Io is a violent world. Tortured by the gravitational forces of Jupiter, it erupts with sulphur and lava. Our own planet has volcanoes and lava flows, but it pales in comparison to Io.
From Earth, astronomers can observe changes in the brightness of Io’s surface, but we had to send probes such as Voyager to Jupiter to see active and erupting volcanoes on Io. This proved that Io is the most geologically active body in the solar system. We can infer much from Io by comparing it to volcanic activity seen on Earth, but there are significant differences. Earth is a water-rich planet with a significant atmosphere, while Io is dry with little atmosphere. This raises questions about how things like lava flows behave on the small moon.
On the surface of Io, Loki Patera is a hot spot that brightens and dims every 400 – 600 days. The most popular explanation is that Loki Patera is a lava lake more than a million times larger than any on Earth. But even high resolution images of Io from the Galileo mission have failed to confirm this idea.
Recently, astronomers used a fortunate celestial event to solve this mystery. Io is the closest Galilean moon to Jupiter. Periodically the next closest Galilean moon, Europa, passes in front of Io as seen from Earth (known as a transit). As it does so, Io’s surface is gradually blocked and revealed by icy Europa. This lets astronomers create a map of Io, particularly Loki Patera. By observing Io during a transit of Europa, they found that the brightness and temperature of the region increased steadily from one end to the other.
This is consistent with the behavior of a lava lake that is overturning. That is, cold lava at the surface of the lake sinks, churning hotter lava to the top. The astronomers also confirmed an island in the center of Loki Patera that has been there since Voyager photographed the region in 1979. This again supports the lava lake model. All it took to verify a lava lake on Io was a fortunate transit by an icy world.
Paper: K. de Kleer, et al. Multi-phase volcanic resurfacing at Loki Patera on Io. Nature 545, 199–202 doi:10.1038/nature22339 (2017)