Regions of the moon known as irregular mare spots, formed by magma cooling as a result of a volcanic eruption, have almost no large craters, indicating that they must be relatively young. By examining the distribution of the craters in them, we can estimate when these regions were formed: no more than 100 million years ago.
The moon isn’t very active these days, and to be honest, it hasn’t been very active in a long time. By examining the craters on the surface – specifically, how many there are and how big they are – we can get a handle on the ages of different parts of the lunar surface.
This is because volcanic activity wipes away all of the craters and the moon reappears. The longer the time since last surfacing, the more craters we will see. They will also tend to be larger as it takes a long time to be hit by that unfortunate blow from a massive asteroid or comet.
Almost the entire surface of the moon is incredibly old and well over a billion years old. But regions on the near side of the moon known as irregular mare spots (IMPs) tell a different story. These IMPS, usually no more than a dozen kilometers wide, do not have large craters.
This means that the IMPs are regions where volcanic activity occurred relatively recently, only 100 million years ago. This makes IMPs one of the youngest regions on the lunar surface.
We don’t currently know what kept the moon so warm that it has recently triggered volcanic eruptions. An alternative hypothesis suggests that the IMPs are not young at all, just made up of some form of magma with very low density. Perhaps a mission devoted to the study of IMPs will give us the answer.
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