View Full Version : Earth's Astroblemes

21st June 2015, 19:19
Dual asteroid strike hints at chaos in the inner solar system (http://www.newscientist.com/article/mg22630264.000-dual-asteroid-strike-hints-at-chaos-in-the-inner-solar-system.html#.VYb3FRNg7fE)

by Jeff Hecht (http://www.newscientist.com/search?rbauthors=Jeff+Hecht) 17 June 2015
Magazine issue 3026 (http://www.newscientist.com/issue/3026).

Popigai crater (https://en.wikipedia.org/wiki/Popigai_crater)

SOMETIME between the demise of the dinosaurs and the rise of primates, Earth suffered a one-two punch from two unrelated asteroids. The double whammy could signify chaos in the inner solar system, triggering a cascade of events that perhaps plunged Earth into the recent ice age.

The impacts that made the Popigai crater in Siberia and the Chesapeake Bay crater (http://www.newscientist.com/article/mg18324581.200-dual-asteroid-strike-hits-comet-theory.html) on the US East Coast were two of the largest asteroid strikes (http://www.newscientist.com/article/mg17623662.200-its-only-the-really-big-asteroid-impacts-that-are-a-threat-to-life-on-earth.html) since the one that killed the dinosaurs, so big that their dust left thin layers of debris all over the globe.

Chesapeake Bay impact crater (https://en.wikipedia.org/wiki/Chesapeake_Bay_impact_crater)

The depth of debris layers shows that the asteroids struck in quick succession – within 20,000 years of each other, around 36 million years ago. A Popagai-sized impact happens just once every 20 million years. So the thinking was that the two rocks were fragments of one larger asteroid.

But when Birger Schmitz (http://www.lunduniversity.lu.se/lucat/user/2374cda9e07da82f19e893d3eeee7974) of Lund University in Sweden examined sediment from a site in central Italy containing debris from both impacts, he found two distinct types of meteoritic grains. Those from the part of the deposit associated with Popigai were rich in iron; those from the part associated with Chesapeake Bay were iron-poor. So the pair can't be explained by a single body breakup – they must be from two separate asteroids (Earth and Planetary Science Letters, doi.org/5cb (http://doi.org/5cb)).

But how could that happen? One explanation is that chaos in the inner solar system (http://www.newscientist.com/article/mg18124365.100-chaotic-heavens.html) disturbed asteroids' orbits, sending several Earthwards.

It's even possible that a change in Earth's own orbit sent the asteroids on their paths. Such a shift might explain why Earth entered an ice age around 35 million years ago: a new orbit meant less energy from the sun.

"The analysis of the grains is very good and convincing," says David Nesvorny (http://www.swri.org/iprofiles/ViewiProfile.asp?k=i60j401q6e7508n) of the Southwest Research Institute, San Antonio, Texas. But some meteorites are simply made of many kinds of rock, he says.

This article appeared in print under the headline "Dual strike hints at chaos in the asteroid belt"

For similar stories, visit the Solar System (http://www.newscientist.com/topic/solar-system) and Comets and Asteroids (http://www.newscientist.com/topic/comets-asteroids) Topic Guides

22nd June 2015, 12:53
And then also there's the strike that nobody talks about. The strike that separated Antartica from South America. Its' path is clearly visible on Google Earth, or any globe for that matter, when one knows what to look for.

28th July 2015, 14:29
Comprehensive analysis of impact spherules supports theory of cosmic impact 12,800 years ago (http://phys.org/news/2013-05-comprehensive-analysis-impact-spherules-theory.html#nRlv)

May 21, 2013

http://cdn.phys.org/newman/csz/news/800/2013/2-comprehensiv.jpg (http://cdn.phys.org/newman/gfx/news/hires/2013/2-comprehensiv.jpg)
The researchers studied the impact spherules in 18 sites in nine countries on four continents for this study. Credit: YDB Research Group

About 12,800 years ago when the Earth was warming and emerging from the last ice age, a dramatic and anomalous event occurred that abruptly reversed climatic conditions back to near-glacial state. According to James Kennett, UC Santa Barbara emeritus professor in earth sciences, this climate switch fundamentally –– and remarkably –– occurred in only one year, heralding the onset of the Younger Dryas cool episode.

The cause of this cooling has been much debated, especially because it closely coincided with the abrupt extinction of the majority of the large animals then inhabiting the Americas, as well as the disappearance of the prehistoric Clovis culture (http://phys.org/tags/clovis+culture/), known for its big game hunting.

"What then did cause the extinction of most of these big animals, including mammoths, mastodons (http://phys.org/tags/mastodons/), giant ground sloths, American camel and horse, and saber- toothed cats?" asked Kennett, pointing to Charles Darwin (http://phys.org/tags/charles+darwin/)'s 1845 assessment of the significance of climate change. "Did these extinctions result from human overkill, climatic change (http://phys.org/tags/climatic+change/) or some catastrophic event?" The long debate that has followed, Kennett noted, has recently been stimulated by a growing body of evidence in support of a theory that a major cosmic impact event (http://phys.org/tags/impact+event/) was involved, a theory proposed by the scientific team that includes Kennett himself.

Now, in one of the most comprehensive related investigations ever, the group has documented a wide distribution of microspherules widely distributed in a layer over 50 million square kilometers on four continents, including North America, including Arlington Canyon on Santa Rosa Island (http://phys.org/tags/santa+rosa+island/) in the Channel Islands. This layer –– the Younger Dryas Boundary (YDB) layer –– also contains peak abundances of other exotic materials (http://phys.org/tags/exotic+materials/), including nanodiamonds and other unusual forms of carbon such as fullerenes, as well as melt-glass and iridium. This new evidence in support of the cosmic impact theory (http://phys.org/tags/impact+theory/) appeared recently in a paper in the Proceedings of the National Academy of the Sciences.

