Illustration of Milky Way Galaxy
In a bizarre geological twist of fate, researchers report that the very continents on which we humans call home were likely a byproduct of four-billion-year-old giant Earth impactors incredibly triggered by passages through our Milky Way’s spiral arms.
In a paper just published in the journal Geology, an Australian-led team details research which indicates that at least some of Earth’s six continents appear to have had a sizeable start from impactors triggered during periodic passes through these galactic arms.
The authors insist that dense interstellar clouds within the galaxy’s spiral arms may have triggered repeated perturbations of our solar system’s outlying Oort cloud of comets onto high-energy inward trajectories towards Earth.
On roughly 200 million-year periodicities, repeated cometary Earth impactors likely seeded enhanced production of our planet’s continental crust, the Geological Society of America (GSA) reports.
The authors say the proof of such cosmic periodicities can be found in zircon mineral crystals, both in the Pilbara region of Western Australia and in Greenland.
Formation of Earth’s continental crust went through cycles, with periods of increased crust production roughly every 200 million years, the GSA notes, corresponding to transits through our galaxy’s four spiral arms. In both Australia and Greenland, decay of uranium in zircon crystals has been used to establish a timeline of formation, spanning the period from roughly 2.8–3.8 billion years ago, the GSA reports.
We infer an increase in Earth impacts on entry into and exit out of the arms, says Kirkland. Entry into the arms is important because of gravitational perturbations during transit into the arm, he says. That’s because the compression of gas passing into the galactic arm may cause a disturbance of our Solar System’s Oort cloud, Chris Kirkland, the paper’s lead author and a Geochronologist at Australia’s Curtin University in Perth, told me.
Illustration of a Voyager probe at the Oort cloud. The two Voyager probes were launched in late … [+]
The exit out of a spiral arm is important, because by that point our solar system would have spent more time in a region of space with higher densities of stars. Thus, this would increase the probability of multiple stellar encounters in a manner consistent with more Oort Cloud-triggered Earth impactors.
“We think that impacting might just have been the first stage process before other crust formation processes started or became preserved,” said Kirkland.
Impacts caused melting that created buoyant crust that allowed our continents to form, says Kirkland.
Earth is connected to the cosmos through periodic comet bombardment, he says.
Although that’s a concept that will undoubtedly be hard to accept by some geologists, Kirkland likens these findings as paradigm-shifting for geology. That’s because, as he points out, the science of geology traditionally views processes on Earth as being driven solely by internal processes on the planet.
Why are zircons a classic signature of such giant impactors?
Zircons are the premier timekeeper for geologists because they contain uranium that decays to lead, so we know their age, says Kirkland. Zircon will also trap other elements as they grow, some of which can tell us about where the liquid that grew the zircon came from, he says.
In other words, did the liquid (magma) come from Earth’s underlying mantle or simply from remelting of our young planet’s existing crust?
By looking at the frequencies of “mantle” versus “existing crust” magma we can see a frequency that matches that for solar system transit through the galactic arms, says Kirkland. Oxygen isotopes in zircon indicate that during these impact periods the melting of Earth’s existing crust was shallow – consistent with a “top-down melting process,” he says.
This data thus points to shallow melting which would indicate crustal melting triggered via giant impactor rather than from magma originating internally from Earth’s underlying mantle.
But how did the researchers successfully trace the motions of our solar system around our spiral galaxy back over billions of years?
We have tracked the known movement of these different parts of our galaxy, our solar system and other stars backwards in time, using both previous models of the movement of the solar system and our own data, Phil Sutton, a co-author and astrophysicist at the University of Lincoln in the U.K., told me.
What’s the biggest surprise?
That what we stand on, and where the majority of Earth’s biomass lies, is connected to rhythmic disturbances of our solar system’s flight through the galaxy, says Kirkland.
