The author suggests that a brine would not freeze - brine at the eutectic point, the strongest possible solution, freezes at -21C. So it might be liquid on a sunny day.

But even the strongest brine would evaporate to dry salt - we have done this merely by sunlight on Earth for centuries. And the brine must freeze in the Martian night. That will cause the dissolved salts to precipitate out, and the much purer ice will sublime.

Is it really possible that liquid brine could survive on the Martian surface? That flat deposit may be the relic of a puddle, but not an actual one.

John