december: micro impact crater
The largest crater on the Moon, and indeed in the solar system, is the South Pole / Aitken Basin which is 2250km (1400 miles) across. At the other end of the scale are the micrometeorite impact craters, such as the one in the upper left part of this 0.25mm diameter glass bead from an Apollo 11 lunar soil sample. The bead itself was created by another impact of intermediate size.
In general the size and frequency of impacts on the Moon and Earth have decreased enormously during the 4.7 billion years since the early days of the solar system. But this trend has been far from consistent. In fact the impact rates, determined by a variety of methods including the analysis of beads such as this, appear now to be four times higher than those of half-a-billion years ago.
It may not be chance that this period of increased activity corresponds closely with the rise of life on Earth. It has been speculated that comets and meteorites may be bringing to Earth the water and organic compounds which encourage life. It may be that the impacts which can threaten life on Earth also play a part in its creation.
Image: Timothy Culler, University of California, Berkeley, NASA.