Effects and signatures of thermal and compositional mantle anomalies induced by giant impacts on Mars

Kurzfassung

Introduction: The early history of the terrestrial planets was strongly shaped by the occurrence of several large meteorite impacts. Such large impacts do not only leave a crater as a mark on the surface of the planet but also penetrate deeply into the planet and deposit a major part of their energy at depth. While in small impacts the energy is quickly transferred to the atmosphere and/or space, the deep burial in large impacts prevents such a rapid loss and facilitates the formation of a large thermal anomaly that may persist for thousands or even millions of years. Moreover, by affecting deeper parts of the planet, the impact heats regions that already are at elevated temperature and thus promotes the formation of large amounts of melt, much of which may be extracted in the aftermath of the impact and form a new crust. The source region in the mantle is left depleted in the more fusible mineral phases and incompatible trace components as is also the case in normal melting processes; however, depending on the depth of penetration and the stage in the planet’s evolution at which it occurs, the impact may affect parts of the mantle that would otherwise be undisturbed or at least melt to a lesser extent. The thermal anomaly is thus accompanied by a compositional anomaly. Several studies on the mantle dynamical effects of giant impacts (e.g., [1, 2, 3, 4]) have been carried out, but most of them ignored the compositional contribution to the density anomaly. Method: We combine fully dynamical numerical mantle convection models that include a detailed representation of mantle mineralogy and chemistry and are coupled with a simple model of core energetics (e.g., [5]) with a parameterization of the effects of a large meteorite impact, in particular shock heating of the mantle. The model accounts for melting and the concomitant changes in material properties, especially density. Melt above a defined retention threshold is extracted from the mantle source region and added to the top to form a basaltic crust. Results: The normal melting processes produce a depleted layer in the mantle beneath the lithosphere, which is characterized by a lower content of incompatible elements and a lower density; these consequences of melting are well known from many geodynamical models of terrestrial planets. Giant impacts such as those that formed the large impact basins on Mars instantaneously produce a large supersolidus region in the mantle that may reach deeper than the normal melting zone in such large events as the Utopia-forming one and attain higher degrees of melting. The massive melt production can result in a thickened crust at the impact site in spite of the erosive action of the impact itself, and a long-lived thermal and compositional anomaly remains in the mantle. The reduced density induces a strong local upwelling that can induce mantle plumes or attract existing ones, and the anomaly rises by its own buoyancy to the base of the lithosphere, where it spreads. The thermal anomaly decays over timescales of tens to hundreds of millions of years, but the compositional anomaly may exist much longer and can become “frozen in” in the growing thermal boundary layer. Its gravity or seismic signature may, in principle, be detectable today, although probably only under very favorable circumstances. References: [1] C. C. Reese, et al. (2002) Journal of Geophysical Research 107(E10):5082. [2] C. C. Reese, et al. (2004) Journal of Geophysical Research 109:E08009. [3] W. A. Watters, et al. (2009) Journal of Geophysical Research 114:E02001. [4] J. H. Roberts, et al. (2012) Icarus 218(1):278. [5] T. Ruedas, et al. (2013) Physics of the Earth and Planetary Interiors.