[meteorite-list] chondrules-- gamma ray lightning?

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Wed Feb 2 17:24:04 2005
Message-ID: <3jk20190sj4f7ehc4kst3a86mtoq9qg20a_at_4ax.com>

http://www.astronomy.com/default.aspx?c=a&id=2853

GRBs spark protoplanetary lightning
Solar-system-scale lightning storms triggered by gamma-ray bursts may be responsible for a key
feature of stony meteorites.

Robert Adler
February 2, 2005
Blasts of gamma rays may have spawned lightning storms as large as the solar system, fusing
primordial dust grains into chondrules ? the mysterious BB-size spheres that abound in stony
meteorites ? that, in turn, seeded the formation of Earth and the other planets 4.6 billion years
ago.

Astrophysicist Brian McBreen, at University College, Dublin, and his colleagues have been working
for years to solve the mystery of how chondrules formed.

Their first model, proposed in 1999, featured direct melting of iron-rich dust grains by a blast of
radiation from a nearby gamma-ray burst (GRB). The researchers were encouraged when a laboratory
experiment in 2002 produced convincingly chondrule-like spheres by searing likely raw material with
intense X rays from the European Synchrotron Radiation Facility.

Still, that model had a major drawback: It could not explain the many chondrules that have melted
repeatedly. The early solar system had less than one chance in 100 of being close enough to a GRB
for its radiation alone to fuse dust grains into chondrules, and a vanishingly small chance of being
blasted by more than one nearby GRB.

"For any protoplanetary disk within about 100 parsecs [326 light-years], a gamma-ray burst will
blast it and form chondrules," says McBreen. But since roughly one-third of the chondrules in our
solar system have melted more than once, he adds, "there have to be other mechanisms out there."

The team now believes it has found the missing mechanism ? solar-system-size lightning storms
induced by bursts of gamma rays from different sources at different times. The scientists envision
lightning storms spanning the entire protoplanetary disk, with individual bolts crackling one-tenth
the distance from Earth to the Sun, each releasing a thousand billion times more energy than a
terrestrial lightning flash. The group's work appears in the January 17, 2005, issue of Astronomy &
Astrophysics Letters.

The team calculates that fluxes of gamma rays powerful enough to spawn giant lightning storms can
come from GRBs located anywhere in the galaxy ? from post-GRB emissions, which last longer and
irradiate larger segments of the galaxy than the GRBs themselves, and from soft gamma repeaters,
thought to represent starquakes in highly magnetized neutron stars.

The researchers believe each of these gamma-ray sources is capable of sparking chondrule-forming
lightning storms, and, taken together, they may be able to account for the abundance and complexity
of chondrules in the solar system.

"GRBs can cause charge separation and lightning in protoplanetary disks," says McBreen. "The
lightning melts the dust grains to form chondrules that later aggregate to form planets."

The mechanism that separates positive and negative charges to power the lightning storms is called
Compton scattering. When a burst of gamma rays strikes molecular hydrogen, it produces a flood of
electrons and positrons moving in the same direction as the radiation. The positrons quickly
annihilate, leaving a wave of electrons to carry a negative charge for millions of miles across the
nebula.

Alan Rubin, a geochemist at UCLA, applauds the team's demonstration that gamma rays can produce
sufficient charge separation to create nebula-size lightning storms as "an advance in lightning
theory."

If McBreen is right, chondrules in many meteorites would have formed simultaneously. "Chondrules
would have been melted all across the disk at the same time by the same GRB," he says. "That should
provide a simultaneous time marker between chondrules in different meteorites."

He and his colleagues also point out that as radiation from GRBs in other galaxies spawns similar
lightning storms in planet-forming disks around young stars, the melting dust grains should produce
infrared flashes that large Earth- or space-based telescopes can detect.

McBreen is eager to see the theory tested by comparing the formation time of chondrules in different
meteorites, and by observers scanning nearby galaxies for the infrared flashes the theory predicts.

So far, the group's proposed link between gamma rays, lightning, and chondrules has sparked more
skepticism than support. Alan Boss, at the Carnegie Institute, sees it as "imaginative" but "highly
unlikely" because of the low frequency of GRBs. John Wood at the Smithsonian Astrophysical
Observatory, in Cambridge, Massachusetts, says the leading chondrule-forming mechanism remains
shock-wave heating of dust grains within the early solar nebula.

McBreen, however, believes his group has made "a credible case" that the chondrules that represent a
critical step in planet formation were spawned in repeated, titanic lightning storms, and that the
process is being repeated today in other galaxies.
Received on Wed 02 Feb 2005 05:24:18 PM PST