Moon and Mars
There's plenty of evidence that Mars was once much warmer and wetter than it is today. Ancient riverbeds and floodplains scar the surface, and many of the Martian rocks formed in a wet environment. A more recent bit of evidence came from beyond Mars: a meteorite that the Opportunity rover discovered this summer.
Opportunity was beetling across the sandy Martian plains when it came across a shiny blue-black rock the size of a watermelon. When it analyzed the rock, it found that it's a meteorite made of iron and nickel.
The meteorite is so big and heavy that it should have been pulverized when it hit the ground. Since it wasn't, scientists concluded that it must have hit Mars at a time when the planet's atmosphere was much thicker than it is today. The air would have slowed the rock's descent enough to keep it from disintegrating when it hit the ground.
But scientists aren't sure just when the meteorite hit. It could have been billions of years ago -- a time when scientists are sure that Mars was much warmer and wetter. Or it could have been more recent -- at a time when the planet warmed up for a while, releasing gas from its ice caps to create a thicker atmosphere. Either way, the atmosphere slowed the meteorite's plunge toward the surface -- preserving more evidence about the planet's past.
Look for bright orange Mars near the Moon late tonight. They rise before midnight, and stand high overhead at first light tomorrow.
Script by Damond Benningfield, Copyright 2009
For more skywatching tips, astronomy news, and much more, read StarDate magazine.