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Narrator: Listen to part of a lecture in an astronomy class. Professor: Today we're going to talk about why Mars appears red. Now, you might think it's because of the rusty rocks on its surface, and that's partly true. But there's more to it. The red color comes from iron oxide, or rust, which covers most of the planet. When sunlight hits Mars, the dust in the atmosphere scatters light. But unlike Earth's atmosphere, which scatters blue light, Mars has a thin atmosphere filled with fine dust particles. These particles are larger, so they scatter red light more effectively. This is why the sky on Mars appears pinkish or butterscotch-colored, and the planet itself looks red from a distance. However, this wasn't always the case. Billions of years ago, Mars had liquid water and possibly a thicker atmosphere. The water would have oxidized the iron in the rocks, creating rust. Over time, as the atmosphere thinned and water disappeared, the rust remained, and the dust storms spread the reddish particles all over the planet. So, the red we see today is a result of ancient water and ongoing dust.