Methane and Organic Molecules on Mars

[BillTre]

Curiosity has found seasonal occurrences of methane and some organic molecules on Mars.
Both or possible indicators of carbon based life processes, but can have other non-biological explanations.

Methane has been intermittently found previously on Mars.
However, lacking a continuous long term record the seasonal variations were not obvious.
The seasonal variation could be explained by either biological or non-biological processes.

The organic molecules are from 5 cm below the surface in the Gale crater where they were shielded from UV irradiation (which would be expected to breakdown organic material).
The Gale crater is an impact crater from 3.5 to 3.8 B years ago, that at some time was filed with water in which the sediments containing the organics were laid down.
The organics were found in mudstones at the base of the craters' deposits.

CNN article here.
Open access Science methane article here.
Open access Science organics article here.

 

[Fewmet]

A few other notes from the news conference:

The methane was atmospheric and the seasonal variation was from about 0.2 to 0.6 ppb. They said that meteoric sources might explain a 20% seasonal variation but not a three-fold change.Levels were too small to get carbon isotope ratios.

The specific organic molecules reported were benzene, propane and thiophene. Those were identified from drilled samples that were heated until the organics volatilized, so they might be fragments of larger organic molecules.

My understanding is that the 5 cm depth means they might have been exposed to ionizing radiation. There was talk about how ESA's Exomars will drill to depths of 2 meters , producing samples not (or less) impacted by radiation.

I think they said the drilled samples were up to three years old and it took that long to make sense of the results. Can anyone speculate about why it would take that long?

 

[enorbet]

I was trained in Engineering and I'm weak in Organic Chemistry so I don't yet understand why sulfur may play a role in the preservation of these samples so near the surface. Can anyone here shed some light on this process?

 

[Fewmet]

I was trained in Engineering and I'm weak in Organic Chemistry so I don't yet understand why sulfur may play a role in the preservation of these samples so near the surface. Can anyone here shed some light on this process?

What is your source for sulfur taking a role in preservation?

 

[Alex Coopman]

Well, in chemistry they still call "organic" any molecule containing carbon...Moreover, the most pro-life compound is water, which is not technically "organic"...I think there's a long way to go befofe we can highlight any findings on Mars as something life-related...but anyway new discoveries are always challenging... ;-)

 

[mfb]

I think they said the drilled samples were up to three years old and it took that long to make sense of the results. Can anyone speculate about why it would take that long?

You need several measurements - data collection takes time. In addition data analysis takes time. You want to make sure that you understand the results, that there is no possible contamination and so on.

What is your source for sulfur taking a role in preservation?

I'm not the one you asked, but you can find it in the NASA press release: "these fragments contain sulfur, which could have helped preserve them [...]"

 

[enorbet]

What is your source for sulfur taking a role in preservation?

Greetings. I read about the possible role of sulfur here at https://arstechnica.com/science/201...e-found-intriguing-organic-molecules-on-mars/

Here is a short quote

The result of this careful analysis is that scientists have conclusively found thiophenic, aromatic, and aliphatic organic compounds on Mars. These results are reported Thursday in a new paper in Science, of which Eigenbrode is the lead author. She was especially intrigued by the discovery of sulfur within the organic molecules, which probably helps explain how the organic molecules were preserved on the surface of Mars for a long period of time despite the harsh radiation.

Since then I've also read that the presence of sulfur may also help determine if these molecules were from organic processes or geological, so it seems to me a rather important component deserving of some focus. Whatever the result it seems a reasonably exciting find... finally.

 

[BillTre]

Here is a Science mag news article on the findings they published.

I think they said the drilled samples were up to three years old and it took that long to make sense of the results. Can anyone speculate about why it would take that long?

As I recall (no references), the rover has had a series of technical problems that may have impacted this.
For example, at first they were concerned that there was some residual organics in their oven ad were repeatedly heating and exhausting it to get rid of organic traces left from earth. I'm sure that they went through a lot of procedures to ensure they were really looking at Mars rather than Earth stuff in their analysis. This was probably well before drilling though.
The news article (link above) also talks problems they in 2016 with their drill mechanism which kept they from using a test that involved:

nine small cups containing a solvent that frees organic compounds bonded in rock, eliminating the need to break them apart—and potentially destroy them—at high temperatures

.
If I were them, I would wait before saying anything about what was found until I could use a superior test or knew that I could not use it. This may explain the delay.
Now the drill is working and they are going back to the mudstone area.

They are starting at the bottom of a mountain of sediments in the Gale crater and (as I understand it) working their way up toward the top.
Therefore they are looking at the oldest sediments first and as they go up slope will be able to sample more recent sediments.
If the organics they found were due to life processes on the surface, then the organics might possibly go away, at some point (in more recent sediments), as the Martian climate became more untenable for life exposed on the surface (it may have retreated to underground locations).
I guess this would depend on the timing of the loss of surface water delivering sediments vs. the loss of atmosphere blocking solar radiation.
I would not expect non-biological sources that show this kind of cut-off (but I am no geologist).

