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Dynamic climates: what can other planets tell us about the Earth?

Stephen Lewis

The Open University opened its doors to welcome visitors to Stephen Lewis’s inaugural lecture on 9 November to mark the first 2021-22 lecture.

Stephen Lewis, Professor of Atmospheric Physics, delivered his inaugural lecture on the atmospheres, weather and climate of other planets in the Solar System.

The topic was apt given the COP26 gathering and as it focused on what weather and climate mean on these different planets. Professor Lewis pointed out that the atmospheres of our neighbouring planets are fascinating physical systems to study and test our ideas. Our ability to forecast conditions is now vital for safe exploration by spacecraft. He asked if the knowledge gained about other worlds helps us to understand our own climate better.

Watch the video of Professor Lewis’s inaugural lecture:

Nick: Good evening everybody. Thank you for joining us at this first in the academic year’s Inaugural Lecture series. I'm Nick Braithwaite. I'm the Executive Dean of the Faculty of Science, Technology, Engineering and Mathematics. I'm proud and privileged to be hosting one of our inaugural lectures here. The series showcases our research, teaching and knowledge exchange portfolios. I'm also delighted that we can deliver this one here on campus in a COVID compliant way, of course. It's been so long since we've been in this room doing this sort of thing, it’s very exciting. Now each year the Vice-Chancellor invites some of the recently appointed professors to give an inaugural lecture. Over the course of a year our series provides an opportunity to celebrate academic excellence, with each lecture representing a significant milestone in an academic’s career. But before we get a long way into that let me just do the housekeeping bit. The lecture will be followed by a Q&A session. Then we'll invite you to celebrate, if you're in the room here, celebrate with us downstairs and we ask that you follow the exit signs and go that way to leave the theatre. During the presentation here for those in the room, for all of those more remote it will be easier for you to do it I guess, if you want to use Twitter to increase the exposure that we get that would be appreciated. There's a hashtag displayed up there and we also like you to tag it with @OpenUniversity and that lets the world join in. For colleagues who are watching via the Live Stream for your questions please email them. If you keep it succinct it will help because they're going to be relayed to me by somebody reading the email out and then I will relay it to Stephen. The purpose of that is to give him some thinking time but not too much.

So let's talk a little bit about my esteemed colleague, Stephen Lewis, Professor of Atmospheric Physics at The Open University. He's currently Head of the School of Physical Sciences and he researches into the dynamics of planetary atmospheres. Now I want to go back a little bit on this and say that what he's doing is really important in understanding the weather on other planets, and he's going to challenge us with why that might be important for understanding our own weather and climate better. I know first-hand that he does understand the weather on this planet as we worked together with a colleague Shelagh Ross on a course that was essentially of that name, Understanding the Weather. Stephen joined us and it was shortly after he joined us that I lured him into that because I found out he was a meteorologist at heart. He joined us in 2005 as a research fellow, and by 2009 he had advanced to senior research fellow. In 2010 he transferred over to being a senior lecturer, and by 2017 professor, Professor of Atmospheric Physics. Now his understanding includes the dynamics of climate systems, that's why it's part of the title and forecasting the weather for spacecraft missions and interpreting the atmospheric observations that they return is really important. In fact, it is important to space exploration as the early forecasts were on Earth for exploration of all types, by land, by sea and particularly by air. He's won awards for his work on spacecraft teams, including NASA Mars Reconnaissance Orbiter missions, and the Curiosity and Perseverance rovers. Back on Earth it's important to note that he is a fellow of the Royal Meteorological Society, and in that course that we produced together he was very effective in linking us up with that organisation. He's been the academic consultant, as you would expect, on a number of BBC series, Wild Weather that would be one of his and A Perfect Planet. That's a perfect point for me to pause and introduce tonight's speaker, Professor Stephen Lewis.

Stephen: Thank you Nick and good evening everyone. So I was asked to give an Inaugural and I'd already given a talk recently about the planets. So I thought what could I do that was a little bit different and I'd really like to theme this around what can other planets tell us about the Earth? This is, of course, a view of the Earth's atmosphere but it's a view from space. It's actually been taken by an astronaut on the International Space Station and it's a view of sunset, and you can see the various layers of the atmosphere. The weather that we're accustomed to is all in the lowest part, below the bright line where you can the clouds and then the middle and upper atmosphere above the stratosphere and mesosphere.

So that's the Earth. What about other planets in the Solar System anyway? Well, almost all planets have an atmosphere, it's arguable whether Mercury does, but all the other planets have a significant atmosphere and these are our 2 neighbours, Venus, Earth and Mars from left to right. You can see that each of them are dominated by atmospheres to different degrees. Venus completely covered with clouds. Earth, a mixture of clouds and some land, and Mars only very thin cirrus clouds. These planets have suffered different fates. But really the differences in surface conditions on each planet are much more due to the atmosphere than to anything else. Venus, well known for a runaway greenhouse effect, and temperatures hot enough to melt lead. Mars, essentially a frozen desert. They’re certainly not the only atmospheres. In fact, not most of the atmosphere. This is an image of Neptune. It's an image I'm rather fond of actually because it was actually the last planetary picture, just about, taken by the Voyager 2 spacecraft as it flew out of the solar system back in the late 80s, I think 1989, it was the late 80s. I was actually in California at the time this image was taken, it's the first time I was involved in a NASA event in which a spacecraft flew past the planet. So the last of Voyager from the grand tour of the 1970s and the first space mission I was involved in. Beautiful blue planet with some streaks of white cloud just visible above, and the great dark spot which is no longer there in the centre of the image.

Moving on to another planet. This is Jupiter, when it comes down to it, it's the biggest atmosphere of them all. It is basically all atmosphere down to a level at which the physics becomes rather strange and we enter a metallic hydrogen dominated region. This is a view of Jupiter we didn't have in my day anyway when I was first a research student. This is an image from the NASA Juno spacecraft which is operating now and it's actually a view down over Jupiter's North Pole. So until recently we'd never seen the poles of Jupiter because of course from a telescope we tend to see the equator rather well and the poles are very foreshortened, and the same from the Voyager spacecraft, it simply flew by in the plane of the ecliptic, the plane where all the planets orbit the sun. But you can see that Jupiter's atmosphere is just a fantastic fluid dynamics playground. If you're into fluid dynamics, this is the place to go. It's full of whirls and vortices, very active systems, all the time thrashing around. In fact the particular vortex I started my PhD on is this one, this is a more recent image, but it's a nice image of Jupiter's famous Great Red Spot. This is a vast vortex. You could fit at least 2 Earths within it. So that gives you an idea of the scale we're talking about. The vortex rotates every few days, the gas whirls all around the outside and the other kind of interesting thing about it is that it's been there for a very long time. It's changed in size and colour a little bit. It's a little controversial who first saw it, but there's certainly a record of Robert Hooke seeing it in 1664, because he wrote it up in the for the Royal Society. So we know that he saw it, it's likely although it's not clear that Cassini, the Italian astronomer who was operating in Paris at the time, also saw it and it's a little unclear who saw it first, but that doesn't really matter. The point is they both saw it and the reason why it was first seen at that time was not because it formed then probably but because telescopes were just being invented that we're good enough to see it with. So this is a fantastic feature. It's often described as a hurricane, or you’ll see it said it’s to be like a hurricane, it is nothing whatsoever like a hurricane actually. It's completely the opposite. It's actually a high pressure system, an anti-cyclone. It's in the southern hemisphere which is why it's rotating the way it is. I did my research on the dynamics of this feature and it's does have an analogue in Earth and the Earth analogue is actually a blocking high, a very stable high pressure weather system and famously, the winter of 1963 was a particularly bad winter. A good year otherwise I might point out, but it was a bad winter, and the reason why was because a blocking high like this was sitting over the country and just holding that cold weather and it lasted for about a month, which is a long time for Earth weather. Well this feature’s clearly been here for 400 years or getting on for that plus, we don't know when it formed. So an amazingly long lift feature.

