Fossil fractures

While initially more reminiscent of a leathery elephant’s ear than a planetary surface, this image actually shows the inside of a 40-kilometre-wide impact crater lying just at the boundary between the northern lowlands and southern highlands of Mars.

The image was taken by the CaSSIS camera onboard the ESA/Roscosmos ExoMars Trace Gas Orbiter (TGO) on 19 June 2021 and is centred at 32.1°N/62.7°E, near a feature named Antoniadi Crater. ExoMars has imaged this region before and captured leaf-like structures within Antoniadi Crater itself, which lies to the south of this scene within the Syrtis Major Planum region of the planet.

Especially prominent in this image is the complex geometry of ridges seen to the upper right (the southern part of the crater in this orientation). While the eye may initially read these as depressions scratched into the martian surface, they actually rise upwards towards the camera. It is likely that these ridges were originally low-lying fractures that were subsequently filled with highly durable material such as lava, rock or sediment; when the layers that contained these fractures were eroded away, possibly by wind activity, only the durable infills of these fractures remained – creating the positive relief seen here. This phenomenon, known as ‘inverted terrain’, has been observed on both Earth and Mars.

The wider landscape surrounding the patch of Mars shown here also shows evidence of extensive past water activity and chaotic terrain, further indicating the region’s complex geologic history.

TGO arrived at Mars in 2016 and began its full science mission in 2018. The spacecraft is not only returning spectacular images like this one, but also providing the best ever inventory of the planet’s atmospheric gases, and mapping the planet’s surface for water-rich locations. It will also provide data relay services for the second ExoMars mission when it arrives on Mars in 2023.