For researchers based at the Australian National University (ANU) in Canberra made a surprising find when plumbing the ocean depths. The scientist analysed what they believe is extraterrestrial dust from supernovae that, over a period of millions of years, has settled on the ocean floor.
Their findings could change our understanding of supernovae, an event that occurs when a star goes rogue, in the process exploding and ejecting quantities of stellar material far into space.
Supernovae can occur in two ways. One type of supernova can come about in a binary star system — a pair of stars orbiting the same point — when one of the stars, a carbon-oxygen white dwarf, steals matter from its companion star. Eventually, the white dwarf’s voracity for its neighboring star’s material causes a form of stellar indigestion. The white dwarf eventually explodes resulting in a supernova.
A supernova can also occur at the end of a single star’s lifetime. As the ailing star’s supply of nuclear fuel becomes exhausted, some of its mass flows to its core. The end result is that the star’s core becomes too heavy so that it can’t resist its own gravitational forces. A core collapse happens resulting in a massive supernova explosion.
The ANU team analysed what they believe to be stellar dust from the ocean floor to ascertain the amount of heavy elements generated by these super-massive supernovae.
As lead researcher Dr Anton Wallner, based at ANU’s Research School of Physics and Engineering, explained, “Small amounts of debris from these distant explosions fall on the Earth as it travels through the galaxy. We’ve analysed galactic dust from the last 25 million years that has settled on the ocean and found there is much less of the heavy elements such as plutonium and uranium than we expected.”
Their findings are at odds with current theories of supernovae which hold that some of the materials necessary for human life, such as iron, potassium and iodine, are created and distributed throughout space.
But supernovae also create lead, silver and gold, as well as heavier radioactive elements such as uranium and plutonium.
ANU researchers examined plutonium-244. Plutonium-244 is particularly useful as a radioactive clock due to the rate at which it decays radioactively. Plutonium-244 takes a long, long time to break down, having a half life of 81 million years.
That being so, as Dr Wallner explained, “Any plutonium-244 that existed when the Earth formed from intergalactic gas and dust over four billion years ago has long since decayed.”
But, by a process of elimination, “So any plutonium-244 that we find on earth must have been created in explosive events that have occurred more recently, in the last few hundred million years,” added Dr Wallner.
To conduct their analysis scientists analysed a 10 centimeter (4 inches) thick sample of the Earth’s crust; that represents 25 million years worth of accumulated dust. They also looked at deep-sea sediments gathered from a very stable area of the Pacific Ocean’s floor.
They found 100 times less plutonium-244 than they’d expected.
“It seems that these heaviest elements may not be formed in standard supernovae after all. It may require rarer and more explosive events such as the merging of two neutron stars to make them,” said Dr Wallner.
Finding heavy elements like plutonium still present, as well as uranium and thorium, suggests that an explosive event must have occurred close to the Earth around the time of its formation, according to it Dr Wallner.
“Radioactive elements in our planet such as uranium and thorium provide much of the heat that drives continental movement, perhaps other planets don’t have the same heat engine inside them,” he added.
The existence of the supernovae dust in the ocean deep is not a new concept. In a 2013 study, researchers from the Technische Universitaet Muenchen in Munich, Germany, showed how iron-loving ocean sediment-dwelling microbes had metabolised a particular isotope of iron whose source could only have been a supernovae explosion.
Details of the ANU research were recently published in Nature Communications.
