Messenger mission reveals surprises on — and inside — Mercury

The smallest planet in the solar system keeps serving up big surprises. Scientists working on the Messenger mission to Mercury have found that the planet has unexpected inner layers and craters with tilted bottoms, and it may have been geologically active far later into its life than previously imagined.

In the first of two studies released Wednesday by the journal Science, a team led by MIT geophysicist Maria Zuber scanned the surface of Mercury's northern hemisphere and found the planet's surface to be unusually flat when compared with the terrain of the moon or Mars.

The scans also showed that in the 960-mile-wide Caloris impact basin, the northern part of its floor is higher than the south, with parts standing higher than its rim. This strange lopsidedness may have resulted from tectonic forces — and is one of the growing number of clues that the planet's surface must have been changing well past its youth.

In the second study, led by David Smith of MIT, researchers used tiny shifts in radio frequency to measure Mercury's gravitational influence as the Messenger spacecraft orbited the planet. Knowing how much gravity was exerted on the spacecraft would help the team understand how dense the planet must be inside — and thus, what material lies within.

That turned out to be trickier than anticipated because the scientists had to tease the planet's gravitational signal out of all the other forces on the spacecraft — including photons from the sun that pushed on it and gravity from the sun that pulled on it.

In the end, the spacecraft orbited hundreds of times before the team had the information it needed.

"I thought we were going to know a lot of what we needed to know after a couple of orbits — and boy, was I naive," said Zuber, who worked on both studies.

A gravitational field can be broken down into its constituent wavelengths — the shorter wavelengths will reveal what's going on in the planet's shallow depths, and the longer wavelengths can help scientists figure out the gravity being exerted by material deeper in the core.

Scientists already knew that Mercury is rich in sulfur at the surface and has a massive iron core — so massive, in fact, that they wondered if Mercury had once been a much larger planet whose outer layers were stripped off, perhaps by a major impact.

Now, by studying the planet’s gravity, they found that thecore is even larger than they thought — from the center, it takes up 83% of Mercury’s radius. (The Earth’s core, by comparison, is a little more than half of the planetary radius.)

Perhaps most oddly of all, there must be a solid layer of iron sulfide lying between Mercury's liquid outer core and its thin mantle.

"That is the most surprising result that they've come up with, because we don't have anything like that in the Earth," said Gerald Schubert, a UCLA planetary physicist who was not involved in the study.

Schubert was quick to add that the iron sulfide theory is based on mathematical modeling rather than on an actual physical sample — but, he added, "if it's real, it has interesting consequences."

Knowing how different from Earth a rocky planet can look on the inside takes off the blinders in our analysis of potentially habitable planets beyond our solar system, Zuber said.

"People are making models of what 'super-Earths' could be like, and all these models are based on interiors of terrestrial planets as we know them," she said. "This gives people a broader range of things that need to be considered for planets around other suns."