What happens to pure liquid water on the Martian surface when atmospheric pressure drops too low?
Answer
It either freezes instantly or sublimates
A fundamental physical constraint of Mars's current environment is that when surface pressure is too low, liquid water cannot exist stably; it converts directly to ice or vapor (sublimates).

#Videos
They Finally Found Liquid Water on Mars - YouTube
Related Questions
What geological formations provide evidence that liquid water once flowed freely on Mars?What atmospheric and temperature conditions were necessary for ancient stable surface water on Mars?What happens to pure liquid water on the Martian surface when atmospheric pressure drops too low?What are the most compelling features suggesting liquid moisture activity on present-day Mars slopes?If water cannot remain liquid on the surface due to cold and near-vacuum, what must the observed flowing water contain?What is the term used to describe the highly saline liquid flowing on present-day Mars?How deep are the transient liquid flows observed as RSLs?Where are the largest known reservoirs of modern Martian water currently locked up?What scientific evidence strongly suggests the existence of large, stable bodies of liquid water beneath the southern polar ice cap?What two physical factors are hypothesized to maintain the liquid state of deep underground water reservoirs?