Space Rocks II 
Mars is growing brighter now as it moves closer to Earth. Tonight, it climbs into view in the east by around 10 o’clock, and looks like a bright orange star. It’ll continue brightening through early March.
Planetary scientists hope that spacecraft will someday bring pieces of Mars to Earth. But they don’t have to wait for a sample-return mission to study pieces of Mars — they already have about three dozen of them.
The pieces are meteorites — chunks of rock that were blasted off the Martian surface and eventually made their way to Earth. Tiny bubbles of gas inside the rocks contain the same ratios of certain elements as measured in the Martian atmosphere by Mars landers, allowing scientists to identify their origin.
What scientists can’t identify is where on Mars the rocks came from. In addition, the rocks have been altered by their flight through space and their time on Earth. So while they provide important details about the Red Planet, they’re just not the same as samples collected there.
The world’s meteorite collections also include bits of the Moon, which reached Earth in the same way. They may even include bits of Venus and Mercury. But there aren’t enough measurements of the conditions on those worlds to make a good comparison with the meteorites. We’ll need new expeditions to tell us whether pieces of other worlds are already sitting in museums and other collections here on Earth.
More about meteorites tomorrow.
Script by Damond Benningfield, Copyright 2011