Mercury
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BBC Sounds. Music, radio, podcasts. This is In Our Time from BBC Radio 4 and this is one of more than a thousand episodes you can find on BBC Sounds and on our website. If you scroll down the page for this edition, you'll find a reading list to go with it. I hope you enjoy the programme. Hello. Mercury is the planet closest to our sun and as it's visible to the naked eye, it's intrigued humanity for as long as we've been here.

We see it as an evening or a morning star, close to where the sun has just set or is about to rise. And it helped Copernicus to understand that we orbit the sun and Einstein to prove his general theory of relativity. And for the last 50 years, we've been sending missions there to reveal something of its secrets and how those relate to the wider universe, with the latest, the BepiColombo, out there in space now.

With me to discuss Mercury are Emma Bunce, Professor of Planetary Plasma Physics and Director of the Institute for Space at the University of Leicester, David Rothery, Professor of Planetary Geosciences at the Open University, and Carolyn Crawford, Emeritus Fellow of Emanuel College, University of Cambridge and Emeritus Member of the Institute of Astronomy, Cambridge. Carolyn, before the invention of telescopes, what did we know, I think we knew, about Mercury?

As one of the five planets that are visible to the unaided eye, Mercury's been known about since ancient times. And in fact, there are recorded observations of it from the first millennium BC on Babylonian clay tablets.

And it's been observed, it changes in brightness, it can be as bright as any of the stars in the sky, a mental fate of being barely discernible to the unaided eye. But the key thing that makes observing Mercury quite difficult is that proximity to the Sun. Most of the time it's completely lost in the Sun's glare and you only see it when, or you see it best when the Sun is below the horizon, it's masked by the horizon, and

And maybe in the evening twilight for a couple of hours you might see Mercury as an evening star, or if it is on the other side of its orbit, it'll rise before the Sun for a couple of hours and you'll see it as the morning star. But you only ever see it really when it's well away from the glare of the Sun, it's at the maximum separation from the Sun. And this isn't very far, that spans only an angle of 28 degrees, which means it's always near the horizon.

So apart from the fact that it's small, it can be faint, it's near the sun, you've also got to contend with hills, clouds, general haze in the horizon, which make it very difficult to spot. Why is it called Mercury?

One of the things you do notice about Mercury is it moves fast relative to the fixed backdrop of stars. And it's this sort of fleetness that is associated or led it to be associated with the Roman messenger god Mercury. It's just the speed with which it goes around the sun. So what did Copernicus and Kepler learn from observing Mercury?

Mercury and its motions were one of the key lines of evidence that pointed Copernicus to remodel our solar system as being a sun-centered one rather than having everything rotate around the Earth.

I mean, it is alleged that Copernicus never saw Mercury himself, but he used the fact that Mercury and Venus were always seen close to the sun to infer that they actually revolved around the sun and not around the Earth. And the fact that you never saw them on the opposite side of the sky, like Mars, Jupiter and Saturn to the sun, meant that their orbits were within the Earth's orbit.

So it's part of the lines of evidence for building up this new model of the solar system. And out of that, you get two predictions to do with Mercury and Venus. One is that they should show phases. So like the moon shows a phase depending on how much of the sunlit side of the moon you see. It's the same for Venus and Mercury. They should show phases, right?

and also sometimes they would line up between the Earth and the Sun and you'd see what's known as a transit where they're seen in silhouette against the solar disk. So those are the predictions that came from Copernicus and later on, so beginning of the 17th century, Kepler was refining this model, choosing to use elliptical orbits and was able to make predictions of the transit in much more detail.

And he said the next transit will be one of Mercury on the 7th of November, 1631. And people with telescopes all across Western Europe tried to look for it. And as usual with these phenomena, it was cloudy. There was one person, Pierre Gassandi in Paris, who apparently saw the transit of Mercury on the appointed day,

But it threw up a few surprises. First, it appeared really small, much smaller than anybody had expected. In fact, he had to observe it for a while to realise it wasn't a sunspot on the sun that he was observing. It's more like a hundredth the diameter of the sun rather than the tenth that they were expecting.

Observing the phases of Mercury didn't happen until a lot later. Galileo, when he looked through telescopes, he saw the phases of Venus. He couldn't see the phases of Mercury. That didn't happen until later on in the 17th century when telescopes were more advanced. Dave, Dave Rothery, it's 50 years since the space probe Mariner 10 reached Mercury. What did it tell us about the terrain?

Well, it was our first close-up view of the planet. I saw Mercury from my back garden on Easter Saturday, so it's nice to feel I maybe went one better than Copernicus. But you cannot see any details of the surface by observing from Earth. Mariner 10 flew past Mercury, and because its trajectory was quite cunning, it was put in an orbit around the Sun where Mariner 10's closest point to the Sun was Mercury's furthest point from the Sun, and its orbit took...

twice as long as Mercury's orbit. So every time Mariner 10 went round the Sun once, Mercury had gone round twice and they met each other again. So we had three flybys of Mercury from one spacecraft before it ran out of manoeuvring thrust of gas and couldn't work anymore. So we got our first close-up views, unfortunately of the same side of Mercury each time because the same face of Mercury was lit up by the Sun. But we saw wonderful details. It looked deceptively moon-like. There's no atmosphere.

