Astronomy Now - January 2021

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Do some of Europa’s plumes come from its

icy crust?

ot all of Europa’s watery plumes may originate from its deep ocean, according to research based

partly on imagery of Europa’s surface from the Galileo mission.

Jupiter’s moon Europa has a subsurface global ocean, hidden beneath an icy crust dozens of
kilometres thick. Some indirect evidence suggests that the ocean could be habitable (note: this is not
the same as inhabited), and it had been hoped that by sampling the watery plumes that spurt up
from cracks in the surface, a future space mission would be able to ascertain the properties of the
ocean without having to dig down 20 or 30 kilometres. Writing in the 10 November issue of
Geophysical Research Letters, scientists led by Gregor Steinbrügge of Stanford University suggest
another origin for at least some of the plumes. ey focused on a 29-kilometre-wide crater called
Manannán, which formed on Europa from an impact tens of millions of years ago. e heat of the
impact would have melted pockets of water inside the ice crust. ese pockets would have been rich
in salt embedded in the ice, and with the salt acting as an antifreeze, the pockets of water could
remain liquid. ey would then begin to move sideways by melting the adjacent ice, until they
reached the centre of the crater. With nowhere else to go, the pockets of briny liquid would freeze,
but as more pockets pile into the centre, the pressure would increase until something gives, and a
plume of water is driven up out of the ground to over 1.6 kilometres above the surface. On the
surface, the cryovolcanic eruption would leave a spider-shaped mark, just like the one seen in
Manannán.

However, this process cannot explain the biggest of Europa’s plumes, and therefore there may still be
some that originate from the deep ocean.

“e work is exciting, because it supports the growing body of research showing [that] there could be
multiple kinds of plumes on Europa,” says NASA’s Bob Pappalardo, who is a Project Scientist on the
Europa Clipper mission that launches later this decade.

An artist’s impression of a plume erupting on Europa. Image: NASA/ESA/K. Retherford/SwRI.

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Do some of Europa’s plumes co...

January 2021

Astronomy Now