Sky & Telescope - January 2021 - 14

NEWS NOTES

Hubble Images Reveal Dark
Matter Problem
ASTRONOMERS STUDYING IMAGES of
massive galaxy clusters have uncovered
a potential problem with dark matter.
In the September 11th Science, Massimo Meneghetti (National Institute
of Astrophysics, Italy) and colleagues
present a new way of visualizing unseen
dark matter in galaxy clusters - and
their results contradict cosmological
expectations.
Theorists have long known that dark
matter isn't smooth. Instead, like a
botched baking project, the dark matter
halos that surround galaxies and clusters have lumps, or as astronomers term
them, subhalos.
Simulations specify how many subhalos we might expect around the galaxies in a cluster. Some of them could
have stars and be faintly visible as dwarf
galaxies, while others might be entirely
dark. In any case, most of them would
be difﬁcult to observe.

SOLAR SYSTEM

The Origin of Earth's Water
WE MAY FINALLY HAVE FOUND the

missing source of most of Earth's water
in a rare class of meteorites called
enstatite chondrites.
These rocks come from the part of
the protoplanetary disk in which the
terrestrial planets formed. While the

Meneghetti's team came up with a
way around that. Because galaxy clusters are so massive, they act as gravitational magnifying glasses, lensing the
light from background galaxies into arcs
or multiple images. A galaxy cluster's
mass may create images separated by
several tens of arcseconds, but a subhalo
in just the right place in the cluster
might create additional images separated by less than a few arcseconds.
The team used detailed images and
data from the Hubble Space Telescope
and the European Southern Observatory's Very Large Telescope in Chile to
examine several galaxy clusters and
count the number of those small-scale
lenses that subhalos make. Then they
looked at what cosmological simulations predict.
To the researchers' surprise, the
simulations predict far fewer halos than
observed by a factor of 10. Try as they
might to explain the difference, there
were no easy answers.
Frank van den Bosch (Yale), who
was not involved in the study, calls the

meteorites' composition resembles
Earth's, astronomers had presumed
them too dry to supply water. But a new
analysis including the most pristine
samples known shows that the original
population could have supplied more
than enough water to ﬁll our oceans,
Laurette Piani (University of Lorraine,
France) and colleagues report in the
August 28th Science.
Astronomers
think Earth and the
Astr
other tterrestrial planets formed too
close to the young Sun for water to
condense. While water could have
cond
condensed
out near Jupiter, beyond
co
the snow line of the young solar
system, it would have had to
take a roundabout route to
Earth. The leading candidate for
water delivery in this scenario
had been carbonaceous chondrite
asteroids
- rocky leftovers of the
as
protoplanetary
disk that formed
prot
beyond
beyon Jupiter. But these asteroids
W An enstatite
en
chondrite meteorite on display at
the Royal
Roya Ontario Museum

JANUARY 2 021 * SK Y & TELESCOPE

S An artist's impression shows small-scale
dark matter clumps in MACS J0416.1-2403
(left), compared to a visible image of the galaxy
cluster (right).

results "potentially groundbreaking."
"The authors address all the 'obvious'
candidates for what might explain this,"
he says. "I agree with them that none
of them seem to be able to reconcile the
large discrepancy found." He expects
that follow-up studies would examine
the abundance, structure, and other
properties of dark matter lumps.
■ MONICA YOUNG

fail a crucial ﬁngerprint test: They
don't have the same ratio of isotopes as
Earth's material does. Enstatite chondrites, on the other hand, have isotopic
compositions that are nearly identical
to those of Earth and the Moon. "We
consider enstatite chondrites the best
analog we have for Earth's building
blocks," says Piani.
Piani's team analyzed 13 samples of
enstatite meteorites, ﬁnding traces of
water that account for 0.08%-0.5% of
the rocks by weight, with meteorites
altered the least since their formation
carrying the most water. That may not
sound like much, but it adds up: Piani's
group suggests enstatite chondrites
could have supplied all the water in the
mantle and most of the oceans, with
the rest coming from wet asteroids.
While enstatite meteorites provide
water a more straightforward route to
Earth, future research must address
how the water originally came to be in
rocks forming so close to the Sun.
■ JEFF HECHT

G A L A X Y VISIBLE VIE W: ESA / HUBBLE / N ASA / HST FRON TIER FIELDS; ACK NOWLEDG EMEN T: M ATHILDE JAUZ AC (DURH A M UNIV., UK A ND
ASTROPH YSICS & COSM OLOGY RESE A RCH UNIT, SOU TH A FRICA) A ND JE A N-PAUL K NEIB (ÉCOLE POLY TECHNIQ UE FÉDÉR A LE DE L AUSA NNE,
SWIT ZERL AND);DARK MAT TER VIE W: ESA / HUBBLE / M. KORNMESSER; ENSTATITE CHONDRITE: WIKIMEDIA COMMONS / CC BY 3.0

COSMOLOGY

Sky & Telescope - January 2021

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