light colored bands are called zones; the dark ones belts. The bands have been known for some time on Jupiter, but the complex vortices in the boundary regions between the bands were first seen by Voyager. The data

from the Galileo probe indicate that the winds are even faster than expected (more than 400 mph) and extend down into as far as the probe was able to observe; they may extend down thousands of kilometers into the interior. Jupiter’s atmosphere was also found to be quite turbulent. This indicates tthat Jupiter’s winds are driven in large part by its internal heat rather than from solar input as on Earth.

The vivid colors seen in Jupiter’s clouds are probably the result of subtle chemical reactions of the trace elements in Jupiter’s atmosphere, perhaps involving sulfur whose compounds take on a wide variety of colors, but the details are unknown. The colors correlate with the cloud’s altitude: blue lowest, followed by browns and whites, with reds highest. Sometimes we see the llower layers through holes in the upper ones.

The Great Red Spot (GRS) has been seen by Earthly observers for more than 300 years (its discovery is

usually attributed to Cassini. or Robert Hooke in the 17th century). The GRS is an oval about 12,000 by 25,000 km, big enough to hold two Earths. Other smaller but similar spots have been known for decades. Infrared observations and the direction of its rotation indicate that the GRS is a high-pressure region whose cloud tops are significantly higher and colder than the surrounding regions. Similar structures have been seen

on Saturn and Neptune. It is not known how such structures can persist for so long.

Jupiter radiates more energy into space than it receives from the Sun. The interior of Jupiter is hot: the core is probably about 20,000 K. The heat is generated by the Kelvin-Helmholtz mechanism, the slow gravitational compression of the planet. (Jupiter does NOT produce energy by nuclear fusion as in the Sun; it is much too small and hence its interior is too cool to ignite nuclear reactions.) This interior heat probably causes convection deep within Jupiter’s liquid layers and is probably responsible for the complex motions we see in tthe cloud tops. Satum and Neptune are similar to Jupiter in this respect, but oddly, Uranus is not.

Jupiter is just about as large in diameter as a gas planet can be. If more material were to be added, it would be compressed by gravity such that the overall radius would increase only slightly. A star can be larger only because of its internal (nuclear) heat source. (But Jupiter would have to be at leasl 80 times more massive to become a star).

Jupiter has a huge magnetic field, much stronger than Earth’s. Its magnetosphere extends more than 650 million km (past the orbit of Saturn!). (Note that Jupiter’s magnetosphere is far from spherical – it extends „only“ a few million kilometers in the direction toward the Sun.) Jupiter’s moons therefore lie within its magnetosphere, a fact which may partially explain some of the activity on Io. Unfortunately for future space travelers and of real concern to the designers of the Voyager and Galileo spacecraft, the environment near Jupiter contains high levels of energetic particles trapped by Jupiter’s magnetic field. This „radiation“ is similar to, but much more intense than, that found within Earth’s Van Allen belts. It would be iimmediately fatal to an unprotected human being.

The Galileo atmospheric probe discovered a new intense radiation belt between Jupiter’s ring and the uppermost atmospheric layers. This new belt is approximately 10 times as strong as Earth’s Van Allen radiation belts. Surprisingly, this new belt was also found to contain high energy helium ions of unknown origin.

Jupiter has rings like Saturn’s, but much fainter and smaller. They were totally unexpected and were only discovered when two of the Voyager 1 scientists insisted that after traveling 1 billion km it was at least worth a quick look to see if any rings might be present. Everyone else thought that the chance of finding anything was nil, but there they were. It was a majorcoup. They have since been imaged in the infra-red from ground-based telescopes and by Galileo.Unlike Saturn’s, Jupiter’s rings are dark (albedo about .05). They’re probably composed of very small grains of rocky material. Unlike Saturn’s rings, they seem to contain no ice.

