Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p51a1409a&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P51A-1409
Astronomy and Astrophysics
Astronomy
[5417] Planetary Sciences: Solid Surface Planets / Gravitational Fields, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering
Scientific paper
With the major axis to minor axis ratio of about 1.3, Hellas is probably the most elliptical giant impact basin on Mars, even more elliptical than the basin fits to the northern lowland [1]. Because less than 4% of the impacts in the solar system have occurred at impact angles greater than 80o relative to the planet surface [2] Hellas is perceived to be formed by a single oblique impact at the first glance. Numerical simulations of oblique impacts have lead to contradictory conclusions. For 1 km size projectile the shape of a resulting crater is found indistinguishable from circle as long as the impact angle is greater than 30o [3], whereas for a putative planetary scale impact that has created the northern lowland the impact angle as high as 60 degrees fundamentally affects the ellipticity of the supergiant basin [4]. Despite these contradicting results the two numerical models are in agreement in that a single impact creates a single cavity characterized by a single bowel shape with a maximum depth located beneath the basin. The subsequent basin collapse, impact induced melt and later volcanism have resulted in a smooth floor of Hellas, to a point that the present topography does not provide any viable information about the structure of the excavated cavity. Unlike these surface processes, the mantle plug created in the process of isostatic uplift of the mantle, has likely been less modified. Based on this premise, I calculated the shape of the mantle plug beneath Hellas basin using MOLA surface topography [5] and most recent gravity field of Mars, MRO110B2 of Jet Propulsion Laboratory [6], assuming that the gravity field arises from the surface topography and the crust-mantle density contract associated with the mantle plug. Spherical harmonics coefficients of degree 2-20 are retained to suppress small scale features. The resulting crust-mantle boundary shows two distinct mantle plugs one at the northwest and the other at the southeast of the center of the basin, indicating that Hellas basin may have been actually formed by two comparable impacts with a center to center distance of about 640 km. At this harmonic band the floor of Hellas also shows two distinct but slight depressions exactly over the mantle uplifts, but the free Air anomaly is almost featureless indicating that the basin is highly compensated. . [1] Andrews-Hanna, J.C., et al., Nature, 453, 1212-1215, 2008. [2] Shoemaker, E.M., In: Kopal, Z. (Ed.), Physics and Astronomy of the Moon. Academic Press, San Diego, pp. 283-359, 1962. [3] Elbeshausen, D., et al., Icarus, doi:10.1016/j.icarus.2009.07.018, 2009, [4] Marinova, M.M., et al., Nature, 453, 1216-1219, 2008. [5] Smith, D., et al., NASA Planetary Data System, MGS-M-MOLA-5-MEGDR-L3-V1.0, 2003. [6] Konopliv, A.S., personal communication, 2010.
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