Sky and Telescope - February 2017 - 14

NEWS NOTES
SOLAR SYSTEM

W NASA's Cassini spacecraft captured this
view of Dione, with giant Saturn and its rings in
the background, in August 2015.

CASSINI DATA suggest that a subsurface
ocean might lie deep beneath the crust
of Saturn's moon Dione.
Planetary scientists have found
evidence of subsurface oceans lurking
beneath the crusts of several solar system bodies. The most convincing case
is Enceladus, a little icy moon of Saturn
known for its salty geysers. Other
contenders are Saturn's largest moon,
Titan, and Jupiter's big moons Europa
(see below), Ganymede, and Callisto.
At the March 2016 Lunar and Planetary Science Conference, Doug Hemingway (University of California, Berkeley)
and others on the Cassini radio science
team presented a preliminary analysis suggesting that Dione might have
a subsurface ocean, too. They based
this conclusion on the moon's overall

SOLAR SYSTEM

Europa Geysers Point to
Subsurface Ocean
FOR DECADES astrobiologists have

pondered whether primitive life might
exist in a global ocean presumed to
lie beneath the smooth, icy crust of
Jupiter's moon Europa. But this putative
ocean must be at least several kilome-

S Faint light streaks near the south pole of Europa in this composite image of Hubble (blue)
and Galileo data (gray disk) might be towering
geysers of water erupting from the moon.

topography and how strongly it pulls on
the Cassini spacecraft, which has been
touring the Saturnian system since
2004 and made three close, carefully
tracked ﬂybys of the moon between
2011 and 2015.
Using this preliminary work, Mikael
Beuthe and colleagues at the Royal
Observatory of Belgium did a more
detailed analysis of the gravity data. The
Belgian team suggests in the October

ters down - a formidable barrier to
exploring it ﬁrsthand.
Now it appears that sampling this
briny deep might be easier than once
thought. Hints that Europa sports
water-powered geysers ﬁrst came to
light in 2012, when a team led by
Lorenz Roth (Southwest Research
Institute) used the Hubble Space Telescope (HST) to spectroscopically detect
localized clouds of hydrogen and oxygen
atoms - presumably derived from water
- in Europa's vicinity.
More recently, William Sparks (Space
Telescope Science Institute) and others
used HST to record images of Europa
as it crossed in front of Jupiter. They
wanted to see if the moon has a thin
atmosphere, which would show up as a
dark aura around Europa when viewed
in silhouette against Jupiter.
Sparks' team tracked 10 transits of
Europa from December 2013 to March
2015, as well as non-transit sessions to
model the appearance of Europa itself.
They used a far-ultraviolet channel
centered at 150 nm, because that wavelength is scattered by Jupiter's high-alti-

F E B R U A R Y 2 0 1 7 * SK Y & TELESCOPE

16th Geophysical Research Letters that
the best ﬁt to the data is a crust about
100 km thick, overlying a global ocean
35 to 95 km deep. (Dione is about 1,100
km across, or twice Enceladus's size.)
But there may be other ways of
explaining the uncertain Dione data.
Scientists think Enceladus has an ocean
in part because it wobbles, or librates,
signiﬁcantly in its orbit around Saturn,
which wouldn't happen if it were solid
throughout and "all but proves the
ocean is there," says William McKinnon
(Washington University in St. Louis),
whose team is one of several that has
studied the moon. Beuthe's team predicts Dione does librate, but not enough
to show up in Cassini images.
■ CAMILLE M. CARLISLE

Sky and Telescope - February 2017

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