Last week, ESA published an image from its Mars Express Orbiter showing Mars’ icy north pole during summer 2006. The synonymous dark red surface of the planet is streaked with "bright swathes of ice" and "streams of clouds". This image was published to coincide with the Seventh International Conference on Mars Polar Science and Exploration, which took place in Argentina from January 13-17, 2020.
The nature of Mars’ polar regions varies throughout the Martian year. During the summer, the poles are covered predominantly in water ice. However, when winter arrives, temperatures drop below -125°C (-193°F), causing carbon dioxide in the atmosphere to precipitate. This forms a layer of dry ice a couple of meters thick on top of the water ice and creates carbon dioxide clouds.
The image captured by the Mars Express’ High Resolution Stereo Camera (HRSC) is not obscured by these clouds due to its summer capture date. Instead, at resolutions greater than 10 meters per pixel, the HRSC is able to generate beautiful views of Mars’ surface. The image slice of the north pole clearly shows the dark troughs and bright ice in a zebra stripe pattern. Even ripples of clouds perpendicular to the dark troughs can be seen to the left of the image, which ESA attributes to small local storms that have eroded slopes and scarps, progressively changing the Martian terrain.
When viewed in context, the series of depressions are seen to spiral outwards from the pole in an anti-clockwise orientation.
According to ESA, the pattern is "formed via a mix of processes, the most significant one being wind erosion… These winds, known as katabatic winds, move cold, dry air downslope under the force of gravity, often originating in areas of higher elevation (such as glaciers or snow-covered plateaus) and flowing down into lower, warmer areas such as valleys and depressions."
The airflow is then acted upon by the Coriolis force, which causes the otherwise straight path to spiral. This is similarly seen on Earth in large-scale weather patterns like hurricanes.
Imaging the polar regions of Mars enables scientists to not only understand the current conditions of the planet but also the previous ones too. As the polar ice melts and freezes between seasons, dust gets trapped and stored inside the layers. These snapshots of the climate embedded in the ice can unlock the history of the poles.
As the second-longest surviving continually active spacecraft in orbit around a planet other than Earth, Mars Express has been building up a picture of the planet’s surface since late-2003. As well as mapping the surface over time, the orbiter has characterized volcanoes and probed Mars’ sub-surface for liquid water.
To add to our growing knowledge of the Red Planet, the ExoMars Rosalind Franklin rover – set to arrive on the planet next year – will continue the work of Mars Express and the other orbiters, such as the ExoMars Trace Gas Orbiter.