We live at an incredible time, when all of the major planets, and most of their satellites, have been viewed close up with the inquisitive eyes of robotic spacecraft. They have perceived awesome, unanticipated features that are far beyond the range of human vision with even the best telescopes on the ground. No two of these fascinating new worlds are exactly alike. Most of them have been investigated many times, with increasingly sophisticated instruments.
This captivating voyage of discovery began close to home, in the late 1950s, when the first artificial satellites were lofted into orbit around the Earth and the Russian Luna 3 spacecraft swung once around the far side of the Moon, which had never been seen before. It is strongly deficient in the large, dark maria that characterize much of the near side facing Earth.
In all, there have been six manned landings on the Moon, beginning with Apollo 11 in July 1969 and ending with Apollo 17 in December 1972. The actual landings were performed by the bug-like Lunar Module that separated from the main spacecraft, while in orbit around the Moon, and returned to it. At first the astronauts traveled on foot, staying near to the Lunar Module, but they subsequently moved to more remote locations in roving vehicles. Altogether, 382 kilograms of rocks were brought back from the Moon for analysis in the terrestrial laboratory, determining the Moon's age, chemical composition, history and probable origin.
The first spacecraft to be launched on lengthy journeys beyond the Moon were flyby missions, the Mariners, Pioneers and Voyagers, that passed near the planets and their satellites to give us new vistas, unavailable from the ground, and making important discoveries in the process. In 1962, instruments aboard the Mariner 2 flight to Venus detected a perpetual flow of charged particles in interplanetary space, emanating from the Sun. Mariner 10, launched in 1973, made the first spacecraft photographs of Venus, and traveled on to reveal the heavily-cratered surface of Mercury.
In 1972-74, the Pioneer 10 and 11 missions to Jupiter showed that spacecraft could pass safely through the asteroid belt, blazing a trail for the extraordinarily successful Voyager 1 and 2 flyby missions whose itinerary included Jupiter (1979), Saturn (1980, 81), Uranus (1986) and Neptune (1989). Voyager 1 and 2 vastly improved our understanding of the atmospheres of the giant planets, and discovered unexpected rings, moons and magnetic fields. They also transformed the satellites of the giant planets into unique and distinctive places with diverse surfaces and in some cases atmospheres or magnetic fields.
The initial explorations using flybys were followed by orbiters that greatly increased the time available for detailed study, often for years at a time. They revealed many features that previous flyby missions had missed, and forever changed our view of the planets and their satellites.
The Viking 1 and 2 orbiters amplified and enhanced our new perspective of Mars in the late 1970s. Each Viking also had a nuclear-powered, 1-ton (1,000 kilogram) lander that was sent safely to the planet's surface, obtaining beautiful panoramas of the Martian surface and measuring the properties of the thin, freezing atmosphere. The Viking landers were also sent to search for both extant and extinct life on Mars, but the results were inconclusive.
More recently, the exploration of Mars has begun to shift from global to exceptionally close-up, high-resolution views taken with the Mars Global Surveyor. At the end of the 20th century and the beginning of the 21st century, it obtained images that show much finer detail than those obtained with the Viking orbiters, including layered deposits suggesting ancient lakes or shallow seas, and dramatic evidence for recent flows.
In the 1990s, the Magellan orbiter used radar to penetrate the thick, cloudy atmosphere of Venus, mapping the entire planet with a clarity and resolution not available for much of Earth. Since Venus is perpetually shrouded in clouds, this was the only way to detect its surface. Magellan's radar images have revealed an unearthly world that was resurfaced long ago by rivers of outpouring lava, and disclosed numerous volcanoes that now pepper its surface.
The Galileo orbiter-probe spacecraft, launched in October 1989, was so massive that no existing rocket had the power to launch it directly to Jupiter, its primary target. Instead, the spacecraft was placed on a looping trajectory that took it past Venus once and Earth twice. The gravity of these planets was used to accelerate and propel the spacecraft in slingshot fashion toward its eventual rendezvous with the giant planet. While the roundabout route took six years, in comparison to the direct, 21-month flights of Pioneer 10 and 11, it also took Galileo on close encounters with two asteroids along the way.
Galileo carried an entry probe that penetrated Jupiter's kaleidoscopic clouds, obtaining the first direct, or in situ, sampling of a giant planet's atmosphere. The main orbiting spacecraft looped around Jupiter for more than five years, until 2001, obtaining high-resolution images and analysis of the planet, its ring, and the four large moons.
