Gravity Assist Podcast: Venus, with David Grinspoon (Part 1)

The Gravity Assist Podcast is hosted by NASA’s Director of Planetary Science, Jim Green, who each week talks to some of the greatest planetary scientists on the planet, giving a guided tour through the Solar System and beyond in the process.

This week, he spoke to astrobiologist and planetary scientist David Grinspoon, of the Planetary Science Institute, about the second planet from the Sun: Venus, a world with surface temperatures hot enough to melt lead. In the first part of their discussion, they talk about Venus’ volcanoes, its clouds of sulfuric acid and its runaway greenhouse effect. Was Venus once like Earth and what clues might it provide about the future of our own planet? They also explore Venus’ backward rotation and its amazing ‘forever sunsets’.

Here’s a short teaser of this week’s podcast.

You can hear the full podcast here,  or read the transcript below.

Planetary scientist and astrobiologist David Grinspoon. Image: Gerald Shields.

Jim Green:  Our Solar System is a wondrous place, with a single star – our Sun – and everything that orbits around it: planets, moons, asteroids and comets. What do we know about this beautiful Solar System that we call home? It’s part of an even larger cosmos, with billions of other solar systems.

David, you did some early work on Venus for your PhD. What was that like?  And, what did you do?

David Grinspoon:  Well, when I was in grad school in the 1980s, there was this new idea about large impact events affecting planets. We had recently discovered this amazing fact that an event 65 million years ago had knocked out the dinosaurs and caused an extinction.

So, people were starting to wonder, “What else have large impacts done in the Solar System?” I had a couple of mentors who suggested that I look at some other atmospheres and I started to be drawn towards the fact that Venus is so similar to Earth, and yet so different, and wondering what large impacts did to Venus.

That led me to work on things like, what happens when comets hit Venus and what does that do to the amount of water in such a dry planet? What was the early climate of Venus like compared to Earth, under the influence of being pummeled by a lot of impacts?

It’s basically how I learned to do climate modeling, by considering these weird scenarios of early atmospheres and what might have been happening to them under the influence of violent impacts.

Giant impacts could have had different effects on Venus and Earth. They may have forced Venus to slowly rotate backwards, while the last giant impact on Earth formed the Moon. Image: NASA/JPL–Caltech.

Jim Green:  Even though Venus is about the size of the Earth, it’s very different in many ways.  What do you think is happening on Venus that makes it so different?

David Grinspoon:  Well, that’s one of the very compelling questions about Venus, because it’s so Earth-like in terms of its bulk properties. If you just ask the most basic questions, “What’s the size of it?  What’s the mass of it?  Where is it located in the Solar System?”, you’d say, “Wow, it’s just like Earth in all these ways.”

Then you’d start looking at, [and asking] what’s the environment like? And you’d go, “Whoa, that’s not at all like Earth,” because it’s so incredibly hot and so incredibly dry. So, it’s evolved in a different direction.

When you start talking about Venus today and comparing it to Earth, you’re led to these questions of not just what’s happening there now, but how has it evolved and how did those two planets head down such different paths? Obviously, for Venus, it’s a story of somehow having lost its oceans and lost what we think of as a more pleasant climate a long time ago, and trying to understand that sort of divergence of conditions away from what we believe was more Earth-like a long time ago.

Jim Green:  So, how does Venus help us understand what Earth’s climate is like?

David Grinspoon:  Well, it serves as an interesting laboratory for us to test a lot of our ideas about climate and atmospheric processes on a planet that, again, is somewhat Earth-like in some ways, and somehow very, very different in other ways. But, you know, with climate and the environment, a lot of it has the same physics and chemistry in different situations. Everybody knows about the greenhouse effect on Earth and how carbon dioxide is part of that. Well, picture a planet just like Earth, where the atmosphere is almost all carbon dioxide basically. What would that do to the climate? It’s an interesting thought experiment, but it’s also a real experiment because we have this planet next door, Venus, which is basically all carbon dioxide.

Many other aspects of Earth are exaggerated on Venus, too. You’ve heard about acid rain. The clouds on Venus are sulfuric acid, so it’s an extreme case of acid rain. That’s allowed us to study, again, an Earth environmental issue in an exaggerated form.

This is just something that kind of makes us smarter about the problems and the puzzles that we encounter on Earth, by seeing them in an altered, and sometimes exaggerated form, on a nearby planet.

Jim Green:  Venus isn’t actually that very far away from us, nor is it very far away from the Sun in the sense that our two planets have evolved, at the moment, very differently. What was Venus like early on in its evolution, do you think?

David Grinspoon: Well, that’s a great question and, of course, that’s one of the mysteries that compels us in our research and exploration to try to get a clearer picture of the earliest history of Venus. We have a lot of circumstantial evidence that leads us to believe it was a much more Earth-like planet a long time ago.

For instance, there’s the fact that Venus is so incredibly dry today. In fact, if you add up the amount of water on Venus and compare it to what we think is the amount of water on Earth – I say it like that because we don’t actually know how much water is hidden inside the Earth – but, it’s something like 100,000 times as much water on Earth than on Venus, which is really strange, because we picture them probably having formed with roughly the same amount, because they formed nearby out of similar materials. So, we think Venus had oceans and we think it had a more pleasant climate, possibly even for life, early on, and we’ve got some other circumstantial evidence about that.

So, my best guess is that Venus was much more Earth-like early on. I would say an informed guess, but there’s still a lot of mystery there and a lot of experiments [that] we’d like to do and measurements we’d like to make to try to pin down that early history much more clearly.

