Volcanic activity seems to be a common feature in our solar system; we have evidence of it on three planets and two moons and hints of it elsewhere. But that doesn’t mean all volcanic activity is the same. Venus’s activity is driven by a simple version of plate tectonics. On the moon, huge lava flows were released by large impacts, and Mars seems to have simply dissipated heat left over from its formation. There are also hints of cryovolcanoes, which erupted ice instead of lava, on some bodies of the outer solar system.
But when it comes to the sheer amount of activity, all of this comes second to Jupiter’s moon Io. Io has partially melted due to gravitational stress from its proximity to three large moons and a massive planet. The results are active volcanoes and huge pools of molten material on the lunar surface. And we just got a good look at the biggest of them.
Cut Loki into pieces
Loki Patera is the most powerful active volcano in the solar system. It is a huge crater with a central island; surrounding that island is a sea of hot material that covers more than 20,000 square kilometers. It seems that hot material is not stable as the entire surface seems to be reworked every few years, temporarily replaced with new hot material.
Unfortunately, we didn’t get a good picture of the process, instead we’re limited to whimsical snapshots from the Voyagers, the Galileo orbiter, and various telescopes. But now a team of US-based researchers has figured out how to get a high-resolution view of the resurfacing process using an Earth-based telescope. And it involved using another man of Jupiter, Europa.
In March 2015, Europa passed over Io from Earth’s perspective, eclipsing it and Loki Patera over the course of about four minutes. But the process was gradual, with the rim of Europe partially obscuring the volcano both on its way in and out. By quickly capturing infrared images (one every 120 milliseconds), the team was able to capture Loki Patera with different sized slices obscured by Europe.
Due to the speed of the imaging, the amount of Io eclipsed in successive images differed by only two kilometers. So while it’s impossible to image objects at that resolution from Earth, we can reconstruct them based on their disappearance and reappearance from the image stack.
Ride the wave
The wavelength used gave a measure of the amount of heat radiated from the surface of Loki Patera. This, in turn, gave a measure of the age of the surface, as the material there will gradually cool as it is exposed to space (Io’s atmosphere is far too weak to hold much heat).
This analysis revealed two waves moving across the surface of Loki Patera, both starting in the west near where the central island meets the crater wall. It moves clockwise through the north, at a rate of about a kilometer per day, replacing the old cool surface with new hot material. The second does the same, but moves counterclockwise through the south, advancing twice as fast. The authors estimate that the hottest part of the two waves, where molten material had reached the surface, had joined just to the east about 75 days prior to their observations.
The Galileo mission had detected similar temperature gradients at Loki Patera, but had not created a full view of the crater. Monitoring from the ground showed that the volcano was rising and cooling cyclically, suggesting that this resurfacing is a regular cycle.
There are two ideas about what the resurfacing process could look like. In the first case, the molten material simply flows onto the older surface, replacing it and, over time, pushing it deeper into the crust. This process would require two sources of molten material with very different properties to explain the differences in north and south expansion rates. In the second explanation, the surface becomes denser as it cools and sinks into the molten material below. Again, this implies regional differences in the material to explain the different rates of advance north and south.
Whatever the precise mechanism, this wave-like resurfacing has been going on at least since the Galileo mission, and possibly as far back as the Voyagers. The fact that there are two waves also makes sense for sporadic observations that suggested the resurfacing was moving in the opposite direction.
All observations (dating back to the 1975 Voyager visits) agree that the cool central island was present, indicating that it is a stable feature of Loki Patera. The fact that it helps divide the two waves, with their different properties, suggests that it is also an important feature below the surface. With this new data, geologists will likely have fun modeling how Loki Patera’s underground pipes work.
Nature2017. DOI: 10.1038/nature22339 (About DOIs).