In the vast desert landscapes of Mars, peculiar geological formations continue to intrigue scientists. A prime example of this is Acheron Fossae, a dramatic network of chasms and valleys carved into the Martian surface by forces that date back billions of years. The European Space Agency’s (ESA) Mars Express orbiter has provided invaluable insights into this area, revealing how these geological features came to be and what they can tell us about the planet’s history. A recent ESA report revisits Acheron Fossae, bringing new findings that shed light on the planet’s ancient and dynamic past.
The Geology of Acheron Fossae: When The Ground Cracks and Shifts
Acheron Fossae is an extraordinary geological feature on Mars, defined by an extensive series of deep cracks, known as fossae, that stretch across the Martian landscape. These chasms are not just simple fractures; they are the product of complex geological processes that began over 3.7 billion years ago, during the peak of Mars’s geological activity. The cracks in the region exhibit a distinct pattern known as horst and graben, where elevated blocks of ground (horsts) sit next to lower areas (grabens), indicating that Mars’s crust has been pulled apart by the upwelling of molten rock from beneath.
What makes this area particularly intriguing is how it reflects the tectonic activity of Mars. Unlike Earth, which experiences plate tectonics and volcanic activity in a predictable cycle, Mars appears to have undergone intense periods of volcanic activity and stretching, followed by phases of relative calm. The pattern seen in Acheron Fossae is a result of hot material rising from beneath the crust, stretching and breaking the surface. These fractures have remained visible for billions of years, offering scientists a unique window into Mars’s geological evolution.
The most striking feature of Acheron Fossae is the diversity of its terrain. From deep, fault-like cracks to meandering valleys filled with a mixture of ice-rich rock, this region holds numerous clues about Mars’s past. Understanding how such features were created—and how they have evolved—allows scientists to reconstruct the environmental conditions of ancient Mars, shedding light on how the planet transitioned from a potentially habitable world to the cold, arid place we see today.
Context map of Acheron Fossae on Mars. Credit: ESA
Acheron Fossae and the Evidence of Mars’s Ancient Climate
The importance of Acheron Fossae goes beyond its striking geological features; it also serves as a time capsule of Mars’s ancient climate. The valley floors in this region, once filled with flowing ice and rock, tell a story of alternating warm and cold periods on the planet. These flows are often compared to rock glaciers found on Earth, a phenomenon where ice mixes with rock to create slow-moving glaciers. Such glaciers are sensitive indicators of climatic changes, as they form in periods when the environment is cold enough to allow ice to accumulate.
This slow-moving ice, combined with the presence of rock debris, serves as evidence of climatic fluctuations on Mars. These fluctuations are driven by Mars’s tilt—unlike Earth, which maintains a relatively stable axial tilt, Mars’s tilt varies dramatically over time. Between 15 and 45 degrees over the last 10 million years, Mars’s tilt has caused significant changes in the planet’s climate, resulting in periods of warming and cooling. These shifts in tilt—known as Milankovitch cycles—have caused ice to migrate towards the planet’s equator during warmer periods, before retreating back to the poles during colder spells.
The evidence gathered by Mars Express indicates that these climatic shifts played a crucial role in shaping the landscape we see today. The ice-rich rock deposits within the Acheron Fossae are remnants of these periods, offering clues about the cycles of freezing and thawing that have shaped not only the surface features but also the planet’s atmosphere over millennia.
Acheron Fossae from above. Credit: ESA
The Unique Transition from Faults to Flat Plains
Another fascinating aspect of Acheron Fossae is its unique topography. The area’s rugged faults gradually transition into lowland plains, providing a dynamic contrast in the landscape. The western fringes of Acheron Fossae show this shift clearly, where the deep cracks lead into areas of smooth, flat plains. These plains, once part of a continuous rock layer, have been eroded over time by the combined effects of ice and rock flows. The result is a landscape marked by knobs, which are small, rounded hills, and mesas, which are flat-topped plateaus.
These flat-topped plateaus, along with the meandering channels in between, are remnants of what was once a continuous rock formation. Over time, these areas were subjected to erosion by ice-rich flows, which wore down the surfaces and left behind the characteristic topography we see today. The transition from jagged cracks and faults to the smooth plains and mesas speaks to the dynamic nature of Mars’s surface, influenced by both tectonic activity and climatic changes.
To the south, near Olympus Mons, the largest volcano in the Solar System, lies another smooth patch of ground, offering a stark contrast to the rugged Acheron Fossae region. This proximity to Olympus Mons suggests that volcanic activity in the region could have also contributed to shaping the Martian landscape, further compounding the complexity of Acheron Fossae’s history.