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violetX-raysGamma rays Infrared Radio wavesMicrowavesWWW.ASTRONOMY.COM 49recently documented marine megafaunal
extinction at about the same time. This
included Megalodon, which resembled a
great white shark the size of a school bus.
What about ozone depletion, the old
boogeyman? It doesn’t show up at any sig-
nificantly damaging level. Researchers are
used to dealing with ozone depletion sce-
narios from solar events and from gamma-
ray bursts. In both cases, the radiation ends
up mostly in the stratosphere, where it can
easily interfere with the ozone layer there.
What we have found is that the high-
energy cosmic rays from such a supernova
pass right through the atmosphere. Instead
of depositing their energy 20 to 30 miles
(32 to 48 kilometers) up, they dump most
of it about 7 miles (11 km) up, and still
enhance ionization right down to the
ground. This is in the troposphere, where
weather happens and where about 75 per-
cent of the mass of the atmosphere resides.
People have not been used to thinking
about radiation from supernovae affecting
the troposphere. So there is not much effect
on the ozone layer unless, of course, the
supernova is much closer than the ones
that astronomers have documented.
The spark of an idea
We expect the biggest effect to be on light-
ning. Lightning starts when there is a big
voltage difference between two regions,
either within the atmosphere or between
it and the ground. But lightning can’t get
started by itself. It must rely on a leader —
a path of increased ionization in which an
electric field can accelerate free electrons.
This sets up a growing cascade in which
the accelerated electrons knock other ones
loose, and you get a current that grows into
a lightning bolt.
But where does the leader come from?
Atmospheric scientists think the main
mechanism is paths of ionization left by
cosmic rays. So, a twentyfold increase in
tropospheric ionization should lead to a big
increase in cloud-to-ground lightning
(because most of the cosmic ray tracks are
more or less vertical).The big change would be that ordinary
storms would produce a lot more lightning.
In normal conditions, lightning is the main
ignition source for wildfires. Wildfires kill
trees and other woody plants; more fires
mean fewer trees and more grassland. The
American Great Plains was largely kept as
grassland by lightning-set wildfires. Native
Americans set fires to renew the grass,
which attracted bison. Even today, ranchers
there conduct controlled burns on their
rangeland. During the past few millions of
years, there has been a conversion in many
places, including the Great Rift Valley in
East Africa, from forest to grassland.
Finally, we can speculate on the conse-
quences. The conversion from trees to
grassland may have forced our ancestors
out of trees and down to the ground, walk-
ing and using their hands. Once the cosmic
rays (and consequent lightning) slack off,
forest tends to replace grassland until a
new burst of cosmic rays creates increased
lightning. If this happened, it would
require our predecessors to use their brains
to adapt to a new environment.
And so it would go.Adrian L. Melott is emeritus professor of
physics and astronomy at the University of
Kansas, a fellow of the American Physical
Society, and a fellow of the American
Association for the Advancement of Science.Tripping the light fantastic
Cosmic rays are the universe’s most energetic form of radiation. When they enter Earth’s
atmosphere, some create air showers of secondary radiation. ASTRONOMY: ROEN KELLYAir showers produced by cosmic rays can act as leaders for lightning. If the number of air showers
went up significantly, storms would produce a lot more cloud-to-ground lightning. That increase
would lead to greater numbers of forest fires, which may have caused adaptive changes in our
predecessors. © BRUNO ISMAEL DA SILVA ALVES | DREAMSTIME.COM