Sky & Telescope - August 2024 - 11

IN BRIEF
MILKY WAY
Shiva and Shakti: Two New Stellar Streams?
TWO NEWLY DETECTED stellar groups
in the inner Milky Way could be the
remnants of ancient protogalaxies - or,
more prosaically, they might represent
stars in resonance with our galaxy's bar.
Khyati Malhan and Hans-Walter
Rix (both at Max Planck Institute
for Astronomy, Germany) identified
the groups using data from the Gaia
satellite, publishing the results in the
April 1st Astrophysical Journal. The
two groups, which Malhan and Rix
nicknamed Shiva and Shakti after two
Hindu deities, are distributed between
us and the galactic center.
The astronomers teased out the
two subpopulations by the comparable
properties of their orbits. Combined
with other measures of their ages, such
as the stars' compositions, the data suggest
that the two groupings could have
joined the Milky Way between 11 and
12.5 billion years ago.
While it may seem surprising that
such populations would still be identifiable
after more than 10 billion years,
Rix compares the process to observing
small bodies in the solar system. A
comet may disintegrate into fragments
that gradually spread out along its orbit;
however, even when that trail spans
millions of miles, those fragments
would still retain a distinct identity by
their orbits' similarity.
pThis image shows the location and distribution
of stars that are part of the Shakti (yellow)
and Shiva (blue) streams in the Milky Way.
Not everyone is convinced, though.
Adam Dillamore (University of Cambridge,
UK) and colleagues have a
different interpretation for such stellar
groups, contending that they cluster due
to a resonance effect.
For example, passages of the galaxy's
central bar every several hundred million
years can group stars in the same
way that asteroids gather in resonance
along Jupiter's orbit. Dillamore and colleagues
have posted their study on the
arXiv astronomy preprint server.
Dillamore says that the orbits of the
Shakti group correspond to the corotation
resonance, " when the stars orbit the
galaxy with the same average frequency
as the bar's rotation. " The case is less
clear for the Shiva group, Dillamore
acknowledges, but he thinks it's still
possible that those orbits correspond to
a different resonance.
Rix argues that the resonance interpretation
doesn't quite match the data,
but he adds that it's alright if the question
remains unresolved.
" I think sometimes we need to let
people see the soup kitchen of science, "
Rix says. " It's not just instant eternal
truth that comes out. "
¢DAVID L. CHANDLER
Mars Sample Return On
Shaky Ground
NASA's Perseverance rover has collected
two dozen samples from Jezero Crater, but
it's unclear how those samples will come
home. The initial plan - which involved a
lander with helicopters, an ascent vehicle,
and an orbiter/return vehicle - was
planned for launch in 2028, with samples
back on Earth by 2031 at the earliest.
However, while the Mars Sample Return
concept passed its first independent
review board evaluation, a second review
three years later found that the program
would need to either grow its budget (up to
$11 billion) or extend its mission (with return
by 2040). NASA administrator Bill Nelson
called the new budget " too expensive "
and the new timeline " unacceptably too
long. " To avoid canceling the mission altogether,
NASA is now asking for alternative
architectures, with requests out to NASA
centers, the Jet Propulsion Laboratory, and
industry. Associate administrator Nicky
Fox emphasized that the agency is looking
for tried-and-true technologies to lower
risk and cost. Concept studies will be due
by the fall, after which NASA will evaluate
them and decide how to proceed.
¢MONICA YOUNG
Melting Mimas
Until recently, nobody expected an ocean
in Saturn's moon Mimas (see https://
is.gd/MimasOcean). " Usually, we equate
oceans with super-active surfaces, " explains
Geoffrey Collins (Wheaton College).
Mimas's surface is geologically dead. " This
is why a lot of people (including myself)
were incredulous about the recent ocean
result. " However, the moon's rotation and
orbital motion do indicate a subsurface
ocean and, in the June 1st Earth and
Planetary Science Letters, Alyssa Rhoden
(Southwest Research Institute) and
coworkers suggest how it could have
appeared. The ocean might be young,
created when some event not terribly long
ago shifted the moon into an eccentric
orbit, driving tidal heating and melting the
moon's interior. But tides giveth, and they
taketh away: Tidal friction gradually makes
Mimas's orbit more circular, resulting in
less tidal friction and less interior heat.
Rhoden's simulations show that Mimas is
now on its way to freezing solid again. The
odds of such lucky timing improve when
we visit enough moons.
¢EMILY LAKDAWALLA
sk yand tele scope .o r g * AUGUST 2024 11
ESA / GAIA / DPAC / K. MALHAN
http://www.is.gd/MimasOcean http://www.is.gd/MimasOcean https://skyandtelescope.org/
Sky & Telescope - August 2024

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