The biggest volcano on Jupiter's moon Io could be about to blow. Decades of observation have revealed a periodic cycle in the volcano's eruptions; according to past behaviour, it's due for the next one any day.
That potential burst of activity - or lack thereof - could help us to better understand the volcano and Io itself, the most volcanically active object in the Solar System.
The massive volcano, called Loki, was originally discovered to have a cycle of around 540 days. This was based on years of observations between 1988 and 2000, described in a 2002 paper led by physicist and planetary scientist Julie Rathbun of the Planetary Science Institute.
At the start of the eruption, Loki would brighten, and remain bright for around 230 days before falling darker again. Then, the cycle would repeat. This was happening like clockwork until 2001, when the volcano stopped brightening and dimming.
Then, in 2013, Loki started up again, but on a slightly shorter cycle - 475 days, instead of 540. It's been on a 475-day cycle ever since.
"If this behaviour remains the same, Loki should erupt in September 2019," Rathbun said. "We correctly predicted that the last eruption would occur in May of 2018."
Rathbun and her team interpreted Loki as a lake of lava in a crater-like depression called a patera about 200 kilometres (124 miles) across. As the cooling crust on the surface of the lake becomes gravitationally unstable and collapses into it, the pool "overturns", flooded by fresh lava.
This was supported by observations reported in 2017 that saw waves of lava slowly rolling across the patera - a process that can take up to 230 days.
What caused the hiatus in this cycle between 2001 and 2013 is not yet known, but one possible explanation could implicate changes in the volatile content in the magma, which affects the density of both magma and crust. Even a small change can produce large variations in how long the crust takes to sink.
The last eruption started sometime between 23 May and 6 June 2018. That means the 475-day window is between 9 and 24 September. It may have already started.
"Volcanoes are so difficult to predict because they are so complicated. Many things influence volcanic eruptions, including the rate of magma supply, the composition of the magma - particularly the presence of bubbles in the magma, the type of rock the volcano sits in, the fracture state of the rock, and many other issues," Rathbun said.
"We think that Loki could be predictable because it is so large. Because of its size, basic physics are likely to dominate when it erupts, so the small complications that affect smaller volcanoes are likely to not affect Loki as much."
Rathbun presented her findings at the EPSC-DPS Joint Meeting 2019 in Geneva.