The Cassini spacecraft was launched on its seven-year journey to Saturn on 15 October 1997, with arrival expected in June 2004. It includes an orbiter, whose instruments will study the planet's atmosphere, rings, satellites, and magnetic environment. The spacecraft also carries the Huygens Probe that will be parachuted into the hazy, dense atmosphere of Saturn's intriguing moon Titan, determining the properties of its Earth-like atmosphere and its mysterious surface below.
Comets are so tiny and so far away that you cannot detect them until they come near the Sun, and their center is then buried within the brilliant glare of fluorescing gases and reflected sunlight. As a result, no one had ever seen the bare surface of a comet's nucleus until 1986, when the Giotto spacecraft peered into the core of Halley's Comet. It found the nucleus to be a black, oblong chunk of ice and dust, roughly the size of Paris or Manhattan. At the moment of encounter, the comet was spewing out about 25 tons (25,000 kilograms) of water every second, propelled into sunward jets by the vaporizing ice.
There is a growing awareness of the similarities of the major planets and some moons, despite the differences that make each of them unique. They all exhibit common properties and similar processes, such as impact craters, volcanoes, water and atmospheres, reminding us of the basic elements in ancient Greek philosophy - Earth, fire, water and air.
The Moon and Mercury demonstrate the power of impact
The most distinctive features on the Moon are the circular craters that closely pepper its face. Comparatively recent ones still exhibit the details of the impact that created them; older craters have been worn away by small particles that continuously bombard the Moon.
The lunar craters must have been created by solid, rocky objects, named meteoroids, that came from interplanetary space and hit the Moon. When the meteoroids strike the surface of a planet or satellite they are called meteorites. Although the projectile vaporizes on impact, the explosion excavates material and hurls it outward, creating a raised rim, radial ejecta and secondary craters. Meteorites of all sizes have hit the Moon, and its crust records the impact of more small meteorites than large ones.
The largest lunar craters are the impact basins. A typical one is the Imbrium Basin, with a diameter of 1.5 million meters. Its outline can be seen with the unaided eye, forming an ""eye socket"" of the face of the ""Man on the Moon"". Its outer rim is defined by prominent mountain ranges, such as the Apennine mountains. Such basins were created early in the Moon's history, and they were soon flooded and nearly filled with dark molten lava from the interior.
Mercury's surface also contains multi-ringed impact basins. The largest of these has been named Caloris, the Latin name for ""heat"", because it is located at a place on Mercury that faces the Sun when the planet is at the point in its orbit that is closest to the Sun. During the Mariner 10 encounters with Mercury, half of the Caloris Basin was in shadow and the other half in sunlight. An irregular annulus of mountains cuts across the sunlit image, defining the edge of a huge excavation that is 1.34 million meters in diameter.
The cataclysmic impact that created the Caloris Basin occurred an estimated 3.85 billion years ago when a meteorite roughly 150 thousand meters across hit Mercury, like a cosmic bomb with an energy of a trillion 1-megaton hydrogen bombs. The violent explosion reverberated through the young planet, sending strong seismic waves along the surface and through the deep interior. These waves converged to a focus on the side of Mercury opposite to the Caloris Basin, producing a peculiar terrain of cracks, faults, hills and valleys. The similarity of the surfaces of the Moon and Mercury, despite their differing masses and locations in the solar system, suggests that impacting objects were spread throughout the inner solar system during its early days. Mercury could have been bombarded at about the same time as the Moon, for scientists think that the entire solar system, with its Sun, planets and their satellites, formed 4.6 billion years ago.
Ubiquitous impact craters - from Mars and Venus to Callisto and Miranda
When Mariner 4 flew past Mars in 1965, snapping 22 close-up photographs, it revealed a wasteland riddled with the scars of an ancient rain of impacting meteorites. This heavily cratered part of the Martian surface, found in the red planet's southern hemisphere, has undergone little erosion over the aeons. Like the surfaces of Moon and Mercury, the oldest Martian terrain probably bears the scars of the intense cosmic bombardment during the first 500 million years of the solar system, as well as the marks of a continual bombardment since then.
Large impact craters on Venus are relatively scarce when compared with the closely-spaced, overlapping lunar craters. At one time Venus was probably as heavily cratered as the Moon, but the relatively small number and wide spacing of the craters now on Venus indicate that the surface we now see is much younger. When the Moon's cratering rate is scaled to Venus, the relative paucity of craters on its surface indicates an average age of about 750 million years, but the planet originated about 4.6 billion years ago. The relatively few craters we now see are due to meteoritic impact since the entire planet was resurfaced by rivers of outpouring lava about 750 million years ago.
