Life and helium atmospheres

https://earthsky.org/space/life-in-helium-atmospheres-exoplanets-habitability/

Well a couple of paragraphs down they get around to mentioning that helium is inert and hence would not be part of any life, only that maybe it won't interfere.

Exactly. The headline makes it sound much more exotic than the actual claim.

The key distinction is that helium isn't being proposed as the stuff of life. It's being proposed as the background atmosphere in which ordinary biochemistry might operate. The article itself eventually says this quite plainly: helium is chemically inert, so it doesn't interfere with metabolism, cell division, photosynthesis, etc.

And that is actually an interesting result, but considerably less dramatic than “life in helium atmospheres.” A helium atmosphere would be rather like saying, “Could terrestrial organisms live in an atmosphere consisting mostly of nitrogen?” Nitrogen isn't food either; it is simply the surrounding medium. The organisms need oxygen, carbon dioxide, water vapor, nutrients, and so forth in appropriate amounts.

The potentially important part of the study is therefore not helium supporting life, but helium not preventing life. The researchers apparently tested a surprisingly broad range of terrestrial organisms and found that, provided the necessary metabolic ingredients were supplied, helium itself wasn't toxic or otherwise obstructive.

There's also a nice astronomical wrinkle: because helium is so light, a helium-rich atmosphere has a large scale height, which can make the spectral signatures of relatively tiny quantities of biologically interesting gases easier to detect. So helium might actually be useful to astronomers looking for life, even though it contributes essentially nothing to the biology.

So I'd translate the paper's headline claim into something like:

    “Could life exist on a planet whose atmosphere happens to be mostly helium? Apparently yes.”

That's a much less sensational—but actually rather neat—result.

Nitrogen does play an important role in Earth's life, just not atmospheric nitrogen.  Hydrogen could do no such thing.

Yes—and that's an important distinction. Nitrogen is biologically essential; molecular nitrogen in the atmosphere is merely an enormous reservoir of it.

Plants and microbes ultimately need usable nitrogen to make amino acids, nucleic acids, and other compounds. Atmospheric N₂ is remarkably unreactive, though, so most organisms can't simply breathe it in and use it. Nitrogen fixation—by bacteria, lightning, and industrial processes—converts it into chemically usable forms such as ammonia.

Hydrogen is quite different. Hydrogen is certainly an essential element of life, because water and virtually all organic molecules contain it. But molecular hydrogen, H₂, isn't remotely analogous to atmospheric N₂ as a planetary nutrient reservoir. Some microorganisms can use H₂ as an energy source, but that's a specialized metabolism, not hydrogen being a fundamental feedstock in the way fixed nitrogen is.

And helium really is the odd one out: there is no biological role for helium at all. No helium-containing biomolecule, no helium nutrient cycle, no helium metabolism. At most, it can be the chemically boring stuff surrounding the chemistry of life.

So the interesting question in that article isn't really “Can life live on helium?” It's “Can a planet have a chemically inert helium atmosphere without that atmosphere making life impossible?” That's a much more defensible—and much less headline-grabbing—proposition.

My prompt mistakenly said hydrogen, leading to your diversion, when I meant helium.

Ah, yes. I took your “Hydrogen” literally and went charging down the wrong rabbit hole. 😄

You meant: nitrogen plays an important role in Earth's life, though not as atmospheric N₂; helium could do no such thing.

And I agree. That makes your original point even sharper. Nitrogen's atmospheric form is chemically inert but participates in a planetary cycle that makes nitrogen available to life. Helium has no comparable biological role whatsoever. It is essentially a spectator gas.

So when the article says helium “could support life,” the word support is doing a lot of work. It means “could be present without preventing life,” not “could provide something life needs.”

That nitrogen reservoir seems important to me.

Yes, I think that's exactly right. The atmospheric nitrogen reservoir is indirectly one of the great foundations of terrestrial life.

What's fascinating is that N₂'s very inertness is part of the reason it works so well as a reservoir. Earth has an enormous quantity of nitrogen sitting in the atmosphere, largely unavailable to most organisms, while a relatively small but continually renewed fraction is converted into biologically useful forms. That gives the biosphere a huge buffer against losing its nitrogen supply.

It's rather like a bank account that most organisms can't directly access, with specialized organisms acting as the tellers. Nitrogen fixation moves nitrogen from the atmospheric account into the biological economy; decomposition and denitrification eventually move much of it back.

And that makes the comparison with helium particularly revealing. A helium atmosphere would be a reservoir of... helium. But since life has no use for helium, there's no corresponding biological cycle. Helium can be an innocuous background gas, but it isn't an ingredient in the planetary machinery of life.

In fact, I think this is one of those cases where the distinction between “chemically harmless” and “biologically unimportant” matters enormously. An inert gas can be harmless without being irrelevant. Nitrogen is a spectacular example of the latter.