Why some rocks on Earth look like those on Mercury

Curiosity killed the explorer. And Nicola Mari feared he might be next.

Mari navigated her way around her cell phone as she drove through the most remote mountains in Cyprus.

But as night fell, his cell phone battery ran out, until he found himself lost in the middle of nowhere, with no idea how to get back to his accommodation.

“I had traveled more than 50 km without finding another vehicle,” he says.

Mari thought he could remember the route to a restaurant, where he could replenish his stomach, his engine and his cell phone battery. But when we got there, everything was deserted.

With a little luck, Mari ended up arriving at another establishment, but he admits that he feared losing his life on those lonely mountain roads. “I miscalculated,” he says.

But fortunately, his mission was not in vain.

Mari is a planetary geologist at the University of Pavia, in Italy. He studies how our neighbors in the Solar System formed and evolved. And to get his PhD, he studied lava flows on Mars.

This time his eyes were on Cyprus, but turned towards Mercury. His objective was to find a certain type of rock on the island, called “boninite”.

This rock is believed to have a striking similarity to those found on Mercury. And, if this assumption is correct, it could be an indication of that planet’s unique origins.

Mercury is a planet of extremes. With a total volume slightly larger than that of the Moon, it is the smallest planet in the Solar System – and the closest to the Sun.

Mercury has no atmosphere to retain heat. In other words, the temperature on its surface varies from 400°C during the day to -170°C at night.

The planet also has the smallest orbit of any planet in the Solar System. Mercury’s year only lasts the equivalent of 88 days on Earth.

Scientists have a very difficult time studying Mercury due to its location.

One of the reasons is the heat. To get closer to the planet, spacecraft need to withstand scorching temperatures, due to their proximity to the Sun.

The other reason is gravity. The closer you get to the Sun, the greater its attraction force, which increases the spacecraft’s speed. This makes delicate maneuvers much more difficult.

To avoid traveling too fast, the spacecraft can follow a more complex route, making a series of detours around other planets, which helps reduce speed. But she still needs a lot of fuel to slow down and maintain control of her movements.

“From a trajectory perspective, it’s probably more difficult to get there than Jupiter,” says Ignacio Clerigo, spacecraft operations manager for BepiColombo, the European Space Agency’s current mission to Mercury. Mari’s work is in line with this project.

All these difficulties have meant that Mercury has been less studied than the other neighboring planets.

But two previous missions – Mariner 10 and Messenger – came close enough to the planet to map its crater-dotted surface. They revealed big surprises about their structure.

One of these surprises was the planet’s core. All other rocky planets – Venus, Earth and Mars – have a relatively small core, surrounded by a thick mantle composed of magma and a hardened crust.

But Mercury’s crust appears to be surprisingly thin, while its core is unexpectedly large compared to the mantle. “It’s absurd”, according to Mari.

Another even more unexpected discovery brought by the missions is that Mercury is surrounded by a magnetic field.

Combined with its density, it suggests the planet has an iron core – which may be partially liquid, like Earth’s core.

Adding to the mystery, the proportion 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 discovered that Mercury has a much higher concentration of thorium than its closest neighbors.

Thorium should have evaporated in the extreme heat of the early Solar System. But Mercury’s thorium content is closer to the level of Mars (three planets away), which would have formed at lower temperatures due to its distance from the Sun.

These anomalies have led some planetary scientists to imagine that Mercury originally formed further away from the Sun, close to Mars.

And it would have started with a much larger mass, approximately the size of Earth, which would justify its large core.

But the hypothesis is that, at some point in history, Mercury collided with another planetary body that displaced it, causing it to rotate towards the Sun. This collision may have ripped off its crust and much of its mantle, leaving the enormous liquid core of the planet.

“The Mercury we see today may just be the seed of the planet it once was,” according to Mari.

The best way to research this theory would be to analyze rock samples from Mercury’s crust or drill into its mantle. But no probe has managed to land on the planet’s surface to date, which leads scientists on Earth to look for other sources of information.

Some indications may come from a class of meteorites known as aubrites. Their name comes from the French commune of Aubres, where they were first found.

These rocks have a similar chemical composition to Mercury and some scientists have even imagined that they could be fragments of the collision that took the planet to its current position in the Solar System.

The idea is tempting, but Mari is skeptical. He says that the evidence already discovered indicates that the aubrites come from asteroids that formed in the same region of the solar nebula as Mercury, but that were never part of the planet.

An alternative line of evidence may come from “geochemical analogues”, which are rocks formed on Earth that bear a good resemblance to structures found on other planets.

