This post is, effectively, a continuation of a previous discussion about methane but with an important added piece of the puzzle included: a solution which includes the constraint levied by the Trace Gas Orbiter. Since it's very hard to draw/visualize methane, this post like so many others, has a wonderful picture of the spacecraft that made the measurements - the curiosity rover, pictured above, with a jigsaw puzzle mask for effect! (original masking image CC 2.0 description and license here)
You might think it a bit strange, publishing two papers back to back 4 months apart on the same topic. After all, my last paper talking about methane on Mars only came out in Nature Geoscience in March and was fully published in May. And now, here my co-authors and I am again with a paper in Geophysical Research Letters talking about explanations for the seasonal cycle. Part of the story has to do with differences in time to publication for the two journals. But the bigger story is that this past year has seen a surprisingly quick evolution in our understanding of this trace gas.
Of course, with a new spacecraft and a new suite of instruments on-station at Mars and starting to report on their results it's not entirely surprising that things could change. The Trace Gas Orbiter (TGO), which arrived at Mars back in 2016, began taking measurements just before the recent Global Dust Storm after an extended series of orbit lowering maneuvers. At the time, there was no global picture of methane on Mars at the level of a few parts per billion by volume (ppbv*) level. All we had were a few sporadic detections of plumes above 10 ppbv from terrestrial telescopes and orbiting spacecraft and even the Planetary Fourier Spectrometer (PFS) onboard Mars Express in its shiny new spot tracking mode could manage only an uncertainty of about 2.5 ppbv. This was an impressive detection threshold to be sure, but with the Curiosity Rover seeing a seasonal cycle at Gale Crater that averaged a mere 0.41 ppbv, 2.5 ppbv was just not enough.
But TGO was purpose-built to change all that, with a sensitivity to methane in the column down to better than 0.05 ppbv (50 pptv or parts per trillion by volume). Once the ACS and NOMAD instruments started their measurements those of us who work in the field were waiting with bated breath for the results. What would the maps of methane on Mars show? Where were the sources and where were the sinks? What was the relationship between the plumes and the background?
We waited and then waited some more and then, in April of 2019 the answer finally came: all the maps of methane from TGO were blank! No detections had been made.
Even though that first result from the TGO did come with a few caveats, it was shocking! Let's discuss those limitations a little. In their first paper the TGO had not yet observed an entire Martian year**. Therefore, it remains possible that there are times when methane rises above its detection threshold in the normal course of the seasons, though this is looking more and more unlikely. Secondly, while there were some measurements made before the onset of the Global Dust Storm, the filling of the atmosphere with obscuring dust restricted the altitudes and latitudes where observations could be made. In particular, the martian tropics had not yet been observed and this was where many previous plumes seemed to have originated. Thirdly, TGO is not able to sense methane all the way down to the surface. Even in clear atmosphere, 3 km in altitude is the limit with the best sensitivity typically above 5 km. So, unfortunately, there was no way that TGO could peer into Gale crater for an apples to apples comparison.
Nevertheless, the TGO results challenged the Curiosity seasonal cycle. Just as on the Earth, the atmosphere on Mars circulates through lateral winds and vertical convection. In all, it takes about 3 months for martian air to mix completely and about 1 day (or sol) for air to mix vertically up to altitudes where TGO can see and analyze its constituent gasses. In that context, that the TGO sees nothing in the air they can examine provides a powerful constraint. Indeed, as the TGO team pointed out, if the methane concentration in Gale were 0.41 ppbv all the time that means that about 30 kg of the gas was being emitted every day***. That would be enough to fill up the whole atmosphere of Mars to more than the TGO's detection limit in just a few decades. Given that our understanding of the chemistry of methane on mars tells us that the average lifetime of each molecule is centuries long, the TGO should have seen that level of emission, if it was happening.
Something's gotta give.
What were we missing about the methane puzzle? For some, the answer was simple. At the 9th Mars conference in July of 2019, Kevin Zahnle, a researcher from NASA Ames who has previously been skeptical of martian methane claims, expressed the opinion that the observations seen by curiosity were coming from the rover itself and not the martian environment. Indeed, he felt that all the positive detections of methane on Mars from all research groups, telescopes and instruments were flawed in some way and that there actually was no (and had perhaps never been) any methane in the atmosphere.
I find that a difficult argument to accept. For starters, Mars (like all the planets) is constantly accreting an organic carbon-rich dust made up of Interplanetary Dust Particles**** or IDPs. Hundreds of tons of these fall on Mars every year, enough to raise the methane concentration to 10 ppbv planet-wide if all the carbon became methane. Indeed, meteoritic carbon very readily transforms to methane under Mars like conditions in terrestrial laboratories! This accretion should have been going on for all of Mars' history, including in the distant past (3-3.8 Ga) when Gale crater contained a lake that was laying down sediments. Over time, any buried organics should thermally degrade and release methane through seepage to the surface. And we do see ancient organics in those sediments when we drill into them and examine them with SAM.
