MERCURY MISSION: “I’M GLAD WE’RE GOING BACK!”
BepiColombo is shooting for Mercury. Professor David Rothery, a key member of the science team, reveals what’s in the pipleline and what we’ve got to look forward to ahead of its rendezvous with the swift planet in 2025
What will you be doing in preparation ahead of BepiColombo’s arrival at Mercury in 2025?
My role on the MIXS team is to help prepare to make the best observations that we can. MIXS’ role towards understanding Mercury’s surface and composition is to map all the chemical elements it can, and there are some targets which MIXS will try to stare at, making sure it’s collecting observations when it’s over these critical targets where we think the composition is unusual. MIXS will map the whole planet, but some small areas need special attention – places where there have been volcanic explosions, or regions called ‘hollows’, which is where the surface is dissipating away to space. At the present day we think the surface is rotting away. We have very limited compositional measurements of these features from the previous mission, MESSENGER. [BepiColombo] will get much better spatial resolution with the X-ray spectrometer then we’ve achieved before. We’re going to make sure we know in advance where these are and build them into the data-collection schedule for arrival to be sure that it’s collecting data when it’s over the right targets. I’m making sure that MIXS will collect data over the most important regions. I’m also concerned with making sure that we understand the context of the observations we make.
What is the surface of Mercury like?
Mercury is covered in craters, volcanic planes and holes ripped in the ground by violent volcanic explosions, violent events and more passive processes where the surface is just dissipating away somehow. We need to get this mapped. Half the planet was mapped in the late-1970s by the Mariner 10 mission, but we’ve got something that looks better than black-and-white imagery. We’ve now got a whole planet as seen by NASA’s MESSENGER. But the MESSENGER team didn’t undertake the job of making new geological maps of the planet. With our mission coming up, we’re using the MESSENGER data to make geological maps because we want to understand the context of all our observations.
What do these maps look like?
There are geological maps, or what we call a ‘morphostratigraphy map’. The pale-brown areas indicate the smoothest plains, and they’re smooth because they’re young and there hasn’t been time for many craters to accumulate. The mid-brown and the dark brown are progressively older classes of surface. The older they are, the rougher they are, because more craters have occurred. Then you map the individual craters more than 20 kilometres (12 miles) in size and distinguish them by age. The youngest craters sometimes have rays coming out from them and the ejecta is very prominent; you can see the burying of the old terrain and the craters look very fresh. The older craters are partly buried by younger events and they’re more degraded with age. We’ve also broken it down to five degradation classes to try and work out the age relationships.
There have been topographic maps that have just come out from the stereo imaging or the laser altimetry that’s been done. But here [at the OU] we’re interpreting it as a geologist and identifying different surfaces and then saying which is older than which – let’s get the layering worked out or the sequence of events worked out. When you have a geological map like that with the relative ages of surfaces, and then you get a new mission there and it says, “Hey! Over this area here it’s high sulphur and low calcium, but over there it’s the other way around,” we can go straight to the map and say, “Wow! So that’s the oldest stuff that’s high
sulphur and low calcium.” We can understand our observations because we’ve already done the best geological mapping we can.
How does the presence of craters on Mercury relate to the planet’s age or past volcanic activity?
The craters we’re dealing with are usually impact craters, and you can recognise them as they’re almost exactly circular, produced by the shock of an impact. This is because impacts occur at Mercury at tens of kilometres per second. That shockwave radiates out from the point of impact, which is almost always circular with circular ejected patterns. The surface that’s been there for 4 billion years is going to be absolutely covered in craters.
You can see that a surface that has a small number of craters superimposed is younger. But on some of these larger plains you can see the older craters that were flooded by the lava still visible, because they were flooded but not buried at such depths that all trace has been lost. You can see these ghost craters where there are traces of circular features.
And then volcanically, there was also a series of explosive volcanic eruptions where holes were ripped in the ground. For example, there’s a 30-kilometre (19-mile) wide, three-kilometre (1.9-mile) deep hole that was ripped out. Probably not in a single eruption, but a series of eruptions. And each time it went boom, it flung out a plume of material which just fell back to the surface. There’s no atmosphere on Mercury, so it didn’t start convecting like a volcanic eruption on Earth and just fell back down, and that’s given us this 200- or 300-kilometre (120- or 190-mile) wide explosive eruption deposit on the surface.
You can work out the age of events, because the hole was ripped through material of a certain age, and the deposit is sitting on top of some of these young lava flows. To work out the age of events, it’s a detective story. You see what’s on top of what, what has ripped a hole through something else. There are geological faults on Mercury where parts of the surface have been thrust over features.
How does understanding Mercury help us understand Earth?
