A ceramic brick of 238Pu (the isotope 238 plutonium) sits inside a carefully prepared cylinder and behind a protective cylindrical shell of graphite. The characteristic orange-red glow is due to heat radiation of the brick which is used as the main fuel of the thermoelectric power generator mounted on NASA’s Curiosity Mars rover. A similar generator, which is the state-of-the-art way of powering long-term, deep-space missions is also mounted on the Perseverance rover.
238Pu is artifially produced in nuclear reactors. It has a rather long lifetime and it produces sufficient heat to power exploration for a few centuries. As you go deeper into space and away from shiny stellar sources, radioactivity from such thermoelectric generators is the only way to provide continuous, long-term power to probes, rovers and spaceships. Cool, huh?