For Earth, there is lot of free satellite data easily available. For example, I can download the Sentinel-2 optical images freely from https://scihub.copernicus.eu/. What would be the closest thing for Mars? I mean especially in terms of data availability. There are many satellites orbiting around Mars. If there is no remote analogue to Sentinel-2 in terms of data availability, why?
Both ESA and NASA have a policy that data need to be made available after a certain grace period. Look at the respective satellite mission like you look at the sentinel mission pages and you will likely be able to find the ressources or links to them. E.g. for Mars express, the data are all gathered in the data archive on https://www.cosmos.esa.int/web/psa/mars-express