Salt water may form on the surface of Mars for a few days annually, according to a study.
Researchers at the Planetary Science Institute in the US noted that liquid water is difficult to come by on the Red Planet, because ice rapidly dissipates into the atmosphere long before it reaches its melting point.
This is because the atmospheric pressure on Mars lies near the triple point pressure of water molecules (H2O), the minimum pressure necessary for liquid water to exist, they said.
"Mars has plenty of cold ice-rich regions and plenty of warm ice-free regions, but icy regions where the temperature rises above the melting point are a sweet spot that is nearly impossible to find," said Norbert Schorghofer, a senior scientist at Planetary Science Institute.
"That sweet spot is where liquid water would form," Schorghofer said.
In the study, published in The Astrophysical Journal, the researchers explained that a boulder sitting on the surface of Mars at mid-latitudes casts a shadow in winter.
The continually shadowed area behind the boulder is very cold, so cold that water ice accumulates in winter, they said.
When the Sun rises again in spring, the ice suddenly heats up.
In detailed model calculations, the researchers noted that the temperature rises from minus 128 degrees Celsius in the morning to minus 10 degrees Celsius at noon, a huge change over a quarter of a day.
Over such a short time, not all of the frost is lost to the atmosphere, according to the researchers.
Salt depresses the melting point of H2O, so on salt-rich ground, water ice will melt at minus 10 degrees Celsius, according to the researchers.
Brines, or salty water, will form until all of the ice has either turned to liquid or vapour, they said.
Next Mars year, the same process repeats, the researchers noted.
The shadowed areas behind the boulders are so cold in winter that not only water frost but also carbon dioxide ice builds up, they said.