Spirit takes temperature on Mars
From correspondents in Washington
January 10, 2004
"During daytime, martian rocks were colder than objects made up of fine particles, said Phil Christensen, the scientist tasked with the temperature readings.
"On the ground, the warmest temperature is around five degrees Celsius and the coldest is -15 degrees Celsius," he said.
"The temperatures taken by Mini-TES, the nickname for the mini-thermal emission spectrometer, were combined with the panoramic pictures taken earlier of Mars's surface to create an image of chilly blue objects and warmer red ones, changing as the martian day warmed up."
http://www.news.com.au/common/story_page/0,4057,8364604%255E1702,00.html

Also remember the salts and minerals in the soil will significantly lower the freezing point of water. Even -15 C might be within the range of liquid water under these circumstances.

Bob Clark