Introduction: Two scientists discuss the possibility of liquid water on Mars' surface. Scientist 1: Mars currently has an average atmospheric pressure of 0.6 kPa. Because this is below the triple point of water, liquid water cannot exist; it would immediately sublime. The dark streaks seen on craters are likely dry granular flows of dust. Scientist 2: While the average pressure is low, seasonal variations and local depressions allow pressure to exceed 1.0 kPa. At these sites, perchlorate salts lower the freezing point of water, allowing briny liquid water to exist briefly. The dark streaks are evidence of these seasonal flows.