W/o an atmosphere to erode them, they stay fresh because scientists now know the age of the moon, they can count the number of impacts craters to find the rate of the cratering science the birth of out solar system. By knowing the rate of crating, scientists are able to use the number of the craters on any body of water to estimate how old the body of water is. This is easier because scientists dont need to bring back the rock to find out.
