The gorgeous view from the Utah Valley Convention Center in Provo. Today I'm talking about what I'm not presenting here (a project conceived, executed, written up, submitted, reviewed and accepted all after the deadline for DPS abstracts!)
Greetings from the 49th annual meeting of the American Astronomical Society's Division for Planetary Sciences! While a bit on the colder side, it's a gloriously sunny day here in Provo, Utah.
But I'm not here to talk about the conference. I've already submitted my talk and picked up my poster, so I've got a few minutes before the reception starts to catch up on my writing a little.
Instead, I wanted to spend some digital ink on my 50th paper (18th 1st author paper) which just came out in-press at Planetary and Space Science. While that's a milestone worth some commentary, I'd like to focus in this post on the Science itself. This particular paper is a novel one for me in that it is an update of a previous model that I had created back in 2012. In fact, a perceptive reader will notice some parallels between this paper and that of Moores and Schuerger (2012). Those are intentional. Indeed, it was our express intention to re-examine our earlier results in light of just how far research on martian methane has come over the past five years.
You might ask what changed so drastically that this model needed updating? There were several developments. First, and foremost, the REMS instrument obtained our very first ultraviolet measurements from the bottom of the martian atmosphere. Back in 2012, we had ultraviolet imaging from above the atmosphere with HST along with plenty of surface images in the visible and near infrared. Extrapolating those images and combining them with a radiative transfer model and Andrew Schuerger's laboratory analysis of UV production of methane from samples of murchison meteorite is what we used to create the 2012 model. With actual measurements to which we could now compare our model, we were finally able to go through a much desired validation step. This work was led by my postdoctoral fellow, Dr. Christina Smith, based off of Michael Smith's comprehensive 2016 analysis of the REMS UV data-set. At the end of that process, we discovered that I had overestimated the amount of UV reaching the surface in 2012 by 35%, on average.
The second development was that we now had measurements of methane from the surface via the Sample Analysis at Mars' Tunable Laser Spectrometer (SAM-TLS). These measurements were much lower than we had predicted back in 2012, surprisingly so at less than 0.7 ppbv. Such a low value meant that, if estimates of the amount of organic carbon falling on Mars were correct, less than 10% of all that carbon was being chemically converted into methane or the atmospheric lifetime of methane was much smaller than we expected it to be. Where was all that organic carbon? It was a puzzle I thought might be examined more thoroughly with the updated model.
Thirdly, a controversial paper citing us has been making the rounds over the past two years. While others have challenged the model of Fries et al (notably in timing and in mass balance) they had not opposed the mechanism for producing methane; Fries had explicitly invoked our 2012 work in support of their hypothesis of methane production from Interplanetary Dust Particles (IDPs) while they were still in the atmosphere. This was an intriguing possibility, given that very small particles can remain aloft for extremely long periods of time. As such, we sought to address the UV photolysis not only of IDPs on the surface, mixed into the soil, but also during their journey through the atmosphere.
Finally, following a successful orbit insertion, the European Trace Gas Orbiter is preparing for their first measurements of methane from orbit. The instruments they brought with them, and in particular NOMAD, are the first space-borne instruments intentionally designed to map the atmospheric methane on Mars. Over the next year, we anticipated that they would be collecting this data and attempting to reconcile their results with each of the different processes which could be acting on Mars to produce methane. We felt that we ought to make certain that our model was refined in time for the team to consider the production of methane from IDPs alongside other theories.
So those were our reasons for writing. What did we find? Well, first of all, if the UV flux is reduced compared to what we though, pre-MSL, then the particles at the surface should take longer to be consumed. This increase in particle lifetime leads directly to a greater loading of surface organic carbon needed to maintain a mass balance between incoming and outgoing organic carbon. But the rate of production of methane from IDPs should be unchanged so long as the supply of Carbon is correct and the amount of carbon passing through photolytic pathways is correct.
Production is only one part of the equation. The other side of the equation is the atmospheric lifetime of methane. If methane is shorter lived then we hypothesize (e.g. from Atreya et al, 2007, who suggest 329 earth years) the same amount of production will lead to a lower equilibrium concentration in the atmosphere. This then gives us three potential sources of the low values seen by SAM-TLS - either less organic carbon is entering the system, less of it gets turned into methane by interaction with UV radiation or the atmospheric lifetime is shorter than we think (some combination of these is also possible).
A good hint is that if there was less carbon becoming methane that makes a testable prediction about the surface levels of organic carbon. Our refined model predicted a reduction and, in fact, MSL also found relatively little organic carbon in the soils of Gale Crater. What they did see, about 0.3 ppm, is about what we would expect from only about 5-7% of the organic carbon being processed by UV into methane. Recent work by Matteo Crismani from the MAVEN mission tends to support our conclusions with MAVEN seeing substantially less dust impinging on the martian atmosphere than they had expected.
And what about the photolysis of IDPs before they even reach the surface? It turns out that this contribution is relatively minor. As an upper limit, only about 0.32% of organic carbon can be converted to methane in this way.
All in all, a short but fun project. With a bit of luck this week's conference will give me even more ideas for future work. But even if that plan doesn't pan out it'll be a fun week. Look to this space in a year or two to see which was the case.
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