Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p33b1452g&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P33B-1452
Physics
5420 Impact Phenomena, Cratering (6022, 8136), 5430 Interiors (8147), 5455 Origin And Evolution, 5480 Volcanism (6063, 8148, 8450), 5499 General Or Miscellaneous
Scientific paper
A large impact not only creates a giant basin on a planet but also results in considerable melting in the mantle, especially if the impact occurs in the early history of the planet. There are generally 4 stages of melting caused by a large impact: 1) Melting of a major part of the impactor and target material beneath the impact site, due to release of the kinematic energy of the impactor that is largely converted to heat energy; 2) Melting in the upper mantle due to immediate depressurization caused by excavation of crustal material from the impact site. This stage of melting is simultaneous with the excavation process and it usually occurs in the upper mantle because the pre-impact temperature is usually close to, or at, the melting temperature, and the sudden depressurization allows melting to occur; 3) Melting in the upper mantle due to its upwelling in order to achieve isostatic compensation, during which rocks from deeper parts of the upper mantle move to low- pressure upper parts. This process involves appreciable displacement of the mantle material when the resulting excavation volume is very large, and it may take up to a few thousand years to accomplish; 4) Melting in the entire mantle by convection circulations that develop in response to the temperature perturbations in the upper mantle caused by the second and third stages of melting. Depending on the size of the impactor and the pre-impact temperature condition of the mantle, this secondary convection may take a much longer time to develop and it usually results in enormous amount of melt and extensive volcanism, which are by far more important than those associated with the first three stages of melting. We study the secondary convection induced in the Martian mantle by large impacts that created giant basins such as Utopia, Acidalia, Ares, Deadalia, Hellas, Isidis, and Argyre, as well as the giant Borealis impact that likely created the major part of the northern low lands. We investigate two-flow convection in an axisymmetric spherical shell mantle model, where the mantle is allowed to melt as it crosses the solidus temperature. We consider two different models, permeable and impermeable. In the permeable model the melt is allowed to migrate inside the partially molten solid matrix which in turn convects, whereas in the impermeable model the melt does not move relative to the convecting solid matrix, but it is extracted once it exceeds a threshold of 3% rock volume. We modeled different sizes of the impactors as well as different thermal stages of the pre- impact mantle and thickness of a stagnant lithosphere. We show that depending on the size of the impactor, the pre-impact temperature conditions in the mantle, and the presence of a thick and strong lithosphere, the major melting induced by the secondary convection may take a very short time for giant impacts, such as Borealis and Utopia, but up to several hundred million years for impacts that created smaller basins such as Isidis and Argyre. It is shown that due to the long delay of partial melting in the mantle the lithosphere beneath these small basins cooled appreciable and gained enough strength to support the excess mass concentrations (mascons) that have survived for 3.5-4 Gyr.
Arkani-Hamed Jafar
Ghods Abdolreza
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