Jupiter’s icy moon Callisto, long hypothesized as a potential human outpost in science fiction, has largely remained a mystery to real-life planetary scientists. As one of the least studied large moons of Jupiter and Saturn, many researchers consider it essentially a dead body.
But a new paper appears in Journal of Geophysical Research: Planets Reveals a new mystery about Callisto. New models indicate that it contains much more molecular oxygen in its atmosphere than previous models have indicated over the decades.
The American Geophysical Union notes that “previous observations of Jupiter’s second largest moon, Callisto, found molecular oxygen in Callisto’s atmosphere.” “Scientists hypothesized that the oxygen in the atmosphere is caused by the interaction of Jupiter’s magnetic field with the moon’s icy surface,” says AGU.
But this new study shows that there isn’t enough molecular oxygen to match the observations. Thus, Callisto must form O2 in some other way.
This suggests that the source of oxygen on Callisto and/or calculations of the age of molecular oxygen need to be reevaluated, Shane Carberry-Maughan, a postdoctoral researcher in planetary sciences at UC Berkeley and the study’s lead author, told me by phone. And email.
Such satellites contain ice, mainly composed of H20, whose molecular bonds are broken by impacting and penetrating charged particles, Carberry-Maughan says. This hydrogen and oxygen will recombine and form new molecules, such as H2 and O2, and sometimes even H2O2, he adds.
Artist’s illustration of the surface of Callisto
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But Callisto was never a candidate for life
It is very cold, has no intrinsic magnetosphere, and no signs of cryovolcanic activity. But Callisto harbors water.
Life on Earth requires water and Callisto contains water, Carberry-Maughan says. But because of the cold temperatures in the outer solar system, the H20 is primarily in the form of ice, with an additional component of water vapor from ice escaping from the surface, he says.
As for the surface of Callisto?
“We assume that Callisto’s surface is divided into patches of relatively cold, bright ice, and relatively warm, non-ice or ice-poor material,” the authors wrote.
Based on data from NASA’s Galileo spacecraft, Callisto’s interior appears to be a mixture of ice and undifferentiated rock. In other words, Callisto’s interior does not have a large contrasting composition, like the core and mantle.
Why does Callisto get so little attention?
There are several reasons, says Carberry-Maughan. When NASA’s two Voyager spacecraft flew by in the late 1970s, they saw a cratered object, and that was it, he says.
Jupiter’s moons Europa and Ganymede were more interesting, says Carberry-Maughan. He says that Callisto is in this neighborhood of all these other strange objects, and yet it is this kind of mysterious moon that is difficult to explain.
Two upcoming missions, NASA’s Europa Clipper probe heading to Jupiter and the European Space Agency’s Jupiter Icy Moons Explorer, will visit the Jovian system. JUICE will fly by Callisto more than two dozen times, and Europa Clipper will likely fly by the icy moon several times as well.
But understanding Callisto is important for exploring these other large icy moons, Carberry-Maughan says. He says it will be useful in interpreting the data we eventually receive from JUICE and Europa Clipper.
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(Signs for translation) Callisto