Ashes from ‘nearby’ supernova explosion 2.3 mn years ago, still raining on Earth

It’s still raining ashes on Earth. A team of scientists has found that majority of cosmic rays raining on earth today were sent by a nearby supernova about 2.3 million years ago.

Traces of 60Fe detected in space indicate that a nearby supernova occurred within the last few million years. The iron isotope 60Fe, which is very rare, is created when a massive star collapses in the form of supernova.

Walter Binns et al. detected 60Fe in cosmic rays flying through space, revealing that the contents of a nearby supernova are being sprinkled on Earth to this day. Some previous reports have found samples of the isotope that have accumulated on Earth and the Moon in the distant past, but this is the first measurement of the present-day rate.

The 60Fe was detected using the Cosmic Ray Isotope Spectrometer (CRIS) aboard NASA’s Advanced Composition Explorer (ACE), which is currently orbiting between Earth and the Sun.

Data collected over nearly 17 years resulted in detection of just 15 of these rare nuclei. Because 60Fe is radioactive with a half-life of 2.6 million years, these nuclei must have formed in a relatively nearby supernova within the last few million years.

The authors note that there is a delay between when these nuclei were formed and when their acceleration occurred, which suggests that at least one more supernova occurred nearby and contributed to the acceleration.

The authors propose that the 60Fe detected by CRIS is most likely coming from supernovae that marked the death of massive stars in the nearby Scorpius-Centaurus association.

The study appears in journal Science.