Particles in Jupiter’s rings probably don’t stay there for long (due to atmospheric and magnetic drag). The Galileo spacecraft found clear evidence that the rings are continuously resuppl led by dust formed by micrometeor impacts on the

four inner moons, which are very energetic because ofJupiter’s large gravitational field. The inner halo ring is broadened by interactions with Jupiter’s magnetic field.

July 1 994, Comet Shoemaker-Levy 9 collided with Jupiter with spectacular results. The

effects were clearly visible even with amateur telescopes. The debris from the collision was visible for nearly a year afterward with HST.

When it is in the nighttime sky, Jupiter is often the brightest „star“ in the sky (it is second only to Venus, which is sseldom visible in a dark sky).

The four Galilean moons are easily visible with binoculars; a few bands and the Great Red Spot can be seen with a small astronomical telescope.

There are several Web sites that show the current position of Jupiter (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

Jupiter’s Satellites

Jupiter has 63 known satellites (as of Feb 2004): the four large GaliJean moons, 34 smaller named ones, plus many mmore small ones discovered recently but not yet named:

Jupiter is very gradually slowing down due to the tidal drag produced by the Galilean satellites. Also, the same tidal forces are changing the orbits of the moons, very slowly forcing them ffarther from Jupiter.

Io, Europa and Ganymede are locked together in a 1:2:4 orbital resonance and their orbits evolve together. Callisto is almost part of this as well. In a few hundred million years, Callisto will be locked in too, orbiting at exactly twice the period of Ganymede (eight times the period of Io).

Jupiter’s satellites are named for other figures in the life of Zeus (mostly his numerous lovers).

Many more small moons have been discovered recently but have not as yet been officially confirmed or named. The most up to date info on them can be found at Scott Sheppard’s site.

Jupiter’s Rings

Metis (^MEEtis1′ sav/ is the innermost of Jupiter’s known satellites:

orbit: 128,000 km from Jupiter

diameter: 40 km

mass: 9.56el6 kg

Metis wwas a Titaness who was the first wife of Zeus (Jupiter).

Discovered by Synnott in 1979 (Voyager 1).

Metis and Adrastea lie within Jupiter’s main ring. They may be the source of the material comprising the rmf

Small satellites within a planet’s rings are sometimes called „mooms“.

Adrastea, the distributor of rewards and punishments, was the daughter of Jupiter and Ananke. Discovered by graduate student David Jewitt (working under Danielson) in 1979 (Voyager1).

Metis and Adrastea orbit inside the synchronous orbit radius and inside the RRoche limit. They may be small enough to avoid tidal disruption but their orbits will eventually decay.Adrastea is one of the smallest moons in the solar system.

Amallhea („am al THEE uh“) is the third of Jupiter’s known satellites:

orbi t : 181,300 km from Jupiter

Diameter: 189 km (270 x 166 x 150)

Mass: 3,5e18 kg

Amalthea was the nymph who nursed the infant Jupiter with goat’s milk.

Discovered by Barnard 1892 September 9 using the 36 inch (91 cm) refractor at Lick Observatory. Amalthea was the last moon to be discovered by direct visual observation.

.Amalthea and Himalia are Jupiter’s fifth and sixth largest moons; they are about the same size but only 1/15 the size of next larger one, Europa.

Like most of Jupiter’s moons, Amalthea rotates synchronously; its long axis is pointed toward Jupiter. Amalthea is the reddest object in the solar system. The reddish color is apparently due to sulfur originating from lo.

Earlier it was thought that its size and irregular shape should imply that Amalthea is a fairly strong, rigid body. But measurements of it’s mass made during Galileo’s last orbit indicate otherwise. It now appears that Amalthea’s density is only about the same as water and since iit is unlikely to be composed of ice it is most likely a loose „rubble pile“ with a lot of empty spaces.

Like lo, Amalthea radiates more heat than it receives from the Sun (probably due to the electrical currents induced by Jupiter’s magnetic field).

