David Grinspoon, an astrobiologist at the Planetary Science Institute, has long advocated sending a spacecraft to look for signs of life in the clouds of Venus. In an email to Cosmic Log, he provides his reaction to the news that a potential biomarker known as phosphine or PH3 has been detected in Venus’ atmosphere:
Well, I think it’s extremely enticing.
I am not an expert in this type of observational astronomy, but from what I can tell, they’ve done things very carefully, and the observation seems solid.
It’s always thrilling to find a glaring anomaly, and this is a gas that really should not be present in such abundance in the atmosphere of Venus. It’s far out of equilibrium and has a short lifetime against chemical destruction. So this demands an active source. What is that source?
There is no obvious mundane answer. Volcanic outgassing and meteoric infall both seem to fail by many orders of magnitude. So, we go to the “extraordinary claim.” Could this be a biosignature? What’s interesting to note about phosphine is that well in advance of this work it had already been singled out as a uniquely ideal biosignature for rocky worlds, in studies that looked at all possible gases, and selected for those that are produced by life and that do not have non-biological sources. Certainly in Earth’s atmosphere all the phosphine comes from biology, and from industry. (Including from meth labs — I don’t think this is specifically relevant to the Venusian PH3 observation…)
Now, personally, I am delighted because I have been talking up the plausibility of a possible cloud-level biosphere on Venus since 1997. That is when my book “Venus Revealed” came out with a chapter on life on Venus, proposing the case that the (then new) post-Magellan picture of Venus is one that could support such a biosphere. It’s not just that the cloud level atmosphere is, unlike the deadly surface, moderate in terms of temperature and pressure, and the clouds themselves represent a stable, long-lived aqueous environment with all the known biogenic elements either observed or strongly suspected, and with copious sources of both radiative and chemical energy which an enterprising (or well-adapted) microbial population could have adapted to. … Venus had newly revealed itself to be an active world, and some modeling by my group at the University of Colorado showed that the clouds themselves were intimately related to the surface and interior chemical cycles of sulfur and other elements, all mediated by both volcanism and solar radiation.
In other words, Venus, we found, has the kind of global geochemical cycles which, on Earth, life surfs upon, turning them into biogeochemical cycles. Further, I noted that there are some strange observations of the clouds which could conceivably have a biological explanation. The “unknown ultraviolet absorber” behaves in some ways like a biological pigment, absorbing a huge amount of solar energy, and coming and going mysteriously, almost like algae blooms in the ocean. The largest cloud particles seem to have some unknown substances residing in their cores. The lifetime of the cloud particles — much longer than cloud particle lifetimes on Earth — could easily support a population of reproducing microbes, which would be self-supporting as long as the reproduction timescale was much shorter than the particle lifetimes. All of this is laid out in “Venus Revealed.”
For years I was the guy who gave that “Life in the Clouds of Venus” talk at the planetary science meetings and then, after astrobiology became a thing, at the astrobiology conferences. Folks would roll their eyes at my conference talks, but I was tolerated because I did a lot of good work on other aspects of Venus, writing papers on the clouds, the surface evolution, the climate, and so forth.
The idea encountered much resistance and some ridicule back then. My editor tried to get me to remove that section from “Venus Revealed,” arguing that an otherwise excellent work would be tarnished by such an outlandish proposal. I still have the correspondence. I had to fight to keep this in the book.
Interestingly, the idea has experienced a bit of a renaissance recently, well before this recent announcement. It has become somewhat more acceptable to talk about possible life in the Venusian clouds. This is in part because of the steady discovery of extremophile organisms on Earth, including those which are happy in extremely acidic conditions, which was always the most serious objection to a cloud biosphere on Venus. Also, the continuing discovery of exoplanets and the realization that a great number of these are in the “Venus Zone” has led to some fresh thinking from new communities outside the usual suspects of planetary science.
Also, our ideas about the evolution of Venus are changing. It used to be standard knowledge that if Venus ever had surface oceans, it lost them very early on in a runaway greenhouse. More recently we have finally been able to apply state-of-the-art, Earth-style GCMs (General Circulation Models) to the problem. Surprisingly, we find that the most likely history for Venus is one in which it enjoyed surface water oceans for most of its history, perhaps losing them relatively recently. So, it may be that Venus, not Mars, is our best hope for a long-inhabited nearby neighbor.
Then the question becomes, when Venus lost its surface water and its mild climate, what happened to the biosphere? One possibility is it migrated to the clouds. It’s true that there is no comparable cloud biosphere on Earth, but I note that compared to the clouds of Earth, which are ephemeral, patchy and transient, the Venus clouds, being globally continuous and very stable, may be a much better place for life.
So … Does this mean we’ve found my Venusian organisms? Of course not. But it sure is an interesting hint. And it demands an explanation. People will immediately begin taking potshots at it, as well they should when such a striking finding is announced. Some will critique the observational technique and interpretation. This is not my forte, but I look forward to the debate. The experiment must and will be repeated. Laboratory studies will be undertaken to see how PH3 behaves in a Venus-like environment and what else could possibly produce it. But the best test, and the one I’m most excited about, is to go back to Venus and investigate the atmosphere in situ.
We have not done this for many decades, and never with 21st-century scientific instruments. This demands that we go there and see what is really going on. Investigate the atmospheric composition and find clues to its history. Determine the composition of the ultraviolet absorbers. See what trace elements (and structures?) reside in the large cloud particles. There are good scientific reasons to do all this anyway, but now that we’ve found a genuine candidate biosignature, we absolutely must go.
And even if this turns out to be a false alarm, it could be productive, in the way that the “Mars rock” (ALH84001) was. That turned out – probably – to be a false alarm, but it got everyone to think about it in a fresh way and ask “Why not?” Why shouldn’t there be Martian microfossils? And thus an Astrobiology Institute was born. In the same way, this announcement should get everyone to say “Why not” to the idea of indigenous life in the Venus clouds. And I believe that when astrobiologists really think it through, they will realize that the idea is at least as plausible as a subsurface biosphere on Mars or Europa today.
The fact is we know nothing about ET life, so any locale that seems to have the necessary ingredients must be considered and explored. The clouds of Venus (as I’ve maintained since I was 37 — and I’m 60 now) are such an environment.
We need to follow up in the ways I’ve suggested, but this could be the first observation we’ve made which reveals an alien biosphere and, what do you know, its on the closest planet to home in the entire cosmos. That is pretty damn exciting!
David Grinspoon is also the author of “Earth in Human Hands: Shaping Our Planet’s Future” and the co-author (with Alan Stern) of “Chasing New Horizons: Inside the Epic First Mission to Pluto.”