Over to a much smaller planet now, but to show another atmosphere in action. This is Mars, this is the North Pole of Mars, I'm sticking to polar pictures for now. So this is a beautiful view of the North Pole of Mars. The ice cap you can see on the right hand side of the image is essentially a water-ice cap. People will often think of Mars as this dry dusty desert which is true to a large extent but look at the active water cycle going on here. There's water-ice on the surface in the polar cap, more towards the centre of the image this huge bank of cloud here. This is stratocumulus cloud, if you’ve done Understanding the Weather you might recognise that. It's quite unusual to see so much stratocumulus cloud on Mars but this is stratocumulus cloud. Then further down over to the left we're going towards the equator and there's fogs actually and thin cirrus cloud. So these are all water-ice clouds. So there's an active water cycle going on even on repeatedly the driest of dry planets. So plenty of atmospheric excitement going on there as well. I'll produce some examples from Mars later in the talk.

So as Nick mentioned, I think actually one of the most interesting or most different parts of my job at the OU has always been to work as an academic consultant on various co-productions with the BBC and a natural one that happened recently in 2019 was The Planets with Brian Cox. I was involved in a lot of the science of this, you may have the poster. So it was natural that I was involved in that. But more recently, actually over a long period of time, but the programme was finally shown this year, A Perfect Planet with David Attenborough which you may notice doesn't involve a lot of other planets. It involves a lot of animals and the Earth and yet I worked on this series. I can remember vividly, I was sitting at home working through some scripts or looking at some images on my computer and with the respect and the deference that my children have always given me, one of my daughters came in and said, ‘Well what do you know about that?’ so that’s the motive for this talk really. What do I know about this? Why do they put a planetary scientist on a programme about the Earth? Well, here's the Earth's atmosphere. Space is nearer than you think I always like to say. You would get a view not unlike this if you travelled about the same distance as you would travel to London from here, upwards. It's not very far away at all. This is obviously a view from the International Space Station so the actual photograph has been taken from higher up, but you can see a Soyuz capsule approaching in the lower part of the image. But all the weather you can see going on down here, it's all within a few kilometres of the surface, the tops of these clouds are all well below 10 kilometres. So a rather sobering thought perhaps is that all the life that we know about in the entire universe exists within a layer of a few kilometres of the surface of this planet at the moment. We're really living in the thinnest of shells around the Earth. If the earth was an apple it would be thinner than the skin of an apple. That's the only place where we know life exists. So it's quite a precarious position to be in perhaps in some ways.

That brings me on to how do we study the Earth's atmosphere? Well what a scientist would normally like to do like me, if you want to study a fluid system well you go into the laboratory and you spin it faster, or you heat it up a bit more, or you do something to it, and change the parameters and see what happens. Probably a bad idea if you've only got one atmosphere to live in. So I'd like to talk a little bit about how we study the Earth and why other planets might help with that study.

Well two ways we could do experiments in atmospheric science is we have to look at different systems that we can do experiments with without affecting the real thing, hopefully. So just two examples I've worked on here in sketch form anyway. On the left a rotating tank experiment. You probably don't get an idea of the scale. This is actually in the EU facility in Grenoble. So I've done some experiments on and this is a 14m rotating turntable several meters deep. If I can just point to, you might see there’s some computer screens up here. You can walk along this plank here and several people could sit on this plank when it's operating. It operates in the dark because the only way we can find out what the fluid is doing is by shooting sheets of lasers at different heights in the tank. This is an example of one in the lower left and then we use essentially tether video tracking technology to track the motions. The reason why it's still valuable to do experiments with actual real fluids is that we can access a range of scales that you just can't access in a computer simulation at the moment even. We have a tank that's 14m across, but fluids move on scales of millimetres so we can access that whole range of scales and range of motions. The other side of the slide is actually what I'm going to talk about much more tonight, because it's more commonly done for the planets I'm going to talk about, is a giant numerical model. Now the way we model an atmosphere is that we effectively split it up into lots of little boxes, at least this is conceptually what happens. It may not actually happen according to the precise technique used. But effectively, we divide the atmosphere up into a whole bunch of boxes, both in the horizontal and in the vertical stacks of boxes in the vertical. Then we solve the various equations we want to solve for each box. So the more boxes you can have, if you like, the more resolution you've got. It's a bit like having an HD TV rather than the old fashioned conventional TV, we can go to a higher resolution with a bigger computer and we can make something that hopefully looks more like an atmosphere. The trouble is atmospheres are rather complicated and I've got to put a lot of different equations into each box. Here's just a few of them really. There will be a test on the equations on the top right at the end before you get your glass of wine. Effectively though it's not that, I'm joking slightly, but I don't really expect you to follow all the bits. But the point is that there's lots of different science. In fact, these might be rather better called kitchen sink models. They’re actually called global circulation models but we effectively have to throw in everything that we think is important and it turns out that a lot of things are important. So the first 3 items are the equations of fluid dynamics. This is just Newton's second law written in a slightly unfamiliar form because you're in a rotating frame of reference on a planet. That's why there's some extra terms there and there's gravity to worry about and there's other things going on. But it's Newton's second law, its acceleration equals force divided by mass. We then have some good old classical physics really, continuity and equation of state. So continuity basically says mass is conserved. The equation of state we're describing a gas, we could equally well put in a different equation of state if we were describing a liquid, an ocean. Then we've got the law of thermodynamics, effectively conservation of energy. There's a little term here that looks rather innocent which is the Q on the right hand side of this. Whenever a physicist writes a scripty letter, that often means there’s something quite dark and deep buried there. In this case it's basically everything else I'm going to talk about and it's about 90% of the computer time involved in solving this equation. So in the boxes what goes on to calculate what Q is. So for example, we need to know about radiative transfer at different wavelengths, we need to understand about sunlight coming in, infrared radiation going out, the scattering of both, the absorption of both. There’s cloud and ice physics. You've seen some clouds and ice, we need to know about how things condense, sublime, precipitate. We need to worry about the surface. So we need to talk to some geologists and geomorphologists because we need to know the shape of the surface and we need to know the properties of the surface, and how the heat is transferred from the surface to the atmosphere. If we were on the Earth we’d need to know about the ocean as well. Then there’s aerosol physics, hugely important particles from volcanoes or from dust storms. Then atmospheric photochemistry. So chemistry turns out to be a huge time hog on these models, because instead of just tracking a gas, we track every species that we think is important, and then we make them all react in the boxes. So we have to talk to chemists and then sometimes if you're working on the Earth you talk to biologists, and sometimes even some people will even put economics models in, all sorts of things. So the point is with my speciality if you like are the top 3 lines but I need to talk to lots of other people. It's a huge, huge collaborative effort. Virtually the whole of STEM, Science and some people from outside STEM are involved in formulating parts of these models. So it's an interesting exercise.

