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Largest Methane Spike Ever, Curiosity Rover to Examine it Further
nprev
post Jul 21 2019, 09:27 PM
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This was a general reminder to remember 1.3 given the nature of the topic.


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serpens
post Jul 22 2019, 09:08 AM
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The previous methane spike was detected by both Curiosity and Mars Express and appears to have originated in Aeolis Mensae, East of Gale. This area is a tectonic transition zone with probable buried ice. I believe the hypothesis is that occasional fracturing of the ice releases small amounts of methane that have accumulated in pockets beneath the ice. If there is liquid water deep down wouldn't ongoing serpentinization be a possibility?
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jccwrt
post Jul 28 2019, 09:22 PM
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A theme from the methane session at 9th Mars was that Curiosity measurements and TGO observations are possibly irreconcilable (supposing the Curiosity measurements are natural methane and not rover-generated). There were a couple of working hypotheses which might be working in tandem with one another to prevent TGO from seeing much methane.

First is that the planetary boundary layer collapses overnight, and within a well-protected crater it might only be several meters thick. If the methane is produced locally, it could build up to a relatively high concentration under a temperature inversion. Then once the sun rises the boundary layer rapidly expands and mixes out the gas. This would prevent TGO from seeing the methane at high levels because TGO takes its observations post-sunset, when the boundary layer is at its most mixed. The problem is that while this goes some way towards explaining the methane measurements it really only works under the presumption that there are only a few localized sources of methane, and that we got lucky and landed at one of those sources.

The other one is some mechanism of fast oxidation near the surface. These could be things like hydrogen peroxide (probably generated by electrostatic effects during dust storms) or some other nasty volatile adsorbed onto dust grains. Any methane released into the atmosphere would quickly interact with these adsorbed oxidants and be destroyed. This mechanism could do the trick, but it's not particularly well-constrained. Short of directly measuring how methane reacts with dust particles (which I don't even know how to go about doing with the current equipment on Mars) I'm not sure how well this works in practice.

At any rate, TGO can only see down to about 5 km above the surface, so if there's some mechanism destroying methane near the surface TGO is never going to see it.
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nprev
post Jul 28 2019, 10:36 PM
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Hmm. On that note, I've heard very little about SAM's ability to detect perchlorates and peroxides. I assume that they are far too volatile to be inferred from GCMS data?


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JRehling
post Jul 28 2019, 11:43 PM
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That's a good point, but also, GCMS only provides the mass of a molecule and that becomes ambiguous for large molecular weights (and sometimes, for smaller ones like CO vs N2). Not only do different substances have the same MW, but different isotopes of one substance will have results 1, 2 (etc) of the mode.

Perchlorate molecules would have a molecular weight of about 100-200, and then you're left to interpret what compound produced a result of, say, 138.
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mcaplinger
post Jul 29 2019, 02:41 AM
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QUOTE (JRehling @ Jul 28 2019, 03:43 PM) *
GCMS only provides the mass of a molecule and that becomes ambiguous for large molecular weights...

Mass spectrometry has that limitation, but AFAIK gas chromatography doesn't. And SAM has many wet-chemistry and evolved-gas modes for perchlorates as well.

See, for example, "Evidence for perchlorates and the origin of chlorinated hydrocarbons detected by SAM at the Rocknest aeolian deposit in Gale Crater"
https://agupubs.onlinelibrary.wiley.com/doi...1002/jgre.20144


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Disclaimer: This post is based on public information only. Any opinions are my own.
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marsbug
post Jul 29 2019, 09:22 PM
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Ahem. If I might pedant.... many moons ago I used to build GCMS. So, for the uninitiated (feel free to skip this first paragraph if you already know how GCMS work): The Gas Chromatograph stage works by flash heating the sample and seeing how long the components of the vapour take to travel down a long, thin, column (more like a rolled capillary tube, usually) with one of a range of coatings on it's inner surface that slow the different vapour components down by differing amounts. The GC gives you a rough idea of what is in the sample based on how long different pulses of vapour components take to hit the detector at the end of the column. In the GCMS the Mass Spectrometer is the detector. This takes the material in the pulses exiting the GC stage, ionises it (aiming to average 1 charge per molecule), and separates the molecules in it according to their charge to mass ratio. So, crudely speaking, you get a rough idea from the GC, then a more detailed analysis from the MS.

OK, here's why I'm pedanting at you all: The important point for large molecules is that when you ionise them they almost always break apart - and for a given type of ionisation method most large molecules break apart in a predictable, repeatable, fashion. So you don't get a big signal at the charge to mass ratio for the parent molecule, you get a 'fingerprint' of smaller peaks that is unique to a given high mass molecule. For example, even something fairly light weight like the amino acid glycine (mass 75 AMU) yields a fingerprint like the one I've attached when ionised by electron bombardment (see attached file). Notice that the biggest peak isn't at 75 AMU, but at 30.

So the MS isn't simply analysing heavy molecules by their charge / mass ratio - in fact that almost certainly wouldn't work. It's using these heavy molecule specific fingerprints. It can be very, very, specific as long as the molecule is known or modelled in how it breaks down under ionisation. When combined with the GC stage, and a database of known GC and MS 'fingerprints' this can be a very precise process for identifying heavy molecules.

It can go wrong : Getting the right rate of GC heating, GC column type, ionisation process and mass to charge ratio analyser stage all needs some idea to start with of what type of thing you'll be analyzing. But in this case the team behind the GCMS on the rover will have had that from previous missions.

Now, that's probably clear as mud. And, if my old boss is here he will probably now tear me apart for that explanation anyway... go ahead Vic, I'm sure I deserve it.
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serpens
post Jul 29 2019, 10:26 PM
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Thanks for the explanation Marsbug. The mud was extremely transparent. So in the event of anomalous breakdown fingerprints, provided that a duplicate GCMS design is held it should be possible to run samples against the same setting variables in a lab to ascertain the identity of a molecule?
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marsbug
post Jul 29 2019, 10:48 PM
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Exactly. Unless I'm very much mistaken there will be a carbon copy of the rover's GCMS sitting in a NASA lab here on Earth, and one of its duties will be exactly that. Replicating a new fingerprint(s) may be a bit of an art, but they will have a good idea of what kinds of materials they need to start with in the Martian environment.

As an aside: I am very interested to see how this system flies when we get to environments with lots of very complex unknown organics, formed under hard to replicate conditions, such as Titan! I suspect they will already be building a database using reactions that can take place at cryogenic temperatures but that, as they say, is a different story.


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marsbug
post Jul 29 2019, 11:47 PM
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I just found this breakdown of the SAM lab, and including the GCMS. Good lord, she's a monster - six columns on the GC. I hated swapping columns on a single column system, this would drive me insane... Not that the columns can be changed on Mars, so I guess that makes sense since on Earth we could just swap the columns out to tackle different sample types. The article really hammers home that the GCMS is part of a battery of interconnected chemical analysis systems in SAM. I suspect that anything SAM as a whole truly cannot identify on its own will be real 'sit up and pay attention' stuff


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JRehling
post Jul 30 2019, 02:30 AM
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That's a superb explanation, marsbug! Thanks for elevating the discussion!
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atomoid
post Sep 9 2019, 08:41 PM
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more recent detail on the 'boundary layer collapse' mentioned earlier by jccwrt
https://www.sciencenews.org/article/overnig...methane-mystery
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