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<\/p>\nThe closest planet to our sun is shrinking. Here\u2019s what that means<\/p>\n
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Telltale wrinkles that cover Mercury, the closest planet to the sun, suggest it was once a larger world that has shrunk in size \u2014 and it may be shrinking more than scientists realized.<\/p>\n
Related video above: New images of the sun show its surface in the finest detail yet<\/p>\n
The smallest planet in our solar system is about 36 million miles (58 million kilometers) from the sun on average and completes one orbit around our star every 88 days, according to NASA<\/a>.<\/p>\n The rocky world formed from swirls of gas and dust roughly 4.5 billion years ago. The early days of our solar system were chaotic, marked by violent interactions as a multitude of space rocks orbiting the sun crashed into other objects.<\/p>\n Newly formed planets are already hot, much like volcanic lava, but the energy from such collisions generates even more heat. As the heat escapes these worlds over time, the interiors of planets such as Mercury shrink.<\/p>\n Mercury\u2019s cracked surface bears scars from such shrinkage \u2014 scientists call these ridges and cliffs shortening structures. Some of the cliffs can reach up to 1 mile (1.6 kilometers) in height and can extend for hundreds of miles in length.<\/p>\n But asteroids and comets have also bombarded Mercury, riddling it with impact craters, much like the pockmarked surface of Earth\u2019s moon.<\/p>\n Debris from the impacts might have hidden just how much Mercury has contracted over time, according to a new study published Thursday in the journal Geophysical Research Letters.<\/p>\n Understanding Mercury\u2019s shrink rate can shed light on how the planet has evolved and reveal secrets about its mysterious interior.<\/p>\n \u201cThere are lessons about the formation and evolution of large, rocky bodies like our Earth, that can be learned on Mercury better than anywhere else in our Solar System,\u201d wrote Hannes Bernhardt, who was not involved in the study, in an email. Bernhardt is an assistant research scientist in the department of geological, environmental and planetary sciences at the University of Maryland, College Park.<\/p>\n The new study also highlights just how many questions linger about this tiny world \u2014 some of which could soon be answered by an unprecedented mission that will release two orbiters around Mercury in November.<\/p>\n A shrinking planet<\/p>\n As Mercury shrank, cracks likely appeared across its surface in a consistent way. But craters and debris from billions of years of impacts covered many of these fissures, said lead study author Gaku Nishiyama, a planetary scientist at the German Aerospace Center\u2019s Institute of Space Research in Berlin.<\/p>\n Mercury\u2019s proximity to the sun has made it a challenging planet to study and explore. Only two missions have ventured into this harsh environment in previous decades: NASA\u2019s Mariner 10 in 1974 and the agency\u2019s MESSENGER mission in 2011.<\/p>\n Mariner 10 conducted flybys of the planet that revealed Mercury\u2019s intriguing wrinkles, while MESSENGER provided a more global portrait of the planet\u2019s features, said Bernhardt, who has also studied Mercury\u2019s shrinking.<\/p>\n Nishiyama and his team used data from MESSENGER to create a global map of Mercury\u2019s surface roughness. They spotted a trend of fewer wrinkles in the roughest parts of Mercury, compared with smoother regions of the surface.<\/p>\n \u201cIt made us think that there\u2019s a process obscuring shortening structures,\u201d Nishiyama said, such as debris cloaking signs of contraction on the surface.<\/p>\n The team estimated how many wrinkles might be present across Mercury\u2019s surface if they were not hidden by debris. The computations determined that the planet may have shrunk 10% to 30% more than previously believed, or a radius change of about 7.2 miles (11.6 kilometers). Previous studies have suggested a change in radius between 0.6 to 1.2 miles<\/a> (1 to 2 kilometers) and up to 4.3 miles<\/a> (7 kilometers).<\/p>\n Paul Byrne, associate professor of Earth, environmental and planetary sciences at Washington University in St. Louis, said it\u2019s plausible that previous studies of Mercury\u2019s shrinking, including one of his own, missed geological cracks on the planet because they were hard to see in rough areas or because they had not formed at all due to Mercury\u2019s extremely rugged crust. Byrne was not involved in the latest study.<\/p>\n \u201cYet if we can accurately measure how much Mercury has contracted, we can make better estimates of its interior layering, the size and make-up of its core, its tectonic and volcanic histories, how its magnetic field is generated, and a whole lot more,\u201d wrote Byrne, also the director of the NASA Planetary Data System Geosciences Node, in an email.<\/p>\n Mercury is the second-densest planet in our solar system after Earth, mainly due to its large metallic core.<\/p>\n Calculating how much the planet has shrunk could provide a window into Mercury\u2019s core and what has transpired inside the planet as it cooled.<\/p>\n \u201cMore shrinking means Mercury could have a larger metal core, less light elements like silicon mixed into the metal core, or a higher starting temperature,\u201d Nishiyama said.<\/p>\n An unprecedented look at Mercury<\/p>\n The BepiColombo mission, which recently began its arrival at Mercury after traveling for nearly eight years, will position two orbiters near the planet at the end of this year.<\/p>\n Topography data that the mission collects could enable scientists to estimate more precisely how much Mercury has contracted over time and showcase the planet\u2019s cliffs, craters and ridges like never before, said Nishiyama, who is a member of BepiColombo\u2019s science team.<\/p>\n BepiColombo will make the first comprehensive global measurements of Mercury\u2019s topographic properties, which is something MESSENGER could not do, Byrne said.<\/p>\n \u201cThose new data will be key to testing whether this and other global contraction papers are right,\u201d Byrne said. \u201cBut I suspect we\u2019ll find that they are, and that we still have so very much to learn about the tiny innermost planet.\u201d<\/p>\n