Two new studies published in Nature today revealed that ancient supernovae fed the planet Earth radioactive stardust approximately two million years ago.
The first paper sheds light on the existence of ancient seabed deposits of iron-60 isotopes and reveals their source as supernovae that took place 325 light-years away from the Earth. The other provides estimates of when these explosions took place, narrowing it down to two events: one 1.7 to 3.2 million years ago and another 6.5 to 8.7 million years ago.
"This research essentially proves that certain events happened in the not-too-distant past," said Adrian Melott, a University of Kansas astrophysicist who wrote an article on the implications of these papers, which will appear in Nature this week. "They make it clear approximately when they happened and how far away they were. Knowing that, we can consider what the effect may have been with definite numbers. Then we can look for events in the history of the Earth that might be connected to them."
The first team revealed that iron-60 isotopes are found in crust and sediment samples from the Indian, Pacific and Atlantic oceans, pointing to a global event outside of our solar system as the source, the most likely being giant stars.
The second team set their sights on ancient supernovae from millions of years ago that could have released the iron-60 isotopes to Earth. Their study revealed that there was a peak enhancement of iron-60 around 2.2 million years ago, indicating the time period of the event as well as the possibility that a nearby supernova exploded during this time at a distance of around 300 light-years from our solar system.
Although both of these supernovae events were past the distance of 30 light-years required to pose dangers to life on Earth, Melott claims that they still could have impacted human evolution.
"Our local research group is working on figuring out what the effects were likely to have been," he said. "We really don't know. The events weren't close enough to cause a big mass extinction or severe effects, but not so far away that we can ignore them either. We're trying to decide if we should expect to have seen any effects on the ground on the Earth."
The method of using iron-60 to unveil ancient supernovae explosions in the distant past was created by astrophysicist Brian Fields.
"In the 1990s he did the calculations and carried it forward," Melott said. "He said, 'Hey, look for iron-60. This is a way to find out if there have been supernova near the Earth.' Five years later came the first indications of supernovae using iron-60. Now, 20 years later, we've got a slam-dunk."
The two papers were both published online in the April 6 issue of Nature.
