Most meteorites that have reached Earth come from the asteroid belt between Mars and Jupiter. But we have 1,000 or so meteorites that come from the Moon and Mars. This is probably a result of asteroids hitting their surfaces and ejecting material towards our planet.
It should also be physically possible for such debris to reach the Earth from Mercury, another nearby rocky body. But so far, none have been confirmed to come from there – presenting a longstanding mystery.
A new study my colleagues and I conducted has discovered two meteorites that could have a Mercurian origin. If confirmed, they would offer a rare window into Mercury’s formation and evolution, potentially reshaping our understanding of the planet nearest the Sun.
Because Mercury is so close to the Sun, any space mission to retrieve a sample from there would be complex and costly. A naturally delivered fragment, therefore, may be the only practical way to study its surface directly – making such a discovery scientifically invaluable.
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Observations from Nasa’s Messenger mission have inferred the surface composition of Mercury. This suggests the presence of minerals known as such as sodium-rich plagioclase (such as albite), iron-poor pyroxene (for example enstatite), iron-poor olivine (such as forsterite) and sulfide minerals such as oldhamite.
The meteorite Northwest Africa (NWA) 7325 was initially proposed as a possible fragment of Mercury. However, its mineralogy includes chromium-rich pyroxene containing approximately 1% iron. This poorly matches Mercury’s estimated surface composition. As a result of this, and other factors, this link has been challenged.
Aubrite meteorites have also been proposed as potential Mercurian fragments. Recent modelling of their formation suggests an origin from a large planetary body approximately 5,000km in diameter (similar to Mercury), potentially supporting this hypothesis.
Although aubrites do not exhibit chemical or spectral (the study of how light is broken up by wavelength) similarities with Mercury’s surface, it has been hypothesised that they may derive from the planet’s shallow mantle (the layer beneath the surface). Despite ongoing research, the existence of a definitive meteorite from Mercury remains unproven.
Our latest study investigated the properties of two unusual meteorites, Ksar Ghilane 022 and Northwest Africa 15915. We found that the two samples appear to be related, probably originating from the same parent body. Their mineralogy and surface composition also exhibit intriguing similarities to Mercury’s crust. So this has prompted us to speculate about a possible Mercurian origin.
Both meteorites contain olivine and pyroxene, minor albitic plagioclase and oldhamite. Such features are consistent with predictions for Mercury’s surface composition. Additionally, their oxygen compositions match those of aubrites. These shared characteristics make the samples compelling candidates for being Mercurian material.
However, notable differences exist. Both meteorites contain only trace amounts of plagioclase, in contrast to Mercury’s surface, which is estimated to contain over 37%. Furthermore, our study suggests that the age of the samples is about 4,528 million years old. This is significantly older than Mercury’s oldest recognised surface units, which are predicted (based on crater counting) to be approximately 4,000 million years.
If these meteorites do originate from Mercury, they may represent early material that is no longer preserved in the planet’s current surface geology.
Will we ever know?
To link any meteorite to a specific asteroid type, moon or planet is extremely challenging. For example, laboratory analysis of Apollo samples allowed meteorites found in desert collection expeditions to be matched with the lunar materials. Martian meteorites have been identified through similarities between the composition of gases trapped in the meteorites with measurements of the martian atmosphere by spacecraft.
Until we visit Mercury and bring back material, it will be extremely difficult to assess a meteorite-planet link.
The BepiColombo space mission, by the European and Japanese space agencies, is now in orbit around Mercury and is about to send back high-resolution data. This may help us determine the ultimate origin body for Ksar Ghilane 022 and Northwest Africa 15915.
If meteorites from Mercury were discovered, they could help resolve a variety of long-standing scientific questions. For example, they could reveal the age and evolution of Mercury’s crust, its mineralogical and geochemical composition and the nature of its gases.
The origin of these samples is likely to remain a subject of continuing debate within the scientific community. Several presentations have already been scheduled for the upcoming Meteoritical Society Meeting 2025 in Australia. We look forward to future discussions that will further explore and refine our understanding of their potential origin.
For now, all we can do is make educated guesses. What do you think?
Ben Rider-Stokes receives funding from the Science and Technology Facilities Council (STFC).
* This article was originally published at The Conversation
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