MESSENGER Data Reveal That Mercury’s Inner Core Is Solid

by |April 19, 2019

How do you explore the interior of a planet without ever touching down on it? Start by watching the way the planet spins, then measure how your spacecraft orbits it — very, very carefully. This is exactly what planetary scientists did, using data from a former NASA mission to Mercury.

It has long been known that Mercury has a metallic core. Like Earth’s, Mercury’s outer core is composed of liquid metal, but there have only been hints that Mercury’s innermost core is solid. Now, in a new study, scientists have found evidence that Mercury’s inner core is indeed solid, and that it is nearly the same size as Earth’s inner core.

“With this study, Mercury is now the second rocky planet — after Earth — for which we have evidence of a solid inner core,” said co-author Sean Solomon, director of Columbia’s Lamont-Doherty Earth Observatory and principal investigator on the NASA mission that collected the critical data for this study.

A graphic of Mercury's internal structure.

A graphic of Mercury’s internal structure. Credit: Antonio Genova

Some scientists compare Mercury to a cannonball because its metal core fills nearly 57 percent of the volume of the planet. This large core — huge compared with those in the other rocky planets in our solar system — has long been one of the most intriguing mysteries about Mercury.

The new findings on the size of Mercury’s solid inner core, described in Geophysical Research Letters, will help scientists model the flow within its molten outer core, said Solomon, to better understand how it gives rise to Mercury’s magnetic field.

The study also offers clues about how the solar system formed and how rocky planets change over time.

“Mercury’s interior has cooled more rapidly than our planet’s,” said Antonio Genova, an assistant professor at the Sapienza University of Rome who led the research while at the NASA Goddard Space Flight Center. “Mercury may help us predict how Earth’s magnetic field will change as the core cools.”

The team used several observations from the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission to probe the interior of Mercury and find out what it’s made of. The MESSENGER spacecraft orbited Mercury from March 2011 until April 2015.

The researchers looked at MESSENGER’s data on the planet’s spin and gravity field. They used radio observations from MESSENGER to determine the gravitational anomalies (areas of local increases or decreases in mass) and the location of its rotational pole, which allowed scientists to understand the orientation of the planet.

Each planet spins on an axis. Mercury spins much more slowly than Earth, with its day lasting about 58 Earth days. Scientists often use tiny variations in the way an object spins to reveal clues about its internal structure. In 2007, radar observations made from Earth revealed small shifts in the spin of Mercury, called librations, that proved that the outer part of Mercury’s core must be molten metal. But observations of the spin rate alone were not sufficient to give a clear measurement of what the inner part of the core was like. Could there be a solid core lurking underneath, scientists wondered?

Gravity can help answer that question, because a planet’s gravity field depends on its internal density structure. The scientists recorded how the MESSENGER spacecraft accelerated and decelerated under the influence of Mercury’s gravity field. During the last two years of the mission, MESSENGER’s closest approach drew progressively closer to Mercury’s surface. The final low-altitude passes before the spacecraft impacted the surface provided the highest-resolution data yet, and allowed the team to make the most accurate measurements about the internal structure of Mercury yet taken.

Then they entered the MESSENGER data into a sophisticated computer program that allowed them to figure out what the interior density structure of Mercury must be like to match the way it spins and the way the spacecraft accelerated around it. The results showed that for the best match, Mercury must have a large, solid inner core. They estimated that the solid, iron core is about 1,260 miles wide and makes up about half of the diameter of Mercury’s entire core (about 2,440 miles wide). In contrast, Earth’s solid core is about 1,500 miles across, compared with a diameter for the outer core of 4,200 miles.

New discoveries about Mercury are practically guaranteed to be waiting in MESSENGER’s archives, with each discovery about our local planetary neighborhood giving us a better understanding of what lies beyond.

“Every new bit of information about our solar system helps us understand the larger universe,” said Genova.

Adapted from a NASA press release

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