Craters everywhere, some densely cratered, some less densely cratered areas. It's actually a rather dark planet. It reflects less than 10% of the sunlight. It's actually a bit darker than the moon.

What it lacks is the bright areas. When you look at the Moon, you see bright areas and dark areas. Everywhere on Mercury is equivalent to the dark areas. There's no bright crust formed of those feldspar minerals, which tells us something about Mercury's early history. The feldspars didn't float and rise to the top of the early Magma Ocean, but it's an ancient surface, heavily cratered,

And one thing we didn't expect to find is great fault scarps crossing the surface. The cratered areas are cut through by escarpments, slopes, sort of linear, but they snake around a bit. They're called low bait scarps.

And this is where we think a thrust fault has cut up towards the surface and the escarpment is the front of the thrust sheet. And this is showing us that Mercury is contracting. And we think this is because it's cooling down. As it's all planned, the Earth is cooling down, but we don't see these contractional faults on the Earth because the Earth's whole outer shell is being shunted around by plate tectonics. But Mercury's got these escarpments two kilometres high,

hundreds, several hundreds of kilometres long, snaking across the surface. So if you stood on the surface at the right place, you'd see this magnificent escarpment going horizon to horizon. They're not cliffs, they're not that steep.

but it'd still be quite a landscape feature and it's very heavily cratered. And if you remember views that we saw from the Apollo astronauts stomping around on the moon, they were stomping around in dust. We're leaving footprints in the lunar dust. It would be just the same on Mercury. Almost no bedrock exposed because it's been pummeled by impacts. And as I understand it, Mariner showed us something of a giant impact crater, Caloris. What was exciting about that?

Caloris is the biggest impact basin that Mariner saw on Mercury. It's still the biggest impact basin on Mercury. It's about 1,500 kilometres across. When Mariner flew past each time, only the eastern half of it was lit up by the sun.

and it's got a rampart around the edge and inside it's got a relatively smooth area with relatively few craters have formed inside colloid itself so the floor is younger than the rim it's been floored over by something that's smoothed out the original rough nature of the crater floor and loads of cracks in it and it's at the point on Mercury that gets hottest because

The longitude where the Caloris Basin is is where the sun is overhead at noon when Mercury is closest to the sun, hence the name Caloris meaning hot. Are we any time-prone for Caloris? Yes, we do. It's used as the benchmark for one of the...

age systems on Mercury. We think Caloris formed about 4 billion, maybe 3.8 billion years ago. 4 billion years is half a billion years after the birth of the solar system, so half a billion years after Mercury itself formed.

At that time on Earth, there may have been very primitive single-celled life, but of course life on Earth didn't really get going until about 600 million years ago. So Caloris is extremely ancient, but most of Mercury's history

is in this time period longer than three billion years ago at least. There's a lot of ancient history on Mercury, heavily pummeled by impacts, the same impacting bodies that form the craters so visible on the Earth's moon, but not preserved on the Earth because the Earth has been resurfaced by active geological processes. Emma, Emma Bunce, 50 years ago, what did Mariner reveal to us about the magnetic field of Mercury?

So Mariner 10, as Dave has already explained, undertook three flybys of Mercury over the space of about a year in the mid-1970s. And we really didn't know very much about Mercury's magnetic or environment or space environment. So during those three flybys, scientists were very surprised to see evidence for an internally driven magnetic field during that first flyby. Well, they were surprised.

Because of the terrestrial planets, the four inner planets, there's the Earth, obviously we know we have a magnetic field, but Mars does not have an internally driven magnetic field today. It has the remnants of one that probably existed long ago. Venus doesn't have an internally driven magnetic field. And so I don't think anyone was expecting little Mercury, very close to the Sun, to have its own internally driven field. What's the consequences of that?

Well, there are some interesting consequences of that. So as the spacecraft flew past, it saw this magnetic signature increase to a maximum when the spacecraft got closest to Mercury. So that was the evidence for a magnetic field being driven within the planet.

And knowing that that planet has an internal magnetic field then tells you something about the interior of Mercury. It tells you that there must be a liquid part in the iron core that can actually convect and allow electrical currents to flow in order to sustain a magnetic field for a long time. It...

forms a sort of protection around the planet to some extent. We have a magnetic field and that extends far out into space and protects us from the sun's radiation, for example, and possibly protects our atmosphere from being stripped away.

But what Mariner found is that Mercury does not have a very strong magnetic field. In fact, it's about 1% of the strength of the Earth's magnetic field. So very weak indeed, which is interesting in itself. But what also happens then is that as the spacecraft flew past...

On those flybys, it saw the evidence for the wider space environment, which we call the magnetosphere. So this is the sort of protective space environment that extends out far beyond the planet itself and into space around Mercury, and

and it contains and is controlled by Mercury's magnetic field. And that magnetosphere forms in the flow of solar wind that leaves the Sun and flows throughout the solar system past all the planets. And if those planets have a magnetic field, there is an interaction between the solar wind carrying the Sun's magnetic field...

and the planet's internally driven field to create this magnetospheric bubble protecting the planet.

And then this century there was the Messenger mission. What did that set out to do that was different from Mariner? So following the Mariner 10 flyby mission in the 1970s, we then had to wait 30 years until the next spacecraft headed out to Mercury. Why that long? Well, it takes a long time for any planetary missions to be established, to be accepted, to be built and to actually travel to their destination. So it can take a surprisingly long time.

