In the long and intermittently very interesting comments thread to the previous post, there's some more discussion of what we should think about formaldehyde. Formaldehyde is one of the trace chemicals that Vittorio Formisano talked about looking for using the Mars Express PFS in his interview with Linda Moulton Howe. And while Formisano does not claim to have detected formaldehyde, such a claim was made a decade ago by scientists using an instrument called Auguste on the 1989 Soviet Phobos mission (Korablev, O.I., Ackerman, M., Krasnopolsky, V.A., et al., Tentative Identification of Formaldehyde in the Martian Atmosphere, Planetary and Space Science vol. 41, pp. 441-451.)
Since formaldehyde is an organic chemical that could most easily be made out of methane, you'd think that the discovery of formaldehyde on Mars would be a big thing, since it would imply the existence of methane. (Though there are other ways to make formaldehyde on Mars, they probably wouldn't make a lot, and they'd be local to the ice caps). The reason the 1993 formaldehyde paper is not seen as a big thing is that, by and large, I don't think anyone -- including the people who made it -- really puts much faith in it these days.
That's not to say no-one talks about it. As Rick Sterling points out in those previous comments, the tentative detection made in 1993 is cited in a 2003 paper by Formisano and colleagues entitled "Atmospheric Photochemistry Above Possible Martian Hot Spots", (a paper presented at COSPAR, I think, not one published in a journal). This paper asks what an instrument like the Mars Express PFS might expect to see if there's a little volcanism still going on on Mars. The authors say that one explanation for the formaldehyde might be a "large flux of methane from a localized source in the Martian atmosphere" such as a volcanic hotspot. Rick also points out a 1999 paper (Weiss B, Yung, Y & Nealson K, Atmospheric Energy For Subsurface Life On Mars, PNAS, vol 97, pp1395-1399) where the authors say "Formaldehyde, which has no known geological source on Mars & has a lifetime in the Martian atmosphere of only 13 hours, may have already been detected by the Phobos spacecraft, although this is uncertain". On the issue of uncertainty the paper references a 1997 paper by Krasnopolsky, Mumma and colleagues in which formaldehyde was not detected in earth-based measurements at anything like the level apparently seen from the Phobos spacecraft (Krasnopolsky, V., Bjoraker, G., Mumma, M. & Jennings, D. J. Geophys. Res. vol 102, pp6525–6534).
To me, though, the key is a 2002 paper in which the first author on the original formaldehyde detection paper provides a detailed summary of the Auguste instrument's measurements (Korablev, O. I., Solar Occultation Measurements of the Martian Atmosphere on the Phobos Spacecraft: Water Vapor Profile, Aerosol Parameters, and Other Results, Solar System Research vol 36, pp12-34). It's a paper written largely from a desire to dispel the idea that the Phobos mission was an ignominious failure; as Korablev argues, it sent back more useful data than all the USSR's previous Mars missions combined (though that's not as grand an achievement as it might sound). To make this case Korablev goes through the relevant data very carefully, seeking to provide a definitive account of the achievements.
In his discussion of the features in the 3.7 micron region of the infrared spectrum that he and his colleagues had tentatively described as absorption by formaldehyde, Korablev shows that in the final analysis, if you explain the data from the relevant two pixels (yes, just two) of the spectrum by invoking formaldehyde, you get a formaldehyde level of between 200 and 1,300 parts per billion, with a best estimate of 500 parts per billion. That's slightly lower than the figure they originally published -- but still a staggering fifty times the level currently being quoted for methane. And Korablev himself does not believe it.
He goes on to compare this interpretation of the data to ground based observations -- some published, some not -- including those of his co-author on the original paper, Krasnopolsky (who, in case anyone has forgotten, leads one of the teams that reported methane earlier this year). Those observations, which have much better spectral resolution, set limits on the formaldehyde level of 3 parts per billion or less. That's a two or three order of magnitude difference between what the spacecraft seems to be saying and what the earthly observers say. What's more, it's very hard to imagine what might be producing formaldehyde in the huge amounts implied by the Phobos data, since methane levels are so much lower; if formaldehyde is being made from methane, which has a longer atmospheric life, you'd expect there to be a lot less of it, not a lot more. (As I read it, the Formisano et al “Hotspots” paper mentioned above calculates that formaldehyde should be about ten million times less common than methane if it’s being made from methane in the atmosphere). Nor does Korablev think the formaldehyde hypothesis can be saved by suggesting that it's a very local phenomenon related to a hotspot or a life-bearing oasis. The relevant spectra were measured over completely different locations on Mars.
On the basis of all this, Korablev concludes that the features seen by Auguste in the 3.7 micron region cannot have been due to formaldehyde. And I conclude, yet again, that making sense of spectra is a very tough and time-consuming affair.