The sulfate unit at gale crater: what to expect there and how to explore it

R.Gellert, J.A.Berger, N.I.Boyd, C.D.O’Connell-Cooper, M.McCraig, L.M.Thompson, S.J.VanBommel, A.S.Yen. Univ. of Guelph (Guelph, ON, N1G2W1, Canada; rgellert@uoguelph.ca), Johnson Space Center, Houston, TX,Univ.of New Brunswick, Fredericton, NB, Washington University in St. Louis, St. Louis, MO, Jet Propulsion Lab, Pasadena, CA Introduction: Gale Crater was selected as landing site for the Curiosity rover because of a sequence of clay, hematite and sulfate signals detected from orbit on the slopes of Mount Sharp [1]. The transition in minerals is expected to shed light in the changing environmental conditions over the first one billion years on Mars, a time frame when life arose on Earth. As of sol 2985, the rover has traversed ~23km from its landing site and climbed across the first two areas, closing in on the sulfate unit. Here, we will speculate, what the rover might encounter in that unit, based on earlier sulfate detections in previous rover missions and in-situ sulfates already seen in Gale Crater. We will discuss, how the rover can efficiently explore the sulfate unit and how optimal drill samples for SAM and Chemin might be selected, in particular by the use of the APXS [2]. Method: The APXS is an arm-mounted X-ray spectrometer using a combination of PIXE and XRF to quantify 16 standard elements, among them sulfur, with high precision, good accuracy and low detection limits. Starting with Na, elements are quantified by their characteristic X-ray peaks. Their usual oxides are normalized to 100%, assuming a homogeneous and water and carbonate free sample. The assumption that sulfur is fully oxidized as SO3 in the overwhelming number of cases can be confirmed by quantification of the invisible elements – essentially oxygen – using the APXS scatter peak method [3] or other mineralogical results like CheMin or Moessbauer. Identifying and quantifying sulfate candidates is straight forward using APXS results in many cases through mass balance or elemental correlations; however, detailed structural information, including the hydration state is not possible. These details require dedicated drill campaigns of several weeks for CheMin and SAM. We will first discuss possible settings the sulfate unit could represent, based on already encountered sulfate deposits from MSL and MER. Soils: The unconsolidated material (soil) at all landing sites, including Pathfinder and Viking, shows similarity in overall composition – interpreted as average martian crustand a significant abundance of sulfur, about 5% SO3. Sulfur correlates well with chlorine and zinc in soils (fig 1), and with the Fe/FeT content from Moessbauer on MER. These phases likely represent the fine Martian dust [4] and are usually linked to the amorphous component in CheMin XRD, since the amount of crystalline sulfates in a few soils cannot account for the SO3 abundance from APXS. Meridiani Planum: The dominant bedrock at Meridiani is the Burns Formation, a sandstone with up to 25% SO3 from APXS and hematite and jarosite from Moessbauer. Over the traverse of ~45km, the bedrock is remarkably homogeneous, with some variations with depth in impact craters, interpreted as dissolved Mg-sulfates caused by changing ground water levels. The observed 1:1 molar decrease in Mg and S is one example for the application of bulk chemistry deduced mineralogy [2]. Plotting the sulfur content against the major possible cations in fig 2 reveals that no single cation can be identified in the Burns formation; however, Mg and Fe are clearly not diluted by the addition of sulfur, although Fe is also attributable to hematite in the bedrock. Abraded interiors have the highest sulfate content, while brushed and as-is surfaces have lower values due to soil/dust cover or preferential abrasion of sulfate minerals from the rock surface, which ultimately could be a source of sulfur in the global soil. Figure 1 S vs Cl and Zn in soils 0 1 2 3 4 5 6 7 8 9 10 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0