Tiny and relatively ignored, Mercury holds outsize mysteries. Only two spacecraft have made the difficult journey to its sunbaked environs. Now comes the planet's third and most ambitious visitor, the European-Japanese mission BepiColombo, a pair of probes due to launch on 20 October. Picking up where the last visitor, NASA's MESSENGER mission, left off in 2015, BepiColombo will probe puzzles including Mercury's skewed magnetic field, its overstuffed iron core, and strange lakelike depressions perhaps carved by escaping volatile elements.
"MESSENGER really threw into question many theories about how this planet came to be," says BepiColombo team member Emma Bunce of the University of Leicester in the United Kingdom. BepiColombo "is perfectly timed and set up to answer these questions," says mission scientist Johannes Benkhoff of the European Space Agency's (ESA's) technology center in Noordwijk, the Netherlands.
The first Mercury probe, NASA's Mariner 10, made a series of flybys in 1974 and 1975, some 40 years before MESSENGER. But getting a spacecraft into orbit around Mercury without it plummeting into the sun was a tough problem. In the 1980s, mission planners worked out the complex series of gravity assists from other planets needed for the journey, building on work by Italian astrophysicist Giuseppe "Bepi" Colombo, now honored by the new mission. A second challenge—how to endure solar radiation 10 times as strong as at Earth while observing a surface heated to 400°C—also delayed a second mission. "Spacecraft had never spent so long close to the sun," says Sean Solomon, director of the Lamont-Doherty Earth Observatory in Palisades, New York, who led the MESSENGER mission.
The ESA and NASA missions were both approved in the late 1990s, but the smaller $450 million MESSENGER mission got to the launchpad first and reached Mercury in 2011. The more ambitious €1.65 billion BepiColombo hit a few bumps during development, including solar arrays that degraded too quickly, which delayed its launch about 5 years. Japan joined the mission in 2003, eager to build on its success observing a planetary magnetic field in the 1992 Geotail mission, which studied the tail of Earth's magnetosphere. "Mars and Venus don't have magnetic fields, so the target was Mercury," says Go Murakami, project scientist for Japan's part of BepiColombo at the Japan Aerospace Exploration Agency's Institute of Space and Astronautical Science (ISAS) in Sagamihara.
ISAS's contribution is a separate spacecraft with five instruments that focus on the magnetosphere, whereas the larger ESA orbiter, with 11 instruments, is equipped to study the planet itself. Following launch from ESA's spaceport in French Guiana, the two stacked craft will embark on a 7-year journey, swinging once past Earth, twice past Venus, and six times past Mercury before finally separating and entering orbit around the planet in 2025.
One mystery awaiting the mission is MESSENGER's discovery of many volatile elements on the planet's surface, including chlorine, sulfur, potassium, and sodium, which should have been boiled off by the sun's heat long ago. "There is something odd in the formation history of Mercury," Benkhoff says. A clue comes from the ratio of potassium to thorium, which indicates a planet's temperature during formation. Benkhoff says Mercury's ratio points to a cooler origin, farther out than Mars. Volatiles are more abundant at those distances, and if Mercury formed beyond Mars and drifted in only later, it would have retained a larger supply of volatiles.
BepiColombo will make a sharper map of the volatiles than MESSENGER, thanks to an imaging spectrometer that identifies elements by how they fluoresce when hit by solar x-rays. That could help it track the loss of volatiles today. Depressions spotted by MESSENGER, tens of meters deep and hundreds wide, could be formed by escaping gases. If any have changed since MESSENGER's visit, it will suggest a large volatile supply that continues to vent into space.
BepiColombo will also scrutinize the peculiar contrasts between the planet's north and south. The north has large areas covered by smooth volcanic material that must have erupted relatively recently, whereas the south is cratered and ancient. Mercury's magnetic field mirrors the divide, as it, too, is shifted to the north. "How are these asymmetries connected?" Solomon asks.
Inside Mercury is another anomaly for the mission to explore: the planet's huge iron core, extending out to 80% of its radius, surrounded by a relatively thin layer of rock. One theory is that early in life, Mercury suffered a collision with another body that blasted off most of the lighter rocky material, leaving the heavier iron behind. Researchers would expect the iron core to have cooled and solidified by now, yet at least some of it is still liquid and churning, generating a magnetic field.
Although the field is 100 times weaker than Earth's, it accelerates magnetospheric electrons to energy levels approaching those seen in Earth's magnetosphere. Magnetometers aboard both the European and Japanese spacecraft should help the team understand the processes behind the energy boost, says James Slavin, a lead MESSENGER investigator at the University of Michigan in Ann Arbor, who expects "definitive answers to the mystery."
Studies of the magnetosphere could have implications beyond the solar system. Exoplanets found orbiting cool red dwarf stars could host liquid water—and conceivably life. But because they orbit their stars more closely than Mercury orbits the sun, they likely face strong stellar winds and radiation levels inimical to life—unless the planet is protected by a magnetosphere like Mercury's. "If we want to understand if life can [survive] on such planets, one of the important bits of information is the magnetosphere," Murakami says.
*Correction, 9 October, 6:05 p.m.: An earlier version of this story incorrectly described the acceleration of electrons in Mercury’s magnetosphere.