Mercury surprises: tiny planet has strange innards and active past

Using observations from NASA’s Messenger spacecraft in orbit around Mercury, researchers observed that the floors of many craters have been tilted substantially. Part of the floor of the Caloris basin has even been raised above its rim. This suggest that internal forces pushed the craters up after the impacts created them, providing strong evidence that Mercury remained geologically active long after its formation.

Researchers also estimated Mercury’s gravity field by precisely radio-tracking Messenger’s movements around the planet. From these estimates, they determined that Mercury has “mascons” (short for “mass concentrations”), which are large positive gravity anomalies associated with big impact basins.

The team’s gravity calculations also suggest that Mercury has an iron core that comprises roughly 85% of the planet’s radius. (For comparison, Earth’s iron core covers about half of its radius.) Further, it looks like a layer of solid iron sulfide overlies Mercury’s core — a feature not known to exist on any other terrestrial planet.

The new findings shed light on Mercury’s past, and on the formation and evolution of rocky planets in general. But they also serve to remind scientists that they’re in for many more surprises as they continue to probe the solar system’s many mysteries.

Above: Perspective view of ancient volcanic plains in the northern high latitudes of Mercury revealed by NASA’s Messenger spacecraft.


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