It also takes quite a long time for a spacecraft like MESSENGER to arrive at Mercury because it actually has to spend quite a lot of time slowing down in order to get into orbit around Mercury. So Mariner 10 was a series of flybys. It didn't orbit the planet, but MESSENGER...

actually wanted to go into orbit around Mercury. So that's the key difference between those missions. And to get into orbit, the spacecraft has to spend a long time going around the inner solar system using Venus and Earth and Mercury itself to actually slow the spacecraft down, put the brakes on and get into orbit around Mercury. And that difference between those missions is incredible.

It really allows a huge amount of extra science to be obtained because MESSENGER went into orbit. It took four years to orbit around Mercury before the end of the mission, so it was there between 2011 and 2015. And once you're in orbit, the amount of science that you can do substantially increases compared to the Mariner 10 mission.

We got so much science from MESSENGER because we were in orbit and we saw the whole globe for the first time. We saw the other half of a Caloris basin. But another reason for why it took so long to get a mission to Mercury is it's a very hostile environment to operate in. Apart from working out how to slow down and get into orbit when you get there,

you've got to survive the temperatures. When you're on the day side of Mercury, you've got the Sun 10 times as strong as it is here at Earth because you're that much closer. And below you, you've got a planetary surface at 450 degrees. So your spacecraft's being cooked from both sides. And it's very hard to keep the electronics at a working temperature. So it's very complex designing a mission to get into orbit at Mercury and that will stay cool enough for the instruments to keep working.

Caroline, it's the smallest planet and the fastest. Can you tell us about Mercury's days and years? You're right, it's the fastest. It travels around its orbit at an average of 48 kilometres per second and it only takes 88 Earth days to go once round its orbit.

And it's worth mentioning that it doesn't travel in a nice sort of circular path around the sun. It's quite elliptical. It's a squash circle. And the distance between Mercury and the sun ranges between about 46 million kilometres out to 70 million kilometres. So that's the year, 88 days.

What is interesting is when you try and determine the rotation of Mercury, the planet itself, which defines things like the day. And as Dave has already said, when you look from Earth through a telescope, you can't really see much in the way of surface features, right?

And it was always assumed that Mercury was what is known as tidally locked to the sun. This is when you've got a very massive object like the sun and something much smaller in orbit around it, like Mercury around the sun, like the moon around the earth.

There's the way the gravitational tidal forces act to sort of slow down or accelerate the spin of the smaller body such that it then synchronises that the day and the year are the same length of time. So with the moon, that's why it always has its face towards us because it goes once on its axis in the same time it takes to revolve once round the axis.

So it was assumed that Mercury was tidally locked to the Sun and it was only in the mid-1960s that they determined how fast it was rotating. They did that by bouncing radar signals on the planet and receiving the signal back in the echo and seeing that the signal had been smeared out. And this is due to the Doppler effect and due to the fact it was bouncing off a rotating surface.

And from the amount of that broadening of the signal they could work out, it was rotating once every just short of 59 days. Now, 59 days is shorter than 88 days. So it is not tidally locked to the sun to always show the same face, but it's got a different kind of resonance inside.

in that every two times it goes around the Sun, it orbits exactly three times on its axis. So it's called a three-to-two resonance. And it's just quite a peculiarity of Mercury in the Sun system.

Thank you. Dave Rothery, Messenger revealed volcanic activity, which was surprising. Why was it surprising? It was surprising. Mercury is a small world. You don't expect it to retain a lot of heat. The fact that the outer core is still liquid tells you something about heat inside.

It's not volcanic activity today as far as we can tell, but there are two kinds of volcanic activity that we can see traces of. There's vast plains of lava, the smooth infill of the Chloris Basin, whose nature wasn't apparent really from the mariner data, but we see smooth plains all over the globe now and we're pretty convinced those are vast fields of lava, just like the vast lava fields on the Moon that form the dark areas.

So that's lava plains from big, enormous, really, lava flows.

But there are also holes ripped up through these lava plains, which are not impact craters. They're nowhere near circular, rather irregular in shape and sometimes they're overlapping. And we're very sure that these are volcanic vents. Volcanic explosions have ripped up through the surface rocks. And you can see surrounding these big holes, which are tens of kilometres across and up to three and a half kilometres deep.

surrounding these to a distance of the order of 100 kilometres, you've got these bright red, relatively bright red deposits with diffuse edges. And that's material that's been flung out explosively by these explosions. So the volcanic explosions have not only ripped a hole in the ground, they've flung material out. And the only way to do that...

is to have gas interacting with the hot rock, with the magma, and expanding with sufficient violence to fling out the fragmented rock. So we didn't expect Mercury to have volatile materials.

but it clearly has. So either the magma is coming up from depth, bringing gases in it, which come out of solution when the magma nears the surface, just like carbon dioxide and water come out in volcanoes on Earth and drive explosions, or else you've got volatile-free magma reaching a volatile-rich layer in the shallow crust as the magma nears the surface. Either way, the

The volatiles turn to gas, expand, and drive out these explosions. So this is a piece of early evidence that Mercury is rich in volatiles. Now, so far as we can tell, these explosive volcanic eruptions were mostly 3 billion years ago, but there are some that appear to be younger. It's very hard to date features on Mercury's surface, but if you look at the impact craters, the younger they are, the crisper they are, the less they've been smeared out by

debris being flung over them from other craters and so on. So some of the freshest impact craters have small volcanic pits ripping through them. So those volcanic eruptions have to be younger than these relatively young impact craters. So the most recent explosive volcanic eruptions could be as young as one or 200 million years. And that was a surprise. Emma, Emma Bunce, at the heart of this, we don't know how Mercury was formed.