Next I thought I’d talk a little bit about weather and climate because there's always some confusion between the 2 and there's a rather infamous quote “Climate is what we expect. Weather is what we get.” which to some extent tells you something about what's going on. Often attributed to Mark Twain, although that apparently is not correct and then it's since been attributed to Robert Heinlein but in the 1950s, but I don't think that's right either because there's records that it came from a book of American student answers to exams or something from the early 20th century. So we don't know really where it comes from but it perhaps sums up the point. So individual weather events are how the atmosphere changes from day to day. Why is today different from yesterday and why will tomorrow be different again? That's weather if you like. Actually we can predict these with large numerical models of the type I’ve described. Surprisingly our skill is actually getting better. You may not believe it, but our weather forecasts are about a day better every decade as a rule of thumb. So what that means is that a weather forecast now for 5 days ahead is as accurate or as skilful as a weather forecast was 3 days ahead 20 years ago. There’s a combination of reasons, partly the models and partly the fact that we've got better observations, we've got more satellites and so we know more about what the atmosphere is like now. Climate on the other hand is a sort of average, you can think of it as an average, perhaps on a decadal timescale, formerly on a 30 year timescale, although often the 10 year type climates are published. It's also about the pattern of variations about that average. So if we're thinking 10 or 30 years ahead, the question I'm sometimes asked is, ‘Well you can only predict the weather a week ahead, how do you know anything about climate 10 or 20 years ahead?’ The answer actually stems from something that one of the members of my family was fond of saying when I was younger which is ‘You never meet a poor bookie.’ and that's essentially the answer here. This is a rather simpleminded analogy, but you can think of it this way. I believe that there are games on which people wager money which involves throwing 2 dice. It's something to do with the score you get. So if I threw 2 dice and you asked me what will the score be next time I throw them? That's an incredibly difficult problem actually. On the other hand if you asked me what would I expect the score to be I can solve that rather easily in my head. So to give you an example. We can think of predicting what's going to happen next time as a weather forecast. It will actually be very hard, but there's nothing fundamental that would stop you from doing this. You just need to know exactly where the dice were at the start of the throw. Exactly what orientation they're in. Exactly what velocity they were moving with and you'd need to know all about the material properties of the dice, the material properties of the surface they we're going to hit, the air, and you could throw a massive supercomputer calculation at this. In principle you could try to work out what those dice were going to land on. I have to say I think it would be very, very difficult, it would require vast resources and it would also be very subject to tiny uncertainties in the initial conditions of the dice because they're going to bounce and collide and a lot of things will happen. So you can think of that a bit like an analogy for weather forecasts, it’s probably even harder. This is really quite a hard problem, you'd have to throw a lot of physics at it. On the other hand, if you asked me what I would expect the score to be, well assuming the dice aren't biased of course, I know that there's 36 possible answers and 6 of them are going to be 7. So a 1/6th of the time I expect to get 7 as the total. That's the climate. Equally, there's a distribution and I'm progressively less likely to get numbers that are different from 7 until I get to 2 or 12 which are both going to happen one time in 36. So I can actually very easily describe the climate. You might even think that you could push this analogy a bit further, I don't want to push it too far, that if we started to bias the dice a little bit that might be climate change.

So why do we get weather anyway? What's going on? This is a simplified picture of the Earth, but it could be any planet actually. A beam of sunlight hitting somewhere near the equator falls on less surface area than it would do if it hits at higher latitudes. Effectively what happens is that the tropics on average day and night over the year absorb 300Wm² from the sunlight, but they actually only emit 250Wm². In contrast the poles absorb 50 Wm² but omit 150 Wm², so what's going on? Well clearly the energy isn't being created or destroyed. What's happening is that the atmosphere and the oceans to some extent on Earth is transporting the heat from the equator to the poles. In classical thermodynamics, a lot of which came about in the 19th century for reasons not unconnected probably with the industrial revolution, this is called a heat engine. Certain types of engines move because you heat parts of them to hotter temperatures than other parts and they move in response. Effectively that's what the atmosphere is doing as well. You might wonder how powerful an engine is it. Typically the Earth's atmosphere, which is not a particularly big atmosphere, is about a 5 petawatt heat engine. Now that's quite a hard number to visualise. So I've tried to break it down for you. 5 petawatts is 5 with 15 zeros after it. I tried to think of the biggest mechanical engine that I could think of and imagine the Space Shuttle just after its lifted off. The Space Shuttle with all its boosters firing at maximum as it tries to accelerate away from the launch pad. You would need half a million of those to get to 5 petabytes, all the maximum thrust. Well, another way of thinking about it is what's the peak electricity consumption of Great Britain? Actually the most ever recorded, which was recorded in winter 2 years ago on a cold day, was about 100,000 times less than this. So it's a vast amount of power. More loosely somebody has tried to estimate the total power consumption in all forms of the human world. So everything we do, oil, coal, wood, electricity, all added up and it's still 25 times less than this steam engine that's cranking on above our heads right now. So weather is pretty powerful.

So how does the atmosphere actually move? Well this is a classic figure from our Understanding the Weather book actually from the OU. It’s a chap called Hadley explained the tropical circulation as large cells and these are the blue cells near the equator here. So rising motion at the equator and then falling motion away from the equator. The reason why he was interested in these was this was the 18th century and he was interested in the trade winds. He's trying to explain the trade winds and effectively it's not exactly what happens but it's a good picture to have in your mind. But then about 100 years later or a bit more, in the 19th century an American meteorologist called William Ferrel suggested that there must be some other cells rotating the other way to carry the heat onto the poles, because we know the poles are emitting about 3 times as much heat as they're receiving. The poles would be a lot colder if we didn't have an atmosphere. We’ve drawn these in purple, you will often still see them to this day in books and all over the internet, Ferrel cells. There’s a small problem with Ferrel cells which is that they're not actually there if you go and look, there's nothing like that happening. Well there is something like that happening but it's only like that in the average sense. What actually happens is that the motion breaks down. It's not this simple 2 dimensional overturning circulation like a cell, it actually breaks down into wave-like motions. We're familiar with these actually because when the peaks and crests of the waves pulse over us we say we're having a high or a low pressure system and this is exactly the weather that's passing over us right now actually, we're in a low, not a very strong low but a low nonetheless. So the motion up here is not this Ferrel cell at all. So if you ever see that picture view it with a pinch of salt or colour it purple as we did and try to remember that actually what’s going on here is waves.

