Giant Waves on a Lava Lake the Size of Wales Seen on Jupiter's Volcanic Moon Io

Jupiter's moons
Jupiter's four Galilean moons. From top to bottom, the moons shown are Io, Europa, Ganymede and Callisto. NASA

A rare cosmic alignment has allowed scientists to observe a never-before seen phenomenon on Jupiter's volcanic moon Io—two huge waves sweeping across the surface of a lava lake the size of Wales.

Io is the most volcanically-active object in the whole solar system. It is the innermost of Jupiter's Galilean moons—the four largest of the planet's 64 known moons—and has the highest density of them all.

Changes to the brightness of Io's surface have been documented for almost 50 years and in 1979, when NASA's Voyager 1 and 2 flew past, scientists realized this was due to volcanic activity on the moon.

One spot in particular stood out—a region named Loki Patera after the Norse god, Loki.

The patera—a bowl-shaped volcanic crater—brightens every 400 to 600 days and there is an ongoing debate about what causes it. Either it is a massive lava lake being churned up, or regular and massive eruptions causing lava to flow over a huge area.

Scientists led by Katherine de Kleer, from the University of California, Berkeley, have now taken advantage of a rare cosmic alignment between Io and Europa (Jupiter's icy moon) that allowed them to isolate the heat coming from the volcanic moon's surface—and work out what is causing the observed changes.

In an email interview with Newsweek , de Kleer explains why Io is so volcanically active: "Io's volcanic activity is powered by tidal heating: Io is locked into an eccentric orbit because it's in an orbital resonance with the other satellites Europa and Ganymede [Jupiter moons]. Because of its eccentric orbit, the gravity it feels from Jupiter changes during its orbital period (which is only 1.8 days), which causes friction in its interior that is so strong it melts portions of the mantle, generating magma."

But volcanic activity is normally difficult to observe. That is until the rare alignment of Io and Europa gave scientists to peer down on the moon and obtain a detailed map of its huge lava lake.

Jupiter moon Io
Galileo image of Io. Colours have been adjusted for contrast, but are based on real composite infrared, green and violet-light images. NASA/JPL

As Europa passed in front of Io, it blocked out the light from the volcano moon. Because Europa's surface is frozen it reflects little sunlight at infrared wavelengths, meaning the team could isolate heat coming from Io's surface. Their findings are published in the journal Nature.

"There was so much infrared light available that we could slice the observations into one-eighth-second intervals during which the edge of Europa advanced only a few kilometers across Io's surface," study co-author Michael Skrutskie said in a statement.

"Loki was covered from one direction but revealed from another, just the arrangement needed to make a real map of the distribution of warm surface within the patera."

The thermal map provided scientists with a picture ten times better than would normally be possible. The map showed temperatures varied across the lake from 270 Kelvin (minus 3.1 Celsius) in the west to 330 Kelvin (56 degrees Celsius) in the southeast.

This map then allowed the team to work out how often the magma is exposed at the surface. Their analysis indicates the lava was turned over in two waves—one moving west and the other moving east—at a rate of around 0.6 miles (3,300ft) per day.

"Loki Patera has been active for decades, so we were not surprised to see the lava lake itself," de Kleer says. "However, all past observations could be explained by a single wave propagating in one direction around the entire patera, so we were indeed surprised to see two waves, and in particular two waves moving in opposite directions at different speeds."

"The patera itself is enormous—covering the same surface area as Wales or the state of Massachusetts. So if you were standing at the edge of it, you wouldn't be able to see the other side; it would look like an ocean of lava. The lava gives off a lot of heat, but in fact much of it is below freezing, although it is considered 'warm' because it is still more than 100 degrees warmer than Io's surface."

"The wave 'front' spans the entire width of the patera, or 50-100km (31-62 miles), and travels all the way around the horseshoe-shaped lake, covering the whole surface area of 21,500 square kilometers (8,300 square miles)."

Researchers say the findings show the Loki Patera is far more complex than was previously thought and that the overturn of lava observed could be related to the composition of the magma, or the amount of dissolved gas it contains. "These results give us a glimpse into the complex plumbing system under Loki Patera," de Kleer said in a statement.

They hope to make more observations of Io to confirm their findings, but they will not be able to do this until 2021, when Io and Europa align again: "We would like to make observations of a similar nature but at different points in the overturn cycle at Loki Patera," she says.

"This would help with confirming the two-wave model and/or uncovering additional details of the overturn process. Unfortunately, we have to wait four more years until the geometry aligns again to make this experiment possible."

Europa Io
Europa crossing the disk of Io. Loki Patera is the bright hot spot in the upper part of the disk. Large Binocular Telescope Observatory