"You're more likely to have a collision when you're in a galactic arm and the increased density sends a comet towards Earth".
The timing of some major extinction events on Earth coincides with the solar system's journey through Milky Way's spiral arms, suggests a new study. The research, published on the pre-press website ArXiv.org
, supports the idea that mass extinction events were not always random.
The Sun spends 50 to 60 per cent of its 220-million-year journey around the galaxy passing through its spiral arms, says study co-author Dr Jonti Horner of the University of New South Wales
"These are regions of higher than average density, where there are more stars and molecular gas and dust clouds," says Horner.
"It could be argued that the increase in the number of stars encountered as the Sun moves through a galactic arm, can trigger gravitational perturbations, sending comets from the Oort cloud towards the inner solar system, where the Earth is."
The Oort cloud is a hypothetical reservoir of comets and other icy bodies half way to the Sun's nearest stellar neighbour. Together with vast volcanic outpourings of flood basalt magmas, such as the Deccan and Siberian Traps, and snowball Earth periods of global glaciation, asteroid or cometary impacts are considered a likely cause of mass extinction events on Earth.
According to Horner, the Earth impact database currently lists 182 large craters caused by asteroid and comet collisions, and these only represent a tiny fraction of Earth's true impact history, the rest being erased by weathering and geological events.
He says the far more heavily scarred lunar surface, provides a better indication of the true level of major impact events.
While both flood basalt outpouring and global glaciations appear rare and occur randomly in Earth's history, one of the most intriguing ideas suggests mass extinctions are not randomly distributed, but follow a specific timing pattern.
"The timing's never going to be perfectly precise, but when you look at the data, there's always been this nagging doubt that the extinctions won't totally randomly distributed through time, but were vaguely periodic," says Horner.
"We can't be sure [extinctions were periodic], but if they were, we wanted to look at what could possibly cause them."
The six largest and best-known extinction events are the Late Cambrian 488 million years ago; the Late Ordovician 445 million years ago; the Late Devonian 375 million years ago; the Permian-Triassic 251 million years ago; the Triassic-Jurassic 200 million years ago; and the Cretaceous-Tertiary 66 million years ago.
Recently, researchers have also identified five additional extinction events, during which time significant drops were recorded in species diversity. These have been dated to about 415, 322, 300, 145 and 33 million years ago.
"If we imagine for a moment that the only cause for extinction events involved collisions with comets, then you're more likely to have a collision when you're in a galactic arm and the increased density sends a comet towards Earth," says Horner.
Horner and colleagues used an existing model of the Milky Way to determine the position of the Sun around the time of each of event.
They found most, but not all, matched up with the Sun passing through one of the known galactic arms.
Then Horner and colleagues developed a second model to predict what happens on the far side of galaxy, which we can't see and know very little about.
"We hypothesised that on the far side of the galaxy there's a bit of structure like we have locally, there's a little spur in the spiral arm," says Horner.
"If you do that, then suddenly every one of these mass extinctions lands in a period when we're in a spiral arm.
"That's not saying every mass extinction is caused by us moving through a spiral arm, you still have snowball Earth and the traps, but it's a tantalising hint."