NASA Mars Reconnaissance Orbiter HiRISE Imagery Release 30 Apr 2008

This image shows defrosting sand dunes near the north polar region of Mars.

Around Mars’ north pole is a vast region or “sea” of sand dunes that become covered with carbon dioxide frost or ice in the northern hemisphere’s winter. The light areas indicate that parts of the dunes are still covered in frost or ice.

As Mars’ northern hemisphere enters into spring and begins to warm, the carbon dioxide sublimates (turns directly from a solid to a gas). The carbon dioxide sublimates in surprising ways, with trapped gas bursting through the ice in jets that leave dark streaks when the wind is blowing

During the summer, all the frost will have sublimed leaving dark sand dunes. The unfrosted dunes are dark because the sand is derived from dark volcanic rocks.

	Lava Flows at the Summit of Olympus Mons Olympus Mons, the largest volcano in the solar system, has a large depression called a “caldera” at its top.

	Lava Flows from Pavonis Mons This image shows geologically young lava flows west of Pavonis Mons, part of a group of giant shield volcanoes.

	Ejecta Blanket of an Impact Crater Since the impact event, this ejecta has been subject to millions of years of wind erosion.

	Defrosting Dunes in the North Polar Sand Sea Around Mars’ north pole is a vast sea of sand dunes that become covered with carbon dioxide frost or ice in the winter.

	Segment of a Crater Ray There are more than half a million small craters visible in this observation, which covers less than 1 percent of the ray system.

	Exposed Layers in Crater Wall Near Mawrth Vallis The crater gives us a peek into what lies beneath, which will help interpret the history of this part of Mars.

	North Polar Layered Deposits in Springtime These layered deposits are thought to be ice and dust deposits that record climate changes over thousands of years.