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J. Ormo (International Research School of Planetary Sciences IRSPS, Pescara, Italy.)
Most bolides that collide with the Earth hit the sea. All marine-target craters so far found on Earth have formed in epicontinental seas. Among them are the famous Chicxulub crater, Mexico, as well as a group of Middle Ordovician craters formed at different water depths. Rapid coverage by sediments helps the marine-target crater to survive later erosion. Therefore, some of these craters are among the best preserved crater structures in the world. In the oceans, the great average water depth (> 4 km) prevents the smallest, and most frequent, impacts from forming craters on the seabed. The large impacts necessary for cratering the ocean floor are rare. This circumstance, combined with the young age of the oceanic crust, explains why only few craters are likely to be found in the oceans. The geological features of craters formed at sea are most likely a result of influence of target on the cratering process. When the crater diameter is large compared to the water depth, the crater resembles counterparts formed on land. Craters formed in deeper water are concentric, often lack melt sheets and rim walls but have deposits and radial gullies formed by the resurge of the sea. These features would exist at other craters in the Solar System with similar target environments. A number of Martian impact craters resemble craters formed in shallow epicontinental seas on Earth. The best examples are situated in an intermediate zone between the northern plains and the highlands. However, at the moment only the morphological features can be investigated, although lithological evidence is needed to prove a marine target. An outer shallow crater must be considered when estimating marine-target crater dimensions. These impacts are probably much more energetic than is suggested by the dimensions of the more often preserved, inner part of the crater.
The author(s) of this abstract have provided an email address for comments about the abstract: ormo@sci.unich.it