Ashes of Supernovas Litter Ocean Floor

Scientists have discovered radioactive debris from relatively nearby stars that exploded a few million years ago, raising questions about whether cosmic rays released by the supernovae impacted Earth’s past climate.

Scientists have discovered radioactive debris from relatively nearby stars that exploded a few million years ago, raising questions about whether cosmic rays released by the supernovae impacted Earth's past climate.

The evidence comes from samples taken from the floors of the Pacific, Atlantic and Indian oceans. All contained a radioactive isotope of iron, known as iron-60, which is produced in the cores large stars and in supernovae, which are exploding stars.

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The particles were transported via interstellar dust grains to Earth between 1.7 million and 3.2 million years ago, according to a paper published in this week's Nature.

A second analysis, also published in Nature, traces the iron-60 to two explosions, the first occurring about 2.3 million years ago and the second 1.5 million years ago.

The exploded stars, which were roughly 9.2 and 8.8 times bigger than the sun, respectively, were about 300 light-years away at the time -- close enough to be visible during the day.

"We have shown that the iron-60 must come from outer space, it cannot originate from the solar system," lead researcher Anton Wallner, with the Australian National University's Department of Nuclear Physics, wrote in an email to Discovery News.

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Wallner and colleagues also were able to ferret out when the iron-60 was deposited, indicating that there was more than one nearby supernova explosion.

"It's not a single event in the last 10 million years or so, it's rather a series of supernovae," Wallner said.

The timing of the supernova explosions coincides with a period of time when Earth cooled, shifting from what is known as the Pliocene into the Pleistocene periods.

"We have now a consistent and coherent picture of what happened around the solar system in the last 20 million years and we know how close these supernovae were. We can now proceed to find out if there might have been any biological effects," astronomer Deiter Breitschwerdt, with the Berlin Institute of Technology, wrote in an email to Discovery News.

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The supernova explosions would have generated cosmic rays, which although unlikely were powerful enough to kill off life on Earth, might have triggered increased cloud cover, lightning and other climate changes.

"We do not know if there is a link between supernova activity and colder temperature" – a variation that may have been one of the conditions that led to human evolution, University of Kansas astronomer and physicist Adrain Melott wrote in an related essay in Nature.

"The new studies will open up ... deeper insight into what might have happened on Earth over the past 10 million years as a result of nearby stellar fireworks," Melott said.

The Cassiopeia A supernova remnant is the left-overs of a stellar explosion that seeded its interstellar environment with heavy elements.

NASA's Spitzer Space Telescope was launched 10 years ago and has since peeled back an infrared veil on the Cosmos. The mission has worked in parallel with NASA's other "Great Observatories" (Hubble and Chandra) to provide coverage of the emissions from galaxies, interstellar dust, comet tails and the solar system's planets. But some of the most striking imagery to come from the orbiting telescope has been that of nebulae. Supernova remnants, star-forming regions and planetary nebulae are some of the most iconic objects to be spotted by Spitzer. So, to celebrate a decade in space, here are Discovery News' favorite Spitzer nebulae.

First up, the Helix Nebula -- a so-called planetary nebula -- located around 700 light-years from Earth. A planetary nebula is the remnants of the death throes of a red giant star -- all that remains is a white dwarf star in the core, clouded by cometary dust.

Spitzer will often work in tandem with other space telescopes to image a broad spectrum of light from celestial objects. Here, the supernova remnant RCW 86 is imaged by NASA's Spitzer, WISE and Chandra, and ESA's XMM-Newton.

Staring deep into the Messier 78 star-forming nebula, Spitzer sees the infrared glow of baby stars blasting cavities into the cool nebulous gas and dust.

The green-glowing infrared ring of the nebula RCW 120 is caused by tiny dust grains called polycyclic aromatic hydrocarbons -- the bubble is being shaped by the powerful stellar winds emanating from the central massive O-type star.

Spitzer stares deep into the Orion nebula, imaging the infrared light generated by a star factory.

In the year 1054 A.D. a star exploded as a supernova. Today, Spitzer was helped by NASA's other "Great Observatories" (Hubble and Chandra) to image the nebula that remains. The Crab Nebula is the result; a vast cloud of gas and dust with a spinning pulsar in the center.

The Tycho supernova remnant as imaged by Spitzer (in infrared wavelengths) and Chandra (X-rays). The supernova's powerful shockwave is visible as the outer blue shell, emitting X-rays.

Over 2,200 baby stars can be seen inside the bustling star-forming region RCW 49.

The "Wing" of the Small Magellanic Cloud (SMC) glitters with stars and warm clouds of dust and gas. By combining observations by Spitzer, Chandra and Hubble, the complex nature of this nebulous region can be realized.

The giant star Zeta Ophiuchi is blasting powerful stellar winds into space, creating an impressive shock wave in the interstellar medium.