The discovery of hundreds of huge comets in the outer Solar System over the last 20 years means that these ancient objects pose a real hazard to our civilization, says a group of astronomers led by University of Buckingham scientist Bill Napier.
Saturn’s small moon Phoebe, depicted in this image, seems likely to be a centaur that was captured by the gas giant’s gravity at some time in the past. Image credit: NASA / JPL-Caltech / Space Science Institute.
Massive comets, also known as centaurs, are 30 to 50 miles (50 to 100 km) across, or even larger. They move on unstable orbits crossing the paths of Solar System’s gas giants – Jupiter, Saturn, Uranus and Neptune.
The planetary gravitational fields can occasionally deflect these ancient comets in towards our planet.
Calculations of the rate at which centaurs enter the inner Solar System indicate that one will be deflected onto a path crossing the Earth’s orbit about once every 40,000 to 100,000 years.
Whilst in near-Earth space they are expected to disintegrate into dust and larger fragments, flooding the inner Solar System with cometary debris and making impacts on Earth inevitable.
“The disintegration of such giant comets would produce intermittent but prolonged periods of bombardment lasting up to 100 000 years,” said Prof. Napier and his colleagues from the University of Buckingham and Armagh Observatory, UK.
“Mass extinction/geological boundary events on Earth show such a pattern, as do levels of dust and meteoroids in the upper atmosphere.”
Specific episodes of environmental upheaval around 10,800 BC and 2,300 BC are also consistent with this new understanding of cometary populations.
“Over the past 10,000 years, Earth has been experiencing the intermittent arrival of dust, meteoroids and comet fragments from the disintegration of comet 2P/Encke, trapped within the orbit of Jupiter,” the scientists said.
Prof. Napier and co-authors have also uncovered evidence from disparate fields of science in support of their model.
For example, the ages of the sub-millimeter craters identified in lunar rocks returned in the Apollo program are almost all younger than 30,000 years, indicating a vast enhancement in the amount of dust in the inner Solar System since then.
“Our work suggests we need to look beyond our immediate neighborhood too, and look out beyond the orbit of Jupiter to find centaurs,” said Prof. Napier, who is the lead author of a paper published in the journal Astronomy & Geophysics.
“If we are right, then these distant comets could be a serious hazard, and it’s time to understand them better.”
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Bill Napier et al. 2015. Centaurs as a hazard to civilization. Astronomy & Geophysics 56 (6): 6.24 – 6.30; doi: 10.1093/astrogeo/atv198
