有life on Venus?

有life on Mars? David Bowie and a host of space scientists have asked this question for many, many years. But what about our other neighbouring planet, Venus? Studies of our morning star have led to some strange observations about Venus’ atmosphere, which have led to speculation that maybe life might inhabit the clouds. A study published this week in Nature put this hypothesis to the test, using what we know about digestion and a computer. Sean Jordan, from the University of Cambridge, who conducted this research, first gave Julia Ravey an idea about what it might be like to live on Venus…

Sean - Venus is very, very hot and high pressure at its surface. The atmosphere is made of carbon dioxide. So when we talk about greenhouse gases on earth, like carbon dioxide in the atmosphere, we're talking about hundreds of parts per million. But on Venus, the whole atmosphere is made of CO2. So it's really, really hot. And it's also got this really thick cloud layer that's made of sulphuric acid and this cloud layer acts like an insulating blanket across the whole planet. While the surface is completely uninhabitable, we have conditions in the cloud layer that are suitable for life, as far as we know on Earth, in terms of pressure and temperature.

Julia - And so what are the hypotheses for why there are such high levels of sulfur in Venus' atmosphere?

Sean - Venus is in a regime called a stagnant-lid regime; whilst earth has plate tectonics, which helps to recycle materials from the surface and atmosphere back into the interior, all of these mechanisms for drawing down gases from the atmosphere back into the interior, Venus doesn't have this. So that means that when you have volcanoes releasing gases into the atmosphere, these gases just build up and build up and build up until they become really high pressure.

Julia - And these conditions, as you mentioned before, are potentially life-permitting. If there were to be life up in the clouds of Venus, what would that life be?

Sean - Well, that would be microbial life. Nothing like you or me., not even things like birds flying around, it would just be tiny little microbes and they would be living inside of the cloud droplets. And basically the whole biosphere would have to be in this permanently aerial regime where you have microbial life in the droplets, replicating, metabolising, doing all of its processes that it needs. And then when they fall to the hotter region at the base of the cloud deck, the cloud droplets would evaporate, the microbes would turn into spores and then some proportion of those spores would get uplifted back up to the cloud layer. And it would repeat this cycle.

Julia - If life was in Venus' atmosphere, what would we expect to see in terms of alterations to the gases that are present there?

Sean - It would have to use the chemicals in the atmosphere in order to generate energy via some sort of energy metabolism. And then if that life is sufficiently abundant, it would have to be altering the chemistry of the atmosphere via that energy metabolism, by the loss and gain of different chemicals from the atmosphere. So what we would expect to see is some depletion or some release of gases in the atmosphere that isn't otherwise consistent with an atmosphere without life.

Julia - And are there any models which you can use to test if there is life out there and it's changing the atmosphere in this way?

Sean - There have been energy metabolisms proposed that life on Venus, if it exists, would be able to use based on what we know is in the atmosphere and based on known terrestrial metabolisms that some microbes on earth use. So what we can do is we can test whether life is there and altering the atmosphere via these metabolisms by doing computer simulations, where we couple the metabolism occurring due to a biosphere in the crowd layer with the full atmosphere of Venus. And we can see what effects that biosphere would have on the atmosphere.

Julia - And you've done this. So what did you find?

Sean - We found that the main feature in the sulphur chemistry in the clouds of Venus that currently is not fully explained, where sulfur dioxide is kind of sucked out of the atmosphere in the cloud layer; this is exactly the kind of feature that could potentially be indicative of a biosphere using the suggested metabolisms. And when we ran the models, we found that actually whilst each metabolism that was proposed could, in principle, reproduce the observations of this sulphur dioxide being sucked out of the atmosphere, what it meant was that other species in the atmosphere would then be violating their observational constraints. And so we can rule out the hypothesis that life is responsible for sucking this SO2 out of the atmosphere in the cloud layer.

Julia - Does this mean that there isn't any life on Venus?

肖恩——好吧,不一定。这仅仅意味着we can place an upper limit on how much life could potentially be there using the suggested metabolisms before violating the observational constraints. And that upper limit turns out to be quite small. So it's only possible that a relatively small biosphere or low biomas biosphere could be there in the clouds, but you could never rule out the possibility that there's any life there. The other implication is that even if our own Venus in the solar system turns out to not have life in the clouds, if it's possible, even in principle that you could have this kind of life that lives permanently within a cloud layer, then that also has enormous implications on life elsewhere in the universe, because that means that we might be able to find life on planets whose surfaces are too hot to be habitable, but which will also have cloud layers that could be at the right pressures and temperatures.