Scarps that cut across the surface of Mercury suggest the planet has been shrinking (Image: Science/AAAS)

The solar system’s smallest planet has been shrinking at an unexpected rate, researchers announced on Thursday.

When NASA’s Mariner 10 probe flew by Mercury in 1974 and 1975, it returned images of strange cliffs called ‘scarps’ that cut across all sorts of geological formations. That suggested that the planet’s surface has contracted over time.

Now, pictures of Mercury’s surface taken with NASA’s Messenger spacecraft confirm that the crust appears to have buckled. In fact, the planet seems to have shrunk more than previously thought – and may still be shrinking.


The new result comes from an analysis of pictures snapped when the spacecraft whipped past the planet on 14 January and photographed a previously unseen 20% of the surface.

In order for such shrinking to take place, Mercury must have had a molten, liquid core that has cooled and contracted over time.

That may settle a debate that has raged for more than 30 years about the planet’s magnetic field. Researchers have wondered whether the field is more like Earth’s, fed by a dynamo of circulating fluid, or like that of Mars, which hosts solid, magnetised rocks.

Liquid motion

In 2007, a team bounced radar signals off Mercury, revealing slight variations in the speed of the planet’s rotation that suggested the planet’s core is partly liquid.

Now Messenger researchers say additional images of scarps, along with a better map of the planet’s magnetic field, show Mercury has a constitution similar to Earth’s.

“Both of these results point therefore to an active source for Mercury’s magnetic field, a dynamo stirred by motions in Mercury’s fluid outer core,” says the mission’s chief scientist, Sean Solomon of the Carnegie Institution of Washington in Washington DC, US.

Mariner 10 images suggested the planet’s radius, which is roughly 2400 kilometres, has shrunk by 1 to 2 km since its formation more than 4 billion years ago. “What Messenger is showing us is that’s an underestimate,” says Solomon.

The new scarp images, which were taken from a better angle relative to the Sun than Mariner 10, show the planet seems to be shrinking by at least a third more than previously thought.

A second Messenger flyby on 6 October, which will image another 30% of the planet’s surface, could show shrinkage that is even more dramatic, says Solomon.

Further study may help date the scarps, revealing whether Mercury is still shrinking or has stopped.

Journal reference: Science (vol 321)