I wouldn’t describe myself as an ‘expert’ on astrobiology; more of a knowledgable enthusiast with enough background in biochemistry, systems ecology, and planetology to be both intrigued by what Perseverance found and skeptical that this alone indicates a high probability for life having developed on Mars. As you note, there is a storied history of discovering strong evidence (‘signals’ in the parlance of astrobiologists) for former or extant life on Mars, mostly in plumes or atmospheric concentrations of organic molecules or in geochemical deposits which on Earth are biomarkers associated with the waste products of microbiota.
In this case, the Perseverance rover found vivianite (which is often found attached to fossilized bone or within the shells of bivavles) and greigite (which is formed by magnetotactic bacteria that occupy hydrothermal vents on Earth). To my understanding (I am definitely not an expert it geochemistry), the known pathways for forming these minerals at terrestrial temperatures requires the decay of organic matter or transformation by bacteria in iron-bearing substrate, and the only pathways of developing these minerals without the presence of living system require very high temperatures for which this is no evidence were present on the surface of Mars which this material was deposited. That leaves open the possibility of formation elsewhere near a volcanic or hydrothermal vent during the period that Mars was still tectonically active and was then deposited later at this location through hydrologic activity, The Nature paper linked in the article expresses all of this in the abstract:
Here we report a detailed geological, petrographic and geochemical survey of these rocks and show that organic-carbon-bearing mudstones in the Bright Angel formation contain submillimetre-scale nodules and millimetre-scale reaction fronts enriched in ferrous iron phosphate and sulfide minerals, likely vivianite and greigite, respectively. This organic carbon appears to have participated in post-depositional redox reactions that produced the observed iron-phosphate and iron-sulfide minerals. Geological context and petrography indicate that these reactions occurred at low temperatures. Within this context, we review the various pathways by which redox reactions that involve organic matter can produce the observed suite of iron-, sulfur- and phosphorus-bearing minerals in laboratory and natural environments on Earth. Ultimately, we conclude that analysis of the core sample collected from this unit using high-sensitivity instrumentation on Earth will enable the measurements required to determine the origin of the minerals, organics and textures it contains.
Perseverance used the Mastcam-Z ,Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) and Wide Angle Topographic Sensor for Operations and Engineering (WATSON), SuperCam, and Planetary Instrument for X-ray Lithochemistry (PIXL) instruments to examine the Bright Angel outcrop area and extracted the ‘Sapphire Canyon’ core sample in which these minerals were found by X-ray spectroscopy (using PIXL) coincident with the authigenic nodules and distributed in such a way that they appear to have been formed in place rather than produced elsewhere and later transported into the mudstone substrate. From the paper:
A striking feature observed in the Cheyava Falls target (and the corresponding Sapphire Canyon core sample), is distinct spots (informally referred to as ‘leopard spots’ by the Mars 2020 Science Team) that have circular to crenulated dark-toned rims and lighter-toned cores (Fig. 3a–c). The spots range in size from about 200 μm to 1 mm in diameter and their cores are less red than their surrounding mudstone. Like the previously described authigenic nodules they co-occur with, the spots are not concentrated in layers or laminae; together with their irregular shapes, this indicates that they were not deposited as grains. Instead, these multi-coloured features appear to represent in situ reaction fronts.
I hesitate to further summarize the petrography and geochemical speculation because it is far enough from my domain of knowledge that I’ll likely misstate some critical piece of information but in essence they suggest that the iron–phosphate bearing minerals are unlikely to have formed absent of the presence of carbon-rich organic matter in “post-depositional redox reactions“ (in other words, reduction-oxidation actions catalyzed in place by the activity of some energy-mediating system). The section of the paper labeled “An exploration of reaction mechanisms“ posits a null hypothesis—that is, a thesis that would provide an abiotic (non-living) path to producing these minerals—and then disprove it by arguing that there was no evidence of sufficient conditions for this process to have occurred:
However, geological constraints demand that this sulfide migrate in from a distal, high-temperature sulfide-gas-producing system, to the low-temperature depositional-diagenetic environment of the Bright Angel formation. No evidence for sulfide-producing hydrothermal or magmatic systems was observed in the Crater Floor, Western Fan or Margin Unit before investigation of the Bright Angel formation. Abiotic reduction of sulfate to sulfide by organic matter is another possible source of dissolved sulfide that could both reduce Fe3±bearing sediment and provide the reduced sulfur required to form Fe-sulfide minerals37. However, sulfate reduction by reduced carbon compounds is energetically demanding and kinetically inhibited by the symmetry of the SO42− ion38, so abiotic reaction rates are exceedingly slow at temperatures <150–200 °C (refs. 37,38). As discussed previously, the Bright Angel formation shows no unambiguous evidence that it was heated in contact with adjacent geologic units, and burial to depths in excess of about 5 km would be required to achieve temperatures >150 °C during the Noachian39.
