ANU: Supernovae Showered Earth with Radioactive Debris

[bystander]

Supernovae Showered Earth with Radioactive Debris
Australian National University | via EurekAlert | 2016 Apr 06

A series of massive supernova explosions near our solar system showered the Earth with radioactive debris between 3.2 and 1.7 million years ago

Artist's impression of a supernova - Credit: Greg Stewart, SLAC

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An international team of scientists has found evidence of a series of massive supernova explosions near our solar system, which showered the Earth with radioactive debris.

The scientists found radioactive iron-60 in sediment and crust samples taken from the Pacific, Atlantic and Indian Oceans.

The iron-60 was concentrated in a period between 3.2 and 1.7 million years ago, which is relatively recent in astronomical terms, said research leader Dr Anton Wallner from The Australian National University (ANU).

"We were very surprised that there was debris clearly spread across 1.5 million years," said Dr Wallner, a nuclear physicist in the ANU Research School of Physics and Engineering. "It suggests there were a series of supernovae, one after another.

"It's an interesting coincidence that they correspond with when the Earth cooled and moved from the Pliocene into the Pleistocene period." ...

A possible source of the supernovae is an ageing star cluster, which has since moved away from Earth, independent work led by TU Berlin has proposed in a parallel publication. The cluster has no large stars left, suggesting they have already exploded as supernovae, throwing out waves of debris.

Proof that ancient supernovae zapped Earth sparks hunt for aftereffects
University of Kansas | 2016 Apr 06

Recent near-Earth supernovae probed by global deposition of interstellar radioactive ⁶⁰Fe - A. Wallner et al

• Nature 532(7597):69 (07 Apr 2016) DOI: 10.1038/nature17196

The locations of recent supernovae near the Sun from modelling ⁶⁰Fe transport - D. Breitschwerdt et al

• Nature 532(7597):73 (07 Apr 2016) DOI: 10.1038/nature17424

[neufer]

[b][color=#0000FF][size=150]Artist's conception of the Local Bubble (containing the Sun and Beta Canis Majoris) and the Loop I Bubble (containing Antares)[/size][/color][/b]

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<<In 2002, Narciso Benítez et al. calculated that roughly 2 million years ago, around the end of the Pliocene epoch, a group of bright O and B stars called the Scorpius-Centaurus OB association passed within 130 light-years of Earth and that one or more supernova explosions gave rise to a feature known as the Local Bubble. Such a close explosion could have damaged the Earth's ozone layer and caused the extinction of some ocean life (at its peak, a supernova of this size could have the same absolute magnitude as an entire galaxy of 200 billion stars).>>

[Ann]

<<In 2002, Narciso Benítez et al. calculated that roughly 2 million years ago, around the end of the Pliocene epoch, a group of bright O and B stars called the Scorpius-Centaurus OB association passed within 130 light-years of Earth and that one or more supernova explosions gave rise to a feature known as the Local Bubble. Such a close explosion could have damaged the Earth's ozone layer and caused the extinction of some ocean life (at its peak, a supernova of this size could have the same absolute magnitude as an entire galaxy of 200 billion stars).>>

Click to view full size image

Upper Centaurus Lupus and Lower Centaurus Crux. Photo: Akira Fuji.

Fascinating. The old Akira Fuji image at left is still my favorite portrait of the Sco-Cen association, even though the youngest subgroup of the association, Upper Scorpius, is not seen in the image. What Fuji's image brings out is the wealth of rather similarly bright, hot massive blue stars, spread out over a long swath of sky. The fact that they are so similarly bright and spread out so wide brings home the fact that these stars are rather similar in age and rather nearby.

There is to my knowledge a single O-type star in the Sco-Cen association, and that is one of the components of binary star Sigma Scorpii. (Admittedly Antares was born as an O-type star, too.) All the other blue stars in Sco-Cen belong to spectral class B. It is impossible for a starforming cloud to produce so many B-type stars without hatching a few O-type stars as well. These O-type stars must have exploded as supernovas, not all that many dozens of light-years from the Earth. Today, the mean distance to the Sco-Cen association is between 380 and 470 light-years, but according to neufer's source they were much more nearby some 2 million years ago. It seems highly likely that the Sco-Cen association may have produced several supernovas going off so close to the Earth as to do some damage, and this may indeed have happened roughly 2 million years ago. Or, as bystander's source proved, it may have happened between 3.2 and 1.7 million years ago.

