Other
Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lpico.953r...3b&link_type=abstract
The First International Conference on Mars Polar Science and Exploration, Proceedings of the Conference held at Camp Allen, TX.
Other
Atmospheric Composition, Calorimeters, Gas Analysis, Mars Atmosphere, Mars Surface, Polar Regions, Soil Sampling, Tunable Lasers, Absorption Spectroscopy, Mars Polar Lander
Scientific paper
The Thermal and Evolved Gas Analyzer is an instrument in the MVACS (Mars Volatiles and Climate Surveyor) payload on the 1998 Mars Polar Lander. It is due to reach the layered terrain at around 70S latitude on Mars in December 1999. The instrument will heat soil samples acquired with a robotic arm to determine their volatile content with a differential scanning calorimeter (DSC) and an evolved gas analyzer (EGA). Instrument Objectives: The instrument aims to measure the volatile content of the martian soil at depth, specifically to determine the water and CO2 content. These greenhouse gases may be present in large quantities as ices and locked chemically in the soil, particularly at high latitudes. Understanding the martian climate history and the future resource potential of Mars requires that we measure the abundance of volatile-bearing material in the soil and the minerals with which they are associated. Secondary objectives include the identification of other minerals and the detection of oxidizing compounds in the soil. Instrument Description: The instrument comprises a set of eight thermal analyzers, each of which will be used only once. Each analyzer has two identical ovens, one for the sample and one (empty) for a reference. The DSC identifies the temperature and enthalpy of phase transitions by carefully determining the difference in the energy required to heat the reference and sample ovens at a controlled rate. The DSC digitally controls the duty cycle of the power to the ovens to maintain each of them at the programmed ramp temperature. The output of the DSC is simply the difference in power required by the two ovens. The EGA analyzes the evolved gases as the ovens are heated to provide knowledge of correlated gas release associated with the phase transitions. The correlated gas release will aid in the identification of the phase responsible for the phase transition. The EGA will determine water and CO, contents via a high-resolution tunable diode laser-absorption spectrometer. The spectrometer comprises two tunable laser diodes, which are swept around a water and a CO2 absorption line, and a small mirrored Herriott cell that provides a 1.0 m absorption pathlength in a 5-cm-long cylinder. The O analyzer uses an yttria-doped zirconia amperometric cell, the current of which is proportional to O partial pressure. Instrument Operation: A sample of soil from depth will be acquired with a robotic arm and deposited into a hopper over the selected thermal analyzer.Me sample is loaded into the sample oven from the hopper with an agitator/impeller, which prevents the sample from becoming blocked. A photodiode/LED beam-break detector provides an indication that the sample oven is full, at which point it is commanded to close. The ovens are made from tapered nickel cylinders that seal together even in the presence of particulates. The temperature of the sample is controlled and measured by Pt windings around 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 the ice's latent heat on the power required to heat the sample. The sample will then be heated up to around 1000C with N carrier gas flowing over it. The robotic arm will acquire samples from several depths, perhaps as deep as 1 m. We expect the volatile content to increase with depth, and the oxidant content may decrease with depth. The eight analyzers in TEGA will allow eight different samples to be analyzed taken from different positions and depths. Mineral phase transformations will be calorimetrically detected, while evolution of adsorbed water and decomposition products (e.g., CO, from carbonates, water of crystallization from evaporite minerals, and 0 from peroxides) will be carried to the 0 sensor and finally into the TDL spectrometer. The spectrometer will enable the quantitative determination of the volatile content of the sample, and may constrain the isotopic ratios of the evolved gases. The comparison of isotopic composition of the gases locked in the soil with that of the present atmosphere will be a valuable clue in understanding the role of the polar deposits in Mars' history.
Bailey Sterling H.
Boynton Willam V.
Hamara David K.
Lorenz Ralph D.
Williams Michael S.
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