DESCRIPTION |
Instrument Overview
===================
The Thermal and Evolved Gas Analyzer (TEGA) on the Mars Phoenix
Lander is composed of two separate components which are closely
coupled: a Scanning Calorimeter (SC) and a mass spectrometer as
an Evolved Gas Analyzer (EGA). TEGA has the capability of
performing scanning calorimetry on eight small soil samples
selected in the vicinity of the lander. The samples will be
heated in ovens to temperatures up to 1000C, and the volatile
compounds (e.g.water and carbon dioxide), which are released
during the heating, will be analyzed in the EGA. The power
required by the sample oven is continuously
monitored during the heating, allowing analysis of both
endothermic and exothermic phase transitions, which can be used
to identify the phases present. By correlating the gas release
with the calorimetry, the abundance and composition of the
volatile compounds associated with the different phases can be
determined. The EGA mass spectrometer is sensitive to detection
levels down to 10 parts per billion, a level that may detect
minute quantities of organic molecules potentially existing in
the ice and soil. Please see [BOYNTONETAL2008] for a
comprehensive descrition of the TEGA instrument.
Scientific Objectives
=====================
The main scientific objectives of TEGA are to determine the
abundance of climatologically important compounds (e.g. water,
carbon dioxide) in the Martian soil, and to determine the
mineral phases with which they are associated.TEGA. TEGA will
also be looking for traces of organic material that may
potentially exist. Additionally,the EGA mass spectrometer will
perform measurements of atmospheric constituent abundances and
isotopic ratios.
Operational Considerations
==========================
TEGA is well suited to analyze the important volatile
constituents water and carbon dioxide. It combines two
instruments, a thermal analyzer and an evolved gas analyzer,
to make a synergistic analysis of the amount of volatiles in the
soil and the mineral phases with which they are associated.
The eight thermal analyzers that compose TEGA are scanning
calorimeters (SC). After being filed with sample, each oven is
heated according to a programmed temperature ramp, and the heat
needed to maintain the oven at the ramp temperature is monitored.
The heat required by the sample is attributed to that needed to
heat the sample at the commanded temperature ramp. The output of
the instrument is simply the heat flow to the oven plotted as a
function of time (or temperature).
When a phase transition occurs, the enthalpy associated with the
transition will be noted in the output. If the transition is
endothermic, for example, the melting of ice, the heat required
to effect the melting, 334 J/g, will be added to the sample oven
as the sample is heated past the ice melting point. This heat is
in addition to that required by the heat capacity of the sample
and results in a positive peak in the output. In contrast, if
there is an exothermic transition, the sample will require less
heat than otherwise, and a negative peak in the output will be
observed. In both cases the area of the peak is equal to the
enthalpy (deltaH) of the reaction.
While the sample is being heated, a carrier gas of N2 is passed
through the ovens to transport any evolved gases to the EGA. The
EGA continuously monitors the gas stream, and the output is time
correlated to the SC output in order to associate gas release
with any observed phase transitions.
The EGA consists of magnetic sector-field mass spectrometer. The
key EGA mass spectrometer instrument parameters are as follows:
Mass analyzer: Magnetic sector-field.
Electron source: Thermal emission type.
Mass ranges: 4 - extending from 0.7 Da to 140 Da.
Channel 1 - 0.7-4 Da,
Channel 2 - 7-35 Da,
Channel 3 - 14-70 Da
Channel 4 - 28-140 Da.
Mass resolution: M/deltaM = 140
(highest mass range- others
appropriate to the mass range)
Volume: 9.5 x 9 x 7 inches
Mass: 5.7 kg.
Operating Power: 13 watts
The EGA measures relative abundances of evolved gases and the
isotopic ratios of its principal constituents. In addition to
measuring evolved gasses the EGA mass spectrometer makes two
different kinds of atmospheric measurements. The atmosphere is
sampled through a constricted flow tube to measure the relative
abundances and isotopic ratios of carbon, nitrogen, oxygen,
hydrogen and argon. Atmospheric samples are also collected in
the Gas Enrichment Cell. The atmospheric sample is admitted to
a small volume. A getter removes active gases increasing the
partial pressure of noble gases sixty times. The sample is then
admitted to EGA mass spectrometer to measure noble gas isotopic
ratios and methane.
Calibration
===========
Calibration information for the TEGA instrument is provided
in the TEGA Calibration Report (calib/tega_calibration_report.pdf).
Operational Modes
=================
The TA assembly has several modes of operation. First the TA is
operated with the oven empty and open. As the oven is filled with
sample, the sample fill mechanisms and the sample fill indicator
LED are operational. Once the oven is full, it is closed, and the
oven heating portion of the operations begins. Heating continues
until a maximum temperature is reached and sample is finished
evolving gasses.
The EGA has two operational modes, sweep mode and mass hopping
mode. In sweep mode, the EGA collects count data in four channels
with discrete mass ranges. In mass hopping mode, the EGA collects
counts in 5 or 7 contiguous masses, then hops to the next mass
peak area of interest, thus ignoring uninteresting protions of the
mass spectrum.
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REFERENCES |
Boynton, W. V., H. Enos, C. Fellows, D. K. Hamara, K. Harshman, M. S. Williams,
M. R. Fitzgibbon, M. Finch, G. Droege, J. Hoffman, D. W. Ming, and P. Niles,
The Thermal and Evolved-Gas Analyzer on the Phoenix Mars Lander, J. Geophys.
Res., in prep.
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