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Figure: A watering hole for Martian thoats? Gullies seen on the canyon walls of Nirgal Valis may be subsurface water draining down-slope, causing mile-long channels. (Credit: NASA/JPL/Malin Space Science Systems)
The destiny of Mars was sealed by something as simple as its smaller mass, which forced a rapid evolution of its atmosphere and interior. It may have been an entirely Earth-like world three billion years ago. Eons later, it has become a cold planet that has largely lost its atmosphere. Yet in it’s exposed, ancient crust we see written a fascinating story of rivers that once flowed, and vast, but shallow, oceans that once graced its rusty-red lowlands.
We can see these gullies carved into valley walls and running down-slope. Since the earliest telescopic viewing of Mars, its north polar cap has given us tantalizing hints that water-ice (not just carbon dioxide-ice) may be present at the surface in vast, exposed deposits. Intense solar ultraviolet radiation breaks water molecules into free hydrogen and oxygen. Released from its molecular prison, oxygen flows into the atmosphere and is quickly lost by incessant collisions with solar wind particles at a rate of something like 70,000 tons per year. On Earth, this loss is slowed to only a few hundred tons per year.
Part of the vast difference is in the masses of the planets, but it is also a matter of a much less obvious ingredient: Their magnetism. Earth's magnetic field deflects much of the solar wind away from Earth's atmosphere like an invisible umbrella that prevents the wholesale erosion of our atmosphere. Even the feeble magnetism of Mars seems just enough to slow down the loss of its atmosphere solar wind erosion.
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This answer was updated in 2011.
See my books:
The Astronomy Cafe (1998) and
Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011
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