The electricity needed to operate NASA's Mars 2020 rover is provided by a power system called a Multi-Mission Radioisotope Thermoelectric Generator, or MMRTG. The MMRTG will be inserted into the aft end of the rover between the panels with gold tubing visible at the rear, which are called heat exchangers.
Essentially a nuclear battery, an MMRTG uses the heat from the natural radioactive decay of plutonium-238 to generate about 110 watts of electricity at the start of a mission. Besides generating useful electrical power, the MMRTG produces heat. Some of this heat can be used to maintain the rover's systems at the proper operating temperatures in the frigid cold of space and on the surface of Mars. Some of it is rejected into space via the rover's Heat Rejection System.
The gold-colored tubing on the heat exchangers form part of the cooling loops of that system. The tubes carry a fluid coolant called Trichlorofluoromethane (CFC-11) that helps dissipate the excess heat. The same tubes are used to pipe some of the heat back into the belly of the rover.
MMRTGs are provided to NASA for civil space applications by the U.S. Department of Energy (DOE). The radioisotope fuel is inserted into the MMRTG at the DOE's Idaho National Laboratory before the MMRTG is shipped to the launch site. Electrically heated versions of the MMRTG are used at JPL to verify and practice integration of the power system with the rover.