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In its latest postcard from Mars, the European Space Agency's returns to Acheron Fossae: a dramatic network of chasms carved into the surface of the red planet.

In April, we highlighted the eastern end of —an amazingly diverse mix of rugged and smooth terrain covered in ancient ditches, craters, and solidified lava—as seen by Mars Express's . Today we head back to show the region's western edge, where the features are equally diverse: deep cracks, valleys, and meandering channels sculpted and filled by slow-moving flows of ice and rock.

Demonstrating just how intriguing these features are, also explored Acheron Fossae, all the way back in April 2004—just months after Mars Express began investigating the planet.

Broken ground

Acheron Fossae is an extensive system of deep, fault-like cracks (known as fossae), with alternating chunks of raised and lowered ground (, with "horst" referring to a raised block of ground formed in this way, and "graben" a lowered one). This pattern can be seen most clearly in the prominent channels slicing vertically through the frame just right of center.

Likely dating back over 3.7 billion years to when Mars was most geologically active, such a pattern was created as hot material rose upwards beneath the Martian crust. This upwelling of molten rock stretched and pulled the surface apart, creating kilometer-deep cracks and valleys—some of which are many hundreds of kilometers long.

What lies beneath

Once created, these valleys continue to transform. As shown clearly in the generated bird's-eye view, the valley floors are relatively smooth, marked by gently weaving lines reminiscent of a flowing river. Rather than water, these valleys have been filled by a slow, viscous flow of ice-rich rock, a lot like the rock glaciers we see here on Earth.

, and so act as good markers for how a planet's environment has changed over time. Here, they indicate that this region of Mars has experienced alternating periods of cool and warm, freeze and thaw.

These temperature fluctuations are driven by the tilt of Mars's rotational axis; unlike Earth, which sticks at a reasonably steady and moderate tilt due to the stabilizing influence of our large moon, Mars's tilt varies dramatically over time. This leads to alternating warm spells and ice ages, periodically letting ice creep near to the planet's equator before shrinking back to its poles.

While Earth and Mars both experience tilt fluctuations in this way, the variations are far bigger on Mars. Mars's tilt has swung between 15 and 45 degrees in the last 10 million years, while Earth's has varied between 22 and 24.5 degrees. These regular shifts—known as —play an important role in our planet's climate, but their effects are more subtle than those on Mars.

Left behind

To the right of frame the fossae transition into flat, dark lowland plains, with a knobbly strip of raised mounds and rocky hills in between. These are the remains of what was once a continuous rock layer; over time, flows of ice and rock have worn away the ground here, leaving behind rounded hills (knobs) and flat-topped plateaus (mesas) interspersed with narrow, meandering channels.

This transition—from fossae through knobs and mesas to lowland plain—is seen especially clearly in the associated topographical image of this region, where red and yellow tones (indicating higher ground) melt into light and darker blues (indicating lower elevations).

Another patch of smooth ground can be seen to the left. This plain lies next to the largest volcano in the solar system, , which lies about 1,200 km out of frame to the south (left).