David Rothery: exploring Mercury

31 March 2020

Interview with 

David Rothery, Open University

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Volcano expert and professor of planetary geosciences at The Open University, David Rothery tells Chris Smith about his work, first off, how he first became involved in the BepiColombo mission...

Chris - Now. David, you told us some interesting insights into the BepiColombo mission and Mercury at the beginning. So chance now to dwell it more on that theme. So first of all, tell us how you got involved in that, because it's not everyone who gets to send a space probe to another planet.

David - Oh, I was invited to go to Paris to make the geological case for going to Mercury in 1997, because the guy who was meant to go, couldn't make it, and I got roped in. And I was thinking, Mercury's a boring place, but hey, we've only had one spacecraft there, so we really ought to go and take another look. Since then, we've had NASA's mission Messenger, which orbited Mercury and produced wonderful data. And we now know Mercury is a planet which just doesn't fit. It's the closest one to the Sun. It's rocky, but it's got lots of volatile substances there. We don't know what they all are. We've measured a lot of sulphur, which is a surprise. You wouldn't think that close to the sun sulphur would condense, but there's all these visible signs on the surface as well. There have been exploding volcanoes, where something in the magma was turning into gas and blasting great holes in the surface. We don't know what that substance is that's available to turn to gas in volcanic eruptions.

Chris - What is it actually like on Mercury though? If we were to go there, not that we could, but if we were to go there, what would it be like.

David - If you were standing on the surface of Mercury, it would look like standing on the surface of the Moon. It's airless, it's bare rock, but most of the rock is broken up to powder by continual bombardment, by meteorites hitting the surface. So it's very powdery. You might find a few fist sized lumps of rock, but no cliffs of solid rock. The outer part is all very powdery, but it's not so badly mixed that there are no compositional variations. We do know the composition varies from place to place, and it's been resurfaced by lava flows. And then what's happening today in some places, is either the heat or the sunlight or the solar wind is attacking the surface and turning some of it to vapour, because we've got steep sided, flat bottom depressions, 20 meters deep, that's all widening, with cliffs at their edges, getting wider with time taking the top 20 meters of the surface away, and we don't know how it's happening. We don't know what's being lost.

Chris - What's the temperature, how hot is it there? If I had a thermometer, what would it read?

David - Noontime, about 400 degrees. By night about minus 150. So there's a quite wide temperature range. But that's not enough to boil rock.

Chris - When you say night time, is it turning like Earth is or not?

David - It is turning. It rotates on its axis three times for every two orbits it makes round the Sun.

Chris - So the day is longer than the year?

David - Three spins for two orbits actually makes the day, measured from the Sun, twice as long as each year. So sunrise to sunrise is two Mercury years, which is 176 earth days. So we've got wild temperature extremes, slow spin, and weird things happening on the surface, which we do not understand. But we'll measure much better when BepiColombo gets there. We hope.

Chris - Yeah. Yeah. After 15 years in space, I'm sorry...

David - Well, it's about a nine year cruise and then at least a year working in orbit.

Chris - And then you hope it wakes up at the end of that because that's obviously a risk, isn't it? When the probe is mobile through space, of course, you hope it's going to work when it gets there, because you've got no way to control it.

David - It hasn't been turned off during cruise, the instruments are being tested. When it comes back past the Earth. In mid April, we'll run some experiments and the two Venus fly-bys, we'll collect data. But because it's two spacecraft traveling together, most of the instruments can't see the sky properly, so we won't be operating in full operational mode when we fly by Venus, or when we fly by Mercury several times. It's only when we get into orbit and separate the spacecraft and point instruments down to the ground, that we start getting our real science, but we do know it's working.

Chris - Has it got batteries or solar panels? How are you powering it?

David - Solar panels. Panels to power it when we're in orbit around Mercury, and while it's going to Mercury powered by the ion drive, it's solar electric propulsion. We've got seven metre long solar panels either side of the spacecraft. And blimey, that was tense when we got into orbit and unfurled them, I was thinking if they don't open out, we're in trouble. But they did. We collect electricity from sunlight, use that to ionise xenon gas, which has vented out through the exhaust very fast, and that's our ion drives, xenon ions. It's basically the Starfleet impulse drive.

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