Depleted carbon isotope compositions observed at Gale crater, Mars | Hacker News Reader

Depleted carbon isotope compositions observed at Gale crater, Mars

pnas.org

50 pointsareoform39 comments

Recent HN post of a science.org article on this topic: https://news.ycombinator.com/item?id=29972342

TL/DR: A paper reports (appropriately conservatively) on tantalizing evidence from Gale crater. Curiosity's SAM lab heated 24 powdered rock samples to 850 °C, releasing their solid carbon as methane gas. The onboard Tunable Laser Spectrometer then measured the carbon isotopes in that gas and found more 12C than would be expected based on the 12C to 13C ratio in the modern Martian atmosphere. On Earth, life preferentially uses 12C over the heavier 13C, bioaccumulating it, so we observe significantly more 12C than 13C in ancient rocks where life grew. Did ancient Martian microbial methanogenesis concentrate this isotope at Gale crater as it built up complex organic molecules? Maybe. Two other hypotheses offer abiotic explanations: one suggests that UV light could have caused CO2 in the Martian atmosphere to form isotopically enriched carbon monoxide molecules that settled to the surface, the other wonders if the isotopes arrived from space when our solar system drifted through a 13C-depleted giant molecular cloud hundreds of millions of years ago. “On Earth, processes that would produce the carbon signal we’re detecting on Mars are biological,”[1] but we don’t understand the Martian carbon cycle well enough yet to have any real confidence. As with all tantalizing results about microbial extraterrestrial life so far, this will probably turn out to be due to a physicochemical process we don’t yet understand. Or maybe not.

I'll be covering this more in the next issue of Orbital Index (https://orbitalindex.com/).

[1] https://www.nasa.gov/feature/goddard/2022/nasa-s-curiosity-r...

> On Earth, life preferentially uses 12C over the heavier 13C, bioaccumulating it, so we observe significantly more 12C than 13C in ancient rocks where life grew.

If (like me) you're wondering why life prefers to use carbon-12 over carbon-13, it appears to be because carbon-12 is simple lighter and thus more likely to diffuse into an organism:

From [1]: Due to differential uptake in plants as well as marine carbonates of 13C, it is possible to use these isotopic signatures in earth science. Biological processes preferentially take up the lower mass isotope through kinetic fractionation.

NOAA has a bit more detail on the diffusion of carbon isotopes into organism [2].

[1] - https://en.wikipedia.org/wiki/Carbon-13#Uses_in_science

[2] - https://gml.noaa.gov/outreach/isotopes/stable.html

From [1] above:

> atmospheric, [marine] carbonate, and plant derived δ13C values all differ.

So, there is a mineral process that can explain change in isotope ratio, which requires water. And also a biological process with the same outcome, also requiring water.

In other words, this looks like a solid new piece of evidence in support of presence of liquid water on Mars in the past. Evidence of life though? That is less certain.

Is heating rocks a method of producing methane on Mars?

How does that work?

Note that the atmosphere is more likely to loose 12C over time than 13C. (It's the lighter molecule and will have larger speeds at equal temperatures, making Jeans escape more likely). The paper itself says "the δ13C composition of the atmosphere may have been less 13C enriched when Gale crater sediments were deposited" (compared to now). I am not sure how they exclude that as an explanation why deposits are more 12C rich then the atmosphere is now.

That's a really interesting point. I'd like to know the answer.

> but we don’t understand the Martian carbon cycle well enough yet to have any real confidence. As with all tantalizing results about microbial extraterrestrial life so far, this will probably turn out to be due to a physicochemical process we don’t yet understand. Or maybe not.

That's right. I think NASA has learned its lessons from the original Viking missions that initially offered tantalizing evidence of biological chemistry only to turn out that there are non-biological explanations for the data. The lesson there is you have to understand the inorganic chemistry of Martian geology before going further in the search for biosignatures.

To add to Tectonic's amazing comment. There's another reason why this is an extremely important find. The 12C concentration exists in a gradient.

I can't say who this person is, but they're a senior scientist involved with the project. They pointed out something profound based on the history of the crater. water came over the rim of the crater and it slowly evaporated, down to the bottom, until it evaporated completely.

If there was life there, then it followed this water all the way down, before its eventual death (?) due to dehydration. And the signatures match that hypothesis! The most anomalous signatures are the ones from the samples taken at the bottom of the crater.

What's even more exciting is that the other two phenomena that are used as examples would have left a wider distribution. The specific pattern in which the signatures were found are even more suggestive of life, but all other hypotheses need to be eliminated first.

