Pictures from the Mars Pathfinder mission showed that there are lots of rocks on the surface of Mars. Pathfinder's Sojourner rover examined these rocks and found iron in them. Also, a dozen Martian rocks have been found here on Earth - they fell from the sky as meteorites. Those rocks have iron in them too. It's not pure iron, like you'd find in an old nail or a cannonball, but iron that's mixed into the minerals of the rock itself.

So why have things turned a rusty red color on Mars? After all, iron was found in rocks from the Moon too and the Moon isn't red. The answer may lie in Mars' distant past.

If a planet always stays cold and dry, rocks with iron in them will tend to look - well - like just plain rocks. But suppose that it gets warm and wet on a planet... warm and wet enough for there to be water in the atmosphere for awhile, or maybe even water on the surface. If that happens, rocks that have iron in them can change. This change is called weathering and it can be brought about by water. Pure iron weathers to make rust. And iron-rich rocks can weather to make reddish, rust-like minerals.

What you see when you look at Mars is just a beautiful, red dot shining in the night sky. But that red color is telling you something important: that long ago Mars may have been warmer and wetter than it is today. It's one of the reasons we think there could have once been life on Mars, and also one of the reasons why we explore there.