Measuring just 4880 kilometers across, Mercury is a small world. The planet became slightly smaller as its interior cooled, which caused Mercury to shrink, buckling its surface and creating numerous cliffs and ridges. Now, after studying 5934 of these features, researchers report online today in Nature Geoscience that Mercury's contraction was much greater than previously thought : During the past 4 billion years, the planet's diameter decreased by 7 to 14 kilometers. Earlier estimates date back to data from Mariner 10, a spacecraft that flew past Mercury three times during the 1970s but saw less than half the planet. In 2011, however, the MESSENGER spacecraft began orbiting the sun's innermost planet, imaging the entire surface. The greater estimate of shrinkage accords with models that predict how much a rocky planet should contract as its interior cools; the new work may also lend insight into the evolution of extrasolar planets that, like Mercury and unlike Earth, lack any moving continents.
ScienceShot: Honey, I Shrunk the Planet