Interplanetary storm

A meteoric storm raged over the Earth 13,000 years ago as thousands of pieces of rock each the size of the Tunguska comet rained down over the course of an hour. The end result was a dramatic cooling of the planet, according to astronomer Bill Napier of the Cardiff University Astrobiology Centre.

Writing in the journal Monthly Notices of the Royal Astronomical Society, Napier suggests that the temperature drop was as high as 8 Celsius and interrupted global warming at the end of the last ice age, causing glaciers to re-advance.

Scientists have puzzled over a boundary layer marked by the occurrence of a “black mat” tens of millimetres thick present at sites throughout the United States, which contains high levels of soot from continental-scale wildfires and nanoscopic hexagonal diamonds found only in meteorites or impact craters. The evidence hinted at a catastrophic change at that time caused by the impact of an asteroid or comet 4 km across on the Laurentide ice sheet, which at that time covered what would become Canada and the northern part of the United States.

Napier points out that the cooling lasted a more than a millennium and led to the rapid extinction of 35 genera of North American mammals, as well as the disruption of the Palaeoindian culture. However, the chances of an asteroid impacting the Earth during that period were extremely low. Moreover, the heat generated by the rising fireball would be limited by the curvature of the horizon and could not have led to the continent-wide occurrence of wildfires.

Napier has now devised a model that can account for the evidence.
According to Napier’s model, the Earth ran into a dense trail of material from a large disintegrating comet. He points out that there is compelling evidence that such a comet entered the inner planetary system between 20,000 and 30,000 years ago and has been fragmenting ever since, giving rise to a number of closely related meteor streams and asteroids known as the Taurid Complex.

As the comet disintegrated, the Earth would have ploughed through at least one dense swarm of cometary fragments over an hour-long period.
Thousands of individual impacts would have occurred across what is now continental America, each releasing the energy of a megaton atomic bomb and triggering extensive wildfires.

2005 Hubble Space Telescope image of the breakup of a comet (73/P Schwassmann-Wachmann 3).
2005 Hubble Space Telescope image of the breakup of a comet (73/P Schwassmann-Wachmann 3). Credit: NASA / ESA / H.Weaver (JHU/APL) / M. Mutchler / Z.Levay (STScI)

“A large comet has been disintegrating in the near-Earth environment for the past 20,000 to 30,000 years, and running into thousands of fragments from this comet is a much more likely event than a single large collision. It gives a convincing match to the major geophysical features at this boundary,” says Napier. Indeed, a recent meteorite which may have come from this giant comet progenitor fell on Yukon Territory in January 2000 and has the highest abundance of nanodiamonds of any meteorite so far analysed.

Links.
Monthly Notices Royal Astronom Soc, 2010, in press Preprint link
Cardiff staff

Over and Oort on the comet’s tale

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An enormous asteroid or comet smashing into the Earth 65 million years ago killed off the dinosaurs. But, according to a new study by US scientists, published in the wake of an impact event on Jupiter, cometary collisions with Earth probably didn’t cause any more than one other extinction event during life’s history.

US researchers have looked to the Oort Cloud to help them work out just how many comets may have struck the earth during the time complex life has existed. The Oort Cloud is a remnant of the nebula from which the solar system formed 4.5 billion years ago. It starts about 93 billion miles from the sun (a thousand times as far from the sun as the Earth) and stretches to an incredible three light years away. It is thought that the Oort Cloud contains billions of comets but most are so small and distant that we will never see them.

Now, Thomas Quinn, Nathan Kaib and colleagues at the University of Washington, have used computer models to simulate the evolution of comet clouds in the solar system over the last 1.2 billion years. They found that even outside the periods of comet showers, the inner Oort Cloud was a major source of long-period comets. Some of these can cross Earth’s path but the simulations show that no more than two or three comets could have struck the Earth even during the most powerful comet shower of the last 500 million years.

