What a martian ice age left behind

Earth isn’t the only planet to go through ice ages. A handful have hit our home planet in the last 2.5 billion years, with the most recent – which peaked around 20 000 years ago – taking Earth’s average temperature down to around 7–10 °C (up to 8 °C cooler than today).

Ice ages are an entirely separate phenomenon to more recent human-caused global warming. They are part of an ancient geological rhythm primarily caused by changes in a planet’s path around the Sun and the wobble of its rotational axis. During an ice age, ice is more widespread in the form of glaciers and ice sheets, and fluctuating temperatures enable flows of ice to advance and retreat across the globe.

We see the telltale signs of previous ice ages on other planets, too – their impact on modern-day Mars is evident in these new images from the High Resolution Stereo Camera on ESA’s Mars Express.

The roughly parallel lines slicing diagonally through the image are known as Coloe Fossae, a feature created as alternating chunks of ground fell away. Many craters, formed as incoming space rocks collided with the surface, are seen here: large, small, overlapping, irregular, clustered, older and more recent, both well-defined and smoothed away by erosion. On the floors of the valleys and craters is something exciting: patterns of swirling lines that indicate where material flowed during a previous martian ice age.

Icy flows across Mars

These patterns hint at the past climate of the Red Planet. They’re known more technically as lineated valley fill (in valleys) or concentric crater fill (in craters). They formed as icy debris flowed slowly across Mars’s surface – reminiscent of glaciers seen here on Earth – and became topped by a thick layer of rocky material.

However, this region sits at a latitude of 39°N, nowhere near Mars’s north pole (at 90°N). How did ice accumulate here?

The answer lies in the pulse of advancing-retreating glaciers during an ancient ice age. Although Mars is currently dry, it has experienced alternating periods of warm and cold, freeze and thaw, throughout its history, driven by changes in the tilt of its axis.

During cold periods, ice spreads from the martian poles all the way down to these mid-latitudes, before retreating back when things get warmer – but leaving telltale signs behind. We see lineated valley and concentric crater fill consistently throughout this latitude band of Mars, suggesting that the entire global climate of Mars changed. This area may have been covered in ice as recently as half a million years ago, when Mars’s most recent ice age ended.

These features – signs of glacial flows, Coloe Fossae, craters – are marked on the annotated image, and the split between Mars’s northern and southern terrain is best seen in the topographic and context maps. This divide encircles the entire planet; in some places it’s sharply marked by a two-km-high cliff, while in others – such as here – it’s more of a broad, broken-up transitional zone (known as Protonilus Mensae).

Signs of lineated valley and concentric crater fill were also highlighted in our August Mars Express feature on Acheron Fossae.

The Mars Express High Resolution Stereo Camera (HRSC) was developed and is operated by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). The systematic processing of the camera data took place at the DLR Institute of Planetary Research in Berlin-Adlershof. The working group of Planetary Science and Remote Sensing at Freie Universität Berlin used the data to create the image products shown here.