The decade of Venus: an interview with David Grinspoon
by Arwen Rimmer
|In recent decades we have put almost no resources towards exploring Venus. Yet it is a goldmine of information that’s really nearby on how Earth-sized planets can evolve.|
It should be. The only reason I’m hedging is because I’m part of a community that has been advocating for a long time for a new Venus mission and it’s overdue. We are like Charlie Brown going to kick the football, while Lucy always pulls it away. So, there’s a part of me that is hesitant to get my hopes up. However, I do think it ought to be the decade of Venus. There’s a growing realization beyond our small group of believers that we’re finally getting traction by gaining allies in the wider planetary science community. Because if we’re serious about trying to understand Earth-sized planets and how they evolve, then there’s something ridiculous about putting all this effort into just building huge, expensive space telescopes and trying to glean a few pixels of light from somewhere very distant, while completely ignoring the closest planet to us. It happens to be an Earth-sized planet, with probably a very similar origin to our own planet, and an evolution that we still have not deciphered because we hadn’t really explored it in detail.
Exoplanet science is certainly having its moment. Do Veneraphiles hope to capitalize on the popularity of the field?
The intense interest and effort to understand Earth-sized planets elsewhere, which are very hard to study, is coming together in a way that is inadvertently helping the Venus community. In recent decades we have put almost no resources towards exploring Venus. Yet it is a goldmine of information that’s really nearby on how Earth-sized planets can evolve. At Rocky Worlds in Cambridge I was very gratified at that meeting, as a Venus guy, to hear all these exoplanet people saying to me, “Wow, Venus is really important. Why aren’t we exploring more? We need to understand it to understand exoplanets.”
Beyond enthusiasm from the exoplanet community, who else has been showing interest in Venus, either by contributing to the literature or joining mission proposals?
The other community that is getting onboard is astrobiology. Personally, I’m really motivated by the question of how planets evolve as homes for life, how they gain and lose habitable conditions, and what that even means to have habitable conditions. The most recent climate model for Venus suggests it could have been an Earth-like habitable world for most of its lifetime. We used to have this idea that Venus was always in its current dried-out, hot state. But since then we have finally been able to apply decent climate evolution models to Venus, which are modified from the models we use to predict climate change on Earth. When we apply these general circulation models to Venus, what we find is kind of surprising: that Venus could have had liquid water oceans on its surface for most of its history, and probably only lost them recently. So that creates a very tantalizing picture of our solar system, where there may have been two habitable planets right next door to each other, and both with oceans, maybe even exchanging material and exchanging life between them.
This is why Venus has become a target for astrobiology, so as to understand other places where life could evolve. But to really understand its history, we need more missions because all the current models have caveats. They say, “Well, if we assume this, and if we assume that, then we get a very enticing result showing a previously habitable planet.” But the only way to verify those assumptions, or to refute them, is to go there with new missions and make some crucial measurements that have never been made about the atmosphere, such as identifying the rare gases and trying to understand the minerals on the surface.
David Grinspoon (credit: Gerald Shields - CC BY-SA 3.0)
In your book Venus Revealed, you discussed the “current state of knowledge” on the Venusian surface. Twenty-three years later, we still don’t know much more about it than we did then. Why the long wait?
|When we apply general circulation models to Venus, what we find is kind of surprising: that Venus could have had liquid water oceans on its surface for most of its history, and probably only lost them recently.|
That book came out right after Magellan. Sadly, we haven’t gotten a good look at the surface since then. There have been a few observations, because it turns out that you can see the surface of Venus in the infrared to a limited degree. So we have learned some things from the Venus Express and Akatsuki missions. But for the most part Venus Revealed is not out of date. Further Venus missions were put on pause because of its harsh environment; it was thought to be too high risk. But recent advancements have changed everything; the microelectronics revolution especially. Think about digital cameras, like the one on your phone, how that’s so much better than what we could get in the ’70s, ’80s, and ’90s, which is when we went to Venus last. The process of things getting smaller and lighter, that’s really the key thing, because when you’re sending a mission to another planet, it’s all constrained by the weight of your instruments. That constrains the size of the launch vehicle, which constrains the cost, and also how much power you need to draw when you get there. Materials now can actually survive the environment. I would like to write an addendum to my book, but I have to wait until DAVINCI+, or one of the other missions gives us more data.
What is the most important thing about Venus that we don’t know?
