The planet Mars is cold and dry, with an atmosphere that's less than one percent as thick as Earth's -- a desert world that's not especially friendly for life. But that hasn't always been the case. Billions of years ago, it was much warmer and wetter. An ocean may have covered much of the planet, surrounded by a thick, warm atmosphere.
Much of the water may still be there -- frozen beneath the dusty surface or in the polar ice caps. But much more probably wafted off into space -- along with a good chunk of the Martian atmosphere.
In fact, wisps of Martian air are still escaping into space today. They're carried off by the solar wind -- a stream of charged particles from the Sun.
The solar wind erodes the Martian atmosphere all the time. But a recent study found that it carries off more of the atmosphere at some times than at others. The heaviest erosion occurs when fast streams of the solar wind overtake slow streams, creating pressure waves. As these waves break across Mars, they grab almost three times more of the atmosphere than average -- making the desert planet even less friendly for life.
Look for Mars high in the western sky at nightfall. It looks like a bright orange star. And it's especially easy to find tonight because it has an equally bright companion: Regulus, the brightest star of Leo, the lion. Regulus stands just a whisker below the Red Planet.
We'll have more about this pairing tomorrow.
Script by Damond Benningfield, Copyright 2010