Variations in Craters Across the Solar System

Although impact craters on all the solid bodies of the solar system are grossly similar, their appearances from body to body can vary dramatically. The mostnotable differences are a result of variations among the bodies in surface gravity and crustal properties. A higher surface gravitational acceleration creates a greater pressure difference between the floor of the crater and the surface surrounding the crater. That pressure difference is thought to play a large role in driving the collapse process that forms complex craters, the effect being that the smallest complex craters seen on higher-gravity bodies are smaller than those on lower-gravity bodies. For example, the diameters of the smallest craters with central peaks on the Moon, Mercury, and Venus decrease in inverse proportion to the bodies' surface gravities; Mercury's surface gravity is more than twice that of the Moon, whereas Venus's gravity is more than five times that of the Moon.

The inherent strength of the impacted surface has an effect similar to that of surface gravity in that it is easier for craters to collapse on bodies with weaker near-surface materials. For example, the presence of water in the near-surface materials of Mars, a condition thought to be likely, would help explain why the smallest complex craters there are smaller than on Mercury, which has a similar surface gravity. Layering in a body's near-surface material in which weak material overlies stronger strata is thought to modify the excavation process and contribute to the presence of craters with flat floors that contain a central pit. Such craters are particularly prominent on Ganymede, the largest moon of Jupiter.

Observations of the solid planets show clearly that the presence of an atmosphere changes the appearance of impact craters, but details of how the cratering process is altered are poorly understood. Comparison of craters on planets with and without an atmosphere shows no obvious evidence that an atmosphere does more than minimally affect the excavation of the cavity and any subsequent collapse. It does show, however, that an atmosphere strongly affects emplacement of the ejecta blanket. On an airless body the particles of excavated material follow ballistic trajectories. In the presence of an atmosphere most of this material mixes with the atmosphere and creates a surface-hugging fluid flow away from the crater that is analogous to volcanic pyroclastic flow on Earth. On an airless body an ejecta blanket shows a steady decrease in thickness away from the crater, but on a planet with an atmosphere the fluid flow of excavated material lays down a blanket that is relatively constant in thickness away from the crater and that ends abruptly at the outer edge of the flow. The well-preserved ejecta blankets around Venusian craters show this flow emplacement, and field observations of Earth's impact structures indicate that much of their ejecta were emplaced as flows. On Mars most of the ejecta blankets also appear to have been emplaced as flows, but many of these are probably mudflows caused by abundant water near the Martian surface.

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