For liquid water to exist on or near the surface of Mars,
you don't need above zero Celsius.
What you need is above -56 degrees Celsius,
plus good relative humidity, plus salts.
We found microbes in soil with 3,000 ppm salt concentrations.
That's like vodka.
That's so much salt, temperatures can drop to minus 56 degrees Celsius before there's frost bite.
Some places on Mars, like Valles Marineris,
have relative humidity so high, sometimes,
that you get not only frost but visible fog, e.g.
But, of course, under current policy, NASA will not send
a life-seeking laboratory to such a place.
Thus far, it looks to me like Gusev was more humid than Glenelg.