No, we don't, and we would like to. What's your guess? I don't think I should guess. Not my field of expertise. But we do want to know the answer to that question because Mercury is the sort of most extreme, one of the most extreme end of the planets in our solar system, so close to the sun, such a small planet, and with some really interesting features. So MESSENGER has made some measurements which really challenge formation theories and

confirming, for example, that 85% of Mercury's radius is iron core, which is a much more substantial fraction of the interior compared to the Earth. How on Earth did it form in order to be like that? As well as the iron core being a substantial fraction of the radius, as Dave has just mentioned, there are also volatile elements in abundance on the surface, which was unexpected for a planet so close to the Sun.

So those challenge the formation theories and the theories sit in a couple of different categories. One are sort of chaotic events that occurred. So, for example, was Mercury impacted by another large object like itself early in its formation phase? So we're talking back at the very beginning of the formation of the solar system.

And was it hit by something which actually stripped off part of its mantle, leaving it behind with a large core? But there are problems with that theory because then you would not expect to see, for example, volatiles on the surface, given the energy involved in a collision like that.

And there are other theories that suggest Mercury could have been in a sort of hit-and-run type incident with a very large object, Mercury being much smaller, that could have actually knocked it towards the inner solar system and could explain its unusual characteristics in terms of its interior.

Or there are some theories that suggest there's sort of more orderly things that occurred at the beginning in this formation. So, for example, that the hot solar nebula that Mercury would have been embedded within actually evaporated part of the outer region's

But again, how do we see these volatiles on the surface? Or was Mercury embedded within a proto-planetary disk that was already sort of sorted into being very metal-rich and it just happened to form in a very metal-rich environment, creating that iron core. So fundamentally, at this point in time, the data that we have so far really challenges the formation theories that exist. And so we need more.

more data, more detail that MESSENGER wasn't able to obtain for a number of reasons, partly because it was in an elliptical orbit with the closest part of its orbit in the northern part of Mercury's hemisphere.

And so it could only really obtain close measurements of the surface in the northern hemisphere. And so we have yet to see all of the details on the composition, for example, of Mercury's surface, which should really help us to understand and to rule out what some of those theories might tell us. Thank you. Caroline, can we come back to Earth for a second? How did Einstein make use of Mercury?

Before we get to Einstein, we go back to Newtonian mechanics, which were very successful at explaining everything in the solar system. And indeed, in the 19th century, Urban Le Verrier and John Cooch Adams used the Newtonian mechanics, the effect of the gravity of this undiscovered planet Neptune acting on Uranus and perturbing its orbit to show that there was another planet there. And the

The trouble was, Newtonian mechanics explained everything in the solar system except for the motion of Mercury. Now, as we said, Mercury goes around in a very elliptical orbit, and that orbit pivots slightly about the Sun with time. That means that Mercury never goes around in exactly the same orbit each time.

And if you measure this by looking at what we call perihelion, it's where Mercury is closest to the Sun in its orbit. It's just a little bit forward. It advances a little bit each time. And it advances too much to be explained by Newtonian mechanics. Now, all the planets do this in their orbits, and it's because of the gravitational effect of not just the Sun, but all the planets combined. The trouble is that Mercury's advancement was too high. It was a big anomaly that could not be explained anymore.

So Urbain Le Verrier in 1859, I mean, buoyed with the success of discovering Neptune and predicting where it was, tried the same trick of Mercury and said, OK, maybe these orbital variations are due to another planet, which was tentatively called Vulcan, and which would lie closer into the sun and would perturb Mercury's orbit. And so people went and looked for this tentative planet Vulcan,

Some people claim to have seen it, but it was difficult to know whether they maybe had seen a transit of the planet or maybe tiny sunspots crossing the surface of the Sun.

But more importantly, there were a whole succession of total solar eclipses at the end of the 19th century, beginning of the 20th century. When the glare of the sun is blotted out, you can see all the interior planets. There was no sign of this planet Vulcan. So it was discredited, but nobody could still explain the anomaly of this precession of Mercury.

until Einstein combined gravity and special relativity in 1915. And one of his major successes was to be able to reproduce Mercury's precession exactly. And this success was one of the first ways that his work came to be accepted. Thank you. Dave, in what way does the geology of Mercury appear to be similar to other rocky planets? In what ways was it very different from other planets?

Well, I spent a long time saying to people, Mercury is not the moon, but actually it's most similar object to the moon. The moon's not a planet, but to a geologist, it's a planetary body. Lots of craters, lots of areas formed volcanically, but it has no atmosphere, so it's not like Mars or Earth or Venus. But in

In all the rocky bodies in the solar system, we see the same processes blended together in different proportions. Mercury's got these big faults we've already talked about with one slab of crust being thrust over another. But the movement there is just a few kilometres, whereas on the Earth, because of plate tectonics, it goes on for thousands of kilometres and never really stops.

