Exploding stars showered the Earth with radioactive debris (or star dust) two million years ago, lighting up our sky as much as the moon, according to two studies published in the journal Nature. Scientists found evidence of nearby supernova explosions by tracing the radioactive iron-60 detected in samples from the bottom of the major oceans.
They wanted to find out if those fireworks had somehow altered the course of life on our home planet.
Astronomers have speculated for more than half a century that supernovae have taken place close enough to Earth to contribute to climate change or mass extinctions on the planet, as Adrian Melott of the University of Kansas commented on the paper, which was published in the journal Nature. He was not involved in the study.
How supernovae occur
Supernovae happen when massive stars run out of fuel and experience extremely violent deaths, shedding heavy elements to other stars and their planets. These dying, colossal stars seed the rest of the universe with unique debris that cannot be produced through any other process.
Scientists have discovered in recent years that they can track the iron isotope iron-60, first detected in 1999 in samples from the bottom of the Pacific Ocean.
Astronomers believe the radioactive element must have come from a supernova because it takes just 2.6 million years for half of the iron-60 atoms to disintegrate. Given that Earth has more than 4 billion years of history, all the iron-60 that may have originated in our planet’s formation should already be gone.
Besides, scientists have realized that the element is less stable and therefore degrades faster than the most similar form of iron that’s native to Earth, Fe-56. Knowing this, both research teams used this isotopic evidence to pin down any recent supernova that might have seed our home planet with the radioactive element.
Supernova activity has coincided with changes in Earth’s temperature
A research team led by astronomer Anton Wallner of Australian National University sifted through rare isotopic marks in Earth’s crust beneath the Pacific, Atlantic and Indian oceans. The lead author said in a Nature podcast that the team covered all of the major oceans and demonstrated that the signal must be global, since they found iron-60 in all those samples.
The scientists discovered presence of iron-60 in two spans, from 8.7 million to 6.5 million years ago and from 3.2 million to 1.7 million years ago. That means that there might have been two supernovae showering Earth with star dust, one after the other.
The study authors wrote in the paper that both periods matched up with significant climate changes. The more ancient star explosion happened to overlap with a strong increase in 3He and a temperature variation about 8 million years ago. The more recent event coincides with the onset of the little ice age.
“The more recent activity starting about 3 [million years] ago occurred at the same time as Earth’s temperature started to decrease during the Plio–Pleistocene transition”, researchers wrote.
Both explosions occurred too far away to destroy our planet
The second study was led by led by Dieter Breitschwerdt of the Berlin Institute of Technology. Researchers calculated the supernovae’s possible trajectory based on the isotopic marks found on Earth.
The star that was closer to the blue planet weighed in at 9.2 solar masses and exploded about 2.3 million years ago, while the farther one held the mass of 8.8 suns and went supernova just 1.5 million years ago.
These supernovae didn’t cause major damage to Earth because they are well outside the calculated kill distance of about 26 light-years, or 8 parsecs. However, both explosions were close enough to deposit debris into our planet’s crust.
Further research needs to be done in order to find out whether supernova activity is actually linked to colder temperature. But Melott said that such climatic variation was possibly an important condition that led to human evolution.
“Ionization of the atmosphere by supernovae may also lead to an increase in lightning and possibly other climatic effects”, he added.
Source: Washington Post