Chapter 1: Introduction to Supernovae and Earth’s History

In February 1987, Neil Gehrels, a young researcher at NASA’s Goddard Space Flight Center, embarked on a journey aboard a military aircraft heading to the Australian Outback. Gehrels was equipped with an unusual load: a polyethylene space balloon and a set of radiation detectors he had recently constructed in his lab. His mission was urgent; he aimed to reach Alice Springs, a secluded outpost in the Northern Territory, where he would deploy these instruments into the atmosphere to observe an extraordinary event occurring in our cosmic neighborhood: a supernova erupting in one of the Milky Way's nearby satellite galaxies.

Supernova SN 1987A marked the dramatic collapse of a massive star. Its significance lay in its closeness to Earth; it was the nearest stellar explosion witnessed since Johannes Kepler recorded one in our Milky Way in 1604. This event spurred scientists to ponder numerous questions, particularly: How close must a supernova be to inflict damage on Earth?

During the 1970s, researchers speculated that radiation from a nearby supernova could potentially devastate the ozone layer, exposing terrestrial life to harmful ultraviolet radiation and possibly triggering mass extinction events. With fresh data from SN 1987A, Gehrels was equipped to estimate a theoretical danger zone, within which a supernova could have severe consequences, and to assess how frequently dying stars might wander into this region.

To comprehend the impact of supernovae on life, it was crucial for scientists to correlate the timing of these explosions with significant events on Earth, such as mass extinctions or pivotal evolutionary transitions. “The conclusion was that a supernova capable of significantly affecting the ozone layer would occur roughly once every billion years,” notes Gehrels, who continues his work at Goddard. Although this frequency is infrequent, and no menacing stars currently threaten our solar system, Earth has existed for 4.6 billion years, with life for about half of that time. This suggests a strong possibility that a supernova impacted the planet in the past. However, identifying when that occurred remains a challenge, as supernovae primarily influence the atmosphere, making it difficult to find definitive evidence.

While astronomers have scoured the cosmos for clues, the most persuasive evidence of a nearby supernova paradoxically comes from oceanic depths. Here, a seemingly unremarkable black mineral formation known as ferromanganese crust grows on the barren bedrock of underwater mountains at an astonishingly slow rate. These thin, layered structures chronicle Earth's history and may provide the first direct evidence of a past supernova.

Chapter 2: The Role of Ferromanganese Crusts

These clues regarding ancient cosmic explosions are invaluable to researchers who suspect that supernovae have played a little-known yet significant role in the evolution of life on Earth. “This might be part of the narrative of life's journey, encompassing the challenges it has faced,” explains Brian Fields, an astronomer at the University of Illinois at Urbana-Champaign. To truly understand the influence of supernovae on life, scientists must correlate the timing of these explosions with key events on Earth. The only method to achieve this is by tracking the debris they left behind, specifically the elements on our planet that are produced in supernovae.

Fields and his colleagues identified several such elements, primarily rare radioactive metals that decay slowly, thus serving as reliable indicators of a deceased star. A particularly promising candidate was Fe-60, a heavy iron isotope with four additional neutrons compared to the standard isotope and a half-life of 2.6 million years. However, locating Fe-60 atoms scattered across Earth's surface proved challenging. Fields estimated that only a minuscule quantity of Fe-60 would have reached our planet, and on land, it would likely be diluted by natural iron or eroded away over millions of years.

The growth of these crusts is one of the slowest processes documented in science, accumulating approximately five millimeters every million years.

To circumvent this issue, scientists turned their attention to the ocean floor, where they discovered Fe-60 atoms within ferromanganese crusts—rocks that form similarly to stalagmites. These crusts precipitate from liquid, adding successive layers, and consist primarily of iron and manganese oxides, along with trace amounts of nearly every metal in the periodic table, from cobalt to yttrium.

As iron, manganese, and other metal ions flow into the sea from terrestrial sources or erupt from underwater volcanic vents, they react with oxygen in seawater, creating solid substances that either settle onto the ocean floor or float until adhering to existing crusts. James Hein from the United States Geological Survey, who has studied these crusts for over three decades, admits that the exact process of how they establish themselves on rocky stretches of seafloor remains a mystery. However, once the first layer forms, additional layers continue to accumulate, potentially reaching thicknesses of up to 25 centimeters.

