B Where has all the methane gone?

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gleem

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Recent assays of atmospheric methane by the Trace Gas Orbiter did not find any quantifiable amounts of methane in Mars atmosphere. Previously amounts in pp billion have been documented by surface survey vehicles but the TGO put an upper limit in the atmosphere of 12 pp trillion. It is known that the concentration does vary but this current measurement was unexpected. The human settlement of Mars has been depending on using methane as a fuel for transportation back to Earth.
 

BillTre

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Well, that's interesting.

I am assuming that the methane isn't just freezing out of the atmosphere (due to its low freezing point) and not just quickly getting lost to space, which seems to leave some chemical reaction or a biological cause.

I am no expert on the possible chemistry of this, so leaving that open.
A biological source (something similar to methanogens: microbes that produce methane) has often been hypothesized, but there is also the possibility of a biological sink (which would seem to have to be very efficient).
A terrestrial analogue may be methotrophs which "metabolize methane as their only source of carbon and energy. They can be either bacteria or archaea and can grow aerobically or anaerobically, and require single-carbon compounds to survive".
 
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Could some of this "now-you-see-it, now-you-don't" methane be due to small meteorite falls ? Should such fall into seasonal ice-fields, there could be a significant delay before surface release, making correlation hard...
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@gleem:
"The human settlement of Mars has been depending on using methane as a fuel for transportation back to Earth."
IIRC, it's the other way around: Methane is to be taken to Mars and reacted with Martian CO2 to produce fuel for the return flights...
 
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gleem

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IIRC, it's the other way around: Methane is to be taken to Mars and reacted with Martian CO2 to produce fuel for the return flights...
That wouldn't make much sense. The idea is producing methane from H2O and CO2 on Mars.
The reason Elon Musk has developed his methane fueled raptor engine was so he did not have to carry enough fuel to return to Earth. There is the well know Sabatier process and more recently another catalysis process recently discover by Australian scientists for producing methane from CO2 and H2O. Methane also has a higher boiling point compared to hydrogen currently favored as a fuel by other rocket manufacturers.
 

BillTre

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OK.
So, back to the Weather on Mars:
Different possible methane sources
Intermittent: not there all the time, why?
  1. traditional non-life chemical reaction based on the chemicals interacting in the environment: (something building up underground and then burping out through a weak point (temp might affect likelihood of rupture))
  2. life (such as a methanogen (eats CO2 + H2) and releases methane), seasonal due to temperature or availability or energy sources?
  3. meteors (constant rain of carbon, in micrometeorites, get broken down to smaller molecules, like methane, and are more likely to get released where its warmer). Temperature could cause seasonal changes.
All of these would have to explain why methane is only present intermittently if it is a kind of seasonal cycle based on temperature, especially the second two.
However, lack of a regular periodicity would rule out the third and possibly the second, but not the first.

different possible methane sinks:
  • freezing out of the air
  • getting blown off the planet by solar wind
  • reacting chemically with something to make iit not longer methane and/or bound to a solid and not floating around in the air
  • getting eaten by something like a methanotroph
Fast changes,
What's going on there?
Is something missing?
 

stefan r

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which seems to leave some chemical reaction or a biological cause.

I am no expert on the possible chemistry of this, so leaving that open.
A biological source (something similar to methanogens: microbes that produce methane) has often been hypothesized, but there is also the possibility of a biological sink (which would seem to have to be very efficient).
A terrestrial analogue may be methotrophs which "metabolize methane as their only source of carbon and energy. They can be either bacteria or archaea and can grow aerobically or anaerobically, and require single-carbon compounds to survive".
Oxygen in Mars' atmosphere is measured in parts per thousand. Methane, when it was measured, came out in parts per billion.
Ultraviolet light can remove a proton from methane and make methenium, CH3+. That could react with carbon dioxide or oxygen.

I am assuming that the methane isn't just freezing out of the atmosphere
Methane is dissolved in parts per billion. The entropy would prevent freezing. Methane also has a lower boiling point than carbon dioxide.
 

Zeke137

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I have read very recently that the differences between the measurements taken by the surface vehicles and those taken by TGO may be due, at least in part, to the different measurement methods and strategies used by each vehicle.

As I understand it, the Curiosity rover on the surface takes direct samples of the atmosphere around it and analyzes the samples to find levels of trace gases like methane. The PFS instrument on the Mars Express orbiter looked directly down onto the planet's surface and took spectroscopic measurements in IR ranges 1.2 though 45 microns. Analysis of data gathered by PFS in 2004 seemed also to indicate the presence of trace amounts of methane.

TGO, on the other hand, points its' NOMAD instruments either at the nadir (straight down), or at the planet's limb where it uses solar occultation to achieve very high resolution spectroscopic surveys of the Martian atmosphere. In solar occultation mode, the instruments are peering through a fairly thick layer of mainly middle and upper atmosphere, unlike Curiosity which is sampling at ground level.

So it could be that, if Curiosity is indeed measuring methane, it may also be that this methane is not being transported into upper levels of the atmosphere, or is being decomposed before it is able to get there.

From the ESA Mars Express site:
...there are many gaps in our knowledge. We still don't know, for example, how temperature and pressure vary with altitude, what the global circulation patterns are, how the composition of the atmosphere varies with time and place, and what all the trace constituents are. We also don't know for sure how much dust there is in the atmosphere, how it's transported there, what it's made of and how it affects the Martian weather
 

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