Ice-Quake Echoes Could Lift Lid on Oceans within Pluto and Europa

Measuring outer solar system ice-quakes could identify if Europa’s subsurface ocean is an oxygen-rich, leading candidates for life, or uncover the origin of Enceladus’ giant water plumes, says a team from NASA’s Jet Propulsion Laboratory. They propose that cryo-seismology techniques could also ‘explore’ Pluto’s own newly predicted subsurface sea.

Artist’s concept of ocean on Jupiter’s moon Europa. Image credit: NASA / JPL-Caltech.

Watery oceans are thought common below the surface of outer solar system moons.

The Voyager, Galileo and Cassini missions sent back not just evidence of Europa’s 100 mile deep salty sea, but also liquid water layers within its Jovian neighbors Ganymede and Callisto, as well as Saturn’s Enceladus and Titan, and Neptune’s Triton.

Just last week data from New Horizons has been interpreted as evidence of a cold, slushy ocean lying deep beneath Pluto.

So what are these seas like?

“If you wanted a swim, Europa’s is probably the most Earth-like,” says Steve Vance, who leads the Habitability team within NASA-JPL’s Icy Worlds Astrobiology group.

“Enceladus’ ocean has a more soapy, caustic feel that could harm your skin over time.”

Hopes of more detailed exploration have been hampered by limitations of traditional radar and magnetic field techniques, which are unable to identify clear boundaries between surfaces below a few miles.

Closer to home, however, a new field, cryo-seismology, is raising hopes for future missions.

Making use of techniques that once mapped the Earth’s structure down to its center (by measuring the arrival time of seismic waves from earthquakes and nuclear tests), it has recently been used to analyze the best analogues of outer solar system seas on Earth — Alaskan and Antarctic ice fields. Here they investigated glacier cracking and the flow of liquid water within and beneath the ice.

Out in space seismology’s success in mapping the Earth’s interior led to similar studies of our Moon, and the inclusion of seismographs on 2018’s Mars Insight mission.

However, the data from Alaskan and Antarctic glaciers have raised hopes of taking similar measurements from the surface of outer solar system moons — monitoring the propagation of fractures and other seismic vibrations generated by anything from gravity induced flexing as the moon’s orbit their giant planetary hosts, to meteorite impacts.

“Tidal induced flexing alone should provide ample seismic waves to measure as they echo around the rocky and icy interiors,” says Vance, whose new paper, posted on the arXiv.org site, analysed the potential for such measurements.

“The starting point was to find out if a seismic experiment was worth doing. We believe by listening for echoes from the interfaces of the ice and ocean, and from the ocean and rocky ocean floor, we could investigate the ice’s thickness and the depth of these seas.”

In fact, Vance’s paper set out a range of features and properties that could be uncovered using instruments only slightly more sensitive than those due onboard Insight. These include the source of surface plumes and water jets discovered emanating from Europa and Enceladus, as well as the makeup of each moon’s mantle and core, and presence of seafloor volcanism. These later properties could provide a process for allowing life-giving hydrogen to dissolve into ocean water.

However, in terms of the habitability, there is one unresolved alien ocean property that interests Richard Greenberg, author of ‘Unmasking Europa,’ most of all — the presence of oxygen.

“We know there is oxygen at Europa’s surface,” says Greenberg.

“The ice is bombarded by energetic charged particles from Jupiter’s magnetic field that release it from water molecules.”

“And there is some evidence that ice material from below gets piled on top, suggesting a potential conveyer belt system pushing oxygen down into the ocean.”

So could seismology provide conclusive proof of this potentially life-giving cycle?

“You might get a sense of how thick the ice is, which is critical to the oxygen reaching below,” says Greenberg.

“But I think they are being optimistic over how much detail you will be able to infer with so many potential sources and reflective surfaces, as well as an irregular structure unlike layered rock strata.”

Greenberg also has concerns for the survival of any instrumentation facing the same radiation that releases oxygen from the ice.

“It could fry any solid state electronics,” he says.

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Steven D. Vance et al. 2016. Vital Signs: Seismology of ocean worlds. arXiv: 1610.10067