This week, Madeline discusses a critical component of her research into how methane is vertically distributed in the martian atmosphere. Read on for some details about the present state of the ongoing debate about Methane on Mars.
(Image source: https://mars.nasa.gov/system/feature_items/images/6037_msl_banner.jpg)
by Madeline Walters
On Earth, we’ve often heard of methane being produced as a result of living beings-microbes that help with livestock digestion. Though when we found methane on Mars, we were puzzled by its origins. Are there microbes helping the digestion of Martian cattle? Most signs point to no, however, we are still unsure of what may be producing the gas on Mars. Besides biogenic sources, methane can also be produced by geological processes, so being able to identify the sources of methane is a tricky yet interesting problem.
The issue with identifying the sources of methane is finding the methane in the first place. Since landing in Gale Crater in 2012, the Tunable Laser Spectrometer (TLS) instrument onboard NASA’s Curiosity rover detected background levels and a few higher spikes of methane from the surface, however, ESA’s ExoMars Trace Gas Orbiter (TGO) wasn’t able to detect any methane from higher up in the sunlit atmosphere.
TLS lead scientist Chris Webster [1] comments: "When the Trace Gas Orbiter came on board in 2016, I was fully expecting the orbiter team to report that there's a small amount of methane everywhere on Mars, but when the European team announced that it saw no methane, I was definitely shocked."
The results were certainly unexpected after other detections of methane from other instruments, leading to new questions about whether the detections from TLS perhaps originated from the rover itself. Some scientists suggested the rover detected methane after crushing rocks, or perhaps wheel degradation, not willing to rule out any possibilities. However, the Planetary Fourier Spectrometer onboard the Mars Express (MEx) spacecraft observed higher levels of methane in 2013, after Curiosity also reported a methane spike, bringing back the question of how to make sense of these detections.
So why are some instruments reporting methane while others aren’t? This is something that is puzzling scientists almost as much as the source of the gas itself. Because of the conflicting reports of detection from different instruments, the key is observing how methane diffuses through the atmosphere at different times of day and through different seasons to see if perhaps the reports of methane from different instruments can still make sense.
Moores et al. [2] suggests a small amount of methane seeps out of the ground continuously such that during the day, it mixes well with the atmosphere, which results in very low levels of methane further up. Meanwhile at night, the methane can build up near the surface from the lack of convection. From this approach, we can make sense of both the ExoMars and Curiosity observations. While this could explain the discrepancies in methane detection from different instruments, we still have yet to determine the origin of the gas itself and if that origin perhaps can explain how the gas is being destroyed much quicker than it should. Because solar radiation and oxidation should be destroying the produced methane after a lengthy 300 years, the excess methane buildup should be detectable by TGO. This points to some destruction or sequestration mechanism that is getting rid of the methane quicker than expected such that the detected amounts make sense.
"We need to determine whether there's a faster destruction mechanism than normal to fully reconcile the data sets from the rover and the orbiter," says Webster.
One possible explanation for this is the gas’ reaction with the surface components. A chemical compound called perchlorate, which has been detected by Mars landers, may be acting as a sink for methane due to oxidation reactions [3]. When exposed to ultraviolet radiation from the sun, perchlorate accelerates the destruction of methane-from over 300 years to just days or hours. However, scientists are still exploring this possibility and as of right now, there’s still no way to be sure this is the reaction responsible for the gas’ quick destruction. While there are still many questions surrounding Martian methane, we are getting closer to explaining the mysteries of the gas.
___
References:
[1] https://www.jpl.nasa.gov/news/first-you-see-it-then-you-dont-scientists-closer-to-explaining-mars -methane-mystery
[2] Moores, J. E., King, P. L., Smith, C. L., Martinez, G. M., Newman, C. E., Guzewich, S. D., et al. (2019). The methane diurnal variation and microseepage flux at Gale crater, Mars as constrained by the ExoMars Trace Gas Orbiter and Curiosity observations. Geophysical Research Letters, 46, 9430– 9438. https://doi.org/10.1029/2019GL083800
[3] Zhang, Xu & Berkinsky, David & Markus, Charles & Chitturi, Sathya & Grieman, Fred & Okumura, Mitchio & Luo, Yangcheng & Yung, Yuk & Sander, Stanley. (2021). Reaction of Methane and UV-activated Perchlorate: Relevance to Heterogeneous Loss of Methane in the Atmosphere of Mars. Icarus. 376. 114832. http://dx.doi.org/10.1016/j.icarus.2021.114832.
No comments:
Post a Comment