Astronomy teaches us that life on Earth does not exist in isolation. We are citizens of a larger cosmos, and the cosmos intervenes in our lives – often imperceptibly, but sometimes ferociously.
When Stars Attack! Radioactive Signatures of Prehistoric Supernovae
The most massive stars are the celebrities of the cosmos: they are very rare, but live extravagantly and die in spectacular and violent supernova explosions. While these events are awesome to observe, they can take a sinister shade when they occur closer to home, because an explosion inside a certain “minimum safe distance” would pose a grave threat to Earthlings.
Recently we have learned that a star exploded near the Earth about 3 million years ago. Radioactive iron atoms (iron-60, or 60Fe) have been found in ancient samples of deep-ocean material found around the globe, and also on the Moon. These unique atoms are tiny, telltale samples of debris from the supernova explosion. Thus, for the first time we can use sea sediments and lunar cores as telescopes, revealing explosions before recorded human history, and probing the nuclear fires that power exploding stars. Furthermore, an explosion so close to Earth was probably a “near miss,” which emitted intense and possibly harmful radiation.
Plutonium and the r-Process from Near-Earth Explosions
Plutonium-244 bears witness to the most extreme conditions that give rise to the heaviest elements. 244Pu is a heavy and long-lived radioactive nucleus–the longest lived radioactivity not also contaminated by remainders of Solar System formation. It is only made the astrophysical r-process whereby an abundant supply of neutrons are captured onto seed elements over timescales no longer than a few seconds.
Early data in deep-ocean ferromanganese crusts showed hints of 244Pu in the past millions of years. These results motivated a systematic study of the implications if this signal is real. Group alumna Xilu Wang led the study (Wang et al 2021), which made detailed predictions for other r-process radioisotopes that should be found along with 244Pu and which can test whether these arose in a rare supernova explosion or in a neutron star merger leading to a kilonova. Indeed, existing measurements of 129I already probes models.
New data by Wallner et al (2021) show that indeed the 244Pu signal is real, showing that followup measurements are now a top priority. And beyond that they find an earlier 60Fe pulse at about 6 Myr ago.
Supernova Explosions and the End-Devonian Extinctions
Recent data points to a global, catastrophic loss of ozone at the end of the Devonian period 359 million years ago. We point out that a nearby exploding star–a supernova–can do this and thus may be the trigger for one or more late Devonian biological extinctions. We present tests for this scenario, including the search for trace amounts of radioactivity possibly created by the supernova(e).
Fortunately, today there are no supernova candidates close enough to be menacing. But if the Devonian extinctions were caused by supernovae, then we literally can count they ways these explosions have affected us: the extinctions saw the deaths of many land-dwelling tetrapods, but the ones that survived and happened to have five toes–as do their descendants, including us!
This far-reaching subject weaves together the work of a diverse community of scientists. Many of them were a part of a recent near-Earth supernova White Paper for the Astro2020 Decadal Survey of Astronomy and Astrophysics:
- “Near-Earth Supernova Explosions: Evidence, Implications, and Opportunities” (2019) arXiv:1903.04589
Supernova Collisions with the Solar System.
Images from simulations of a supernova impact on the solar system. The supernova blast violently pushes the solar wind, compressing it to within the inner solar system, more than tenfold times closer than its current reach far beyond Pluto. Citations: Fields et al (2020); Fields, Athanassiadou, and Johnson (2008).
Radioactivity-Based DISTANCE TO THE SUPERNOVA
The measured abundance of supernova-produced radioactive iron-60 (60Fe) depends on: (a) how much 60Fe the supernova makes (the nucleosynthesis yield), and (b) how far away the supernova explodes. Thus if we use supernova models to estimate the 60Fe yield, we can infer the supernova distance. For the experts, this analysis is completely analogous to the calculation of a luminosity distance: here, the yield plays the role of luminosity, and the 60Fe surface density plays the role of flux. Hence we refer to this as a “radioactivity distance.” This idea goes back to the original Ellis, Fields, and Schramm (1996) analysis. Figure citation: Fry, Fields, and Ellis (2015).
Supernova Dust distribution on the Moon
If supernova dust is entrained with (that is, moves along with) the gas component of the blast, then the dust grains should arrive in the inner solar system as a hail of particles with high velocities and a uniform direction that points back to the supernova. In this case, the distribution of supernova grain deposition on the Moon will reflect the direction of supernova origin. The distribution is averaged over longitudes due to the Moon’s rotation, but the latitude distribution retains information about the supernova direction. That is, the Moon acts as an antenna to point to the supernova! Citation: Fry, Fields, and Ellis (2016).
- General public talk at Uof I Saturday Physics for Everyone: YouTube
- Fermilab Colloquium: video
- Nuclear Physics in Astrophysics 8 Conference talk: pdf of slides
- Ada Ertel on Prehistoric Supernovae, a finalist entry in the Illinois Research Live! Competition
Popular Media Coverage
Deep-Ocean Plutonium and Nearby Explosions
- Commentary on Wallner et al 2021 discovery: National Public Radio online article and All Things Considered on-air story
- PNAS article
- Illinois News Bureau Press Release
- Physics World, Eureka Alert
- Podcasts: PNAS Science Sessions, The Star Spot, The Cosmic Connections