Arcadia Planitia Dark Splotch

Arcadia Planitia occupies a region just north of Amazonis Planitia, one of the brightest and dustiest regions on Mars. Along the boundary between these two regions is a dark splotch roughly 600 km long that is relatively free of dust. This THEMIS image straddles the border of the dark splotch, producing the dramatic change in brightness from north to south. Note that many of the small craters in the dark portion of the image contain bright material on their floor. It may be that a mantle of dust has been stripped back revealing the dark surface of the splotch but the crater floors have retained the dust. Alternatively, the bright material may represent a more resistant unit that once covered the area and is difficult to erode from the crater floors. Either way, the presence of the bright material in so many craters produces a scene that is quite unusual for the surface of Mars. http://photojournal.jpl.nasa.gov/catalog/PIA04038