Weather on Other Planets

16 May 2013

Interview with

John Zarnecki, Open University

The weather on Earth may sometimes seem pretty extreme to us, but across the Solar System, even wilder weather phenomena are raging. The famous red spot on Jupiter for example is a hurricane. It's larger than Earth and it's been blowing for hundreds of years. To find out what we can learn from the weather systems on other planets, Dominic spoke to Professor John Zarnecki from the Open University. He started by asking, where else in the Solar System we might find weather that we would recognise here on Earth.

John - Well, of course we're talking about places that have atmospheres. So, we're generally talking about Mars, Venus, and actually, my favourite place which is Titan, the largest moon of Saturn, which also has an atmosphere. And we see some aspects which are quite familiar - We see clouds, we see winds, we see temperature variations between night and day, we see pressure systems. So, in some respects, we see things that are familiar, but actually, if you look a bit more closely, then things start looking a bit different. I mean, for example, the temperatures are much more extreme and then if you take Mars as an example, we have incredible dust storms, even sometimes dust storms covering the whole of Mars. So, it's really a mixture of the familiar and the rather unfamiliar.

Dominic - So, is that just because these planets are at different distances from the sun and we see different amount of heating or does the chemistry of their atmospheres really affect the kind of places they are?

John - It's a combination of quite a few factors really. Certainly, the distance from the sun has a big influence because as we know, it's the radiation, the energy from the sun that drives much of weather. So, we've got Venus, which is closer to the sun and then Titan at the other extreme which is about 10 times further from the sun than us. So, that's one factor that there are other things, the pressure of the atmosphere, the amount of atmosphere is very important. For example, Venus, very roughly has a surface pressure about 100 times greater than us here on Earth. Mars on the other hand is about a factor of 100 times less than our surface pressure. The composition of the atmosphere comes into play, but perhaps less so than those other factors. And again, going back to the dust on Mars, that plays a big part because dust, you might think it's a bit inconsequential, but they're very efficient at absorbing sunlight then that is reradiated into the atmosphere. So, this is a factor of various differences which collectively make up these differences that we see.

Dominic - I guess the big surprise for me is that we're talking about Titan because we think of Venus and Mars as being quite Earth-like because they're relatively close to us in the Solar System and they're being quite well-studied. How does Titan manage to be similar to the Earth despite being 10 times more distant from the sun?

John - Yes, it is fascinating and Titan is basically an icy moon. It's mostly, at least the surface and the shell is made of ice; and as you say, it's a lot further from us, a lot further from the sun than we are. And yet, strangely there are remarkable similarities and parallels, and that I think has surprised us. We weren't quite expecting that when we started studying Titan in great detail in the last 10 years or so. In particular, there is a material, liquid methane which plays the same sorts of role on Titan as water does here on Earth. It can exist as both a gas, a liquid, and a solid. And as a result, there is the equivalent of the water cycle on Titan. So methane sits in seas on the surface. It evaporates into the atmosphere, it forms clouds and then it falls as rain onto the surface. So, in that respect, we see systems on Titan that seem remarkably familiar. It's just that the materials are different, but the physical principles are essentially the same.

Dominic - Now obviously, we got very detailed measurements of the weather that we have on Earth. We've got weather stations. We've satellite images and radar. How does that compare with how much we know about the weather on other planets?

John - Well, that of course is an excellent point. We have compared with Earth of quite sparse data. Having said that, Mars is now moderately well-studied. We've had quite a few space craft visited which have taken observations over long time spans and that is what you need of course to better understand weather. So, we're beginning to build up a picture of the variability, particularly of Mars and in fact for Mars, there are in existence, what we called Global Circulation models. So, these are computer models which try to explain and predict how weather will vary across the whole planet. So in that respect, Mars is similar to Earth in at least in our depth and range of our knowledge. For Venus and especially Titan, the data is much more sparse just I suppose because they're more difficult to observe. Venus of course is a terrible place to try and make measurements from the surface. It's horrifically hot and the surface pressure as I said is about 100 times more than on Earth, so it's terribly difficult to put down a lander and have it survive for more than a few minutes. But we're gradually building up these models and of course, why it's important apart from intrinsic scientific interest, it does allow us to look at weather and physical processes which occurring on the Earth, but we can observe them in a totally different environment, and that certainly helps us to make better models of weather here on our own Earth.

Dominic - That's an interesting point of course because there are always people who will say, given the problem we've got on Earth, worries about climate change and so forth, is it worth investing in going to understand other planets when we could be worrying about our own planet. So, what you're saying is that, looking at weather elsewhere in the Solar System can help us understand where the Earth came from and where it's going.

John - Absolutely so. Now, I wouldn't say that the entire business of space exploration is justified purely for that reason, but it is one of the bonuses that we get from it. And of course, the greenhouse effect for example, which as we know is very important for us here on Earth. It's not something that is unique to the Earth. It occurs on other planets certainly in an extreme case on Venus. We have an extreme greenhouse effect there which is partly responsible for the very high surface temperature and even on Titan - cold and distant Titan. There is also a small greenhouse effect in operation. So, these are the sort of things that we study, we hopefully understand better, and who knows, in the long term, this might be quite significant in the way we better understand climate change and maybe respond to it also.

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