P14A-01
QUANTITATIVE REMOTE LASER-INDUCED BREAKDOWN SPECTROSCOPY BY MULTIVARIATE ANALYSIS
The ChemCam instrument selected for the Mars Science Laboratory (MSL) rover includes a remote Laser- Induced Breakdown Spectrometer (LIBS) that will quantitatively probe samples up to 9m from the rover mast. LIBS is fundamentally an elemental analysis technique. LIBS involves focusing a Nd:YAG laser operating at 1064 nm onto the surface of the sample. The laser ablates material from the surface, generating an expanding plasma containing electronically excited ions, atoms, and small molecules. As these electronically excited species relax back to the ground state, they emit light at wavelengths characteristic of the species present in the sample. Some of this emission is directed into one of three dispersive spectrometers. In this paper, we studied a suite of 18 igneous and highly-metamorphosed samples from a wide variety of parageneses for which chemical analyses by XRF were already available. Rocks were chosen to represent a range of chemical composition from basalt to rhyolite, thus providing significant variations in all of the major element contents (Si, Fe, Al, Ca, Na, K, O, Ti, Mg, and Mn). These samples were probed at a 9m standoff distance under experimental conditions that are similar to ChemCam. Extracting quantitative elemental concentrations from LIBS spectra is complicated by the chemical matrix effects. Conventional methods for obtaining quantitative chemical data from LIBS analyses are compared with new multivariate analysis (MVA) techniques that appear to compensate for these chemical matrix effects. The traditional analyses use specific elemental peak heights or areas, which compared with calibration curves for each element at one or more emission lines for a series of standard samples. Because of matrix effects, the calibration standards generally must have similar chemistries to the unknown samples, and thus this conventional approach imposes severe limitations on application of the technique to remote analyses. In this suite of samples, the use of traditional methods results in chemical analyses with significant uncertainties. Alternatively, greatly-improved quantitative elemental analysis was accomplished by using a Partial Least Squares (PLS) calibration model for all of the major elements of interest. Principal Components Analysis (PCA) and Soft Independent Modeling of Class Analogy (SIMCA) are then employed to predict the rock-type of the sample. These MVA techniques appear to compensate for these matrix effects because the analysis finds correlations between the spectra (independent variables), the individual elements of interest (dependent variables such as Si) as well as the other elements in the matrix.
P14A-02
Aqueous Solution Chemistry on Mars
Currently en route to Mars, the Phoenix mission carries four wet chemistry cells designed to perform basic solution chemistry on martian soil. The measurement objectives are typical of those that would be performed on an unknown sample on Earth, including detection of common anions and cations, total conductivity, pH, redox potential, cyclic voltammetry (CV), etc. Both the challenge and the novelty arise from the necessity to perform these measurements with severely constrained resources in a harsh and (literally) alien environment. Sensors for all measurements are integrated into a common "beaker," with the ability to perform a two-point calibration of some sensors using a pair of low-concentration solutions. Sulfate measurement is performed with a crude titration. While most measurements use ion selective electrodes, halide interferences are resolved by independent chronopotentiometry (CP) measurements. No preconditioning of the soil-water mixture is possible, nor is any physical characterization of the introduced soil sample beyond coarse visual inspection. Among the idiosyncrasies of the measurement is the low external pressure, which requires that the analysis be performed close to the boiling point of water under an atmosphere consisting almost entirely of water vapor. Despite these liabilities, however, extensive laboratory characterization has validated the basic approach, and protocols for both CV and CP have been developed and tested. Enhancing the value of the measurement is the suite of coordinated observations, such as microscopy and evolved gas analysis, to be performed by other Phoenix instruments.
