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One of the things that Earth and Mars have in common is the seasons. The two planets are tilted at almost the same angle. So the amount of sunlight received by their northern and southern hemispheres varies by quite a bit during the year.

And the seasons are changing on Mars this week. It’s the autumnal equinox — the beginning of autumn in the northern hemisphere, and spring in the southern hemisphere. The season will last until the winter solstice, on September 2nd.

Mars will be moving closer to the Sun throughout the season. It’ll be closest to the Sun around the start of southern summer. So the southern hemisphere will be warming up in a hurry. It’ll still be icy cold by Earthly standards. But the extra heat is enough to trigger big dust storms. And some of them can get big enough to cover the entire planet.

The extra heat also causes part of the southern polar ice cap to vanish. Carbon dioxide atop the ice cap vaporizes and enters the atmosphere. That causes the air pressure to go up a bit across the entire planet. Later, as the hemisphere cools, the CO2 recondenses on the ice cap, making it bigger and thicker — one of the cycles of the Martian seasons.

And Mars remains in good view in the early morning sky now. It forms the lower left end of a lineup of three planets at first light. Saturn is to the upper right of orange Mars, with brilliant Jupiter farther along the same line.

Tomorrow: listening in on a nearby star.

Script by Damond Benningfield