Science - USA (2021-07-16)

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270 16 JULY 2021 • VOL 373 ISSUE 6552 SCIENCE

expanding fireball also accelerated cos-
mic rays—mostly nuclei of hydrogen and
helium—to close to the speed of light.
These projectiles arrived stealthily, de-
cades later, ramping up into an invisible
fusillade that could have lasted for thou-
sands of years and might have affected the
atmosphere—and life.
In a flurry of studies and speculation, as-
tronomers have sketched out their potential
effects. A cosmic ray barrage might have
boosted mutation rates by eroding Earth’s
protective ozone layer and generating show-
ers of secondary, tissue-penetrating parti-
cles. Tearing through the atmosphere, the
particles would have also created pathways
for lightning, perhaps kindling a spate of
wildfires. At the same time, atmospheric

reactions triggered by the radiation could
have led to a rain of nitrogen compounds,
which would have fertilized plants, draw-
ing down carbon dioxide. In that way,
the celestial event could have cooled the
climate and helped initiate the ice ages
2.5 million years ago, at the start of the
Pleistocene epoch. Even taken together,
the effects are “not like the dinosaur ex-
tinction event—it’s more subtle and lo-
cal,” says Brian Thomas, an astronomer at
Washburn University who has studied the
earthly effects of cosmic catastrophes for
nearly 2 decades.
Few astronomers are suggesting that the
supernovae caused any great extinction at
the time, and even fewer paleontologists
are ready to believe them. “Death from

space is always really cool,” says Pincelli
Hull, a paleontologist at Yale University.
“The evidence is interesting but has not
quite really reached the threshold to incor-
porate into my mental register.”
Yet the supernova hunters believe other
blasts, more distant in time, went off closer
to Earth. And they think these supernovae
could explain some extinction events that
lack customary triggers such as volcanic out-
bursts or asteroid impacts. Adrian Melott,
an astronomer at the University of Kansas,
Lawrence, who explores how nearby cosmic
cataclysms might affect Earth, says it’s time
to more carefully probe Earth’s history for
ancient supernova strikes. Not only will that
help astrophysicists understand how the
blasts shaped the neighborhood of the Solar







μμμμ μμμμ

UV light


N 2



O 3

O 2



CO 2

CO 2





Menace from afar
Astronomers have discovered traces of a supernova blast
2.5 million years ago. The stellar explosion is thought to have
occurred 150 to 300 light-years away—too distant to drive
massive extinctions. But it nevertheless may have affected
Earth’s biosphere, mostly through a barrage of cosmic rays.

2 Cosmic rays
Trailing the light burst
would be a pulse of near–
light speed cosmic rays—
high-energy protons and
other nuclei—that lasted
for thousands of years
and potentially left scars
on the biosphere.

Core-collapse supernova
When a massive star runs
out of fusion fuel, gravity
collapses its core into a
neutron star or black hole.
A rebounding shock wave
scatters the star’s
outer layers in a bright,
explosive event.

3 Supernova remnant
An expanding shell of
glowing gas and dust would
dissipate long before it
reached Earth, but could still
ferry radioactive atoms like
iron-60. Trace amounts have
been found in seabed crusts,
Antarctic snow, and lunar soil.

1 Light burst
For 1 month or more, the
light from the supernova
would have been as
bright as the full Moon.
But the gamma rays and
x-rays in the light were
probably not powerful
enough to cause harm.

A cosmic rain
Researchers are modeling the many effects that a cosmic ray
barrage would have on Earth.

Fertilizing rain
Cosmic rays can split
nitrogen molecules,
creating nitrogen
oxide compounds
that fall with rain and
fertilize plants. The
surge in growth could
draw down carbon
dioxide (CO 2 ) and
cool the climate.

Ozone depletion
The excess
nitrogen oxide (NO)
compounds would
destroy ozone (O 3 ),
allowing in more of
the Sun’s mutation-
causing ultraviolet
(UV) light.

Piercing muons
Particle showers from
cosmic ray strikes
include muons (μ),
heavy cousins of the
electron that can
penetrate animal
tissue and lead
to cancer.

Greased lightning
As particle showers
split air molecules,
they forge ion
paths for lightning
to follow. Increased
lightning may have
sparked wildfires that
transformed forests
to savanna.

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