The most prominent features on Mars are its polar ice caps. They’re easily visible through any telescope — small white splotches against the orange landscape.
The ice caps undergo major changes over the course of a Martian year. And as they change, so does the Martian atmosphere.
Both ice caps are made primarily of frozen water. The northern cap is a lot wider than the southern one, but it’s not as thick. So they contain about the same amount of water — each one holds roughly half as much water as the ice cap that covers Greenland.
During winter, the caps grow thicker. That’s because they get so cold that carbon dioxide in the atmosphere freezes atop them — a quarter or more of the total atmosphere. During spring and summer, the CO2 vaporizes and returns to the atmosphere. That can stir up big dust storms, like the one that covered much of Mars earlier this year.
All of the carbon dioxide vanishes from the northern cap. But the southern cap is at a much higher elevation, so it stays a lot colder. So it retains a thin layer of carbon dioxide all year.
Both ice caps consist of layers of ice mixed with dust. Probing those layers can reveal the planet’s geologic history. Radar aboard a Mars orbiter, in fact, confirmed that an ice age gripped the planet about 400,000 years ago — making the ice caps even more prominent.
Look for Mars quite close to the upper left of the Moon this evening. It looks like a bright orange star.