Volcanoes from Iceland to Turkmenistan have been described as gateways to hell. But nowhere in the solar system is this term more apt than on Io. That moon of Jupiter, we now know, is in an almost constant state of eruption.
Io’s molten insides refuse to lie hidden beneath its frozen, radiation-bathed surface, giving us a literal window into the moon’s gooey centre. And one of the lava’s most persistent escape outlets, Loki Patera – the most powerful volcano in the solar system – is changing its ways as we watch.
First spotted by Voyager 1 in 1979, Loki is thought to be a horseshoe-shaped sea of magma 200 kilometres wide that hasn’t stopped burbling since we first discovered it.
Thermal exhaust port
Despite its fiery face, Io is far enough from the sun that its average surface temperature is about -130 °C. Those cold surfaces collect a frost of sulphur dioxide that gives them a shiny appearance.
But Loki is warm and therefore looks dark. However, at infrared wavelengths it glows brightly, giving off more than 10 per cent of the entire moon’s heat emission. You could think of it as a thermal exhaust port.
As Io orbits Jupiter, gravitational tides yank at its interior, depositing massive amounts of energy that must then rise to the surface. But the moon has no tectonic plates that allow magma to easily slip through.
Instead, the magma rises through pipes, not unlike the volcanism that built Hawaii. Across Io’s surface, periodic eruptions flare up and then die down. But elsewhere – most notably at Loki – the channel seems to stay open continuously.
And yet Loki still changes over time to its own beat. Over decades of study, observers have noticed a pattern: about every 540 days, a wave of brightness starts at one end of the lake of lava and pivots anticlockwise like a windshield wiper.
That front of warmer lava moves about 1 kilometre per day until the whole lake glows hot. Then the surface of Loki cools until the process starts again.
This pattern may be occurring because new layers of lava are periodically spreading over the lake’s surface. When fresh lava meets Io’s icy, barely existent atmosphere, it might be cooling and eventually sinking back down, allowing new lava to swallow it from underneath.
But in 2002, right when we thought we had Loki pegged, those phases stopped. Then in 2009, according to recent work by Katherine de Kleer at the University of California, Berkeley, they started again – but are now moving clockwise. Appropriate behaviour for a feature named after the Norse trickster god.
We don’t yet understand why. De Kleer and others have speculated that a change in the composition of the lava may have led to the pause, and that the new direction of movement might be caused by a nearby eruption that triggered the “wrong” end of the lake surface to start sinking first.
No lava lakes of the same scale exist on Earth – unless you go back to the very beginning of our planet, when even larger ones sprawled out over its surface. The folklore origins of volcano names weren’t wrong: a volcano can be a window into another world, or maybe our own.
Article amended on 13 January 2017