Things are changing on Mars. The Red Planet is about half way through spring in the northern hemisphere and autumn in the southern hemisphere. The switch triggers a kind of “see-saw” effect. As the north gets warmer, its polar ice cap gets smaller. And as the south gets colder, its ice cap gets bigger. That transition can cause the air pressure to vary by as much as a third across the entire planet.

The ice caps are made mainly of frozen water. They’re up to a couple of miles thick. They’re marked by spiral patterns of deep valleys – perhaps carved by strong winds.

A layer of dry ice forms atop each cap every autumn, and vanishes every spring. The carbon dioxide is pulled from the atmosphere, which is much thinner than Earth’s. So as the dry-ice cap moves from pole to pole, the atmospheric pressure changes by quite a bit.

There’s disagreement about what lies below the southern ice cap. A few studies have suggested there could be liquid water – perhaps in a thin layer, or mixed with the dust and rock in the ground. But other studies say that liquid water couldn’t survive in the conditions below the ice at the south pole of Mars.

Mars is in a crowded region of the sky tonight. It’s close to the upper right of the Moon at nightfall, and looks like a bright orange star. The true star Pollux is closer to the lower left of the Moon, with Castor, the other twin of Gemini, to their upper left.

Script by Damond Benningfield