POSSIBILITIES AND SIGNIFICANCE OF MARINE-TARGET CRATERS ON MARS

Observations of impact craters on Earth show that a water column at the target strongly influences lithology and morphology of the resultant crater. The degree of influence varies with the target water depth and impactor diameter. In cases of strong influence, a concentric shape with a crater inset within a shallower outer crater can occur. Studies of the Lockne crater, Sweden, show that a thick target water layer influences the ejecta flow, such that the outer crater can become covered by extensive flaps of coherent, relatively low-shock ejecta from the inner crater. The Lockne example also shows that a resurgence of water can dissect the flaps and emplace ejecta and rip-up material back into the crater, resulting in deep channels and conspicuous polymictic breccias, respectively. If oceans existed on Mars, marine-target craters may have formed. Based on published data on estimated water depths, extents, and durations of putative oceans (e.g., within “Contacts 1 and 2”), cratering rate approximations for the reported phase, and computer modeling of minimum impactor diameters required to form long-lasting craters, we estimate the number of marine-target craters that may exist on Mars. For the smaller “Contact 2” ocean, with an estimated duration of 100,000 years and the low crater formation rate approximated for late Mars, only about 2 detectable marine-target craters would form. Considering a maximum duration of 0.8 Gy, as many as 1400 craters may have formed. For the larger “Contact 1-Meridiani” with an estimated duration of 100,000 years, no craters are calculated despite the higher crater formation rate estimated prior to 3.5 Ga. On the other hand, 160 seafloor craters may have formed during a 0.8 Gy duration. Though terrestrial investigations show that evidence may be obscured by thick sediments, discoveries of marine-target craters on Mars would help address the ongoing debate of whether large water bodies occupied the northern plains of Mars and constrain future paleoclimatic reconstructions.