MARS ANALOG SOILS IN THE ATACAMA DESERT, CHILE. B. Sutter1, J. Owen2, and R. Amundson2. 1SETI Institute, MS 245-3 NASA Ames, Moffett Field, CA 94035, 2 Div. Ecosystem Sci., UC Berkeley, Berkeley, CA 94720.

Introduction: The Atacama Desert of northern Chile is one of the driest deserts on Earth where precipitation levels occur on the order of 2-3 mm decade-1-[1]. Soils in the hyperaird Atacama accumulate salts (e.g., sulfates, chloride) in concentrations detected on Mars. The Atacama climate is relatively wetter than Mars, yet the transient nature of aqueous activity in the Atacama may mimic transient aqueous activity (e.g., percolating snow melt) that may have existed on a paleo-Mars. The objective of this work is to assess the amounts and distributions of sulfate, chloride, nitrate, and carbonate bearing Atacama soils. Results from this work will be used to assist the understanding of formation of salty soils on Mars.

Materials and Methods: Three soils were sampled in the arid core of the Atacama Desert; one near Cerro Buenos Aires (24o43.7´S; 69o42´W) and two others near the abandoned mining town of Catalina (25o12.7´S; 69o.41.9´W). The Catalina soils occurred at the top (Catalina Summit) and the base of the same hill (Catalina Base). The Buenos Aires soil occurs at the top of a small hill, as well. Soil sampling depths ranged from 40 to 400 cm. All soils developed on basalt lava flows. Eolian inputs may also supply parent material. Total chemical analyses of soils along with mineralogical identification by X-ray diffraction were conducted on all soils.

Results and Discussion: Basalt mineralogy consisted of plagioclase feldspar and pyroxene (augite). A minor amount of quartz was present and is likely of hydrothermal origin. All soils have high levels of sulfate mostly as gypsum (up to 20 wt% S) (Fig. 1). Atacama S concentration at the surface is relatively low, increases to a maximum, and then decreases with depth (Fig. 1). A similar S distribution occurs at Endurance crater where S concentration is ~ 3 à 10 à 7 wt.% S along the surface à Tennessee à Mackenzie transect, respectively [2]. Similarities of S distributions between Atacama soils and Endurance crater suggest pedogenic redistribution of deposited S in Mars soils may be a viable hypothesis.

Unlike the other soils, the Catalina Base soil possesses significant amounts of anhydrite (CaSO4), thenardite (Na2SO4), glauberite [Na2Ca(SO4)], blodite [Na2Mg(SO4)2•4H2O] and has the highest levels of nitrate (1.6 wt%) and chlorine (0.5 wt%) relative to the other soils (0.01 wt% Cl and NO3). Receiving material from the summit and slope, the Catalina Base soil is deeper (~400cm) than the Catalina Summit soil (~40cm). Accumulation of nitrate, chloride and Na-sulfate salts in the Catalina Base soil may be the result of mass wasting and leaching processes. A thicker soil would have the additional effect of inhibiting subsequent aqueous activity from leaching these salts from the profile.

The Buenos Aires soil resides at the top of a small hill, but is 130 cm deep. The concentrations of chloride and nitrate are much less than the Catalina Base soil and no Na-sulfate was detected. The Buenos Aires soil has the highest carbonate concentration (1.1 wt% CO3) relative to the other soils (<0.55 wt% CO3). The lack of Na-sulfates, low NO3 and Cl and higher CO3 concentrations suggests that the Buenos Aires soil is exposed to more aqueous activity than the Catalina soils. Alternatively, the carbonates may not be authigenic, but eolian derived. However, if carbonate had an eolian origin, then carbonate would be expected to occur at Catalina. Soil micromorphological investigations will assess the origin of the carbonate material. The phyllosilicate mineralogy is being examined to determine if phyllosilicates are authigenic or eolian derived, as well.

Atacama soils possess salts that occur at levels found on Mars. Pedogenic processes may explain S distributions as function of depth on Mars. Mass wasting processes on Mars may concentrate salts (e.g., chlorides, nitrates, sodium sulfates) at the bases of Martian hills. The Atacama carbonate concentrations are below the detection limit of instruments deployed to Mars. If the levels of aqueous activity in the Atacama are similar to paleo-Mars levels, then carbonate may be present, albeit below the detection limits of current Mars instrumentation.

References: [1] Ericksen, GE (1981) Geo. Surv. Pro. Paper 1188. [2] Clark, BC (2005) EPSL 240,73