Experiments under Mars conditions show water could remain liquid down to -20C to -50C in salt solutions:
Water Could Stay Liquid on Mars.
By Bjorn Carey
15 November 2005
"Using a planetary environmental chamber - a tank that mimics the atmosphere, temperature, and pressure of other planets - the team exposed various concentrations of briny water to conditions that match Mars' colder, less pressurized environment. Based on these experiments, salty water, it seems, can exist as liquid on Mars.
"It was thought that any liquid on the surface would evaporate almost immediately,' Julie Chittenden, a graduate student with the Arkansas Center for Space and Planetary Sciences told SPACE.com. 'These brine solutions enable water to stay liquid at colder temperatures. If you expose these brine solutions to cold temperatures, they can exist for a very long period of time.'
"While pure water freezes at zero degrees Celsius, water mixed with sodium chloride and calcium chloride salts - the two salts used in these experiments - remains liquid down to -21 and -50 degrees Celsius respectively."
http://www.space.com/scienceastronom...e_tuesday.html
The Phoenix lander is to land in the Mars north polar region. It might be thought there would be little chance for liquid water here.
But this report shows modeled maximal temperatures on Mars according to latitude:
Title: On the possibility of liquid water on present-day Mars.
Journal: Journal of Geophysical Research, Volume 106, Issue E10, p.
23317-23326 (JGR Homepage)
Publication Date: 10/2001
http://www.agu.org/pubs/crossref/200...JE001360.shtml
It appears in Fig. 4 on page 23,231. The maximal temperature at 70 degrees North latitude is given as 250 K, -23 C.
This is below the freezing point of pure water but the experiments show it is in the range for liquid water brines. This fact is also discussed here:
Making a Splash on Mars.
"On a planet that's colder than Antarctica and where water boils at ten
degrees above freezing, how could liquid water ever exist? Scientists
say a dash of salt might help."
...
"One thing we have to be careful of is our everyday experience that
water always freezes at zero degrees," noted Hoover. "It doesn't.
Water containing dissolved salts freezes at a significantly lower
temperature. Don Juan Pond in Antarctica is a good example. It's a
high salinity pond with liquid water at temperatures as low as -24 °
C."
http://science.msfc.nasa.gov/headlin...st29jun_1m.htm
Some images of Don Juan Pond:
http://www.science.siu.edu/microbiol.../DJP.view.JPeG
http://www.science.siu.edu/microbiol.../M2JC.DJP.JPEG
Some reports argue there is no life in Don Juan Pond but this article discusses life found on its periphery:
An extraterrestrial habitat on Earth: the algal mat of Don Juan
[correction of Jaun] Pond.
Adv. Space Res. 1983; vol. 3, 8:39-42.
http://www.ncbi.nlm.nih.gov/entrez/q..._uids=11542753
Bob Clark