From: LARRY KLAES (ljk4_at_msn.com)
Date: Wed Jan 08 2003 - 12:38:14 PST
----- Original Message -----
From: Mark Hess
Sent: Wednesday, January 08, 2003 2:45 PM
To: News Media list.serv
Cc: William.A.Steigerwald.1_at_gsfc.nasa.gov; Edward.S.Campion.1_at_gsfc.nasa.gov
Subject: EARTH LIKELY SPARED FROM ONE FORM OF COSMIC DOOM
Bill Steigerwald
January 8, 2003
NASA Goddard Space Flight Center
(Phone: 301/286-5017)
William.A.Steigerwald_at_nasa.gov
Release: 03-03
EARTH LIKELY SPARED FROM ONE FORM OF COSMIC DOOM
We have one less thing to worry about. While the cosmic debris from a
nearby massive star explosion, called a supernova, could destroy the
Earth's protective ozone layer and cause mass extinction, such an
explosion would have to be much closer than previously thought, new
calculations show.
Scientists at NASA and Kansas University have determined that the
supernova would need to be within 26 light years from Earth to
significantly damage the ozone layer and allow cancer-causing
ultraviolet radiation to saturate the Earth's surface.
An encounter with a supernova that close only happens at a rate of
about once in 670 million years, according to Dr. Neil Gehrels of
NASA's Goddard Space Flight Center in Greenbelt, Md., who presents
these findings today at the American Astronomical Society meeting in
Seattle.
"Perhaps a nearby supernova has bombarded Earth once during the
history of multicellular life with its punishing gamma rays and
cosmic rays," said Gehrels. "The possibility for mass extinction is
indeed real, yet the risk seems much lower than we have thought."
The new calculations are based largely on advances in atmospheric
modeling, analysis of gamma rays produced by a supernova in 1987
called SN1987a, and a better understanding of galactic supernova
locations and rates. A supernova is an explosion of a star at least
twice as massive as our Sun.
Previous estimates from the 1970s stated that supernovae as far as 55
light years from Earth could wipe out up to 90 percent of the
atmosphere for hundreds of years. The damage would be from gamma rays
and cosmic rays, both prodigiously emitted by supernovae. Gamma rays
are the most energetic form of light. Cosmic rays are atomic
particles, the fastest-moving matter in the Universe, produced when
the expanding shell of gas from the exploded star runs into
surrounding dust and gas in the region. Gamma rays, moving at light
speed, would hit the Earth's atmosphere first, followed closely by
the cosmic rays moving at close to light speed.
Gamma-ray light particles (called photons) and the cosmic-ray
particles can wreak havoc in the upper atmosphere, according to Dr.
Charles Jackman of NASA Goddard, who provided the atmospheric
analysis needed for the new calculation.
The particles collide with nitrogen gas (N2) and break the molecule
into highly-reactive nitrogen atoms (N). The nitrogen atoms then
react fairly quickly with oxygen gas (O2) to form nitric oxide (NO)
and, subsequently, other nitrogen oxides (NOx). The nitrogen oxide
molecules can then destroy ozone (O3) through a catalytic process.
This means that a single NOx molecule can destroy an ozone molecule
and remain intact to destroy hundreds of more ozone molecules.
The new calculations -- based on the NASA Goddard two-dimensional
photochemical transport model -- show that a supernova within 26
light years from Earth could wipe out 47 percent of the ozone layer,
allowing approximately twice the amount of cancer-causing ultraviolet
radiation to reach the Earth's surface. Excessive UV radiation is
harmful to both plants and animals, thus a doubling of UV levels
would be a significant problem to life on Earth.
The gamma-ray irradiation would last 300 to 500 days. The ozone layer
would then repair itself, but only to endure cosmic-ray bombardment
shortly after, lasting at least 10 years. (Cosmic rays are
electrically charged particles whose paths are influenced by magnetic
fields, and the extent of such fields in the interstellar medium is
not well understood.)
The calculations simultaneously point to the resilience of the ozone
layer as well as its fragility in a violent Universe, said Dr. Claude
Laird of the University of Kansas, who developed the gamma-ray and
cosmic ray input code and performed the atmospheric model
simulations. Although the ozone layer should recover relatively
rapidly once the particle influx tapers off -- within about one to
two years, the Goddard models show -- even this short period of time
is sufficient to cause significant and lasting damage to the
biosphere.
"The atmosphere usually protects us from gamma rays, cosmic rays, and
ultraviolet radiation, but there's only so much hammering it can take
before Earth's biological defenses break down," he said.
Dr. John Cannizzo of NASA Goddard and University of Maryland,
Baltimore Country, initiated and coordinated the new calculations.
"I've long been fascinated by the possibility of extinction from
something as remote as a star explosion," he said. "With this updated
calculation, we essentially worked backwards to determine what level
of ozone damage would be needed to double the level of ultraviolet
radiation reaching the Earth's surface and then determined how close
a supernova would need to be to cause that kind of damage."
These results will appear in the Astrophysical Journal 2003, March
10, vol. 585. Co-authors include Barbara Mattson of NASA Goddard (via
L3 Com Analytics Corporation) and Wan Chen of Sprint IP Design in
Reston, Virginia.
For images and more information, refer to:
http://www.gsfc.nasa.gov/topstory/2003/0108supernova.html
- end -
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