Following impact, large objects left craters on Venus that at first sight resemble those on the Moon, with central peaks, flat floors, and distinct circular rims. But the dense atmosphere on Venus affected both the incoming projectile and its ejected debris, creating features that are unlike any other craters in the solar system. The bright apron of debris that surrounds large craters on Venus often has a lobate, petal-like appearance with an unexpected asymmetry. Material that was ejected from the crater became entrained in the hot, thick atmosphere, transforming it into a turbulent, fluid-like substance. The material flowed and spread out from the crater, creating patterns that resemble flowers or butterflies, rather than hurtling away from it to great distances.
Ancient impact scars are found on the rigid icy crusts of satellites in the cold outer parts of the planetary system. Jupiter's satellite Callisto, for example, has a rocky core surrounded by a deep layer of ice that is heavily scarred by impacts of meteorites. The icy moons of Saturn, such as Mimas and Tethys, are also heavily cratered. Further out we find Miranda, the satellite of Uranus, with the most bizarre surface of all. It has regions of distinctly different terrain. Some astronomers have argued that Miranda was once shattered into large fragments by a powerful collision, but that it managed to pull itself together again into a single body.
Volcanoes on Earth and Venus
Volcanoes, another common aspect of the solar system, are driven by internal heat. For a large rocky planet, internal heat is continuously generated by the slow decay of radioactive material. Satellites can be heated by tidal interaction with their planet. Heat was also provided when the planets and satellites originated, as the result of high-speed collisions between smaller bodies.
Two planets now have a large number of volcanoes - the Earth and Venus. Because of their large size and rocky composition, both planets have internal heat powered by radioactive decay. They have become hot enough inside to melt solid rock into liquid magma that is bottled up within their deep interior. The magma is swollen by heat, becoming lower in density, and rises through the cooler, higher-density material. A volcano is formed and lava flows across the planet's surface. Molten rock trapped beneath a planet's crust is called magma; when molten rock issues from the crater of a volcano or a fissure in the crust it is called lava.
The Hawaiian islands are giant volcanoes, formed when magma moved up from inside the Earth. Mauna Kea and Mauna Loa, on the big island of Hawaii, together form a mountain of lava that is much broader than it is tall; it is more than 200 thousand meters across and rises 9 thousand meters above the ocean floor. Such shield volcanoes have gentle slopes that have been built up from hundreds and even thousands of eruptions and individual flows of highly fluid lava. Mauna Loa is still erupting and growing, with repeated surges of lava that flow down its flanks.
The upwelling of pent-up heat and magma also forms rift valleys on Earth, with steep sides, sunken floors, and copious outpouring of lava. An example is the Great Rift Valley in Africa, a long forking gash that crosses 4.5 million meters of the continent. It extends from Mozambique in the south to Ethiopia in the north, branching out through the Red Sea in one direction and diverging through the Gulf of Aden in another.
Tens of thousands of shield volcanoes have been identified on the face of Venus, by their round shapes and gentle slopes. They range in size from major, Hawaii-sized edifices that are hundreds of thousands of meters across to more numerous, smaller domes that pop up everywhere on the surface. These shield volcanoes have been built up from runny lava that spreads out over great distances with the ease of spilt olive oil.
A smaller number of volcanic flows on Venus appear to be built from lava that is as stiff and thick as batter. In places, the sluggish lava has oozed onto the hot, flat surface of Venus, forming volcanic domes as round and flat as pancakes. Each one has a dark feature almost precisely at the center, suggesting a vent from which the pasty lava flowed, like pancake batter on a hot griddle.
Volcanoes on Mars
The large volcanoes on Mars have the gentle slopes and round shapes of shield volcanoes on Earth, but the volcanoes on Mars stand higher. A striking example is Olympus Mons - Mons is a Latin term for ""mountain"". Olympus Mons is very much larger than any volcano on Earth, and has been active over a long period of time. Rising to a height of at least 23 thousand meters above the surrounding plains, it has a diameter of about 600 thousand meters at its base, giving it a volume as much as one hundred times that of any large terrestrial shield.
Another type of large volcanic structure on Mars is the tholus, which is similar to the shield type of volcano but with somewhat steeper slopes, perhaps due to eruptions of more viscous lava or to a lower eruption rate. The term tholus designates ""a small domical mountain or hill"". An example is Ceraunius Tholus, with an estimated age of about 2.4 billion years.