And then we have lots of erosion on the Earth, so ancient features are destroyed. We don't see the ancient impact craters on the Earth have been eroded by wind and water or by the action of plate tectonics. Mercury preserves the ancient record. We don't understand the inner solar system unless we look at the evidence that's on all the planets.

we have to compare everything to understand what's gone on. So you want to comment, Caroline? Well, I was just going to say, is it fair to say that Mercury is like the moon on the surface only, though? Because from what Emma was saying earlier about the interior structure of Mercury, that is in itself more like the Earth. Well,

Yeah, Mercury has a much thinner rocky carapace than the Earth does. But it's only superficially lunar, only superficially moon-like as well. Because everywhere on Mercury's surface, we think now, has formed volcanically. It was pretty clear from the detailed observations by MESSENGER that everywhere on the surface has been flooded by lava flows. We can see ancient craters have been partly flooded and so on.

Mercury doesn't have the contrast between the dark areas and the light areas that the Moon has. The reason is that on the Moon, when the Moon was very young, the outer part was molten, we call it a magma ocean, and as that cooled down, some light-coloured crystals, a mineral called feldspar, was able to rise, bob towards the surface because it was buoyant. And that's the bright lunar highlands that were later flooded by lava flows. On Mercury...

Mercury's rocky part is deficient in iron. All the iron's gone into the core. And that meant that Mercury's magma ocean was lower in density than the moon's magma ocean. So when these feldspar crystals crystallized out of the magma ocean, they couldn't float. They sank. And the only thing that would float on Mercury that we can think of is graphite carbon. So Mercury's dark because of carbon that rose towards the surface there.

It's fascinating how these properties get blended together in different proportions. To me as a geologist, you should never look at just one planet. You should always consider the others as well. Emma, with what's been learned since MESSENGER, what can you say about Mercury's magnetosphere and its magnetic field?

The first thing that Messenger found about Mercury's internally driven magnetic field, which is unusual, is that it is offset towards the north. So you would normally expect the magnetic field to be generated sort of centrally in the core of the planet. But for Mercury, the data suggested that the field is actually the centre of the field is offset by 20% of the

of the radius of Mercury. So that's unusual and again leads to some scratching of heads around how that magnetic field is being generated in the core. The second thing that MESSENGER was able to find by spending four years in orbit is just really to measure the magnetosphere and how it behaves over time. And what was found is that Mercury's magnetosphere is extremely dynamic.

In some ways, perhaps this is no huge surprise. Mercury is very close to the sun compared to the Earth, and therefore the solar wind and the interplanetary magnetic field is much more intense. And Mercury is sitting in a strongly varying pressure of solar wind and also strength of magnetic field. And those are important factors for the magnetosphere.

So a key process that occurs in the Earth's magnetosphere is as a cycle of opening and closing of magnetic fields of the planet and connecting out into the solar wind. And that circulation at the Earth takes about 10 hours, but at Mercury it takes one minute. So it's a sort of extreme version of driving of the magnetosphere.

And then the other thing that was also found was that this large iron core actually also plays a really interesting role. So what Messinger discovered was that as the solar wind gets stronger in intensity during, for an example, the passage of a coronal mass ejection from the sun...

As that pressure pushes on the magnetosphere, it actually generates through electromagnetic induction an additional magnetic field within the core of the planet that pushes back and holds off the solar wind. Thank you. Carolyn, there may be water on Mercury, we understand. Why might that be and what does it mean? Well, first of all, it's interesting there's water ice seen on Mercury at all. I mean, there were indications of this from radar signals, but it was...

really confirmed when MESSENGER was in orbit around Mercury. And the thing about Mercury's rotation is that the axis it rotates about is perpendicular to its orbit. So it's not tilted like the Earth. It doesn't have seasons. And that means you can have regions around the poles, and here we're talking mainly about the deep craters around the North Pole, that are in perpetual shadow. They never get that blast of sunlight around.

And it's quite possible that you can have water ice, despite this being an enormously hot surface to the planet, that you've got these sheltered regions, deep north polar craters, where it has been, we don't know how long it's been then, perhaps it was delivered by ice-rich asteroids and comets during the early heavy bombardment period, so billions of years ago.

It's just curious that it's there at all. Does it lead to anything? Well, it's another strange fact about Mercury. In terms of going there, it's hard enough getting, as Dave said earlier, it's hard enough getting spacecraft to function in that hostile environment. The last thing you do is send any people there. Dave, what do we learn from the hollows that seem to cover the face of Mercury? These great craters we've just talked about, one of the biggest. What's going on there?

Well, the hollows are another part of a volatile story. You shouldn't really think of them as craters. Hollows are areas on the floors, mostly, of craters. When we flew past with Messenger, before we got into orbit, we saw these bright regions on crater floors. When we got into orbit, we had enough detail to see that, actually, in these bright regions, what you've got in detail is steep-sided, flat-bottomed depressions. They're only about 10 or 20 metres deep and hundreds of metres across,

where the surface level has just dropped down, material's been eaten away, and they're surrounded by these bright halos, which is why we could see them from a distance. And it's as if the top 10 or 20 metres of the surface has just dissipated away into space somehow, and that's very hard to explain.

