Sky and Telescope - June 2018 - 9
hydrogen gas in the early universe was
In a companion paper, Rennan Barkana (Tel Aviv University, Israel) argues
that non-gravitational interactions
with dark matter particles would have
naturally cooled the gas. Based on the
EDGES observation, Barkana predicts a
relatively low mass for the dark particles - at most a few times the mass of
a proton - and relatively low velocities. "These results indicate that 21-cm
cosmology can be used as a dark-matter
probe," he writes.
But ﬁrst, Bowman wants to see the
results repeated. "The next step in the
scientiﬁc process is for another group
using a different instrument to conﬁrm
■ GOVERT SCHILLING
Giant Impact May Have Vaporized Earth to Make Moon
SY NESTIA: SA R A H STE WA RT (UNIV. OF CA LIFOR NIA , DAVIS) / N ASA;
A13 A ND TRIA ND: © T. M UELLER / N ASA / JPL- CA LTECH
This artist's concept
shows the hot, molten
Moon emerging from
a synestia, which is in
the process of condensing to form Earth.
A NEW TWIST ON the giant impact
theory posits that the Moon formed
from Earth's vaporized remains after an
Most planetary scientists agree that
our Moon was created when a Marsmass body delivered a glancing blow to
Earth, launching large amounts of rock
into an orbiting ring that coalesced to
form the Moon.
However, problems have emerged
with this scenario. For one, most of the
Moon would have been made from the
impactor's mantle, but measurements
of isotopic ratios show that the Moon
and Earth are made of exactly the same
stuff. All other solar system bodies with
known isotopic ratios have their own
To address this inconsistency, Simon
Lock (Harvard University) and Sarah
Stewart (University of California,
Davis) proposed a radical approach.
They developed computer models last
year showing that when two planetmass objects collide, they could form
a synestia. This mass of vaporized rock
and metal takes the shape of a giant,
spinning donut, which is connected to a
metal-rich central bulge - the surviving
core of the planet.
In the model, the Moon forms
within the orbiting torus of the synestia. As the rock vapor cools, it begins to
condense onto bits of solid rock, which
gradually merge into a fully formed but
still molten Moon.
The researchers' follow-up with colleagues, published online February 28th
in the Journal of Geophysical Research:
Planets, indicates that a synestia would
have been the ultimate mixer, erasing chemical differences between
the impactor and the impacted body.
Moreover, this scenario explains the
Moon's lack of volatiles, as the most
easily vaporizable material would have
remained in a gaseous phase rather
than glomming onto the Moon.
So far, the synestia model has produced mixed reactions within the ranks
of planetary scientists. Some of them
welcome it as a potential ﬁ x for the
limitations of the giant impact theory,
but others remain skeptical.
■ JAVIER BARBUZANO
Dwarf Galaxy Evicted
Milky Way Stars
MOST STELLAR STREAMS in the
Milky Way's halo are ghosts of dwarf
galaxies past (S&T: Apr. 2017, p. 22),
long ago torn into shreds after encounters with our more massive galaxy.
Now, new research that appeared online
February 26th in Nature shows that
some of these stars might not be dwarf
remnants at all - they might have come
from the Milky Way's own disk.
Maria Bergemann (Max Planck
Institute for Astronomy, Germany) and
colleagues studied 14 stars in two halo
populations, known as A13 and Triangulum-Andromeda (TriAnd), using the
Keck I telescope in Hawai'i and the Very
Large Telescope in Chile. After collecting the stellar spectra, the astronomers
measured the abundances of elements
heavier than hydrogen and helium.
Unlike most of the stellar halo, these
stars are rich in heavy elements, more
akin to stars in the galaxy's disk.
Moreover, TriAnd and A13 have similar abundances to each other, implying
a common birthplace - even though
they're separated by 30,000 light-years.
Bergemann and colleagues simulated
a possible origin scenario. The Sagittarius dwarf galaxy, now stretched into
a thin stream that wraps around the
Milky Way, careened into our galaxy
several billion years ago. Such an interaction would have disrupted the Milky
Way's disk and sent swirls of stars above
and below the galactic plane.
■ MONICA YOUNG
p The A13 and TriAnd clouds are in the outer
galaxy, roughly 15,000 light-years above and
below the galactic plane, respectively.
sk yandtelescope.com * J U N E 2 018