Science - USA (2021-07-16)

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SCIENCE 16 JULY 2021 • VOL 373 ISSUE 6552 271

System and seeded it with heavy elements,
but it could also give paleontologists a new
way to think about bouts of global change.
“This is new and unfamiliar,” Melott says.
“It will take time to be accepted.”

vae go off in the Milky Way every century.
By the law of averages, a handful must
have exploded very close to Earth—within
30 light-years—during its 4.5-billion-year
lifetime, with potentially catastrophic ef-
fects. Even blasts as far as 300 light-years
away should leave traces in the form of
specks of dust blown out in the shell of de-
bris known as a supernova remnant. When
physicist Luis Alvarez set out in the 1970s
with his geologist son Walter Alvarez to
study the sediment layers as-
sociated with the dinosaurs’
extinction 65 million years ago,
they were expecting to find
supernova dust. Instead, they
found iridium, an element that
is rare on Earth’s surface but
abundant in asteroids.
The Alvarezes didn’t have
the tools to look for super-
nova dust, in any case. Because
Earth is already largely made
of elements forged in super-
novae billions of years ago,
before the Sun’s birth, most
traces of more recent explo-
sions are undetectable. Not
all of them, however. In the
1990s, astrophysicists realized
supernova dust might also de-
posit radioactive isotopes with
half-lives of millions of years,
far too short to have been
around since Earth’s birth.
Any that are found must come
from geologically recent sprin-
klings. One key tracer is iron-
60, forged in the cores of large
stars, which has a half-life of
2.6 million years and is not
made naturally on Earth.
In the late 1990s, Gunther
Korschinek, an astroparticle
physicist at the Technical Univer-
sity of Munich (TUM), decided
to look for it, partly because
the university had a powerful
accelerator mass spectrometer
(ASM) suited to the task. Af-
ter ionizing a sample, an ASM
boosts the charged particles to
high energies and shoots them
through a magnetic field. The
field bends their path onto a
string of detectors; the heavi-
est atoms are deflected least

because of their greater momentum.
Separating atoms of iron-60 from the
similarly hefty but differently charged
nickel-60 is especially challenging, but
TUM’s ASM, built in 1970, is one of the
few in the world powerful enough to tease
them apart.
Korschinek also needed the right sam-
ple: a geologic deposit laid down over mil-
lions of years in which an iron signal might
stand out. Antarctic ice cores wouldn’t
work: they only go back a couple of mil-
lion years or so. Most ocean sediments ac-
cumulate so fast that any iron-60 is diluted
to undetectable levels. Korschinek ended
up using a ferromanganese crust dredged
from a North Pacific seamount by the Ger-
man research ship Va l d i v i a in 1976. These

crusts grow on patches of seabed where
sediments can’t settle because of a slope or
currents. When the pH of the water is just
right, metal atoms selectively precipitate
out of the water, slowly building up a min-
eral crust at the rate of a few millimeters
every million years.
Korschinek and his team sliced their
sample up into layers of different ages,
chemically separated out the iron, and
fired the atoms through their mass spec-
trometer. They found 23 atoms of iron-
among the thousands of trillions of atoms
of normal iron, with the highest abun-
dance from a time less than 3 million years
ago, the team reported in Physical Review
Letters in 1999. The era of supernova geo-
chemistry had begun. “We were the first
ones to start experimental
studies,” Korschinek says.

found in ocean crusts from
other parts of the world and
even in ocean sediment micro-
fossils, remains of living things
that, helpfully for the superno-
vae hunters, had taken up and
concentrated iron in their bod-
ies. Most results pointed to a
local supernova between 2 mil-
lion and 3 million years ago—
with hints of a second one a
few million years earlier.
Although the remnants from
these blasts have long since
swept past Earth, a drizzle of
the atoms they blew out contin-
ues. In 2019, Korschinek’s team
ran iron from a half-ton of
fresh Antarctic snow through
its ASM and found a handful
of iron-60 atoms, which he
estimates fell to Earth in the
past 20 years. Another team
found a smattering of the at-
oms in cosmic rays detected by
NASA’s Advanced Composition
Explorer at a position partway
between the Sun and Earth.
Researchers have even found
iron-60 in lunar soil brought
back by the Apollo missions.
“The Moon confirmed that it
was not just some Earth-based
phenomenon,” says astronomer
Adrienne Ertel of the Univer-
sity of Illinois, Urbana-Cham-
paign (UIUC).
Dieter Breitschwerdt is try-
ing to trace the iron to its
source in the sky. When the
astronomer at the Technical
PHOTO: TIM WETHERELL/RESEARCH SCHOOL OF PHYSICS/ANUTo detect trace ions, an Australian accelerator fired samples through a magnet. University of Berlin learned

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