Fabrication of Sulfate-bearing Ceramic Calibration Targets for the Chemcam Laser Spectroscopy Instrument, Mars Science Lander

Introduction: The ChemCam (chemistry/camera) instrument on the Mars Science Lander (MSL) images outcrop or regolith at high resolution and uses a laser source to generate high-temperature plasma from spots of ~100 μm diameter, at ranges up to ~7 m [1,2]. The lander will carry body-mounted calibration targets consisting of synthetic silicate glasses, a natural volcanic glass (macusanite), and four sulfate-bearing ceramics. The synthetic silicate glasses and the macusanite are highly homogeneous at the scale of laser spot sampling; they are described in detail elsewhere [3]. This abstract describes the fabrication and characterization of ceramic calibration targets. Rationale for ceramic targets: The synthetic and natural silicate glass targets are formed at temperatures that do not retain sulfur. Targets made of sulfate and sulfide minerals were investigated but found unacceptable because of fragility, instability, or inadequate plasma production. Because sulfate-bearing lithologies are important at all potential MSL landing sites, it was decided to prepare low-fire ceramics with firing temperatures below sulfur volatilization (~900-1000 °C). In addition to this thermal requirement, a secondary goal was to make ceramics based on three major components expected at the potential landing sites: a sulfate mineral, phyllosilicates, and basaltic detritus. The sulfate phase selected was anhydrite, which produces no water on heating and retains sulfur at temperatures to at least 850-900 °C. Phyllosilicate components were selected from Clay Minerals Society source samples of Australian nontronite NAu-2 (representing very iron-rich smectite identified at several localities on Mars[4]) and Georgia kaolinite KGa-2 (representing iron-poor kaolinite-group clay minerals identified at some localities[4]). Although these samples were heated to remove water and hydroxyl (see methods below), the refractory chemical constituents remain representative of clay mineralogy. Basaltic detritus was represented by a tholeiitic lava of the Cerros del Rio lava field, NM, that has been used for several years as an internal standard for X-ray fluorescence analyses at Los Alamos National Laboratory. Three of the ceramic targets were based on differing ratios of anhydrite versus basalt mixed with devolatilized NAu2; the fourth target was composed of equal amounts of basalt and anhydrite mixed with devolatilized KGa-2. These mixtures are summarized in Figure 1. Figure 1: Ternary plot of phyllosilicate, basalt, and sulfate components in four ChemCam ceramic targets as prepared (weight basis).