A Supernova at 50 pc: Effects on the Earth's Atmosphere and Biota

Abstract

Recent ⁶⁰Fe results have suggested that the estimated distances of supernovae in the last few million years should be reduced from ∼100 to ∼50 pc. Two events or series of events are suggested, one about 2.7 million years to 1.7 million years ago, and another about 6.5-8.7 million years ago. We ask what effects such supernovae are expected to have on the terrestrial atmosphere and biota. Assuming that the Local Bubble was formed before the event being considered, and that the supernova and the Earth were both inside a weak, disordered magnetic field at that time, TeV-PeV cosmic rays (CRs) at Earth will increase by a factor of a few hundred. Tropospheric ionization will increase proportionately, and the overall muon radiation load on terrestrial organisms will increase by a factor of ∼150. All return to pre-burst levels within 10 kyr. In the case of an ordered magnetic field, effects depend strongly on the field orientation. The upper bound in this case is with a largely coherent field aligned along the line of sight to the supernova, in which case, TeV-PeV CR flux increases are ∼10⁴; in the case of a transverse field they are below current levels. We suggest a substantial increase in the extended effects of supernovae on Earth and in the "lethal distance" estimate; though more work is needed. This paper is an explicit follow-up to Thomas et al. We also provide more detail on the computational procedures used in both works.

Keywords: astrobiology; cosmic rays; stars: supernovae: general.

Figure 1

Cosmic-ray flux spectrum (times E²) for cases A and B at several times as noted in the figure legend along with Galactic cosmic-ray background flux (GCR; black line).

Figure 2

Atmospheric ionization rates for each case at several times, along with Galactic cosmic-ray background ionization rate (GCR; black line).

Figure 3

Globally averaged change in atmospheric O₃ column density, assuming steady-state irradiation in Case A at 100 years (solid line) and Case B at 300 years (dashed line).

Figure 4

Differential muon spectrum at sea level at a time of 100 years, when it is at a maximum. The black line represents the typical present spectrum, primarily as a result of showers initiated by galactic cosmic rays. The upper boundary of the banded region represents Case A; the lower boundary represents Case B. Of course, negligible doses are induced in the case of a transverse ordered galactic magnetic field, which is not shown.

Figure 5

Time evolution of the muon and neutron radiation dose on the ground. The horizontal lines represent the current average dose on the ground. The upper boundary of the banded region represents Case A; the lower boundary represents Case B.