The smallest planet in our solar system has a massive solid inner core.
In its final trip around Mercury before crash-landing in 2015, NASA’s MESSENGER mission zoomed in close to the planet, enabling scientists to make detailed measurements of its gravity, spin and internal structure. Those data, researchers report April 10 in Geophysical Research Letters, suggest Mercury has a solid inner core about 2,000 kilometers in diameter, making up about half of Mercury’s entire core.
Scientists already knew that Mercury’s core was huge, taking up about 85 percent of the planet (SN: 4/21/12, p. 8). In 2007, radar observations made from Earth detected small oscillations in Mercury’s spin rate that suggested the core was at least partially liquid (SN: 5/5/07). Then MESSENGER data revealed that the planet has a weak magnetic field generated by circulation of molten metal in that liquid core (SN Online: 5/7/15). But it wasn’t clear if Mercury, like Earth, also has a solid inner core.
To study the planet’s interior structure, MESSENGER measured Mercury’s distribution of mass by tracking tiny shifts in the spacecraft’s orbiting speed caused by subtle variations in gravitational pull. Using those data, scientists were able to estimate what sort of interior composition would best explain how Mercury spins.
Of the rocky planets, only Earth and Mercury still have magnetic fields generated by their cores. Such fields can shield planets from being battered by charged particles constantly streaming from the sun.
But Mercury’s core is cooling and solidifying faster than Earth’s. Earth’s solid inner core currently makes up only about a third of the total core. So observing how the innermost planet’s interior evolves, and how its magnetic field adjusts, may give a peek into the future of our own planet’s magnetic field, the researchers say.