[HN Gopher] Depleted carbon isotope compositions observed at Gal...
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Depleted carbon isotope compositions observed at Gale crater, Mars
Author : areoform
Score : 46 points
Date : 2022-01-21 14:28 UTC (8 hours ago)
(HTM) web link (www.pnas.org)
(TXT) w3m dump (www.pnas.org)
| tectonic wrote:
| TL/DR: A paper reports (appropriately conservatively) on
| tantalizing evidence from Gale crater. Curiosity's SAM lab heated
| 24 powdered rock samples to 850 degC, 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...
| thatcherc wrote:
| > 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
| nousermane wrote:
| From [1] above:
|
| > atmospheric, [marine] carbonate, and plant derived d13C
| 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.
| mrfusion wrote:
| Is heating rocks a method of producing methane on Mars?
|
| How does that work?
| petschge wrote:
| 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 d13C 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.
| tectonic wrote:
| That's a really interesting point. I'd like to know the
| answer.
| julienchastang wrote:
| > 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.
| areoform wrote:
| 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.
| d883kd8 wrote:
| Why do you assume Earth microbes would outcompete martian
| microbes in their native environment?
| areoform wrote:
| 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.
| AnimalMuppet wrote:
| 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...)
| echelon wrote:
| >> 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.
| AnimalMuppet wrote:
| > I'd wager it is statistically less likely.
|
| Yeah... let's _not_ wager _the entire human race_ on
| "statistically less likely, OK?
| echelon wrote:
| 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...
| ms512 wrote:
| Recent HN post of a science.org article on this topic:
| https://news.ycombinator.com/item?id=29972342
| plutonorm wrote:
| 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.
| q1w2 wrote:
| 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?
| kadoban wrote:
| Images under a microscope may not do it. They could be
| contamination from Earth either at time of test or before
| that.
| hyperbovine wrote:
| Genetic analysis to the rescue. It will be glaringly
| obvious whether billions of years of divergent evolution
| have occurred or not.
| AnIdiotOnTheNet wrote:
| 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".
| plutonorm wrote:
| You may be correct, but there is also a huge taboo here and
| it's damaging to the pursuit of truth.
| biorach wrote:
| 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.
| AnimalMuppet wrote:
| 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.
| s1artibartfast wrote:
| 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?
| yupper32 wrote:
| 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.
| kadoban wrote:
| 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.
| yupper32 wrote:
| > 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
| kadoban wrote:
| 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.
| Scarblac wrote:
| Like where?
| kadoban wrote:
| South pole would be my bet.
| nescioquid wrote:
| 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.
| plutonorm wrote:
| 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.
| nescioquid wrote:
| 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.
| areoform wrote:
| 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/Scientif
| ic_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.
| [deleted]
| Beltiras wrote:
| I don't think you deserve downvotes, but your appeal to an
| authority (yourself), is not very convincing on it's own.
| queuebert wrote:
| What if he's a Martian in disguise?
| djmips wrote:
| Reddit's calling you.
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