Mercury may be hiding diamonds, researchers say
The planet Mercury could be hiding a glittering secret behind its unusually dark colour, according to researchers at Sun Yat-sen University in Zhuhai.
The scientists said their observations and modelling suggested the graphite content that gave the planet its distinctive appearance could be much lower than previously estimated, with diamonds and other carbon forms possibly also present.
If previous estimates of carbon levels on the planet’s surface were accurate, a substantial portion of the element must be present in other forms, such as tiny diamond particles and amorphous carbon, which has no crystalline structure, they said.
The study, published by the peer-reviewed journal Nature Astronomy on Thursday, was based on previous research in the United States into data collected by Nasa’s Messenger, the first spacecraft to orbit the planet.
Mercury is the smallest planet in our solar system, only slightly bigger than the moon. It is also the closest planet to the sun – an average 77 million km from Earth.
The Messenger probe took nearly seven years to reach the planet, entering orbit around Mercury in 2011, and ended its mission in 2015.
In 2016, a team from the Johns Hopkins University Applied Physics Laboratory in the US determined that carbon was likely to be responsible for Mercury’s dark appearance – a reflection of its geochemistry and key to revealing its origin and evolution.
The carbon probably originated deep below the planet’s surface, “within an ancient, graphite-rich crust that was later buried by volcanic material”, according to the study published in the peer-reviewed journal Nature Geoscience.
But the latest research suggests the carbon detected by the Messenger mission “may not entirely occur as graphite”, the Chinese scientists said.
“Our results indicate that most carbon on Mercury may occur in forms other than intergrain graphite and that carbon did not entirely drain from the mantle during magma ocean crystallisation,” they wrote.
The researchers said the previous study had explained Mercury’s “mysteriously low surface reflectance” on the graphite form of carbon accounting for up to 4 per cent of the weight of deposits covering the planet’s bedrock.
However, their study showed that “a combination of less than 1 per cent [in weight] of microcrystalline graphite and similar amounts of metallic iron is adequate for explaining [the phenomenon]”, they said.
“Crystalline graphite in the hypothesised primary crust may have undergone intensive metamorphism or destruction by post-differentiation surface processes, such as impact bombardment, magmatism and space weathering.”
According to the paper, carbon on Mercury “may mainly occur as nanophase diamonds due to the long-term impact of metamorphism or as amorphous carbon due to space weathering of graphite, both of which are efficient darkening phases”.
Graphite – used in pencil leads – is the most stable form of carbon on Mercury’s surface. With very high pressure and temperatures of less than 3,000 degrees Celsius, it can be converted into diamonds. The reverse occurs from 1,000 degrees to 1,600 degrees.
Lead author Xiao Zhiyong, a professor with Sun Yat-sen University’s school of atmospheric sciences, said most of Mercury’s graphite may have been transformed into other forms of carbon after more than 4 billion years of weathering.
“If the primary crust of Mercury was made of graphite, we can imagine that the continuous evolution in 4.56 billion years – with countless impact events, mixing and destruction – would have seen most of the early graphite undergo phase changes and become other forms, including diamonds,” he said.
Xiao said he was looking forward to the findings of the second mission to Mercury, set to arrive in December 2025. The high-resolution data it collected could help scientists identify and study meteorites on Earth that had come from the planet, he said.
According to Xiao, meteorites originated on Mercury could serve as direct evidence of the composition of the planet’s surface until it becomes possible to retrieve samples from the planet itself.
The European-Japanese mission BepiColombo left Earth in 2018. It will be the second mission to orbit Mercury and the most advanced, according to the European Space Agency.