Our understanding of heavy element production in supernovae, exploding stars way beyond our solar system, may need to change following some discoveries we have made looking not to the skies, but deep under our oceans.
Supernova explosions are one of the most violent events in our galaxy and are thought to produce elements essential for life such as iron and iodine but also some of the heaviest elements existing in nature.
When a star goes supernova and explodes, these heavy elements are thrown out into space as dust and debris.
This material, freshly produced or already available from previous stellar burning phases, is returned to the interstellar medium and available for new stars. For example, our solar system is the product of many preceding star generations.
Extraterrestrial material in the form of interstellar dust and meteorites is entering the solar system as it travels through the galaxy.
Some of that debris can actually make it to Earth over millions of years and settle as galactic dust in terrestrial archives such as deep-sea sediments or deep-sea encrustations.
Under very stable conditions this material can stay captured there for millions of years.
Searching for heavy elements
We looked for evidence of the cosmic production of heavy elements found on Earth, with the results published this week in Nature Communications.
The sites as well as the production rates of such elements are still an open question. We know production of the heaviest elements requires explosive scenarios – such as a supernova – but there could be other potential candidates.
The number of such galactic particles from such explosions that are expected to reach Earth in recent time is very small. Thus, we have to search for unique extraterrestrial signatures.
Supernovae produce, for example, lead, gold or mercury, but these are also abundant on Earth. Thus, such elements are not suitable as a marker for cosmic dust.
We searched for extraterrestrial fingerprints at the ocean floor at about 5,000m depths far off from any continents. Close to the land masses terrestrial particles from soil erosion and river sediment influx would dilute the extraterrestrial signatures.
Our archives accumulated trace elements from the ocean as well as interstellar particles dating back over 25 million years. We searched for a specific plutonium isotope (plutonium-244) in such deep-sea archives.
This plutonium isotope is a radionuclide with a half-life of 80 million years and does not exist naturally on Earth. Thus, it is a sensitive marker of cosmic and explosive element synthesis. In contrast to the lighter elements it is exclusively produced in such explosive events.
Through its decay it serves also as a radioactive clock. Any presence of this radionuclide in terrestrial archives tells us it’s coming from space.
We know Pu-244 existed when Earth and the solar system formed about 4.5 billion years ago. But that Pu-244 has decayed in the long time since the solar system formation.
As a consequence any Pu-244 that we find on Earth must have been created in explosive events that have occurred more recently – more specifically, in the last few hundred million years.
Finding atoms of interstellar origin
We analysed about 2kg of deep-sea encrustations from the Pacific Ocean for its plutonium content. Because of the small number of atoms of extraterrestrial origin expected in our sample, the most sensitive technique for detecting such radionuclides was applied.
We used a specific technique of atom-counting called accelerator mass spectrometry (AMS). This technique is best known for radiocarbon (carbon-14) dating so we used it to detect spurious amounts of Pu-244.
This technique is able to measure plutonium with a sensitivity equivalent to filtering one salt grain from the total water volume of a big lake, for example Lake Geneva, the largest lake in Central Europe.
Surprisingly, we found 100 times less Pu-244 than suggested from continuous supernova production. It seems that the heaviest elements, such as plutonium, uranium or thorium, are not produced in standard supernova explosions – at least for the past few hundred million years.
Our data suggest that production of the heaviest elements requires rarer events. This can be a specific subset of supernovae or also the merging of two neutron stars to make them.
The latter are 100 to 1,000 times less frequent than a core-collapse supernova but their huge detonation can produce the heaviest elements also in sufficient quantities.
So heavy element production seems to be a very rare process now.
We know that heavy elements such as plutonium were present when the solar system and Earth formed billions of years ago, and uranium and thorium are still present owing to their long half-life.
Thus, such an explosive as well as rare event must have happened close in time and space to the early solar system.
Interestingly, radioactive decay from those heaviest elements – such as uranium and thorium – provides much of the heat that drives continental movements and vulcanism. Given that their origin is thought to be from such a rare explosive event, perhaps other planets do not have the same heat engine inside them.