Mars Express has spotted a rare case of a crater once filled by a lake, revealed by the presence of a delta.
This delta is an ancient fan-shaped deposit of dark sediments, laid down in water. It is a reminder of Mars’ past, wetter climate.
The delta is in the Eberswalde crater, in the southern highlands of Mars. The 65 km-diameter crater is visible as a semi-circle on the right of the image and was formed more than 3.7 billion years ago when an asteroid hit the planet.
The rim of the crater is intact only on its right-hand side. The rest appears only faintly or is not visible at all. A later impact created the 140 km diameter Holden crater that dominates the centre and left side of the image. The expulsion of large amounts of material from that impact buried parts of Eberswalde.
However, within the visible part of Eberswalde, the delta and its feeder channels are well preserved , as seen near the top right of the crater. The delta covers an area of 115 square kilometres. Small, meandering feeder channels are visible towards the top of the crater, which would have filled it to form a lake.
After the deposition of the delta sediments in the crater’s ancient lake, fresher sediments accumulated to cover up a major part of both the channels and the delta. These secondary sediments, presumably deposited by the wind, were later eroded in the delta area, exposing an inverted relief of the delta structure.
This delta structure, first identified with NASA’s Mars Global Surveyor spacecraft, is characteristic of the presence of a lake in the crater at that time. Such features provide a clear indication that liquid water flowed across the surface of Mars in the planet’s early history.