Mercury, the smallest planet in the solar system, has gotten even smaller according to scientists. The latest data gathered by NASA's Messenger spacecraft indicates that the planet has shrunk by around nine miles.

After analyzing images sent back by Messenger, scientists have calculated that the planet is now 8.6 miles smaller in diameter compared to the original size of the planet when it was formed billions of years ago.

Scientists believe that the planet's shrinking is due to the fact that Mercury is getting cooler. This explanation is also consistent with what is currently known about planetary behavior. All planets, including our own, have been gradually cooling down causing a shrinking effect that was empirically verified. However, Mercury's case is interesting because the planet's shrinking has deviated from previously expected results.

"Mercury, a planet with a lithosphere that forms a single tectonic plate, is replete with tectonic structures interpreted to be the result of planetary cooling and contraction," said Carnegie Institution's planetary geologist Paul Byrne and his colleagues. However, the amount of global contraction inferred from spacecraft images has been far lower than that predicted by models of the thermal evolution of the planet's interior." Byrne is the lead author of a study published in the online journal Nature Geoscience.

Earlier images gathered by the spacecraft Mariner initially suggested that Mercury has lost around .6 to 1.9 miles from its radius since its formation around four billion years ago. However, this initial estimate contradicted what was currently known about the planet's geology. According to planetary models, Mercury should have shrunk even more. The more recent images taken by Messenger has resolved many of these contradictions.

"We show that Mercury's global contraction has been accommodated by a substantially greater number and variety of structures than previously recognized, including long belts of ridges and scarps where the crust has been folded and faulted," said the study's authors. "The tectonic features on Mercury are consistent with models for large-scale deformation proposed for a globally contracting Earth-now obsolete-that pre-date plate tectonics theory." 

When Mariner took photos of the planet Mercury, it was only able to cover around 45% of the planet's surface. Messenger, on the other hand, was able to successfully photograph the entire planet. 

"We find that Mercury has contracted radically by as much as 7 km, well in excess of the 0.8-3 km previously reported from photogeology and resolving the discrepancy with thermal models," the study said. "Our findings provide a key constraint for studies of Mercury's thermal history, bulk silicate abundances of heat-producing elements, mantle convection and the structure of its large metallic core."

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