You can't turn rock into vapour without breaking all the chemical bonds and letting it drift away element by element, atom by atom. So what happened? Well, it seems to happen in lower latitudes rather than pole latitudes. It's slightly concentrated at areas where it's noon when Mercury's at its closest to the sun. So there's a link to temperature or sunlight. It's only a weak link, though. So the contenders are it's just heat from the sun...

It's the sun's radiation. Ultraviolet photons can break chemical bonds. Or it's the solar wind. Now, as Emma said, the solar wind is held off by Mercury's magnetosphere. But in times of solar storm, it is depressed down to the surface. The magnetopause is down below the surface. And the solar wind can impinge upon the surface, sometimes we think. So it could be charged particles from the sun breaking chemical bonds.

or it could be micrometeorites because even tiny objects don't get slowed down as they arrive at Mercury because there's no atmosphere. So there's all kinds of ways where you could break chemical bonds and let stuff drift away. But we still don't understand that the whole top 10 or 20 metres...

has gone without leaving a residue behind. It's very mysterious. We think hollows are still growing today. This is the active geological process on Mercury. The slow recession of these cliff-like edges of these hollows at rates of a millimetre per thousand years or something. But it's still going on today.

Thank you. Emma, there are X-rays coming from the dark side of Mercury. What ideas does that send running? Dave mentioned earlier some measurements that MESSENGER made from the X-ray spectrometer, which help us to understand the composition of the sunlit side of the planet. So that's where we would expect to see an X-ray emission. And that's a process that we can use to work out the elemental composition of the surface of a body that's illuminated by the sun.

On the dark side of the planet, though, you would not necessarily expect to see an X-ray emission coming from the planet. However, MESSENGER measured X-rays coming from the surface of Mercury. And when we looked at all of those X-ray emissions from the surface at the end of that mission and put them all together in a map...

we found that the X-ray emissions were coming from bands in the northern and southern hemisphere that were centred on that offset magnetic field that I mentioned earlier. So bands of X-ray emission that appear to be in the north and in the south, but shifted towards the north, just like the magnetic field is. So that tells us that...

that that X-ray emission must be organised by the magnetosphere and by Mercury's magnetic field. So it's likely to be the result of charged particles, probably electrons, impacting the surface and producing X-rays that MESSENGER has measured. Now, this is interesting because...

if we think about the Earth, when the magnetosphere interacts with our planet, it interacts with the top layers of the atmosphere, the ionized part of our atmosphere known as the ionosphere.

And charged particles from the magnetosphere move in electric currents that run parallel to magnetic field lines and accelerate charged particles into the atmosphere and cause it to glow. And that we know as the aurora. But we don't have an atmosphere at Mercury. We don't have an ionosphere. So the particles, it seems, are interacting directly with the surface of Mercury and producing this strange energy

X-ray emission, which I can only describe as auroral-like. I wouldn't describe it as an aurora because there is no atmosphere, but this tells us something quite unexpected.

quite unusual in the solar system, about a magnetosphere interacting directly with the surface of the planet. Thank you. Dave, out there at the moment is the BepiColombo mission. What is that and why is it so significant? BepiColombo is a joint European Space Agency, Japanese Space Agency mission to Mercury.

It's had three flybys of Mercury already, one more coming up in September, two more flybys at the end of the year. And then the next time we come to Mercury, we'll be going slowly enough to get into orbit. And we separate the Japanese orbiter flies free, the European orbiter flies free, and we can really start doing science. There's a British-led instrument on there, which Emma's a principal investigator, which will

look at the x-rays, we'll get the elemental abundances at the surface. I said we don't know what the volatiles are in the hollows or what the volatiles are in the explosive volcanic vents. We don't know what minerals these elements are bonded in. We'll get that from the thermal infrared spectrometer and the visible light spectrometers. We'll get compositions of the surface and stereoscopic images and more detailed finer resolution images. We'll really...

really understand the geology of Mercury and its compositions much, much better. And we'll understand the magnetosphere. I mean, Emma, you've got wonderful things for magnetosphere.

Yeah, I think one of the things to say about BepiColombo in terms of the progression of our exploration, we go from flyby mission 50 years ago to the first orbiter messenger between 2011 and 2015. And now BepiColombo, as Dave's just described, is a two spacecraft mission. So this allows two vantage points online.

within the system, one orbiting close to Mercury, one in a larger elliptical orbit. So there's no compromise between surface measurements, which we'll be able to get equally in the north and south of the planets on the planet's surface.

and a spacecraft that is dedicated to studying the wider space environment and magnetosphere and that solar-wind interaction. So we expect our knowledge to hugely increase in terms of using those two spacecraft together...

to study the solar wind and magnetosphere interaction and those induction magnetic fields that we see when there's strong solar wind driving of the system. There's never been a spacecraft measuring the magnetic field of a planet in two places at the same time before. I'm happy we'll be absolutely groundbreaking with that, yeah. Carolyn, what do you hope will come from this?

This mission is going to just expand on a lot of the questions we have, that we've alluded to in this programme. Did it have this chaotic event early in its formation, or is it a by-product of where it was in the solar nebula, right at the beginning of the formation?

of the solar system. And what Dave said is right. You never learn anything from just studying one planet. You need different types of planets to look at, to contrast with. And Mercury is certainly a case of extremes that will allow us to understand our own Earth-Moon system and other planets in the solar system better. Emma?

