What does it take for a rock older than the Earth to appear in a Georgia living room?
It was on June 26 that a smoldering fireball flashed across the daytime sky over the southeast United States at an estimated 29,000 miles per hour. NASA called it a bright daylight bolide perhaps a superbolide formed by an asteroidal fragment weighing over a ton. The sphere, one meter in diameter, fell into the atmosphere 48 miles over Oxford, Georgia, and disintegrated 27 miles over West Forest, releasing energy equivalent to a 20-ton TNT blast. Sonic booms did cross five states, leading some residents to question if it was an earthquake.
One, still traveling at more than 1 kilometer per second, pierced the roof of a McDonough, Georgia, residence, slashing through ceiling and air conditioning ductwork before denting the living room floor. Impact broke the meteorite into fragments, dispersing fine space dust that the homeowner continues to find weeks later.
University of Georgia analysis found the meteorite’s incredible age: 4.56 billion years about 20 million years older than the planet itself. “This particular meteor that entered the atmosphere has a long history before it made it to the ground of McDonough,” said Scott Harris, a planetary geologist at UGA. The rock originated beyond Mars, part of a larger asteroid in the main belt between Jupiter and Mars that fragmented around 470 million years ago. Some fragments of that catastrophic impact strayed into Earth-crossing orbits, their orbits slowly synchronizing with Earth’s until impact became inevitable.
Harris classified the specimen as a stony meteorite, a chondrite with minimal metal content. Chondrites are primitive, un-differentiated meteorites packed with chondrules millimeter-sized spherules of silicate minerals that solidified in the early solar nebula. Their mineralogy is usually characterized by olivine and pyroxene, sulfides, and oxides, and they preserve chemical imprints from the early days of the solar system. In other studies of carbonaceous chondrites such as the Allende meteorite, researchers have used multimodal x-ray and electron microscopy to visualize mineral phases at nanometer scales, mapping elemental distributions of Fe, Mg, Ni, and Al and identifying shock veins and melt pockets from ancient impacts.
The likely parent body of the McDonough meteorite was an L chondrite asteroid, a member of the largest known breakup event in the asteroid belt over the last three billion years. Such breakups can release billions of fragments, some of which will end up in resonances that cause them to evolve inward toward the inner solar system. Millions of years later, gravitational interactions with planets, particularly Jupiter, distribute the fragments onto Earth-crossing orbits.
The journey from asteroid belt to Georgia roof is governed by atmospheric physics and celestial mechanics. During atmospheric entry, the intense compression of air ahead of the meteoroid causes it to heat up to temperatures that can vaporize the surface, producing the bright fireball that one sees. The object’s initial speed of 29,000 mph was slowed way down by air drag, but even at meteoritic particle terminal speeds of descent hundreds of meters per second the kinetic energy still had the potential to penetrate a roof.
NASA radar detected a relatively short but extensive strewn field near Blacksville, Georgia, bent by high-level winds moving at about 150 mph. The bigger fragments fell along more linear paths, with the smaller fragments carried westward before they reached the ground. A number of discoveries have been made, and a number of specimens will be deposited in the Tellus Science Museum in Cartersville.
Just 27 meteorites are recorded in the history of Georgia, and six falls have only been seen. However, as Harris indicated, “This is something that used to be expected once every few decades and not multiple times within 20 years. Modern technology in addition to an attentive public is going to help us recover more and more meteorites.” As dashcams, security cameras, and satellite-mapping lightning imagers that record the flash of bolides have proliferated, meteorite recovery has come to involve being able to trace the recovered material back to its path in the atmosphere, and even to individual asteroid families.
To solar system scientists, each recovered meteorite is a physical history of the solar system. Isotopic ratios and mineralogy of the McDonough meteorite will enable the constraining of models of early solar nebula processes, asteroid differentiation, and collisional evolution. Its survival in intact form from atmospheric entry and the preservation of ancient minerals offer a special window to study material older than Earth, carrying within it the record of an age when planets were under formation and the solar system was a disk of gas and dust that swirled.
