Mercury Could Be Littered With Diamonds

Source: Wired, Ramin Skibba
Photo: Jeffrey Hamilton (Getty Images)

Scientists think the diminutive planet’s surface could be covered with space gems, thanks to an abundance of carbon and pressure from colliding asteroids.

DESPITE—OR RATHER, BECAUSE of—Mercury’s tumultuous early years, it could now be a diamond-encrusted world. Space rocks that smashed into the graphite that blankets much of the planet could have crushed it into diamond shards, according to new research.

“The pressure wave from asteroids or comets striking the surface at tens of kilometers per second could transform that graphite into diamonds,” says Kevin Cannon, a geologist at the Colorado School of Mines, who presented his latest findings at the Lunar and Planetary Science Conference in Houston last Thursday. “You could have a significant amount of diamonds near the surface.”

It turns out that Mercury isn’t just a hot hunk of rock closely orbiting the sun; it’s a complex world. Cannon’s and others’ findings reveal new details about its unique geological history, including the likely presence of plenty of bling.

The diminutive planet is smaller than two of the moons in our solar system (Titan and Ganymede), and it’s known for its short years and long days, orbiting the sun every 88 Earth days and rotating every 59. Daytime temperatures reach 800 degrees Fahrenheit—second only to Venus—while Mercury’s lack of an atmosphere means nighttime temperatures plunge to -290 Fahrenheit. But these mind-boggling stats aren’t what set it apart, geologically speaking: It’s the planet’s abundant carbon (in the form of graphite) and the extreme pummeling it received from asteroids some 4 billion years ago. During a violent, destructive period called the Late Heavy Bombardment, Mercury took maybe twice as much battering as the moon did—and our lunar neighbor is completely pockmarked with craters.

Like many other worlds in our solar system, including our own, the young Mercury was covered with oceans of magma, which later cooled and hardened. But unlike elsewhere, a layer of graphite floated atop all that molten rock. In his work in progress, Cannon modeled the effects of frequent impacts on the upper 12 miles of Mercury’s crust over billions of years. The graphite could have been more than 300 feet thick, and the asteroids’ impact pressure would have been enough to turn 30 to 60 percent of it into what he calls “shock diamonds.”

That adds up to a lot of space gems: maybe 16 quadrillion tons of them, he estimates, although the diamonds are likely to be minuscule, scattered, and buried.

Evidence from other research also supports that conclusion. Some meteorites, like the rock fragments known as Almahata Sitta that fell on the Nubian desert of northern Sudan in 2008, contained tiny diamonds, possibly produced by the shock of collisions between asteroids. And planetary scientists like Laura Lark, a researcher at Brown University in Providence, Rhode Island, believe they’ve seen dark spots of graphite on the surface of Mercury in images taken by cameras aboard NASA’s Messenger spacecraft, which orbited and mapped the planet between 2011 and 2015. The false-color maps made from those images—the most detailed currently available—show areas of ancient “low-reflectance material,” thought to be graphite.

“We used these large basins as natural samples of Mercury’s outer layers,” says Lark, who studied the 450-mile-wide Rembrandt basin, among others. (A basin is basically a very large crater.) “If the low-reflectance material in these basins are darkened by graphite, which is what we think, then the layers I’m seeing are thick. It’s more carbon than I’d expect from a magma ocean,” she says. That could mean Mercury was particularly carbon-rich from the start, she argues. Lark also presented new research from herself and colleagues at the LPSC conference last week.

As Mercury was forming, elements joined together mostly as metals or rocks. The metals sank and eventually built the planet’s core, with the rocks solidifying on top. On many planets, most of the carbon ends up becoming part of the metallic core in the mantle above it. But Mercury seems to have ended up with lots of carbon embedded in the rind of the planet, rather than lower down, Lark says. By contrast, on Earth diamonds only arise from carbon deep underground, under intense pressure.

Temperature and commuting issues aside, space miners probably won’t want to head to Mercury anytime soon, despite the copious carbon that allowed crystal creation. That’s because the diamonds are probably impure. “You’ll end up with a messy mixture of graphite, diamond, and maybe some other phases as well, so you won’t have nice, beautiful crystals that you could polish up and put on a ring,” Cannon says.

New research on the asteroids that smashed into the young Mercury could resolve another mystery, too: why the planet has an abnormally large core despite its small size. Some scientists believe its core would make more sense if the planet used to be much bigger and then withstood a giant impact that flung bits of it throughout the solar system. Currently, Mercury is an eighteenth the mass of Earth. “I calculate that the proto-Mercury could have been between 0.3 and 0.8 Earth masses. This is consistent with simulations” that always produce bigger versions of Mercury than the one we currently have, says Camille Cartier, a planetary scientist at the University of Lorraine in France, who also presented new work at the conference.

Based on her models, she argues that as Mercury and the rest of the solar system were still coming together, about 10 or 20 million years after the planets formed, a huge object could have slammed into Mercury, blowing most of its upper layers into space. Some of those chunks of rocks later ended up on Venus, Earth, and the inner asteroid belt. A few later returned to Earth as meteorites.

The next spacecraft to call on Mercury could shed more light on its turbulent past and whether it’s hoarding diamonds today. The European and Japanese space agencies’ joint BepiColombo mission launched in 2018, and its pair of orbiters will finally arrive in 2025. It will bring higher-resolution cameras that probe at longer wavelengths, allowing scientists to look for more direct signs of diamonds on the enigmatic planet.

Cannon wonders if more distant planets may also harbor diamonds—shock diamonds at the surface and others formed through pressure deep underground. “It’s exciting to think about exoplanets that might have even more carbon,” he says. “You could have a sandwich structure of diamonds, graphite, and more diamonds.”

https://www.wired.com/story/mercury-could-be-littered-with-diamonds