ExoMars Trace Gas Orbiter Instruments
CaSSIS - Colour and Stereo Surface Imaging System
The Colour and Stereo Surface Imaging System (CaSSIS) is part of the instrument payload on the ExoMars Trace Gas Orbiter (TGO), launched in March 2016.
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.
By acquiring colour and stereoscopic images of surface features, CaSSIS will allow scientists to investigate whether specific types of geological processes might be associated with trace gas sources and sinks. The NOMAD and ACS instruments, also being carried by the orbiter, will identify sources of trace gases that could be evidence for biological or geological activity. The CaSSIS imaging system will have a horizontal scale of about five metres per pixel; stereoscopic reconstruction will enable a vertical resolution of about six metres.
CaSSIS will be located on the Mars-surface-facing side of the orbiter. The orbiter science payload will be primarily nadir pointing to keep the Martian surface in constant view. The orbiter will rotate about an axis that will maintain its solar panels oriented towards the Sun while avoiding solar illumination of its thermal radiators. CaSSIS can compensate for the spacecraft's yaw rotation using a drive mechanism, but during nominal stereoscopic imaging the orbiter will pause its yaw-rotation to maximise the paired-image accuracy. The rotation mechanism will be able to turn the entire telescope system by 180° while its support structure remains fixed.
This rotation system will also enable the camera to acquire stereo images with only one telescope and focal plane assembly. A stereo image pair will be acquired by first rotating the telescope to point 10° ahead of the spacecraft track to acquire the first image, then rotating it 180° to point 10° behind for the second stereo image. Optimal correlation of the stereo signals will be ensured as there will be identical illumination conditions every time a stereo image pair is acquired.
The camera will consist of a three-mirror off-axis telescope with an additional slightly powered folding mirror, which will project an image onto the focal plane. The instrument electronics unit will be mounted separately on the spacecraft deck, next to the telescope. The imager will cover an eight-kilometre-wide swathe of the planet's surface in four different wavelength ranges.