GEORGIA, USA: Was the demise of the dinosaurs 65 million years ago somehow linked to our Solar System’s journey through the galaxy?
The way our Sun and its retinue of planets bounces through the galactic plane may occasionally send comets hurtling to Earth, say researchers.
In a study to appear in the U.K. journal Monthly Notices of the Royal Astronomical Society, experts at Cardiff University in Wales used computer modelling to establish a causal link between our movement through the Milky Way and periodic cometary bombardments capable of creating mass extinctions.
Distant comet reservoir
For years, astronomers and palaeontologists have argued as to whether there’s a periodicity in Earth’s giant impact record that is somehow related to an increased influx of comets from the Oort cloud – a distant reservoir of comets circling the Solar System’s outer fringes.
We all know the Earth circles the Sun. What is less appreciated is just how the Earth and Solar System rotates through the spiral arms of the Milky Way. It crosses through the galactic disk once roughly every 41.7 million years, all the while oscillating up and under the discs mid-plane.
Astrobiologist William Napier and his team ran simulations of the Solar System’s two directions of motion, while also factoring in the Sun’s random encounters with giant molecular star forming clouds. Napier said that passages through these denser parts of the galaxy are responsible for the Oort cloud disturbances.
His team argues that the Solar System passed through the Sagittarius-Carina arm about 34 million years ago, and the Scutum-Crux arm about 142 million years ago. Both these dates coincide with heavy bombardment episodes involving large impact craters and mass extinctions. The periodicity argument, however, is not just based on the impact that killed the dinosaurs but also the Permian-Triassic mass extinction event that took place 250 million years ago.
Michael Rampino, a geologist at New York University in the U.S., routinely tracks putative periodicities in the extinction record, and he’s not convinced on this point. Rampino, who was not an author of the new paper, maintains that this 250 million year-old extinction puts a squelch on Napier’s periodicity claims since it has now been attributed to Earth-bound volcanic eruptions rather than bombardment.
Even so, the Cardiff group argues that each new massive cometary impact not only brings about massive destruction to the home planet, but also a chance for such impacts to literally throw microbial life out into space. Known as panspermia, the idea is that microbial life can move either from one planet to another within the same Solar System; or even to nearby giant molecular clouds thought to harbour planetary systems.
Astrobiologists have no evidence that microbes from the impact that led to the dinosaur extinction may have escaped earth’s atmosphere; much less made their way to Mars or even beyond the outer envelope of our own Solar System. Yet Napier and colleagues believe that given the number of random encounters an Earth-like planet may have in its journey through the galaxy, such mechanisms could lead to the propagation of microbial life throughout significant sectors of the galaxy.
“There are perhaps a billion microbugs in a gram of modestly fertile soil,” said Napier, meaning a big impact could eject a massive quantity of microbes into space. If so, large scale cometary impactors may signal the death knell for a wide variety of species, but could possibly lead to a whole new generation of others in a neighbouring star system.
Yet Max Bernstein, an astrobiologist at NASA Ames Research Centre in Mountain View, California, remains sceptical, however. “No matter what their model says about the frequency of impacts from the Oort cloud,” he said. “The notion that micro-organisms survive ejection from Earth by giant impacts, interstellar travel, and incorporation into newly forming planetary systems remains fantastic.”
In other news this week, even if the 65-million-year-old impact was not due to a periodic encounter with a nearby molecular cloud, it likely set off a global firestorm of oily sediments.
As they detail in the journal Geology, researchers at Indiana University in Bloomington, U.S., found evidence that this in turn pumped huge amounts of carbon dioxide into the atmosphere and may have contributed the extinction of many dinosaurs.