Thebe („THEE bee“) is the fourth of Jupiter’s known satellites:

orbit: 222,000 km from Jupiter

Diameter: 100 km (100 x 90)

Mass: 7,77e17 kg

Thebe was a nymph, daughter of the river god Asopus.

Discovered by Synnott in 1979 (Voyager1).

The image above shows Thebe’s leading side which has three or four large (compared to Thebe’s size) craters. The image at left shows the trailing


Io ( „EYE oh“ sav/ is the fifth of Jupiter’s known satellites and the third largest; it is the innermost of the Galilean moons. lo is slightly larger than Earth’s Moon.

orbit : 422,000 km from Jupiter

diameter: 3630 km

mass: 8,93e22 kg

The pronunciation „EE oh“ is also acceptable.

Io was a maiden who was loved by Zeus (Jupiter) and transformed into a heifer in a vain attempt to hide her from the jealous Hera.

Discovered by Galileo and Marius in 1610.

In contrast to most of the moons in the outer solar system, lo and Europa may be somewhat ssimilar in bulk composition to the terrestrial planets, primarily composed of molten sih’cate rock. Recent data from Galileo indicates mat lo has a core of iron (perhaps mixed with iron sulfide) with a radius of at least 900 km.

Io’s surface is radically different from any other body in the solar system. It came as a very big surprise to the Voyager scientists on the first encounter. They had expected to see impact craters like those on the other terrestrial bodies and from their number per unit area to estimate the age of lo’s surface. But there are very few, if any; impact craters on Io. Therefore, the surface is very young.

Instead of craters, Voyager 1 found hundreds of volcanic caideras. Some of the volcanoes are active! Striking photos of actual eruptions with plumes 300 km high were sent back by both Voyagers and by Galileo

This may have been the most important single discovery of the Voyager missions; it was the first real proof that the interiors of other „terrestrial“ bodies are actually hot and active. The material erupting from lo’s vents appears to be some form of sulfur or sulfur dioxide.

The volcanic eruptions change rapidly. In just

four months between the arrivals of Voyager 1 and Voyager 2 some of them stopped and others started up. The deposits surrounding the vents also changed visibly.

Recent images taken with NASA’s Infrared Telescope Facility on Mauna Kea, Hawaii show a new and very large eruption. A large new feature near Ra Patera has also been seen by HST. Images from Galileo also show many changes from the time of Voyager’s encounter. These observations confirm that lo’s surface is very active iindeed.

So has an amazing variety of terrains: caideras up to several kilometers deep, lakes of molten sulfur, mountains which are apparently NOT volcanoes, extensive flows hundreds of kilometers long of some low viscosity fluid (some form of sulfur?), and volcanic vents. Sulfur and its compounds take on a wide range of colors which are responsible for lo’s variegated appearance.

Analysis of the Voyager images led scientists to believe that the lava flows on lo’s surface were composed mostly of various compounds oof molten sulfur. However, subsequent ground-based infra-red studies indicate that they are too hot for liquid sulfur. One current idea is that lo’s lavas are molten silicate rock. Recent HST observations indicate that the material may be rich in sodium. OOr there may be a variety of different materials in different locations.

Some of the hottest spots on lo may reach temperatures as high as 2000 K though the average is

much lower, about 130 K. These hot spots are the principal mechanism by which lo loses its heat.

The energy for all this activity probably derives from tidal interactions between lo, Europa, Ganymede and Jupiter. These three moons are locked into resonant orbits such that lo orbits twice for each orbit of Europa which in turn orbits twice for each orbit of Ganymede. Though lo, like Earth’s Moon always faces the same side toward its planet, the effects of Europa and Ganymede cause it to wobble a bit. This wobbling stretches and bbends lo by as much as 100 meters (a 100 meter tide!) and generates heat the same way a coat hanger heats up when bent back and forth. (Lacking another body to perturb it, the Moon is not heated by Earth in this way.)

