Astronomy Picture of the Day

Explanation: Mercury, the closest planet to the Sun, has a surface with so many craters it resembles the Earth's Moon. The largest surface feature on Mercury is the Caloris Basin, which resulted from a collision with an asteroid. The basin, which is more that 1000 kilometres across, is visible as the large circular feature at the bottom of the above photograph. Similar features, such as the Mare Orientale, are seen on the Moon. The Caloris Basin gets very hot because it is near the "sub-solar point" - the point on Mercury's surface that is directly under the Sun when Mercury is closest to the Sun.