How Is Methane Disappearing from Mars' Atmosphere?

[gleem]

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]

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".

 

[Nik_2213]

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...
==
@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...

 

[DrStupid]

Could some of this "now-you-see-it, now-you-don't" methane be due to small meteorite falls?

Not only due to fresh meteorites:
https://scitechdaily.com/high-energy-uv-radiation-triggers-methane-on-mars

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.

 

[gleem]

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]

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]

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]

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

 

[Steelwolf]

"Where has all the Methane Gone,
Long time Outgassing,
Where has all the Methane gone,
From oh so long ago
Where has all the Methane Gone,
Long time Outgassing,
Where has all the Methane Gone,
We would really like to know.(I keep making these science puns,
Maybe it's time to go...)

 

[Steelwolf]

Well, today has interesting News, this is NYTimes, but it says NASA has not released 'data' yet, but is doing more experimentation to confirm:

https://www.nytimes.com/2019/06/22/...ml?action=click&module=Latest&pgtype=Homepage

 

[Zeke137]

So, according to the NYT article you link to, a spike of 7 ppb of methane in the Martian atmosphere was recorded by the Curiosity rover in 2013, and now just this week a spike up to 21 ppb has been recorded. As the article points out, these findings are preliminary, but it does seem as though the spikes "rise and fall with the planet's seasons".

As ever, more data are needed, but it's watch this space time again.

 

[Nik_2213]

Is there a correlation with meteor showers ?

 

[Vanadium 50]

Remember that ppt amounts of methane correspond to only a few hundreds of pounds, planetwide. One can easily imagine ppb levels from concentrated sources that eventually mix planetwide.

 

[Zeke137]

Is there a correlation with meteor showers ?

A Google search on "mars methane correlation with meteor showers" came up with a number of sources suggesting that there may be some degree of correlation between meteor shower incidence and the appearance of methane plumes.

One idea from 2016 is that material from cometary debris falling onto Mars during such encounters might generate methane via UV photolysis: MARTIAN ATMOSPHERIC METHANE PLUMES FROM METEOR SHOWER INFALL: A HYPOTHESIS . The paper mentions debris from comets C/2007 H2 Skiff and 275P/Hermann, as well as 1P/Halley, 5335 Damocles, 13P/Olbers, and Marsden group comets as possible sources.

Any methane produced in such an encounter would be formed either:

• at high altitudes (90 km +), and would then have to percolate downwards through the atmosphere to be detectable by Curiosity;
• alternatively, a significant infall mass fraction, depending on infall geometry, would need to survive to low altitudes to generate enough methane for detection by Curiosity.

In any case, the persistently negative detection results so far for methane by the Trace Gas Orbiter (TGO) gives cause to throw at least a shadow on the meteor shower hypothesis. More data needed!

 

[stefan r]

Remember that ppt amounts of methane correspond to only a few hundreds of pounds, planetwide. One can easily imagine ppb levels from concentrated sources that eventually mix planetwide.

50 kilo would be a great sample size. E coli are around 10^-15 kg. That would mean around 350 per square meter. If they are concentrated it might make sampling easier.

The population's mass could be much higher than the methane released. Humans, for example, have close to 100 kilograms of mass but generate less than 100 grams of methane. Metabolic rates may be slower in Mars' cold climate.

Of course there may be some completely different explanation.

 

[BillTre]

A new NY Times article says the methane pulse from last week has already dissipated.

 

[Nelli]

The methane is still there. It could be released from the planet by means of a shock underground caused by many forms of contact like maybe a landing of our rover or meteor, a measurement is found as it dissipates quickly into the already very thin atmosphere. Great topic.

 

[Zeke137]

A new article has been published by researchers in Denmark which proposes a different mechanism for removing methane from the Martian atmosphere.

The team, based at Aarhus University, has been studying the erosion of minerals by wind-driven sand grains in the laboratory under Mars-like conditions. Their experiments have shown that sand-grain erosion of minerals like basalt and plagioclase (both common on Mars) can lead to oxidization of the surfaces and to the ionization of gases during the erosion process.

In particular they note that argon can be so ionized: they note also that the ionization energy of argon is higher than the energy required to ionize methane to a reactive cation. The ionization energy is also higher than the energy to dissociate methane to highly reactive species like CH₃, CH₂, and CH.

Since this mechanism is much more effective than photochemical processes (e.g. UV photolysis - see post #14 above), it could lead to the rapid removal of methane from the atmosphere, and the dissociation by-products deposited in the Martian surface.