8 Jun 2017

• Impact crater in the Mars region Margaritifer Terra shows various geological features such as 'chaotic terrain' and light-toned deposits.
• Chaotic areas indicate ice in the subsoil.
• Focus: space exploration

'Chaos terrain' and unusually light-toned deposits inside an impact crater in the southern highlands of Mars can be seen in this set of images acquired by the High Resolution Stereo Camera (HRSC), operated by the German Aerospace Center (Deutsches Zentrum für Luft-und Raumfahrt; DLR) on board the ESA Mars Express spacecraft. Remnants of valley networks in the vicinity indicate the surface runoff of melt water in the Martian past.
Impact craters on Mars are very diverse; on the crater floors we find, for example, sediment deposits, landslide remains, dunes, and river deltas fanning out into the crater from the side, as well as ejecta material from other craters. Often, such geological landforms also occur in combination with one another and craters become more and more complex as their diameter increases. Small craters a few kilometres across have a simple, bowl-like profile, while craters approximately 100 kilometres across have terraces at their edges created within a few seconds of the crater’s formation and, at even larger diameters, even a central mountain can form due to the rebounding of the crater floor.

Fascinating crater in the southern highlands
Some of these phenomena can also be observed in the crater shown in these HRSC images. Located in the Margaritifer Terra region, this crater has a diameter of approximately 70 kilometres and a flat floor surrounded by a very eroded, almost encrusted crater rim, the inner wall of which is partly covered by the remains of earlier landslides. Terrace-like steps are still visible in the crater rim. The smooth crater floor in the north-western section transitions into blocky, 'chaotic' material in the western part. Some light-toned deposits are visible in the southeast of the crater. Thus far it has not been possible to determine what these bright, solid deposits consist of exactly.

Chaotic terrain points to ice in the subsoil
Numerous so-called chaotic terrain are found on Mars, and they are characterised by an irregular accumulation of rock blocks of various sizes and plateaus. "It is assumed that chaotic areas are formed when underground ice caves melt by volcanic heat and the water is suddenly released," says Ralf Jaumann, DLR planetary researcher and Principal Investigator of the HRSC experiment. "When the water drains away, the surface collapses over the newly created cavities and the landscape caves in. Chaotic terrain is left behind with the remaining plateaus, which are separated by a deeply carved valley network."

The chaotic terrain in the interior of the crater is characterised by plateaus, buttes and knobs. The same features are also found north of the crater (on the right in images 1, 3, 5), in an area called Erythraeum Chaos that is assumed to have contained a lake. Sediments on the floor of Erythraeum Chaos were possibly transported through the Paraná Valles network, which is located further east, and deposited there. The Paraná Valles, together with the Loire Valles, form one of the largest and best integrated valley systems on Mars (upper right corner of images 1, 3, 5). However, these river systems and lake existed more than 3.5 billion years ago.
"In general, such chaotic terrain and valley networks – many of which are still found today – are evidence of the transport, storage and release of water in the Martian past,' explains Jaumann.

[Image]
(A) View of a crater with light-toned deposits and ‘chaotic’ areas in the Margaritifer Terra region
(B) Anaglyph image of the crater in the southern highland Margaritifer Tera region
(C) False color representation of the topography of the crater in the Margaritifer Terra region
(D) Crater with light-toned deposits in the southern highland Margaritifer Terra region
(E) Topographic overview map of the border region between Margaritifer Terra and Noachis Terra