potential to unleash a cascade of events that would
dramatically impact our planet. Visible light from
the supernova would reach Earth first, and the
burst star would appear to shine as bright as the
moon for months. While not ultimately harmful to
humans, it could possibly be bright enough to alter
the biological systems of nocturnal animals, says
Adrian Melott, Ph.D., an astronomer at the Univer-
sity of Kansas, Lawrence.
Soon after the first light of a supernova appears,
a blast wave packed with cosmic rays, an amalgam
of high-energy particles, would begin its race toward
Earth. “The guts of the star are launched into space
at speeds that are a few percent of the speed of light,”
says Brian Fields, Ph.D., an astronomer at the Uni-
versity of Illinois Urbana-Champaign. This wave
radiates out into space, sweeping gases and other
interstellar matter up like a cosmic snowplow. It
could take thousands of years for these rays to reach
Earth because their trajectory is inf luenced by the
magnetic fields they encounter. If their path is free
of magnetic field lines, they’ll travel in a straight
line, says Melott.
Earth’s atmosphere would bear the brunt of
these charged particles, according to a 2016 study
in Astrophysical Journal Letters, led by Melott and
his colleagues. They suggest that the cosmic rays
generated by a supernova 300 light-years away
would slice through nitrogen molecules in the air,
generating nitrogen oxide compounds that can
rain down and fertilize vegetation. This would spur
Earth’s plant life to gulp up carbon dioxide from the
atmosphere and cool the climate.
The excess nitrogen oxide in the atmosphere
could wash away as much as 7 percent of Earth’s
ozone layer, too, according to the study. Erasing
this protective shield would subject animals and
plants to sun damage, potentially altering the food
web for thousands of years. “You and I would put
on a hat and some sunblock—but if you’re a phyto-
plankton, you don’t have that option, and just get
cooked,” says Fields.
Furthermore, when cosmic rays pierce through
Earth’s atmosphere, they generate secondary par-
ticles called muons, which are similar to electrons
but heavier. “[Muons] can come all the way down to
the ground and even under the ground,” Fields says.
“You can’t hide from them.” These muons would
subject animals on Earth’s surface to three times
the normal amount of radiation.
Scouring the planet for geologic evidence of
a near-Earth supernova is more difficult than,
say, searching for a five-mile-wide asteroid cra-
ter. Still, researchers like Melott and Fields are
combing through the geologic record for instances
where supernovae could have played a role in shap-
ing Earth’s environment and the evolution of life
on Earth.
Last year, for instance, a team of researchers
studying fossilized leaves from a notable extinc-
tion event in the late Devonian period, roughly 359
million years ago, found evidence of warped plant
spores—suggesting these plants may have absorbed
excessive amounts of ultraviolet radiation. Fields
and his colleagues argued in a subsequent paper
published in the Proceedings of the National Acad-
emy of Sciences last September that the jump in
radiation could be the result of an ozone-free Earth.
And in a study published in The Journal of Geol-
ogy last year, Melott and his colleagues suggest
that widespread wildfires—possibly spurred by
cosmic ray–induced lightning—helped push our
early human ancestors to move from forests to the
savanna and embrace bipedalism at the beginning
of the Pleistocene, 2.5 million years ago. Depos-
its of the radioactive isotope Iron-60 (see sidebar)
found on Earth and on the moon seem to corre-
spond with this timing.
Fields admits more evidence is needed to under-
stand exactly what role these stellar explosions may
have played in charting our evolutionary path. Pin-
pointing the exact causes of global-scale changes
across the geologic record is a difficult task.
One thing is certain, however: Earth is safe
from future supernovae. Of the stars in our gal-
axy that are approaching the end of their life cycle
and might go supernova in the near future, none
of them are likely to cause catastrophic damage.
At most, Fields says, they’ll just provide a captivat-
ing show.I Spy: Iron-60
One telltale sign of a near-Earth super-
nova is the presence of the radioactive
isotope Iron-60. The isotope, which
is carried to Earth by the gaseous
remnants of these burst stars, has a
half-life of millions of years, meaning itmust have arrived on Earth long after
our planet formed. Traces of Iron-
60 have been found in rocky crusts
plucked from the seafloor, in Antarctic
snow, and even in lunar soil collected
during the Apollo missions.24 January/February 2022
Space
7