There is a weird cold spot on Europa, and nobody knows what it is. The first full thermal map of Jupiter’s icy moon has thrown up one spot about 300 kilometres across that seems colder than it ought to be, and we don’t have enough data on the area to figure out why.
Samantha Trumbo at the California Institute of Technology and her colleagues used the Atacama Large Millimeter Array (ALMA), a set of 50 radio telescopes in Chile, to measure the heat radiating from Europa’s surface in a set of four overlapping thermal images. Their measurements represent the first full temperature map of the icy moon.
“We weren’t so interested in cold spots, we were more interested in hot spots because those could indicate geological activity,” says Trumbo. “But we saw this one cold spot repeated in two of the images.” The spot is only about 9°C colder than the surrounding areas, but it is different enough to be surprising, she says.
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There are two potential explanations for the cold spot: either it is simply emitting less heat than the rest of the surface, or it takes longer to warm up in the Europan morning.
It may be because the ice there has a different composition than the surrounding area, making it harder for heat to travel through it. Or it could have a blockier, less grainy texture. A large block of ice would remain cooler for longer, like how an ice sculpture can last an entire afternoon but a bowl of ice cubes will melt within a few minutes.
“If this spot is real, I would expect it to be a large ice sheet, maybe younger than the rest of the surface because it hasn’t been broken up yet,” says Jessica Noviello at Arizona State University. “On Europa I would be interested in studying something young to see what this moon really looked like in its earlier state and how it changed over time.”
The model that Trumbo and her team used to spot this anomaly was partially based on limited data from the Galileo spacecraft on the area’s albedo, or reflectiveness. Because of that, Noviello and Julie Rathbun of the Planetary Science Institute in Arizona doubt that the cold spot is even there. “I looked back over the Galileo data, and there is no good data on that area, so we have no idea what it looks like,” says Rathbun.
Despite the lack of data, Trumbo insists that there’s something odd going on. “It is possible that our albedoes are not 100 per cent correct – actually it’s likely,” she says. “But it would be strange if they were wrong significantly in only one location without that spot being somehow different.”
Figuring out exactly how it’s different will have to wait for more data. “For now, you can imagine almost anything,” Trumbo says.
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