SKY_September2014.pdf

News Notes

LUNAR I New Moon Results Explore Lunar Origin, History

IN BRIEF

New Meteor Shower Disappoints. Some

dynamicists had predicted that Comet 209P/

LINEAR would create an active meteor dis-

play in the early morning of May 24th (S&T:

May 2014, p. 30). But reports compiled by the

International Meteor Organization suggest

that the Camelopardalids hit a maximum

zenithal hourly rate of 20. The low rate was in

part because the particles moved slowly for

meteoroids (about 20 km/sec, or 45,000 mph),

making them dim. The poor display might also

be because this year’s shower involved comet

particles shed more than a century ago and

not the most recent or densest debris strands.

A full analysis is at skypub.com/camels2014.

■ J. KELLY BEATTY

Active Galaxies Actually Different. The

zoo-like variety of active galaxies might be due

to evolution, argue two teams in the Astro-

physical Journal and Nature Physics. Every

active galaxy contains a gorging supermas-

sive black hole at its core. Astronomers think

complex structures including a dusty torus

encircle the black hole, obscuring it from view

from some angles. Until now they thought

the diff erent types of active galaxies they

observe are merely the same structures seen

at diff erent angles. But the new studies show

that obscured active galaxies behave diff er-

ently than their exposed counterparts: they

cluster closer together and tend to reside near

gas-rich, star-forming galaxies. The results

suggest that the structures surrounding

supermassive black holes are not universal

but instead evolve over time.

■ MONICA YOUNG

Star Cluster Runs Away. One of the sev-

eral thousand globular clusters orbiting the

elliptical galaxy M87 appears to be scream-

ing toward us at 1,025 km/sec. The cluster,

dubbed HVGC-1 (for hypervelocity globular

cluster), is going fast enough to escape the

entire Virgo Cluster in which M87 resides,

Nelson Caldwell (Harvard-Smithsonian Center

for Astrophysics) and colleagues report in the

May 20th Astrophysical Journal Letters. The

team suggests that, if a double supermassive

black hole lurks in the galaxy’s core, a three-

body interaction with the cluster could have

slingshotted HVGC-1 out. There’s circumstan-

tial evidence that M87 might have a double

black hole, but for now astronomers don’t

know how the galaxy ejected the cluster.

■ MONICA YOUNG

First Direct Exoplanet Spin Measurement.

Ignas Snellen (Leiden Observatory, The Neth-

erlands) and colleagues have used infrared

observations to clock the spin of the young

exoplanet Beta Pictoris b, the team reports

in the May 1st Nature. The planet’s mass is

roughly 11 times that of Jupiter and rotates at

25 km/sec, much faster than Jupiter (13.3 km/

sec) and Earth (0.5 km/sec). This spin seems

to uphold the correlation seen in the solar

system between spin and mass, with more

massive planets rotating more quickly. ✦

■ SHANNON HALL

Three new results are bringing planetary

scientists closer to understanding how

Earth’s natural satellite was born and why

it looks the way it does.

The Moon likely formed when a Mars-

size object (often called Theia) struck the

young Earth. This glancing blow ejected

a huge jet of debris that formed a ring

around our young planet and rapidly

coalesced into the Moon.

Until recently, the best answer for

when this big splat occurred was roughly

4.47 billion years ago. However, in June

Guillaume Avice and Bernard Marty (Uni-

versity of Lorraine, France) presented an

earlier date at the Goldschmidt Geochem-

istry Conference. Their analysis of xenon

trapped inside quartz-bearing rocks from

South Africa and Australia allows them to

estimate how much xenon early Earth lost

to space (during, say, a major impact). The

team’s analysis suggests the Moon formed

4.53 billion years ago, just 40 million years

after the solar system did.

If the giant-impact hypothesis is

correct, the Moon should be a mélange

of proto-Earth and Theia. Yet previous

analyses have failed to fi nd a distinction

between the ratios of oxygen’s three iso-

topes in terrestrial and lunar rocks.

In Science for June 6th, Daniel Her-

wartz (University of Göttingen and

University of Cologne, Germany) and col-

leagues report detecting that long-sought

diff erence. The team used improved

techniques to derive the isotopic ratios

of oxygen-16, -17, and -18 and found a dif-

ference of 12 ± 3 parts per million in the

(^17) O: (^16) O ratios. This result puts strict limits
on Theia’s composition.
The third lunar result off ers a pos-
sible solution to the age-old question of
what put the Man in the Moon. Various
researchers have tried to explain why
dark maria cover the lunar nearside but
cratered highlands cover the farside.
Writing in the June 20th issue of
Astrophysical Journal Letters, Penn State
researchers Arpita Roy, Jason Wright,
and Steinn Sigurðsson think they have
the answer. When the Moon fi rst formed
it was much closer, just 5% to 10% of its
present distance, and its rotation quickly
became tidally locked. Both bodies were
still hot from the collision, with Earth’s
surface a red-hot 2500°°C (4500°F). This
meant the lunar nearside was continually
seared by the glowing Earth that domi-
nated its sky, making it too hot for any
lunar minerals to condense.
But the farside, out of sight from Earth,
cooled much faster. The fi rst solids to form
on the Moon were therefore more likely to
condense there, building up a crust with
minerals rich in aluminum and calcium.
Later on, titanic collisions battered the
young Moon. The nearside’s relatively
thin crust, fractured deeply by the largest
impacts, provided an easy conduit for
dense, metal-enriched magmas to rise
to the surface. These formed the pattern
of dark maria seen today. But due to the
farside’s thicker crust, similar eruptions
were almost nonexistent.
■ J. KELLY BEATTY
18 September 2014 sky & telescope
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