Using sophisticated software to precisely analyse how Mercury’s gravity affected the trajectory of NASA’s MESSENGER spacecraft during multiple low-altitude orbits, researchers have been able to show the small planet must have a large, solid-iron inner core measuring some 2,000 kilometres (1,260 miles) across, almost as large as Earth’s.

Including its molten metal outer layer, Mercury’s metallic core fills nearly 85 percent of the planet’s volume, huge compared to the cores of the solar systems other terrestrial planets.

“Mercury’s interior is still active, due to the molten core that powers the planet’s weak magnetic field, relative to Earth’s,” said Antonio Genova, an assistant professor at Sapienza University of Rome who led the research. “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.”

To determine the nature of Mercury’s inner core, researchers used radio observations from MESSENGER to determine the precise position of the planet’s rotational pole and incorporated ground-based radar data collected in 2007 showing small shifts in the planet’s 58-day spin, called librations, that indicated the presence of at least a partially molten metal core.

Data collected during the MESSENGER mission indicates Mercury has a solid inner core. Image: Antonio Genova.

Then they entered MESSENGER trajectory data to find out how the planet’s gravity affected its orbit during the late stages of its mission when the spacecraft flew as low as 100 kilometres (65 miles) above the surface. A sophisticated software package allowed researchers to “adjust” various parameters and the best fit with the data was a solid inner core 2,000 kilometres across. For comparison, Earth’s core measures 2,400 kilometres across (1,500 miles)

“We had to pull together information from many fields: geodesy, geochemistry, orbital mechanics and gravity to find out what Mercury’s internal structure must be,” said Erwan Mazarico, a planetary scientist at NASA Goddard and co-author of the new study.

The findings of a solid inner core were published in the American Geophysical Union journal Geophysical Research Letters.