Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p21b1669a&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P21B-1669
Physics
[6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
When the core dynamo of Mars initiated and why it ceased is not well understood. Attempt is made in this study to constrain the activation period of the core dynamo, assuming that it was powered by vigorous thermal convection in the iron core. Two distinct periods of the planet are investigated: the latest stage of accretion when the growing Mars embryo likely experienced high-velocity embryo-embryo collisions, and during the late bombardment of 20 large impacts at ~4 Ga that created the giant basins on Mars. The impact heating of Mars by a large embryo, 1000 or 2000 km diameter, results in thermal stratification of the core where temperature increases with radius. The thermally stratified core requires 100-200 Myr to cool and regain vigorous convection, powering a strong core dynamo. The weak magnetization of an extended primordial curst in the southern hemisphere supports this scenario. The impact heating of Mars by the 7 largest of the 20 impacts is studied in detail. The collective battering the core dynamo by the impacts eventually kill the dynamo. A strong core dynamo existed for ~300 Myr, until Ares impact which introduced substantial perturbations to the core temperature and together with the following Amazonis impact constrained the core convection to the upper ~200 km for ~28 Myr, during which the temperature gradient deep in the core gradually reduced from super-adiabatic to sub-adiabatic. The strong dynamo that existed prior to Ares impact retained its strength in the subcritical condition until Acidalia impact which killed the dynamo
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