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The best evidence seems to be that in the early part of the life of the solar system, there were numerous small bodies moving around, and collisions (and cratering) was common. After some period (a few hundred million years) most of these smaller bodies had either hit something or been gravitationally flung out of the system, or had been segregated into stable orbits, and in any case the result was a marked decrease in cratering events. So surface areas which are old tend to be very cratered (e.g. the surface of Luna or Mercury or Ganymede) while younger surfaces (Europa, Io) or surfaces subject to weather (Earth, Venus, Titan) are not. Mars is curious because some parts of its surface are heavily cratered and some are not. There are three relatively uncratered features in particular which stand out. First, there is a massive impact crater in the southern hemisphere at (70,-40). There are a few craters in that area but not many; it appears to be the result of a really big impact after the main cratering interval was over. Second is a large area of active vulcanism centered at -110 degrees longitude, on the equator. This includes the largest known mountain in the solar system, Olympus Mons, which is so tall that its peak is outside the atmosphere of Mars. Given that these volcanos were erupting long after the cratering event, it's not surprising that the surface there is quite smooth since the craters have long since been buried by lava flows. The third, and largest, uncratered region is the northern depression. It is not an impact crater and there's no evidence of vulcanism there. With just an occasional crater it contrasts strongly with the heavily cratered ancient areas of the surface. So why is the surface of the northern depression so free of craters? I think there can be no doubt: water. For most of the interval of heavy cratering, that part of the surface was under water. That did two things: it prevented most of the craters from forming at all, and any really big ones which did form were eliminated by erosion and sedimentation. After the oceans vanished, later rare impacts formed craters sitting in isolation. (There are features which are clearly rivers.) So where did the water go? Not yet known, and it may have gone a lot of different places. First, some of it may be underground in permafrost. Second, a fair amount of it is probably locked up in the ice caps. But it is unlikely that all the water could be stored in that way. The majority of it (IMHO) probably evaporated off into space along with most of Mars' atmosphere. The problem Mars had was that it just didn't have enough mass to hold an atmosphere for 4 billion years (especially since it also ceased being volcanically active a long time ago, removing a major source of new atmospheric gasses), and once most of the air was gone, the water would go next. But that process is a slow one, taking hundreds of millions of years. Mars is the most exciting body in the solar system (besides Earth) because it is the one which future humans are most likely to terraform and colonize. It's almost tailor-made for it, in fact, given that its day is nearly the same as that of Earth. (Agriculture on the Moon would require substantial infrastructure because a lunar "day" is 29 earth-days long.) If we could create a new atmosphere on Mars (not impossible; the required gasses are there, stored in rocks) it would last longer than the human race. (discuss) Update: If you have a very fast PC running Windows, you should definitely go here and download the Mars Explorer program, and also the Mars screen saver. |
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