More on Mars Chemistry

From Bruce Moomaw:

This July 27-28, the "New Mars Chemistry Workshop" will be held in Massachusetts. The abstracts are already online ( http://www.lpi.usra.edu/meetings/marschem2009/pdf/program.pdf ); and they include one ( http://www.lpi.usra.edu/meetings/marschem2009/pdf/8005.pdf ) -- co-written by the experimenters for the "Urey" organic detection instrument planned for ExoMars -- that seems to firmly answer my question about whether the "subcritical water extraction" process intended to flush organic compounds out of Martian material for this instrument would be vulnerable to the destruction of those organics by perchlorates (as attempted pyrolytic detection of organics is vulnerable):

To put it briefly, it is not vulnerable to the same problem as pyrolytic organic detectors (such as ExoMars and MSL carry).-- at least judging from experiments on Atacama Desert soil, which contains a lesser fraction of perchlorates -- and so the experimenters regard it as an important backstop to the pyrolytic "MOMA" organics detector on ExoMars:

"Results indicate that sub-critical water extraction liberates amino acids from Atacama Desert soils quickly and efficiently despite the presence of perchlorate. Aqueous heating experiments at 100 deg C. show minor differences in the rate of amino acid degradation over timescales of weeks, and therefore perchlorate should minimally affect SCWE at the short exposure times characteristic of these optimized extraction conditions."

The abstract implies that this applies to organic compounds in general -- not just amino acids -- and so both the Urey instrument and the "Life Marker Chip" currently scheduled for ExoMars (which also uses sub-critical water extraction) are immune to the harmful effects of perchlorates. It also implies that this is the case despite the fact that Phoenix found perchlorate concentrations in Mars near-polar soil two orders of magnitude higher than that in the Atacama. The implications of this for the design of any Mars exploration program are obvious. It would be a great pity -- to say the least -- if MSL's lack of any non-pyrolytic organic detection system made it insensitive to organics in its Mars samples.

Similarly, from the TEGA team itself ( http://www.lpi.usra.edu/meetings/marschem2009/pdf/8004.pdf ):

"If perchlorates are ubiquitous on Mars and if organic detection techniques require heating, then leaching of the soil will be required to remove soluble perchlorate. Solution techniques (e.g., supercritical water extraction) that extract organics without heating may provide an alternative way to obtain organics for analysis without organic destruction." (Note: I suspect that "supercritical" is a typo and the authors meant to say "subcritical", since all other references to this particular extraction technique say that it utilizes water in a subcritical state.)

Other interesting notes from the abstracts:

(1) From Phoenix experimenter Michael Hecht ( http://www.lpi.usra.edu/meetings/marschem2009/pdf/8030.pdf ):

"From the perspective of searching for life, Capone et al recently suggested that 'follow the carbon' may not be the best strategy. While the carbon cycle may dominate biology, the reverse is not true. The nitrogen cycle, on the other hand (and particularly the denitrification process) is strongly influenced by biology, and the lack of nitrogen in the Martian atmosphere may in and of itself contra-indicate life. In light of Phoenix findings, it might similarly be argued that dechlorification is predominantly a biological process, and the presence of large quantities of perchlorate may therefore contra-indicate extant biology."

But: "Most recently, localized release and eventual decomposition of methane has been detected in the Martian atmosphere. Lacking any evidence of current volcanism on Mars, the source of such releases seem limited to sublimation of existing clathrates or biogenic sources. Confirming this observation and determining the genesis of the methane is clearly a high exploration priority."

(2) Similarly, from Howe et al ( http://www.lpi.usra.edu/meetings/marschem2009/pdf/8014.pdf ):

"Although methane in the Martian atmosphere is globally very dilute, about 10 ppb, there are localized areas where the concentrations are as high as 35 ppb and must be constantly replenished due to photochemical losses. These localized areas and concentrations cannot be explained by impacts or volcanism, and may be areas where methanogens are producing methane."

(3) Levy et al ( http://www.lpi.usra.edu/meetings/marschem2009/pdf/8015.pdf ):

"No morphological evidence for geologically recent saturated soil conditions is observed at the Phoenix landing site or across the Martian northern plains. These observations suggest that chemical species requiring abundant water to form were not produced recently in-situ at the Phoenix landing site, but rather have been mixed into the surface regolith from alternate (potentially impact-related) sources."

(4) J.A. Hurowitz ( http://www.lpi.usra.edu/meetings/marschem2009/pdf/8006.pdf ) concludes that -- even given the apparently high acidity of Martian soils at the MER landing sites -- soil from these sites dunked in water by a duplicate of Phoenix's "Wet Chemistry Lab" would turn into a slightly alkaline solution (pH 7.2) after only 8 hours (from a starting point pH of 5.6). That is, the supposed differences between Mars soils at the MER sites and Phoenix's site may be far less than was initially assumed.