So let's look at a real picture of the earth and this is an image of the Earth in February actually from a satellite that's actually just for convenience sitting right over the equator and right over the prime meridian. So right over the Greenwich Meridian and right over the equator. The Hadley cells you can see really here or their impact anyway. There's a band of white cloud near the equator, it wanders around a bit with the seasons and this is February so it has wandered around a little bit. But you can certainly see the impact of the Hadley cells because the equator is quite green isn't it under those bands of clouds. There's rainforest in South America and across through Africa near the equator. This is where there's rising motion, it rains out and the moisture leaves the air as it rises. Now you can also see the effects of the Hadley cells when they come down again at the end of the Hadley cells because that's when the air which has now dried out through rain is descending and it's pretty clear that there's some deserts. The Sahara is impossible to miss in this image but there's also drier more desserty land in the southern hemisphere at a similar latitude away from the equator. So this is the impact of the Hadley cells. But if you start to look here at higher latitudes in both the northern hemisphere, Europe maybe, and across the North Atlantic and in the southern hemisphere as well, you can see that the motion looks different. In fact it's rather swirly, the clouds are in a spiral pattern and this here is a low pressure system and there’s another low pressure system here. These are cyclonic weather systems. So this is what I was talking about. This is where the Ferrel cell is really the average activity of a lot of waves and instabilities going on. That's what gives us our weather. So it’s a nice picture of the Earth's atmosphere.

Why do we study other planets? I've talked about the Earth and the importance of the Earth is clear because we live in it. We need to understand our atmosphere. But why study other planets? I think the first motivation is fairly obvious that they're just fascinating places in their own right. I hope you liked the picture of Jupiter, I certainly enjoyed that, I find that fascinating understanding the flows and similarly Mars fascinating, and why it's different from the Earth. But we also want to understand things like how the solar systems evolved and life. Perhaps the Earth wasn't always the best place for life 4 billion years ago, it might well not have been. It might have been Mars then. So understanding atmosphere is important for understanding the solar system. Then as Nick mentioned, spacecraft exploration, operational safety, we need to understand atmospheres. It is actually becoming more important as we get more advanced with our spacecraft because we’ve tried to push closer to the engineering envelope, we don't build in such huge margins and so when we're pushing close to that envelope we need to understand the density of the atmosphere we're flying through very often, and what its variability is. But actually what I'd like to talk about tonight is none of those things. It's the idea of comparative planetology, which is often cited by some scientists, but rarely fully explored. We certainly understand other planets by understanding a bit about the Earth. But I'd like to ask really can we look at other planets and can we learn something that we can bring back to Earth? I'm going to argue that there have been a few examples from Mars research that we have done. The things that we've learnt are more important on some of the planets so we studied them a bit more. Then actually they turn out to happen on the earth as well, but often to a lesser extent on the modern Earth.

Also this comes back to the idea of experiments again maybe. We're looking at things that are way outside, we're going to really stress test are models on objects that are way outside our current experience of climate on Earth.

So a few examples, all these examples are from Mars. So how do we understand Mars? These are all models that are run at The Open University now. So we have, as I was describing, a global circulation model of Mars and we can run it in what I like to think of as a climate mode, which is on the left, it's a global model, but it's a fairly coarse resolution picture if you like because we're going to run the model for a long time. We can run it in a weather mode, which is the same model, but simply turned up to a higher resolution so we can't run it for so long but we're going to look at more individual events in more detail. Then if we want to get really detailed we have to go down to the limited area models, or in this case what we call a Messer scale model, a medium scale model and those who know a little bit about Mars might recognise why this particular model was run because this is Gale crater in the centre, the purple is Gale Crater, the little bit of yellow in the middle is Mount Sharp and if you've got very good eyesight, there's a little black circle in the purple and that is actually where the Curiosity rover landed. So all of these models are run at the OU which we've developed over a period of time and in collaboration with many other universities as well, of course, I should say. But you also need data. It's no good having a great model but if you don't know how to start your model off, what your initial conditions are if you like, then what good does it do you? Another mission I've been involved with is Mars Reconnaissance Orbiter. This is the Mars Climate Sounder instrument and this is real Mars data you're looking at, the image of Mars is just there for reference, the picture of Mars with the dust and the surface but that is an image of Mars taken contemporaneously with this data so the dust storms that you can see are reflected in the data. The data are these coloured curtains, we call them data curtains that are surrounding the planet. This particular instrument flies around Mars, it orbits Mars roughly every 2 hours. So it orbits the planets between 12 and 13 times a day from pole to pole. Every time it passes over it makes thermal soundings. So the colours represent temperature in the atmosphere with a much exaggerated vertical scale so that you can see them. You can see there's all sorts of issues with this. In some places the bottoms of profiles are cut off, sometimes that's because you're going through a dusty region and they can't retrieve the temperature when the atmosphere is too dusty. Sometimes it's just because of various things that have happened on the spacecraft, it's had to do a roll or yaw manoeuvre or something so you have to turn the instrument off for a while. But this is a good day, we've got about 13 orbits. You can see that each of these curtains is drawn down, it crosses the equator. But another issue with these data, it’s not a synchronous picture, it's asynchronous data. So in other words we fly and take one data curtain, and it doesn’t come back to take the next data curtain until 2 hours later. The problem with atmospheres, unlike planetary surfaces, is that they move and in that 2 hours in which you've been flying round the other side of the planet, everything's moved a bit. So it's a bit like the old wagon wheels on the movie problem where everything's aliased and things change while you're looking at them. We never really get a global picture of the planet. So how do we cope with that? Well, a technique which I've been involved in developing for the last 30 years really called data assimilation which is used on the Earth for getting the initial state for a weather forecast, we use it for a slightly different purpose on Mars. So in other words we've got our observations which have the problems of the limited coverage uncertainties, but nonetheless they're real data. So they're massively valuable. That's actually what the atmosphere is doing now, which is what we want to find out, and we've got the model which is great because it's got global coverage and we've got nice uniform sampling but we don't know what the initial state should be. Data assimilation is all about bringing those 2 things together, combining them in a statistical way and eventually coming up with if you like a model state that most plausibly best fits the data that we've got. Another way of thinking about that is that we're using our knowledge of physics to interpolate between the observations. So we see something, then we see something else 2 hours later, but we know that it's moving, we can think we can work out what the winds are so we can make some forecast as to how it's changed during that time and we can combine our knowledge of physics with our simple knowledge of the observations to make something that's a bit more than the sum of the parts. We hope.

