This Nasa image shows a new colour mosaic created from MESSENGER which is part of a release by NASA's Planetary Data System (PDS). It spent four years orbiting Mercury before it ran out of fuel and crashed into the planet's surface on April 30.

Cape Canaveral - Detailed maps of Mercury's cliffs and ditches show the solar system's innermost and smallest planet Mercury has lost much more real estate due to cooling over four billion years than scientists thought, according to a report published on Sunday.

Cooling of Mercury's massive iron core has pared about nine miles (14km) from the planet's diameter, more than twice as much as previous estimates.

“When you look at the actual number, it's really pityingly small, compared to the size of a planet. But it doesn't need to change very much to have some effect,” said planetary scientist Paul Byrne, with the Carnegie Institution's Department of Terrestrial Magnetism.

Scientists studied more than 5 900 surface features, including cliff-like scarps and wrinkle ridges, to calculate how much Mercury has condensed.

Unlike Earth, which as several plates of crust, Mercury has just one rigid, rocky layer which bears telltale cliffs and chasms caused by global contraction.

The measurements, made with Nasa's Mercury-orbiting MESSENGER spacecraft, match computer model predictions, which scientists use to determine the planet's inner composition, chemistry and structure.

“An awful lot of a planet's processes are driven by its heat loss - that's a primary thing that drives a planet's evolution,” Byrne said. “We didn't set out to prove the models right, but it turns out this number is exactly what the models have been predicting for 40 years.”

Previous maps of Mercury's surface map date back to the mid-1970s. Nasa's Mariner 10 spacecraft made three flybys of the planet, imaging about 45 percent of its surface.

That data indicated Mercury had lost 1.2 to 3 miles (2 to 4km) in diameter, a finding that clashed with scientists' heat dissipation models for the planet.

In addition to learning more about how Mercury evolved, the discovery has implications for assessing the compositions of planets beyond the solar system.

“It may be that Mercury is an archetypal example of what a planet does and how it behaves as it has cooled in time,” Byrne said.

The research is published in the journal Nature Geoscience. - Reuters