The HRSC image shows a remarkable scene in the highlands of Noachis Terra, one of the oldest regions on Mars (see context map). The highlands are characterized by ancient terrain dating back to the Noachian period, which lasted from approximately 4.1 to 3.7 billion years ago. The old age of the landscape is evident from the densely cratered surface that dominates this region. The image features part of Kaiser Crater, a large impact crater located among numerous neighboring craters, including Neukum, Greeley and Le Verrier (see context map). Kaiser Crater measures about 180 kilometers in diameter and reaches a depth of nearly two kilometers. The right side of the image shows the southern part of Kaiser Crater, while the left side displays a few smaller impact craters near its rim. What makes the HRSC scene particularly distinctive is the extensive dune field covering much of the crater floor, with its dark, metallic appearance (see annotated image).

This dune field and individual dunes are composed of dark, volcanic material. The metallic appearance is caused by bright frost deposits on their south-facing slopes. This frost can also be seen on some south-facing crater slopes. This scene was taken during the second half of the southern winter on Mars, when the seasonal frost cover grows from the South Pole up into the mid-latitudes of Mars.

The Kaiser Crater dune field consists of various dune types, some of which merge into one another. The individual dunes, mostly at the edge, are classic crescent dunes, also known as bachans (see annotated image). As more sand becomes available, the individual bachans merge with one another, giving rise to so-called bachanoid ridges. These represent a transitional form towards the dune type that occupies a large part of the dune field and emerges as sand availability increases: transverse dunes. Their dune ridges are oriented perpendicular to the wind direction and, just like those of the bachans and bachanoid ridges, indicate a prevailing wind direction from the west (the top of the image). Bachans can reach a height of up to more than hundred meters.

Such intra-crater deposits are common in this region – including nearby Neukum Crater, named after planetary scientist Gerhard Neukum (see 2018 press release). 

Another dune-like feature, which is known as Transverse Aeolian Ridges (TARs) can be found in Moni Crater, located within Kaiser Crater. These exhibit characteristics of both ripples and dunes (also see HiRISE image). With up to 14 meters of height they are significantly larger than ripples but smaller than dunes. They were probably formed by martian winds and might be made up of fine grained bright sand.

 In some places near the dune field, wind erosion has eroded the soil, exposing the underlying layers. These appear as light-coloured deposits (see annotated image) and suggest the presence of clay minerals, which typically form through the weathering of basalt – the volcanic parent rock – in the presence of water.

 If you look closely, you can also see the wind in action here: a small light-coloured spot with a red fringe is a dust devil currently moving across the Martian surface. The long shadow cast by this dust devil reveals that it reaches a height of approximately 1.9 kilometres (calculated from the shadow’s length and the position of the Sun). Dust devils are a widespread phenomenon on Mars and have often been observed in HRSC images (see 2026 press release of Mamers Valles).

 Another intriguing geological feature, also attributed to water activity, can be found within some of the smaller craters, such as the small impact crater located inside Kaiser Crater itself. Gullies are clearly visible along its steep inner crater walls, forming narrow channels that run downslope (see annotated image). These gullies may have formed when water from melted ice or from groundwater reservoirs triggered debris flows, transporting material down the crater wall.