2. The new, close-up view from space
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.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University