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Lunar interior has
very little water
By Alexandra Witze
The moon’s insides might not be all wet
after all. A new study suggests that, contrary to other recent work, the lunar
interior is as bone-dry as scientists
thought when NASA astronauts first
lugged home moon rocks 40 years ago.
New analyses of chlorine in those
rocks, published online August 5 in
Science, indicate that the moon contains
just one- 10,000th to one-100,000th the
water that the Earth’s interior does.
Researchers have long argued over
whether the moon has water on its surface, delivered there over eons by comet
impacts. But the new studies tackle a
more fundamental question: How much
water did the moon contain when it
formed, 4. 5 billion years ago?
Nearby galaxy is a very dark place
Holds record for concentration of mysterious missing mass
By Ron Cowen
Devotees of the dark side have reason
to rejoice. Observations confirm that a
faint group of stars in the Milky Way’s
backyard has the highest density of dark
matter — the invisible material thought
to account for 83 percent of the mass
of the universe (see Page 22) — of any
galaxy known.
The findings, reported online July 28
at arXiv.org by Joshua Simon of the
Carnegie Observatories in Pasadena,
Calif., along with Marla Geha of Yale
University and their colleagues, provide a bonanza for astronomers trying
to unveil the nature of dark matter.
When astronomers discovered the
galaxy Segue 1 in 2007, they weren’t sure
if it was anything more than a cluster of
stars, perhaps stripped from the nearby
Sagittarius dwarf galaxy. But observa-
tions with the Keck II telescope atop
Hawaii’s Mauna Kea now confirm the
status of Segue 1 as a galaxy by show-
ing that its stars have a diverse chemical
makeup, Simon says.
stars in Segue 1 have a combined mass of
no more than about 1,000 suns, the mass
of the whole galaxy is more than 500
times larger. “That tells us that Segue 1
is made almost entirely of dark matter,”
Simon says.
Segue 1 is both dark matter–dominated
and compact, yielding a dark matter density higher than any other known galaxy.
The galaxy’s high density and proximity to Earth —about 80,000 light-years
distant — make it an ideal place to look for
proposed signatures of dark matter.
“It’s extremely important to figure out
the dark matter properties of galaxies,”
says Rosemary Wyse of Johns Hopkins
University in Baltimore. Galaxies such as
Segue 1, which have such a tiny amount
of visible material to gravitationally disturb the dark matter, are the best places
to reveal the true distribution and nature
of the unseen material.
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