Tiny Mercury may have a solid metallic core that rivals Earth's

Asolid metallic core nearly as large as Earth's might be lurking deep within the Solar System's smallest planet, according to new research based on data gathered by NASA's Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) mission. That spacecraft spent four years circling the tiny planet before intentionally crashing on Mercury's surface. Now, scientists have used data from near the end of the probe's mission to peer deeper than ever into the planet.

As the MESSENGER spacecraft orbited the tiny planet, the probe pinpointed the poles that Mercury rotates around. One of the instruments aboard also carefully measured small variations in local gravity.

Those variations cause a spacecraft to speed up or slow down, just a hair, and are the result of patches of

the planet that are more or less dense than the average. They were particularly detailed near the end of the spacecraft's mission, when engineers sent the probe as close as 105 kilometres (65 miles) above the planet's surface.

Combining that information, scientists could build a detailed model of the tiny planet, then play around with variables like the size of a solid inner core and compare the results with reality. That modelling suggested both that Mercury should have a solid inner core and that this core should be quite large.

In fact, their most compelling model setting featured a solid inner core 2,000 kilometres (1,260 miles) across. That's about half the size of Mercury's entire core, which takes up 85 per cent of the planet. Even by terrestrial standards it's huge; Earth's solid inner core is about 2,400-kilometres (1,500-miles) wide.