The rapid recovery and analysis of a meteorite that fell onto a frozen lake in Michigan two years ago is illuminating the pivotal role these rocks played in delivering the basic building blocks for life to Earth.
As the meteorite was falling toward our planet on January 16, 2018, at speeds reaching 57,936 km per hour, its bright fireball could be seen in parts of the U.S. midwest and portions of Ontario. Hundreds of witnesses were privy to the lightshow, in an event that was captured by multiple security cameras.
And with that, the race was on to retrieve the meteorite as quickly as possible. The more time spent on the surface, and with possible exposure to water, the more meteorite would become contaminated, preventing scientists from studying the rock as it existed in space. The big prize, as it were, is the recovery of pristine, untainted extraterrestrial organic compounds, that is, carbon-based molecules that formed within the rock’s parent asteroid.
“As soon as you get water, the metal starts to rust, and minerals like olivine get altered,” Philipp Heck, a curator at the Field Museum in Chicago and the lead author of a new paper describing the meteorite, explained in an email. “Water also brings in contaminants through the many cracks that usually crisscross meteorites — cracks that formed when the meteorite got ejected from its parent asteroid during a previous impact event,” he added.
Using NASA’s weather radar, meteorite hunters tracked the velocity and trajectory of the meteorite, allowing them to pinpoint the likely location of the fallen object. In under 48 hours, a private meteorite hunter named Robert Ward found a 22-gram chunk of meteorite resting on the frozen Strawberry Lake near Hamburg, Michigan. Ward and private collector Terry Boudreaux decided to quickly donate and deliver the piece, dubbed the Hamburg meteorite, to the Field Museum in Chicago.
“This study is a beautiful example of how citizen scientists such as Boudreaux and Ward can make meaningful contributions to science in collaboration with scientific institutions,” said Heck, who’s also an associate professor at the University of Chicago. “Hamburg is one of the very few meteorites that was quickly recovered from a frozen surface and delivered to scientific institutions, and that is what makes this meteorite remarkable.”
Several other smaller pieces belonging to the same meteorite were recovered on the same day, while an additional 13 pieces were found within two weeks of the fall, according to the new paper, published today in Meteoritics & Planetary Science.
Now in possession of the rock, Heck, along with University of Chicago graduate student Jennika Greer, went to work studying it. They employed a variety of different methods, including weather radar, microscopy, spectroscopy, different types of mass spectrometry, magnetometry, and CT scanning.
The Hamburg meteorite was classified as an H4 chondrite, which is relatively rare, as only 4% of all meteorites that currently fall to Earth belong to this group. H4 chondrites are interesting because they were blasted with heat while being ejected from their parent asteroid. That means some of the original components, such as chondrules (for which these rocks are named after), are “still preserved and visible,” said Heck, adding that chondrules are solidified droplets of melted rock.
The quick retrieval of the meteorite paid off, as the team was able to analyse a high diversity of untainted organic compounds. Meteorites like these may help to explain how these compounds arrived on Earth during its primordial period. Importantly, these compounds aren’t a form of extraterrestrial life, nor are they biomarkers, but they do constitute some the basic ingredients from which life was able to emerge over 3 billion years ago.
In total, the team found 2,600 different organic compounds in the Hamburg meteorite.
“These compounds formed in the parent asteroid right after it formed, when it was still hot from accretion and from the decay of radioactive elements that were still present in the early solar system,” Heck explained. “Even though there are some meteorites, like carbonaceous chondrites, which are a thousand times richer in organics, the fact that this ordinary chondrite meteorite was rich in organics provides support of the hypothesis that meteorites played an important role in delivering organic compounds to early Earth.”
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Because the Hamburg meteorite was quickly recovered, it experienced minimal contamination, but Heck said the only truly uncontaminated samples will be those collected directly from asteroids, such as the samples recently scooped up by NASA’s OSIRIS-REx spacecraft and JAXA’s Hayabusa2 probe.
“It will be interesting to compare meteorites to the mission returned samples,” said Heck.