NASA’s Juno spacecraft, which has been studying Jupiter and its large inner moons up close for the past six and a half years, is currently humanity’s most distant planetary orbiter. The giant planet and its stormy atmosphere were initially Juno’s main focus, but in the mission’s current extended phase, close fly-bys of the Jovian satellites are on the menu as well.
While Europa and Ganymede (the Solar System’s largest moon) have typically been of greater interest to astrobiologists due to their subsurface oceans, another Jovian satellite — hot, volcanic Io — has largely been dismissed as a possible abode for life now or in the past. That may be about to change, however. Juno recently passed by Io at a distance of 80,000 km, and over the course of this coming year will get much closer, within 1,500 km.
Most of what we know about Io dates from more than 20 years ago, when the Galileo spacecraft toured the Jovian system. A quick recap: Io is roughly the size of our Moon, but otherwise couldn’t be more different. Galileo found, among other things, an active lava lake (perhaps crusted over) and a lava “curtain,” along with active lava flows, calderas, mountains, plateaus, and plains.