Plume-Surface Interactions due to Spacecraft Landings and The Discovery of Water on Mars.

Abstract

Pulsed supersonic jets or rocket plumes have different surface flow physics than steady jets, in particular in tenuous atmospheres such as that of Mars where jets are collimated over large distances compared to their diameters. We show that plate shock formation and collapse during each cycle of pulsed jets impinging on a surface causes large pressure fluctuations capable of producing extensive erosion during spacecraft landings. Here, we study the pressure loads and erosion caused by pulsed jets of the Phoenix spacecraft on the surface of Mars and its implications to engineering and science. While steady thruster jets caused only modest surface erosion during the landings of previous spacecraft on the moon and Mars, the pulsed jets from Phoenix led to extensive alteration of its landing site on the martian arctic, exposed a large fraction of the subsurface water ice under the lander, and led to the discovery of evidence for liquid saline water on Mars. We report the discovery of the ‘explosive erosion’ process that led to this extensive erosion and evidence for liquid water. We show that the impingement of supersonic pulsed jets fluidizes porous soils and forms cyclic shock waves that propagate through the soil producing erosion rates more than an order of magnitude larger than that of other jet-induced processes. The understanding of ‘explosive erosion’ allows the calculation of bulk physical properties of the soils altered by it, provides new insights into the behavior of granular flow at extreme conditions, and explains the alteration of the Phoenix landing site at the northern arctic plains of Mars. We then show new photometric evidence that the Phoenix spacecraft imaged liquid saline water in the arctic, and that deliquescence causes liquid water to sporadically flow in the polar region. This finding also corroborates the hypothesis that the thermodynamics of freezing/thaw cycles leads to the seasonal formation of liquid saline water where ice and salts exist near the surface. Finally, we show broadband spectral signature of liquid brines in flow-like and pond-like features on defrosting polar dunes. This has important implications for geology, geochemistry and the habitability of Mars.Ph.D.Atmospheric and Space SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/78975/1/manishm_1.pd