Mike Caplinger, Malin Space Science Systems February 1994
Martian craters show a unusually wide variation in characteristic shapes and features when compared, for example, to craters seen on the Moon. Some do resemble those seen on the Moon -- for example, those smaller than 5 km in diameter are usually bowl-shaped, with raised rims and slightly flat floors, just like similarly sized craters on the Moon.
Small, simple crater (5 km diameter)
Very large craters -- those larger than about 50 to 70 km in diameter -- resemble flat plains ringed by a circle of hills. These craters also look something like their lunar counterparts, although they appear shallower and their rims are often worn down.
Large crater (60 km diameter)
Large craters often have central peaks, and the larger ones may have concentric rings of hills within them. In the latter case, they are called multi-ring basins, and are characteristic of the larger impacts on Mars (and other bodies in the solar system -- for example, Mare Orientale on the Moon, Valhalla on Callisto, and the Caloris Basin on Mercury.)
The craters on Mars that are most unusual generally occur between the sizes of 5 km and 50 km. They are distinguished by variations in the material thrown out from the crater during the impact that formed it -- the ejecta blanket. Some are surrounded by single lobes of material:
Single lobe ejecta (7 km diameter)
while others may have double lobes.
Double lobe ejecta (16 km diameter)
Some have complex sprays of material resembling the petals of a flower,
"petal" ejecta (18 km diameter)
and some are surrounded by flat, pancake-shaped ejecta.
Many scientists have speculated that the form of the ejecta blankets of medium-sized craters depends on the amount of subsurface water present when the impactor hit the surface. Certainly the ejecta resembles patterns that one might see throwing rocks into fluid mud, and for this reason these craters are often called "splosh" or "fluidized ejecta" craters. However, one must be careful not to predict the behavior of impactors moving at many kilometers per second from pebbles tossed into mud, and the question of exactly what factors contribute to the form of these ejecta patterns must wait for the further exploration of the planet.
Mike Caplinger (firstname.lastname@example.org)