Thermal Evolved Gas Analyzer (TEGA)
mission specific
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 description 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 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 portions of the mass spectrum.