Why is there a spiral around the North Pole of Mars? Each winter this pole develops a new outer layer about one meter thick composed of carbon dioxide frozen out of the thin Martian atmosphere. This fresh layer is deposited on a water-ice layer that exists year round. Strong winds blow down from above the cap’s center and swirl due to the spin of the red planet — contributing to Planum Boreum’s spiral structure. The featured image is a perspective mosaic generated in 2017 from numerous images taken by ESA’s Mars Express and elevations extracted from the laser altimeter aboard NASA’s Mars Global Surveyor mission. [via NASA] https://ift.tt/OkVi5y0