Mercury Has A Very Large Solid Inner Core

Mercury as seen by MESSENGER during its second flyby. NASA / JHUAPL / CIW / color mosaic by Jason Perry

Mercury might be the smallest planet in the Solar System but its mysteries are as big as any other planet's. A particularly intriguing one is to do with its disproportionally large metallic core, which occupies 85 percent of its volume.

The latest analysis suggests that its inner core is solid just like Earth’s. What's more, despite the fact that Mercury is so small, its inner core might be almost as big as our planet's. As reported in Geophysical Research Letters, the team estimates that the inner core's radius is 30 to 70 percent that of the outer (and molten) core.  

“Mercury’s interior is still active, due to the molten core that powers the planet’s weak magnetic field, relative to Earth’s,” lead author Antonio Genova, an assistant professor at the Sapienza University of Rome, said in a statement. “Mercury’s interior has cooled more rapidly than our planet’s. Mercury may help us predict how Earth’s magnetic field will change as the core cools.”

The study is based on observations collected by NASA’s MESSENGER probe, which studied Mercury for over four years, from March 2011 to April 2015. The spacecraft recorded both the gravity of the planet and its spin in precise detail. This allowed researchers to gain insights into the planet's interior.

An illustration of Mercury’s interior based on new research that shows the planet has a solid inner core. Antonio Genova

Mercury spins on its axis much more slowly than the other planets, completing a full rotation every 58 days. This rotation is not perfect. Similar to the spinning eggs experiment, the interior of the planet influences its overall rotation. Observations from Earth in 2007 suggested that the planet’s core was molten, but scientists couldn’t work out if there was a solid part within the molten core.

MESSENGER brought us new spin observations and was also able to produce detailed gravity maps of the planet. It flew about 200 kilometers (120 miles) from the surface, halving this distance in its final 12 months. The final low-altitude orbits provided excellent gravity measurements that made this study possible.

The researchers loaded the data into a computer program. By adjusting parameters, they were able to match the observations to the best model for the planet. And the best scenario sees Mercury with a large metallic inner core. Understanding this planet tells us more about Earth given the similarities between the cores, but it might also shed light on what is happening in planets around other stars.

“Every new bit of information about our Solar System helps us understand the larger universe,” Genova added.

The research highlights the importance of sending probes to investigate planets up close. The team thinks many more discoveries remain hidden in the extensive MESSENGER data.

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