CaSSIS

CaSSIS. The Colour and Stereo Surface Imaging System is a high-resolution scientific camera operating aboard the European Space Agency's ExoMars Trace Gas Orbiter spacecraft. Developed by a team led by the University of Bern with contributions from Istituto Nazionale di Astrofisica and Space Research Centre, it is designed to photograph the surface of Mars in color and stereo. Since beginning its operational phase in 2018, CaSSIS has provided critical context for the detection of atmospheric gases and has significantly advanced the study of dynamic surface processes on the Red Planet.

Overview

Launched from the Baikonur Cosmodrome aboard a Proton-M rocket, the instrument arrived at Mars in October 2016. After a lengthy aerobraking period to circularize its orbit, CaSSIS commenced its primary science mission in April 2018. The camera serves as a pivotal complement to other instruments on the ExoMars Trace Gas Orbiter, such as the NOMAD spectrometer and the Atmospheric Chemistry Suite. By providing detailed geological context, it helps scientists determine potential sources and sinks for trace gases like methane detected in the atmosphere of Mars. Its observations are coordinated with other Martian orbiters, including the Mars Reconnaissance Orbiter and the MAVEN mission, to create a multi-faceted understanding of the planet.

Instrument design

The core optical design of CaSSIS is a three-mirror anastigmat telescope with a 135 cm focal length, providing a ground sampling distance of approximately 4.5 meters per pixel from its nominal 400 km altitude. A key innovation is its rotating telescope assembly, which allows the capture of stereo images by pointing off-nadir on consecutive orbital passes over the same terrain. The detector is a 2K x 2K CMOS sensor from Teledyne e2v, coupled with a filter wheel containing four color bands: panchromatic, blue-green, red, and near-infrared. This configuration enables the creation of color composites and the derivation of spectral reflectance data to study surface composition. The instrument's compact and robust design was managed by a consortium including RUAG Space and Oerlikon Space.

Scientific objectives and mission profile

The primary scientific goal is to characterize features on the Martian surface that may be linked to trace gas sources, such as geyser-like activity, aeolian processes, or geothermal hotspots. Its mission profile involves targeted imaging of specific regions of interest identified by NOMAD or from previous data from the High Resolution Imaging Science Experiment. CaSSIS also conducts systematic monitoring of dynamic phenomena, including the evolution of polar ice caps, dust devil tracks, and seasonal changes in recurring slope lineae. The orbital parameters of the ExoMars Trace Gas Orbiter, with a local solar time of approximately 3 PM, are ideal for imaging under consistent lighting conditions, enhancing the quality of morphological and color analysis.

Key discoveries and observations

CaSSIS has documented a wide array of geological activity, providing evidence that the surface of Mars is more dynamic than previously understood. Notable observations include detailed imagery of fresh impact craters, revealing subsurface ice, and the monitoring of avalanches on the rim of the North Polar Layered Deposits. The camera has captured the formation and migration of large barchan dunes within Proctor Crater and other regions. Its color data have been instrumental in identifying compositional variations associated with clay minerals and hydrated salts, offering clues to the planet's aqueous history. These findings are frequently analyzed in conjunction with data from the Compact Reconnaissance Imaging Spectrometer for Mars.

Data processing and public archive

Raw image data are downlinked via the European Space Agency's ESTRACK network and processed through a pipeline developed at the University of Bern. The pipeline performs radiometric calibration, geometric correction, and the generation of coregistered color products and digital terrain models from stereo pairs. All validated data are delivered to the Planetary Science Archive, a publicly accessible archive maintained by the European Space Agency, where they are available to the global scientific community. Processed images are also regularly released through outreach channels, contributing to public engagement with the ongoing exploration of the Solar System.

Category:Spacecraft instruments Category:Mars spacecraft Category:ExoMars program