A third kind of Martian volcano is the patera, with an exceptionally low summit and complex caldera. The name patera is Latin for ""shallow dish or saucer"". They sometimes exhibit the worn-down appearance of old age. Images of the Martian surface suggest that volcanic activity has persisted from the planet's youth into relatively recent times. Like the Moon and Mercury, the red planet bears the scars of a steady rain of meteorites. Relative ages of volcanoes and lava flows can be determined from the density of impact craters on them. The paucity of impact craters near the top of Olympus Mons suggests that the latest lava flows are relatively young, whereas the higher crater density on the volcano's flanks and outer edges indicate a ripe old age for the edifice itself.
Close-up images of other volcanic landforms, taken with cameras on Mars Global Surveyor in 1999-2000, indicate a distinct lack of craters and a fresh, young surface. The impact crater densities on some lava flows in the Martian plains, within the Elysium Planitia region, are up to a thousand times less than those on the lunar maria. The low crater density indicates that volcanism is a continuing process in the recent geologic history of Mars, within the past 100 million years and probably even the last 10 million years. The presence of young lava flows and volcanoes implies that Mars may still be volcanically active today.
Active volcanoes on Jupiter's satellite Io
There is one place in the solar system that is now more volcanically active than any other place; it is Jupiter's satellite Io. It is the hottest satellite in the solar system, so hot that you can see it melting before your eyes. Io is now spewing out 100 times more lava than all the volcanoes on the Earth. This is a totally unexpected discovery, made by the inquisitive camera eyes of Voyager 1 in 1979. Volcanoes are literally turning the satellite inside out, so parts of Io's surface are younger than your backyard.
Galileo returned for a close-up view of Io's volcanoes in 1999-2000, providing a better understanding of the sizzling world. Instruments on Galileo measured the temperatures of the volcanoes, showing that the lava is at 1700 to 2000 degrees kelvin, up to twice the temperature of volcanoes on Earth. The high-temperature eruptions emit gaseous sulfur and sulfur dioxide; the bright surface flows are attributed to sulfur and the white surface deposits to sulfur dioxide. The very high temperatures apparently rule out liquid sulfur as a dominant volcanic fluid, and they have certainly driven off any water that might have been on Io.
Volcanism on Neptune's satellite Triton
Neptune's largest satellite, Triton, is the coldest moon ever recorded, with a temperature of just 38 degrees kelvin, approaching absolute zero where all motion stops. It is so cold because Neptune is so far away from the Sun, therefore receiving little sunlight, and also because Triton reflects more of the incident sunlight than most satellites - only Enceladus and Europa are comparable. Yet, the frozen moon is a dynamic, alive world, set in motion and molded by volcanic eruptions.
Numerous dark plumes and streaks, found in the midst of the bright southern cap of Triton, suggest a different kind of volcanic activity, propelled by relatively recent eruptions of nitrogen gas. Nitrogen boils at very low temperatures, at just 77 degrees kelvin on Earth, and when it boils it expands, producing enormous pressures that can shoot gas and other material high into Triton's thin nitrogen atmosphere. Thus, geyser-like eruptions may have lofted the dark material outward from beneath the surface. The prevailing winds would then carry it across the satellite, depositing it on the ice as dark streaks.
Earth, the water planet
From outside, our home planet Earth looks like a tiny, fragile oasis in space, a glistening blue and turquoise ball of water, flecked with delicate white clouds and capped with glaciers of ice. Seventy one percent of the Earth's surface is now covered with water. The oceans contain so much water that if the Earth were perfectly smooth the oceans would cover the entire globe to a depth of 2.8 thousand meters. They contain about one billion trillion (1021) kilograms of water.
Possible water ice at the poles of Mercury and the Moon
A strong tidal lock keeps the Sun directly over Mercury's equator at all times. This means that crater interiors near the poles are never exposed to direct sunlight. Calculations indicate that the permanently shadowed spots may have remained colder than 112 degrees kelvin for aeons, permitting substantial quantities of water ice to accumulate at the frigid crater floors near the poles. When Mercury's poles tip toward the Earth, while never deviating from the north-south direction and remaining hidden from the Sun, astronomers have beamed radio signals at them and examined the echoes. The radar observations have revealed that the planet's north and south poles may contain substantial deposits of water ice, at least a couple of meters thick. If Mercury has reservoirs of frozen water at its poles, then the Moon might have some ice in craters at its polar regions that are also permanently in shadow, remaining eternally dark and cold.
A former ocean on Venus
Today the surface and atmosphere of Venus are exceptionally dry, which is what you would expect for a planet whose surface temperature is now a blistering 730 degrees kelvin. The planet may nevertheless once have had vast quantities of water. If the very small quantities of water vapor now found in the atmosphere of Venus are a remnant of an ancient reservoir of liquid water, then Venus has lost the equivalent of a very large lake or a small ocean.
