It's not generally known that there were actual *ground* temperature sensors on that mission. They were attached to wheels on the Sojourner rover:

Detailed answers to selected Frequently Asked Questions about the Mars Rover and Lander.
"Warmest External Rover Temperature 19.2 °C Measured at rover RF
wheel on Sol 3."
http://mars.jpl.nasa.gov/MPF/rovercom/rovfaq.html

DUST ON MARS:
Materials Adherence Experiment Results From Mars Pathfinder.
"The MAE mechanism actuates on command from the rover's regulated 5-
Volt DC power supply. Since the SMA action is thermal in nature,
ambient temperature plays an important role in determining how much
current is required to activate the rotating arm. The actuator is
specified to operate over a temperature range of -50 C to +30 C. This
is a large enough range that the time required to rotate the cover
glass can vary from 2 to 0.5 seconds. The experiment includes a
temperature measurement at the time the actuator is used, and the
opening time is set based on a look-up table of opening time vs.
temperature. We have found actual operating temperature range on Mars
to be typically in the range of 5 C to 13 C for noon operation, and 3
C to 7 C for the 2PM operation. This is slightly warmer than the air
temperature, which reaches a maximum of about 0 C in the early
afternoon. Shortly after landing, we performed a check of the opening
with several different opening times, and verified that the cover
opened at all of the durations tested."
http://powerweb.grc.nasa.gov/pvsee/publications/mars/Dust_97.html

See also this plot of the external temperature sensors on Sojourner:

This confirms that actual ground temperatures are significantly higher than the air temperatures, going above freezing daily during the daytime.
This is quite likely also the case for the near equatorial site Gusev during the daytime Summer hours.

Bob Clark