Paper No. 9
Presentation Time: 4:10 PM
SEEKING CONSTRAINTS ON THE CLIMATE OF EARLY MARS FROM THE MINERALOGY OF ANCIENT SOILS PRESERVED AT MAWRTH VALLIS
HORGAN, Briony, EAPS Department, Purdue University, West Lafayette, IN 47907, BISHOP, Janice L., Carl Sagan Center, SETI Institute and NASA-ARC, Mountain View, CA 94043, CHRISTENSEN, Philip R., School of Earth and Space Exploration, Arizona State University, PO Box 876305, Tempe, AZ 85287-6305 and BELL III, James F., School of Earth & Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ 85287, briony@purdue.edu
Outcrops in the Arabia Terra region of Mars expose an ~250 meter thick stack of light-toned layered deposits that have near-infrared spectra consistent with a variety of clay minerals, and these units are best exposed in the region surrounding Mawrth Vallis. Because of the high modeled clay abundances (30-60 wt.%) and the clay diversity exposed at Mawrth Vallis, this site has been proposed as a landing site for a future NASA or ESA robotic mission, and was one of the final four possible landing sites for the Mars Science Laboratory mission. However, one of the potential weaknesses of Mawrth Vallis as a landing site is the lack of a clear sedimentary context for evaluating the origin of the clays. To address this issue, we propose here that the layered deposits are volcaniclastic sediments, deposited gradually over a period of up to 200 million years, and concurrently sub-aerially weathered to form a sequence of clay-rich soils. If this hypothesis is true, then the Mawrth Vallis paleosols could be a stratigraphic and geochemical record of an era when significant precipitation and surface habitability were sustained on Mars. In this study, we seek constraints on the paleoclimate required to form the Mawrth Vallis layers by comparing their mineralogy and stratigraphy to terrestrial paleosols.
Our primary terrestrial analog to the proposed Mawrth Vallis paleosols is the paleosol sequence preserved in the John Day Fossil Beds National Monument (eastern Oregon). This 440 meter thick volcaniclastic deposit contains over 500 individual clay-rich (30-90 wt.%) paleosols, and the mineralogy, layering, and volcanic source for these deposits bear a striking similarity to the Mawrth Vallis units. The deposits span 15 My around the Eocene-Oligocene boundary, a period of global cooling. The mineralogy of the paleosols reflects this climatic shift through a transition from high kaolinite and oxide abundances at the base, to high smectite abundances, to low abundances of smectites, poorly-crystalline phases, and zeolites at the top of the section. Intriguingly, the Mawrth Vallis clays exhibit a reversed mineralogical trend, from glass and possible zeolites or sulfates at the base, to Fe/Mg smectites, and Al-rich phases and kaolinite at the top, suggesting that the Mawrth Vallis layers may record a shift to a warmer and wetter climate.
