A European Space Agency probe launched in 2016 March that went into orbit around Mars in 2016 October to study gases present in small amounts in the planet’s atmosphere, such as methane and water vapour. It also carried the Colour and Stereo Surface Imaging System (CaSSIS) for high-resolution imaging of the surface, and the Fine Resolution Epithermal Neutron Detector (FREND) for detecting subsurface water ice. TGO was part of the ExoMars programme.
http://exploration.esa.int/mars/46475-trace-gas-orbiter/ Information page at the European Space Agency.