Is Jupiter's moon Callisto hiding more than we thought?

A new study finds more oxygen in Callisto's atmosphere than previously thought, posing questions about its formation and making a case for future exploration.
Rizwan Choudhury
Callisto in Color.
Callisto in Color.


Jupiter's moon, Callisto, has often been imagined as a future outpost for humanity. But in real-life planetary science, Callisto, one of Jupiter and Saturn's larger moons, has primarily been left in the shadows, dismissed by many researchers as a "dead body" in our celestial neighborhood.

However, a recent study published in The Journal of Geophysical Research: Planets throws a curveball, suggesting that Callisto's atmosphere contains a higher concentration of molecular oxygen than previously thought. This revelation is a surprising deviation from past models and poses new questions about the icy moon's atmospheric dynamics.

Defying previous assumptions

"For years, we believed that the molecular oxygen in Callisto's atmosphere was a result of Jupiter's magnetic field interacting with the moon's icy surface," said the American Geophysical Union (AGU).

But the new research conducted by Shane Carberry Mogan, a postdoctoral scholar in planetary science at the University of California at Berkeley, and his team presents an intriguing puzzle. "The levels of molecular oxygen we are currently observing simply don't match the earlier theories. There must be another method of O2 formation at play," Carberry Mogan tells Forbes which reported on the same.

Is Jupiter's moon Callisto hiding more than we thought?
The New Horizons Long Range Reconnaissance Imager (LORRI) captured these two images of Jupiter's outermost large moon, Callisto, as the spacecraft flew past Jupiter in late February 2007.

Revisiting the source of oxygen

"The findings strongly suggest that we need to re-evaluate Callisto's O2 source or reconsider the existing calculations for molecular oxygen's lifespan," Carberry Mogan pointed out. These moons, including Callisto, consist primarily of ice—chiefly H2O. Incoming charged particles can break molecular bonds, resulting in the recombination of hydrogen and oxygen into new molecules like H2, O2, and even H2O2, added Carberry Mogan.

While the presence of water (H2O) on Callisto tantalizes astrobiologists, it's crucial to note that conditions on the moon are far from hospitable for life as we know it. Callisto's bitterly cold temperatures, lack of intrinsic magnetosphere, and absence of active cryovolcanism make it an unlikely candidate for life. "It's primarily ice, with an additional component of water vapor released from the surface," clarified Carberry Mogan.

The moon's surface is thought to be a blend of patches comprising "relatively cold, bright ice and relatively warm, dark non-ice or ice-poor material," according to the study's authors. Furthermore, data from NASA's Galileo spacecraft suggest that the interior of Callisto is a haphazard mixture of ice and rock, lacking a differentiated core and mantle.

Why the cold shoulder?

So, why has Callisto received so little attention from the scientific community? "When NASA's Voyager spacecraft flew by in the late 1970s, they merely witnessed a cratered celestial body," Carberry Mogan explained. Moons like Europa and Ganymede have captivated researchers, relegating Callisto to the backdrop of more exotic celestial bodies.

However, upcoming missions may offer a fresh perspective. NASA's Europa Clipper and the European Space Agency's Jupiter Icy Moons Explorer (JUICE) are set to explore Jupiter's moon system. "JUICE will fly by Callisto more than twenty times, and Europa Clipper will likely make a few visits as well," said Carberry Mogan.

Understanding Callisto, he emphasized, is crucial for interpreting the data from these forthcoming missions and for a broader exploration of Jupiter's other icy moons. "Even though it has been overshadowed, Callisto holds keys that could unlock many mysteries in the Jovian system," Carberry Mogan concluded.

The study was published in The Journal of Geophysical Research: Planets

Study abstract

Observations of Callisto's atmosphere have indicated an O2 component should exist, but the evolution from its initial source to its inferred steady-state abundance is not well understood. Herein we constrain the production of O2 via radiolysis within Callisto's exposed ice patches and determine the corresponding O2 column density. To do so, for the first time we simulate the thermal and energetic components of the Jovian magnetospheric plasma irradiating Callisto's atmosphere and estimate energy deposited therein by the impinging charged particles along their trajectories to the surface. We then calculate O2 source fluxes corresponding to the energy of the impacting plasma fluxes, which is coupled with estimated atmospheric lifetimes to determine the steady-state abundance of O2. Our results suggest that production of O2 via radiolysis within the exposed ice on Callisto's surface does not produce a sufficiently dense atmosphere relative to the column densities inferred from observations by about 2–3 orders of magnitude. To resolve this discrepancy between estimated and observed abundances, we provide the first estimates for other potential sources of atmospheric O2. We also make similar estimates for the production of H2 in Callisto's atmosphere relative to constraints provided in the literature, and the conclusion is the same: a sufficiently dense atmosphere is not produced. Thus, we have shown that a better understanding of the production and fate of radiolytic products in Callisto's regolith is required in order to place firmer constraints on the generation mechanisms of its atmosphere in preparation for future observations.

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