On January 14, 2008, the MESSENGER spacecraft observed about half of the hemisphere not seen by Mariner 10. These images, mosaicked together by the MESSENGER team, were taken by the Narrow Angle Camera (NAC), part of the Mercury Dual Imaging System (MDIS) instrument, about 20 minutes after MESSENGER's closest approach to Mercury (2:04 pm EST), when the spacecraft was at a distance of about 5,000 kilometers (about 3,100 miles). The image shows features as small as 400 meters (0.25 miles) in size and is about 370 kilometers (230 miles) across.
The image shows part of a large, fresh crater with secondary crater chains located near Mercuryís equator on the side of the planet newly imaged by MESSENGER. Large, flat-floored craters often have terraced rims from post-impact collapse of their newly formed walls. The hundreds of secondary impactors that are excavated from the planetís surface by the incoming object create long, linear crater chains radial to the main crater. These chains, in addition to the rest of the ejecta blanket, create the complicated, hilly terrain surrounding the primary crater. By counting craters on the ejecta blanket that have formed since the impact event, the age of the crater can be estimated. This count can then be compared with a similar count for the crater floor to determine whether any material has partially filled the crater since its formation. With their large size and production of abundant secondary craters, these flat-floored craters both illuminate and confound the study of the geological history of Mercury.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington