Nearby Supernovae Were Essential to Life on Earth

It’s almost impossible to comprehend a supernova explosion’s violent, destructive power. An exploding supernova can outshine its host galaxy for a few weeks or even months. That seems almost impossible when considering that a galaxy can contain hundreds of billions of stars. Any planet too close to a supernova would be completely sterilized by all the energy released, its atmosphere would be stripped away, and it may even be shredded into pieces.

But like many things in nature, it all comes down to dose.

A certain amount of supernova activity might be necessary for life to exist.

Many Universe Today readers know about nucleosynthesis, and how supernova explosions forge heavy chemical elements necessary for life. Supernovae explosions create and spread elements like iron out into space to be taken up during the formation of stars and planets. Without them, we wouldn’t be here.

“Life on Earth appears to have evolved under the influence of supernovae activity.”

But a new research article sheds light on another way that supernovae support life. Supernova activity in Earth’s neighbourhood may have led to more oxygen in the atmosphere. And oxygen is necessary for complex life.

The oxygen is at the end of a long chain of cause and effect, and it all begins with the Galactic Cosmic Rays (GCR) released by supernovae.

The title of the article is “Supernova Rates and Burial of Organic Matter.” The sole author is Henrik Svensmark, a physicist and professor in the Division of Solar System Physics at the Danish National Space Institute in Copenhagen. The paper is published in the journal Geophysical Research Letters.

“Life on Earth appears to have evolved under the influence of supernovae activity in the solar neighbourhood,” the letter begins. Evidence shows a connection between climate, clouds, and cosmic rays from supernovae.

“When heavy stars explode, they produce cosmic rays made of elementary particles with enormous energies. Cosmic rays travel to our solar system, and some end their journey by colliding with Earth’s atmosphere. Here, they are responsible for ionizing the atmosphere,” Professor Svensmark said in a press release.

The ionizing energy from those cosmic rays creates aerosols in Earth’s upper atmosphere. That increases cloud formation. Clouds block solar radiation from reaching Earth’s surface, cooling the climate. A cooler climate has greater temperature differences between polar regions and mid-latitudes. Those differences create stronger winds and ocean currents, which in turn drive stronger nutrient cycles.

This figure from the research letter shows the correlation between supernova rates and trace elements, or nutrients, in the ocean. The nutrients are found in pyrite and are a proxy for nutrients in the ocean. The colored band at the top of the figure indicates climatic warm periods (orange), cold periods (blue), glacial periods (white and blue hatched bars), and finally peak glaciations (black and white hatched bars). Abbreviations for geological periods are Cm Cambrian, O Ordovician, S Silurian, D Devonian, C Carboniferous, P Permian, Tr Triassic, J Jurassic, K Cretaceous, Pg Palaeogene, Ng Neogene. <See the study for a more detailed explanation.> Image Credit: Svensmark 2022.

Stronger nutrient cycles mean that more chemical elements necessary for life are delivered to the upper 200 meters of the ocean, near continental shelves, where bio-productivity is highest. When there’s higher bio-productivity, more organisms live and die, and when they die, they fall to the ocean floor as organic matter, to be encased in sediments. Hence the title of the paper, “Supernova Rates and Burial of Organic Matter.”

On geological time scales, supernovae activity can fluctuate wildly, by several hundred percent. So the effect on climate can be pronounced on long time scales.

This figure from the research letter shows supernova activity over time and the increased levels of organic matter that result from supernovae. The top panel shows supernova activity and the lower panel shows the organic matter content in ocean sediments. The frequency of supernova explosions comes from star cluster data and open cluster data from previous studies. The top panel clearly correlates with the lower panel. The author says that the correlation is clearer for the last 500 million years and is less clear the further back they look. Image Credit: Svensmark 2022.

So how does the increased organic matter lead to more oxygen? Read on.

The organic matter in ocean sediments in the form of Carbon 12. Life prefers the lighter C12 isotope over C13, and the ratio of C12 to C13 in the sediments reveals the presence of life over geological timescales.

All of this activity has consequences for Earth’s oxygen. When organic matter moves into sediments, it becomes an indirect source of oxygen. If all of that organic matter were exposed to the atmosphere, then it would react with atmospheric oxygen as it decomposed and pull the oxygen out of the atmosphere. Instead, since the organic matter is buried, the oxygen remains in the atmosphere. And complex life needs oxygen.

This wouldn’t happen without nearby supernova and the GCRs they produce. Without enough nearby supernova activity, the climate would be warmer. The winds and ocean currents would be weaker, and would move fewer nutrients around. The strong upwelling ocean currents required to deliver chemical nutrients to the ocean’s bioproductive zone would be absent. The consequence of a warmer climate would be less bioproductivity because ocean currents and atmospheric winds would be weaker. Less bioproductivity would mean less organic material (C12) in the ocean sediments. “The available kinetic energy in the ocean-atmosphere system determines the mixing and transport <of nutrients> in the oceans and atmosphere,” the author writes.

This image shows the modern Earth’s ocean currents. All those currents combine to create thermohaline circulation, also called the ocean conveyor belt. That belt, along with winds and surface run-off from rivers,. drives Earth’s nutrient cycle. Image by Dr. Michael Pidwirny (see http://www.physicalgeography.net) – http://blue.utb.edu/paullgj/geog3333/lectures/physgeog.html, [http://skyblue.utb.edu/paullgj/geog3333/lectures/oceancurrents-1.giforiginal image], Public Domain, https://commons.wikimedia.org/w/index.php?curid=37108971

“A fascinating consequence is that moving organic matter to sediments is indirectly the source of oxygen. Photosynthesis produces oxygen and sugar from light, water and CO2. However, if organic material is not moved into sediments, oxygen and organic matter become CO2 and water. The burial of organic material prevents this reverse reaction. Therefore, supernovae indirectly control oxygen production, and oxygen is the foundation of all complex life,” says author Henrik Svensmark.

“Oxygenic photosynthesis and organic matter burial is the primary source of oxygen, and oxygen underpins the evolution of complex life,” Svensmark writes in his conclusion. In a press release he says that, “The new evidence points to an extraordinary interconnection between life on Earth and supernovae, mediated by the effect of cosmic rays on clouds and climate.”

Clearly, supernova activity and life on Earth come down to dosage. According to some scientific evidence, some supernovae have been close enough to Earth to contribute to partial extinctioce. A supernova explosion may have triggered the Ordovician Extinction, the second-largest extinction in Earth’s history by number of species killed off. And if one were too close, it would sterilize Earth completely. But according to this research some supernovae activity helped drive life on Earth by stimulating the nutrient cycle and increasing atmospheric oxygen.

We’re accustomed to thinking of nearby supernovae as potentially devastating to life on Earth, and they are. But this study shows that, like many things in nature, it’s the dosage that matters.

If there were no supernova activity in our neighbourhood, life on Earth might look much different than it does now.