Sky and Telescope - August 2017 - 10

NEWS NOTES

SOLAR SYSTEM

Enceladus's Hydrothermal Heating, Europa's Leaks
that the hidden ocean inside Saturn's
moon Enceladus is heated by hydrothermal activity.
The Cassini spacecraft detected
geysers spouting from Enceladus's
south pole soon after its arrival in the
Saturnian system in 2004. Subsequent
work showed that these are probably high-pressure leaks from a global
subsurface ocean. Chemically speaking,
the water in the plumes is tainted from
being around rocks. There's ample evidence that this taint is hydrothermally
produced, from the salt in the water to
silica nanoparticles coughed out by the
moon, the latter of which would have
precipitated out of hot, rock-interacting
water when it cooled.
One of the chemical ﬁngerprints
scientists have been hoping to ﬁnd is
molecular hydrogen. Rocks in a hydrothermal environment will leach oxygen
from water, producing hydrogen.
In October 2016, Cassini scientists
did their penultimate ﬂyby of the moon
with the speciﬁc goal of ﬁnding this
hydrogen. Reporting in the April 14th
Science, J. Hunter Waite (Southwest
Research Institute, San Antonio) and
colleagues conﬁrm they've succeeded.
The heat in Enceladus's ocean exists
because of Saturn. The giant planet
10

A U G U S T 2 0 1 7 * SK Y & TELESCOPE

tidally wrenches the little moon, and
this tidal energy warms and cracks the
moon's rocky core. Seawater ﬂows into
these cracks, heats up, and comes back
out. The inferred temperature is high
enough that the process might drive
large-scale convection within the ocean.
This new result has crucial implications for astrobiology. Molecular hydrogen's production creates a chemical
imbalance in the surrounding seawater,
because the hydrogen "wants" to react
with other things. Some of the most
ancient forms of life on Earth exploit
this imbalance by eating hydrogen and
metabolizing it, living off the chemical
energy and making methane. (Cassini
has previously detected methane, CH4,
but don't get too excited: The methane could be produced abiotically, no
microbes required.) The new result will
help astrobiologists understand the
energy available to any potential organisms inside the moon.
Among those speculating about the
implications for life, astrobiologist Mary
Voytek (NASA) says that the abundant
hydrogen and carbon dioxide Cassini
sees from Enceladus actually disfavor
the presence of life in her mind. "If
there is life, it's not very active," she

p This artist's illustration depicts Cassini diving
through geyser plumes over Enceladus.

surmised in an April 13th press conference. Think of hydrogen like pizza.
"When you have stacks of pizza, much
like in a graduate school department, it
disappears. So we have this buildup of
food that's not being used."
In tandem with Cassini's Enceladus
result, astronomers also announced
news about a second icy moon: Jupiter's
Europa. The Hubble Space Telescope
had previously detected a hint of geyser
activity from this moon, too (S&T: Feb.
2017, p. 14). Reporting in the April 20th
Astrophysical Journal Letters, William
Sparks (Space Telescope Science Institute) and colleagues say they've now
also seen a possible plume in 2016 data,
in the same place as one detected in
2014 - and where the Galileo orbiter
saw a "heat anomaly" way back in 1999.
The detection is at the limit of what
Hubble can record, so it's not unequivocal, Sparks cautions. But like Enceladus,
Europa is tidally squeezed due to its
parent planet. Maybe that creates some
sort of pseudo-plate tectonics, explaining the moon's young-looking surface.
■ CAMILLE M. CARLISLE

* To get astronomy news as it breaks, visit skypub.com/newsblog.

N ASA / JPL- CA LTECH

ASTRONOMERS HAVE more evidence

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Sky and Telescope - August 2017

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