This allows crusts to function as cosmic historians that retain records of seawater chemistry, including elements that serve as timestamps of dying stars. One of the oldest crusts, retrieved by Hein southwest of Hawaii in the 1980s, dates back more than 70 million years, a period when dinosaurs roamed Earth and the Indian subcontinent was a solitary island in the ocean, situated halfway between Antarctica and Asia.

The crusts grow at an incredibly slow pace, a stark contrast to the rapid nature of supernova explosions. The most prevalent type of supernova occurs when a star exhausts its hydrogen and helium fuel, leading to the core burning heavier elements until iron forms. This process spans millions of years, but the star's final moments transpire in mere milliseconds. As heavy elements accumulate in the core, it becomes unstable and collapses inward at a quarter of the speed of light. The density of particles in the core eventually repels the implosion, resulting in a colossal explosion that ejects a cloud of stellar debris into space—including Fe-60 isotopes, which may ultimately end up in ferromanganese crusts.

Chapter 3: Discovering Supernova Evidence

The first researchers to search for Fe-60 in these crusts were Klaus Knie, an experimental physicist formerly at the Technical University of Munich, and his team. Initially, they were not focused on supernovae or crusts; instead, they were developing techniques to measure rare isotopes of various elements, including Fe-60. After another scientist measured an isotope of beryllium—useful for dating crust layers—Knie decided to analyze the same specimen for Fe-60, knowing it was a product of supernovae. “We are part of the universe and have the opportunity to hold 'astrophysical' matter in our hands by examining the right locations,” states Knie, now at the GSI Helmholtz Center for Heavy Ion Research.

Knie's innovative approach has enabled scientists to date other, potentially older, supernovae that may have occurred near Earth and to investigate their impacts on our planet. The crust, also sourced from the seafloor not far from Hawaii, proved to be the ideal specimen: Knie and his colleagues identified a spike in Fe-60 in layers dating back approximately 2.8 million years, suggesting the death of a nearby star around that time. This discovery was significant for multiple reasons. It provided the first evidence that supernova debris can be located on Earth and identified the approximate timing of the most recent nearby supernova explosion. Moreover, it allowed Knie to propose a compelling evolutionary hypothesis.

Based on the concentration of Fe-60 within the crust, Knie estimated that the supernova exploded at least 100 light-years away from Earth—three times the distance at which it could have devastated the ozone layer—yet close enough to potentially influence cloud formation and climate. Although no mass extinction events occurred 2.8 million years ago, significant climatic shifts did take place, which may have facilitated human evolution. Around this time, the African climate experienced aridity, leading to a reduction in forests and the emergence of grassy savannas. This transition may have encouraged our hominid ancestors to descend from trees and eventually adopt bipedalism.

While this theory remains speculative and faces skepticism, as some scientists propose that Fe-60 might have arrived on Earth via meteorites, or that these climatic shifts can be attributed to decreasing greenhouse gas levels or geological changes, Knie's method provides a means to examine other supernovae that may have influenced Earth’s history. It is remarkable that we can utilize these dull, slow-growing formations to investigate the swift and brilliant phenomena of stellar explosions, Fields remarks. Their narratives are far from complete.

The Impact of Supernovae on Earth’s Evolution

The first video titled "The History of Supernovae (1836 Astronomy Book) | Geography of the Heavens: Pt 1 | ASMR soft spoken" delves into the historical significance of supernovae in shaping our understanding of astronomy and the cosmos.

The second video, "The Universe: Supernova Consumes the Galaxy (S2, E9) | Full Episode | History," explores the catastrophic effects of supernovae on galaxies and their potential implications for life on planets like Earth.

Julia Rosen is a science writer based in Portland, Oregon. Her work has appeared in outlets such as the Los Angeles Times, Science News, and EARTH Magazine. Prior to her writing career, Julia earned a Ph.D. in geology, focusing on air bubbles trapped in polar ice cores. Originally published at Nautilus on March 19, 2015.