Moon and Mars
Mars is the most familiar of all the planets. In part, that's because we've seen a lot of it. Spacecraft have transmitted hundreds of thousands of pictures from orbit and from the surface. And the landscape itself is familiar -- Earth-like deserts, canyons, and mountains.
Yet much about Mars remains hidden. We know, for example, that water once flowed across the surface, and that much of it is now frozen below the surface. But we don't know how much water remains on the planet, or whether any of it is still in liquid form -- perhaps in aquifers far beneath the surface.
We don't know if life ever took hold on the planet, either. The half-dozen craft that have landed on Mars haven't found any living organisms. In fact, they've found that the top layers of Martian dirt are pretty sterile. But if life ever did evolve on Mars, a record of it could be preserved as fossils. It'll take a lot more time and effort to scour the planet for such evidence.
We're also a little fuzzy on the details of how the Martian climate has evolved over the years. Like Earth, the planet has undergone major shifts over the eons, but it'll take a lot more work to figure out how and when these changes took place.
So there's still a lot of work to do to completely familiarize ourselves with this familiar world.
Look for Mars near the Moon tonight. They're high in the sky at nightfall, with Mars a little above the Moon. It looks like a bright orange star.
Script by Damond Benningfield, Copyright 2010
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