Researchers may have solved mystery of Mercury's missing meteorites, but doubts remain
The least studied and most mysterious of the solar system's rocky planets, Mercury is so close to the sun that exploring it is difficult even for probes. Only two uncrewed spacecraft have visited it to date — Mariner 10, launched in 1973, and MESSENGER, launched in 2004. A third, BepiColombo, is en route and due to enter orbit around the planet in late 2026.
Scientists know little about Mercury's geology and composition, and they have never been able to study a fragment of the planet that landed on Earth as a meteorite. In contrast, there are more than 1,100 known samples from the moon and Mars in the database of the Meteoritical Society, an organization that catalogs all known meteorites.
These 1,100 meteorites originated as fragments flung from the surfaces of the moon and Mars during asteroid impacts before making their way to Earth after a journey through space.
Not every planet is likely to eject fragments of itself Earth-ward during collisions. Though Venus is closer to us than Mars is, its greater gravitational pull and thick atmosphere may prevent the launch of impact debris. But some astronomers believe that Mercury should be capable of generating meteors.
'Based on the amount of lunar and Martian meteorites, we should have around 10 Mercury meteorites, according to dynamical modeling,' said Ben Rider-Stokes, a postdoctoral researcher in achondrite meteorites at the UK's Open University and lead author of a study on the Sahara meteorites, published in June in the journal Icarus.
'However, Mercury is a lot closer to the sun, so anything that's ejected off Mercury also has to escape the sun's gravity to get to us. It is dynamically possible, just a lot harder. No one has confidently identified a meteorite from Mercury as of yet,' he said, adding that no mission thus far has been capable of bringing back physical samples from the planet either.
If the two meteorites found in 2023 — named Northwest Africa 15915 (NWA 15915) and Ksar Ghilane 022 (KG 022) — were confirmed to be from Mercury, they would greatly advance scientists' understanding of the planet, according to Rider-Stokes. But he and his coauthors are the first to warn of some inconsistencies in matching those space rocks to what scientists know about Mercury.
The biggest is that the fragments appear to have formed about 500 million years earlier than the surface of Mercury itself. However, according to Rider-Stokes, this finding could be based on inaccurate estimates, making a conclusive assessment unlikely. 'Until we return material from Mercury or visit the surface,' he said, 'it will be very difficult to confidently prove, and disprove, a Mercurian origin for these samples.'
But there are some compositional clues that suggest the meteorites might have a link to the planet closest to the sun.
It's not the first time that known meteorites have been associated with Mercury. The previous best candidate, based on the level of interest it piqued in astronomers, was a fragment called Northwest Africa (NWA) 7325, which was reportedly found in southern Morocco in early 2012.
Rider-Stokes said that was the first meteorite to be potentially associated with Mercury: 'It got a lot of attention. A lot of people got very excited about it.' Further analysis, however, showed a richness in chrome at odds with Mercury's predicted surface composition.
More recently, astronomers have suggested that a class of meteorites called aubrites — from a small meteorite that landed in 1836 in Aubres, France — might come from Mercury's mantle, the layer below the surface. However, these meteorites lack a chemical compatibility with what astronomers know about the planet's surface, Rider-Stokes said. 'That's what's so exciting about the samples that we studied — they have sort of the perfect chemistry to be representative of Mercury,' he said.
Most of what is known about Mercury's surface and composition comes from NASA's MESSENGER probe, which assessed the makeup of the planet's crust from orbit. Both meteorites from the study, which Rider-Stokes analyzed with several instruments including an electron microscope, contain olivine and pyroxene, two iron-poor minerals confirmed by MESSENGER to be present on Mercury.
The new analysis also revealed a complete lack of iron in the space rock samples, which is consistent with scientists' assumptions about the planet's surface. However, the meteorites contained only trace amounts of plagioclase, a mineral believed to dominate Mercury's surface.
The biggest point of uncertainty, though, is still the meteorites' age. 'They are about 4.5 billion years old,' Rider-Stokes said, 'and most of Mercury's surface is only about 4 billion years old, so there's a 500 million-year difference.'
However, he said he thinks this discrepancy is not sufficient to rule out a Mercurian origin, due to the limited reliability of MESSENGER's data, which has been also used to estimate the age of Mercury's surface layer.
'These estimates are based on impact cratering models and not absolute age dating, and therefore may not be entirely accurate,' Rider-Stokes said. 'It doesn't mean that these samples aren't good analogs for regional areas on the surface of Mercury, or the early Mercurian crust that is not visible on the modern surface of Mercury.'
With more modern instruments now available, BepiColombo, the European Space Agency probe that will start studying Mercury in early 2027, may be able to answer long-standing questions about the planet, such as where it formed and whether it has any water.
Having material confirmed to have come from other planetary bodies helps astronomers understand the nature of early solar system's building blocks, Rider-Stokes said, and identifying fragments of Mercury would be especially crucial since a mission to gather samples from the planet closest to the sun and bring them back would be extremely challenging and expensive.
Sean Solomon, principal investigator for NASA's MESSENGER mission to Mercury, said in an email that he believes the two meteorites described in the recent paper likely did not originate from Mercury. Solomon, an adjunct senior research scientist at Columbia University in New York City, was not involved with the study.
The primary reason Solomon cited for his doubts is that the meteorites formed much earlier than the best estimates for the ages of rocks now on Mercury's surface. But he said he thinks the samples still hold research value.
'Nonetheless, the two meteorites share many geochemical characteristics with Mercury surface materials, including little to no iron … and the presence of sulfur-rich minerals,' he added. 'These chemical traits have been interpreted to indicate that Mercury formed from precursor materials much more chemically reduced than those that formed Earth and the other inner planets. It may be that remnants of Mercury precursor materials still remain among meteorite parent bodies somewhere in the inner solar system, so the possibility that these two meteorites sample such materials warrants additional study.'
Solomon also noted that it was difficult to persuade the planetary science community that there were samples from Mars in meteorite collections, and that it took precise matching of their chemistry with data about the surface of Mars taken by the Viking probes to convince researchers to take a closer look. Lunar meteorites were also not broadly acknowledged to be in meteorite collections until after the existence of Martian meteorites had been demonstrated in the 1980s, he added, even though the Apollo and Luna missions had returned abundant samples of lunar materials more than a decade earlier.
Once samples are confirmed to be from a planetary body, Solomon said, they can provide crucial information not available from remote sensing by an orbiting spacecraft on the timing of key geological processes, the history of internal melting of the body, and clues to planet formation and early solar system processes.
Rider-Stokes plans to continue the discussion around these meteorites at the annual meeting of the Meteoritical Society, which takes place in Perth this week. 'I'm going to discuss my findings with other academics across the world,' he said. 'At the moment, we can't definitively prove that these aren't from Mercury, so until that can be done, I think these samples will remain a major topic of debate across the planetary science community.'
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