So the first example of the value of this is landing one of these rovers. This is the Curiosity rover sitting in Gale Crater. I showed you before how the Gale Crater model was embedded within the global models, so we ran the global model, we wanted to land Curiosity. The problem with landing these things is it's a big beast, you often see these images on the news and it's unclear how big they are. So Curiosity won't park in a standard American parking space. We've tried it and it takes about 1½ parking spaces. It's like a big 4 wheel drive vehicle. It weighs about a metric ton which is heavy for a spacecraft and you have to land safely on the surface. When it arrived at Mars it was traveling at 21,000km an hour and it has to stop in about 6 minutes but it doesn’t want to stop too soon or you are left dangling up in the air or stop too late in which case you hit the surface rather hard. So that involves knowing a detailed density profile of the atmosphere. But the problem is on Earth you would solve this problem by probably going and looking at some past record, you would get a weather station nearby where you were landing and go and look at the record. Unfortunately no one has set up a weather station network on Mars. So we had to try to predict from first principles and I'll show you some forecasts. These are real genuine forecasts, made for both Curiosity and Perseverance. On the left is Curiosity which landed in 2012. On the right is Perseverance which landed this year. In each case the continuous line is the forecast. So in the case of Curiosity, it's actually a forecast, it's not the day of landing because there wasn't any observation. The instruments weren't all turned on at that point. But we just compared after the event. The blue line here is my forecast of the surface pressure at the Curiosity site for a week from about 9 days after landing. On the right is a forecast of surface pressure at the Perseverance site. It’s a little different because I've just folded it in by time of day. So the black line is now an average and the grey line is some distribution about that average of what I think the surface pressure would be over several days. On the left the black little bars are the actual measurements made by the Curiosity lander. On the right the coloured dots are the actual measurements made by the Perseverance lander. You can see we're not totally without skill. Firstly, the biggest worry was would we even get the surface pressure right because the pressure on Mars varies very strongly for the time of year and we do seem to have got that nailed down which was one of the biggest concerns. But the second thing is how does surface pressure change with time of day, and in the case of Curiosity, it changes rather simply. The surface pressure is high early in the morning and low late in the afternoon. It falls very rapidly during the day and the cycle repeats. It's a bit like Los Angeles every day is the same, or at least it is at this time of year. Perseverance is a bit more interesting. So we actually predicted this that we would get a high pressure at 8 o’clock in the morning, but we’d get another high pressure at 8 o'clock in the evening. If you think about this, you might understand, are you familiar with something that is high twice a day? Well it's a tide, it's actually a thermal tide though. It's not a gravitational tide. It's a thermal tide from the sun going around. But the fact that we were able to relatively well, it's not perfect certainly, I wouldn't claim perfection but we got the right phase and amplitude I would say of the main components of the tides right for each site, it’s quite satisfying. The other aspect of these forecasts is they were both made about 2 to 4 years before the landings happened. So they're quite long range forecasts. They are really climatology forecasts, they’re climate forecasts not weather forecasts. But there wasn't a lot of weather certainly not at the Curiosity side. On the other hand you could argue we predicted there wouldn’t be a lot of weather. There’s a bit more weather going on at Perseverance. That's one of the reasons why the fit isn't perfect. But there's not a lot, they're both quite near the equator.

The next topic I'd just like to talk about is Mars weather. We've talked a bit about weather systems. This again is a visualisation from our global circulation model at the OU. The colour scale, the label hasn’t come out unfortunately, the colour scale is water-ice clouds and the little vectors give you an idea of winds. Actually what you're looking at here, this is wintertime in the northern hemisphere on Mars and you're seeing a jet stream around the pole, very familiar to us from Earth and in fact the jet stream splits in this example, you can see where a puff of cloud towards the centre of the image has come off more towards the equator and that's a splitting jet stream, it is a classic phenomenon in terrestrial meteorology as well. So just like the Earth, there's a winter jet stream and there are waves that go around this jet stream, I'll show you some of the waves now. This is now a real image of Mars, it's in the spring, it's not the winter, because you can see the North Pole. So the North Pole has just lit up in the spring. The white towards the top is the polar ice cap. But I think you can see hopefully pretty clearly actually there's a spiral system here. There's another puff of dust here, this is all dust you can see, there's not much cloud, there's a little cloud over on this side but all of this stuff that looks like cloud is actually dust that's been thrown up. So that's what's showing you the weather. But there's certainly a cyclonic low pressure system here and in fact if you look very closely you might see over here an arc of a weather front again shown up by dust which stretches over the edge of the polar cap. Now we can run our model for the same time as this. It won’t look immediately like it because I'm showing you air temperatures in our global model but it's from the same time as this image. What you can hopefully see here is that it's not simply warmer at the equator and colder at the pole, there's something going on and that something I would suggest is that warm air is coming up, it's being drawn up this way and cold air is being been pushed down this way from the pole and in fact this triangular pattern is very familiar to a terrestrial meteorologist. It's called a warm sector. This is exactly what's happening. A low pressure system is up here and these are weather fronts. Here's a cold front that’s been forming. Our model can't quite resolve a cold front because it's got a limited resolution, but quite a good resolution in this, it’s a weather resolution. So there's a cold front happening down here and a warm front here. In fact where I've put the laser pointer now is pretty much exactly where we would be sitting right now. We're in a warm sector tonight. In fact the warm front passed over us very early this morning, there's a cold front sitting up in northern England right now near the Scottish border and it's going to push down over us tomorrow morning, it will reach Milton Keynes tomorrow morning some time before lunchtime. So sometime before lunchtime tomorrow we're going to get a band of rain come down which is associated with this air being pushed together at the front, the cold air being pushed down. So this is a very terrestrial-like situation. In fact back to Understanding the Weather, here's the picture from the classic Earth, if you do Earth meteorology, this is a classic low pressure system over the North Atlantic that everybody will learn about from the Norwegian School of Meteorology through to the modern day and this is the warm sector, cold air, a cold front here, there's the warm sector in that triangle, and warm air being pushed and drawn that way. That front there is what you could see delineated by dust in the image of Mars.

Onto something a little bit more complicated if I've got time, I think. This is something from a paper I published with colleagues in 2016. It's actually quite a complicated figure so I won't have time to explain it all but I'll try to explain the main features. So what you're looking at here is if you like a map of storminess and where it's red there's a lot of weather going on, there's a lot of highs and lows passing over the location. Where it's green it's quite calm, the weather is the same every day if you like, it's a bit like Curiosity. This is the North Pole. This is the South Pole. There's not much weather at the equator and this is time along this axis. We've reconstructed the Mars weather for several years, the Mars years are all numbered now for convenience, it's not an international system yet but this is Mars years 24 to 26. A Mars year is about 2 Earth years long so it's quite a lot of data. So if we just focus on the northern hemisphere where we are up to about 50º, let's put on a few boxes to draw your attention to, so just look at these mid latitudes. This is autumn, this is where we are right now. That would have been 21st September in Earth units there. So we're sitting about here now and this is where all the storms happen. So the storms come south from the polar cap as the polar jet stream come southwards. But then something rather unexpected happens, we would expect certainly, and higher in the atmosphere this does happen, the storms just get stronger and stronger and they're strongest in the middle of winter which is when the temperature gradient is the largest, that's what you'd expect. Then they die away again in spring. But down near the surface, this is only 2km above the ground, this the weather you'd be experiencing if you were on the surface of Mars. In fact something rather bizarre happens, all the big storms happen during the autumn, they build up and up and then they calm down right in the middle of winter. So this would be Christmastime on Earth, the winter solstice. So the middle of this box is if you like late December on Earth or the equivalent. So this was a rather remarkable feature that's very strongly apparent in this dataset which models didn't produce until I had done this reanalysis, this combination of data with models. It’s become known now as the ‘solstitial pause’. It's known on Mars that dust storms are slightly less common at this time because the weather is less active and rather intriguingly the idea has come back to Earth and there are just hints. It's not so strong as this example, there are just hints that actually this happens on Earth too, that the strongest storms happen in the autumn and in the early spring, but don't happen right in the middle of winter. You tend to get quite calm weather on Christmas Day if you like. Think of it that way and the same thing happens to a much weaker extent in the southern hemisphere. I haven't got time to discuss why now but the same sort of phenomenon seems to happen. So there's a phenomenon that we discovered on Mars because it's much stronger, but actually people have come back and you can see this in the North Pacific weather on Earth apparently.

