关于这张图
On this part of Mars there is an extensive surface brightness (albedo) boundary. One hypothesis is that this boundary marks a shoreline, from a time long ago when liquid water could have been stable on the surface of Mars. With the resolution of HiRISE we can see that the reason one side of the boundary is darker than the other is due to the many boulders strewn across the darker side. https://photojournal.jpl.nasa.gov/catalog/PIA25183