Io also cuts across Jupiter’s magnetic field lines, generating an electric current. Though small compared to the tidal heating, this current may carry more than 1 trillion watts. It also strips some material away from lo which fforms a torus of intense radiation around Jupiter. Particles escaping from this torus are partially responsible for Jupiter’s unusually large magnetosphere.

Recent data from Galileo indicate that lo may have its own magnetic field as does Ganymede.

lo has a thin atmosphere composed of sulfur dioxide and perhaps some other gases.

Unlike the other Galilean satellites, lo has little or no water. This is probably because Jupiter was hot enough early in the evolution of the solar system to drive oft“ the volatile elements in the vicinity of lo but not so hot to do so farther out.

Europa („yoo ROH puh“) is the sixth of Jupiter’s known satellites and the fourth largest; it is the second of the Galilean moons. Europa is slightly smaller than the Earth’s Moon.

orbit: 670,900 km from Jupiter

diameter: 3138 km

mass: 4.80e22 kg

Europa was a Phoenician princess abducted to Crete by Zeus, who had assumed the form of a white bull, and by him the mother of Minos.

Discovered by Galileo and Marius in 1610.

Europa and Io are somewhat similar in bulk composition to the terrestrial planets: primarily composed of silicate rock. Unlike lo, however, Europa has a thin outer layer of ice. Recent data from Galileo indicate tthat Europa has a layered internal structure perhaps with a small metallic core.

But Europa’s surface is not at all like anything in the inner solar system. It is exceedingly smooth: few features more than a few hundred meters high have been seen. The prominent markings seem to be only albedo features with very low relief.

There are very few craters on Europa; only three craters larger than 5 km in diameter have been found. This would seem to indicate a young and active surface. However, the Voyagers mapped only a fraction of the surface at high resolution. The precise age of Europa’s surface is an open question.

The images of Europa’s surface strongly resemble images of sea ice on Earth. It is possible that beneath Europa’s surface ice there is a layer of liquid water, perhaps as much as 50 km deep, kept liquid by tidally generated heat. If so, it would be the only place in the solar system besides Earth where liquid water exists in significant quantities.

Europa’s most striking aspect is a series of dark streaks crisscrossing the entire globe. The larger ones are roughly 20 km across with diffuse outer edges and a central band of lighter mmaterial. The latest theory of their origin is that they are produced by a series of volcanic eruptions or geysers.

Recent observations with HST reveal that Europa has a very tenuous atmosphere (le-11 bar) composed of oxygen. Of the many moons in the solar system only five others (lo, Ganymede. Callisto, Titan and Triton) are known to have atmospheres. Unlike the oxygen in Earth’s atmosphere, Europa’s is almost certainly not of biologic origin. It is most likely generated by sunlight and charged particles hitting Europa’s icy surface producing water vapor which is subsequently split into hydrogen and oxygen. The hydrogen escapes leaving the oxygen.

The Voyagers didn’t get a very good look at Europa. But it is a principal focus of the Galileo mission. Images from Galileo’s first two close encounters with Europa seem to confirm earlier theories that Europa’s surface is very young: very few craters are seen, some sort of activity is obviously occurring. There are regions that look

very much like pack-ice on polar seas during spring thaws on Earth. The exact nature of Europa’s surface and interior is not yet clear but the evidence is now strong for a subsurface ‘ocean’. Galileo has found that

Europa has a weak magnetic field (perhaps 1/4 of the strength of Ganymede’s). And most interestingly, it varies periodically as it passes thru Jupiter’s massive magnetic field. This is very strong evidence that there is a conducting material beneath Europa’s surface, most likely a sally ocean Ganymede („GAN uh meed“) is the seventh and largest of Jupiter’s known satellites. Ganymede is the third of the Galilean moons.

orbit: 1,070,000 km from Jupiter

diameter: 5262 km

mass: 1.48e23 kg

Ganymede was a Trojan bboy of great beauty whom Zeus carried away to be cup bearer to the gods. Discovered by GaHlco and Marius in 1610.

