Cooling of Mercury's massive iron core has pared about nine miles
from the planet's diameter, more than twice as much as previous
"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
Unlike Earth, which as several plates of crust, Mercury has just one
rigid, rocky layer which bears telltale cliffs and chasms caused by
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
"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
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.
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That data indicated Mercury had lost 1.2 to 3 miles in diameter, a
finding that clashed with scientists' heat dissipation models for
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.
(Editing by Marguerita Choy)
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