This image shows a very young impact crater in the moon's Balmer basin. The dark streamers are impact melt splashes thrown out during the crater formation. The entire image is 1,302 meters (0.8 miles) wide. Credit: NASA/Goddard Space Flight Center/Arizona State University
› Larger image
When you look at the moon, you'll notice right away that the moon is covered with impact craters. The size of an impact crater is related to the size of the object that struck the moon (such as an asteroid or comet). Smaller impacts happen much more frequently than large impacts, so the moon has many small craters. Smaller impact craters also tend to be younger than larger impact craters because they are easily destroyed during the formation of other craters.
The unnamed crater shown here is notable for several distinct features. It is not circular in form like most impact craters, perhaps due to a pre-existing crater on the surface or a weakness in the moon's crust. The dark streamers are the result of rock that melted and was sprayed out as part of the debris thrown out during the impact. The jumbled floor of the crater is made of rock broken during the impact mixed with impact melt and soil. It has probably changed little since the crater formed.
Most impact craters on Earth are poorly preserved due to rain, wind and other forces not present on the moon. Meteor Crater
in Arizona is one of the best-preserved examples. Its diameter of 1,200 meters (about 0.75 miles) is a just a little larger than the lunar example shown here. Meteor crater is also distinctly non-circular, and when viewed from above looks a bit squared off. The strikingly square-like shape is due to perpendicular faults that provided a path of weakness for material to move during the impact.
Mark Robinson, ASU