The Zodiacal Light (left) is made up of small dust particles orbiting in the plane of the solar system that scatter light towards the Earth (and everywhere else) after sunset. The particles themselves consist of tiny silicate grains glued together with an organic-rich rime. By weight, such particles can be up to 24 wt% organic carbon, but are more typically around 10%. Hundreds of tons of these fall on Mars and all of the planets each year. Original images: Left and Right
So if both sets of measurements are real, how can they be reconciled? It turns out that one of the keys has to do with the time when the measurements were acquired. TGO observes the sun filtered through the very edge of the atmosphere (what we refer to as a limb observation), so the location where those measurements occur are experiencing sunrise or sunset. But the TLS instrument aboard Curiosity measures methane in the middle of the night. At that time of the night, the atmosphere is at its most stable with mixing homogenizing the air above the surface only up to a few tens of meters, instead of the deeper kilometers-thick mixed layer that exists during the day that we call the Planetary Boundary Layer or PBL for short.
A
photo of the Earth's "limb" acquired from the space shuttle. Note how the sun filtering through the
atmosphere in this configuration really lights up all the gas and
aerosols making it possible to analyze what is present in the atmosphere
and to separate out those gasses and aerosols that are closer to the
surface from those that are higher up. Unfortunately, sometimes the
topography can block out parts of the view and it's always sunrise or
sunset where spacecraft observe this phenomenon.
Because that near-surface layer is so thin, trace gasses can become very concentrated without requiring much gas. Once the sun comes up in the morning, even a ppbv of methane in this thin layer would get mixed and diluted away nearly to nothing. Where does this gas originate? Perhaps it is seeping out of the subsurface, as many geologists suspect.
If you have seepage of methane from below that happens at a constant rate all the time and an atmosphere that is nearly methane free, you will naturally set up a daily (or "diurnal") cycle. During the day, vigorous convection keeps the concentration of the seeping methane low - it's mixed away before the concentration can get too high. But at night, the concentration climbs as the night wears on. That means that the concentration of gas that the TLS measures depends in large part on how late at night that measurement was made.
In our GRL paper we simulated that cycle and used the TLS measurements and the TGO constraint to derive how quickly the methane was seeping out of the ground. Even after correcting for the time of night, there was still a seasonal component to the variation. So we went back to our model of the subsurface from Nature Geoscience and plugged in the new near-zero atmospheric concentration. It turns out that the seasonal component matches up very nicely with a constant rate of production and seepage from depth, modulated by adsorption and diffusion through the seasonal temperature wave!
So does this solve the mystery of methane on Mars? Well not so fast! While we are able to reconcile the TLS and TGO constraints, very little of Mars' surface can have microseepage if the chemistry of methane that has been predicted by others holds. In fact, only 140% of Gale can emit and not be seen by the TGO. That seems to be much too little.
But where it does help us is in reducing the magnitude of the needed new chemistry. Whereas before others had talked about the need to bring the lifetime of methane down from 300 years to as little as 1 day (!), now even a correction by an order of magnitude in the lifetime of methane down to a few years or even a few decades is enough to allow a significant portion of the surface to breathe in this way. That would line up with what geologists predict should be the methane emitting regions on Mars.
So where do we go from here? The answer is more measurements. We now know that measurements like those that the TLS performs can be used overnight to measure how much methane seeps from the ground. If you can do that in multiple places you can start to address what sorts of processes created that methane in the first place. If you can acquire those measurements quickly, say multiple times a day, then you can disentangle the plumes from the background and start to understand them as well.
Sadly, neither the Rosalind Franklin nor Mars 2020 rovers are equipped to perform sensitive methane measurements of the atmosphere so the answer will have to wait. But we still have lab and theoretical work we can do on Earth to help understand how methane on Mars should evolved. In the interim, as it has been said: "sometimes it has to be enough to love the questions themselves."
___*ppbv is a way of measuring the concentration of a trace gas in terms of the number of molecules as a fraction of all molecules in a given volume. So 1 ppbv means that in a volume that encloses 1,000,000,000 molecules, just one of those molecules will be your trace gas. For spectroscopic measurements especially, this is a more natural scale in which to work and the "v" part has to do with the fact that at the same temperature all gas species occupy the same volume. If you don't see that "v" in there, as in the case of "ppb" it means that mass is being compared to the total mass of gas in a given volume.
**Results presented in Korablev's paper covered dates through approximately November 2018, though an update presented at 9th Mars in July 2019 did not contain any detections of methane either.
***Our Nature Geoscience paper made this assumption as well, assuming that the 30 kg of methane escaping the crater was composed mostly of methane coming from other parts of the planet, wafting in on the winds, and up to a few kg contributed by Gale itself through seepage.
****These small particles orbiting between the planets in the plane of the solar system give rise to the Zodiacal Light just after sunset by scattering light from the sun towards our eyes.
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