On May 16, 2023, Juno passed within 35,000 kilometers of Io's surface. JunoCAM acquired these eight views over a period of an hour and fifteen minutes as the spacecraft approached and receded from Jupiter’s volcanic moon. These are the sharpest visible-light images of Io acquired since the New Horizons flew past Jupiter on its way to Pluto in 2007. They reveal a landscape of mountains and volcanoes, some revealing changes over the course of sixteen years and others revealed due to the unique lighting and viewing geometry of Juno’s flybys.
Several surface changes are visible compared to images taken by New Horizons sixteen years ago and Galileo more than twenty years ago. Some were seen in previous Juno encounters such as fresh red material surrounding Chors Patera and a darkening at a volcano east of Girru Patera. Revealed for the first time in these images are changes at the volcano Volund. Thermal data from the JIRAM instrument, also onboard Juno, in previous encounters of Io showed two prominent areas of activity at Volund on its northwest end and to the west of a circular feature thought to be the vent area of Volund. Both these areas are now darker than they appeared in New Horizons LORRI images. These two dark areas are best seen as the upper two dark spots near the terminator (the boundary between day and night sides) in the middle of the third image from left in the top row. Images from encounters in July and October may help to reveal how these two changes are connected.
The polar geometry of Juno’s encounters is helpful for mapping the terrain in Io’s north polar region. For example, a triangle patch of three bright regions, best seen above center in the second image from left in the bottom row, is revealed to be host to two mountains, one with a height of approximately 5 kilometers. Mountains in Io’s polar regions, like Haemus Montes, are often surrounded by moats of bright sulfur dioxide frost, and these two mountains appear similar. Confirmation of a third mountain will need to wait for imaging from the July and October encounters.
The Io images show here have an original pixel scale of 24 to 52 kilometers per pixel and have been enlarged by 5x to improve the visibility of surface features.