This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Of Jupiter's four Galilean moons, Callisto is the one that gets the least attention. Io is constantly being resurfaced by volcanoes.
Europa has a giant liquid water ocean. And Ganymede has its own magnetic field that interacts with its parent planet in weird ways. Callisto, by comparison, seems sedate, with its ancient, crater-saturated surface seemingly frozen in time.
But new data from the James Webb Space Telescope (JWST) shows that even this most benign of the Big Four moons is more active than previously realized. The data, by an international team of astronomers led by Maria Camarca of Caltech, was based on JWST's Near-Infrared Spectrograph (NIRSpec) instrument. It covered three major perspectives of Callisto itself—its trailing hemisphere, the giant Asgard impact basin and a view centered squarely on Valhalla, the largest multi-ring impact structure in the solar system.
The paper was recently accepted for publication in the Planetary Science Journal and is available on the arXiv preprint server. One notable feature in the data was water ice. NIRSpec picked up the 3.1 µm "Fresnel peak" indicative of ice crystals and mapped it onto the moon's surface in unprecedented detail.
The map revealed a notable divide between the leading and trailing hemispheres. On the leading hemisphere, the water ice directly tracks with geography. Bright impact basins like Valhalla and Asgard, along with younger craters like Lofn and Heimdall, show sharp spikes in water ice where ancient impacts excavated fresh, brighter material that contrasts sharply with the dark albedo of the rest of Callisto.
On the trailing hemisphere, on the other hand, the ice forms a distinct "bullseye" pattern. Its signature is weakest near the equatorial center and strengthens toward higher latitudes. Most likely, that pattern is shaped by plasma bombarding the moon's surface from Jupiter's magnetosphere.
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