What’s new at Mercury?
Scientists are still busy analysing data from the MESSENGER mission – and it has plenty of surprises
MESSENGER might be long gone, but the results are still pouring in
Which planet is Earth’s nearest neighbour in the Solar System? The obvious answer is Venus, which makes the closest approach to us – but it spends half its orbit on the other side of the Sun, when it’s further away from us than Mercury. It was only last year that Tom Stockman, a graduate research assistant at Los Alamos National Laboratory, New Mexico, and colleagues crunched the numbers to work out which planet is actually closest on average – and they were as surprised as anyone by the answer: “When averaged over time, Earth’s nearest neighbour is in fact Mercury,” they wrote.
Despite its proximity, Mercury has always been a mysterious planet due to the difficulty of observing it through Earth-based telescopes. That’s down to a combination of its small size and the fact that it never gets very far from the Sun in the sky. The only time it makes a really spectacular sight, in fact, is when it passes directly in front of the Sun during a transit of Mercury – like the one that took place a few months ago in November 2019.
If Mercury is a difficult planet to observe from Earth, it’s not an easy destination for spacecraft either. That’s partly because a spacecraft speeds up under the effect of gravity as it falls towards the Sun – and then its rocket engine has to work hard to lose that excess speed when it gets to Mercury.
Another problem is the extreme heat – well over 400 degrees Celsius (752 degrees Fahrenheit) – in the vicinity of Mercury, which poses serious challenges for spacecraft designers. Since the dawn of the space age, only two space probes have been to Mercury – Mariner 10 in the 1970s, followed by MESSENGER more recently, both by NASA.
The photographs sent back by Mariner 10, which made three close passes of Mercury in 1974 and 1975, revealed a desolate, crater-studded landscape that looks a lot like our Moon. But the mission had a surprise for scientists in its discovery of a welldefined magnetic field around the planet. It’s a hundred times weaker than Earth’s field, but Venus and Mars don’t have internal magnetic fields at all, and Mercury wasn’t expected to either.
A closer look at Mercury’s magnetic field was one of the key objectives of NASA’s follow-up mission, MESSENGER, which entered orbit around the planet in March 2011. It remained there until it ran out of manoeuvring propellant four years later – and in the final few months the mission controllers got increasingly bold. They dipped the spacecraft to just 15 kilometres (9.3 miles) above the planet, allowing them to measure relic magnetism in the surface rocks. “The signals we detected are really small, and very hard to measure,” explains planetary geophysicist Catherine Johnson. “We’d never have been able to measure them if not for these really
risky low-altitude observations in the last few months of the MESSENGER mission.”
The measurements indicated that not only is Mercury’s magnetism very old – going back at least 3.8 billion years – but it was much stronger in the past, “comparable to the strength of Earth’s magnetic field today,” according to Johnson.
When all its propellant was used up, MESSENGER was deliberately crashed into Mercury’s surface – the first Earth-made artefact on the planet – on 30 April 2015. From NASA’s point of view the mission was a huge success, repeatedly surprising researchers with its discoveries. “In the end, most of what we considered to be gospel about Mercury turned out to be a little different than we thought,” as mission scientist William McClintock said at the time.
“We’d never have been able to measure them if not for these really risky low-altitude observations”
Catherine Johnson
Among MESSENGER’s biggest surprises was the discovery of water ice on Mercury. This seems odd, given the extremely high surface temperatures in direct sunlight. But there are spots inside some of the craters near the planet’s poles that never see the Sun at all, and consequently the temperature there goes to the opposite extreme – closer to minus 200 degrees Celsius (minus 328 degrees Fahrenheit).
There had been hints of ice inside these permanently shadowed craters as long ago as the 1990s, when radar measurements indicated the presence of highly reflective material. The issue wasn’t clinched, however, until MESSENGER detected hydrogen atoms in the same locations. “Water ice is the only candidate we’ve got that fits all those observations,” as principal investigator Sean Solomon explained. It’s not just a few traces of ice, either – according to Solomon it’s “enough ice to encase Washington DC in a frozen block two-and-a-half miles deep”.
Another unexpected finding was a brand-new type of surface feature that isn’t seen anywhere else in the Solar System except Mercury. Called ‘hollows’, these are shallow depressions found inside many of the planet’s craters. They’re believed to have formed when volatile components in the surface material evaporated, causing the remaining material to collapse. “The hollows are one of the most viscerally interesting discoveries from the mission,” according to another MESSENGER scientist, Steve Hauck. “They were completely unexpected – a new landform, and one that appears to form by loss of rock to space.”
The idea that volatile materials boiled off Mercury in the distant past seems reasonable enough, given its hot location close to the Sun. What would be more surprising would be to find volatile materials still on the planet’s surface today – and yet that’s exactly what MESSENGER did find. It’s a subject Brian Cox talked about in his TV series The Planets last year. “The discovery of relatively large concentrations of elements like sulphur and potassium on Mercury’s surface was a huge surprise,” he said. “So Mercury is an enigma, and discoveries like these force us to completely
“Most of what we considered to be gospel about Mercury turned out to be a little different than we thought”
William McClintock
rethink our theories about the formation of the planet.”
The problem with this new discovery was that it didn’t fit in with existing ideas about Mercury’s origin, which had been developed over the years to explain its unusual internal structure. Like Earth, the planet is made up of a rocky crust and mantle surrounding an iron-rich core – but in Mercury’s case the core is huge, making up almost 85 per cent of its volume. For a long time it was assumed that Mercury must have started out looking very similar to Earth in size and composition. Then, billions of years ago, it had its outer layers knocked off in a collision with a huge asteroid.
The problem with that theory is it doesn’t explain why there’s so much sulphur and potassium on Mercury’s surface today. It now seems likely that Mercury formed much as it is now, and that its large core was a consequence of the different physical conditions in the inner parts of the early Solar System compared with further out where the other planets formed. That’s supported by the discovery of exoplanets orbiting close to other Sunlike stars, which also appear to have large metallic cores like Mercury.
Despite its numerous discoveries, MESSENGER left plenty of unanswered questions – but fortunately there’s another mission, BepiColombo, on its way to Mercury right now. Made up of two separate spacecraft, the ESA’s Mercury Planetary Orbiter and the Japan Aerospace Exploration Agency’s Mercury Magnetospheric Orbiter, it’s due to arrive in 2025. With 16 scientific instruments, researchers are hoping BepiColombo will make just as many discoveries as MESSENGER did.
“The discovery of relatively large concentrations of elements like sulphur and potassium on Mercury’s surface was a huge surprise”
Brian cox