HR: 14:15h
AN: P23A-04    [Abstracts]
TI: Gravity Models of the Argyre and Isidis Impact Basins, Mars: The Relative Importance of Surface Loading and Mantle Uplift
AU: * Kiefer, W S
EM: kiefer@lpi.usra.edu
AF: Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058 United States
AB: Several large martian impact basins, including Argyre and Isidis, have large positive free-air gravity anomalies, and in this regard are similar to the lunar mascon basins. Such gravity highs are not intuitively expected over topographic basins. Previous modeling emphasized two possible causes, flexurally-supported surface loads emplaced after basin formation and super-isostatic uplift of the crust-mantle interface. Quantitative constraints on the relative importance of the two mechanism have not previously been presented for Mars. Distinguishing between these two models is important, because the super-isostatic uplift model places significant constraints on both the impact process, which presumably produces the uplift, and on the post-impact thermal environment, which controls the relaxation of the uplift. Recent work on the morphometry of large impact structures on Mars constrains the expected basin depth for pristine impact structures as a function of basin diameter (Howenstine and Kiefer, Lunar and Planetary Science Conference, 2005). Basins that are shallower than expected are interpreted as being partially filled by post-impact deposition of volcanic or sedimentary material. These results indicate that there is 2.6 ± 1.0 km of post-impact fill on Argyre's floor and 2.9 ± 0.7 km of fill on the floor of Isidis, providing important new constraints on the causes of their mascon gravity anomalies. The gravity high in Argyre, 142 mGal, can be explained by post-impact deposition alone provided that this load is less than 40-50% compensated. Super-isostatic moho uplift is not required at Argyre, although it is permitted if the surface load is close to isostatic compensation or relatively thin. The gravity high in Isidis is much larger, 415 mGal, and thus harder to explain by surface loading alone. In the limiting case of uncompensated basaltic fill of the maximum allowed thickness, the surface load can reproduce no more than 75% of the peak gravity anomaly. Thus, some super-isostatic moho uplift (at least 5-6 km) is required at Isidis. However, it is likely that at least part of the fill on the Isidis floor is sedimentary (Vastitas Borealis Formation). This reduces the mean fill density and increases the required amount of super-isostatic moho uplift.
UR: http://www.lpi.usra.edu/science/kiefer/home.html
DE: 5417 Gravitational fields (1227)
DE: 5420 Impact phenomena (includes cratering)
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2005 Joint Assembly