In the mountains of Cyprus, identical rocks found only on the planet Mercury: The enigma of the Lilliputian planet, boninite and the ancient ocean of Tethys [video]

An undated handout picture made available by the European Space Agency (ESA) shows an artist impression of the BepiColombo probe flying by Mercury (issued 01 October 2021). BepiColombo is an international collaboration between ESA and the Japan Aerospace Exploration Agency (JAXA). EPA, ESA/ATG MEDIALAB HANDOUT

Full of surprises not only for its origin and the enigmatic chemical composition of its surface is the planet Mercury the closest planet to the Sun and the smallest in the solar system.

Mercury is one of the four rocky planets of the solar system. The other three are Venus, Earth and Mars. It has a negligible atmosphere and this results in the temperature on its surface showing huge fluctuations.

Some answers to the mysteries of the smallest planet in the solar system may be hidden in rocks found in Cyprus. Specifically, as reported by the BBC, Nicola Mari, a planetary geologist at the University of Pavia in Italy, studies the ways in which our “neighbors” in the solar system formed and evolved.

For his doctoral thesis, Marie had studied the lava flows of Mars. This time, his target was the planet Mercury – via Cyprus.

In particular he looked for a specific type of rock, called boninite (a high-magnesium form of basalt, distinguished by its low titanium content and trace element composition) and is thought to bear an eerie resemblance to rocks found on Mercury – a hypothesis that, if correct, could be a clue to the unique origin of the planet.

The planet of extremes

Mercury is a planet of extremes. With a total volume slightly larger than the Moon, it is the smallest planet in the solar system and the closest to the Sun. Mercury has no atmosphere to retain heat, meaning its surface temperature ranges from 400C during the day to -170C at night. It also has the shortest orbit of any planet in the solar system – each year lasts just 88 Earth days.

Mercury’s position has made it very difficult for scientists to study it. One reason is heat. Spacecraft approaching the planet must be able to withstand the scorching temperatures as they orbit so close to the Sun.

The other reason is gravity. The closer to the Sun, the stronger its pull, accelerating the speed of the spacecraft. This makes fine maneuvering much more difficult, with many “detours” around other planets.

“From an orbital point of view, it’s probably harder to reach than Jupiter,” says Ignacio Clerigo, operations manager of the BepiColombo spacecraft, the European Space Agency’s current mission to Mercury, a project to which Mari’s work contributes.

These difficulties mean that the planet Mercury has been less well studied than our other “neighbors”. Two previous missions—Mariner 10 and Messenger—had flown close enough to map its cratered surface, discovering—and revealing—some major surprises about its structure.

One of them concerned the core of the planet. The other rock-based planets—Venus, Earth, and Mars—all have a relatively tiny core, surrounded by a thick mantle of magma and a hard crust. Mercury’s crust, however, appears to be surprisingly thin, while its core is unexpectedly massive compared to the mantle. “It’s absurd,” says Marie.

Even more unexpectedly, these missions revealed that Mercury is surrounded by a magnetic field. This, combined with its density, suggests that it has an iron core – and, like Earth’s core, the core may be partially molten.

But the mystery continues, as the ratio of chemicals on Mercury’s surface is highly unusual. Using a technique known as spectrometry to analyze the planet’s chemical composition from a distance, scientists know that Mercury has a much higher concentration of thorium than its closest neighbors.

Thorium should have vaporized in the extreme heat of the early solar system. Instead, its thorium content is closer to that of Mars – three planets away – which would have formed at cooler temperatures due to its distance from the Sun.

According to the BBC, scientists speculate that Mercury began with a greater mass, comparable to that of Earth. An early colossal impact is believed to be responsible for reducing its size and changing its orbit, resulting in the compact world we observe today.

The ocean of Tethys

Further studies of similar Earth rocks may provide valuable clues about Mercury’s past geological activities and its amazing journey through our Solar System.

Cyprus is a piece of crust that formed under the Tethys Ocean more than 90 million years ago. As the tectonic plates collided, it was eventually pushed to the surface, becoming the island we know today. The landscape still has an otherworldly feel, says Mari, with greenish rocks rich in minerals.

“In some areas of the mountains of Cyprus, it’s like we’re still walking over an ancient ocean,” he says. Eventually, he found the specific pieces of lava he was looking for, as boninite is formed by the solidification of molten lava

Marie returned home and, in collaboration with her colleagues at Nasa and the Planetary Science Museum in Italy, analyzed the composition of the rocks and compared them to samples taken from Mercury. When the results came in, he was stunned. “They weren’t just similar, they were identical.”

The mix of elements such as magnesium, aluminum and iron was the same as that seen in the mysterious planet with the massive core. The only difference was that the rocks from Cyprus had oxidized – which is inevitable given Earth’s oxygen-rich atmosphere. This makes it the first real terrestrial analogue for Mercury, Mari says – providing a valuable additional data point for our understanding of the planet.