Inquiries into the Consequences of Planetary-Scale Impacts and the Implications of Carbonates in the Hyper-Arid Core of the Sahara

Citation

Marinova, Margarita M. (2010) Inquiries into the Consequences of Planetary-Scale Impacts and the Implications of Carbonates in the Hyper-Arid Core of the Sahara. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/AA8Q-2954. https://resolver.caltech.edu/CaltechTHESIS:05272010-184707226

Abstract

This thesis focuses on the geophysical and morphological consequences of planetary-scale impacts – the last remnants of planetary accretion. In this size regime, the impact crater size is a significant fraction of the size of the planet, and the finite size of the target is important: its surface curvature, radial gravity, and large relative size of the impactor with respect to the target. A fully three-dimensional hydrodynamics model is used to simulate the events, thus capturing these finite-size effects. Simulated are a range of impact energies (0.02–5.89x10^(29) J), velocities (6–50 km/s), and angles (0º–75º) into a Mars-like planet. In addition, the variation in results with impactor type, for both single-material and differentiated impactors, is also examined. For this range of impact conditions, the crater size can span up to ~60% of the planetary circumference, and the ellipticity of the crater can be significant even for intermediate angle impacts. This is consistent with the observed large craters, which are commonly elliptical. Despite the large melt volumes produced, the planetary surface is preserved in most cases, as much of the melt is placed in the mantle. Antipodal crustal removal is common for the more energetic cases. For impacts with more than about three times the mutual impactor-target escape velocity, the impact has a net erosive effect, with more mass being removed than deposited. These large impacts are sufficiently massive that they can give an initially stationary Mars a rotation period of less than a day. The simulation results suggest that the Mars hemispheric dichotomy may have formed by a single, planetary-scale impact; the required impact conditions are consistent with accretion models.

The last chapter of this thesis examines the paleoclimate implications of reef-like carbonate structures in the currently hyper-arid core of the Sahara (southwestern Egypt). The carbonates suggest a wetter epoch about 9,000–10,000 years ago, and the presence of long-term, standing water. Despite the higher precipitation, the chemical composition of the carbonates suggests that the vegetation cover was sparse.

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