Water on Mars
Mars is a much drier, colder planet than the Earth. Over most of the surface of Mars, the temperature is below the freezing point of water at 273 degrees kelvin, so nearly all of the water on the Martian surface is now frozen solid. Liquid water cannot now exist for any length of time on the surface of Mars. It would immediately begin to boil, evaporate and freeze - all at the same time. Because of the low pressure of the thin Martian atmosphere, any liquid water would quickly vaporize. And because it is now so cold on Mars, any liquid water or water vapor would soon freeze into ice.
Although there is now no liquid water on Mars, vast quantities of liquid water flowed over its surface in the distant past. Huge, dry river beds and flood channels, imaged by Mariner 9 and the Viking 1 and 2 orbiters, provide unmistakable signs of former, water-charged torrents that cascaded down the broad hills of Mars. The flow channels that have been carved and etched into the surface of Mars are immense by terrestrial standards, as much as 100 thousand meters wide and 2 million meters in length. The amount of water required to gauge out these river-like outflow channels is enormous, requiring catastrophic floods containing million of tons, or billions of kilograms.
As everyone knows, water flows downhill, so one clue to the water's fate is provided by the topography of Mars. The laser altimeter on board the Mars Global Surveyor made height measurements, with a precision of 5 meters, by bouncing laser beams off the planet's surface, determining land elevations by the time of the round trip. The dominant feature of the global topography is the six-thousand-meter difference in elevation between the low northern hemisphere and the high southern hemisphere. The northern plains have been resurfaced to a nearly billiard-ball smoothness, free of the canyons, craters, volcanoes and valleys visible in the southern hemisphere. This suggests that the flood waters that cut the outflow channels drained northward and pooled in the vast northern lowlands at the ends of the channels.
The crisp, sharp images obtained with the cameras aboard Mars Global Surveyor have recorded deep and widespread layered deposits that might have been laid down long ago in numerous lakes and shallow seas when Mars was able to sustain liquid water for long periods. They are now seen just south of the equator, in exposed walls within ancient impact craters or basins and the chasms of the Valles Marineris. These startling images remind us of the rock layers in the Grand Canyon. The sedimentary layers do not show evidence of any gullies, streams or channels that might have filled the layered regions with water, so the source of water would also likely be subterranean.
Water ice in the outer solar system
Dense rocky substances dominate the four terrestrial planets (Mercury, Venus, Earth and Mars) that are nearest the Sun, while the lighter gaseous and icy substances dominate the outer giant planets (Jupiter, Saturn, Uranus and Neptune). These compositional differences appear to result from the fact that the terrestrial planets formed close to the hot, bright, young Sun, and they suggest that water ice might be common in the colder, outer parts of the planetary system.
Jupiter's satellite Europa has a mass density of 2,970 kilograms per cubic meter, so it could be mostly rock. Yet the surface of Europa is almost perfectly smooth and exceptionally bright, with no mountains and valleys in sight. All of the light material in Europa must have once melted, floating to the top and freezing into a crust of ice.
Very few impact craters are present on Europa's face, indicating that its smooth surface was formed relatively recently, geologically speaking. Some process must be keeping it young on time scales of a few hundred million years or less. Liquid water or slush apparently oozes out within cracks in the ice, resurfacing the globe. Some cracks in the icy moon are as deep as the distance form Los Angeles to New York, and when you look down them you might see water rising.
Ganymede is the largest moon in the solar system, with a radius that exceeds that of the planet Mercury, but the satellite's mass density is relatively low, at 1,940 kilograms per cubic meter, so it probably contains substantial amounts of water. Its surface has large dark plates separated by lighter regions, and impact craters that are surrounded by bright material. The dark regions are believed to be part of the original crust of Ganymede, that probably cracked and spread apart. The lighter regions are most likely water ice that has moved in, replacing about half of the old, dark surface. The brilliant white material that surrounds some craters is probably clean water ice that splashed out from inside the satellite.
Saturn's moon Enceladus, has a bright, smooth icy surface that contains cracks and grooves, suggesting the release of water from below the surface within the last one or two billion years. This would be consistent with the satellite's low mass density of just 1240 kilograms per cubic meter, suggesting that it is just a big ball of water ice. Enceladus is caught in a gravitational tug-of-war between Saturn and the satellite, Dione, whose orbital period is about twice that of Enceladus. Dione's repeated gravitational tug produces Enceladus' eccentric orbit, and causes recurrent tidal flexing from Saturn that may warm the moon's interior.