Next dust. Mars is all about dust storms. This is actually a pretty small dust storm you can see, it's moving from right to left across one of the northern plains on Mars. Dust is a major driver in the Mars climate. This is some work done by a recent PhD student, Paul Streeter and me. People have been interested in Mars dust since the late 60s. It’s a constant mystery why you sometimes get dust storms and you don't always get them and what impact they have. Dust is obviously a massive factor for the climate. It's well known, I think, that if you have plenty of dust in the atmosphere less sunlight will get through to the surface and the surface will be cooler during the day. That seems fairly self-evident. This is the old nuclear winter idea that people used to worry about for the Earth. But actually at night something different happens and it's a bit like having a cloudy night on Earth. If you have a cloudy night you tend to not get a frost in the morning because the infrared radiation sent up from the surface as the surface cools is instead scattered in all directions by the dust, and some of it is scattered back down to the ground and it actually keeps the ground a bit warmer than it would be if the dust wasn't there. It's a sort of greenhouse effect. Actually the net effect of the 2 in this state comparing a dusty year with a less dusty year that Paul ran, shows you this complex pattern. But certainly some places on Mars are much warmer when there's dust near the surface than they would be without the dust and some places are cooler. So Mars has a complicated greenhouse effect. On Earth dust storms are important now but they tended to be less modelled because Earth's climate is so dominated by water-ice clouds, whereas on Mars water-ice clouds are less relevant. I'll just show you a storm developing in our Mars model. So a little orientation guide first, this is a little map of Mars for those who are not familiar with the planet. This is a standard equal angle map. So the northern hemisphere of Mars is, don't worry about the names, but the northern hemisphere of Mars is essentially flat and it's a big plain, the various Planitia plains. The southern hemisphere on the other hand is a warm colour and that means it's high and rocky, it's rough and some of the most dominant features, of course, are the giant volcanoes, Olympus Mons, 3 times the height of Mount Everest and the footprint of that volcano is the size of France which is quite big for a volcano, if you think about it, especially on a small planet. Tharsis Ridge, 3 volcanoes along here, and a volcano on the other side of the planet which is the Elysium Montes. So that just gives you a little orientation guide to what you're going to be seeing. Because the movie was shot from a different angle we're going to turn the map upside down and rotate it at a bit of an angle. So we're looking down from the north now, so look out for Olympus Mons on the right and the Tharsis Ridge here, and then Elysium this way. So hopefully the movie will work and this is showing the beginning of a global dust storm which started in 2018 in our model. So you can see the dust. So this is actually the dust in our model being blown around with exaggerated vertical heights to Gale, here is Olympus Mons the giant volcano, it's an exaggerated vertical height scale, it would actually be flat because it's got such a large footprint. But you can see the dust blowing around and one thing you might well be able to see is the way it seems to slosh around and that sloshing is the thermal tide, that's the sun moving from left to right. But now you'll see a dust storm really kicking off in the Tharsis region, the dust being lifted up every day when the combination of the thermal tide and this is if you like a 4 dimensional reconstruction of a real dust storm on Mars that we can run in our model.

I said that dust is important on Earth too and I'll just give you an example. This is actually an image, it’s not hard to find a dust storm on Earth actually. This you may be familiar with, finding dust on your car or your windows sometimes it’s always said to be from the Sahara. Well it often is, and this is a fantastic example. This is actually an image of the Earth from 16th June this year. I will show you the dust storm, it’s in this region. We'll blow it up a little bit. There's the Sahara. You can see this rather sickly yellow-orange colour it’s the dust being blown off it and it's being blown all the way to Central America. It's actually raining out in the Caribbean and in Central America. So planetary-scale dust storms happening right now. Well a few months ago anyway. So just understanding dust storms is important. For the present day Earth atmosphere it is clearly even in this image dominated by clouds, but it nonetheless plays a role and we've improved our understanding of dust on Earth through looking at Mars. Dust has probably also been more important in the past on the Earth as well.

Okay, I'll move on to the next topic now, which is about climate history. These are dry riverbeds on Mars but they're not recently dry. They were probably dried up about 4 billion years ago. It's an incredibly ancient surface. So we know Mars was a lot warmer and wetter than it is now because you don't get riverbeds like this without rain. Nowadays Mars is dry but you still see ice. This is actually an image taken by an instrument called CaSSIS on the ExoMars Trace Gas Orbiter. So this is an image taken by an instrument that is actually co-managed at The Open University. So some of the people who choose the images it takes are sitting in the audience now I think. This is Korolev Crater in the northern hemisphere of Mars and the winds here have brought a frost down from the northern polar caps. So this white water-ice doesn't sit there all year, it's just been brought down by the winds now, a very thin layer of frost.

A bit larger region of aqueous alteration. This is an example from a PhD project that I'm running at the moment with an OU student, Lori-Ann Foley, and this is Lyot Crater. It is a huge crater in the northern hemisphere of Mars. In the image on the left there's lots of signs if you're a geomorphologist that there's been various aqueous alteration in the past in this crater. So we've actually zoomed in our limited area model. We're measuring where the ice is accumulating and now the great thing is with our model we can play games that we can't play in real life. We can look back in history. So we can go back to previous times on Mars when the climate was different. We can change the orbital parameters of Mars and see if things changed and can we understand how these features are formed over the history of Mars. So that's another thing we can do with the model. There's no surfaces as ancient as these riverbeds on Earth I should say. Mars preserves features that are much more ancient because of the activity on the Earth.

Then finally looking again at the global scale, so this is going back to this dust storm you saw. This is some images prepared in collaboration with James Holmes and on the left is the state just before that dust storm you saw. So this is the dust over the planet Mars, we're in the south now so the southern hemisphere is towards us. On the right we've put some other features on the model. So the blue is the water vapor in the atmosphere and the pink is water-ice clouds. There are a few clouds just as you saw in the image of Mars way back near the start of the talk and lower clouds over the poles. But this is a typical state when Mars is a bit dusty, but not very dusty. Only a week later this happens. So there the dust storm blows up. You can see the impact of the dust storm on the water. So the dust storm has just kicked up here in the Tharsis region, and the water has been blown much higher in the atmosphere. The atmosphere has warmed up, there’s fewer clouds, water is transported much higher. Why is that significant? Well, if water is transported high in the atmosphere it gets broken down by photochemical reactions to its constituent atoms and they escape to space. So maybe understanding the escape rate from these sort of processes, the role of dust storms in an escape is what is the key to understanding the history of Mars. How did Mars dry out so much? We still don't fully know. But certainly a large amount of water has probably escaped to space and we are trying to work out now how fast it's escaping and what's governing it and it seems that dust storms may be a key to the escape rate.