Venus, captured in 1990 by NASA’s Galileo spacecraft while it received a gravity assist from Venus to send it on towards Jupiter. The blue hue is added false color to emphasize the subtleties in the sulfuric acid clouds. Image: NASA/JPL.

Jim Green: One of the things that’s fascinated me about Venus is that it rotates in the opposite direction, and it rotates very slowly.  How has that affected its evolution, do you think?

David Grinspoon: Yeah, it’s an interesting fact that almost all the planets rotate in the same direction as Earth does. So, if you’re standing on their surfaces, the Sun rises in the east and sets in the west, like we’re used to.

If you were on the surface of Venus, assuming you could see the Sun, which would be hard because it’s so cloudy there, [then] the Sun would actually rise in the west and set in the east. And it would do so very, very slowly, because the planet rotates incredibly slowly. [Editor’s note: a ‘solar day’ on Venus lasts 116 Earth days.]

So, in fact, if you’re on Venus, you could walk fast enough to keep the sunset in the same place. You could walk as fast as the Sun is moving around the planet. I did that calculation once and I was like, “Wow, well that would be kind of neat. You could watch the sunset forever just by walking.”

How that fits into Venus’ evolution is a fascinating question. We don’t fully understand the cause of that. We surmise that it’s related, both to the early impact history of Venus, just as Earth’s rotation and Earth’s moon are related to the early impact history of the Earth and setting the Earth spinning in a certain way.

You know, the planets formed by these big collisions and the final few were probably very violent. So the geometry of those final few collisions, which way they hit, probably really influenced that spin. But, on Venus, there’s also the fact that we have this incredibly thick atmosphere, almost 100 times as thick as Earth’s, and that can cause a drag on the rotation of the planet through what we call tides, atmospheric and solar tides, which are just these phenomena of the mass of the atmosphere itself actually pulling on the planet’s rotation over a long period of time. So, that might have to do with how slowly it’s rotating.

We’re not sure about its total evolution of the rotation rate over time. As far as rotating in a backwards direction, if you will, we think that probably has to do with large impacts early on in its history, when it was still forming.

Jim Green: That’s fascinating. Maybe that means that over time, Venus will end up being tidally locked with the Sun. And then, we’re going to have a completely different environment, perhaps, with the extremes in temperature on both sides.

David Grinspoon: Yeah, that’s really an intriguing possibility that Venus, moving so slowly, could be on its way to being tidally-locked, and that at some point, probably in the pretty distant future – I don’t think we have to start revising our models too quickly – but, it could actually become a locked planet, like we think a lot of exoplanets are, where one side is permanently facing the star.

A computer-generated view of the surface of Venus, based on Magellan radar data and showing two volcanoes, 3km-tall Gula Mons on the right and 2km-high Sif Mons on the left. Image: NASA/JPL.

Jim Green: As you mentioned, Venus has this huge pressure and enormous clouds that are very opaque. But we have been able to penetrate through those. One mission was called Magellan. What was the most important set of observations that came from the Magellan spacecraft?

David Grinspoon: Magellan was an amazing mission. It really revolutionized our understanding of this neighboring planet. Before then, we had a couple of pictures of a couple of spots on the surface from Russian landers, which amazingly were able to operate under those extreme surface conditions briefly in the 1970s and the 1980s. [Editor’s note: Magellan orbited Venus between 1990 and 1994.]

We also had a couple of vague images from radar of parts of the planet, but with Magellan, we were able to orbit and basically map almost the entire surface by using radar, which as you say, penetrates through those clouds.  It does [a] sort of flash photography almost, bouncing radar off the surface, and then you see that image and you build up what the planet looks like.

That revolutionized our view of Venus in so many ways. One thing we learned was how volcanically interesting Venus is. Its surface is almost completely covered, in one way or another, with volcanic features, these broad, flat plains that we think of as flood basalts, like we have some areas on Earth. The Pacific Northwest comes to mind as one of these big flood basalt areas.

Then [there are] other kinds of volcanoes, these steep shield volcanoes, like Hawaiian style shield volcanoes. So I think of Venus almost as [a] volcano world. It immediately sharpened the question, is it still volcanically active?

And, ever since Magellan, we’ve been trying to nail that down. Again, we think we have some clues about that, but we don’t have what we sometimes refer to as the smoking gun, telling us it’s definitely volcanically active. Now that we know from Magellan there’s volcanoes all over the place, that’s sort of the next question. Are they still going?

Jim Green: Right. I mean, is the carbon dioxide in the atmosphere believed to be because of the volcanoes?

David Grinspoon: Well, over the long term, yes. I mean, that’s the main way CO2 gets supplied to planetary atmospheres, but we don’t really know if Venus requires an active supply now, because there may be nothing removing CO2 from the atmosphere.

On Earth we have this cycle where CO2 is supplied by volcanoes and, well, now by factories and cars, too, but, historically, by volcanoes. Then it’s removed from the atmosphere by what we call weathering reactions, that are facilitated by water running over rocks and pulling CO2 out and making carbonate rocks.

Venus has no surface water, so it may not really have any way of removing CO2 from the atmosphere, but we do see other things in the atmosphere that may require an ongoing volcanic source. We see all these sulfur gases, SO2, sulfuric acid, and it may be that chemical reactions are always removing those [by] reacting with surface minerals. So, in fact, the sulfur gases we see on Venus today may require an ongoing source of volcanic gases.

You can find the Part 2 of their discussion here.