The Thermal InfraRed SpaTial System (THIRSTY) is a joint mission concept being developed by CNES and NASA that is nearing the end of its Phase 0 activities. THIRSTY is science-driven and during the Phase 0 activities, the team has utilized the key science objectives to develop the measurement needs and mission concept. The science of the THIRSTY mission is focused on addressing questions related to evaporation stress and water use. However, the measurements will be useful for a broad range of other studies related to urban environments, coastal and inland waters, and solid earth and natural disasters. The THIRSTY mission is driven by the need to address two main science questions related to ecosystem stress and water use. These are 1) how is the terrestrial biosphere responding to changes in water availability and 2) can agricultural vulnerability be reduced through advanced monitoring of consumptive use and improved drought detection. These questions are being further refined by the mission concept team together with input from the science community. THIRSTY represents a confluence of two earlier concepts being developed by CNES and NASA. On the CNES side, the mission draws from the MISTIGRI (Micro Satellite for Thermal Infrared Ground surface Imaging) concept. This 200 kg satellite was composed of a 4 band thermal infrared (TIR) instrument and a 4 band visible-near infrared (VNIR) instrument, mounted on a Myriade spacecraft (CNES low cost product line). MISTIGRI was a sampling mission that would acquire tens of 25km x25 km scenes from parts of the globe with 50 m spatial resolution and a revisit of 1 day. On the NASA side, THIRSTY provides the measurements needed for part of the Hyperspectral Infrared Imager (HyspIRI) mission. The HyspIRI payload includes a visible shortwave infrared imaging spectrometer and a mid and thermal infrared multispectral scanner. THIRSTY would provide the data needed to satisfy the measurement requirements of the mid and thermal infrared multispectral scanner. The HyspIRI mid and thermal infrared instrument has 60 m spatial resolution, 8 spectral channels, global coverage and a revisit time of 5 days both day and night. The THIRSTY concept will provide high spatial resolution data (<100 m) in the VNIR-TIR parts of the electromagnetic spectrum of Earth land surface and coastal regions in multiple spectral channels every 3 days. The mid and thermal infrared measurements will be made both day and night. This presentation will discuss the science questions and how the measurements required to answer those questions were used to define the THIRSTY mission concept.