The bright flashes that lit up the evening skies near Detroit, Michigan earlier this year were not the only signs of the meteor that disintegrated in the atmosphere on 17 January 2018.
The meteor explosion was also captured by infrasonic microphones and seismometers, offering a rare chance to compare these data with satellite and ground camera images.
In a report in Seismological Research Letters, a team of scientists led by Michael Hedlin of Scripps Institution of Oceanography use these data to pinpoint the time, location and height of the bolide disintegration, and to calculate an approximate yield for the explosion. Bolides, sometimes called “fireballs,” are extremely bright meteors that explode in the atmosphere. They estimate a likely yield within the range of .8 to 8.1 tons of TNT, and probably equivalent to 2.2 tons of TNT.
About 2000 bolides this size or larger pass through the Earth’s atmosphere each year, which makes the Michigan bolide a particularly interesting event to study, said Hedlin and colleagues. Scientists would like to know more about how frequently we can expect to see near-Earth objects of this yield, to assess the potential threat they pose on the ground. Researchers also use data from bolide bursts as “test cases” to determine how well infrasound and seismic instruments can locate and characterize secret nuclear test explosions.
Other bolides such as the February 2013 Chelyabinsk, Russia fireball also have been studied intensively through infrasound, seismic and optical observations–and made famous through “dashcam” videos taken by Russian drivers. But “it’s pretty unusual for such immense events to occur over a heavily instrumented area; they could be expected to occur once every several decades,” said Hedlin.
Credit: Alex Alishevskikh / Wikimedia Commons
The infrasound and seismic calculations were consistent with the optical observations, the researchers concluded, although some of the calculations–such as the location and yield provided by infrasound data–contained a large amount of statistical uncertainty. The seismic data, however, were able to pinpoint the location and height of the burst within kilometers, in agreement with the optical data.
If the Michigan bolide is the smallest detectable event that can be measured using CEUSN data, researchers should be able to detect about 15 events of this size or larger each year in eastern North America, Hedlin and his colleagues write.
The Michigan analysis could also help researchers who study clandestine nuclear tests “make some inferences about how detectable similar-sized anthropogenic events would be–how far away from the explosion source we’d be able to detect signals, for example,” said Hedlin.
Contacts and sources: Becky Ham Seismological Society of America
Read more at beforeitsnews.com
Scientists Say Meteorites Created Earth’s Oldest Rocks
By Katyanna Quach
The oldest rock formations on Earth were born when meteorites pummelled into the ground over four billion years ago, according to a Nature Geoscience paper published last week.
A team of geologists have analysed samples of felsic rocks known for containing high concentrations of silica near the Acasta River near Great Bear Lake, the largest lake in Canada. The rocks are known as the Idiwhaa gneisses, meaning ancient in the Dogrib language spoken by the Tlicho people, an indigenous group in Northern Canada,
These rocks are believed to be around 4.02 billion years old, judging from uranium-lead dating of tiny zircon crystals lodged inside. The researchers modeled the formation process for the rocks by looking at their chemical composition and measuring the melting points for all the different minerals.
“Our modelling shows that the Acasta River rocks derived from the melting of pre-existing iron-rich basaltic rock, which formed the uppermost layers of crust on the primitive Earth,” said Tim Johnson, co-author of the paper and a senior lecturer in applied geology at Curtin University, Australia.
The iron-rich basaltic rock are known as amphibolites and contain high amounts of iron and low levels of quartz. “It would have needed something special to produce the 900oC temperatures needed to generate these early felsic rocks at such low pressures, and that probably means a drastic event, most likely the intense heating caused by meteorite bombardment,” said Johnson.
The meteorite bombardment would have melted rocks within the continental crust stretching to about three kilometers down to make ancient felsic rocks like those found in Canada common. But the surface shifted and changed as Earth’s plates formed and split apart.
“We believe that these rocks may be the only surviving remnants of a barrage of extraterrestial impacts which characterized the first 600 million years of Earth History,” Johnson added.