Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.v13c2048m&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #V13C-2048
Physics
[1507] Geomagnetism And Paleomagnetism / Core Processes, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5440] Planetary Sciences: Solid Surface Planets / Magnetic Fields And Magnetism, [5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution
Scientific paper
Despite recent improvements from cosmochemistry, petrology and numerical modeling, the early state of terrestrial planets is still debated. In particular, crustal magnetic fields recorded in some of the oldest rocks on the Moon and Mars suggest that the thermal regimes of the mantle and core of Mars and possibly the Moon permitted an early but short-lived dynamo. We investigate whether such regimes can result from the mechanics of accretion and core formation. Large meteoritical impacts on an undifferentiated growing planet lead to local heating under the impact site, melting and separation between the metallic and silicate phases. Hence, as the volume of the differentiated zone is of order of the volume of the impactor, consequent volumes of metal can sink towards the center of growing planets and contribute to early core formation. During the sinking of a metallic diapir, viscous heating occurs depending on the size of the blob and the extent of crystallization. One key issue for early dynamo action is the temperature difference between the core and mantle following core formation. We use numerical models to investigate conditions in which the formation and segregation of core material leads to a core-mantle temperature difference sufficently large to drive a dynamo. We build numerical models in axisymetrical spherical geometry to characterize the dynamics of the sinking diapir and its thermal evolution. We show that once the metallic phase has reached the center of the impacted planet, convection occurs in the protocore during several hundred million years and can generate an early magnetic field. We discuss the effect of the crystallization of the metallic phase and of the rheology of the undifferentiated material on the duration of this early magnetic field. Our results underline the importance of the accretionary conditions (size and temporal distribution of impacts) on the magnetic history of growing planets and moons.
Jellinek M.
Johnson Clifton L.
Monteux Julien
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