Europa is one of the largest moons of Jupiter, and its icy shell covers a global subsurface ocean that scientists think could be a promising location to search for life. But where in the ocean could that life be?
A clue may lie in finding Europa’s hot spots—places where warmer waters could create regions more hospitable to organisms.
To that end, scientists have now produced the first global map of Europa’s surface heat. The map has a spatial resolution of 200 kilometers, equivalent to spotting a tennis ball in Boston while standing in New York City.
Using the map, scientists can “search for any obvious correlations between the potential surface thermal properties and the geology or surface composition of Europa,” Samantha Trumbo told Eos. Trumbo is the lead researcher on the study and a graduate student in planetary sciences at the California Institute of Technology (Caltech) in Pasadena.
Thermal hot spots on Europa’s surface might be where warmer subsurface liquid thins Europa’s icy outer layer, said Trumbo. Such thinning might result in water plumes or cracks in the ice, features that scientists have identified in prior studies. Depending on the pattern of hot spots, thermal observations could bolster the hypothesis that plate tectonics exists on Europa. And with plate tectonics come rift zones, which, on Earth, teem with microbial life.
However, although the map reveals one unusually cold region in Europa’s northern hemisphere, the spatial distribution of heat seems to be mostly consistent with the surface reflecting and absorbing sunlight, the researchers note. These results will publish in an upcoming issue of The Astronomical Journal.
Small Heat Variations
The team observed Europa with the Atacama Large Millimeter/Submillimeter Array (ALMA), a radio telescope array in northern Chile. They looked at four ALMA observations taken in November 2015 at a wavelength that probes heat from the outer layers of Europa’s surface.
ALMA measured the heat leaving Europa’s surface, which the team used to make a 360° map of Europa’s surface temperature. They then compared their ALMA map to a simulation of the temperature at Europa’s surface in the absence of interior heat sources. In this scenario, the surface ice, depending on its thickness and roughness, just absorbs and reflects the sunlight that reaches it.
They found that Europa’s surface temperature was no more than 10 K hotter or colder than predicted by the simulation. This suggests that the moon’s surface temperatures are governed mostly by its ability to retain the Sun’s heat.
Still, a few spots on the temperature map seemed just a bit too hot or cold to be fully explained by sunlight. Seeking to explain these regions, the team used the ALMA data to create maps of Europa’s surface thermal properties.
Specifically, they examined Europa’s thermal inertia—how quickly temperatures level out across the surface—as well as the moon’s emissivity (outgoing radiation). Thermal inertia is likely related to surface texture, Trumbo explained, and emissivity is likely related to composition.
Two odd spots stood out to the researchers. One is located at Pwyll crater, the moon’s largest fresh crater, which seems to retain heat—similar to other craters in the solar system—even though the crater is from an impact on ice.
However, another anomalous signature doesn’t seem to be related to an obvious physical feature. It lies at 90°W and 23°N, a spot in the northern part of Europa’s leading hemisphere, which always faces in the direction of the moon’s orbit. This spot is the coldest location on that hemisphere. The model suggests that either it is very resistant to temperature changes or it emits much less radiation than other areas of the moon.
“We don’t yet have a good explanation of what the cold spot means,” Trumbo said. “But, since we see the same anomaly twice, at two different times of the Europa day, we think it must be a region that is distinct in its thermal properties, and that could be interesting.”
The team noted that the location does have Europa’s highest water ice concentration and is also almost directly opposite Pwyll. However, neither of those facts can explain that spot’s strange thermal properties, they say.
“We should be able to learn more about this spot with more ALMA [data] and perhaps spectroscopic observations,” Trumbo said, “but we will probably not know if it correlates with unique geology until the Europa Clipper mission.”
Mapping Now Possible from Earth
Before now, the only spatially resolved thermal data for Europa were taken by NASA’s Galileo mission, which orbited Jupiter. Galileo flew over Europa in the late 1990s and provided thermal data at a resolution of 80–200 kilometers over sections of the moon’s surface.
“Until ALMA, the best that we could do from Earth was to measure an average temperature of the entire disk of Europa,” coauthor Michael Brown, a planetary astronomer at Caltech, told Eos. “Using ALMA,” he said, “we are now able to map temperatures on the surface of Europa better than they were mapped by the Galileo spacecraft.”
The team will be gathering more observations of Europa’s surface during its daytime and when it is eclipsed by Jupiter—a regular occurrence—using ALMA and other high-powered telescopes. The eclipse observations should make it easier to find any hot spots that might suggest geologic activity, Trumbo said. The daytime observations will let them home in more precisely on the moon’s thermal properties to determine if they need to dig deeper to explain the odd spots.
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer
Correction, 5 September 2018: A description of future observations has been updated and clarified.