Mars is similar to Earth. It's got three parts:
The Crust - a thin outer layer of rocks ,
The Mantle - made up of rocks (silicates),
The Core - a small dense ball of metal (nickel and iron).
Since Mars has no detectable magnetic field (it's not a magnet), there probably isn't any liquid metal in the core like the Earth has.

You can't live on Mars, but it would be a pretty good place to have a space station. Without a good space suit and a special station to live in, you wouldn't be able to breathe and there isn't any water to drink. There is, however, some water in the form of ice on the polar ice caps. These ice caps have frozen carbon dioxide in them too. (This is what we breathe out.) Scientists think that there might have been regular water on Mars at one time, but they haven't proven it.

To escape Mars' gravity and get out into space, a rocket has to travel at a speed of 11,000 mph or 5.0 km/sec. That's about 169 times faster than your parents are allowed to drive on a U.S. highway!

Mars has two moons: Deimos and Phobos. They are a lot like the asteroids in the asteroid belt.

Mars has the tallest mountain of any planet in the solar system. It's called Olympic Mons and it's more than twice the height of our tallest mountain, Mt. Everest. The cool thing is that Mars has the largest canyon in the solar system too! It's called Valles Marineris and it would stretch all the way from Los Angeles to Washington D.C. This sure makes our Grand Canyon look pretty pathetic in comparison.

Scientists think that they have possible evidence for life existing on Mars. They found tubular structures in Martian meteorites that landed in the Antarctic. Some think that these structures are the fossils of microorganisms that lived on Mars billions of years ago.

Sources:
In Quest of the Universe, 2nd ed. by Karl F. Kuhn
Voyages Through the Universe, 2nd ed. by Fraknoi, Morrison and Wolff
Universe by Kaufmann
The Complete Idiot's Guide to Astronomy by Christopher De Pree and Alan Axelrod
The Astronomy Cafe by Sten Odenwald