We’ve got four rocky planets in the Solar System: Mercury, Venus, Earth and Mars. They’ve all got factors in common. I mean, they’ve all got cores, which is where the dense, iron-rich material has gone, and they’re surrounded by rock. But only Mercury and Earth have cores that are molten, and we know that because the Earth and Mercury have magnetic fields. You’ve got motion churning around, this electrically conductive fluid, which is what you need to generate a magnetic field. But it doesn’t happen inside Mars or Venus. Why? We’re not sure. Venus is almost the same size as Earth, and its core, as far as we can tell, is about the same size as Earth’s core. It’s completely solid; the core is not in motion. There is something strange going on there, but we don’t understand why some planets generate their own magnetic fields and some don’t.
Then there is how the surface behaves. Earth is the only rocky planet to have a rigid outer shell that can slide around on a very weak interior, hence plate tectonics. For example, the Atlantic Ocean is
getting wider and the floor of the Pacific Ocean is being pushed below South America. That kind of thing doesn’t go on with any other rocky planet.
Each rocky planet has some features in common, but each has a very different history. We need to compare them. If we wanted to understand trees and how they work, you wouldn’t just look at an oak tree. You need to look at a fir tree and a palm tree as well so you can understand how they all work. It’s the same with the planets. You’re not going to understand Earth if you don’t understand how and why each of the three comparison bodies is different.
Would it be too simplistic to say that some features on Mercury were formed in a similar way to plate tectonics?
It’s not that similar because in plate tectonics, when India crashed into Asia and started to go under it, it kept going under indefinitely, throwing up the Himalayas. Here the displacement is only five kilometres [three miles] or so. The faults on Mercury move a little way and then they grind to a halt, because Mercury’s surface is not mobile. On Earth, plates are sliding around continuously. The features on Earth [move around a lot] because the planet is cooling down, and therefore contracting thermally. It’s the effects of thermal contraction that’s driving the thrust movement at the surface.
You’re not going to get 1,000 kilometres [621 miles] worth of displacement [on Mercury], one slab over another, because the planet is not shrinking now. The shrinkage of Earth doesn’t really get manifested because it’s completely outpaced, but the whole idea of plates moving around and some sliding underneath each other, it’s because of tectonic processes. It’s the same kind of fault displacement [on Mercury], except the total amount of movement on it is much less.
Why have there only been two missions to Mercury so far?
I guess it didn’t look that exciting. When Mariner 10 went there in the 1970s, it looked a bit Moon-like because it’s covered in craters, but there are very important differences between Mercury and the Moon. However, it’s less exciting than Mars, where it’s had water flowing on the surface and there might be life, and less exciting than Venus, which is the Earth’s twin except it evolved very differently.
Although you can fly past Mercury relatively easily, if you wanted to send a spacecraft and whiz by it, once you’ve gone by you’ve gone by. The tricky thing is to get captured into orbit. That’s very difficult at Mercury, because the changing velocity you need to be captured into orbit is very, very great. This is because if you send a spacecraft towards Mercury, you’re basically falling towards the Sun, and the Sun’s gravity will grab a hold of you and make you go faster and faster and faster, so when you get to Mercury, you’re going far too fast to stop. It would just swing by the planet and disappear.
The trick that MESSENGER did, and the trick that BepiColombo will do, is to find a way to slow down so you get to Mercury going slowly enough to be captured into orbit. Both space probes had Venus flybys and then a series of Mercury flybys, and each flyby slows you down. We’ve got six or seven flybys for BepiColombo. On the seventh or eighth approach to Mercury, the spacecraft is going slowly enough that it can just use a small amount of fuel and get captured into orbit.
How have these previous missions shaped the science goals of BepiColombo?
I think Mercury has turned out to be far more interesting than we suspected before MESSENGER got there. For example, the explosive volcanism we didn’t expect, or the recent hollows, places where the surface is patchy and stuff has been dissipating away into space. These regions dissipate at a very slow rate, millimetres per thousand years or something, but that’s still young in terms of the Solar System. These are actively growing patches, but we don’t know why. We don’t know what’s being lost, except it’s got to be volatiles somehow. But is it sublimation like when you warm up dry ice and it just turns to vapour? Or is it breaking chemical bonds? Photons can break chemical bonds, micrometeorites can break chemical bonds and charged particles from solar wind can break chemical bonds. Any of those three could be attacking the surface and just helping it strip away to space, and that could be the source of some of the sodium in Mercury’s exosphere.
The science goals of BepiColombo have been rewritten. We still want to understand how Mercury formed and so on. How it got so much iron, but has so little rock. And now we want to know how you couple that with being rich in volatiles. What are the volatiles that are being lost, and how has it hung on to them? Especially as these volatiles seem to have lost so much rock to give it a small amount of rock on a large core. It’s a totally puzzling planet and I’m glad we’re going!