Scientists have revealed the most complete look ever at the largest lake on Io, a stunning glimpse into this moon of Jupiter that’s the most volcanically active place in the Solar System.

In a paper published in Nature, researchers led by the University of California, Berkeley (UCB) described how they were able to study the largest lake on Io not from spacecraft imagery, but from watching its neighboring moon Europa pass in front in March 2015, known as an occultation.

They were able to see waves move across Loki Patera, Io’s biggest lava lake at more than 200 kilometers (125 miles) across. For comparison, Earth’s biggest lava lake is no more than 200 meters (650 feet) across, one million times smaller in volume.

“We saw two waves within the patera that hadn’t been seen before, with different velocities and start times,” Katherine de Kleer from the University of California, Berkeley, the lead author on the paper, told IFLScience. “This tells us there’s some complex system underneath the volcano.”

^{Europa seen passing in front of Io and Loki Patera (the upper bright spot). LBTO}

The lake was seen increasing in temperature from one side to the other, from 270 Kelvin in the west to 330 Kelvin in the east, suggesting it had “overturned” from west to east.

This is the process by which a crust forms on the surface, and then becomes unstable and sinks into the lava, exposing new magma while pulling other crust under with it. The magma then cools, forms new crust, and the process repeats over about five months. It is thought it might be accompanied by fire fountains, which are also seen on Earth.

“This is the first useful map of the entire patera,” said co-author Ashley Davies from NASA’s Jet Propulsion Laboratory in California in a statement. “It shows not one but two resurfacing waves sweeping around the patera. This is much more complex than what was previously thought.”

^{There's also an island present in the middle of the lake, seen here}

The findings were made by watching as Europa passed in front of the moon with the Large Binocular Telescope Observatory in Arizona. Such an event only occurs about once every six years, but by training the telescope on Io, the team was able to get a complete look at Loki.

The occultation was useful because Europa blocked out the light elsewhere, meaning the scientists could isolate the heat coming from the lava lake. An image was taken each time Europa moved an extra 2.4 kilometers (1.5 miles) relative to the surface, giving a complete picture of the lake.

Io is estimated to have more than 400 active volcanoes, but our studies of the moon are few and far between. Voyager had a close look in 1979, as did Galileo two decades later, and New Horizons in 2007, but there are still many unanswered questions.

In particular, while we know its interior is likely melted by the push and pull of Jupiter’s gravity, we don’t know how its magma gets to the surface. Loki is also particularly interesting, as it goes through strange episodic brightening periods, which may be related to this overturning mechanism.

“This particular volcano produces way more total heat flow than any other volcano on Io,” said de Kleer. “And we don’t really understand why it’s unique.”

The team will now wait until the next occultation in 2021 to follow-up their findings. Or, you know, maybe we could send a spacecraft there to take a closer look. Hint hint NASA…