So I'm just going to finish now with this image. I was rather struck by this image. It actually was taken by a camera that is mounted on a small spacecraft but it was taken from Mars. The camera is for looking at the surface in high resolution but it was turned back on Earth at some point and this camera would have roughly the power of a really top of the range amateur telescope. So the sort of telescope that if you're a really keen amateur astronomer you might have in your back garden. So if you were a Martian astronomer this is the view of the Earth so you can actually see we've got clouds, it’s straight out of HG Wells and War of the Worlds. If somebody was sitting on Mars now they could see us, they could see we are different from the moon certainly. You can see the moon in the top right and I hope that gives a little perspective on one reason why you do planetary science because you gain a different perspective on the Earth than you have looking from below only. So with that I would like to thank you for your attention.

Nick: Thank you very much Stephen. We're going to go over there in just a moment or you could set off now and I'll join you. Just to say we're going to hear questions from people in the lecture theatre. We're not going to use the microphones. So if you just speak clearly and loudly I will relay it through my microphone to people online. People online if you'd like to send them in by email to the address which is there on screen. It'll then be relayed by voice back to me and I'll try and repeat it. So that's what we're going to do. It would be good if you could identify yourself by name. It makes it feel a bit more friendly. Say who you are. Have you got a question for us right at the beginning from online?

Helene: It's from somebody called Alex Wood. He's got two questions. You mentioned the large physical modelling spinning tank is still used. At the current of progress how long until our computer models can achieve the same small resolution over the same large spinner.

Nick: I’ll just repeat it, it’s essentially comparing the analogue computer of the big tank experiment and the numerical models that Stephen does before breakfast.

Stephen: It's a very difficult question because I think actually we’ll always want the real fluid experiments because we always want to validate and test our models. It’s a very dangerous situation to get into where you believe one model and you believe that's reality. I'm certainly a modeler and I love my models, but I would still like to test them against reality occasionally. So that's one factor. We’re still well away from getting the full range of scale. So to give you an example on the whole planet, if you like, with a single model we can probably resolve down to scales of several kilometres with a global model of the Earth with the very large computer clusters. But remember real viscosity doesn't happen in the air until you’re on a scale of about one millimetre so we still have this whole grey area between the tiny motions and one millimetre and the motions out at several kilometres. That grey area is a large part of the complications that are in these models. How do you describe the effect of the small scale that we know about? We know how viscosity works in fluids on tiny scales to the large scale. So good examples of that are extreme weather events which often in the past haven't been forecasted very well. So I’m thinking of something like the Camelford floods in Cornwall, for example, where they didn't capture the full extent of the intensity of the rain because the model simply couldn't go down to a small enough scale. So I think there's always going to be a place for looking at reality as well as looking at models. But we're still well away from just thinking that computing models can do everything. We shouldn't think that.

Nick: It’s not just rotating tanks, but presumably it’s rotating the Earth as well and using that as a model and your other planets in your experiments.

Stephen: And other planets, exactly. So we're a million miles away from ever being able to simulate Jupiter which an even vaster range of scales than the Earth. I said several Earths could fit inside the Great Red Spot. Jupiter moves and thrashes around on small scales just like the Earth, but then it's 10 times bigger effectively.

Hélène: He has a second question, you mentioned that our weather forecast accuracy is extending by 1 day per decade. How long do you think that rate of progress will continue and are there theoretical limits?

Nick: So this is the forecast of 1 day per decade improvement. How long can we go on Stephen?

Stephen: I'm not sure I can say precisely. I think we're probably starting to level off now. It's not that we can't carry on improving but improvement gets harder and harder and harder. So famously the whole idea of chaotic systems, the tiniest change in the initial conditions grows exponentially with time, so that if you like buying a day into the future is progressively more and more expensive. The computing power is not increasing at the rate it was actually. The Moore's law, so the computers aren't getting that much more powerful but more importantly we've actually improved our observing systems so much. So in the past the big flaw was that we perhaps had lots of observations in let's say Northwestern Europe, where there's a lot of people, a lot of money as well, and weather stations, very few observations in Africa. But that ballpark has been levelled a lot by the coverage from satellites. So as we have more and more satellites the difference between northern and southern hemisphere of the planet has been evened up. The southern hemisphere forecast was pretty bad a few decades ago certainly, but it's now caught up rapidly with the northern hemisphere forecast. But of course that will tend to even out as we use satellites more and rely on individual people and weather reports less.

Nick: Okay, thank you very much a question in the room, I see one from John Zarnecki.

John: Hi, thank you very much Stephen. A fascinating talk. Exoplanets, we know the 5000 or so planets beyond our own solar system. Do you expect in your lifetime that we will be studying weather on some of those exoplanets or is that fanciful?

Stephen: Weather, I don't know. Climate yes because we’ve done that already. We've modelled large Jupiter-sized planets near stars with different rotation rates. So we can do that in the broadest sense. At the moment you can't measure very well. You can measure only in the simplest, broadest, large scale parameters. So yes, you can't validate the models in the way I'm comfortable with with an Earth model. We should always be humble in the face of how complex these systems are. I talked about the difference between Venus and Titan actually and Mars and Earth. Earth and Mars are rapidly rotating planets and they have similar weather dynamics. Venus and Titan are slow rotators and their atmospheres super-rotate massively. If we didn't know they super-rotated massively, until a few years ago we wouldn't have been able to predict that with models. Models don't reproduce super-rotation that well, and we've made a lot of advances on that but I would argue that had we never seen Venus, we would have a very bad idea of what its circulation is actually. Or at least we wouldn't know we were wrong for a long time.

Nick: Okay, I have another question down here from online. Has the study of atmospheres on other planets made you more anxious about climate change on Earth because of the fragility of our atmosphere?

Nick: Can I just say that’s a very good question. This is, of course, COP26 we are in the middle of and worrying about the fragility of our atmosphere based on insights that you gained from space, Stephen.

Stephen: I think the simple answer is yes. I've been well aware of climate issues since the 80s when I started working in this field. Although I’m a planetary scientist I talk to Earth scientists a great deal of the time and it is a concern. I wouldn't go into the total doom and gloom scenario of thinking, we're going to end up like Venus in a runaway greenhouse, where the oceans are boiled if there were any oceans and the planet is hot enough to melt lead on the surface. Actually maybe we will end up like Venus but probably not for a billion years. That's not human climate change that's way outside that scope. But it does show you the vast range of parameters if you like that atmospheres can settle over and there's no reason why a planet should be as accommodating and as nice as the Earth. Whether you regard it as luck or how you want to regard that we've evolved to enjoy the climate we've got and the fact is that had the Earth had a slightly different climate, we have a natural greenhouse effect of about 33º. If we didn't have that natural greenhouse effect we would be frozen, there probably wouldn't be life on Earth. If we have a greenhouse effect a bit more, we could push over the edge and various drastic effects could happen. So yes, I'm certainly well aware. I don't think we should panic, but equally, I don't think we should sit back and think it's always going to be okay. I’m certainly well aware of just really how thin a skin we live in. It's always quite interesting to remember that if you went 5kms that way, you probably couldn't survive and if you went for a kilometre or 2 that way you couldn’t survive. It's not very far in the scale of the universe.

Nick: I shall never eat an apple and feel the same again. Any other questions from the room?

Question: You mentioned about Mars drying out. How close are we to understanding why it’s drying out, or how it dried out I should say?

Nick: I’ll just repeat it. ~This is the loss of water from Mars. How close are we to understanding all of that?

