Cooling made the crust buckle<\/p>\n
Mercury formed about 4.5 billion years ago and has been losing heat ever since. As the interior cooled and the solid center of its metal core<\/a> grew, the planet lost volume. The rigid shell had to fit a smaller planet, so it broke along faults and pushed the ground on one side of each break over the other.<\/p>\n Earlier teams estimated the loss of radius at 0.6 to 1.2 miles (1 to 2 kilometers), or as much as 4.3 miles (7 kilometers). The estimate depends on how researchers count the smaller cliffs.<\/p>\n A cooling planet should shrink equally in every direction, so its wrinkles should appear evenly across the surface. Instead, some regions have many long, tall scarps, as the cliffs are called, while others have almost none.<\/p>\n Nishiyama\u2019s team suspected that younger events had covered part of the record. They tested that with a global roughness map<\/a> they had published earlier this year. It measures how lumpy the surface is at a scale of 6 miles (10 kilometers).<\/p>\n A fresh impact sprays debris and smaller craters across the surrounding land, making roughness a useful measure of how recently an impact disturbed the surface. <\/p>\n Against a 2025 global catalog of scarps and ridges, the roughest regions had the fewest wrinkles. Around young craters wider than 90 miles (150 kilometers), such as Rachmaninoff, the zone of rough debris has far fewer of them.<\/p>\n \u201cThis correlation was the most surprising because it was well beyond my expectation at that time,\u201d Nishiyama told Earth.com.<\/p>\n Crater debris likely buried the wrinkles<\/p>\n The team weighed three explanations. The first is that debris from big impacts buried older cliffs. Near Rachmaninoff, the few long scarps that cross the rough zone get lower toward the crater center, as buried cliffs would.<\/p>\n The second is that a rough background makes a ridge harder to pick out by eye. That can\u2019t be the whole story, because researchers have mapped scarps in rough terrain south of the Caloris basin.<\/p>\n The third is that the wrinkles never formed. Heavily cratered ground may have a thick layer of loose, porous rubble. A planet can absorb some of its shrinking by crushing those pores shut instead of breaking along big faults.<\/p>\n Whichever is right, some shrinking is missing from the count, and the correction applies either way.<\/p>\n Mercury may have lost 7 miles of radius<\/p>\n Below a certain level of roughness, wrinkles and roughness increase together, because the cliffs themselves add bumps. Above it the trend reverses, and rougher ground means a lower estimate. The team took the smoother zone\u2019s value as the true one for the planet.<\/p>\n When every cataloged scarp is counted, the uncorrected estimate of lost radius is 5.2 miles (8.3 kilometers), and the correction raises it to 7.2 miles (11.6 kilometers). That would take about 14.5 miles (23 kilometers) off the planet\u2019s diameter. When only the main ridges are counted, the shortfall is smaller, and that spread is the 10% to 30% range.<\/p>\n Nishiyama said, \u201c30% is a little bit surprising, but the corrected amount of contraction actually makes sense to me.\u201d<\/p>\n The authors treat it as a lower bound, though. Some strain may be stored in the crust without ever breaking it, and later lava flows may have covered older scarps. And many ridge heights came from a terrain model too coarse to measure them well.<\/p>\n More shrinking means a hotter start<\/p>\n How much a planet contracts depends on what\u2019s inside it, so a bigger number changes the picture of Mercury\u2019s interior. \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 explained.<\/p>\n Asked by Earth.com about the interior, he said thorough work with cooling models is still needed, but that the findings call for a re-evaluation of Mercury\u2019s thermal history. No wrinkles cut across Rachmaninoff or its debris, and that crater is about 3.6 billion years old, so the cliffs around it are probably older.<\/p>\n BepiColombo will map the southern half<\/p>\n The analysis rests on data from NASA\u2019s MESSENGER<\/a> mission, which ended in 2015. Its laser altimeter, which measures heights, covered mainly the north pole. Everywhere else, the roughness comes from a terrain model built from stereo images, which can\u2019t reliably resolve features smaller than about 3 miles (5 kilometers).<\/p>\n That\u2019s why Nishiyama is waiting on BepiColombo<\/a>, which photographed Mercury<\/a> last year and reaches the planet in November. Its laser altimeter, BELA, starts science measurements next April, he said, and will map roughness at scales as fine as 330 feet (100 meters).<\/p>\n Nishiyama said the same method applies to other rocky worlds, above all to the Moon<\/a>, whose highlands are about 40% rougher than Mercury\u2019s cratered plains. Earlier mapping of the Moon\u2019s ridges<\/a> suggests it has lost less than 0.6 miles (1 kilometer) of radius, while models of its cooling interior<\/a> predict more.<\/p>\n \u201cAs the roughness on the Moon is much higher than Mercury, by using the method developed in this study, we can get closer to understanding the thermal history of the Moon as well,\u201d he added.<\/p>\n Until BELA delivers, whether the missing wrinkles are buried or never formed remains an open question, and the true size of Mercury\u2019s shrinking is known only as a minimum.<\/p>\n The full study was published in the journal Geophysical Research Letters<\/a>.<\/p>\n \u2014\u2013<\/p>\n Like what you read? Subscribe to our newsletter<\/a> for engaging articles, exclusive content, and the latest updates.<\/p>\n Check us out on EarthSnap<\/a>, a free app brought to you by Eric Ralls<\/a> and Earth.com.<\/p>\n \u2014\u2013<\/p>\n","protected":false},"excerpt":{"rendered":"Scientists found that Mercury’s roughest terrain may conceal ancient cliffs, suggesting the planet shrank substantially more than estimates…\n","protected":false},"author":2,"featured_media":598514,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[23],"tags":[111,139,69,147,392],"class_list":["post-598513","post","type-post","status-publish","format-standard","has-post-thumbnail","category-space","tag-new-zealand","tag-newzealand","tag-nz","tag-science","tag-space"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts\/598513","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/comments?post=598513"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts\/598513\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/media\/598514"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/media?parent=598513"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/categories?post=598513"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/tags?post=598513"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"\"](\"https:\/\/www.newsbeep.com\/nz\/wp-content\/uploads\/2026\/09\/cropped-mercury-scarp-fig1-credit-Hokkaido-University.webp\")
<\/a>A contractional landform on Mercury with arrows marking the scarp. Credit: Hokkaido UniversityThe roughest ground has the fewest wrinkles<\/p>\n