After following up on many of the social media reports regarding the Chelyabinsk meteoroid impact last February in Russia, scientists, including those at the University of New Mexico, contributed to present a comprehensive overview of what occurred that day in a report published online recently by the journal Science.
The report was led by Dr. Olga Popova of the Institute for Dynamics of Geospheres of the Russian Academy of Sciences in Moscow, and by NASA Ames and SETI Institute meteor astronomer Dr. Peter Jenniskens, who participated in a fact-finding mission to Chelyabinsk in the weeks after the fall.
"Our goal was to understand all circumstances that resulted in the damaging shock wave that sent over 1,200 people to hospitals in the Chelyabinsk Oblast area that day," said Jenniskens, who is best-known for investigating the asteroid impact over Sudan in 2008 and last year's Sutter's Mill meteorite fall in California. "Based on infrasound data, the brightness of the fireball, and the extent of the glass damage area, we confirm that this event was 100 times bigger than Sutter's Mill."
Conducting the Oxygen Isotope Analyses
UNM Sr. Research Scientist Karen Ziegler in the UNM Institute of Meteoritics (IOM), was one of 57 contributors by other researchers from nine countries. Ziegler’s research group studied samples provided by Chelyabinsk State University and U.S. scientists to identify the oxygen isotope composition of this space rock. The measurement of oxygen isotope ratios is a useful tool to identify the different groups of meteorites. The Ordinary Chondrite meteorites have three groups that have oxygen isotope compositions that fall in similar but resolvable ranges. Ziegler used a modified analytical procedure from colleague and Regents’ Professor Zach Sharp at UNM’s Department of Earth and Planetary Sciences.
“We crushed the samples to gather tiny fragments from inside the rock to avoid possible contamination from the fusion crust of the meteorite sample and from contamination from the Russian soils,” Ziegler said. “We used a laser fluorination method for oxygen isotope analyses of our samples.”
With the fluorine mixture, the researchers zapped the rock with a laser for about two minutes under low laser power when it starts melting, and with high power until it vaporizes. In order to assure that only the oxygen gas is collected, they put it through a series of cleanup steps. After, the samples went into a mass spectrometer to determine the relative proportions of the three different stable isotopes of oxygen. Mineralogy, petrography and chemistry of the sample are studied on different aliquots of the meteorite.
“It was highly shocked and filled with sulfide-rich impact melt veins,” Ziegler said of the Chelyabinsk meteorite.
One of Ziegler’s samples was light grey in color with abundant relict chondrules and recrystallized matrix containing metal and sulfide.
“Another sample was a dark grey/black impact melt material, consisting of a matrix that is quenched silicate melt, including finely disseminated sulfides. It was marked with abundant partly resorbed grains derived from the Chondrite host entrained in the melt material.” said Dr. Rhian Jones from the Department of Earth and Planetary Sciences, who studied the petrography of this meteorite in an independent study.
There are several ways to classify (meteorites) through mineralogy, petrography, chemistry and abundance of certain metals that test for different meteorite classes. The results from Ziegler’s study showed that all the data from the Chelyabinsk meteorite fell into the oxygen 3-isotope space occupied by the LL-group of the Ordinary Chondrite meteorites and indicated that Chelyabinsk is heterogenous both on a large (different stones) and small scale in terms of oxygen isotopes.
“It was an ordinary chondrite,” Ziegler said. “There weren’t really surprises with the results of the oxygen isotope classification. It was very rewarding because we get the oxygen isotope results back right away. The classification matched what others had found based on mineralogy, petrography, and chemistry. However, the impact material was more interesting than initially thought, as it is the same material as the meteorite itself is made of. This meteorite fireball was interesting because we were able to observe the meteoroid’s spectacular entry into the Earth’s atmosphere, and saw it explode and rain down over Russia, and this is a major, once in a lifetime event.”
Mapping the Extent of the Damage
The team led by Jennisken visited 50 villages in the area to map out the extent of the glass damage created by the shockwave and interview eye witnesses. The map of damaged buildings has the form of a butterfly, stretching in a narrow swath out to 90 kilometers on either side of the trajectory. The city of Chelyabinsk, with over a million inhabitants, was right in the path of the wave. The team showed that the shape of the damaged area could be explained from the fact that the energy was deposited over a range of altitudes.
"Most of the shockwave destructive power was obtained during the fragmentation event", says Popova who is an expert on bolide entry modeling, "but in order to explain the arrival times of the shockwave, we calculated that the fragmentation of the larger fragments that survived this breakup down to 23 km altitude contributed as well."
When it reached 30 km altitude, the meteor brightness peaked. For observers near the trajectory, it was brighter than the sun at that time, creating so much ultraviolet emission that some people were severely sunburned and lost skin later on. Most avoided lasting eye damage by looking away.
At that moment, the meteoroid catastrophically fragmented, creating the debris that later would be recovered as meteorites on the ground just south of the trajectory in a long stretch from Aleksandrovka to Deputatskiy and Timiryazevsky. The team estimated that only 4,000 to 6,000 kilogram of meteorites fell on the ground, only 0.03-0.05 percent of the initial mass of the meteoroid. Seventy-six percent of the meteoroid evaporated, with most of the remaining mass converted into dust.
"The reason so little survived is that the radiation was so intense it contributed to evaporating the fragments before they could fall as meteorites out of this cloud,” Popova said.
That cloud was so hot it glowed orange and settled to 26 km altitude. Above this location, a small mushroom cloud developed by the rising hot air. From the very beginning, a large amount of dust was created and the dust cloud split in two due to buoyancy of the hot air. That splitting was also seen in the wake of the fragments that survived the main fragmentation event, one of which impacted Lake Chebarkul.
Ural Federal University researchers, led by meteoriticist and co-author of the report Professor Viktor Grokhovsky, initially found small pieces of the meteorite around a 7-m sized hole in the ice and set out to determine the position of the fallen rock in a systematic manner using sophisticated metal detectors. After many months of searching, eventually a 650 kg meteorite was recovered from deep in the mud by professional divers on Oct. 16.
Shoreline security camera video recorded how this meteorite impacted the ice. A cloud of ice and smoke is seen rising in the air and then drifting away in the wind. This is the first time that the actual meteorite impact was recorded on video. The timing of the fall enabled Jenniskens and Popova to calculate an impact speed of 225 meters per second.
Researchers were most interested in finding out why the meteoroid fragmented so readily at 30 km altitude and did not penetrate deeper.
"We suspect it is the abundance of shock veins in this rock," says NASA Ames meteoriticists Derek Sears. "When we pressed on the rock, it broke along one of these shock veins." NASA JSC cosmochemist Mike Zolensky may have found why these shock veins were so frail. They contained a layer of small iron grains at each rim, which had precipitated out of the glassy material when it cooled.
"Impacts on the parent body fractured the rock and pushed molten iron and iron sulfides through the cracks," says Zolensky. "There are cases where this increased a meteorite's mechanical strength, but Chelyabinsk was weakened by it."
Jenniskens speculates that Chelyabinsk belonged to a bigger rubble pile asteroid before it broke apart 1.2 million years ago, possibly in an earlier close encounter with Earth.
"The rest of that rubble could still be around as part of the near-earth asteroid population," says Jenniskens, "but based on our calculated orbit it is not one of the earlier proposed parent body objects. We are still looking for it."
For more on the Chelyabinsk Meteorite, visit the Field Campaign Blog.