Irregardless, impressive technical job getting all this done.
Well Done NASA!

 

[mfb]

I would not expect non-biological sources that show this kind of cut-off (but I am no geologist).

I'm not a biologist either, but the loss of surface water should also influence the chemical processes and the distribution of chemicals.

 

[Fewmet]

I don't yet understand why sulfur may play a role in the preservation of these samples so near the surface. Can anyone here shed some light on this process?

I've read through the technical paper (Eigenbrode, et al) in the current issue of Science. There is fairly extensive discussion about preservation of organics. One point made is
Further, iron sulfides detected in the Sheepbed mudstone (29) and suspected in the original Murray mudstone detritus (7, 8) may have aided organic matter preservation by providing an additional oxygen sink during diagenesis.
I am reading that as the iron sulfide reacts with free oxygen, consuming it so there is less to attack the organics.

They mention that reducing permeability of the surroundings limits exposure to fluids and gas, and that is partly attributed to sulfur cements among the grains. Finally,, they explain that "natural vulcanization" occurs when organic combine with sulfur, making the products resistant to degradation by heat.

 

[rootone]

At the moment there just isn't enough data to distinguish between biological or geological souces of CH4 and other simple organic molecules found on Mars.
It is definitely interesting that a seasonal effect is occurring, though it doesn't prove anything.
The next Mars landers will do more than scrape 5cm of the surface, and that might deliver information which sways the debate

 

[alantheastronomer]

Organic molecules have been detected for some time now by radio astronomers in the interstellar medium and have also been found in carbonaceous chondrite meteorites on Earth. Perhaps the organics found on Mars in the Gale crater came from the meteor that formed the crater. As far as the seasonal variation in atmospheric methane is concerned, it could be due to sublimation from methane ice at the poles.

 

[Ian J Miller]

In my opinion, the chondritic explanation is highly unlikely because there are just insufficient of them to get the seasonal effect for methane. I am also skeptical of the organics showing the presence of life, the reason being the absence of nitrogen. Now, it might be argued that the equipment was not suitable for finding nitrogen, but that raises another question - if you are going to do this and spend this much, why not go the extra bit? Nitrogen is not that difficult to detect, especially if you go for molecular ions (which means low energy ionisation - a standard mass spec approach) because nitrogen gives odd numbered ions, and to anyone with some experience in the mass spectroscopy of organic compounds, they stick out like the proverbial organ stop. The really unfortunate part of this experiment is that they pyrolyse the sample, which makes it hard to comprehend the significance of what they found. If these were the original compounds they would be most unlikely to come from living entities - nothing like those would come fromTerran life. However, the pyrolysis can give rearrangements, and what we see are the "bottom end" of the stability scale, which makes it really difficult to interpret the significance. The fact that there was kerogen-like material does not mean life. Quite complicated hydrocarbons can be made geochemically, and even now I have been to a geothermal area that contains a place called "Kerosine Creek". In fact I am a firm believer that life requires some lipid equivalent to contain entities while they develop. As an aside, Mars is certainly an interesting place because one of the deep questions that is plaguing me while I am preparing a paper to present at an Astrobiology Conference shortly is whether life needs felsic material, or can it be generated using basalt.

 

[BillTre]

I am preparing a paper to present at an Astrobiology Conference shortly is whether life needs felsic material, or can it be generated using basalt.

Is this based on possible chemical reactions these rocks might have with water, or what?

Sounds intriguing.
Please elaborate.

 

[Ian J Miller]

Assume we start with an RNA world. There are a number of reasons why this is so, which loosely fall under the requirements: (a) for reproduction one needs to be able to copy information, which means as you build the copy, the CORRECT mer has to have a significantly stronger attraction to that position that anything else; (b) You need catalytic activity, and RNA has that; (c) You need the polymer to be strong enough to last, but you need it also to be plausibly able to be degraded, the reason being you have to be able to delete "mistakes"; and (d) you need the polymer to be soluble in water, and once you get up to very large molecular weight, this is difficult. We solve (a) with nucleobases, and you solve (c) and (d) by joining the polymer through phosphate ester. Phosphate is the only plausible entity that has three functions - strong enough links and the ability to solubilise and cross-link with cations such as Mg++ to get the catalytic activity. Now there is no way to form two phosphate links to the nucleobases without eliminating the functional bits, so something else is required, and that is a sugar intermediate. Now the problem is, how do you make phosphate esters? The only way that I know of without using reagents that simply would not exist in water for any length of time is to heat the phosphate and the alcohol to 200 degrees C in the absence of water, when the vibrational energy is great enough to do the job. The trouble with that is it has to be in the absence of water at that temperature, because water at that temperature hydrolyses the phosphate esters, so the trick is to get the vibrational energy into the alcohol group corresponding to 200 degrees C in cold water. That eliminates black smokers as an option (unless someone comes up with some hidden reaction we don't know about) - currently life is there, but the phosphates are protected by various ion pumps etc that have evolved - they were not there originally. The answer is hinted at by a paper by Saga et al. - he shone UV on adenine, ribose and phosphate and got ATP and AMP, and some hint of maybe a AMP link to the 3 position of another adenosine. The reason this works is that the adenine excited state collapses and generates a lot of vibrational energy.

