Department of Earth Sciences

Following up on our April 2025 release, which explored the eastern part of Acheron Fossae, this HRSC image reveals the western edge of the region. Just a quick reminder: Acheron Fossae is located about 1200 kilometers north of Olympus Mons – the tallest shield volcano in our solar system. The entire ridge stretches roughly 800 kilometers and rises in the shape of an arc on the Martian surface – perfectly reflecting its name. On its northern and western side, the arc-shaped ridge transitions into the flat plains of Arcadia and Amazonis Planitia. To the south, the rift system scratches at the vast Aureole deposits – massive landslide accumulations formed by repeated collapses of Olympus Mons’ flanks (see context map).

Acheron Fossae is marked by large, deep cracks on the surface known as ‘fossae’. These linear depressions are a textbook example of what’s called a ‘horst and graben’ landscape – a pattern of raised and lowered blocks of crust. The formation of such a structure is driven by the planet’s internal geological activity, where hot mantle material from deep inside rises toward the surface – a process known as mantle upwelling. This causes the ground to stretch and pull apart, leading blocks of crust to sink down (graben) relative to the neighboring higher blocks (horst). Since Mars was most geologically active during the Noachian period, around 3.7 to 3.9 billion years ago, Acheron Fossae likely formed in that period. Over time, many of these grabens have been filled with different types of material, especially deposits rich in ice.

In the HRSC images, multiple grabens of varying depths cut across the right (northern) side of the scene (see annotated image). A closer look reveals smooth material with a streamlined pattern on the floors of these grabens. These features are known as lineated valley fills and typically form through the slow, viscous flow of ice-rich debris (see annotated image). The deposits are believed to be mostly made up of ice covered by a layer of rock – similar to rock glaciers found here on Earth.

The flow-like feature is common in periglacial environments – not only on Earth but also on Mars. Their presence suggests that the region experienced alternating cold and warm periods, driven by repeated freeze–thaw cycles. These climatic fluctuations are caused by changes in Mars’ orbital parameters – especially the tilt of its rotation axis, known as obliquity. Unlike Earth’s relatively stable tilt of about 23.5°, thanks to our loyal companion the Moon, Mars’ axial tilt varies more dramatically and frequently due to gravitational influences from other planets. These obliquity cycles cause periodic shifts in Mars’ climate – and with them, changes in the distribution of ice. Simply put: during periods of high obliquity, ice spreads from the poles toward the mid-latitudes. When the tilt is lower, as it is today, the ice retreats back to the poles, leaving behind visible traces in the landscape.

Between the grabens and the adjacent lowlands, isolated remnants of what was once a continuous rock layer mark the landscape. These erosion-resistant remnants come in different shapes – some appear as rounded hills known as knobs, while others have flat tops and resemble small plateaus called mesas. Interestingly, many of them are partially surrounded by lineated valley fill material, which seems to have traveled all the way down to the lowlands. The knobs are most prominent on the right side (north) of the image. Toward the center and left-hand side, they gradually become fewer – and eventually disappear. All around the knobby terrain small, narrow meandering channels mark the surface (see annotated image). Meandering channels have a serpentine morphology formed by erosion and sedimentation of material.

The left side of the scene looks much smoother, which may point to a different geological origin. One possibility is that this smoother material may be linked to the massive landslides triggered by the collapse of Olympus Mons’ flanks forming its Aureole (see annotated image). If that is the case, the debris would have traveled a huge distance to reach the foot of Acheron Fossae.

Read more about the eastern part of Acheron Fossae in the press release from April 9, 2025 and explore all press releases with the interactive map.

→ Acheron Fossae – Glaciers and Volcanos in a giant Graben System

Find out more about the extensive Auerole of Olympus Mons in the press release from Aug 23 2023:

→ Lycus Sulci – The wrinkled Aureole of Olympus Mons