P14A-03
Pore-Filling Ice Diffusively Derived From Atmospheric Water Vapor Under Mars Conditions
Conditions during previous climate epochs on Mars may have allowed subsurface ice to form via diffusion from a moist atmosphere. The deposition and recharge of such reservoirs is driven by subsurface humidity gradients; an atmospheric frostpoint greater than that of the subsurface results in a net influx of vapor which deposits in pore space as ice. Observations of the hydrogen distribution by Mars Odyssey indicate that the ice content of some high-latitude regions (e.g. Olympia Undae) exceeds 70% by volume. Reconciliation of this concentration with typically lower porosities of soils demands a process of ice segregation (lensing) and mechanical expansion, or direct precipitation. We investigate the possibility and consequences of volumetrically significant subsurface ice derived from the Mars atmosphere by vapor diffusion, at present and in the past. Experiments conducted at the Mars Simulation and Ice Laboratory at Caltech demonstrate that diffusion processes produce significant pore-filling ice under controlled lab conditions. Atmospherically derived water vapor is deposited within an initially dry porous medium subject to a strong (~15~K/cm) temperature gradient forcing a humidity gradient. This mimics the humidity gradient caused by time varying temperatures in the shallow subsurface of Mars with a static experimental setup. The vertical profile of water content is determined at the end of the experiment by gravimetric analysis and the thermal conductivity of the ice-bearing sample is calculated. Pore filling fractions up to 100% have been measured. Profiles with a marked transition in ice content at the frostpoint depth are observed corresponding to a subsurface ice table. The data enable calculation of time-varying diffusion coefficients which exhibit a reduction of up to an order of magnitude with respect to ice-free regolith. These are compared to numerical models of vapor diffusion incorporating ice deposition and pore constriction. Formation theories of high-volume subsurface ice deposits are informed by these results. Implications and predictions for the state of the regolith at the Phoenix landing site are discussed.
P14A-04
Electrical Properties of Ice and Ice-Silicate Mixtures for Mars Exploration
We are measuring the complex permittivity of materials relevant to the cryosphere of Mars in order to interpret data from current missions and to plan future exploration. Measurements are made over a temperature range of 180- 273 K and a frequency range of 1 mHz-1MHz. Materials include "pure" ice, doped ice, and saline ices, either alone or mixed with reference solids (glass beads, Ottawa sand) or with Mars-regolith analogs (JSC Mars-1, smectite clays). Initial solute concentrations are always undersaturated. We find a regular increase in overall conductivity of ice with solute content. This occurs at temperatures above the eutectic temperature where brine channels are expected to exist and also deeply below the eutectic temperature where solutes are expected to be almost completely excluded from the ice and precipitated along grain boundaries. We hypothesize this is a kinetic effect of incomplete segregation, leaving hydrated salts in the former brine channels. For CaCl2 as a representative solute, initial concentrations must be much greater than 1 mM to develop significant brine channels. Mixtures of ice and sand or glass beads behave as expected for two-component systems, with conductivity dominated by the ice and precipitated solutes. The conductivity of JSC Mars-1 and ice is much lower than expected and may be due to solute fixation by clays with high ion exchange capacity. We find no evidence of significantly enhanced conductivity due to thin films of adsorbed or capillary water at subeutectic temperatures. The dielectric relaxation frequency of protonic point defects in ice was found to vary regularly between a previously established low value for "pure" ice and a higher limit mapped by us corresponding to 3-10 ppm Cl- saturation of the ice matrix. We found no changes to the ice relaxation due to mixing with sand; however, JSC Mars-1 caused the ice relaxation frequency to decrease, presumably because even trace Cl- was removed and not incorporated into the ice matrix. These results have implications for surface-penetrating radars and microbial habitability of Mars. The DC conductivity of saline ice is too small to cause significant attenuation of sounding radars and the dielectric relaxation is also inefficient unless the ice is warm. The low DC conductivity even for ice-silicate mixtures where water layers of a few monolayers are present further implies that microbes in the cryosphere cannot transport nutrients and waste in quantities sufficient even for dormancy: warming cycles to periglacial conditions are necessary to activate brine channels and solute transport. These measurements can also be used to help interpret the simple electrical properties measurements that will be made by the Phoenix lander. Ongoing investigation includes the effects of dielectric relaxations due to bound water and interfacial polarizations on radar loss, and testing the ability to map subsurface ice using the strength of the dielectric relaxation.