So far, maybe, but the next problem is, how do you get ribose? Only ribose works, and the reason is the pyranose forms are too rigid. The furanose forms, however are rather flexible, and in my opinion, more easily vibrationally excited. In any case, in that experiment, only the furanose gave a phosphate ester. Now we see why ribose was used - only ribose has any appreciable furanose forms in aqueous solution (about 50%, but it is a rapid equilibrium) - all the other sugars are essentially pyranose in solution. Now the problem with ribose is that in any sugar preparation you get hardly any because all its hydroxyls are eclipsing, which makes it the least stable. However, there is evidence that this is overturned if the sugar can be formed complexed with either borate or silica. Borate is rather difficult to locate, but silica is very common dissolved in hot water emerging from fumaroles where the rock is felsic. Granite, for example, has silica in it, and if you go around hot springs in geothermal areas you see where the silica eventually gets deposited. Back to my original issue, I have no idea whether basaltic hot springs will dissolve silica. My gut feel (evidence would turn that around) is the ionic nature of olivine and pyroxenes would be too difficult to break down, but there may be chemical reactions that work. If anyone knows, feel free to let me know.

 

[alantheastronomer]

This is in response to Ian J. Miller's post #13 - I'm not proposing chondrites as a source of methane, I'm proposing them as a source of organics, although the organics could also be primordial - part of the protoplanetary nebula. I'm proposing frozen methane in the polar ice caps as the source of the seasonal variations in atmospheric methane.

 

[Ian J Miller]

The issue of carbon, water and nitrogen from chondrites is a persistent one. The evidence I have collected makes this difficult for me to accept, other than as minor additions. First, the ratios of carbon or nitrogen to other elements such as chlorine do not agree with what we see; if all of Earth's volatiles came from chondrites, the only way isotopes of certain elements can be accepted is if the bulk mass was incorporated into the mantle through plate tectonics, but that wouldn't happen on the Moon, and it should always be hit by some such chondrites. The carbon should be visible, but it isn't. If chondrites delivered the atmospheres of the rocky planets, each planet received a completely different set of chondrites, and since we require these chondrites to come from only the carbonaceous chondrite zone, how did this separation occur. It also requires a monstrous increase in the number of carbonaceous asteroids, but none in any other asteroid. Given they occur in the region of Ceres, why did that not accrete further, and why were these selectively removed? My view is the carbon and nitrogen were accreted as solids and were acted on chemically by geochemical processing with water, and the water was accreted through chemical interaction with silicates. This has the advantage that the different compositions of atmospheres is simply explicable through the planets accreting with different temperature histories. I guess there is bias here - people tend to go to their own training to find explanations, and I am basically a chemist.

 

[alantheastronomer]

The issue of carbon, water and nitrogen from chondrites is a persistent one. The evidence I have collected makes this difficult for me to accept, other than as minor additions. First, the ratios of carbon or nitrogen to other elements such as chlorine do not agree with what we see; if all of Earth's volatiles came from chondrites, the only way isotopes of certain elements can be accepted is if the bulk mass was incorporated into the mantle through plate tectonics, but that wouldn't happen on the Moon, and it should always be hit by some such chondrites. The carbon should be visible, but it isn't. If chondrites delivered the atmospheres of the rocky planets, each planet received a completely different set of chondrites, and since we require these chondrites to come from only the carbonaceous chondrite zone, how did this separation occur. It also requires a monstrous increase in the number of carbonaceous asteroids, but none in any other asteroid. Given they occur in the region of Ceres, why did that not accrete further, and why were these selectively removed? My view is the carbon and nitrogen were accreted as solids and were acted on chemically by geochemical processing with water, and the water was accreted through chemical interaction with silicates. This has the advantage that the different compositions of atmospheres is simply explicable through the planets accreting with different temperature histories. I guess there is bias here - people tend to go to their own training to find explanations, and I am basically a chemist.

Do you mean lithosphere rather than atmosphere?

 

[Ian J Miller]

What I mean by the composition of the atmospheres is the gases in the atmosphere, but carbon and chlorine, of course, have to be assessed as the lithosphere because carbon, for example, is mainly present as carbonate. The solids were accreted as nitrides, carbides, cyanamide, etc. and would be more or less evenly distributed through the planet until differentiation started. The action of water transfers a hydroxyl group to the metal (two such hydroxyls on the same atom eliminate water and give the oxide) and a hydrogen to the carbon or the nitrogen. The chemical isotope effect arises strongly because the transfer will come from the weaker bond, and because of zero point energies, deuterium is bound more strongly by about 8 kJ/mol, from memory. So hydrogen is preferentially sent into the atmosphere in entities prone to being decomposed by UV. That, in my opinion, it the main cause of the Venusian high D/H ratio. It never had a lot of water anyway, accreting in a hot zone, so the deuterium got enriched significantly.