Sky & Telescope - May 2022 - 10

NEWS NOTES
SOLAR SYSTEM
Do the Plumes from
Saturn's Icy Moon Reach
Down to Its Ocean?
ASTRONOMERS long thought that
the watery plumes erupting from the
surface of Saturn's icy moon Enceladus
might provide access to its subsurface
ocean. But computer models designed
for studying sea ice on Earth suggest
that the plumes might have a different
chemistry than the waters below.
NASA's Cassini spacecraft first
imaged and later even sampled the
geyser-like plumes of water vapor and
particles escaping from rifts in the icy
crust of Enceladus's south polar region.
Scientists interpreted the presence of
salts (sodium chloride and sodium
carbonate) and traces of biologically
interesting organic compounds as
evidence that the plumes were erupting
from pipe-like fissures that extended all
MILKY WAY
Celestial Ruins on Our
Galaxy's Edge
LONG AGO, the Milky Way's gravity
rent a globular cluster and crumbled its
thousands of stars across 15 degrees of
sky. Now, new measurements of some
of those stars show that they used to
belong to the oldest stellar association
in or around our galaxy.
The Milky Way itself formed some
200 million years after the Big Bang.
Eons of star formation have rendered
it anew, but hints of its past remain in
its 150 or so globular clusters, each of
pThe Cassini spacecraft imaged dramatic plumes erupting from Saturn's moon Enceladus.
the way from the surface of the thick,
icy crust to a salty subsurface sea.
However, hopes that the plumes
would provide easy access to the subsurface
layers were called into question
at December's meeting of the American
Geophysical Union. Colin Meyer
(Dartmouth College) and colleagues
applied computer models developed for
terrestrial sea ice to Enceladus. They
found that pockets of slushy brine,
produced by the localized heating of
tidal stress and shear, should be present
within Enceladus's icy shell. That brine
can percolate through porous ices to
the surface, where it erupts into the
which is crowded with stars typically
billions of years old.
While the Milky Way's globular
clusters are undeniably ancient, none of
them seems to have survived from the
earliest days. Their abundance of heavy
elements, a proxy for their age, bottoms
out at 0.2% that of the Sun.
A study of a celestial ruin may shed
light on ancient globulars' disappearance.
In the January 6th Nature, Nicolas
Martin (University of Strasbourg,
France) and colleagues report a detailed
study of a collection of stars in the
Milky Way halo dubbed C-19. Based
on their data, the researchers conclude
these stars were
once a globular
cluster. Now they're
a stellar stream
Sun
C-19
Large Magellanic Cloud
Small Magellanic Cloud
10 MAY 2022 * SKY & TELESCOPE
 This illustration
shows the location of
the C-19 stellar stream,
recently discovered at
the edge of our Milky
Way Galaxy. (The Sun
and the Magellanic
Clouds are labeled for
reference.)
vacuum. The concentration of salts in
this meltwater should increase as the
water vaporizes and escapes into space.
While Meyer doesn't doubt that
Enceladus has a global subsurface
ocean, he cautions that the plume's
measured saltiness " may not be representative
of the ocean chemistry or the
chemistry of the shell but actually of
concentrated interstitial liquid. "
Sampling Enceladus's hidden ocean
may be more difficult than many have
imagined, but Meyer notes that pockets
of slushy brine in the icy crust could be
habitable zones in their own right.
¢THOMAS A. DOBBINS
that's orbiting the Milky Way on a
relatively close polar orbit, currently
60,000 light-years from the Sun.
Team member Rodrigo Ibata (also
University of Strasbourg) helped
discover this stream from the massive
data set that the Gaia satellite has
collected. The team then followed up
with ground-based spectroscopy, which
confirmed that stars selected from this
disintegrated cluster have extremely
low amounts of heavy elements: less
than 0.05% that of the Sun. " Such a
low global metallicity has so far never
been observed for any stellar system in
the Milky Way, in its surroundings, or
beyond, " the researchers write.
C-19's ancient remains suggest that
globular clusters could form early on;
it's just that the ones from our galaxy's
earliest years generally didn't survive
the tumultuous gravitational environment.
Or, adds John Norris (Australian
National University), who wasn't
involved in the study, it's still possible
that back then, relatively few such clusters
were forming in general.
¢MONICA YOUNG
PLUMES FROM ENCELADUS: KEVIN M. GILL / JPL-CALTECH / NASA / SSI; C-19 STELLAR STREAM:
INTERNATIONAL GEMINI OBSERVATORY / NOIRLAB / NSF / AURA / J. DA SILVA / SPACEENGINE;
ACKNOWLEDGMENT: M. ZAMANI
Sky & Telescope - May 2022

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