The 2016 ExoMars Trace Gas Orbiter (TGO) is the first in a series of Mars missions to be undertaken jointly by the two space agencies, ESA and Roscosmos. A key goal of this mission is to gain a better understanding of methane and other atmospheric gases that are present in small concentrations (less than 1% of the atmosphere) but nevertheless could be evidence for possible biological or geological activity.

The Colour and Stereo Surface Imaging System (CaSSIS) is part of the instrument payload on the TGO. CaSSIS will characterise sites that have been identified as potential sources of trace gases and investigate dynamic surface processes – for example, sublimation, erosional processes and volcanism – which may contribute to the atmospheric gas inventory. The instrument will also be used to certify potential landing sites by characterising local slopes, rocks and other possible hazards.