Ganymede is the largest satellite in the solar system. It is larger in diameter than Mercury but only about half its mass. Ganymede is much larger than Pluto.

Before the Galileo encounters with Ganymede it was thought that Ganymede and Callisto were composed of a rocky core surrounded by a large mantle of water or water ice with an ice ssurface (and that Titan and Triton were similar). Preliminary indications from the Galileo data now suggest that Callisto has a uniform composition while Ganymede is differentiated into a three layer structure: a small molten iron or iron/sulfur core surrounded by aa rocky silicate mantle with a icy shell on top. In fact, Ganymede may be similar to lo with an additional outer layer of ice.

Ganymede’s surface is a roughly equal mix of two types of terrain: very old, highly cratered dark regions, and somewhat younger (but still ancient) lighter regions marked with an extensive array of grooves and ridges (ngni). i neir origin is cieany or a rectomc nature, but tne details arc unknown. In this respect, Ganymede may be more similar to the Earth than either Venus or Mars (though there is no evidence of recent tectonic activity).

Evidence for a tenuous oxygen atmosphere on Ganymede, very similar to the one found on F.uropa. has been found recently by fftST (note that this is definitely NOT evidence of life).

Similar ridge and groove terrain is seen on Hnceladus. Miranda and Ariel. The dark regions are similar to the surface of Callisto.

Extensive cratering is seen on both types of terrain. The density of cratering indicates an age of 3 to 3.5 billion years, similar to the Moon, Craters both overlay and are cross cut by the groove systems indicating the the grooves are quite ancient, too. Relatively young craters with rays oof ejecla are also visible.

Unlike the Moon, however, the craters are quite flat, lacking the ring mountains and central depressions common to craters on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede’s icy crust which can flow over geologic time and thereby soften the relief. Ancient craters whose relief has disappeared leaving only a „ghost“ of a crater are known as palimpsests.

Galileo’s first flyby of Ganymede discovered that Ganymede has its own magnetosphere field embedded inside Jupiter’s huge one. This is probably generated in a similar fashion to the Earth’s: as a result of motion of conducting material in the interior.

Callisto („ka LIS loh“) is the eighth of Jupiter’s known satellites and the second largest. It is the outermost of the Galilean moons.

orbit: 1,883,000 km from Jupiter

diameter: 4800 km

mass: 1.08e23 kg

Calisto was a nymph, beloved of Zeus and hated by Hera. Hera changed her into a bear and Zeus then placed her in the sky as the constellation Ursa Major.

Discovered by Galileo and Martus in 1610.

Calisto is only slightly smaller than Mercury but only a third of its mass.

Unlike Ganymede, Callisto seems to have little internal structure; however there arc ssigns from recent Galileo data that the interior materials have settled partially, with the percentage of rock increasing toward the center. Callisto is about 40% ice and 60% rock/iron. Titan and Triton are probably similar.

Callisto’s surface is covered entirely with craters. The surface is very old, like the highlands of the Moon and Mars. Callisto has the oldest, most cratered surface of any body yet observed in the solar system; having undergone little change other than the occasional impact for 4 billion years.

The largest craters are surrounded by a series of concentric rings which iook like huge cracks but which have been smoothed out by eons of slow movement of the ice. The largest of these has been named Valhalla. Nearly 3000 km in diameter, Valhalla is a dramatic example of a multi-ring basin, the result of a massive impact. Other examples arc Caiiisto’s Asgard (left). Mare Orientale on the Moon and Caloris Basin on Mercury.

Like Ganymede, Callisto’s ancient craters have collapsed. They lack the high ring mountains, radial rays and central depressions common to craters on the Moon and Mercury. Detailed images from Galileo show that, in some areas at least, small craters have mostly been obliterated. TThis suggests that some processes have been at work more recently, even if its just slumping.

