Some interesting stuff from the Nature paper

> The Perseverance rover has explored and sampled igneous and sedimentary rocks within Jezero Crater to characterize early Martian geological processes and habitability and search for potential biosignatures ..... the 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.

> Organic matter was detected in the Bright Angel area mudstone targets Cheyava Falls, Walhalla Glades and Apollo Temple by the SHERLOC instrument ..... 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

> PIXL XRF analyses of reaction front rims reveal they are enriched in Fe, P and Zn relative to the mudstone they occur in ..... In the reaction front cores, a phase enriched in S-, Fe-, Ni- and Zn was detected

> Given the potential challenges to the null hypothesis, we consider here an alternative biological pathway for the formation of authigenic nodules and reaction fronts. On Earth, vivianite nodules are known to form in fresh water ..... and marine ..... settings as a by-product of low-temperature microbially mediated Fe-reduction reactions.

> In summary, our analysis leads us to conclude that the Bright Angel formation contains textures, chemical and mineral characteristics, and organic signatures that warrant consideration as ‘potential biosignatures’ that is, “a feature that is consistent with biological processes and that, when encountered, challenges the researcher to attribute it either to inanimate or to biological processes, compelling them to gather more data before reaching a conclusion as to the presence or absence of life .....

I had to look up PIXL XRF from this paper - https://arxiv.org/pdf/2402.01544 - it is:

> The Planetary Instrument for X-ray Lithochemistry (PIXL) is an X-ray fluorescence (XRF) spectrometer mounted on the arm of the National Aeronautics and Space Administration’s (NASA) Mars 2020 Perseverance rover (Allwood et al., 2020; Allwood et al., 2021). PIXL delivers a sub-millimeter focused, raster scannable X-ray beam, capable of determining the fine-scale distribution of elements in martian rock and regolith targets. PIXL was conceived following the work by Allwood et al. (2009) that demonstrated how micro-XRF elemental mapping could reveal the fine-textured chemistry of layered rock structures of ~3,450-million-year-old Archean stromatolitic fossils. Their work not only pushed back the accepted earliest possible window for the beginning of life on Earth, but also demonstrated that significant science return might be possible through XRF mapping. PIXL was proposed, selected, and developed to carry out petrologic exploration that provide the paleoenvironmental context required in the search for biosignatures on Mars, analogous to Allwood et al.’s earlier work.

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Anything is possible but unlikely we would find it unless there are buried megaliths signed by humans with graphic pictures. Even on Earth it would be hard to prove we were ever here after half a billion years after we destroy ourselves. It might not even take that long. Most things will rust, decompose, shatter into something resembling dust and many coastal regions will get recycled by plate techtonic movements. Mars has been a dead planet for a long time no magnetosphere, probably how this one will end up at some point.

I think the filter was eukaryogenesis [1]. (Which in turn depends on the endosymbiosis of mitochondria.)

Put simply, I expect the universe is littered with single-celled life. I think multicellular life, on the other hand, is rare.

[1] https://en.m.wikipedia.org/wiki/Eukaryogenesis

It's pretty unlikely that life emerged independently on two planets in adjoining orbits.

The much more reasonable explanation is that life emerged on one planet and transferred over. Earth and Mars aren't particularly close, but they're close enough for material to transfer between them, particularly early in the solar system when there were far more asteroid impacts kicking rocks and dust out into space.

Would you mind elaborating on this? I don’t understand the point. Maybe naively, I would think that evidence of life on mars would increase the probability of life on exoplanets.

The inner planets have exchanged a lot of material it's possible they also exchanged life. In fact I believe some theories for how life formed rely on it.

For all we know, life started on Mars, not here. Hence the "sudden explosion" of life.

Smoking gun needs sample return, and the current outlook for that mission is not great.

You are right: this is indeed no smoking gun (and it isn't hyped to be one). This is more like "we can't rule out life having created this, but there are alternate explanations which have also not been ruled out".

Unfortunately most of the evidence is going to be like this. The chances for better evidence would probably require a sample return of some sort, and even then I wouldn't expect a smoking gun (either way).

This is a silly comparison. The article itself does not contain any phrase like "Clearest Sign Yet of Ancient Life on Mars", it was someone else's decision to give it the clickbait title here.

I'm not an expert on the topic here, but at arm's length this sure seems like responsible scientists doing their best to rigorously study something with some crazy implications. They're not saying "OMG guys there was life on mars!!!!", they're saying from what we can tell with Perseverance's little portable lab these rocks sure seem consistent with a biosignature. Their conclusion is that gee it would be great to have samples brought back to earth for better analysis, which... maybe one day, who knows? Here's what they actually say:

  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.

> Is it more clear than the presence of artificial canals on the planet?

…yes. Exhibit A is TFA. Exhibit B is the claim that there is ancient life has a lower burden of proof than that there was an ancient technological civilization.

I think the notable thing this time is Acting NASA Administrator Sean Duffy referring to the (current) Sep 10th Nature paper by summarizing that “After a year of review, they have come back and they said, listen, we can’t find another explanation, so this very well could be the clearest sign of life that we’ve ever found on Mars, which is incredibly exciting.”

Last I heard there was a lot of back and forth about whether that was a real signal. I guess phosphine is easy to mistake for SO2, which, well, Venus has a lot of SO2.

Theories of the origin of life on earth seem to be leaning towards it happening on/in warm damp rocks where RNA like molecules can accumulate in pores. I think there would be problems on Venus - the rocks are too hot, nothing cell like in the atmosphere. But it may be common with a warm damp rock set up which probably was there on Mars.

Organic molecules are fairly common in the solar system on asteroids and the like and probably of non biological origins.

I'm always amused when sci-fi writers think they're being deep by having ancient humans be aliens from another planet. They just completely forget about the 10 million other species we're so clearly related to? Ancient bacteria on meteorites might explain why DNA looks the same, but it doesn't explain the steady, continuously increasing genetic difference between us and our increasingly distant cousin species. It doesn't explain the dozens of "missing link" fossils between modern humans and ancient primates. If ancient humans came from another planet, then they must have brought billions of other lifeforms from millions of different species with them as well, including millions of fossils that they then buried at the appropriate strata as some sort of prank on future paleontologists, on a giant ark that has completely disappeared. It's such a weak take. The natural record is far too clear for that to make any sense.

Now, Mars / Earth cross-seeding proto-bacteria, billions of years ago, sure. Of course the energy required to kick up a rock to a trajectory where it will hit Mars (or vice versa) is orders of magnitude more than the energy required to vaporize all nearby life, so we've got a pretty big problem already. More likely, they both got seeded when our solar system passed through a cloud of primitive organic proto-life (this is my favorite theory). But eukaryotic transfer? Multi-cellular life transferring between the two worlds? No.

Similar idea I think: organics depositing iron

>Will we ever directly probe somewhere that likely harbors life

I recall a few years ago when they believed they had seen liquid water running down a hill they didn't want to move a probe near it for fear of contaminating it.

> NASA has chosen landing sites that specifically have evidence of ancient rather than more recent or even present day water

Source?