Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p43d1712k&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P43D-1712
Mathematics
Logic
[6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
Geologic activity has been changing the surface of Mars throughout the planet's history. One aspect of this activity is that the topography of large impact craters is generally much shallower than expected, and the large scatter in the depths as a function of diameter suggests this shallowing is a product of post-impact modification processes. While geologic analyses of these craters reveal units superposing the craters that are likely volcanic or sedimentary in nature (i.e., infilling of the craters), here we discuss another process: lower crustal flow and crater relaxation. A corollary observation is the magnitude of the compensating topography on the crust-mantle boundary (CMB) is also far less than expected, even accounting for surface infilling. Here, we simulate the long-term evolution of craters using a viscoelastic finite element approach. Our candidate craters are ~200-500 km in diameter (large enough to be resolved in current gravity models) and span an age range from mid-Noachian to Hesperian. Pressure gradients due to crustal thickness variations cause lower crustal flow, facilitated by high temperatures in the deep crust afforded by high ancient heat flows on Mars and by the remnant impact heat, which serves to decouple mechanically the surface and CMB (though they remain coupled via mass conservation). This lateral movement relaxes the CMB by moving material in from the periphery and thickening the crust beneath the crater. An example is illustrated in the figure below, which shows the evolution of a crater 182 km in diameter. The initial topographies are shown with dotted lines, and our results are shown with solid lines. Lower crustal flow eliminates most of the topography on the CMB. Loss of buoyancy requires transfer of support of the surface topography to the lithosphere, which flexes upwards in response. The mechanical decoupling is revealed by the dashed line, which shows the predicted CMB topography assuming isostatic support of the final surface; the difference demonstrates the support of the surface comes from the lithosphere and not buoyancy. This process is sensitive to the magnitude of the background heat flow, and future work will use this phenomenon as a probe into the thermal history of the Red Planet.
Dombard Andrew J.
Karimi Majid
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