Swirling spirals spotted at the north pole of Mars

European Space Agency's Mars Express has beamed back images that show distinctive dark spiralling troughs on the red planet's north polar ice cap.
The mosaic generated from 32 individual orbit 'strips' captured between 2004 and 2010, and covers an area of around a million square kilometres.
The ice cap is a permanent fixture, but in the winter season - as it is now in early 2017 - temperatures are cold enough for around 30 percent of the carbon dioxide in the planet's atmosphere to precipitate onto the cap, adding a seasonal layer up to a metre thick.
During the warmer summer months most of the carbon dioxide ice turns directly into gas and escapes into the atmosphere, leaving behind the water-ice layers.

Strong winds are thought to have played an important role in shaping the ice cap over time, blowing from the elevated centre towards its lower edges and twisted by the same Coriolis force that causes hurricanes to spiral on Earth.
One particularly prominent feature is a 500 kilometre-long, two kilometre-deep trench that almost cuts the cap in two.

The plunging canyon, known as Chasma Boreale, is thought to be a relatively old feature, forming before the ice-dust spiral features, and seemingly growing deeper as new ice deposits built up around it.
Subsurface investigations by radar instruments onboard Mars Express and NASA's Mars Reconnaissance Orbiter revealed that the ice cap is made up of many individual layers of ice and dust extending to a depth of around two kilometres.

This presents a valuable record for the nature of how the planet's climate has changed as its tilt and orbit varied over hundreds of thousands of years.