An active moon coming into focus

Io close up, lit by Jupiter-shine — NASA/Jet Propulsion Laboratory–Caltech/Southwest Research Institute (SwRI)/Malin Space Science Systems (MSSS); Image processing: Emma Wälimäki © CC BY
Processed image. Emma Wälimäki processed this JunoCam observation from Juno’s 30 December 2023 close flyby, about 1,500 kilometres above Io. Reflected sunlight from Jupiter illuminates the darker left-hand lobe. This is a visible-light photograph with public image processing, not an infrared temperature map or a view beneath the crust. NASA published this unbranded presentation on 7 February 2024, separate from the capture date.
Creative Commons Attribution (version not specified by source) · resized to WebP
· Original image source · Image licence

A citizen-processed approach view meets earlier detailed observations.

World: Io
Case date or period: Observations · 1979–2023

Look closer

Follow Io from Gill’s distant March 2023 view to Wälimäki’s close December encounter. Jupiter-shine reveals terrain on the night side. Galileo’s composite and a separate Voyager 2 photograph add earlier views of plumes and surface patterns. Different camera distances, filters and processing mean that a changed-looking colour or sharper edge alone does not prove a new eruption.

Follow the evidence

These observations span different missions and dates. Colour processing can accentuate deposits, so image-to-image colour differences alone are not a record of new volcanic activity.

Built by people looking closely

The credited image processors worked with publicly available mission observations. Their versions are part of this collection, with each person’s processing and licence retained. Compare the treatments and return to the underlying source when a small detail matters.

The close passes also tested the interior

Ryan Park’s investigation used Doppler tracking during Juno’s December 2023 and February 2024 close encounters to measure how Io’s gravity affected the spacecraft. Comparing the tidal response with interior models argued against a shallow global magma ocean, while allowing substantial melt within a mostly solid mantle. This inference comes from gravity and deformation, not the surface colours in Gill’s approach image. It does not map each volcano’s underground reservoir.

Infrared brightness is not total heat flow

Federico Tosi, Alessandro Mura and Francesca Zambon challenged an earlier inference of a magma ocean from the latitude distribution of JIRAM emission. Their 2024 critique notes saturated measurements and warns that one infrared band is an incomplete proxy for total volcanic power. The distinction matters when combining photographs, thermal observations and interior models: even accurately mapped brightness needs spectral and statistical interpretation before it can diagnose where Io’s tidal heating occurs.