Astronomy Picture of the Day

Explanation: What lies on the floor of this Martian crater? A frozen patch of water ice. The robotic Mars Express spacecraft took the above image in early February. The ice pocket was found in a 35-kilometre wide crater that resides 70 degrees north of the Martian equator. There, sunlight is blocked by the 300-metre tall crater wall from vapourizing the water-ice on the crater floor into the thin Martian atmosphere. The ice pocket may be as deep as 200 metres thick. Frost can be seen around the inner edge on the upper right part of the crater, while part of the lower left crater wall is bathed in sunlight. The existence of water-ice pockets inside craters near the Martian North Pole, like that pictured above and others noted previously, give clues not only about surface conditions in the Martian past but also possible places where future water-based astronauts might do well to land.