Earth bears the scars of nearby supernovae through traces of unique isotopes such as iron-60 and plutonium-244 found in sediments and lunar samples. Detected using advanced methods such as accelerator mass spectrometry, these cosmic remnants reveal the history of massive stellar explosions in our cosmic neighborhood.
Astronomers are looking for "witnesses" to near-Earth astrophysics events. When large stars or objects explode near Earth, their fragments can reach our solar system. There is evidence of these cosmic events on both Earth and the Moon, detectable with accelerator mass spectrometers (AMS). Professor Anton Wallner from the Helmholtz Zentrum Dresden-Rosendorf (HZDR) recently published a review of this exciting research in the scientific journal Annual Review of Nuclear and Particle Science.
In their paper, HZDR physicist Anton Wallner and colleague Professor Brian D. Fields of the University of Illinois at Urbana provide an overview of near-Earth cosmic explosions, focusing specifically on events that occurred 3 million years ago and 7 million years ago.
"Fortunately, these events are still far enough away that they probably won't have a major impact on Earth's climate, nor on the biosphere. However, when cosmic explosions occur at distances of 30 light-years or less, things get really bad," Wollner explained. Converted to the astrophysical unit parsec, this corresponds to less than eight to ten parsecs.
Once a massive star burns out all its fuel, its core will collapse into an ultra-dense neutron star or black hole, and at the same time, high-temperature gas will be ejected outward at extremely fast speeds. Much of the gas and dust dispersed between the stars was carried away by the expanding shock wave. This envelope is like a giant balloon with bumps and bumps on it, and it also sweeps away any matter already in space. After thousands of years, the supernova's remnants have expanded to a diameter of 10 picoseconds, spreading out more and more slowly until the motion finally stops.
A nearby explosion has the potential to severely disrupt Earth's biosphere, causing a mass extinction similar to the asteroid impact 66 million years ago. Dinosaurs and many other animal species were victims of that event. "If we take into account the time span of billions of years since the formation of the solar system, then a very close cosmic explosion cannot be ruled out," Wollner emphasized.
However, supernovae only occur in heavy stars that are 8 to 10 times more massive than the Sun. Such stars are very rare. One of the closest stars of this size is the red supergiant Betelgeuse in the constellation Orion, which is at a safe distance of about 150 picoseconds from the solar system.
Production of interstellar isotopes
Shortly before and during a cosmic explosion or supernova explosion, many new atoms are created, including some radioactive atoms. Wollner was particularly interested in radioactive iron isotopes with an atomic mass of 60. About half of these isotopes, known simply as iron-60, became stable nickel isotopes after 2.6 million years. Therefore, any iron-60 that was present when the Earth was formed about 4.5 billion years ago is long gone.
"Iron-60 is extremely rare on Earth because it is not produced in large quantities in nature. However, it is produced in large quantities before supernovae occur. If this isotope appears now in seafloor sediments or material on the surface of the moon, it is likely to have come from a supernova or other similar space process that occurred near the Earth millions of years ago," Wallner concluded.
The same is true for the plutonium isotope with an atomic mass of 244. However, this plutonium-244 is more likely to have been produced by a neutron star collision rather than a supernova. Therefore, it is an indicator of heavy element nucleosynthesis. Over 80 million years, about half of the plutonium-244 isotope changed into other elements. Therefore, in addition to iron-60, the slowly decaying plutonium-244 is another indicator of galactic events and the production of new elements over the past millions of years.
"Exactly how, where and under what conditions these heavy elements were created is still a subject of intense scientific debate. The creation of plutonium-244 also required an explosive event, and according to theory, its creation process is similar to the creation of elements such as gold or platinum, which have always existed naturally on Earth but are now composed of stable atoms," explains Woerner.
Dust particles as "space cargo ships"
But how did these isotopes get to Earth in the first place? Iron-60 atoms ejected by supernovae like to clump together in dust particles. The same goes for the plutonium-244 isotope, which may have been created in other events and swept away by the supernova's expanding envelope. According to theory, after a cosmic explosion at a distance of more than 10 but less than 150 picoseconds, the solar wind and the magnetic field of the heliosphere prevent individual atoms from reaching Earth. However, the iron-60 and plutonium-244 atoms trapped in the dust particles will continue to fly toward the Earth and the moon, eventually trickling to the lunar surface.
Even if the supernova occurred within the so-called "kill radius" of less than 10 picoseconds, not even a microgram of material would fall from the envelope to the ground per square centimeter. In fact, only a tiny amount of iron-60 atoms per square centimeter reaches Earth every year. This poses a huge challenge for "investigators" like physicist Anton Varner: In a gram of sediment sample, there may be only a few thousand iron-60 atoms distributed like needles in a haystack among billions of ubiquitous stable iron atoms with an atomic mass of 56. Furthermore, even the most sensitive measurement methods can only detect every five thousand particles, which means that in a typical measurement sample, only a few iron-60 atoms can be detected at most.
Such extremely low concentrations can only be determined by accelerator mass spectrometry (AMS for short). The Helmholtz Accelerator Mass Spectrometer for Tracking Environmental Radionuclides (HAMSTER) is about to join them. Because AMS facilities around the world are designed differently, different facilities can complement each other in the search for rare isotopes produced by supernova explosions.
A 20-year search for 1,000 iron-60 atoms
Isotopes of the same element but with different masses, such as naturally occurring iron-56, are removed by mass filters. Atoms of other elements that have the same mass as the target iron-60, such as naturally occurring nickel-60, can also be interfered with. Even after very complex chemical preparation of the samples, they still contain billions of times more iron-60, so they have to be separated using nuclear physics methods in special accelerator facilities.
Ultimately, perhaps only five individual iron-60 atoms could be identified during measurements that lasted several hours. Pioneering work on the detection of iron-60 was carried out at the Technical University of Munich. But currently, ANU Canberra is the only existing facility in the world sensitive enough to make such measurements.
In total, only about 1,000 iron-60 atoms have been measured over the past 20 years. As for the concentration of interstellar plutonium-244, it is more than 10,000 times lower. For a long time, only data on single atoms can be obtained. Until recently, it was possible to measure around a hundred plutonium-244 atoms at a specialized facility in Sydney - similar to the HAMSTER facility currently under development at HZDR.
However, only certain samples are suitable for study, and these can act as archives to preserve these atoms from space for millions of years. For example, samples from the Earth's surface are rapidly "diluted" by geological processes. Sediments and crust from the deep sea, where the seafloor formed slowly and undisturbed, are ideal samples. In addition, samples from the lunar surface are also suitable because the destruction process is hardly a problem.
During the research trip until early November 2023, Varner and his colleagues will search for more cosmic isotopes at particularly suitable AMS facilities in the Australian cities of Canberra (iron-60) and Sydney (plutonium-244). To do this, he obtained some lunar samples from NASA. At the same time, he was also conducting measurements at HZDR. These unique samples will allow us to gain new insights into supernova explosions near Earth and the heaviest elements formed through these and other processes in the Milky Way.
Compiled source: ScitechDaily