Lunar and Planetary Science XXX 1914.pdf

THE THERMAL AND EVOLVED GAS ANALYZER (TEGA) ON THE MARS POLAR LANDER. W. V. Boynton, S. H. Bailey, R. Bode, D. K. Hamara, D. A. Kring, R. D. Lorenz, M. Ward and M. S. Williams, Lunar and Planetary Lab, University of Arizona, Tucson, AZ 85721-0092 ([email protected]).

Introduction: The Thermal and Evolved Gas determination of the gas released. Some isotopic ratio Analyzer (TEGA) is an instrument in the MVACS information may also be obtained. (Mars Volatiles and Climate Surveyor) payload on the The combination of DSC and EGA is particularly 1998 Mars Polar Lander, launched successfully in powerful, since volatile release can be characterized in January. It is due to reach the Martian polar layered a correlated way by both components. terrain around 76ºS in December 1999. The instru- ment will heat soil samples acquired with a robotic arm to determine their volatile content and constrain mineral composition using Differential Scanning Ca- lorimetry (DSC) and an Evolved Gas Analyzer (EGA). Instrument Objectives: The instrument aims to measure the volatile content of the Martian soil at depth, in particular to determine the water and CO2 content (both as ices and mineralogically bound). These basic data will help answer many of the out- standing questions regarding Mars’ present and past climate. In particular, where is the ‘missing’ water needed to form the fluvial features seen on Mars ? The instrument will be able to identify carbonate min- erals – valuable in assessing how unique or otherwise the carbonates found in SNC meteorites may be. Sec- Fig.1 The flight model TEGA instrument. The an- ondary objectives include the identification of other gled rectangular structures are three of the eight minerals and the detection of oxidizing compounds in thermal analyzers (the remaining five are on the op- the soil. posite side). Each analyzer has a sample funnel with Instrument Description: The instrument (figure an agitator, a sample and reference oven, an oven 1) comprises a set of eight thermal analyzers, each of closing mechanism, and a set of doors to prevent which will be used only once. Each includes a cylin- contamination via airborne dust or unintentional drical nickel sample oven, about 1cm long and 2mm spillage from the robotic arm scoop. The box and diameter (inside), and an identical reference oven. cylinder at right contains the Herriott gas absorption After the sample is acquired and the ovens sealed, the cell with the two laser diodes and detectors in the box. ovens are ramped up in temperature at a controlled The section between the two banks of analyzers, with rate by digitally modulating the power supplied to the the rounded top, contains the carrier (purge) gas and oven heaters. Comparing the power required to main- calibration gas tanks, the gas handling manifold and tain this rate on the sample oven with that for the ref- the oxygen cell. erence allows the heat capacity of the sample, and the enthalpies associated with any phase transitions, to be Instrument Operations: TEGA operations are determined. Nitrogen gas is flowed at a controlled rate expected to begin on Sol 2 on Mars: each TEGA ex- through the ovens to sweep away any evolved gas to periment will probably take 2 days. On the first day, the EGA -- an yttria-doped zirconia amperometric cell samples will be acquired from depths of up to 0.5m with the robotic arm and deposited into the funnel of (for O2) and a tunable diode laser (TDL) absorption spectrometer. This spectrometer provides for the de- the selected analyzer. The sample is loaded into the tection of water vapor and carbon dioxide in a small oven with an agitator/impeller which prevents the mirrored Herriott cell, which gives a 1m pathlength in sample blocking the loading port. A photodiode/LED a 5cm-long cylinder. It has two laser diodes (one for beam-break detector provides an indication of sample movement and oven-full detection (the oven will typi- water, one for CO2), which are scanned in wavelength across an absorption band, allowing the quantitative cally hold a sample of 20-50mg). The oven is then commanded to close. Lunar and Planetary Science XXX 1914.pdf

TEGA ON THE MARS POLAR LANDER: W. V. Boynton et al.

The ovens are made from tapered nickel cylinders which will seal even in the presence of particulates. Oven heating and temperature sensing are performed by Pt windings around the female half of the oven. First, the temperature will be ramped up and down around the freezing point, to detect the abundance of water ice by the influence of latent heat on the power required to heat the sample. Then the sample will be held at a modest temperature (typically ~35ºC) to al- low volatiles to evaporate. Then the ovens will be powered down and the instrument turned off for the night. The following day the sample will be ramped up to around 900ºC to measure bound water, decar- Fig.2 An example DSC calibration peak, 15mg of bonation and other mineral phase transitions. Indium melting at 156ºC. The ordinate is the heater In all, up to eight samples will be analyzed, proba- power (ticks at 5mW intervals) : the abscissa is time, bly at intervals of 7-10 days to allow data analysis, with ticks at 200 s intervals. This run was at a ramp sample selection and adjustment of command se- rate of 2K/min. Individual data points shown as quences. In-situ calibration of the EGA can be per- points: data after binomial smoothing shown with line. formed using a calibration gas tank containing known amounts of water vapor, O2, and CO2. Instrument Performance: After a hectic build schedule, instrument performance evaluation and cali- bration are underway with the engineering model. Additionally, a program of laboratory DSC and EGA investigations is underway to evaluate various candi- date minerals and the effects of varying experiment parameters such as heating rate and carrier gas flowrate. The DSC element of the instrument appears to perform well: standard calibration metals show well- resolved peaks. At intermediate temperatures, when the ovens are supplied power from a 15V power sup- ply that is shared with plumbing heaters, there is sub- stantial noise on the DSC signal. However, this noise appears to be amenable to smoothing without seriously degrading the shape of the sample signal peak.