Old News

Phosphine and Venus. Venus image courtesy Venus Ultraviolet Imager (UVI). JAXA / ISAS / Akatsuki Project Team

September 2020. Phosphine on Venus! (and why it matters)

On September 14^th Nature Astronomy published a paper by Prof. Jane Greaves and her collaborators, including me, on the discovery of phosphine in the atmosphere of Venus.

This is huge news. Phosphine is a colourless, highly poisonous gas found in tiny amounts (parts per trillion) in Earth's atmosphere. It is a gas that our MIT group has been interested in for some time (see here for a previous news item on this). It is a robust sign of life, as it cannot be made by any geological process: if phosphine is there, it is made by life. On Earth, anyway. See Here for the Greaves et al discovery paper.

So - phosphine on Venus?

We have spent the last two years trying to work out how a non-living process could produce phosphine in Venus' acid, oxygen-rich atmosphere. We calculated the rate at which it might be produced by reaction of atmospheric gases with each other, with sulfur haze, with cloud droplets, with surface rocks. We worked out if volcanoes or lighning or earthquakes or meteorites could account for what Jane saw. We drew a complete blank. There is no way that Venus can make phosphine, unless our understanding of Venus is seriously wrong. This manuscript is the draft of the paper where we go into detail about how we ruled out a wide range of chemical processes as a source of phosphine on the planet.

But life also seems impossible. Life on the surface is ruled out because at surface temperatures - over 400^oC - any organic chemicals are reduced to char. The clouds are cooler, but they are made of sulfuric acid, which destroys nearly every component of life as we know it. Any life on Venus must be radically different from life on Earth, and I do not mean "has pointed ears and green blood" different. I mean does not use protein. Or DNA. Or maybe even water.

This is spectacularly exciting because, as Isaac Asimov (probably) said, "The most exciting phrase to hear in science is not "Eureka" but "That's odd...". Something inexplicable, strange, can lead to momentous discoveries. Personally I still think the chance of life on Venus is small. I think the chance of life anywhere is small. But - wow! If that is life, if our nearest neighbour in the Universe, a planet completely unlike ours, hosts living organisms, that would be beyond amazing. The tiniest chance that there is life there is something that we have to follow it up, to go there and find out.

Dec 2018. Phosphine - smelly, dangerous, and a great sign of life.

The Seager group has become really interested in phosphine. Phosphine (chemical formula PH₃) is a very toxic gas that is present in a few parts per trillion in Earth's atmosphere. We have published two papers on this work [1] [2]. The bottom line message is complicated, but exciting.

Phosphine is very poisonous. It is used as a fumigant because it kills rodents, insects, almost everything. But that toxicity is dependent on oxygen. Phosphine is actually pretty safe if you do not need to breath. Phosphine is also very rare, but our first paper showed that life could actually get energy from making phosphine if certain environmental conditions were met, including there not being any oxygen around.

And nothing else makes phosphine (apart from human industry). Volcanoes do not make it, it is not formed by itself in the air. Phosphine is a great sign of life, what astrobiologists call a 'biosignature'

So on another world, one where there is no oxygen, a world that is more acid than Earth, could phosphine be a sign of life? We think it could, and our colleague Clara Sousa-Silva, is working on just how we could detect phosphine as a sign of life on worlds around other stars.

Do we have a favourite world in mind? You bet! Watch this space for more.

Dec. 2016. Wherever life is, it is complex

Two papers [1] [2] out recently argue that complex life is not a unique outcome of unlikely events on Earth, but is likely wherever life arises in the Universe.

Dirk Schulze-Makuch and I explored the key innovations in function that have lead from simple microorganisms to plants and animals today. We have three models for this: Random Walk (a step is very rare and unlikely), Critical Path (a step is likely, but only after one of more sets of conditions make it possible), or Many Paths (each mechanism to allow a function is unlikely, but many combinations of mechanisms can work). We show that all the steps apart from the origin of life itself and the appearence of human-style, technological intelligence are probably Many Paths processes, and so likely to occur.

Does this mean that We Are Not Alone? Alas, not necessarily, as the two remaining steps could be very, very unlikely indeed

July. 2016. Looking for signs of life? Look for them all!

Sara Seager's group published a completely new approach to searching for life in Astrobiology this month. The only possible way to detect life on worlds around other stars today and for the next 20 years will be to detect their effect on the atmospheres of those worlds. But what gases to look for? On Earth we know what gases life produces, but different life might make different smells.

Our approach is to compile a list of all possible gases, and work backwards to the ones we would not expect a 'dead' planet to make, and which we can detect from Earth. This paper describes the first steps in this project, the database itself. A major piece of work, this was featured on the cover of Astrobiology. The database itself is now available at this web site.