We're meeting our celestial neighbours, starting with Venus. Eva Higginbotham gives us the low-down on our neighbours, and University College London's Andrew Coates tells us about the Venus missions he's been involved with. Finally, David Rothery, planetary geoscientist from the Open University tells us about the conundrum of Venus' atmosphere...
Eva - Venus is the first of our neighbours we’ll be meeting. Named for the roman goddess of love, Venus is anything but a lovely place to be. Venus is roughly the same size as Earth, and like Earth is a rocky planet. But that’s about where the similarities end. On the Venusian surface, the temperature is hot enough to melt lead, reaching near 500 degrees C. There’s snow on the mountains of Venus, but that snow is made of metal. Venus is stuck in a runaway greenhouse effect, where the atmosphere lets heat in, but doesn’t let heat out. That means that despite not being as close to the Sun as Mercury, it is still the hottest planet in the solar system. If you were to stand on the surface of Venus, and you could somehow withstand the heat, you wouldn’t stand the pressure. The pressure at ground level is roughly 90 times of the pressure on Earth, like being 1km underwater. All that strangeness is enough for scientists to want to go and have a look, although the extreme conditions mean that most missions have not landed on the surface. But more than that, Venus might hold some information about what might happen to Earth, if climate change is left completely unchecked.
Chris - So beautiful, but deadly, and maybe not the nicest place in our solar system. Now we know a lot of this, of course, because missions like the Venus Express probe, which operated between 2005 and 2014, have been dispatched there to study the planet. So what have they discovered, and what new questions have they raised about our celestial near neighbor, where it also - as if molten metal snow wasn't floating your boat sufficiently, rains sulfuric acid.
Adam - If Venus is such a hostile place, how do you go about visiting it? How do you peer through all that thick, heavy atmosphere and actually look at the surface? Well, a few missions have been to Venus and professor Andrew Coates from University College London, who has worked on a few of them, took me through some of those missions.
Andrew - I worked on a mission to Venus a few years ago, which was Venus Express. So this was the European Space Agency's mission to unveil some of the secrets of Venus, which had been left from previous missions. Russian missions had been able to land on the surface, probes to go on down into the atmosphere. Also an American mission mapped the surface in great detail with a radar. That was the Magellan mission. So Venus Express actually was done to try to look down towards the surface and to the lower atmosphere in particular, using windows in the infrared, which had been discovered when Galileo actually, the spacecraft, on its way to Jupiter, when that past Venus on one of its gravity assists. If you approach Venus on a spacecraft, you see these clouds, V-shaped or Chevron shaped clouds really, associated with the planet rotating very slowly, but the atmosphere rotating very quickly. There's something called a super-rotation going on in the atmosphere. So that was another thing which Venus Express was to look at. So Venus Express, European Space Agency mission, the instrument we worked on was to look at the solar wind interaction.
Adam - And what kind of things did the Venus Express and learn about the Venusian atmosphere?
Andrew - Well, we knew already that Venus has a very thick atmosphere, so there's a mainly carbon dioxide atmosphere. One reason we wonder about why the atmosphere is so thick. It's a bit of a conundrum because actually like Mars, Venus lacks a magnetic field. And yet you've got this very thick atmosphere. Despite the fact that solar wind is stripping away the top of the atmosphere all the time. So one of the things we're able to look at is how much, you know, the rate at which material is being lost from the top of the atmosphere of Venus. And it turns out to be about the same as Mars, you know, per unit area. So you have the same process going on in terms of losing material. You have this carbon dioxide rich atmosphere. So one of the conundrums of Venus is how come the atmosphere is so thick now, how is this being replenished? Because over billions of years, you might expect that atmosphere to basically disappear. So one of the possibilities with Venus is, there's active volcanism going on. Nobody has seen that, or nobody had seen it before Venus Express because that window in the infrared hadn't been used to be able to look down towards the Venus surface.
Chris - It's intriguing to think what we know and what we've learned. Isn't it? That was Andrew Coates. He'll be back a bit later on with more insights. So it does appear to be quite a mystery. Doesn't it? How does Venus work then? Well, with us is David Rothery. He's professor of planetary geoscience at the Open University. David, we seem to have some kind of contradiction here. Venus has got this crushing atmosphere, while Mars has the vague vestiges of one. They're roughly similar sort of sizes. Why the difference do we think?
David - Well, the most important contrast is really between Venus and the Earth because they really are the same size and mass. Mars is considerably smaller. What's happened on Venus, with such a dense atmosphere is that all the limestone that's on the Earth as rock with carbon dioxide locked up in it, doesn't exist on Venus. That carbon dioxide is all in the atmosphere. So if you add the carbon on Earth and the carbon on Venus up, they're pretty much balanced, but it's all in the atmosphere on Venus. So it's got this atmosphere 90 times the Earth's surface pressure, made of carbon dioxide, and that's what gives it the greenhouse effect, which currently keeps it so hot. Mars is a much smaller body and it's lost most of its original gases.
Chris - Andrew Coates was saying that the atmosphere's being stripped away from Venus, by the solar wind rather quickly. He cited the reason as, it doesn't have a magnetic field. But where's the CO2 coming from then to replace that, which is being blown away continuously?
David - Well, I think Andrew's right. That Venus is losing atmosphere. I'm not sure we've got the good measure of it's rate. It doesn't have a magnetic field, which is a surprise. It should have a core like the Earth, but maybe its slow rotation is not stirring the core up to generate a magnetic field. So it's not protected from the solar wind, but the stripping away has to compete against the planet's gravity and CO2 is quite a heavy molecule and there's plenty of it there. So I don't think Venus is going to lose this atmosphere quickly. It's in that state for a long time and has been like that as far back in time as we can peer, which is not all that far, because Venus' surface is geologically active. It doesn't have a long cratering record because it's been resurfaced by lava so many times.
Chris - Allegedly it has the most volcanoes in the solar system.
David - Well, we don't know how many are active today. We don't know that any are active today. If you want somewhere that's volcanically active, you go to Jupiter's moon Io, which has got 30 or more erupting at any one time. Pretty much the same numbers as on the earth, but it's much smaller. We don't know if Venus is erupting today. It probably is. There have been hints, localised traces of sulphur oxide, occasional glimpses of a little bit of localised heat through the clouds. But we don't know. That's why we need a mission dedicated to Venus. The current probe, Akatsuki, that's orbiting Venus, doesn't see down to the surface. When BepiColombo, the European Mercury bound spacecraft has a fly-by of Venus in October, it won't see to the surface, but it will get some atmospheric measurements as it whizzes by.