NASA has released a high-resolution image taken by the Juno deep space probe during its close encounter with Jupiter's moon Io on December 30, 2023. The image of the volcanic satellite not only shows remarkable details, but an unusual kind of lighting.
When the robotic Juno orbiter zoomed past Io in December, it was the closest flyby of the moon since the Galileo mission over 20 years ago. It also marked a generational jump in photographic capability thanks to its JunoCam, which can manage a resolution of 15 km/pixel (9.3 miles/pixel) from 4,300 km (2,700 miles).
As Juno passed over the southern hemisphere of Io, JunoCam sent back high-resolution images of many of the moon's features, including active volcanoes spewing clouds of sulfur into space.
However, the latest processed image showcases an odd phenomenon. We're used to seeing objects in space starkly lit by the Sun on one side and sitting in jet-black darkness on the other. The new image shows one side of Io brightly lit, but the night side is almost as bright. This is because the brighter side is illuminated by the Sun, while the other is lit by "Juptershine" – that is, sunlight reflected by Jupiter back at Io.
You can see a similar thing during a New Moon. Even though the Moon should be in complete darkness, it's actually very dimly lit by Earthlight. When there's a New Moon, the Moon sees a full Earth, so a lot of sunlight is being reflected onto it – enough to be visible on Earth under the right conditions.
Of course, since Jupiter has eleven times the diameter of Earth, Io gets almost as much light from it as from the Sun.
Source: NASA