Temperature
While the Rover has been designed to operate at low temperatures, the motors and batteries must be maintained at about +20 degrees C. Read more about how the Rover's Thermal systems operate, and the role of the Thermal Engineer.
The average temperature on Mars is -55 degrees C, though the Mars Rover, Spirit, which operated near the equator, recorded this graph during the Martian summer.
Mars has a very low atmospheric pressure at about 1% of Earth's, which means that water can only exist in two states - as a solid or a gas.
Temperatures on Mars get so low that during winter as much as 25% of the atmosphere condenses at the poles as solid carbon dioxide (dry ice). In summer, the dry ice sublimates back into the atmosphere creating significant variations in atmospheric pressure.
As shown in the H2O phase diagram below, the triple point of water on Mars is between +0.01 - +9 degrees C. Remember that the triple point of any substance is where the temperature and pressure means it can exist in all three phases (states) solid, liquid and gas.