Pictures were captured by ground-based telescopes, providing an exceptionally detailed map of the most volcanically active place in the solar system.
The breakthrough was possible thanks to an ‘occultation’ – a rare astronomical event similar to an eclipse where the larger moon Europa passed in front of Io.
As Europa’s surface is coated in water ice it reflects very little sunlight at infrared wavelengths.
This allowed researchers to accurately isolate the heat emanating from volcanoes on Io.
The infrared data showed the surface temperature of Io’s massive molten lake steadily increased from one end to the other.
It suggested the lava had swept from west to east in two waves – travelling about one kilometre (0.62 miles) per day.
This would confirm the popular idea among scientists that the periodic brightening and dimming of Io’s hot spot – called Loki Patera after the Norse god of fire and chaos – is caused by an overturning lava lake.
Loki Patera – a bowl-shaped volcanic crater – is about 200 kilometres (127 miles) across. Its hot region has a surface area of 21,500 square kilometers (13,400 sq miles), larger than Lake Ontario.
Study lead author Katherine de Kleer, a graduate student at California University in Berkeley, said: “If Loki Patera is a sea of lava.
“In this scenario, portions of cool crust sink, exposing the incandescent magma underneath and causing a brightening in the infrared.”
A bit larger than Earth’s Moon, Io is the third largest of Jupiter’s moons, and the fifth one in distance from the planet.
Io’s orbit keeps it roughtly at a distance of 422,000 km (262,000 miles) from Jupiter
Volcanologist Ashley Davies, of the Jet Propulsion Laboratory in Pasadena, who has studied volcanoes on Io for many years, said: “This is the first useful map of the entire patera.
“It shows not one but two resurfacing waves sweeping around the patera. This is much more complex than what was previously thought.”
The images were obtained by the twin 8.4-meter (27.6-foot) mirrors of the Large Binocular Telescope Observatory in the mountains of southeast Arizona, linked together ausing advanced adaptive optics to remove atmospheric blurring.
Imke de Pater, professor of astronomy at California University in Berkeley, said: “This is a step forward in trying to understand volcanism on Io which we have been observing for more than 15 years and in particular the volcanic activity at Loki Patera.”
The occultation happened on 8 March 2015 – 36 years after Voyager 1 and 2 spacecraft flew by Io and first spotted the volcanic eruptions on its surface.
Despite highly detailed images from NASA’s Galileo mission in the late 1990s and early 2000s astronomers continue to debate whether the brightenings at Loki Patera – which occur every 400 to 600 days – are due to overturning lava in a massive lake or periodic eruptions that spread lava flows over a large area.
The study published in Nature saysEuropa took about 10 seconds to completely cover Loki Patera.