Martian Air 
Every day, Earth loses a tiny bit of its air to space. Atoms at the top of the atmosphere are blown away by the solar wind. The loss doesn't amount to much, though -- in part because Earth's magnetic field acts like a protective bubble -- it deflects most of the solar wind around it.
Mars, on the other hand, doesn't have a planet-wide magnetic field. And that could be one reason its atmosphere is so cold and thin.
The early Mars probably was wrapped in a thick, warm blanket of air. It kept things cozy enough for liquid water to flow across the surface. But over the last few billion years, most of that atmosphere has disappeared.
The atmosphere consists mainly of carbon dioxide. It's possible that a lot of the CO2 in the atmosphere could have been absorbed by minerals on the surface to form rocks -- a process that has pulled much of the CO2 from the atmosphere here on Earth.
It's likely, though, that a lot of the Martian atmosphere was simply blown away into space. The planet's magnetic field is weak and spotty, so it doesn't hold back the solar wind. In fact, the magnetic field may actually help carry away some of the air. Under this scenario, the solar wind rips off chunks of the magnetic field and carries them off into space -- magnetic "balloons" filled with Martian air.
Look for Mars in the east this evening, well below the Moon. It looks like a bright orange star. More about Mars and the Moon tomorrow.
Script by Damond Benningfield, Copyright 2009