If it is indeed life, this changes everything. I have been in favor of orbitals for space settlement for a long time, and this reinforces my beliefs. We might have to declare Mars to be the solar system's largest natural preserve and outlaw human settlement (other than Mars bases for research into this life).

We will carry our microbiome to Mars, and any large scale settlement will have inevitable leakage of our microbiome to the subsurface layers of Mars, causing the microbes we have to out-compete (and perhaps mingle?) the life that exists on Mars. (if it exists)

At the very least, making a settlement on Mars should be put on hold until we have had a definitive detection and have found the native Martians.

This is one of the most important scientific finds in human history.

Why do you assume Earth microbes would outcompete martian microbes in their native environment?

Yeah. More, why do you assume that martian microbes would not kill us?

We don't just need quarantine to protect life on Mars. We need quarantine to protect life on Earth.

(Presuming that this actually is life, which, despite the tantalizing hints, is still a presumption...)

>> Why do you assume Earth microbes would outcompete martian microbes in their native environment?

> Yeah. More, why do you assume that martian microbes would not kill us?

While we can't conclusively argue one way or the other, I wager that life on Earth has probably been subjected to a wider range of states than life on Mars. Our life has explored evolutionary state space from geothermal, oceanic, and hundreds of terrestrial environments. Our microorganisms fiercely compete, and if one of them finds a new optima, the ecosystem adjusts.

While there could be some higher metabolomic maxima that life on Mars discovered, I'd wager it is statistically less likely.

An interesting way this thesis might be compromised however is if alien life uses an altogether different and (probably simpler) biochemistry. All life on Earth sits atop a remarkably uniform platform. Maybe alien life won't pay the same expenses for maintenance of cells and information, and they could compete metabolically in that sense.

> I'd wager it is statistically less likely.

Yeah... let's not wager the entire human race on "statistically less likely, OK?

We've done it several times in the past.

https://en.wikipedia.org/wiki/Safety_of_high-energy_particle...

https://www.insidescience.org/manhattan-project-legacy/atmos...

Because Mars isn't carpeted with life. It is generally thought that if life does indeed exist on Mars, it's likely to be at the poles, in the ice, and/or under the surface where most of the water is.

Life on Earth, on the other hand, carpets the Earth and is almost impossible to completely kill. For example, most space probes are sterilized for forward planetary protection, but they can't be completely sterilized, so they're instead sterilized to different standards. Here's the standard under COSPAR for Mars,

> then the entire landed system shall be sterilized to the following levels: a surface biological burden level of 30 spores (see 5.3.2.4) and a total (surface, mated, and encapsulated) bioburden level of 1.5x10^-4 spores (see 5.3.2.4).

5.3.2.4 states,

> 5.3.2.4 The 30 spore figure takes into account the occurrence of hardy organisms with respect to the sterilization modality. This specification assumes attainment of PP Category IVa surface cleanliness, followed by at least a four order-of-magnitude reduction in viable organisms. Verification of bioburden level is based on presterilization bioburden assessment and knowledge of reduction factor for the sterilization modality.

Life on Earth has survived, amongst other things,

    - On the outside of spacecraft, for several years.
    - Beyond the photosphere in places like geothermal vents or the movile cave, https://en.wikipedia.org/wiki/Movile_Cave
    - Hyperacidic and hypersaline lakes
    - In volcanoes
    - Anoxic lakes

Given that there are - almost certainly - more varieties of life in more niches on Earth, and that these species often co-exist with humans. If any large-scale settlement of Mars occur, then human by-products like sewage, and other by-products produced through agriculture, are likely to cause sub-surface contamination. All it takes is one place for the microbe laden water to seep through into the subsurface water of Mars, and we may end up killing off life on Mars.

More specifically, the worry isn't that they interact with each other, but that our life will out compete the martians for the same resources.

Backwards contamination is less of a concern, at least right now, as we may not be that biocompatible and it is extremely unlikely that the Martians will have the right proteins etc to get past our cell membranes and make us sick. The most plausible disease scenario is the one discussed in the Expanse, where an alien organism finds its way into the vitreous humor of the eyes and starts growing there, causing blindness.

The evidence for extant microbial life on mars is getting pretty solid at this point.

Seasonal methane. Seasonal oxygen. Viking label release experiments. Now accumulation of C12.

At some point someone has to call it.

I'll take my down votes, but I'll wager I've thought about this longer and more carefully than most. I urge you to think critically about this.

Given the milestone that the discovery of extraterrestrial life represents, I would expect nothing short of actually seeing a martian microbe that is provably not a hitchhiker from earth before anyone credible "calls it".