I'm personally looking forward to the fact that we have built an instrument which is on the payload. It's the only UK instrument called the Mercury Imaging X-ray Spectrometer. And we're going to be able to make a significant contribution to understanding both the geochemistry of Mercury, working out what that surface is made of in the greatest detail that we've been able to see, because it is an imaging instrument specifically.

And we're also going to be able to look at this nightside interaction between the magnetosphere and the surface and these auroral-like X-ray missions. So there's a lot of exciting data to come from that mission. Well, thank you all very much. Thanks, Emma Bunce, Dave Rothery and Carolyn Crawford. And our studio engineer, Bob Nettles. Next week, it's Thomas Wyatt.

an ambassador for Henry VIII and alleged lover of Anne Boleyn and sometimes called the father of English poetry. Thank you for listening. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests. What I'd like to do in the next section is to ask what you would like to have said you didn't have time to say. Would you like to kick off there, Caroline?

Well, one thing that would be nice to say is the name BepiColombo of this mission is actually named for an Italian scientist, Giuseppe Colombo, who in 1970 was the person who first showed it was feasible to reach the inner planets using this gravitational assist mechanism that Emma described.

And this was first proven to be the case for Mariner 10. So it's sort of celebrating his achievement because the difficulty is to break the spacecraft enough in order it to fall in but not get captured by the sun is...

If you didn't do this gravitational assist, it would require an awful lot of fuel. So this made a very economical way of sending missions into the inner solar system that worked. And not only getting to Mercury, but having repeated flybys of Mercury. Jeff Colombo also said, if you do this right, once your spacecraft has gone round the sun once, Mercury will have gone round twice and you can meet it again and get flyby after flyby. And that was great. And did you know that...

The messenger trajectory to Mercury was designed with similar cunning by Chen Wang Fen from JPL, who said what you need to do to get into orbit around Mercury is have repeated flybys of Mercury. And first of all, you'd have the same kind of relationship with Mercury that Mariner did, where the spacecraft goes around the sun once and Mercury goes around twice. That's a two to one resonance.

when you slow down into a 3-to-2 resonance, then a 4-to-3 resonance. But eventually, when you creep up on Mercury, the relative speed difference is so slight that you can easily get into orbit. And that's what MESSENGER did, and it's kind of what EPICOLUMBO is doing as well. It's got an ion drive helping it slow down a lot of the time as well. But Chen Wanfeng is in the position of Giuseppe Columbo in designing trajectories which get you there cheaply. Because...

You can't fill the spacecraft with rocket fuel and screech to a halt. If you do that, you've got no mass for your scientific payload and you won't learn anything. It is fascinating that to get to a planet that is so relatively close to us in the solar system, it's actually a very complicated process to do it successfully. Is that because of the separate speeds involved?

It's because you have to, I mean, you've got Earth going around at 30 kilometres per second. You have to lose that orbital velocity. So you're breaking the spacecraft and then speeding it up whilst also manoeuvring it into the right position. It's a very complex trick. And of course, you've got the sun at the centre and the gravitational pull of the sun is overwhelming when you get close to Mercury. So...

So that's a real challenge. But it is a surprise, isn't it, that it takes approximately the same time for BepiColombo to get to Mercury as it is going to take juice to get to Jupiter-1.

Why is it taking so long?

So the BepiColombo was launched in 2018 and it has a seven and a half year journey to the inner solar system. If it went in a perfectly straight line to go between the Earth and Mercury, it would take just a few months to get there. Trouble is, it wouldn't stop.

And it would just keep going and be accelerated into the sun, which would be very disappointing for everybody involved. So what you have to do is actually send your spacecraft on a bit of a circuitous route around the solar system multiple times...

doing actual gravity assist flybys of the Earth, then Venus and then Mercury six times, actually using those bodies and their motion about the sun to slow the spacecraft down enough that it can get captured, weakly captured,

into orbit around the smallest planet in the solar system, which is very close to the star within our solar system. So it's extremely challenging. We are going to have a lot of new features on Mercury that are going to need names. I mean, we're making geological maps of Mercury at the moment across the whole of Europe using messenger data to get the best geological maps we can to set the context for what BepiColombo will see. But

There are a lot of places that are going to need names. Now, this is all controlled by the International Astronomical Union. Craters, for example, are named after artists, painters, musicians, people that have made their name in the arts.

And a lot of the names were given for half the planet when Mariner 10 flew by and then the rest, the other half of the globe in the past 15 years since Messenger. But if you look at the statistics, they're mostly cis white males. There's only less than 20% of the creators on Mercury named after female artists, painters and musicians. So

We'll have to try to redress the balance there. I've got a PhD student that's taken it upon herself to look into that and has taken the IAU to task. But naming is a serious business because we want people to feel included. We want to see that they're represented out there.

Do you think that learning more about Mercury gives you access to a wider view about the creation and development of the universe? It certainly gives you a more flexible view about the kind of planets that are out there. I mean, we're now discovering planets around other stars in their hundreds, in their thousands, and there is a huge diversity in them. And actually understanding some of the diversity just within our own solar system is a stepping stone to understanding

really appreciating what kind of variations there are in planets out there. Lots of the exoplanets, planets round other stars that we've been discovering over the past couple of decades, are really close to their stars, even closer than Mercury is. But nonetheless, Mercury is the closest example we have to a star, so we can learn quite a lot by studying it.

