ASTRONEWS
WWW.ASTRONOMY.COM 11Hubble homes in on orphaned stars
About 2.6 million years ago, an
exploding star lit up Earth’s sky as
a supernova. A few hundred years
later, its cosmic rays reached Earth
and may have caused a mass extinc-
tion, according to a study published
November 27 in Astrobiology.
The extinction, which occurred at
the Pliocene-Pleistocene boundary,
killed about 36 percent of the genera
in coastal waters, including Earth’s
largest known shark, the megalodon.
The event’s cause is unknown, but
radiation from a supernova is a possi-
ble culprit. Such radiation would be
composed of fast-moving muons,
which are a few hundred times the
mass of an electron, according to
lead author Adrian Melott of the
University of Kansas. “They’re very
penetrating. Even normally, there are
lots of them passing through us.
Nearly all of them pass through
harmlessly,” he said in a press release.
But after a supernova, “multiply
those muons by a few hundred. Only
a small fraction of them will interact in
any way, but when the number is so
large and their energy so high, you
get increased mutations and cancer,”
Melott said. His team estimated that
in human-sized animals, the cancer
rate would increase by about 50 per-
cent. In larger animals, that effect
would have been larger still. High-
energy muons can penetrate hun-
dreds of yards into water, peppering
the shallower coastal waters where
the extinctions occurred.Is there evidence for such a wave of
cosmic rays? Iron-60 is a radioactive
isotope of iron with a half-life of about
2.6 million years — meaning all Earth’s
original iron-60 is now gone. Any iron-
60 on Earth today must have arrived
more recently, via cosmic means. And
a huge deposit of iron-60 was laid
down about 2.6 million years ago,
implicating one or more known super-
novae occurring around that time
within about 150 light-years of Earth.“We have evidence of nearby
[supernova] events at a specific
time. We know about how far away
they were, so we can actually com-
pute how that would have affected
Earth and compare it to what we
know about what happened at
that time,” Melott said. “There
really hasn’t been any good explana-
tion for the marine megafaunal
extinction. This could be one.”
— Alison KlesmanQUICK TAKES
POLO
The twin Mars Cube One
(MarCO) CubeSats relayed data
throughout the InSight lander’s
descent and touchdown on
Mars last November.- TRIGGERED
A supernova’s early,
unexpected surge in brightness
may be due to its companion
star glowing as the blast’s
shock wave washed over it.
GEMSTONES
A nearby super-Earth likely
formed under such high
temperatures that its core is
filled with aluminum oxides,
such as rubies and sapphires.
ALL BUT ONE
Water within Saturn’s rings and
moons is surprisingly similar to
that found on Earth, except for
Phoebe, which has water
unlike any other type found
in the Saturn system.
VANISHING ACT
A Neptune-sized exoplanet
caught rapidly shedding its
atmosphere may help explain
why astronomers find so many
hot super-Earths, but so
few hot Neptunes.
-^
GROUNDED
Researchers have discovered a
staggering amount of microbial
life hidden deep below Earth’s
surface, including about
70 percent of the world’s
bacteria and archaea.
COSMIC COUGH
Spaceflight may increase the
body’s risk of infection, says a
new study that analyzed mice
exposed to microgravity.
NEW DAWN
Data from NASA’s Dawn
spacecraft show the carbon
concentration of the dwarf
planet Ceres may be five times
that of the most carbon-rich
meteorites found on Earth.
EXOBALLOON
Astronomers detected helium
being blown into an extended
envelope around a Neptune-
sized exoplanet as it is
bombarded by radiation
from its host star.
GOOD ODDS
In a recent survey, 12 of 32
protoplanetary disks around
stars in a vast cloud of gas and
dust showed evidence
of planets. — J.P.
WEIGH IN. The first detailed observations of gas clouds zipping around an extragalactic supermassive black
hole have allowed astronomers to measure the black hole’s mass with unprecedented precision.Could a supernova explain an ancient mass extinction?
The number of days Expedition 57 crew members
Serena Auñón-Chancellor, Alexander Gerst, and
Sergey Prokopyev spent aboard the International
197 Space Station, before returning to Earth December 20.NA
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SUDDEN BLAST. This composite X-ray and optical image shows supernova remnant
1E 0102.2–7219, which lies 190,000 light-years away. The supernova that may have caused
a mass extinction on Earth was much closer, only about 150 light-years distant.CROWD CONTROL. Hubble
recently snapped this shot of the
Coma galaxy cluster, making the
Milky Way look pretty puny in the
process. While about 150 globular
clusters orbit our home galaxy,
the space telescope revealed a
staggering 22,426 globular clusters
hanging out in Coma. Shining
300 million light-years from Earth,
the cluster is home to over 1,
densely packed galaxies that often
pass each other closely. When they
do, tidal forces fling ancient stars
and globular clusters in a galaxy’s
outskirts out into space. Because
they are so numerous, these
orphaned globular clusters can
help researchers map the Coma
Cluster’s gravitational fields and
track its distribution of dark matter.
— Amber JorgensonX-RAY:^NASA/CXC/MIT/D.^DEWEY^ET^ AL.^ ANDNASA/CXC/SAO/J.DEPASQUALE.^ OP
TICAL:^ NASA/STSCI