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In Nature Geoscience's article Independent confirmation of a methane spike on Mars and a source region east of Gale Crater is announced that the reported methane spike on sol 305 by the Tunable Laser Spectrometer-Sample Analysis at Mars (TLS-SAM) on Curiosity was confirmed by Planetary Fourier Spectrometer (PFS) observations onboard the Mars Express spacecraft in the spot-tracking mode. The combination of PFS and TLS-SAM observations strongly suggested that the emission took place outside the crater.

Using wind fields simulations with a general circulation model (GCM) and independently from this trying to find potential methane release structures around Gale crater led the authors to conclude that the Aeolis Mensae region a few hundred km east of the crater was the most likely source of the methane spike.

In this article the authors used the Mars Regional Atmospheric Modeling System for methane transport simulations and concluded that a methane source just outside Gale crater could be excluded to explain the observed methane spikes and even the lower background methane levels.

In the recent article Mars Methane Sources in Northwestern Gale Crater Inferred From Back Trajectory Modeling the authors used new modeling techniques to identify upstream methane emission regions at an unprecedented spatial resolution. The outcome supports surface emission sites in the vicinity of the Curiosity rover. Only if fast methane removal mechanisms would exist, emission sites could be outside Gale crater, most likely to the north.
From the Conclusions chapter:

Methane concentration data from future in situ measurements, especially those collected in consecutive measurements performed within a few hours, could further improve the emission site identification.

So, after having examined the sulfate unit, instead of going further up Mount Sharp, could not Curiosity turn around and head for the northern plains in Gale crater ?

Travelling just 50 m a day, while doing TLS-SAM measurements every day and/or night, it could reach the northern rim in less than 4 years.

Could the scientific value of going up Mount Sharp and that of intensive methane measurements while going north be quantified scientifically and be weighed against each other ?
And if not, could not the fact that the Curiosity rover will be the only one for a long, long time that can do in situ methane measurements on Mars be an extra motive to change to this new, main scientific objective ?

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    $\begingroup$ It would be cheaper, and quicker, and more likely to succeed, to design a new mission to do just that investigation and let Curiosity get on with its job. $\endgroup$ Dec 30, 2021 at 10:47
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    $\begingroup$ I tend to agree with @CuteKItty_pleaseStopBArking. Traveling 4 years to get to the supposed source of methane emissions is a long time & much can happen in that time. It's also 4 years of traveling & little ease. Does NASA have the budget for this? If Curiosity continues its existing schedule it can investigate a lot of other things & produce other data which would be useful nonetheless. $\endgroup$
    – Fred
    Dec 30, 2021 at 11:41
  • $\begingroup$ related Mars methane/rover questions: Will the Mars 2020 rover's sensitivity to methane be better than Curiosity's? and What new has come from the recent methane findings on Mars? $\endgroup$
    – uhoh
    Dec 30, 2021 at 12:33
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    $\begingroup$ @Cornelis Being a few parts per BILLION, rarely occasionally, I'd say that Curiosity is not suitable to make a hunt for that stuff into its main mission, but just carry on with what is was designed to do. Leave the methane issue to the ExoMars rover which will drill several meters into the ground. It might strike oil! ;-) $\endgroup$
    – LocalFluff
    Dec 30, 2021 at 14:16
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    $\begingroup$ @LocalFluff Like on Earth, subsurface methane will likely be regional, so there's a good chance the ExoMars rover will find nothing of that. The Tunable Laser Spectrmeter was designed to make a hunt for this stuff, as it did. Curiosity has the unique opportunity to do this search, meanwhile it can (occasionally) continue with using the other important instruments. $\endgroup$
    – Cornelis
    Dec 30, 2021 at 14:32

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The comments are degenerating into an opinion-fest, so I'm going with the facts: Curiosity has already been investigating methane apparently emerging from Gale Crater, finding nighttime background levels of 0.2-0.7 ppb with a peak over 20 ppb. Apparently Curiosity's data combined with that of the ESA's ExoMars orbiter are showing a diurnal cycle, with the methane appearing preferentially at night. See a more detailed description of the findings here.

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    $\begingroup$ From the linked article: 'Moore thinks we need more measurements from the ground. Curiosity has made about 16 measurements in a decade. "It's a very coarse data set and you can imagine it must be difficult to figure out what happens over the day if you only measure every six months ". I think since only Curiosity will be able to do such measurements on the ground for a long, long time that could justify to intensify them $\endgroup$
    – Cornelis
    Feb 1 at 23:43
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    $\begingroup$ ExoMars made a day-night-day series when the discrepancy emerged between ExoMars during the day and Curiosity during the night, leading to the finding of the daily cycle. $\endgroup$ Feb 1 at 23:54
  • $\begingroup$ Apparently the methane case is not important enough for ESA to install a TLS on the Rosalind Franklin rover. But this rover should be able to detect carbon-bearing molecules in the subsurface where likely the methane comes from ! $\endgroup$
    – Cornelis
    Feb 2 at 11:40

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