It’s certainly a lot more practical to build a molten-salt reactor than a conventional water-based reactor on Mars, where there’s no water handy! There’s no reason to expect this reactor to use thorium, however, for reasons I give in my other response. Nuclear reactors do require a secondary coolant, which is often water. Air cooling is possible on Earth but the Martian atmosphere is very thin, so I don’t know if it could work there. But bringing water from Earth would cost far too much.

By the way, I haven’t heard of any reactors designed work in space, because conventional designs all rely on circulation caused by gravity for passive safety. I wonder if any exist…

Space probes and Martian rovers have sometimes been powered by RTGs (radioisotops thermoelectric generators) powered by Plutonium 238 (a substance extracted from high-level nuclear waste). The media has occasionally misreported these as “nuclear reactors”. They are not reactors, they are simple machines to generate small amounts of electricity from radioactivity.