Scientists studying a sample of oceanic crust retrieved from
the Pacific seabed nearly 1.5km down have discovered traces of a rare isotope
of plutonium, the deadly element that has been central to the
Atomic Age.
اضافة اعلان
They say it was made in colliding stars and later rained
down through Earth’s atmosphere as cosmic dust millions of years ago. Their
analysis opens a new window on the cosmos.
“It’s amazing that a few atoms on Earth can help us learn
about where half of all the heavier elements in our universe are synthesized,”
said Anton Wallner, the paper’s first author and a nuclear physicist. Wallner
works at the
Australian National University as well as the Helmholtz Center in
Dresden, Germany.
Wallner and his colleagues reported their findings in
Science on Thursday.
Plutonium has a bad reputation, one that is well-deserved.
The radioactive element fueled the world’s first nuclear
test explosion as well as the bomb that leveled the Japanese city of Nagasaki
during World War II. After the war, scientists found the health repercussions
of plutonium to be particularly deadly. If inhaled or ingested in minute
quantities, it could result in fatal cancers. Small amounts also pack a bigger
punch than other nuclear fuels, a quality that aided the making of compact city
busters that nuclear powers put atop their intercontinental missiles.
The element is often considered artificial because it is so
seldom found outside of human creations. In the periodic table, it is the last
of 94 atoms characterized as naturally occurring. Traces of it can be found in
uranium ores. Astrophysicists have long known that it’s also spontaneously
created in the universe. But they have had a hard time pinpointing any exact
sites of its origin.
What makes the deep ocean a good place for the gathering of
extraterrestrial clues is its extreme remoteness from the waves of change near
the planet’s surface. It’s a sanctuary where things can sit undisturbed for
millions of years. In this case, the scientists got lucky when they had a
chance to study material from a Japanese expedition that sampled the seabed in
the equatorial Pacific.
Stars in their cores turn light elements into heavier ones,
creating elements as heavy as iron. The new discovery sheds light on the
relative contributions of two different ways the universe is thought to go
about making all of the elements that are heavier than iron, including many
found in everyday life, like copper and zinc, mercury and iodine.
The exploding stars known as supernovas have long been seen
as a main source. The abrupt gravitational collapse of a massive star turns
much of its matter into heavy elements that shoot back into space as it
rebounds in a colossal blast. These drifting elements ultimately get mixed with
more common atoms to become the raw material for new stars and planets, or for
life itself in the case of humans.
The second path is a variation on the first. A supernova
leaves behind a dense, collapsed core known as a neutron star, which packs as
much mass as the sun into an area about the size of Manhattan. The merger of an
orbital pair of neutron stars is also seen as producing a burst of heavy
elements, including gold and silver. In 2017, for the first time, astronomers
looking for gravitational waves found evidence of two neutron stars smashing together,
giving the cosmic theory a significant boost.
Now, the scientists report finding rare isotopes of iron and
plutonium that suggest the origin of plutonium is less likely in a supernova
than a neutron-star merger.
“On cosmic time scales,” Wallner said, “these are very much
characteristic of recent explosive events.” The scientists date the terrestrial
arrival of these particular plutonium atoms to within the past 10 million
years.
Isotopes are varieties of the same element whose nuclei have
different numbers of a subatomic particle known as a neutron. Most plutonium on
Earth, made in reactors, is plutonium 239. Its nucleus holds 145 neutrons, and
it is the material that is typically used to set off the explosive force of a
hydrogen bomb.
Much of the global seabed is rich in rocky crusts of
terrestrial metals deposited over the eons. The retrieved sample was roughly an
inch thick and 18 inches in area. The scientists looked for extraterrestrial
plutonium in the deepest layers, using an extremely sensitive detector
optimized for the discovery of tiny traces of plutonium. It registered dozens
of detections of cosmic atoms. They are not plutonium 239 but the rare 244
isotope, which has 150 neutrons.
The scientists found that the rare iron and plutonium isotopes
in the undersea sample had a ratio “lower than required” if the plutonium’s
main source was a supernova. The authors concluded that other astrophysical
events, like neutron star mergers, must have contributed to its creation.
Nuclear weapon states have experimented with assorted
isotopes of plutonium since the start of the Atomic Age but have found little
of the 244 isotope on Earth. Wallner says he and his colleagues are hot on the
cosmic trail.
“We’ve already moved on to another, significantly larger
sample,” Wallner noted, adding that they’re eager to learn more.
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