Calcium Perchlorate, which has been found in various places on Mars, is super hygroscopic, meaning that it is able to attract and hold water from its environment.
The resulting perchlorate brine, also called a hydrated salt, is so stable that it dramatically lowers water’s freezing point and the pressure at which it evaporates. So the brine can exist for fairly long periods of time on the surface of Mars, and likely, perpetually beneath the surface.