The planet Mercury’s puzzlingly dark colour could result from its bombardment with carbon-rich micrometeorites, reports a study published online in Nature Geoscience. This may explain why the Moon is lighter in colour than Mercury, even though the Moon’s surface contains more darkening iron.

Observations of Mercury reveal a planet that reflects little light and also has a low surface iron content, suggesting that another material is probably responsible for its observed dark surface. Comets are rich sources of carbon and they more frequently impact planets closer to the Sun.

Megan Bruck Syal and colleagues calculated the total mass of carbon delivered by micrometeorites, which likely originated from comets, to the surfaces of Mercury and the Moon. They estimate that about 50 times as much carbon-rich material per unit area is retained on Mercury, compared with the Moon. Impact experiments at high velocities confirm that delivery of carbon by micrometeorites can lead to a surface colour similar to that of Mercury, in agreement with observations.