I mean, we've got planets with atmospheres made of rock, basically, which are really, really close to the sun. But Mercury is the best analogue we've got. And the easiest to study. The easiest to study. Yeah, I always think the nice thing about the solar system is that you actually can go to your target of interest and make observations in situ rather than only being able to make observations remotely. So...

whilst we are in a solar system which could be completely unique and different to every other planetary system out there, it is the one that we're in and therefore the one that we can use as a scientific laboratory to study all of these interesting different things that we've been talking about. How likely do you think it is that there are other planets

systems like the solar system around this multi-million representation of stars etc that you've alluded to we're finding earth like planets in terms of mass aren't we now the solar system doesn't have water worlds bodies four or five times earth's mass doesn't have many neptunes but

We've found there's quite a diversity of planetary systems. Most planetary systems are around red dwarfs, aren't they? There are an awful lot of them around red dwarfs. And a lot of the exoplanets we find look nothing like ones we've got on our solar system. But there is a bias because up to...

Up to now, it's been easier to find the bigger, more massive planets that are closer to their sun. So we were biased to planetary systems that didn't look anything like our own, just by our detection methods. And I think now the modern tranche of instruments are going to start finding smaller objects, more Earth-like objects, maybe even one day Mercury-sized objects, and get a bit more of a representation of the range of stuff out there.

How do you cope with the idea that what's out there might be infinite? I don't think about it very often. That's one way to cope. It's just some, well, maybe you two have a different view, but it is something you just take on board and accept and

If you're a scientist, you learn not to think in linear terms, but in logarithmic terms. And I think that is one way of appreciating the scales involved. When we're looking for exoplanets, we are really restricting observations to quite a small part of our own galaxy lengthwise.

let alone the whole galaxy, let alone all the other galaxies out there. So planetary scientists don't have to grapple with the infinite in quite the same way that other kinds of astronomers do.

Dave, do you think it's possible now or ever to land on Mercury? Oh, eventually it will be. We've talked about how difficult it is to get to Mercury travelling slowly enough to get into orbit about Mercury. If you want to land, you've got to touch the surface of Mercury travelling now more than two or three metres per second when you get there, so that's even more slowing down.

So it's difficult to do. You've got to be very careful with it and carry plenty of fuel to retro rockets to slow you down. And you've also got to land in the right spot. If you land on Mercury in the daytime, the spacecraft's going to fry because you're touching a surface at 400 degrees and you can't dump your heat. It's hard enough to keep cool when you're in orbit. When you're on the surface, it's worse.

There are desk studies being made of how to get a lander onto Mercury and choosing where to land, maybe on Mercury's most ancient crust. If you were to land at sunset, you would have 88 Earth days of darkness before the sun rose and it then became very hot. So what you need to do is have a spacecraft that can land

operate during the Mercury night, keep itself warm with battery power because the temperature will drop to well below minus 100 centigrade by night on Mercury. It's easier to keep yourself warm than keep yourself cool. So you've got three months that you can work on Mercury during one more Mercurian night before the sun rises and cooks your spacecraft.

So choose your spot carefully and choose a science that you can get done in three months before you die. I was going to say, another complication, of course, is that Mercury doesn't have an atmosphere. So when you think of us landing probes on Mars, which becomes quite commonplace nowadays, the atmosphere plays an important braking effect and slowing the spacecraft down so it can land on the surface. You can't use parachutes to land on Mercury. No.

Simon Tillotson, the producer, is about to make his entrance. Does anyone want tea or coffee? Tea, please. Tea, please. Three teas, my love. Three teas, yeah. I'll have a tea, yes, please. Thank you very much. Thank you. In Our Time with Melvin Bragg is produced by Simon Tillotson and it's a BBC Studios audio production. I'm Helen Lewis and I have a question.

What links family WhatsApp dramas... I flounced off after someone made a particularly ignorant comment. ...Russian state propaganda... It's a very good platform for spreading all this pro-Putin position. ...and a woman who married an AI. 100% I would never go back to humans ever, ever again. No idea? Well, they're all examples of how instant messaging has changed the world. Find out more by joining me for my new BBC Radio 4 series. Helen Lewis has left the chat.

Subscribe to Helen Lewis has left the chat on BBC Sounds. Yoga is more than just exercise. It's the spiritual practice that millions swear by.

And in 2017, Miranda, a university tutor from London, joins a yoga school that promises profound transformation. It felt a really safe and welcoming space. After the yoga classes, I felt amazing. But soon, that calm, welcoming atmosphere leads to something far darker, a journey that leads to allegations of grooming, trafficking and exploitation across international borders.

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And it's done so skillfully that you don't realize. And it's like this, the secret that's there. I wanted to believe that, you know, that...

Whatever they were doing, even if it seemed gross to me, was for some spiritual reason that I couldn't yet understand. Revealing the hidden secrets of a global yoga network. I feel that I have no other choice. The only thing I can do is to speak about this and to put my reputation and everything else on the line. I want truth and justice.

And for other people to not be hurt, for things to be different in the future. To bring it into the light and almost alchemise some of that evil stuff that went on and take back the power. World of Secrets, Season 6, The Bad Guru. Listen wherever you get your podcasts.