Science - USA (2021-07-16)

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SCIENCE 16 JULY 2021 • VOL 373 ISSUE 6552 273

lates. Korschinek, however, says it will take
more data on the plutonium signal and its
timing to convince him that multiple rare
events happened so near and so recently.

BEYOND DUSTING Earth with rare nuclei,
what impact might nearby supernovae
have had? In 2016, a team led by Melott
and Thomas estimated the flux of various
forms of light and cosmic rays likely to reach
Earth from an explosion 300 light-years
away. Writing in Astrophysical Journal Let-
ters, they concluded that the most energetic,
potentially damaging photons—x-rays or
gamma rays—would have minimal impact.
“There is not a lot of high energy radiation,”
Thomas says. They suggested a few weeks
of the bright light would have little more
impact than disrupting sleep patterns.
Cosmic rays—the particles ac-
celerated to near light speed by
shock waves in the supernova’s
expanding fireball—are another
story. Because they are charged,
they can be deflected away from
Earth by galactic magnetic
fields. But the local bubble is
thought to be mostly devoid of
fields, so cosmic rays from just
300 light-years away would have
a relatively clean shot.
The atmosphere would have
been subjected to a drawn-out
barrage, Melott and Thomas
found. “The ramp up is a slow
process, decades at least,”
Thomas says, reaching a peak
about 500 years after the super-
nova flash and causing a 10-fold increase in
ionization of atmospheric gas that would
persist for 5000 years. Using an atmospheric
chemistry model developed by NASA, they es-
timated that chemical changes caused by the
ionization would deplete ozone by about 7%
or more in places and would boost the cre-
ation of fertilizing nitrogen oxide compounds
by 30%. The resulting surge in plants might
be enough to cool the climate and usher in
the Pleistocene.
The cosmic rays weren’t done yet. When
high-energy particles hit the upper atmo-
sphere, they create cascades of secondary
particles. Most fizzle out in further collisions,
but muons—heavy short-lived cousins of
electrons—keep going. Creatures on Earth’s
surface would receive triple the normal ra-
diation dose—equivalent to one or two CT
scans per year. “An enhanced risk [of can-
cer], but not radiation poisoning,” Thomas
says. Overall, the team thought the effects
were “not catastrophic” but could be de-
tectable in the fossil record if, for example,
certain vulnerable species disappeared
while others survived.

In Astrobiology in 2019, Melott and two
colleagues found that if the supernova ex-
ploded just 150 light-years away, rather
than 300, the muon radiation would have
hit marine animals surprisingly hard. Wa-
ter blocks most particles that rain down
from the sky, but muons can penetrate up
to 1 kilometer. Marine creatures, normally
shielded from nearly all radiation, would
experience the largest relative increase
in dose and suffer the most. This chimes
with an extinction of marine megafauna at
the start of the Pleistocene epoch, only re-
cently identified in the fossil record.
Then, last year, supernova proponents
suggested a similar scenario could explain
a major extinction event 359 million years
ago, at the end of the Devonian period. A
team led by John Marshall of the University

of Southampton had found that the spores
of fernlike plants from the time suddenly
became misshapen and dark, blaming the
changes on ultraviolet radiation (Science,
29 May 2020, p. 926). The team didn’t in-
voke an astronomical cause. But writing in
the Proceedings of the National Academy
of Sciences, astronomers saw the possible
signature of a nearby supernova. They sug-
gested a blast maybe just 60 light-years away
could have drenched Earth in ultraviolet by
depleting the ozone layer. “It’s pretty specula-
tive,” admits co-author John Ellis, a theorist
at King’s College London, as it is currently
impossible to identify the radioactive finger-
prints of a supernova that far back.
In a 2020 paper in The Journal of Geo-
logy, Melott and Thomas took a bigger
speculative leap. They noted that by rip-
ping electrons from air molecules, sec-
ondary cosmic rays would have created
pathways for lightning, making storms
more likely, which would not only generate
more nitrogen compounds but also spark
wildfires. Intriguingly, a layer of soot has
been found in the rock record in some par ts

of the world at the start of the Pleistocene.
Melott and Thomas went on to suggest
that those supernova-induced forest fires
may have pushed early humans out of the
trees and onto the savanna, leading to bi-
pedalism, larger brain size, and everything
that followed. “It’s fascinating to say that a
supernova 2.5 million years ago means we
are talking now via Skype,” Korschinek says.
Such scenarios don’t sit well with
paleontologists. “Timing is the trivial an-
swer to everything,” Hull says. “There’s
always something happening when things
become extinct.” Besides, she says, the tran-
sition to the Pleistocene “doesn’t stand out
as needing an explanation.” She says other
events around that time could have had
more impact on the global climate, such
as the closing of the isthmus of Panama,
which profoundly changed
ocean circulation.
To make their case, she says,
astronomers need to pin down
the timing of the ancient su-
pernovae more precisely. They
“need to measure more crusts.”
But hunting for supernova
traces is not getting any easier.
In 2019 TUM closed its AMS,
leaving only ANU with an ac-
celerator powerful enough to
separate iron-60.
In contrast, rarer isotopes
such as plutonium-244 could
enable researchers to look fur-
ther back in time, but they re-
quire an AMS that emphasizes
sensitivity rather than raw
power, and Wallner says only a few in the
world are up to the job. He has secured
funding to build a new AMS facility in Dres-
den, Germany, specializing in the heaviest
elements, that should be open by 2023. To
renew the hunt for iron-60, his team has
also made a pitch for national funding to
build a new high-energy AMS, which could
be up and running in 7 years.
For astronomers, a sudden flash of light
in the sky today would be the best chance
to see how supernova affects Earth. But
the odds are slim that we will see a light
show like the one that may have dazzled
our distant ancestors. Betelgeuse, a res-
tive red giant likely to blow up sometime
in the next 100,000 years, has settled
down in recent months, and in any case,
it lies more than 500 light-years away. Sco
OB2 is now heading away from the Sun.
And using data from Hipparcos’s succes-
sor, Europe’s Gaia mission, Breitschwerdt
has tracked another 10 clumps of stars.
“None are coming closer,” he says. “The
future”—for Earth, not the supernovae—
“is bright.” j

A mineral crust from the Pacific Ocean floor held both iron-60 and plutonium-244.


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