English: Mercury's giant Caloris basin is the best-preserved large impact basin known on Mercury, and the high density of craters on its floor indicates that the basin is fairly old and probably formed about 3.8 billion years ago. This NAC image shows an area on the plains that partially fill the Caloris basin floor. On the right portion of this image, the light-colored rays emanating from Cunningham crater (named for the American photographer Imogen Cunningham) show that this crater is relatively young; bright ejecta rays tend to darken with time, as the ejected material is gradually modified by impacting micrometeoroids and solar particles (a suite of different processes that together are called “space weathering”). Relative age relationships such as this one are used to unravel Mercury's geologic history. The similar-sized Kertész crater is also visible on the left side of this image.