Conditions would be favorable for liquid water only during the martian day. The temperature falls precipitously at night, so any liquid would re-freeze. The massive Hellas impact basin in the southern hemisphere of Mars is nearly 9 kilometers deep and 2,100 kilometers across. The air pressure at the bottom of the basin is about twice the global average. In this false-color image based on measurements from the Mars Global Surveyor laser altimeter, red and white colors denote high elevations and blue denotes low.
Water containing dissolved salts freezes at a significantly lower temperature.
Don Juan Pond in Antarctica is a good example. That wasn't the first clue that Mars was salty, though. In 1976 the two Viking landers analyzed martian soil and found that it probably contained 10 to 20 percent salts. Martian rocks, like those on Earth, react to form salt and clay minerals when exposed to water. On our planet this process gives rise to a variety of brines in the western salt lakes of North America.