Of the known moons in the solar system, four have been found to have atmospheres: Jupiter's volcanically active moon Io (tenuous sulfur dioxide), Jupiter's ice-coated moon Europa (tenuous oxygen), Saturn's largest moon Titan (dense nitrogen and methane), and Neptune's largest moon Triton (tenuous nitrogen and methane). The ability of a moon to retain an atmosphere is related to its mass and its distance from the Sun, although tenuous atmospheres may be regenerated by a variety of processes.