Older Smooth Plains on Mercury
There is abundant evidence that smooth-floored impact basins have been filled with lava. But less clear is the nature of more heavily cratered terrain surrounding smooth-floored basins and craters, such as that seen in this image. This hilly, rough terrain may have resulted from the piling up of ejecta from nearby craters. But looking more closely, there are several older, less well-preserved craters that also have smooth floors — implying that they too were filled with lava after they formed. Similar observations elsewhere on Mercury suggest that many older portions of the planet's surface were originally smooth volcanic plains, before subsequent impacts rendered them hilly and rough.
Date acquired: October 12, 2012
Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington