Original Post:
1. If we assume that all the centimeter- to millimeter-scale nodules and concretions on Mars are microbialites then what is the most likely redox couple for metabolism?
2. What type of process is likely limiting the size of these putative clusters of microbes?
3 With the discovery by MSL of co-existing oxidized and reduced sulfur in sedimentary rocks that contain abundant small nodules, what addition physical or chemical evidence would be needed to call this a microbialite?
4. If we could accurately measure any light stable isotope on Mars what element would provide the most convincing evidence of a metabolic process associated with the formation of nodules and concretions? Is there a pair of elements that would be particularly insightful for discriminating abiotic from biotic processes?
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I am very concerned about the first two questions for the forum discussion:
1. If we assume that all the centimeter- to millimeter-scale nodules and concretions on Mars are microbialites then what is the most likely redox couple for metabolism?
2. What type of process is likely limiting the size of these putative clusters of microbes?
Nodules and concretions are not "microbialites" on earth the way sedimentologists, at least, use the term. We often assume, and sometimes prove, that they are influenced by microbial processes, but there are also abiotic mechanisms for forming nodules. So assuming they are microbial on Mars seems like a very unproductive starting place for a conversation on analog sites. For Curiosity, we don't even know their mineralogy. For Oppy, they are hematite now, but that won't say anything about possible microbial redox couples for ancient putative microbial processes. It would be much more productive to address the question of how they might form in the absence of biology to give a null hypothesis to work from.
For the second question, I have never heard of sedimentary nodules and concretions being interpreted as "clusters of microbes". The sizes of concretions depend on diffusion gradients, the rates of reactions, and duration. Thus, I don't see this question as being a useful starting place for a conversation on mars analog sites. We don't know any of the sediment properties, and it is crazy to think of them as physically being "clusters of microbes".
The choice of analog sites needs to be well grounded in good scientific questions that take advantage of what we do know and can actually learn. Maybe a rewording of these questions can help clarify what is meant, but as stated now, they make the astrobiology community sound like they are grasping for evidence of life rather than rigorously evaluating the scientific information that mars can give us if we ask the right questions.
We have no idea what putative Martian life looks like so why not speculate on the possibility that some biological process on Mars is associated with formation of a distinctive sedimentary feature. I think we have restricted the conversation on Martian nodules and concretions to abiotic processes for far too long. There are dozens of petrographic and geochemical articles in well regarded journals on small concretions from terrestrial muds, mudrocks, and carbonates. The authors of many papers infer direct involvement of microbial metabolism in the formation of these sedimentary and diagenetic features. Microbial sulfate reduction coupled to decomposition of organic matter and microbial methanotrophy (both aerobic and anaerobic settings) are the most common examples of metabolism driving patchy spheroidal mineralization on Earth.
Is it the accumulation of metabolic by-products or is it the presence of methane (is it a gas at this point?) that drives nucleation of spheroidal mineralization?
Is your issue with assuming - at least for the sake of argument/study - that the concretions are microbiolites? Or is your issue with the lack of inclusion of the null hypothesis that the concretions are purely aboitic? Or both?
If it's just an exclusion you take issue with, I think adding a question (or sub-question) addressing that would be appopriate.
The nodules accumulate around some focal point, correct? Do we know under what condtions such cumulates form around physical, chemical, or hydrological focii? Any of these could be initiated by biological or abiological processes. I am wondering whether there has been any systematic study of nodule 'centers'.
Edge-to-core transects along both the horizontal and vertical axes are a common strategy for study but I do not know of a systematic overview or nodule centers. Pyrite at the center of ankerite nodules is a common theme and organic matter or a fossil are common in the core of carbonate nodules. When the consumed substrate is a gas like methane, the evidence is usually isotopic.
Maybe a better way to frame the discussion topic questions is to pose the working hypothesis that the martian concretions are microbially mediated, and to pose experiments and investigations that can be done in analog environments hosting similar structures that would prove/disprove a microbial role in concretion formation. These would then lead to possible ways to test the hypothses with future investigations on Mars. I don't think that the current instrumentation tells us much and I doubt that the concretions can even be ground in such a way to introduce them into SAM and find out if they contain more organics than background.
The Martian hematite nodules certainly have terrestrial analogs, although it is not at all clear that they are biogenic in character. Nonetheless, there are terrestrial analogs that do have a biogenic component, as in deep sea manganese nodules.
