Wrinkled Mercury's shrinking history

Mercury Mercury is 4,880km wide and is dominated by its giant iron core

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The planet Mercury is about 7km smaller today than when its crust first solidified over four billion years ago.

The innermost world has shrunk as it has cooled over time, its surface cracking and wrinkling in the process.

Scientists first recognised the phenomenon when the Mariner 10 probe whizzed by the planet in the mid-1970s.

But the latest images from the US space agency's Messenger satellite have enabled researchers to refine their estimate for the amount of contraction.

And, as they report in the journal Nature Geoscience, it is significantly more than previously realised.

Mariner made two passes of Mercury, in 1974 and 1975, photographing about 45% of the planet's surface.

Evident in those pictures were long scars where rock had been thrust upwards as the body shortened.

These lobate scarps, as they are known, typically run for hundreds of kilometres, and separate terrains that can differ in height by thousands of metres.

Mercury: The inner-most planet

Mercury eight colour map
  • Mercury was visited first by the Mariner 10 probe in the 1970s; and by Messenger currently
  • The planet's diameter is 4,880km - about one-third the size of Earth
  • It is the second densest planet in Solar System; 5.3 times that of water
  • The Caloris Basin is the largest known feature (1,300km in diameter)
  • Scientists speculate there is water-ice in the planet's permanently shadowed craters
  • Mercury's huge iron core takes up more than 60% of the planet's mass
  • It is an extreme place: surface temperatures swing between 425C and -180C
  • Mercury is the only inner planet besides Earth with a global magnetic field
  • Messenger is the first spacecraft to go into orbit around the planet

From the Mariner evidence, researchers calculated Mercury must have decreased its radius by about 1-3km over its history.

But that figure was in conflict with modelling studies that suggested a cooling object like Mercury should have contracted much more in four billion years.

Messenger helps to resolve the inconsistency. Since entering into orbit in 2011, it has photographed 100% of the planet.

This has allowed for a more extensive study of the scarp features and the more subtle wrinkled ridges that also criss-cross its surface.

The new assessment now brings the observed shrinkage - a reduction in radius of 7km - into the realm expected by the models.

Dr Paul Byrne from the Carnegie Institution in Washington DC is the lead author on the Nature Geoscience paper. He marvelled at the surface features on planet.

"Some of these lobate scarps are enormous," he told BBC News.

"There's a structure called Enterprise Rupes in the southern hemisphere that is a single scarp system. It's 1,000km long and in places has 3km of relief. Imagine standing in front of it. It's Mercury's version of a mountain belt.

"It utterly dominates the topography and it is astounding given the diminutive size of Mercury."

The innermost world is a fascinating oddball. Whereas the Earth has an extensive crust and mantle shrouding its metal core, Mercury is very different.

Estimated to be nearly 4,000km in diameter, the planet's metal core is its defining feature. It is covered only by a thin rocky veneer that may be little more than 400km thick.

Although some of the core must still be liquid, part of it will have cooled and solidified, losing volume as a result. This will have scrunched the thin, overlying layer of rock.

Europe and Japan plan to launch a joint mission to Mercury to follow up Messenger's observations

The BepiColombo venture should launch in 2016. One of its principal investigators will be Prof Dave Rothery from the UK's Open University.

"People used to think the Earth was shrinking - which it is a little bit, but we can't see it because of the way tectonic plates are created and destroyed on the Earth," he explained.

"Before we understood plate tectonics, people thought mountain belts on Earth were because the planet was shrinking and forcing stuff upwards, and areas of thick accumulation of sediment were where the crust was being forced down by contraction. We now know that's broadly speaking wrong, but this is the process on Mercury because it's a one plate planet."

This has relevance as scientists try to understand planets beyond our Solar System. Many of these, too, may have just the single plate and exhibit very similar surface-crumpling features to those seen on Mercury.

Enterprise Rupes Mountain ranges: Mercury is covered with massive scarps where the surface has shortened

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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