In a recent paper presented at the 31st Lunar and Planetary Science Conference two Russian researchers highlight various common salt solutions that have surprisingly low melting points. Mars' soil is now thought to be very salt-rich.
They suggest that such liquid brines may frequently erupt to the surface during Mars' periodic axial tilts to 45 degrees or more every hundred thousand years.
Another abstract presented at the Conference suggests a solution of magnesium sulfate (Epsom salt) has a melting point of minus 35 deg C. and may make up 30% of Mars' soil by itself.
A solution of calcium sulfate — also thought to be very common on Mars — melts at -55 deg; and a mixture of the two melts at only -63 deg.
These are temperatures which surface the of Mars reaches often, even right now.
If these outbursts really are water, it may be explanations such as this that reconciles the problem of how liquid water can exist this close to the surface — but it doesn't explain why they only occur on cold slopes.
Seasonal Salts Could Rub Mars Raw
— Science Abstract non-PDF version
