Mercury is the least understood planet in the inner solar system, and that is unlikely to change until a spacecraft is put down on its surface, according to reporting by Universe Today. The European Space Agency's and Japan's space agency's joint BepiColombo mission is due to enter orbit around the planet late next month, but it is not expected to begin collecting science data until April 2027. After that, no further Mercury mission is currently planned, planetary scientist Paul Byrne of Washington University in St. Louis told Universe Today.
Why it matters
Studying Mercury is unusually hard. Optical telescopes on the ground avoid pointing at it because of the risk that the nearby Sun's glare could damage their instruments, Universe Today reports. The result is a planet that has received far less attention than its neighbors, even though its unusual properties bear directly on how rocky worlds form.
Only two spacecraft have visited Mercury so far. NASA's Mariner 10 flew past it three times between 1974 and 1975, and NASA's MESSENGER mission orbited the planet from 2011 for roughly four years. Universe Today notes that reaching the icy moons of Saturn or Jupiter, or even Pluto, is in some respects an easier proposition than landing on the planet closest to the Sun.
Byrne told Universe Today that a lander is the logical next step, even though it would be technically demanding and costly. Such a mission would give researchers a much clearer picture of what Mercury's surface is made of and what lies beneath it.
A core that challenges planetary formation models
Mercury's iron-rich core is thought to account for about 85 percent of the planet's radius, a proportion that stands out sharply among rocky planets. Byrne told Universe Today that astronomers are increasingly finding evidence of large, massive rocky exoplanets with outsized iron cores, which raises the question of whether Mercury is an oddity or a common outcome of planet formation.
Anne Pommier, a staff scientist and high-pressure experimentalist at the Carnegie Laboratory in Washington, D.C., outlined two leading explanations in comments to Universe Today. One holds that Mercury formed from building blocks already rich in metal. The other suggests the planet was originally larger, with a crust-to-mantle-to-core ratio closer to Earth's. Pommier, a co-investigator on BepiColombo's magnetometer instrument, told Universe Today that large impacts probably tore away much of Mercury's rocky mantle, leaving behind its disproportionately big core. She said the planet remains difficult to account for and pushes against existing ideas about how planets are assembled.
A weak magnetic field and a shrinking surface
Apart from Earth, Mercury is the only rocky planet in the solar system known to have a global magnetic field. Pommier told Universe Today that Mercury's field is weak and is believed to arise in the liquid part of its core, where circulating material drives electric currents that sustain a dynamo over billions of years. A 2024 paper in the journal Earth and Planetary Science Letters, cited by Universe Today, describes the surface field as mostly dipolar and about 100 times weaker than Earth's. The same authors point to magnetized crust dated at 3.7 to 3.9 billion years old as evidence that the dynamo was running early in Mercury's history.
A magnetic field alone does not keep an atmosphere in place. Byrne told Universe Today that a planet also needs enough mass to replenish an atmosphere and enough distance from its star to avoid having it stripped away. Mercury sits close to the Sun and takes a constant battering from the solar wind. What it has instead is a thin exosphere containing small amounts of oxygen, hydrogen, sodium and helium.
Pommier also told Universe Today that surface features indicate the planet has been contracting, possibly as its core cools, though the process is not well understood.
Dark material and a missing meteorite record
Large parts of Mercury's surface are covered by a low-reflectance material whose origin is unclear. Pommier told Universe Today that it appears both at the surface and below it, sometimes buried under other deposits in craters. One idea is that it formed as a magma ocean crystallized early in the planet's history. Byrne described the substance as likely graphite left over from a thin, carbon-rich crust that may once have covered the whole planet, later buried, churned up by impacts, recycled by volcanism or lost to space.
Universe Today also reports that no meteorite found on Earth has been officially confirmed as coming from Mercury, even though some studies have proposed that certain meteorites, including enstatite chondrites, might have originated there.
BepiColombo's orbital insertion late next month is the next milestone to watch, with its science phase set to begin in April 2027.
