Science - USA (2021-07-16)

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272 16 JULY 2021 • VOL 373 ISSUE 6552 SCIENCE

of Korschinek’s results, he was studying
the local bubble, a region of space around
the Solar System swept clear of most of its
gas and dust. Supernovae were the likely
brooms, and so he began to track gangs of
stars in the Solar System’s neighborhood to
see whether any passed close enough to the
Sun to deposit iron-60 on Earth when some
of their members exploded.
Using data from Hipparcos, a Euro-
pean star-mapping satellite, Breitschwerdt
looked for clumps of stars on common
trajectories and rewound the clock to
see where they would have been millions
of years ago. Two clumps, now a part of
the Scorpius-Centaurus OB Association
(Sco OB2), seemed to be in the perfect
spot—300 light-years from Earth—about
2.5 million years ago. “It looked like a mira-

cle,” he says. The odds of a detonation at the
right time were good. Core-collapse super-
novae take place in massive stars. Based on
the ages and masses of the 79 stars remain-
ing in the clumps, Breitschwerdt estimates
that a dozen former members exploded as
supernovae in the past 13 million years.
Visible evidence for these supernovae in
Sco OB2 is long gone: Supernova remnants
dissipate after about 30,000 years, and the
black holes or neutron stars they leave be-
hind are challenging to spot. But the arrival
direction of the iron dust could, in theory,
point back to its source. Samples from the sea
floor provide no directional information be-
cause wind and ocean currents move the dust
as it settles. On the Moon, however, “there is
no atmosphere, so where it hits is where it
stops,” says UIUC astronomer Brian Fields.

Because it spins, the Moon cannot provide
longitudinal direction, but if more iron-
was detected at one of the poles than at the
equator, for example, that could support Bre-
itschwerdt’s Sco OB2 as the source. Fields
and several colleagues want to test that idea
and have applied to NASA for samples of lu-
nar soil, to be collected and returned by any
future robotic or human missions.

KORSCHINEK’S TEAM now has a rival in
the hunt for supernova iron: a group led
by Anton Wallner, a former postdoc of
Korschinek’s, who has used an upgraded
ASM at Australian National University
(ANU) to analyze several ferromanganese
crusts dredged off the Pacific Ocean floor
by a Japanese mining company. “Now we
pushed Munich,” Wallner says.
This year, in Science Advances, Wallner’s
team probed the timing of the recent super-
novae more precisely than ever by slicing a
crust sample into 24 1-millimeter-thick lay-
ers, each representing 400,000 years. “It’s
never been done before with this time reso-
lution,” says Wallner, now at the Helmholtz
Center Dresden-Rossendorf. The 435 iron-
60 atoms they extracted pinned the most
recent supernova at 2.5 million years ago
and confirmed the hints of an earlier one,
which they pegged at 6.3 million years ago.
Comparing the abundance of iron-60 in the
crust with models of how much a supernova
produces, the team estimated the distance
of these supernovae as between 160 and
320 light-years from Earth.
Wallner’s team also found 181 atoms of
plutonium-244, another radioactive iso-
tope, but one that may have been forged in
the supernova blast itself rather than in the
precursor star, like iron-60. But its source
is hotly debated: Some researchers think
plutonium-244 is tough for supernovae to
make in any great amounts. Instead, they
see it as the product of collisions between
neutron stars—cinders left behind by super-
novae (Science, 20 October 2017, p. 282).
These collisions, called kilonovae, are
100 times rarer than supernovae, but are
much more efficient at making the heavi-
est elements. “Neutron star mergers have
an easy time making plutonium,” says
Rebecca Surman, an astrophysicist at the
University of Notre Dame. “For supernovae
it’s much harder.”
Surman still sees a role for supernovae. She
takes the reported seafloor plutonium-
as a sign that a kilonova, deep in the past,
dusted our interstellar neighborhood with
heavy elements. When the two recent su-
pernovae went off, their expanding rem-
nants may have swept up and delivered
some of that interstellar plutonium-
along with their own iron-60, she specu-




Sco OB


Dust and gas









million years


Milky WayMilMilky Waay



600 light-years




Sculpted by supernovae
The Sun is surrounded by the local bubble, a region swept clear of most gas and dust by a series
of supernovae. Some astronomers believe the Scorpius-Centaurus OB Association (Sco OB2), a clump
of several hundred stars, was responsible for these blasts.

Tracking the culprit
By measuring the motions
of Sco OB2’s stars today
and rewinding the clock,
astronomers have calculated
that parts of it were closer
to the Sun 2.5 million years
ago—in a good position
to leave traces of
supernova dust on Earth.

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