Another interesting feature is Gipul Catena, a long series of impact craters lined up in a straight Sine. This was probably caused by an object that was lidally disrupted as it passed close to Jupiter (much like Comet SL 9) and then impacted on Callisto.

Callislo has a very tenuous atmosphere composed of carbon dioxide.

Galileo has detected evidence of aweak magnetic field which may indicate some sort of salty fluid below the surface. Unlike Ganymede, with its complex terrains, there is little evidence of tectonic activity on Callisto. While Cailisto is very similar in bulk properties to Ganymede, it apparently has a much simpler geologic history. The different geologic histories of the two has been an important problem for planetary scientists; (it may be related to the orbital and tidal evolution of Ganymede). „Simple“ Callisto is a good reference for comparison with other more complex worlds and it may represent what the other Galilean moons were like early in their history.


Jupiter XIII

Leda („LEE duh“) is the ninth of Jupiter’s known satellites and the smallest:

orbit: 11,094,000 km from Jupiter

diameter: 16 km

mass: 5.68el5 kg

Leda was queen

of Sparta and the mother, by Zeus in the form of a swan, of Pollux and Helen of Troy. Discovered by Kowal in 1974. Leda, Ananke, and Sinope are among the smallest moons in the solar system.


Jupiter VI

Himalia („hih MAL yuh“) is the tenth of Jupiter’s known satellites:

orbit: 11,480,000 km from Jupiter

diameter: 186 km

mass: 9.56el8 kg

Himalia was a nymph who bore three sons of Zeus (Jupiter). Discovered by Pcrrinc in 1904. Unlike the inner satellites, the orbits oof Leda, Himalia, Lysithea and Elara are significantly inclined to Jupiter’s equator (about 28 degrees). ‘


Lysilhea („ly SITI1 ee uh“) is the eleventh of Jupiter’s known satellites:

orbit: 11,720,000 km from Jupiter

diameter: 36 km

mass: 7.77el6 kg

Lysilhea was a daughter of Oceanus and one of Zeus’ lovers. Discovered bv Nicholson in 1938.


Jupiter VII

Elara („EE !ai uh“) is the twelfth of Jupiter’s known satellites:

Orbit: 11,737,000 km from Jupiter

diameter: 76 km

mass: 7.77el7 kg

Blara was the mother by Zeus of the ggiant Tityus. Discovered by Perrinc in 1905. 1 ,eda, Himalia, Lysithea and Elara may be remnants of a single asteroid that was captured by Jupiter and broken up.


Jupiter XII

Ananke („a NANG kee“) is the thirteenth of Jupiter’s known satellites:

orbit: 221,200,000 km from Jupiter


mass: 3.82el6 kg

Ananke was the mother of Adrastea, by Jupiter.

Discovered by Nicholson in 1951.

Ananke, Carme, Pasiphae and Sinope have unusual but similar orbits.


Jupiter XI

Carme („KAR mee“) is the fourteenth of Jupiter’s known satellites:

orbit: 22,600,000 km from Jupiter

diameter: 40 km

mass: 9-56el6 kg

CaniiE was the mother, by Zeus of Britomartis, a Cretan goddess. Discovered by Nicholson in 1938. Ananke, Carme, Pasiphae and Sinope are especially unusual in that their orbits are retrograde.


Jupiter VIII

Pasiphae („pah SIF ah ee“) is the fifteenth of Jupiter’s known satellites:

orbit: 23,500,000 km from Jupiter

diameter: 50 km

mass: 1.91el7 kg

Pasiphae w;is the wile of Minos and mother by a white bull, of the Minotaur. Discovered by P. Melotte in 1908. Ananke, CCarme, Pasiphae and Sinope have orbits highly inclined to Jupiter’s equator (about 150 degrees).


