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| Mercury |
One of the smallest planets in our Solar System has a huge solid core.
In its last trip around Mercury before crash-landing in 2015, NASA's Messenger mission zoomed closer to the planet, making scientists able to measure its gravity, spin and internal structure in detail. Those data, researchers have reported on Geophysical Research Letters on April 10, suggest that Mercury has a solid internal core of approximately 2,000 km in diameter, which is roughly half of the entire core of Mercury.
Scientists already knew that Mercury's core was very large, which was taking about 85 percent of the planet's part (SN: 4/21/12, p. 8). In 2007, radar observations from the Earth detected small oscillations in the spin rate of Mercury, which suggested that the core is at least partially liquid (SN: 5/5/07). Messenger data then revealed that the planet has a weak magnetic field that is produced from the molten metal movement in that liquid core (SN Online: 5/7/15). But it was not clear if Mercury, like Earth, is also a solid internal core.
To study the internal structure of the planet, Messenger measured the distribution of Mercury keeping in mind the small changes in the orbital speed of the spacecraft due to subtle changes in the gravitational pull. Using those data, scientists were able to guess what kind of internal structure would best explain how Mercury rotates.
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