Astronomers have known for some time that the inner solar system of rocky planets – Earth, Mars, Venus, and Mercury – is protected from comets by the gas giants Saturn and Jupiter. The enormous gravitational fields of these planets essentially eject comets into interstellar space or act as traps for them. This latter point was reinforced on 20th July when a huge scar appeared on Jupiter’s surface, likely evidence of a cometary impact.

There are about 3,200 known long-period comets of which Hale-Bopp is perhaps the most famous as it was visible with the naked eye for much of 1996 and 1997. Some long-period comets take thousands if not millions of years to make a single orbit of the sun. Comet Halley by contrast, which reappears every 75 years, is a short-period comet, which originate in a different part of the solar system known as the Kuiper Belt and are much more predictable. The computer simulation by Quinn and colleagues assumed that the inner Oort Cloud is the only source of potentially dangerous long-period comets.

The long-period comet 2001 RX14
The long-period comet 2001 RX14

“For the past 25 years, the inner Oort Cloud has been considered a mysterious, unobserved region of the solar system capable of providing bursts of bodies that occasionally wipe out life on Earth,” Quinn explains, “We have shown that comets already discovered can actually be used to estimate an upper limit on the number of bodies in this reservoir.”

With three major impacts taking place almost simultaneously, researchers had suggested that the minor extinction event of 40 million years ago resulted from a comet shower. Kaib and Quinn’s research implies that if that relatively minor extinction event was caused by a comet shower, then that was probably the most-intense comet shower since the fossil record began.

Further resources

Science Express, 2009, Jul 30, online

Tom Quinn’s home page

Comet shower movie from Nathan Kaib

Cosmic nanodiamonds

Tiny particles of crystalline carbon found in sediments at six sites in North America dating back almost 13000 years, suggest that a swarm of carbon-and-water-rich comets smashed into the Earth triggering extinctions of large mammals, including the woolly mammoth, across the globe.

The nanodiamonds from these carbonaceous chondrites are formed under high-temperature and high-pressure conditions created at impact and have been found in meteorites in similarly aged sediments at Murray Springs, Arizona, Bull Creek, Oklahoma., Gainey, Michigan, and Topper, South Carolina, as well as at Lake Hind, Manitoba, and Chobot, Alberta, in Canada. Their existence represents evidence for the cause of a severe cold snap that last 1300 years known as the Younger Dryas period.

Douglas J. Kennett

Douglas J. Kennett

The Clovis culture of hunters and gatherers was named after hunting tools referred to as Clovis points, first discovered in a mammoth’s skeleton in 1926 near Clovis, New Mexico. Researchers later identified Clovis sites later across the American continent. The Clovis people presumably reach the Americas via a land bridge from Siberia and their culture is thought to have peaked around 10,900 years to 12,500 years ago. The Younger Dryas period is thought to have fragmented the prehistoric Clovis culture and led to mass extinctions of many animals across North America.

In 2007, a 26-strong research team from sixteen institutions proposed that a cosmic impact event, possibly due to multiple airbursts of comets was the trigger for the Younger Dryas. Now, a team led by the University of Oregon’s Douglas Kennett, a member of the original research team, report the discovery of billions of nanometre-sized diamonds concentrated in sediments in the same six locations. One of the diamond-rich sediment layers reported sits directly on top of Clovis materials at the Murray Springs site.

The nanodiamonds that we found at all six locations exist only in sediments associated with the Younger Dryas Boundary layers, not above it or below it, explains Kennett. These discoveries provide strong evidence for a cosmic impact event at approximately 12,900 years ago that would have had enormous environmental consequences for plants, animals and humans across North America.

Further reading

Science, 2009, 323, 94
http://dx.doi.org/10.1126/science.1162819

Douglas J. Kennett homepage
http://www.uoregon.edu/~dkennett/Welcome.html

UO researchers involved in Clovis impact theory
http://pmr.uoregon.edu/science-and-innovation/uo-research-news/research-news-2007/september-2007/uo-researchers-involved-in-clovis-age-impact-theory