## 7 Conclusions  
The aim of this work was to develop a methodology to map a hydrothermal field in a volcanic environment, by applying spectral unmixing on multispectral data from the ASTER, the Landsat-8/OLI, and the Sentinel-2 MSI sensors. The case study was the hydrothermal field in the volcanic environment of Nisyros. The unmixing results obtained by ASTER and Sentinel-2 MSI were better than those of Landsat-8/OLI. However, Landsat-8/OLI also followed the general alteration pattern (as was given in a sketch map in literature that was used as reference), however, at a lower degree of accuracy. This behavior may be attributed to the fact that the general pattern of the spectral signatures of each material endmembers used in this study was similar for all three sensors. The main challenges faced in this study were the spatial resolution and the complexity of the composition of the existing formations. Despite these difficulties, the proposed methodology successfully mapped the hydrothermal alteration field (in reference to the produced classification map issued from the only existing sketch map of the distribution of hydrothermal alteration in the hydrothermal field of the southern Lakki plain in order to perform quantitative assessments). In addition, a new finding was revealed, namely, the fact that abundances can highlight different degrees of alteration better than conventional methods such as the use of spectral indices, especially in the case of ASTER and Sentinel-2 MSI. Concerning the hydrothermal field of Nisyros caldera, our findings showed that mainly acidic to intermediary lavas and breccias are affected by alteration. Among these lavas, strong alteration affects only the rhyodacitic lava domes and lavas of the last volcanic cycle of Post-Caldera Eruptive Cycle located at Lofos Dome and the central sector craters. On the other hand, while only a relatively small fraction of the surface of the Quaternary deposits seems to be affected by hydrothermal alteration, the other main lithologies affected by the hydrothermal alteration, both in terms of affected area and intensity of the alteration, were the debris flows and lacustrine deposits at the center of the caldera. These formations are mainly related to the historical hydrothermal explosions and belong to the Hydrothermal Explosive Cycle. In addition, our results are in agreement with previous studies according to which the hydrothermal field of Nisyros caldera presents a characteristic spatial pattern with three main centers of strong alteration located at Lofos Dome and the central sector craters, Stefanos crater and Kaminakia crater. Future investigations include the use of other spectral unmixing methods, both supervised (where the endmembers are predefined) and unsupervised (where both endmembers and abundances are estimated simultaneously), the investigation of the distribution of alteration-related minerals and the full exploitation of the spectral information (e.g., thermal bands) of the datasets. A.-M.T., O.S., K.K., I.P. participated on the conceptualization of the project; [PERSON]-[PERSON] implemented most of the data processing; O.S., K.K. supervised the implementation of the methodology; O.S., K.K. were involved in the funding acquisition; [PERSON]-[PERSON], O.S., K.K. participated in the original draft preparation. All authors participated in the editing process of the manuscript. All authors have read and agreed to the published version of the manuscript. This research was funded by the project \"PROTEAS II--Advanced Space Applications for Exploring the Universe of Space and Earth\" (MIS 5002515), which is implemented under the Action \"Reinforcement of the Research and Innovation Infrastructure\", funded by the Operational Programme \"Competitiveness, Entrepreneurship and Innovation\", National Strategic Reference Framework (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).