Recent near-Earth supernovae probed by global deposition of interstellar radioactive 60Fe

Abstract

The rate of supernovae (SNe) in our local galactic neighborhood within a distance of ~100 parsec from Earth (1 parsec (pc)=3.26 light years) is estimated at 1 SN every 2-4 million years (Myr), based on the total SN-rate in the Milky Way (2.0±0.7 per century1,2). Recent massive-star and SN activity in Earth’s vicinity may be evidenced by traces of radionuclides with half-lives t1/2 ≤100 Myr3-6, if trapped in interstellar dust grains that penetrate the Solar System (SS). One such radionuclide is 60Fe (t1/2=2.6 Myr)7,8 which is ejected in supernova explosions and winds from massive stars1,2,9. Here we report that the 60Fe signal observed previously in deep-sea crusts10,11, is global, extended in time and of interstellar origin from multiple events. Deep-sea archives from all major oceans were analyzed for 60Fe deposition via accretion of interstellar dust particles. Our results, based on 60Fe atom-counting at state-of-the-art sensitivity8, reveal 60Fe interstellar influxes onto Earth 1.7–3.2 Myr and 6.5–8.7 Myr ago. The measured signal implies that a few percent of fresh 60Fe was captured in dust and deposited on Earth. Our findings indicate multiple supernova and massive-star events during the last ~10 Myr at nearby distances ≤100 pc.

RADIOACTIVE IRON RAIN: TRANSPORTING60Fe IN SUPERNOVA DUST TO THE OCEAN FLOOR

“…Feige (2014) found the signal to extend in time beyond the Knie et al (2004) time interval with a total time-integrated fluence of ( )  = ´-2.32 0.60 10 atoms cm Feige 7 2 . In addition, Wallner et al (2016) found a larger total time-integrated value of  = Wallner ( ) ´-3.5 0.2 10 atoms cm 7 2 . For the purposes of this paper we will focus solely on the fluences that overlap with the Knie et al fluence.…”

“…These studies have considerable overlap in their time periods and greatly varying locations onEarth. We do not examine the Wallner et al (2016) measurements in detail, first, because the bulk of the analysis for this paper was completed and submitted for review prior to the publication of Wallner et al (2016), and second, because many of the samples included in Wallner et al (2016) arealready included in the other studies, do not cover the period around the 2.2 Myr signal, or were drawn from similar latitudes tothe other samples.…”

“…Sediment accepts nearly all deposited elements, so we assume = U 1 sediment . However, the Fe-Mn crust incorporates iron through a chemical leaching process, so the uptake for iron into the crust is thought to lie in the range -» U 0.1 1 crust (for more discussion, see Feige et al 2012;Feige 2014;Fry et al 2015;Wallner et al 2016). In order to account for concentrations and dilutions in the deposition of SN material, we include a factor ψ to represent the deviation from a uniform distribution ( ) y = 1 , where [ ) y Î 0, 1 implies a diluted deposition and y > 1 implies a concentrated deposition.…”

Did high-energy astrophysical sources contribute to Martian atmospheric loss?

“…Recent reports provide evidence of a 2.2 Myr old supernova at a distance between 60-130 pc based on measurements of 60 Fe deposition on the ocean floor (Binns et al 2016;Breitschwerdt et al 2016;Wallner et al 2016) and on the Moon (Fimiani et al 2016). They also report the possibility of 16 events in the past 13 My within 100 pc in the Local Bubble.…”

“…They also report the possibility of 16 events in the past 13 My within 100 pc in the Local Bubble. They estimate the frequency of such nearby explosions, with at least one event occurring every 2-4 My based on SN rate of ∼ 2 events/century (Wallner et al 2016). On longer timescales of ∼ Gyr, one can assume that a large number of such events would have deposited energy in the Martian atmosphere.…”