When a cosmic impact strikes an aquatic target environment a crater may develop in the seafloor. The effects of the water layer on the cratering process vary with the relation between the magnitude of the event and the water depth. For a relatively deep target water the transient crater becomes concentric with a wide outer crater in the water column and weaker sediments, and a deeper, nested crater in the basement. The final crater often has under-developed or collapsed rim, sometimes crossed by gullies eroded by the resurging water. If the water resurge can overcome the rim, the crater gets filled with debris, so-called resurge deposits. Studies of drill cores from resurge deposits at several impact craters show a direct relation between clast frequency (<N>), event magnitude (i.e., the projectile diameter, d) and target water depth such that any of these factors can be calculated if the others are known. At relatively shallow-water impacts the ejecta may be forming concentric debris-flow ridges although easily eroded in a terrestrial environment. Alltogether, these special features of marine-target craters can indicate a past marine environment even in locations where the envoronment today is completely different. This can help identify past or present Life habitats.

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