There is a good summary of information about how the instruments listed above were used but that is probably only of interest to scientists and instrumentation geeks, so I won’t try to summarize it. For those who are interested there is an excellent book on the Curiosity rover (The Design and Engineering of Curiosity: How the Mars Rover Performs Its Job by former The Planetary Society “Planetary Evangelist” Emily Lakdawalla); the pertinent instrumentation is essentially the same as on Perseverance with some upgrades in capability. This is really an incredibly piece of engineering with a laboratory’s worth of geoscience equipment integrated into a single one ton rover but it is limited by both energy and what it can do to prepare and manipulate the samples. Doing further direct investigation really requires returning the samples to a terrestrial laboratory for more analysis. However, based upon the hypothesis, experiments can be run to simulate different possible conditions to see if those minerals can be formed at low temperatures via some abiotic catalytic system.
I would describe the paper as being ‘cautiously optimistic’ that this is a good biosignature and recommends further work to evaluate it. The paper is full of a lot of geochemical and jargon is really pretty straightforward, and a layman with basic understanding of chemistry and the patience to look up the terms can understand the gist of it. I am expecting that Rachel Phillips (the YouTube geoscience communicator known as “GeoGirl” on YouTube) will put together a video on this discovery as abiogenesis and astrobiology are particular interests of hers, and will go into more depth in an accessible way than I am able with my limited knowledge on the topic. For what it is worth, the broad consensus of the planetology and astrobiology fields is that Mars almost certainly had conditions that would support terrestrial life at some interval during the Noachian and early Hesperian periods (and not even extremophiles but just ordinary bacteria and archaea that we find on the surface and soil biomes) as well as extensive oceans but by three billion years ago was probably close to its present state, albeit with more tectonic and geological activity than today. Whether life actually developed is the open question and the lack of clear chemical signals is somewhat confounding but billions of years of exposure to harsh, unfiltered ultraviolet and cosmic radiation would have scoured the surface of most organic residues.
Extant life on Mars is unlikely to say the least, not only because we do not see any clear signals of it but because the conditions to support a continuously operating biome (specifically, reliable energy gradients, a sustainable nutrient cycle, and self-regulating ecosystem) do not appear to exist. It is possible that some form of life could exist in the Martian regolith powered by ultraviolet solar insolation driving reactions that are somehow generating energy for chemosynthetic organisms but UV is pretty destructive to all complex organic compounds, and the ambient surface temperature on Mars is just too cold for chemical reactions which would catalyze life-supporting processes to occur without some kind of other energy source. Life as we know it also requires a polar solvent–on Earth that is water which is highly abundant–and while there are occasional flows of a slurry of water-bearing salts (recurring slope lineae) the lack of open bodies of free-flowing liquid water on the surface or subsurface (as far as we know) make conditions unlikely to support even microscopic life.
It is certainly possible that there is life buried deep within the Martian mantle, and there are still geological processes at least periodically occurring within Mars that could provide an energy gradient but that begs the question why life on Earth has been so successful at modifying the climate, geology, and hydrology of our planet for its continued existence regardless of natural hazards but failed to colonize the surface of Mars and produce a dynamic, self-perpetuating system. The lack of magnetosphere and stripping of the Martian atmosphere may be a partial explanation, but as the saying goes, “Life will find a way,” in even the harshest conditions on Earth and yet it apparently failed to take hold on Mars. We may yet be surprised but I personally doubt that there is any existing life on Mars, and almost certainly not at or near the surface.
If we are looking for existing life today in our solar system the best places to look are the icy-surfaced moons of Jupiter and Saturn with subsurface oceans, i.e. Enceladus, Europa, Ganymede, Titan, with energy gradients created by tidal heating. In fact, there is an increasingly prominent view in astrobiology that the most likely places for life to develop are not on the delicate balance of surface conditions of terrestrial-like worlds around F and G class main sequence stars but in the water-bearing moons of gas giants or ‘superearth’ planets around longer lived, low mass, metal-rich K and M class stars where the solid crust protects incipient life from the sterilizing harsh UV and solar particle emissions of those stars. Our Sun is atypically calm to the extent that Earth’s magnetosphere absorbs or deflects most charged particles and our just-so mostly transparent atmosphere ameliorates highly energetic cosmic radiation without trapping a killing amount of insolation but those are delicate conditions that were created and are maintained by the biosphere. Conditions under the protective crust of a moon with the regular energy gradients of tidal interactions may be much more hospitable to nascent life.
Stranger
