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Comment by julienchastang | Hacker News Reader

This story reminds me of the Viking lander biological experiments where NASA initially discovered carbon chemistry favorable to life on Mars. But it turned out, there were inorganic explanations to the observed data. In short, they had not done the preliminary work of understanding the inorganic chemistry of Mars. The recent life on Venus story reminds me of that Viking experiment in that our understanding of inorganic chemistry of the atmosphere of Venus is probably far from complete.

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To say "they had not done the preliminary work of understanding the inorganic chemistry of [Venus]" seems uncharitable, in this case. IIUC, the original paper is exactly an attempt to do that work, and the discussion in this blog post recognizes plenty of possibilities other than life.

Those Mars papers weren't necessarily any less rigorous. In context, I take "preliminary work of understanding the inorganic chemistry" to imply something like in situ sampling and analysis, which in some of the Mars cases revealed new geology, or at least hypotheses which were considered extremely unlikely given contemporary understanding--understanding that also included satellite imagery and other cutting edge remote sensing--were catapulted into hard fact shortly after the Exploration rovers landing. The paper covers its butt with qualifications like, "Even if confirmed, we emphasize that the detection of PH3 is not robust evidence for life, only for anomalous and unexplained chemistry." But the overall emphasis of the paper is rather optimistic, to say the least, very much like prior claims regarding Mars' chemistry and geology.

IMO, arguably we know far more about biological chemistry than we do about geology even on Earth. Even when we have a solid idea of the boundaries of what's geologically possible and impossible, we aren't nearly as knowledgable in terms of being able to order the possibilities in terms of relative likelihood. The Chicxulub impact and its effects were only confirmed within the lifetimes of most people on HN, for almost any reasonable value of confirmed--1990s, 2016 drilling, w'ever. We were already well along on our way to understanding the detailed chemistry of DNA before even the general concept of plate tectonics (let alone the finer details) became established scientific fact. Studies from the 2015 Mercury MESSENGER impact have overturned and are continuing to overturn established consensus on planetary geology. We should be skeptical of claims (implicit or explicit) regarding the relative likelihood of biologic vs non-biologic processes.

OTOH, perhaps governments aren't apt to fund those experiments without the enticement. Extravagant claims are useful to help make that happen, and either this group had that in mind or it's simply the case that we only ever hear about such studies when they do make extravagant claims. Either way, I would certainly support more Venusian missions even if my money is in history repeating itself--new geology, not new life.

I agree. As much as I would like there to be microbial life in the atmosphere of Venus producing this excess phosphine, I suspect that there is some abiotic process happening that we don't understand as yet. Also, I didn't have "finding alien life" on my 2020 bingo card. :P

In any case, even if this should turn out to be a previously unknown, abiotic chemical process, it is still a significant discovery. It's just not time to run out and say "we found aliens!" yet.

Layperson, but my intuition is the same. Also, as far as I understand it, phosphine is seen on other planets (gas giants?). If phosphine is seen on these planets, is it really a stretch to think anything but similar explanations?

Is there a TLDR for why we understand that phosphine can be on other planets without life but not on Venus without life?

The temperature and pressure near the center of gas giants is crazy high. For Jupiter [0]:

> The temperature at the core boundary is estimated to be 36,000 K (35,700 °C; 64,300 °F) and the interior pressure is roughly 3,000–4,500 GPa.

It's similar to why you wouldn't be surprised to find diamonds deep underground in a mine, but if you found tiny diamonds forming in clouds that would be a major revelation.

[0]: https://en.wikipedia.org/wiki/Jupiter#Physical_characteristi...

As I understand it, the big deal about this discovery wasn't the presence of phosphine, but the amount of phosphine they found. The authors of the original paper went through all of the well-known scenarios for generating phosphine abiotically, and reasoned that the concentrations these would produce were too small for them to be the cause. That left either a biological mechanism (most likely microbial life in the atmosphere), or some unknown abiotic mechanism.

Edit: Fixed typo, phosphate -> phosphine.

The larger-scale lesson is that life has proved difficult to identify. We don't know all the inorganic paths to life-like signs. It will probably take a microscope to give definitive evidence, not chemistry.

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