So as a departure point, let us for the sake of argument start with a hypothetical possibility: assuming they are biogenic or have a biogenic component involved in their formation, what could sustain microbial growth that would allow this to occur. Given the paucity of Martian organic molecules, these reactions would have to be driven by chemo-autotrophy. Oxidation of entrained sulfide perhaps with Fe(III) and fixation of CO2 into cellular components. But what oxidant would oxidize Fe(II) to Fe(III)? Perhaps anoxygenic photosynthesis if they are exposed to light, or another chemoautotrophic reaction with the abundant perchlorate oxidizng Fe(II) and/or sulfide.
PS: Is there a phone connection involved in this discussion or is it all typed on-line?
Athough the jury is still out on methane plumes in the atmosphere, it still seems worth thinking about methane migration along fractures zones or methane release during seasonal decomposition of hydrates in a active layer above permafrost. If there is an active methane-generating process on Mars then there is substrate for methanotrophy.
Let's assume for the sake of argument that there is no methane in the modern Martian atmosphere. I would still contend the history of methane sources is a relevant topic for study.
Our climate models have difficulty reproducing a warmer, wetter ancient Mars... that's even with methane included in the simulations! So unless someone comes up with some *very* creative solutions to the "faint young Sun padadox: Mars edition" I think we're going to be left asking ourselves where the ancient methane came from, and what chemical and morphological signals those sources may have left behind.
So, I agree! We should pursue this.
and guess what...methane seeps on Earth are often recognized by nodular carbonates or grapestone concretions with anomalously light carbon isotopic compositions. If there was diffuse methane emission over permafrost rather than a discrete seep, is it possible that multicellular communities of single-celled organisms could bloom and sweep the available methane out of thee immediate microenvironment by diffusion such that nodules formed at mm or cm spacing? One can image a similar process for evenly spaced nodules of sulfate or sulfide nodules if there was limited availability of a metabolizable substrate.
Ron, we are using real-time chat line to start up the discussions. Afterwards people can log in and comment at their own conveniece.
No phone, just on-line chats, yes?
Exactly.
We'll have in-person conversations later. But for now it's just the text. No phone.
At least on modern Mars perchlorate is a good candidate for the oxidant.
In my opinion, the key here is finding not the right element but the right combination of elements and the requisite context for the measurement. Fe isotopes were once proposed as a potentially ideal biosignature. But even if we got an instrument capable of measuring Fe isotopes to the surface of Mars (or got a sample back to our multicollector mass specs on Earth), we'd have to contend with discriminating biological fractionation from abiotic redox processes.
That said, there has been some great work at discriminating biotic and abiotic effects by looking at the isotopic composition of Fe, along with that of C, S, and O. Similar work has been done with C, H, and carbon-chain length.
So for me, the question isn't "which isotope?" but "which set of isotopes?" I think that's even more important when the measurement of heavier isotopes is (likely) out of the question.
My brain's grasp of research in this specific area is a little out-of-date, so I'd love to hear others' thoughts on this.
I also believe we need to "think beyond the δ." Mass-independent fractionation of O and S can tell us a lot about the atmospheric inventory history of the planet, and potentially about the presence/absence of biotic processes. Clumped isotope techniques also carry the potential for new information - specifically on temperature histories.
Granted, these may be more difficult measurements to make but I think they're worthy of being ground-truthed on Earth and potential instruments for this developed/tested.
4. If we could accurately measure any light stable isotope on Mars what element would provide the most convincing evidence of a metabolic process associated with the formation of nodules and concretions? Is there a pair of elements that would be particularly insightful for discriminating abiotic from biotic processes?
Certainly there are several stable isotopes of sulfur (eg, 32S, 34S) that can be used to screen for biogenic vs. abiotic signatures within these nodules.
If perchlorate and/or chlorate are involved stable isotopic work can be extended to both Cl and O. Dissimilatory perchlorate reducing bacteria will achieve a substative fractionation of these stable isotopes.
If we imagine that we are performing isotope analysis on the concretions on Mars. Would we be able to say something about biological vs. abiotic origin of the concretions based only on the bulk isotope fractionations (without performing sulfur-compound extractions, or without removal of inorganic carbon) … Can we think of a set of isotope elements that might be considered as potentially indicative of either of the processes/origins?
Thank you Lisa for great start up of the forum.
