ATTEMPTING TO QUANTIFY METEORITIC CONTAMINATION OF MARS SEDIMENTS AT THE MSL LANDING SITE

Because Mars has long-exposed, ancient surfaces, it is reasonable to expect that its soils and upper crust are contaminated with meteoritic materials. Nickel is one potential tracer of exogenic addition because it is a common element in asteroids but uncommon in basalt. Thus, Ni abundances (and other trace elements such as Mn, Cr, Co, and Zn) are useful for assessing meteoritic contamination in Mars sediments. Ideally, variation in the observed levels of meteoritic tracers could be used to track variations in exposure history, although this has not yet been successfully demonstrated in practice.

To examine this problem at the MSL landing site, we are using three strategies. First, we have gathered data from the Alpha Particle X-ray Spectrometer (APXS) instrument for quantifying Ni abundances, for which the reported precision is ±30 ppm and the accuracy is 16% relative. However, the number of samples used to calibrate Ni for the APXS and their range of concentrations are unclear.

Second, we studied ChemCam-derived abundances of Ni on the martian surface using a model informed by our laboratory doping experiments. We created 180 doped standards and also acquired LIBS calibration data on >1000 natural samples under a 700 Pa CO₂ atmosphere in our laboratory. As expected, high-Ni doped samples had more intense elemental lines, and matrix effects proved to be critically important to the uncertainty in the quantitative information that can be inferred.

Preliminary LIBS results show systematically lower average Ni abundances than those measured with the APXS instrument. Either ChemCam and/or our LIBS calibration may be insufficiently sensitive to Ni to provide reliable results, or APXS calibration of Ni may need revisiting. Both explanations are defensible. The low Ni abundances inferred with ChemCam are comparable to those seen in the SNC meteorites. On the other hand, the higher Ni seen with APXS is more consistent with exogenic contamination levels that might be expected from impact processes.

Third, we are examining the cratering and exposure records at Gale crater, in order to estimate the expected magnitude of meteoritic contamination and the implications for Ni (and other trace element) geochemistries. Results from this effort will be discussed at the meeting.