The detection of methane in the Martian atmosphere has made trace gas measurements at this world a priority. The discovery implies that Mars is currently active -- methane has a short lifetime in the Martian atmosphere and must be regularly refreshed. However, the discovery brought with it two mysteries. First, is the source of the methane geological or biological activity? Either would have major implications for our understanding of this planet and the latter would have major implications for our understanding of our place in the universe. The second mystery is what is removing the methane so quickly, much more quickly than known processes could account for? The issue of trace gases is not limited to methane. Current plans are to explore concentrations of CO2, CO, H2O, H2O2, NO2 N2O, O3, CH4, C2H2, C2H4, C2H6, H2CO, HCN, N2S, OCS, SO2, HCl, and CO .
NASA and ESA have committed to flying a highly capable Mars Trace Gas Orbiter (MTGO) to address these questions in 2016. To give an idea of how capable this mission will be, the two space agencies are planning on a 125 kg science payload, slightly more mass than the instruments on either the Mars Reconnaisance Orbiter and the Mars Express missions. With launch, the mission is likely to be in the $700 - 750 M range.
This configuration does not include a high resolution imager, which is now included in the straw man instrument definition.
However, the investigation of trace gases will begin in 2012 with the Mars Science Laboratory, Curiosity. This rover will carry a tunable laser spectrometer which will measure the atmospheric composition. In addition to measuring the concentration of gases in the atmosphere, it will be able to measure carbon istotope in the methane. On Earth, life preferentally uses the lighter carbon-12 isotope; a similar bias on Mars would hint at a biological origin for the methane.
The 2016 orbiter has three goals related to trace gases; in order of priority they are (from the Joint Instrument Definition Team report):
- "Detection of a broad suite of atmospheric trace gases and of key isotopologues;
- "Characterization of the spatial and temporal variability of key species, including methane and ideally representing each family of photochemically important trace gases (HOx, NOx, hydrocarbons, etc.) and their source molecules (e.g., H2O); and
- "Localization, including deriving the time histories of key species (again including, but not limited to, methane) and their possible interactions, including interactions with atmospheric aerosols and as affected by atmospheric state (temperature and the distribution of major source gases; e.g., water)."
Achieving these goals will require measuring changes of gas concentrations as they change with time of day, season, and transport from their sources by winds. Localization of sources is expected to be difficult given the dynamic nature of the atmosphere and changing concentrations of gases.
While the payload is justified on the basis of studying trace gases, the instrument payload will also continue climatological studies that began with NASA missions in the 2000s and carry out high resolution (1 - 2 m) imaging of the surface. The high resolution camera expected to be carried by the orbiter is likely to be less capable than the MRO HiRISE instrument. The former has approximately 25 kg allocated while the latter weighed in at 63 kg. Improvements in electronics will help make up the difference, but as I understand the mass allocations, substantial portions go to the the mechanical structure. (These imagers are essentially small telescopes.) In order of priority, the high resolution cameras goals are resolution, color differentiation, and stereoscopic capabilities.
In addition to the science goals, the mission will deliver an ESA technology demonstration lander and provide data relay for the 2018 rover(s) and possibly later landed missions. The mission will be jointly implemented by the two space agencies. NASA will provide the launch vehicle (> $200M), the Electra relay package, a Ka-band (high data rate) string for the orbiter's communications subsystem, $100M for the science package, and science operations lead. ESA will provide the spacecraft and manage the mission. Individual European nations will also contribute towards the instrument costs. (For ESA missions, the instruments are paid for by the individual nations whereas NASA pays for the instruments in its missions.)
If MSL and MTGO substantiate the presence of trace gases tied to geological or biological activity, several types of follow up missions are possible:
- A Mars Organics Observer could continuously image Mars from the Mars-Sun L1 Lagrange point, and would eventually pinpoint sources of trace gases to within a handful of kilometers. This would be a Discovery class mission.
- Balloons could monitor trace gases from within the atmosphere at different altitudes and locations
- Aircraft could study trace gas concentrations within a local area
- And if sources of trace gases can be localized to within a few kilometers, rovers or static landers with deep drilling capabilities could explore these sources in detail.
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