Lost rocks of Mercury found on Earth

Most of the meteorites that reach Earth come from the asteroid belt between Mars and Jupiter. However, there are also about a thousand meteorites known to have come from the Moon and Mars. It is thought that these rocks were thrown to Earth by fragments of asteroids that hit the Moon and Mars.

Theoretically, it is possible for such pieces to reach Earth from Mercury. However, no meteorite has been found that has been proven to be of Mercury origin. This has long been a question mark in the scientific world.

But a team from The Open University in the UK has found two new meteorites that they think may have come from Mercury. If confirmed, the discovery would provide a rare window into the formation and evolution of Mercury, the planet closest to the Sun, and potentially revolutionise what is known about the planet.

WILL THE DISCOVERY BE CONFIRMED?

"Because Mercury is so close to the Sun, sending a spacecraft there to collect samples would be extremely complex and costly. Therefore, a naturally occurring fragment that reaches Earth would be the most realistic way we have to directly examine the planet's surface," said Ben Rider-Stokes, one of the researchers, in an article published in The Conversation.

Observations from NASA's Messenger mission indicate that Mercury's surface contains sodium-rich plagioclase (e.g. albite), iron-poor pyroxene (e.g. enstatite), iron-poor olivine (e.g. forsterite), and sulfide minerals such as oldhamite.

Previously, the meteorite named Northwest Africa (NWA) 7325 had been on the agenda with the claim that it could be a piece of Mercury. However, this claim was weakened because it contained pyroxene, a chromium-rich mineral that contains about 1 percent iron, and it did not match the surface composition of Mercury.

The latest study examined two unusual meteorites, Ksar Ghilane 022 and Northwest Africa 15915. It was determined that these two samples probably broke off from the same parent body. Their minerals and surface composition are also strikingly similar to Mercury's crust. Therefore, it was suggested that they may have originated from Mercury.

Both meteorites contain olivine, pyroxene, and small amounts of albitic plagioclase and oldhamite. These minerals are consistent with the predicted surface composition of Mercury, and their oxygen compositions also match those of aubrite. These shared characteristics make the samples strong candidates for having broken off from Mercury.

But there are also some important differences. For example, plagioclase is only present as traces in both meteorites, while the proportion of this substance on Mercury's surface is over 37 percent. In addition, the samples were determined to be approximately 4.528 billion years old. This indicates that Mercury's oldest known surface units (about 4 billion years according to crater counts) are much older.

This suggests that if these rocks came from Mercury, they may have been broken off from very old layers of the planet that are not preserved on the surface today.

GOING TO MERCURY

Matching a meteorite to a specific asteroid, moon or planet is difficult. For example, samples from the Apollo missions confirmed lunar meteorites found in deserts, while Martian meteorites were identified when the gases trapped inside them matched measurements in the Martian atmosphere.

“Unless we go to Mercury and get a sample, it is almost impossible to definitively identify a Mercury-origin meteorite,” Rider-Stokes said. “However, the joint BepiColombo mission of the European and Japanese space agencies is currently in orbit around Mercury and will soon be returning high-resolution data from the planet. This could help us understand the origins of Ksar Ghilane 022 and Northwest Africa 15915.”

If confirmed they did indeed break away from Mercury, it could answer many questions, including the age and evolution of the planet's crust, its mineral and chemical composition, and the properties of its gases.