Stephen: Well, it's certainly a topic that many people are working on very hard. I suppose what do we know? We know it was wetter than it is now but we're talking a long time ago, 4½ billion years ago the planets formed, the sun formed about 4.6 billion years ago, the planets formed about 4½ billion years ago. We're talking within the ½ billion years. So this is incredibly far back in time, there's nothing that old that we can find on the surface of the earth, for example, it's all been churned around a lot more since then. So it's not an easy problem. It's a deep time. We know that Mars was wet. I think we can be pretty confident that it had precipitation of some kind, whether that was snow or rain. I can't swear to but I suspect it was both. We know that Mars went through a phase of being much more icy. But there were still mega floods, there's signatures and mega floods all over the surface where the ice has broken and washed away features and that was up to about 3½ billion years ago. Then we know that for the last 3 billion years Mars has varied. Its climate has changed, but it's not changed to that extent. It's varied between different states. So we know that there has been this big change. Why is a question that many would like to answer and how it actually got from one to the other? But it's a big challenge. I think the story I would give you is something along the lines of we know that Mars had a magnetic field in the past and that magnetic field is really only fossilised now. The active magnetic field is almost gone. Now is it the case that the magnetic fields stopped for some reason and was disrupted and when the magnetic field stopped the atmosphere was less protected and so all this water that was been transported into the upper atmosphere by the wind that's certainly part of the story. But there's a chicken and egg here because was it the case that the magnetic field stopped because the water stopped? Mars doesn't have active plate tectonics in the same way as the Earth does. It doesn't have moving plates anyway. So it's a very different planet and I think the answer is we're gradually piecing it together and its missions like the upper atmosphere satellite MAVEN that’s operating now, and our studies of the lower and middle atmosphere trying to tie it together and trying to work out what the loss rate is. If you can pin down what the water loss rate is now and in the recent past, and the recent past might be 3 billion years, then maybe we can have a better estimate. There's probably a lot of water under the surface of Mars as well. We know there’s a lot of ice under the surface, so water has sunk down. We could expect there to have been a 500m deep sea in the northern hemisphere of Mars in the past, just on the basis of thinking it's probably about like the Earth, it’s not that different in terms of composition. So that's a long answer. The short answer is I wouldn’t say I know but there's lots of ideas and it's probably connected with a series of catastrophic events which are to do with the atmosphere cooling, probably the atmosphere lost a lot of mass, the atmosphere was probably a much thicker atmosphere in the past because it would have to have been to have active liquid water exposed to the surface. If you expose water on the surface of Mars now it would rapidly freeze and then sublimate. You won’t have liquid water. So the answer is something pretty bad happened but it's hard to say what at the moment still, but it's an interesting question.

Hélène: There’s some comments about it being a fantastic talk but there is one more question online from Dr Liz Parvin from the School of Physical Sciences here. So you've talked about morning and evening, spring and summer and so on but each planet has a different year length and day length, and in some cases the axis of the planet has a different orientation. How does this affect the prediction of weather on different planets?

Nick: So the relationship between the planetary cycles of day and night and years and so on and the weather?

Stephen: One of the reasons I talked quite a bit about Mars is probably because in those respects Mars is the most like Earth, it's year is almost twice as long as the Earth's, but actually Mars is tilted with respect to its orbit around the sun by almost the same angle as the Earth is currently. That means you get the same sort of pattern of spring, summer, autumn winter. Also Mars rotates only a bit slower than the Earth. A solar day on Mars is about 24 hours and 40 minutes in our time. This is a great problem if you work on a Mars mission actually because I know a lot of colleagues in JPL in California who drive these rovers are driven mad because they basically have a nap and work on Mars time and of course if you’re slipping 40 minutes every day it does horrors to your body clock. But nonetheless, Mars is very similar to the Earth, it rotates for the same sort of period, the days are about the same length, it has the same pattern of seasons. Other planets are different. So Jupiter, for example, rotates very fast. It rotates through a 9 hour period but also its sitting almost upright, so it doesn't really have seasons. It's spring all the time. It’s the same on Venus actually, Venus is sitting almost upright, but it rotates very slowly and Venus has a very strange situation where it rotates the other way. Depending on which way you look at it, you can either say it rotates backwards or it's upside down, it doesn't matter which view you take, but it rotates the opposite way to all the other planets. But it also rotates very slowly. So on Venus a solar day is very different from a rotation period. On Earth we talk about a day quite casually, there are different sorts of days, there's a rotation period of the planet and there's also a solar day which is when the sun is in the same position in the sky. They are about 4 minutes different on earth, 24 hours is actually a solar day, the Earth rotates a bit faster than that. But on Venus they're hugely different. So it's a very strange place. If you were walking on the surface of Venus, not that I'd recommend it, you could keep up with the same local time of day pretty much as you walked round the planet. So the day moves around the planet at that sort of speed. So yes in each case and that's part of testing the models. So if I go back to my tank in the laboratory, what do I want to do? Well I want to spin it twice as fast or I want to spin it half as fast and that's exactly what the planets give you, systems that rotate at slightly different rates.

Nick: Okay, thank you very much. I think we're going to let Stephen off at that point. I'm going to walk over here and at the same time say, Stephen that was an absolutely fabulous lecture. I have never understood the weather, even when we were writing a course of that title itself, quite so well as I do for having spoken to you this evening. So I thank you very much for that Stephen.

Now we do strive for continuous improvement and so feedback is always welcome. There are some feedback forms for people in the room. If you've been watching online we're very happy to receive your comments by email of course. I'd like to thank everyone for joining us this evening, whether or not it's been online or it's been here in the room and for those who are in the room there is a treat downstairs that we can go to in just a moment. But before that I'd like to leave you with details of the next Inaugural Lecture event on the theme of Children of migration as brokers of ‘care’. This will take place on Tuesday 30th November at 6pm and will be delivered by Sarah Crafter, Professor of Cultural-Developmental Psychology in the School of Psychology and Counselling in the OU’s Faculty of Arts and Social Sciences, and it will look at young people's migration experiences. Details are available on The Open University Research website and we've got a little trailer I think just to tease you with at the end

[video]

“The transition to adulthood holds particular challenges for children of migration. In my Inaugural Lecture I will bring some of those challenges to life using examples from my own research with child language brokers who translate and interpret for family and friends and lone child migrants, children migrating without kin. I will draw on the concepts of care and critical childhood to show how anti-immigration contexts frame children's lived experience.”

There we are, come back on the 30th if you want to see that and I guess that will be streamed as well. So thank you very much. Have a good evening.

About Stephen Lewis

Stephen Lewis, Professor of Atmospheric Physics at The Open University, is Head of the School of Physical Sciences. He researches the dynamics of planetary atmospheres. This includes understanding the dynamics of climate systems, forecasting the weather for spacecraft missions and interpreting the atmospheric observations that they return. He has won awards for work on spacecraft teams, including NASA Mars Reconnaissance Orbiter, and the Curiosity and Perseverance Rovers. Back on Earth, he is a Fellow of The Royal Meteorological Society. Stephen has been the academic consultant on BBC series including Wild Weather, The Planets and A Perfect Planet.

Read more about Professor Lewis

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