For more than a decade, methane has been detected in the atmosphere of Mars by various instruments and groups. The ESA and IKI ExoMars Trace Gas Orbiter (TGO), launched in March 2016, will investigate the presence of methane and other trace gas sources in the Martian atmosphere starting from May 2018. Methane does not stay longer than days or months in the Martian atmosphere, which implies that if methane is detected, its source must be active. It can be either biological (methanogenic archaea) or geological (serpentinisation of olivine in the Martian crust, hydrothermal activity, outgassing from clathrates or minerals). On Earth, both types are frequently associated. The EXOMHYDR project focuses on the potential geologic sources, especially in volcanic regions and some fractured regions above which some strong emissions have been detected in the past. Current emissions will be identified using the NOMAD and ACS spectrometers of TGO, which will probe the atmosphere in a wide range of wavelengths, and will be correlated with possible sources identified using the colour and stereo CaSSIS camera of TGO and other instruments from previous missions (HiRISE, CRISM, HRSC, MOLA, SHARAD etc.). Ancient sources may be identified as well by interpreting the data from CaSSIS and earlier instruments. In addition to a better understanding of the structure of the Martian crust, we expect that this work will also contribute to the debate as to the current existence of microscopic life on Mars preserved from the lethal atmospheric ultraviolet radiations, and to the possibility of much more ancient biological activity. Hydrothermal sites are also sites where mineral/metal resources concentrate on Earth, shedding light into interesting sites for future space mining activities.