Everyone, the discussion will continue so please participate at your own conveniece and feel free to start your own discussions too.
Thanks everyone for participating!
I agree with Dawn's general comment that the questions should be rephrased---particularly if they find their way into a Roadmap or MEPAG document because some parts of the questions are loaded and some use terms (e.g., "microbialite") too loosely.
The evidence that Meridiani spherules are concretions is good. At the moment, the MSL spherules are not well characterized. So a basic starting point is to know whether what are seen are concretions or not, before question of biological vs. abiotic origin:
1) With future measurements, how do we establish that apparently widespread spherules in sedimentary layers on Mars are concretions and therefore associated with groundwater?
2) What is the full range of possible formation mechanisms for concretions on Mars?
3) What geologic and environmental conditions would allow concretions to be widespread on Mars? Can we determine these past environmental conditions from in situ measurements?
4) What evidence would we look for to distinguish biological versus abiotic production of concretions?
Regarding the size and distribution of concretions: Diffusion theory (whether biological or not) says it's a matter of timescale, solute concentrations, and physical conditions (e.g., temperature). Elliot Sefton-Nash and I modeled this for Meridiani: E. Sefton-Nash, D. C. Catling (2008) Hematitic concretions at Meridiani Planum, Mars: Their growth timescale and possible relationship with iron sulfates, Earth Plan. Sci. Lett. Download at: http://faculty.washington.edu/dcatling/Sefton-Nash2008.pdf
Whether the "jury is still out" about methane plumes on Mars depends on who the jury is.
Those that have published in the atmospheric chemistry of Mars--- and given considerable thought to the atmospheric redox chemistry of Mars and the Earth-- think that plumes of methane are utterly implausible and a consequence of a mistakes buried in the data reduction techniques, which have not been published in any detail at all. Zahnle et al. (2011) Icarus, provide gory details. The tunable diode laser on Curiosity measures no methane to an upper limit of ~1 ppbv, consistent with prediction in the abstract of Zahnle et al. of an upper limit of <3 ppbv.
Those that haven't given much thought to atmospheric redox chemistry (perhaps quite reasonably because it's outside of their area of expertise) form the vast majority of the jury that's still out.
In my view, it's possible that atmospheric CH4 might exist at some level. But, if so, CH4 will be below the upper limit of what's being measured by the tunable diode laser on Curiosity, i.e., <1 ppbv. The inertness of a exceedingly stable molecule such as CH4 (residence time ~300 years in the martian atmosphere) means that it would be at a steady-level, well-mixed by dynamics throughout the martian atmosphere. It would never be in the form of chemically-implausible, short-lived plumes.
I read the excellent Catling paper last night. How much is known about the source of iron at the west australian site? Perhaps two hydrothermal systems intersecting along differentially reactivated fault zones: one carrying Fe2+ and sulfur, the other Fe3+ and relatively more oxygen. In the Catling paper, many of the sulfate minerals have waters of hydration. The D/H ratio of this mineral bound water might help identify different reservoirs of water. At Houston, the Thiemens group presented data that suggested different water reservoirs on Mars via d17O data. I suggest finding an analog site where sulfides are oxidized to sulfates and measuring oxygen isotope values of sulfate oxygen and D/H of bulk water of hydration. The water of hydration could also come from clays so other chemical data would be needed to contrain relative mineral abundances. Also, could the recent 33S data on black shales look at d17O oxygen isotope data to contrain the atmospheric contribution to the sedimentary record during and prior to 2.5 Ga on earth?
Microbialites are complex communities that have 2 basic characteristics from the point of view of life.
1. They are communities that function because they have a fixed structure in space, such "building" is made by complex polisacarides, and many different "glues", these glues make concresions and clumps in sediments. It is not crazy at all to imagine that the same mineral stabilization process would evolve in an independent mode in life somewere else, because the advantages of having predictable parthers and foes stabilize not only the mineral structures but also the evolution of complex communities.
2. Due to their spatial structure the microbes that live in a microbialite can coexist not only if they have different metabolisms but also different redox by-products of such metabolites. In such case, I would expect that if clumps of minerals in Mars were or are produced by microbial communities they will have a layer of reduced elements followed by layer of oxidized elements. This is because coupled metabolic functions are a thermodinamically stable evolutionary stategy.
The stromatolites and microbial mats of Cuatro Cienegas Basin are extraordinarly diverse in composition but all of them share this basic building rules.