by When a massive star explodes as a supernova, it does more than just release an extraordinary amount of energy.
Supernova explosions are responsible for creating some of the heavy elements, including iron, which are shot into space by the explosion.
On Earth, there are two accumulations of the iron isotope Fe60 in seafloor sediments that scientists trace to about two to three million years ago and about five to six million years ago.
The explosions that created the iron also provided the Earth with cosmic rays.
From new research submitted to the Astrophysical diary lettersScientists investigate how much energy reached Earth through these explosions and how that radiation may have affected life on Earth.
The article is entitled ‘Life in the Bubble: How a near supernova left ephemeral footprints on the cosmic-ray spectrum and indelible imprints on life.’ The lead author is Caitlyn Nojiri of UC Santa Cruz.
“Life on Earth is constantly evolving under constant exposure to ionizing radiation of both terrestrial and cosmic origin,” the authors write.
Earth’s radiation decreases slowly over billions of years. But no cosmic rays. The amount of cosmic rays Earth is exposed to varies as our solar system moves through the galaxy.
“Nearby supernova (SN) activity has the potential to increase radiation levels at the Earth’s surface by several orders of magnitude, which is expected to have a profound impact on the evolution of life,” they write.
The authors explain that the two-million-year-old accumulation is a direct result of a supernova explosion, and the older accumulation dates back to when Earth went through a bubble.
The bubble in the title of the study comes from a certain type of star called OB stars. OB stars are massive, hot, and short-lived stars that usually form in groups.
These stars emit powerful outflowing winds that create ‘bubbles’ of hot gas in the interstellar medium. Our solar system is located in one of these bubbles, the so-called local bubble, which is almost 1000 light-years wide and was formed several million years ago.
Earth entered the local bubble about five to six million years ago, which explains the older accumulation of Fe60. According to the authors, the younger Fe60 accumulation from two to three million years ago comes directly from a supernova.
‘It is likely that the 60Fe peak at about 2-3 Myr comes from a supernova occurring in the Upper Centaurus Lupus association in Scorpius Centaurus (~140 pc) or the Tucana Horologium association (~70 pc) . While the ~5-6 Myr peak is likely attributed to the entry of the solar system into the bubble,” the authors write.
The Local Bubble is not a quiet place. It took multiple supernovae to create this. The authors write that it took fifteen SN explosions over the past 15 million years to create the LB.
“We know from the reconstruction of LB history that at least 9 SN exploded in the last 6 million years,” they write.
The researchers collected all data and calculated the amount of radiation from multiple SNe in the LB.
“It is not clear what the biological effects of such radiation doses would be,” they write, but they do discuss some possibilities.
The radiation dose may have been strong enough to cause double-strand breaks in the DNA. This is serious damage and can lead to chromosomal changes and even cell death. But there are other effects in terms of the development of life on Earth.
“Double-strand breaks in DNA could potentially lead to mutations and jump-start the diversification of species,” the researchers write. A 2024 paper showed that “the rate of virus diversification accelerated in Africa’s Lake Tanganyika 2-3 million years ago.” Could this be related to SN radiation?
“It would be attractive to better understand whether this can be attributed to the increase in cosmic ray dose that we predict occurred during that period,” the authors tease.
The SN radiation was not powerful enough to cause an extinction. But it could have been powerful enough to induce more mutations, leading to more species diversification.
Radiation is always part of the environment. It rises and falls as events unfold and as Earth moves through the galaxy. Somehow it must be part of the equation that has created the diversity of life on our planet.
“It is therefore certain that cosmic rays are an important environmental factor in assessing the viability and evolution of life on Earth, and the key question concerns the threshold for radiation to be a beneficial or harmful trigger when considering of the evolution of species,” the authors write in their conclusion.
Unfortunately, we do not understand exactly how radiation affects biology, what thresholds may exist, and how these may change over time.
“The exact threshold can only be determined with a clear understanding of the biological effects of cosmic rays (especially muons that dominate at ground level), which is still very unexplored,” Nojiri and her co-authors write.
The study shows that whether we can see it in everyday life or not, and even if we are aware of it or not, our space environment exerts a powerful force on life on Earth. SN radiation could have influenced the mutation rate at critical moments in Earth’s history, helping shape evolution.
Without supernova explosions, life on Earth could look very different. A lot of things had to go just right for us to be here. Perhaps supernova explosions in the distant past played a role in the evolutionary chain that leads to us.
This article was originally published by Universe Today. Read the original article.
