This image shows a fresh impact crater about 2 kilometer (1.2 miles) across. How do we know it is fresh?
The crater walls are steep and rocky, and fine striated texture is still visible on the ejecta. Over time, erosion and dust settling out of the atmosphere will smooth out such details. However, these processes are slow on Mars, and the crater is probably at least several million years old.
Craters like this are important targets for HiRISE for several reasons. The details of the fresh crater are interesting in themselves for studying impact processes, but crater walls can also provide great exposures of bedrock. The steep slopes are also good places to look for active processes like rockfalls happening today, especially when we can compare a series of images taken over several years.
Photo credit: NASA/JPL/University of Arizona