The Earth is mostly water, so most meteors crash into the middle of the ocean quietly. Even those we notice dry lands are rare and, for astronomers, precious. They are fragments of the mysterious environment in that vast dark space between the planets of our solar system … or even in the darkness beyond the solar system.
The Desert Fireball Network, a free organization of Australian scientists centered around Curtin University in Perth, went on a search for the August 2016 meteorite, which is what we call a meteorite after it lands. Noting that the rock had briefly orbited the Earth before plunging into the desert, they even gave it a clever nickname: Minimoon.
They finally found it two years later! The celebrations were in order. Astronomers could add the 1.2-ounce rock, the size of an AA battery, to their small but growing collection of recovered meteorites, each a piece of the interplanetary puzzle.
But the party didn’t last. The DFN’s closer inspection of the meteorite found in the Australian orange desert has led to a shocking conclusion. It was a rock from space, for sure. But it was the wrong rock from space.
The DFN meteorite recovered from the sand, roughly within the predicted impact zone of the 2016 fireball, was not the same meteorite that had caused the fireball. “A rogue meteorite,” is how Martin Towner, a Curtin University researcher and DFN operations manager, described the recovered rock to The Daily Beast.
Incredibly, the Australians went in search of a rare space rock and found an unrelated one rare space rock. The odds of this happening are hard to calculate, but the Australian team tried. Spoiler: they are low.
Now the Minimoon mishap is a warning. As astronomers scour the planet for meteorites, they must be careful to trace the origins of each rock. If they link a meteorite to the wrong fireball, evidence of a rock’s travel through the atmosphere, they risk drawing the wrong conclusions about the region of space where a given meteorite originated. They could corrupt entire fields of science.
“This is a key example to demonstrate that fireball-meteorite pairings should be carefully tested,” Towner and other DFN team members wrote in a new study.
By calculating the trajectory of the fireball, the DFN team had narrowed the impact zone to an area of approximately 170 acres. It took a couple of years to organize an expedition due to the extreme remoteness of the impact zone – a two-day drive from Perth.
“Searching on the clays was good,” Towner told the Daily Beast, using a term for a clay-rich depression, “but on the dunes it was a bit hit and miss, with some loose sand that could bury stuff and dense. bushes under the trees. “
“If they link a meteorite to the wrong fireball, evidence of a rock’s travel through the atmosphere, they risk drawing the wrong conclusions about the region of space where a given meteorite originated. They could corrupt entire fields of science.“
The four-person team crawled over the potential impact zone, looking for the telltale signs of an extraterrestrial rock. Round in shape. Dark in color. Dense and therefore heavy. When they finally found a meteorite, after six days of searching, it was only a hundred meters from where they expected to find one.
In case there were multiple fragments of the same meteor, the team searched for another two days and found nothing. They ran back to their labs to analyze the rock. An obvious test was to judge how smooth the meteorite was. The smoother a space rock is, the longer it has remained on Earth undergoing a slow and steady process of smoothing by windswept earth or sand.
The DFN team rated the newly recovered meteorite as only “slightly” altered. “It may have been tempting to attribute the mild degree of weathering to the two years spent on land,” the team wrote in their study (which appeared online July 12 and has not yet been peer-reviewed). In other words, the smoothness was consistent with a recently landed rock.
Scientists therefore had reason to believe they had found Minimoon. “It was sitting on the sand, it was about the right size, about the right place, and it looked pretty cool, and it’s not like you often find meteorites when searching,” Towner said. “So we were happy enough at the time!”
But the next test shattered their joy. The DFN team chiseled a quarter-gram piece of the meteorite, crushed it and burned it. Using a technique called accelerator mass spectrometry, they bombarded the resulting gas with electrons. Different elements collected different charges, modifying their weight and making their differentiation possible.
It’s all very technical, but the result, after careful analysis, was that the DFN team was able to estimate how many rapidly decaying radioactive particles the rock still possessed. Some subatomic particles called radionuclides, coming from cobalt and manganese atoms among others, come from space and do not survive on Earth for long.
If a meteor still has these nuclides, it is “fresh”. That is, having landed in the last thousand years or so. If it doesn’t have nuclides, it has had a greater impact on Earth compared to a thousand years ago.
The rock had no telltale nuclides. The DFN team estimated it hit the desert at least 1,900 years ago. In other words, it wasn’t Minimoon. It was completely different meteorite that just landed in the same area that Minimoon probably did, based on the latter’s trajectory.
Scientists sifted through past investigations and concluded that the wrong fireball-meteorite pairings – in which scientists observe a falling meteor, go looking for it on the ground, and find the wrong space rock – are likely rare. As in, it occurs in no more than one in 50 meteorite surveys that also involve strong fireballs.
These misdirected identities, while rare, are a big problem. We are improving more and more in fireball detection and analysis. The US military even tracks them using various sensors and periodically releases the data. The most recent version, this spring, included data on about a thousand fireballs dating back to 1988.
The data, speed, duration, brightness, and color of a fireball offer clues to a meteor’s internal structure. The faster a meteor is, the farther from Earth it could have originated. The color, brightness, and duration could indicate the composition and size of a meteor’s minerals.
Finding a meteorite on the ground gives scientists the opportunity to confirm and add any conclusions they might draw from observing a fireball. Perhaps a particularly fast fireball appears to come from far away, perhaps even traveling to Earth from beyond the solar system. Scientists would like to know which minerals make up such a strange and distant rock. The implications for planetary formation are profound.
“Scientists would like to know which minerals make up such a strange and distant rock. The implications for planetary formation are profound.“
But these comprehensive analyzes of fireball-meteorite pairings only work if space rock hunters pair the right fireballs and meteorites. Don’t mix fireballs and meteorites and they could all draw the wrong conclusions.
Since fireballs are rare and finding an intact meteorite is even rarer, complacency can arise. Scientists witness a fireball, search for the meteorite, find one in or near the predicted impact zone, and assume the two are related.
As the Australians have discovered, this is not a sure assumption. There are enough meteorites scattered around Earth that scientists sometimes search for one space rock and accidentally find another.
Together, the Minimoon fireball and the unrelated South Australian meteorite are “a warning,” Towner said. “Just because it looks right and is in the right place is not enough – you have to go through the entire chain of analyzes in the lab where possible to confirm that it is the right one.”
If you don’t, you could end up doing bad science.
There is a corollary to this unlikely story. Minimoon should still be out there, somewhere in the Australian desert. “If he landed, he’d still be around,” Towner said. “Even though it’s been a while and the autumn area has sand dunes and plants that can move in the wind or grow, so there’s a chance she’s been buried and lost by now.”