After all, our knowledge of Earth’s geological processes is much better and we can use this knowledge to develop theories about the formation of our Solar System partners.

This was the aim of Mari’s mission to Cyprus. According to the available geological data, that location was the most viable to find the specific composition he was looking for.

When she set out on her journey through the deserted mountains, Mari felt like a “modern Indiana Jones”, he says.

Cyprus is a piece of crust formed under the Tethys Sea more than 90 million years ago. With the collision of tectonic plates, it ended up being thrown to the surface, where it became the island we know today.

The scenery brings a feeling of another planet to this day, according to Mari, with its greenish rocks and rich in minerals.

“In certain regions of the mountains of Cyprus, it’s as if you’re still walking on an ancient ocean floor,” he says.

During her search, Mari found the specific pieces of lava she was looking for, known as boninites.

He returned home and, working with his colleagues at NASA and the Museum of Planetary Sciences in Italy, he analyzed the composition of the rocks, comparing them with measurements from Mercury. The results left him perplexed.

“They weren’t just similar; they were identical.”

The mixture of elements such as magnesium, aluminum and iron was the same as that observed on the mysterious planet with its enormous core.

The only difference was that the rocks of Cyprus had oxidized – which is inevitable considering Earth’s oxygen-rich atmosphere.

Boninite rocks are the first true terrestrial analogue of Mercury, according to Mari. They provide precious additional data for our understanding of that planet. And additional studies may reveal clues about Mercury’s past geological activity.

We know that Cypriot boninites formed with lava from eruptions in a shallow spot in the Earth’s crust. Therefore, its almost perfect similarity with the rocks of Mercury supports the idea that the mantle of that planet is abnormally close to the surface, according to Mari.

And this conclusion is consistent with a violent origin that blew away much of Mercury’s original crust.

Mari’s discoveries are one piece of a very big puzzle.

Much more knowledge can come from the BepiColombo mission, carried out in collaboration between Japan and the European Space Agency, which was launched in October 2018.

Its name is a tribute to the Italian mathematician and engineer Giuseppe (Bepi) Colombo (1920-1984). Among other things, he helped plan the complicated path of the Marine 10 spacecraft.

Following its winding route to Mercury, BepiColombo has already come close to the planet three times. These approaches are part of the strategy to reduce your speed.

The spacecraft will make its last approach in 2025, when it will split into two orbital vehicles. One of them will measure the magnetic field and the other will study the surface and internal composition of Mercury.

Mari states that her research on geochemical analogues could be relevant to these studies, as it could serve as a reference for some of these measurements.

“Laboratory measurements of Mercury analogues help us to better interpret the measurement results we obtained with our infrared and thermal infrared spectrometers, as well as some type of X-ray spectrometer,” explains Johannes Benkhoff, project scientist at BepiColombo.

For a year after arrival, the orbiters will take more precise measurements of Mercury’s mineral composition, its topography and its internal structure.

By comparing this data with past missions, scientists will even be able to determine whether the planet is still geologically “alive.”

There are depressions on the surface that appear to have been formed by the evaporation of material from Mercury’s interior, but we don’t know for sure whether this process is still active.

Taken together, these measurements could finally allow us to get to the bottom of Mercury’s mysterious origins – and, by extension, they could teach us much more about our own place in the cosmos.

“The questions about Mercury’s high density and why its core is so large are very important for understanding the formation and history of our Solar System,” explains Benkhoff.

“The spacecraft has a full suite of payloads and instruments and we hope they will really advance our scientific knowledge.”

Even before the mission arrived, many of our impressions about the first planet from the Sun had already been altered.

“Fifteen years ago, Mercury was considered a dull planet,” says Benkhoff. “But I hope to find many more surprises.”

For Nicola Mari, Mercury is just the beginning.

“In Lanzarote [ilhas Canárias], we found lava similar to the mantle of Mars. And to find traces of Venus, we are searching Sicily [Itália]Hawaii, Indonesia and Kamchatka, Russia.”

When BepiColombo fully begins its scientific operations, which is expected to occur in 2026, we will be able to better understand how much these Earth rocks can tell us about our neighbors in the Solar System.

Alessia Franco is a writer and journalist specializing in history, culture, society, storytelling and its effects on people.

David Robson is an award-winning science writer. His next book (in English) is called The Laws of Connection: The Transformative Science of Being Social, to be published in June 2024 by Canongate (in the United Kingdom) and Pegasus Books (in the United States and Canada). His account on X (formerly Twitter) is @d_a_robson. He can also be found under the handle @davidarobson on Instagram and Threads.