Final answer:
The geological activity on Triton, particularly the geyser-like nitrogen plumes, is driven by the Sun's energy warming the surface and causing evaporation of the moon's polar caps, rather than by internal heat sources.
Step-by-step explanation:
The probable source of heat that drives activity on Neptune's moon Triton is the sunlight warming its surface, rather than internal heat sources like radioactive decay or tidal flexing. Unlike Jupiter's moon Io, which receives heat from tidal interactions with Jupiter, Triton's geological activity, specifically the geysers or volcanic plumes of nitrogen gas, is driven by the evaporation of nitrogen and other substances from the polar caps due to the sunlight. When the Voyager spacecraft flew by Triton, it observed these plumes, which are propelled to heights of 10 kilometers by nitrogen gas that evaporates as Triton's polar regions are warmed by the Sun, especially when the southern pole is tipped toward the Sun.
Even though Triton's general surface temperatures range between 35 and 40 K, which are considered extremely cold by Earth standards, the sunlight is still strong enough to drive geological activity on its surface. This heating can cause a polar cap, likely consisting of frozen nitrogen, to evaporate and form plumes. This observation leads to the conclusion that the Sun is the primary source of the thermal energy driving these processes on Triton.