Surface of Mercury formed within the planet

The chemistry of Mercury’s most ancient plains was likely forged near the planet’s core, lab experiments have shown.

NASA scientists recreated material resembling volcanic deposits on the planet’s surface by simulating its early conditions, 400 kilometres beneath the surface.

Before NASA’s MESSENGER spacecraft crashed on Mercury’s surface last year, it spent four years in orbit, snapping close-ups and sniffing out the planet’s chemistry.

It found Mercury has a low iron content and alkaline surface, and the highest concentration of sulfur of any terrestrial planet in the solar system in its mantle – at least 10 times more sulfur Earth, Mars and the moon.

The probe also revealed two types of regions on the surface: young northern volcanic plains, 3.7 to 3.8 billion years old, and older inter-crater plains and heavily cratered terrains, four to 4.2 billion years old.

But how these different regions could have developed has been a mystery – until now.

NASA scientists from the Johnson Space Center in Houston took a mix of the same chemicals found in “enstatite chondrites” – a rare form of meteorite which is thought to represent Mercury’s building blocks.

“Mercury is a unique terrestrial planet. Unlike the Earth, it has a large core and a comparatively shallow mantle, meaning that the mantle-core boundary is only around 400 kilometres below the planet’s crust,” explained study author Asmaa Boujibar.

Her team squeezed the chondrite chemicals under the pressure of Mercury’s core-mantle boundary – five gigapascals, or 50,000 times Earth’s atmospheric pressure – a pressure high enough to form diamonds.

The team discovered that by mixing just two melting products of enstatite chondrites they could replicate the chemical compositions of most of Mercury’s surface.

“By varying pressure and temperature on only one type of composition, we could produce the variety of material found on the planet’s surface,” Boujibar explained.

“These findings indicate that the older terrains are formed by material melting at high pressures up to the core-mantle boundary, while the younger terrains are formed closer to the surface,” she added.

While the authors say the effects of both the pressure and sulfur explain the composition of the two major types of regions on Mercury’s surface, their experiments have not been able recreate all of its chemistry.

A few regions “remain difficult to explain”, Boujibar admitted, such as the High-Al plains which would have required a source rich in aluminium to form.

“But [this work] does go a long way to helping us understand why we find such a variety of features,” Boujibar added.

The findings were reported at the Goldschmidt Conference in Yokohama, Japan.