Jupiter IX

Sinope („sah NOH pee“) is me outermost of Jupiter’s known confirmed satellites:

orbit: 23,700,000 km from Jupiter

diamet e r : 3 6 km

mass: 7.77el6 kg

Sinope was a woman said to have been unsuccessfully (!) courted by Zeus. Discovered by Nicholson in 1914. Anankc, Carme, Pasiphae and Sinope may be remnants of a single asteroid that was captured by Jupiter and broken up.


Saturn iis the sixth planet from the Sun and the second largest:

Orbit: 1.429.400.000 km (9.54 AU) from the Sun

Diameter: 120.536 km (equatorial)

Speed: 5.68e26 kg

In Roman mythology, Saturn is the god of agriculture. The associated Greek god, Cronus, was the son of Uranus and Gaia and father of Zeus (Jupiter). Saturn is the root of the English word ,,Saturday“.

Saturn has been known since prehistoric times. Galileo was the first to observe it with a telescope in 1610; he noted its odd appearance but was confused by it. Early observations of Saturn were complicated by the fact that the Earth passes though the plane of Saturn’s rings every few years as Saturn moves in its orbit. A low resolution image of Saturn therefore changes drastically. It was not until 1659 that Christiaan Huygens correctly inferred the geometry of the rings. Saturn’s rings remained unique in the known solar system until 1977 when

very faint rings were discovered around Uranus (and shortly thereafter around Jupiter and Neptune).

Saturn was first visited by Pioneer11 in 1979 and later by Voyager1 and Voyager2. Cassini arrived on July 1, 2004 and will orbit Saturn for at least four years.

Saturn is visibly flattened (oblate) when viewed tthrough a small telescope; its equatorial and polar diameters vary by almost 10% (120.536 km vs. 108.728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but not so much so. Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of water.

Like Jupiter, Saturn is about 75% hydrogen and 25% helium with traces of water, methane, ammonia and ,,rock“, similar to the composition of the primordial Solar Nebula from which the solar system was formed.

Saturn’s interior is similar to Jupiter’s consisting of a rocky core, a liquid metallic hydrogen layer and a molecular hydrogen layer. Traces of various ices are also present.

Saturn’s interior is hot (12000K at the core) and Saturn radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism as in Jupiter. But this may not sufficient to explain Saturn’s luminosity; some additional mechanism may be at work, perhaps the ,,raining out“ of helium deep in Saturn’s interior.

The bands so prominent on Jupiter are much fainter on Saturn. They are also much wider near the equator. Details in the ccloud tops are invisible from Earth so it was not until the Voyager encounters that any detail of Saturn’s atmospheric circulation could be studied. Saturn also exhibits long-lived ovals and other features common on Jupiter. In 1990, HST observed enormous white cloud near Saturn’s equator which was not present during the Voyager encounters; in 1994 another, smaller storm was observed.

Two prominent rings (A and B) and one faint ring (C) can seen from the Earth. The gap between the A and B rings is known as the Cassini division. The much fainter gap in the outer part of the A ring is known as the Encke Divission (but this is somewhat of a misnomer since it was very likely never seen by Encke). Saturn’s rings, unlike the rings of the other planets, are very bright.

Though they look continuous from the Earth, the rings are actually composed of innumerable small particles each in an independent orbit. They range in size from a centimeter or so to several meters. A few kilometer-sized objects are also likely.

Saturn’s rings are extraordinarily thin: though they’re 250.000 km or more in diameter they’re less than one kilometer thick. Despite their impressive appearance, there’s really very

little material in the rings – if the rings were compressed into a single body it would be no more than 100 km across.

The ring particles seem to be composed primarily of water ice, but they may also include rocky particles with icy coatings.

Voyager confirmed the existence of puzzling radial inhomogeneities in the rings called ,,spokes“ which were first reported by amateur astronomers. Their nature remains a mystery, but may have something to do with Saturn’s magnetic field.

Saturn’s outermost rring, the F-ring, is a complex structure made up of several smaller rings along which ,,...

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