My first Magazine Sky & Telescope - 01.2019 | Page 13

SOLAR SYSTEM
Dawn Probes Role of
Cryovolcanism on Ceres
NASA’S DAWN MISSION has lifted the
veil on Ceres, a dwarf planet in the
asteroid belt. Now, a study published
September 17th in Nature Astronomy
has revealed this world’s fascinating
history of cryovolcanism, the supercold
watery equivalent to magma eruptions
on our planet.
Led by Michael Sori (University of
Arizona), researchers carefully ana-
lyzed the appearance and location of 22
domes on Ceres. Using the 4-kilometer-
high (2.5-mile-high) mound Ahuna
Mons as a reference, the team estimated
the ages of the other domes by assum-
ing that they started out as tall piles
of cryovolcanic material (like Ahuna
Mons) that then slumped into shorter,
broader mounds at a predictable rate.
The resulting age estimates range from
hundreds of millions to 2 billion years
old. “We’ve shown Ceres may have been
cryovolcanically active throughout its
entire history,” Sori says.
The team then used the distribution
of domes and their ages to estimate
how often cryogenic activity should
occur on Ceres. They deduced that a
cryovolcano forms on average once
every 50 million years. Over time,
these eruptions could disgorge about
10,000 cubic meters (350,000 ft 3 ) of
briny slush per year. That’s equivalent
to four Olympic-size swimming pools
— but it’s a tiny fraction of the erup-
tion rate on Earth, where volcanoes
spew more than 1 billion cubic meters
of molten rock each year.
“Cryovolcanism is important on
Ceres,” says Sori, “but not nearly as domi-
MILKY WAY
Star Pattern Suggests Recent Galactic Whack
AN UNUSUAL PATTERN in the motions
of the Milky Way’s stars suggests that
a neighboring galaxy swept by our own
within the last billion years.
Reporting in the September 20th
Nature, Teresa Antoja (University of
Barcelona, Spain) and colleagues dis-
covered the pattern when they looked
at data on 6 million nearby stars from
the Gaia satellite, which is providing
precise distances and speeds for more
than 1 billion stars. Looking at just the
positions of the stars, the astronomers
didn’t see anything unusual. But when
they plotted the stars’ altitudes above
or below the disk compared to their
velocities perpendicular to the disk, a
spiral showed up.
This phase spiral represents a large-
scale gravitational disruption. Some-
thing with the size and mass of a small
galaxy is the most likely culprit. These
types of encounters were probably com-
mon when our galaxy was fi rst form-
ing, but the disturbances would have
smoothed out over the hundreds of
millions of years that followed. Seeing
a phase spiral today suggests something
jostled the Milky Way’s disk relatively
recently, between 300 million and 900
million years ago.
Another team led by Joss Bland-
Hawthorn (University of Sydney,
Australia) studied the phase spiral in
a complementary way, using not only
the motions of the stars but also their
heavy-element content, which is a proxy
for age. The researchers estimate that
the Milky Way encountered another
p Ahuna Mons, a large mountain on Ceres, is
seen in this simulated view based on images
from the Dawn spacecraft. Elevation is exag-
gerated by a factor of two.
nant or important as rocky volcanism has
been on Earth or Mars.”
The most likely source powering cryo-
volcanism on Ceres is radioactive decay of
certain isotopes in the rocky parts of the
tiny world, as happens on other planets.
Another more controversial hypothesis is
that large impacts on the surface jump-
start the process locally.
■ DAVID DICKINSON
galaxy roughly 500 million years ago,
consistent with the other team’s esti-
mate. They published their results Sep-
tember 7th on the astronomy preprint
server arXiv.org.
The dwarf Sagittarius Galaxy passed
close by the Milky Way about 200
million to 1 billion years ago, making
it a good candidate. That encounter is
ongoing: Our galaxy is in the process of
tearing the dwarf to shreds.
■ JOHN BOCHANSKI
• See a video of the phase spiral at
https://is.gd/GalacticWhack
Artist’s impression of a perturbation in the velocities of stars in the Milky Way revealed by
the European Space Agency’s star-mapping mission, Gaia
sk yandtele scope.com • JA N UA RY 2 019
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