re:They use RHU (Radioactive Heater Units)
From The Wikipedia
The rovers run a VxWorks embedded operating system on a radiation-hardened 20 MHz RAD6000 CPU with 128 MB of DRAM with error detection and correction and 3 MB of EEPROM. Also, the rovers each have 256 MB of flash memory. To survive during all of the various mission phases, the rover's "vital organs" must not exceed extreme temperatures of -40 ºC to +40 ºC (-40 ºF to 104 ºF). At night the rovers are heated by eight RHUs which each continuously generate 1 W of thermal energy from the decay of radioisotopes, along with electrical heaters that operate only when necessary. A sputtered gold film and a layer of silica aerogel are used for insulation.