You may be correct, but there is also a huge taboo here and it's damaging to the pursuit of truth.

There is not a taboo. There is a very very high standard of proof required, rightly so because of the sensational nature of the subject.

How damaging is it to not call it until there's proof?

How damaging would it be to call it and later have to retract it?

Seems to me it's better to wait for definitive proof than to go without it.

What is wrong with saying that it is increasingly likely that there is life on mars and waiting to call it until we see it.

Whats the rush and who is it hurting?

As a very casual observer who hasn't thought long and carefully on this topic, it seems we've been looking for life on mars since the 70's with nothing conclusive having been found (not that we've applied exhaustive efforts over the last 50 years).

> At some point someone has to call it.

With humility, why would that point be anywhere prior to actually finding a Martian organism? I recall people seemed willing to "call it" after President Clinton's remarks on rock 84001, and so on.

Genuinely curious if there is some criteria to conclusively show past/present life on Mars in the absence of the discovery of an organism. I'm thinking also of the headline-grabbing announcement of biological processes on Venus owing to phosphine observations.

If you are genuinely asking - there has not been a concerted effort to find extant life on mars. Missions since the viking era have been strictly geological in nature. As so often happens in science, certain theories become taboo. Martian life was one of these and as a result the targeting of the missions became ultra focused on geology. It is only as the sophistication of instrumentation has improved that these signs have emerged as incidental observations outside of a targeted search for life.

By saying "call it", I am fighting against the prevailing taboo of seriously considering and acknowledging the possibility of life on mars.

As regards phosphine in Venus's atmosphere, the jury is most definitely still out on that one.

Thanks, I was genuinely asking and I see I misunderstood what you were saying. I hadn't realized that looking for Martian life was scientifically taboo, though I vaguely recall seeing something about funding sources being skittish about it at some point (maybe that's tantamount to the same thing).

At any rate, I agree this shouldn't be closed to inquiry, though I imagine there's close competition for which experiments and observations to run on Mars. If that's true, does that amount to a taboo or does it just indicate that other lines of investigation have a higher expected utility? Or perhaps you are just saying that at this point, you feel there is enough evidence to warrant a more full-bodied investigation?

Again, genuinely interested.

Knowing people at NASA, this hasn't been the case in my life-time. Most of the flagship rover missions have been designed principally to find evidence of water and life on Mars. Those "geological" missions have been dedicated to finding the evidence of water (something that has been done in my lifetime. https://en.wikipedia.org/wiki/Scientific_information_from_th...

Once detection was confirmed, the mission brief for Curiosity and Perseverance has evolved to finding life. One of the largest science initiatives at NASA right now revolves around sample return to perform a more exhaustive search for Martian life using Earthen laboratories. https://www.nasa.gov/press-release/nasa-moves-forward-with-c...

I have had the good fortune to get to know some of the senior scientific leadership at NASA. I can confidently say, based on their budget and talking to them, that detection of life in the solar system and other planets is the #1 scientific priority at NASA.

Past life. Not extant.

I don't think you deserve downvotes, but your appeal to an authority (yourself), is not very convincing on it's own.

What if he's a Martian in disguise?

Reddit's calling you.

I think the "point someone calls it" will be when we see images of them under a microscope.

Is the theory that they are too deep under ground for a rover to dig up and look at?

Meh. It's not like there are immediate action items if they just call it and say "there was life on Mars". Might as well take their time and be as sure as they can.

It may change research priorities if we knew/admitted that life used to exist. If there _was_ life, we should search harder and see if it still exists and if we can find it. We could learn a lot of really exciting things from a potentially alien set of living things.

As I understand it, we're really not looking that closely for life at the moment.

> As I understand it, we're really not looking that closely for life at the moment.

???

Perseverance: https://mars.nasa.gov/mars2020/mission/overview/

Two out of the four science objectives:

- Looking for habitability: identify past environments that were capable of supporting microbial life.

- Seeking biosignatures: seek signs of possible past microbial life in those habitable environments, particularly in specific rock types known to preserve signs over time.

Curiosity: https://mars.nasa.gov/msl/mission/science/objectives/

Three of the eight science objectives:

- Determine the nature and inventory of organic carbon compounds

- Inventory the chemical building blocks of life (carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur)

- Identify features that may represent the effects of biological processes

That's, as I understand it, looking for markers that _may_ mean there was, at any point in the past, life. I think you'd look other places, and use other instruments if you're looking for actual, current, life.

Like where?

South pole would be my bet.

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