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.