Fascinating indeed!

Ann

[bystander]

Microscopic "Timers" Reveal Likely Source of Galactic Space Radiation
NASA | GSFC | ACE | 2016 Apr 21

[img3="A cluster of massive stars seen with the Hubble Space Telescope. The cluster is surrounded by clouds of interstellar gas and dust called a nebula. The nebula, located 20,000 light-years away in the constellation Carina, contains the central cluster of huge, hot stars, called NGC 3603. Recent research shows that galactic cosmic rays flowing into our solar system originate in clusters like these.
Credits: NASA/U. Virginia/INAF, Bologna, Italy/USRA/Ames/STScI/AURA"]http://www.nasa.gov/sites/default/files ... iation.jpg[/img3][hr][/hr]

Most of the cosmic rays that we detect at Earth originated relatively recently in nearby clusters of massive stars, according to new results from NASA's Advanced Composition Explorer (ACE) spacecraft. ACE allowed the research team to determine the source of these cosmic rays by making the first observations of a very rare type of cosmic ray that acts like a tiny timer, limiting the distance the source can be from Earth. ...

Cosmic rays are high-speed atomic nuclei with a wide range of energy -- the most powerful race at almost the speed of light. Earth's atmosphere and magnetic field shield us from less-energetic cosmic rays, which are the most common. However, cosmic rays will present a hazard to unprotected astronauts traveling beyond Earth's magnetic field because they can act like microscopic bullets, damaging structures and breaking apart molecules in living cells. NASA is currently researching ways to reduce or mitigate the effects of cosmic radiation to protect astronauts traveling to Mars.

Cosmic rays are produced by a variety of violent events in space. Most cosmic rays originating within our solar system have relatively low energy and come from explosive events on the sun, like flares and coronal mass ejections. The highest-energy cosmic rays are extremely rare and are thought to be powered by massive black holes gorging on matter at the center of other galaxies. The cosmic rays that are the subject of this study come from outside our solar system but within our Galaxy and are called galactic cosmic rays. They are thought to be generated by shock waves from exploding stars called supernovae.

The galactic cosmic rays detected by ACE that allowed the team to estimate the age of the cosmic rays, and the distance to their source, contain a radioactive form of iron called Iron-60 (⁶⁰Fe). It is created inside massive stars when they explode and then blasted into space by the shock waves from the supernova. Some ⁶⁰Fe in the debris from the destroyed star is accelerated to cosmic-ray speed when another nearby massive star in the cluster explodes and its shock wave collides with the remnants of the earlier stellar explosion. ...

Microscopic 'Clocks' Time Distance to Source of Galactic Cosmic Rays
Washington University in St. Louis | 2016 Apr 21

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In 1999, astrophysicists proposed that a supernova explosion in Scorpius might explain the presence of excessive radioactive iron in 2.2 million-year-old ocean crust. Two research papers recently published in Nature bolster this case. One research group examined ⁶⁰Fe deposition worldwide, and argued that there might have been a series of supernova explosions, not just one. The other simulated by computer the evolution of Scorpius-Centaurus association in an attempt to nail down the sources of the ⁶⁰Fe.

Lunar samples also show elevated levels of ⁶⁰Fe consistent with supernova debris arriving at the moon about 2 million years ago. And here, too, there is recent corroboration. A paper just published in Physical Review Letters describes an analysis of nine core samples brought back by the Apollo crews. ...

Observation of the ⁶⁰Fe nucleosynthesis-clock isotope in galactic cosmic rays - W. R. Binns et al

• Science (online 21 Apr 2016) DOI: 10.1126/science.aad6004

[bystander]

Supernova Footprint on the Moon
Physics | American Physical Society | 2016 Apr 13

Lunar soil samples reveal an unusually high level of radioactive iron, which suggests a supernova in our vicinity within the last few million years.

Interstellar ⁶⁰Fe on the Surface of the Moon - L. Fimiani et al

• Physical Review Letters 116:151104 (13 Apr 2016) DOI: 10.1103/PhysRevLett.116.151104