Astronomers recently observed "grooves" on the dark side of Lutetia, an asteroid encountered by ESA's Rosetta spacecraft that provides evidence of a large impact crater on the asteroid.
Rosetta flew past Lutetia at a distance of 3168 km in July 2010, en route to its 2014 rendezvous with its target comet.
The way in which grooves are formed on these bodies was still widely debated, but it likely involves impacts. Shock waves from the impact travel through the interior of a small, porous body and fracture the surface to form the grooves.
One of the groove systems on Lutetia is associated with the Massilia crater and another with the North Pole Crater Cluster, which comprises a number of superimposed craters. Both are on the asteroid's northern hemisphere.
But another group of grooves points to a crater not seen during Rosetta's brief flyby, in the asteroid's southern hemisphere.
Its implied presence has earned it the nickname 'Suspicio'. The grooves related to Suspicio cover a large area on the asteroid, suggesting it might span several tens of kilometres . By comparison, Massilia, the largest known crater on Lutetia, was about 55 km wide, and the largest of the polar cluster was about 34 km across.
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By observing how subsequent small craters lie over the grooves on Lutetia, the scientists determined the relative ages of the three larger cratering events. Massilia is thought be the oldest of the three craters and the polar cluster the youngest, with Suspicio between.
The study is published inPlanetary and Space Sciencethis month.