lunar image of the month

february: ray crater on mercury

In many respects the Moon and Mercury are quite similar. Mercury has only 1.4 times the Moon's diameter and twice its gravity, so both are airless worlds. With no atmosphere to erase the evidence, both carry the scars of impacts dating back to the early stages of the solar system.

Mercury's crater Degas, as seen in this Mariner 10 composite image taken in 1974, is 45km (28 miles) wide and has a large ray system (though smaller than the lunar equivalent due to the higher gravity). The rays are formed when material displaced by the original impact (called ejecta) sprays out, covering surrounding features. As with the Moon, this implies a relatively young crater, though some even younger ones can be seen on top of the layer of ejecta.

Image: Mariner 10 Project, NASA.