The distinctive form of a delta arises from sediments that are deposited by a river as it enters slower-moving water, like a lake or a sea, for example. The Nile River delta is a classic example on Earth, and uncannily similar features have been spotted on Saturn’s moon Titan and – closer to home – Mars. While liquid water is no longer present on the surface of Mars, features in the left portion of this image provide strong evidence of it having played an important role in the history of the Red Planet. Furthermore, water-ice is still stable on the surface today, and a recent discovery from Mars Express detected a pocket of liquid water below the surface.
The 100-metre-thick fan-shaped deposit seen in this image is found in Eberswalde crater in the southern hemisphere of Mars (326.33ºE/23.55ºS). The image covers an area of 31 x 7.5 km and was taken on 16 November 2018 by the Colour and Stereo Surface Imaging System (CaSSIS) of the ESA-Roscosmos ExoMars Trace Gas Orbiter.
While presented in beautiful aqueous blues and greens, the image is false-colour. The layered rocks that comprise the delta deposits are indicated in white/yellow to purple/blue. The yellow represents the presence of oxidised iron deposits, indicating that the rocks were altered by the presence of water, while the blues signify less altered materials. This suggests that the influence of liquid water reduced over time, perhaps relating to a change in environmental conditions… [More at link]