I think there are two things. Firstly, does it have active volcanoes today? We think it does. We have circumstantial evidence pointing in that direction. This is very important because so many of our ideas about Venus compared to other places in the solar system are about how it is currently active. The forensic geology of the Moon and the forces that shaped Mars are very interesting. But In terms of real geological action, like volcanoes and earthquakes, it’s all billions of years in the past. Venus could be alive now in the sense that Earth is alive, with active volcanoes and so forth. And that would be a really exciting thing to characterize. A mission that could go and measure the atmosphere, map the surface in detail, and see changes happening in real time would establish what we currently only have circumstantial evidence for: that Venus is geologically active.
The second, equally important question is its oceanic history. Actually, it’s a two-part question: Were there really oceans? If so, how long did they last? This is really fundamental to the Venus-Earth comparison, and to the question in general of habitability on Venus-like planets. To do this we need to measure the atmosphere and map the surface in detail.
Will the proposed missions, among them DAVINCI+, VERITAS, EnVision, VENERA-D, and Shukrayaan, be able to answer these two vital questions on Venus’s past and present?
They are all different and will all add to our understanding. But as you know, I’m on the DAVINCI+ team, and it’s designed largely around trying to understand aspects of the atmosphere, like detecting disequilibrium and chemicals produced by active volcanoes. It will also do some mapping and will help to characterize the composition of the surface. But I’ll happily extol the virtues of other mission concepts, whether entry probes, rovers, landers, or orbiters, because these things are all complimentary, and will all contribute to our overall goal of understanding Venus. One thing I’m not going to do is give a hard sell on one mission over another because, as a member of the Venus community, I would be excited to see any of these missions done.
With Discovery there are four missions that have made it to phase A, and supposedly two of them are going to be selected to actually fly. I think there’s a case to be made for selecting both the Venus missions, DAVINCI+ and VERITAS. I have my private thoughts, and over a beer, I could tell you why I love DAVINCI+ the best, but honestly, if this is going to be the decade of Venus, we need to do all these things and even more. We need an entry probe, because the last American mission that dropped anything into the atmosphere of Venus was in 1979. We need to go to Venus with 21st century instruments and ask the questions we finally know how to answer. We also need an orbiter with advanced infrared and radar surface mapping capability. These kinds of missions will be groundbreaking in themselves, but will also ultimately pave the way for rovers, because there are certain kinds of questions that can only be answered on the ground.
You mentioned before that the astrobiology community is interested in Venus. And you yourself have speculated on the clouds as a kind of “habitable zone.” Will any of the proposed missions shed light on this topic?
I’ve been advocating for a long time that there could be a niche, a place where life could exist in the clouds of Venus. The conditions there are unlike the surface, where it’s just too hot and organic matter could not exist. But the clouds are mild. It’s sort of like Earth over there in terms of temperature and pressure. But then you have to ask, well, but what about the fact that the clouds are made out of concentrated acid? And the answer is, that we know of life on Earth that lives in conditions as acidic as the clouds of Venus. And there are energy sources there and nutrients. And we know there are aerial microbes on Earth. But then again, the clouds here aren’t green. Life could just be bouncing from mountain top to mountain top. Maybe it’s just passing through, which wouldn’t work on Venus because of the surface conditions.
|I’m not saying I think there is life on Venus, and we have to go find it. But there’s a whole industry in looking for life on Mars in aquifers that may or may not be there. And I would put Venus in the same category of plausibility.|
But there are a few ways in which Venus might be more habitable for an aerial microbe than Earth. Here the clouds are ephemeral, places that sort of come and go. And if you’re an organism that depends on this for your habitat, Earth wouldn’t be a good place to live because your home will just sort of disappear. But the clouds of Venus are continuous and stable. If you lived on an ocean world and found another planet that was mostly dry, with brief little puddles here and there, you would say that world’s not habitable. Venus’s sky is like an ocean. It’s a ten-mile-deep reservoir of clouds that are there all the time. This could allow for particle lifetimes, for a reproductive cycle.
I’m not saying I think there is life on Venus, and we have to go find it. But there’s a whole industry in looking for life on Mars in aquifers that may or may not be there. And I would put Venus in the same category of plausibility. And there’s the famous unknown ultraviolet absorber, this mysterious stuff in the clouds of Venus that absorbs more than half of the solar energy. And we don’t know what it is. The idea that I’ve been pushing, and actually discussed in my book, is that it’s a bio pigment, like an analogue to chlorophyll on Earth. Data from hi-res spectroscopic instruments is needed if we want an answer to this question, as to what is actively absorbing UV in the atmosphere, and whether or not it is life.
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