By Chris Skeldon 
On the third day, NASA's Juno probe made its second encounter with Io, Jupiter's third-largest moon and the most volcanic world in our solar system.
The Juno spacecraft, which arrived at the gas giant in 2016, is on an extended mission to explore Jupiter's rings and moons. Its latest pass, which was complementary to the mission's first approach on December 30, provided more views of the lunar landscape.
Violent explosions of sulfur and additional compounds from Io give the moon its orange, yellow and blue colors. The process is similar to what happens around volcanoes in Hawaii or geysers in Yellowstone National Park, according to Scott Bolton, a physicist at the Southwest Research Institute who leads the Juno mission. “This must be what Io is like – when he's on steroids,” he said. Maybe it smells like those places, too, he added.
The latest images of Juno were released on the 4th and are ready to be discovered. On Io's surface, Bolton saw what appeared to be a twin volcanic plume shooting into space, something Juno had never been able to capture before. Other scientists are monitoring new lava flows and changes in familiar features seen in previous space missions, such as the Galileo probe, which made several close approaches to Io in the 1990s and 2000s.
“That's the beauty of Io,” said Gianni Radibo, a planetary scientist at Brigham Young University, who is not part of the Juno mission but is collaborating with the team in observing Io. Unlike our Moon, which remains frozen over time, “Io changes every day, every minute, every second,” Radibeau said.
Images from the two close passes, during which the spacecraft arrived within about 1,500 kilometers of Io, will be combined with previous shots taken by NASA of the Jovian moon. The goal is to understand “what's the real mechanism that drives all volcanoes, because they're everywhere,” Bolton said.
This could be a global ocean of magma just below Io's crust, or simply pockets of molten rock beneath the surface, like those that fuel Earth's volcanoes. It may take weeks, or even months, before scientists begin to find answers in the data.
This is the last close pass Juno will make to Io. But the mission will continue to make distant observations every 60 days, providing mission experts with an ever-changing picture of the Moon as a whole.
This data will be equally valuable, according to Bolton.
“All the pictures are incredible,” he said. “We never know what to expect.”