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<\/p>\nThe Solar System\u2019s smallest planet has contracted by nearly 19 km (12 miles) across, as crater debris masked telltale wrinkles on its surface, according to new research led by the German Aerospace Center\u2019s Institute of Space Research.<\/p>\n
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This color image of Mercury was generated from three MESSENGER images taken through filters sensitive to light in different wavelengths. Image credit: NASA \/ Johns Hopkins University Applied Physics Laboratory \/ Carnegie Institution of Washington.<\/p>\n
Like the other rocky planets in our Solar System, Mercury formed through a flurry of violent collisions between rocks and asteroids orbiting the Sun around 4.5 billion years ago.<\/p>\n
The energy from these impacts generated huge amounts of heat, which Mercury has been losing ever since.<\/p>\n
Like a balloon left out in the cold, Mercury\u2019s interior shrank as it cooled, and the outermost rocky layers compensated for the planet\u2019s changing size by crumpling and cracking to form tectonic features such as scarps and ridges.<\/p>\n
\u201cDetermining how much Mercury\u2019s radius has decreased over time upon planetary cooling is essential for understanding the planet\u2019s geodynamic evolution,\u201d said lead author Dr. Gaku Nishiyama and colleagues.<\/p>\n
\u201cThis decrease is estimated by analyzing shortening structures, which are the tectonic expression of faults that have accommodated the planet\u2019s global contraction.\u201d<\/p>\n
\u201cInterestingly, the observed heterogeneous distribution of these landforms does not match the isotropic pattern expected from global cooling.\u201d<\/p>\n
\u201cOne possibility is that some structures have been obscured by resurfacing events, such as impact ejecta deposition.\u201d<\/p>\n
\u201cTo test this hypothesis, we use surface roughness as a proxy for geological freshness.\u201d<\/p>\n
In the study, the researchers combined earlier maps of geologic evidence for contraction with new maps of surface roughness for the entire planet\u2019s surface.<\/p>\n
They showed that the roughest areas on Mercury have fewer visible wrinkles.<\/p>\n
Next, they used the contraction required to form shrinkage ridges and scarps in less disrupted areas to estimate how much contraction likely occurred planet wide, including under rough patches.<\/p>\n
They found that missing features in rough areas could amount to 10-30% more shrinkage over Mercury\u2019s lifetime \u2014 a total change of up to 23 km (14.5 miles) in the planet\u2019s diameter rather than the currently estimated 4-16 km (2.5-10 miles).<\/p>\n
\u201cAround 30% is a little bit surprising, but the corrected amount of contraction actually makes sense to me,\u201d Dr. Nishiyama said.<\/p>\n
\u201cWith his updated shrinkage estimate, observations of how much Mercury has cooled and shrunk are now more in step with predictions based on physics, suggesting we are closer to understanding what is going on inside the solar system\u2019s smallest rocky planet.\u201d<\/p>\n