Some of our Solar System\u2019s moons have become very enticing targets in the search for life. There\u2019s growing evidence that some of them have oceans under layers of ice and that these oceans are warm and rich in prebiotic chemistry. NASA\u2019s Europa Clipper is on its way to examine Jupiter\u2019s moon Europa, and the ESA\u2019s Jupiter Icy Moons Explorer is also on its way to the Jovian system to explore some of its icy moons. <\/p>\n
While the presence of an ocean on Europa is becoming widely accepted, there\u2019s more uncertainty about the other Galilean moons. However, new evidence suggests that Callisto is very likely an ocean moon, too.<\/p>\n
<\/p>\n Callisto is Jupiter\u2019s second-largest moon, the third-largest moon in the Solar System, and the outermost Galilean moon. The Voyager probes gave us our first close looks at Callisto in 1979, and the Galileo spacecraft gave us our best images and science data during flybys between 1996 and 2001. Galileo provided the first evidence that Callisto may harbour a subsurface ocean.<\/p>\n Callisto has a different appearance than other suspected ocean moons like Europa and Saturn\u2019s Enceladus. Europa clearly has a white, icy surface, although it has other brownish colours, too. Enceladus has an extremely bright, icy surface and has the highest albedo of any object in the Solar System. Callisto, on the other hand, has a dark, icy surface and is covered in craters. <\/p>\n However, the evidence for its ocean is unrelated to its surface appearance and any visible ice. <\/p>\n The main evidence supporting an ocean on Callisto comes from the moon\u2019s magnetic field. Unlike Earth\u2019s internally generated magnetic field, Callisto\u2019s is induced. That means the field is created from Callisto\u2019s interactions with Jupiter and its extremely powerful magnetic field. For Callisto to induce a magnetic field, it has to have a layer of conductive material.<\/p>\n The question is, is the layer an ocean or something else?<\/p>\n Different researchers have been trying to answer that question since Galileo gathered its data. One of the spacecraft\u2019s instruments was a magnetometer, a type called a Dual-Technique Magnetometer (DTM). There are multiple types of magnetometers, and each one works differently. Galileo\u2019s DTM provided redundancy and allowed for cross-checking, which increased the accuracy and reliability of its data. It was especially good at detecting the subtle magnetic fields of Jupiter\u2019s moons, including Callisto. It also collected data continuously, which let scientists gain insights into how the magnetic fields of Jupiter and its moons varied over time due to different interactions. <\/p>\n In a 2017 paper, researchers pointed to the ionosphere as the primary cause of Callisto\u2019s magnetic fields. \u201cWe find that induction within Callisto\u2019s ionosphere is responsible for a significant part of the observed magnetic fields,\u201d the authors wrote. \u201cIonospheric induction creates induced magnetic fields to some extent similar as expected from a subsurface water ocean.\u201d<\/p>\n New research in AGU Advances based on Galileo data strengthens the idea that Callisto has a subsurface ocean and that it\u2019s responsible for the moon\u2019s magnetic field rather than its ionosphere. The paper is titled \u201cStronger Evidence of a Subsurface Ocean Within Callisto From a Multifrequency Investigation of Its Induced Magnetic Field.\u201d The lead author is Corey Cochrane, a scientist at JPL who studies planetary interiors and geophysics. An important part of this research is that they considered data from multiple Galileo flybys (C03, C09, and C10).<\/p>\n \u201cAlthough there is high certainty that the induced field measured at Europa is attributed to a global-scale subsurface ocean, there is still uncertainty around the possibility that the induced field measured at Callisto is evidence of an ocean,\u201d Cochrane and his co-researchers write. \u201cThis uncertainty is due to the presence of a conductive ionosphere, which will also produce an induction signal in response to Jupiter\u2019s strong time-varying magnetic field.\u201d<\/p>\n In short, Callisto\u2019s magnetic field could be caused by its ionosphere, an ocean, or a combination of both. The problem is that Callisto\u2019s conductive ionosphere creates a magnetic field that can mask the presence of an ocean. To get to the truth, the authors used previously published simulations of the moon\u2019s interactions combined with \u201cboth an inverse and an ensemble forward modeling method.\u201d The authors write that this brings some clarity about the possible range of Callisto\u2019s interior properties. <\/p>\n The researchers created a four-layer model of Callisto, including its ionosphere. \u201cAmong these models, we vary the thickness of the ice shell, the thickness of the ocean, and the conductivity,\u201d the authors write. They also varied the seafloor depth and the ionosphere\u2019s conductance. <\/p>\n The researchers concluded that the moon\u2019s ionosphere alone cannot explain the magnetic field. Instead, it \u201cmore likely arises from the combination of a thick conductive ocean and an ionosphere rather than from an ionosphere alone.\u201d <\/p>\n They also concluded that the ocean is tens of kilometres thick from the seafloor to the ice shell, and the ice shell could also be tens of kilometres thick. \u201cAs our results demonstrate, both the inverse and forward modelling approaches support the presence of an ocean when considering data acquired from flyby C10 alongside C03 and C09,\u201d the researchers explain. \u201cOur analysis, the first to simultaneously fit C03, C09, and C10 flyby data together, favours the presence of a thick and deep ocean within Callisto.\u201d<\/p>\n The models also favour a thick ice shell \u201cconsistent with Callisto\u2019s heavily cratered geology,\u201d they explain. <\/p>\n Galileo wasn\u2019t dedicated to studying Callisto, so there is a dearth of data in all research into its magnetic fields. \u201cIt is challenging to place tighter constraints on the properties of Callisto\u2019s ocean because of the limited number of close Galileo flybys that produced reliable data and because of the uncertainty associated with the plasma interaction,\u201d the authors write in their conclusion. <\/p>\n Better and more complete data is in the future, though. Both NASA\u2019s Europa Clipper and the ESA\u2019s JUICE mission will gather more data, some of it from very close to Callisto\u2019s surface. <\/p>\n The Europa Clipper is scheduled to make nine flybys of Callisto. Seven will be within 1800 km of the surface, and four of those will be within 250 km. Its magnetometer will operate continuously during those flybys. The ESA\u2019s JUICE mission is scheduled to perform 21 flybys of Callisto. All of them will be within 7000 km of the surface, and most will be below 1000 km.<\/p>\n Both the Europa Clipper and JUICE have instruments that Galileo didn\u2019t have. Though Galileo came within about 1100 km of Callisto\u2019s surface, it simply could not provide the same kind of data that these newer missions will. The Clipper and JUICE are scheduled to reach the Jovian system in 2030 and 2031, respectively. <\/p>\n As their data starts to arrive and reaches scientists, we will likely determine for sure if Callisto is yet another of the Solar System\u2019s ocean moons. <\/p>\n![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2025\/02\/Jupiter_s_magnetosphere-1024x576.jpg\")
Licence: ESA Standard Licence<\/figcaption><\/figure>\n![\"Observations](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2025\/02\/Callisto-magnetosphere.jpg\")
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