Bob Clark wrote:
However recent TES temperature measurements suggest the
temperature on Mars can be above zero even at southern latitudes into
the 60's:

http://wwwflag.wr.usgs.gov/USGSFlag/Space/MGS_TES/spole99.html

Then depending on the altitudes and pressures at these sites it is
possible that liquid water could have flowed in current times at least for
short times.

Alex writes:
There is quite a bit of confusion about this subject floating around on
the Internet because of most persons' lack of knowledge in placing the
TES bolometer readings in context. Most people do not understand the
issue of adiabatic and isothermal heat transfer at the Martian
atmosphere/surface interface, as well as the meaning of "daily mean
temperature" and "diurnal temperature range." Conway Leovy, of the
Department of Atmospheric Sciences at the University of Washington,
recently (July 12, 2001) published a paper in Nature, an excerpt of which
is below, that sums the issue up:

"Near winter solstice, Mars' distance from the Sun approaches its
minimum, and the atmosphere becomes dusty (optical depth at visible
wavelengths >> 1). Absorption of solar radiation by dust increases the
daily mean temperature and diurnal temperature range above the
surface, but the surface diurnal temperature range decreases and the
daytime convective layer disappears. Daily average temperature near the
surface is not affected by dust loading."

Reference:
Leovy, Conway, Weather and climate on Mars, Nature 412, 245-249 (2001) .

Alex R. Blackwell
University of Hawaii