Data from the MESSENGER spacecraft suggest that Mercury’s core is much bigger than researchers had realized, and that the planet’s surface has been through an unusually active evolution.
MESSENGER, which was launched by NASA back in 2004 to explore the planet closest to our Sun, has been circling Mercury in an eccentric, 12-hour orbit for more than a year. Now, two separate teams of researchers have characterized the planet’s internal structure, the thickness of its crust, and the size of its core—and their findings are forcing experts to reconsider what they know about planetary formation.
The teams’ reports, which detail the surface of Mercury’s northern hemisphere and suggest that a deep reservoir of high-density material exists within the planet, appear in the 22 March issue of Science Express.
Ancient volcanic plains in the northern high latitudes of Mercury, revealed by instruments onboard MESSENGER. The 4-billion year-old plains comprise about 6% of Mercury’s surface. Purple colors are low, white areas are high. | Image: NASA/JHUAPL/CIW-DTM/GSFC/MIT/Brown University. Rendering by James Dickson and Jim Head
Maria Zuber from the Massachusetts Institute of Technology, along with colleagues from around the world, used a laser altimeter onboard the MESSENGER spacecraft to generate a detailed elevation model of the northern half of the planet. This topographical map of Mercury—the first of its kind—reveals that the elevation of the planet’s surface is much less varied than that of Mars or the moon. According to these researchers, the most prominent feature of Mercury’s northern hemisphere is an extensive lowland region that hosts a volcanic plain.
“A lot of scientists thought Mercury was like our moon, which cooled off very quickly after it formed and didn’t experience much activity afterward,” Zuber said. “But it turns out that this planet had a very different evolution compared to the moon. It’s actually very difficult to compare it to anything else.”
The researchers were able to examine the details of a 932-mile-wide (1,500-kilometer-wide) impact basin, known as Caloris, and determine that part of the basin’s floor now stands taller than its rim. According to the researchers, observations like this suggest that Mercury experienced intense geophysical activity for most of its history.
“On Earth, convective forces in the mantle cause tectonic plates to shift and uplift, creating mountain ranges and canyons,” Zuber explained. “On Mercury, it looks like there were large parts of the surface that were uplifted by similar processes. Caloris basin, for instance, had its floor uplifted higher than its rim in some places. So it’s obviously been deformed in some unusual kind of way. And there are other expanses of terrain on Mercury that have been lifted up several kilometers.”
It also turns out that Mercury’s interior is much different from what researchers had expected. David Smith, also from MIT, and colleagues from across the United States and Canada have provided the first measurements of Mercury’s gravity field. These measurements, in turn, provide insight into the planet’s interior.
“We thought that Mercury had a big, iron core,” said Zuber. “But once we analyzed the gravity data, we realized that it was actually much bigger than we had anticipated. So if you pictured Mercury as an orange, the core would basically be the size of the whole orange and the mantle and the crust would be just the peel.”
Smith and the other researchers found that Mercury’s crust is thicker at low latitudes and thinner toward the northern polar region. They suggest that the outer shell of the planet is much denser than researchers thought it was, and that a deep reservoir of material—probably a layer of iron sulfide—is buried below its surface. Such an interior would be much different from those of other terrestrial planets, they said.
“We use our knowledge of terrestrial planets, like those in our own solar system, to imagine what other, faraway planets might be like,” said Zuber. “But no one ever thought that this could exist inside of a planet. It raises all sorts of possibilities and questions about the planets orbiting other stars.”
Mercury has always been somewhat of a mystery within our solar system. Its surface, for example, is hot enough to melt basically any material, yet researchers believe that they have glimpsed water ice at the bottom of some of the planet’s craters. And the planet hosts many volatile elements, such as sulfur, that would ordinarily vaporize under such extreme temperatures.
“It’s clear that we’re not understanding all of the thermodynamics of planet formation—and how ancient chemical building blocks eventually result in what we see today,” said Zuber. “But these findings certainly expand our horizons in terms of possibilities. I’m really glad to be on this mission, working on such a mystery.”
Read the abstract, “Gravity Field and Internal Structure of Mercury From MESSENGER,” by David Smith and colleagues.
Read the abstract, “Topography of the Northern Hemisphere of Mercury From MESSENGER Laser Altimetry,” by Maria Zuber and colleagues.