This cosmic impact, said Kennett, caused major environmental degradation over wide areas through numerous processes that include continent-wide wildfires and a major increase in atmospheric dust load that blocked the sun long enough to cause starvation of larger animals.

http://cdn.phys.org/newman/csz/news/800/2013/3-comprehensiv.jpg (http://cdn.phys.org/newman/gfx/news/hires/2013/3-comprehensiv.jpg)
These are examples of impact spherules collected from different sites. Credit: YDB Research Group

Investigating 18 sites across North America, Europe and the Middle East, Kennett and 28 colleagues from 24 institutions analyzed the spherules, tiny spheres formed by the high temperature melting of rocks and soils that then cooled or quenched rapidly in the atmosphere. The process results from enormous heat and pressures in blasts generated by the cosmic impact, somewhat similar to those produced during atomic explosions, Kennett explained.

But spherules do not form from cosmic collisions alone. Volcanic activity, lightning strikes, and coal seam fires all can create the tiny spheres. So to differentiate between impact spherules and those formed by other processes, the research team utilized scanning electron microscopy and energy dispersive spectrometry on nearly 700 spherule samples collected from the YDB layer. The YDB layer also corresponds with the end of the Clovis age, and is commonly associated with other features such as an overlying "black mat" –– a thin, dark carbon-rich sedimentary layer –– as well as the youngest known Clovis archeological material and megafaunal remains, and abundant charcoal that indicates massive biomass burning resulting from impact.

The results, according to Kennett, are compelling. Examinations of the YDB spherules revealed that while they are consistent with the type of sediment found on the surface of the earth in their areas at the time of impact, they are geochemically dissimilar from volcanic materials. Tests on their remanent magnetism –– the remaining magnetism after the removal of an electric or magnetic influence –– also demonstrated that the spherules could not have formed naturally during lightning strikes.

"Because requisite formation temperatures for the impact spherules are greater than 2,200 degrees Celsius, this finding precludes all but a high temperature cosmic impact event as a natural formation mechanism for melted silica and other minerals," Kennett explained. Experiments by the group have for the first time demonstrated that silica-rich spherules can also form through high temperature incineration of plants, such as oaks, pines, and reeds, because these are known to contain biologically formed silica.

Additionally, according to the study, the surface textures of these spherules are consistent with high temperatures and high-velocity impacts, and they are often fused to other spherules. An estimated 10 million metric tons of impact spherules were deposited across nine countries in the four continents studied. However, the true breadth of the YDB strewnfield is unknown, indicating an impact of major proportions.

"Based on geochemical measurements and morphological observations, this paper offers compelling evidence to reject alternate hypotheses that YDB spherules formed by volcanic or human activity; from the ongoing natural accumulation of space dust; lightning strikes; or by slow geochemical accumulation in sediments," said Kennett.

"This evidence continues to point to a major cosmic impact (http://phys.org/tags/cosmic+impact/) as the primary cause for the tragic loss of nearly all of the remarkable American large animals that had survived the stresses of many ice age periods only to be knocked out quite recently by this catastrophic event." http://cdn.phys.org/tmpl/v5/img/1x1.gif (http://phys.org/news/2013-05-comprehensive-analysis-impact-spherules-theory.html#)

Explore further: Six North American sites hold 12,900-year-old nanodiamond-rich soil (http://phys.org/news/2009-01-north-american-sites-year-old-nanodiamond-rich.html)

More information: Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago, www.pnas.org/cgi/doi/10.1073/pnas.1301760110 (http://www.pnas.org/cgi/doi/10.1073/pnas.1301760110)

28th July 2015, 14:34
A cataclysmic event of a certain age (http://phys.org/news/2015-07-cataclysmic-event-age.html)

July 27, 2015

The Younger Dryas Boundary locations that provided data for the analysis. Credit: UCSB

New research by UC Santa Barbara geologist James Kennett and an international group of investigators has narrowed the date to a 100-year range, sometime between 12,835 and 12,735 years ago. The team's findings appear today in the Proceedings of the National Academy of Sciences.

The researchers used Bayesian statistical analyses of 354 dates taken from 30 sites on more than four continents. By using Bayesian analysis, the researchers were able to calculate more robust age models through multiple, progressive statistical iterations that consider all related age data.

"This range overlaps with that of a platinum peak recorded in the Greenland ice sheet and of the onset of the Younger Dryas climate episode in six independent key records," explained Kennett, professor emeritus in UCSB's Department of Earth Science. "This suggests a causal connection between the impact event (http://m.phys.org/tags/impact+event/) and the Younger Dryas cooling."

In a previous paper, Kennett and colleagues conclusively identified a thin layer called the Younger Dryas Boundary (YDB) that contains a rich assemblage of high-temperature spherules, melt-glass and nanodiamonds, the production of which can be explained only by cosmic impact. However, in order for the major impact theory to be possible, the YDB layer would have to be the same age globally, which is what this latest paper reports.

"We tested this to determine if the dates for the layer in all of these sites are in the same window and statistically whether they come from the same event," Kennett said. "Our analysis shows with 95 percent probability that the dates are consistent with a single cosmic impact event."

All together, the locations cover a huge range of distribution, reaching from northern Syria to California and from Venezuela to Canada. Two California sites are on the Channel Islands off Santa Barbara.

However, Kennett and his team didn't rely solely on their own data, which mostly used radiocarbon dating to determine date ranges for each site. They also examined six instances of independently derived age data that used other dating methods, in most cases counting annual layers in ice and lake sediments.

Two core studies taken from the Greenland ice sheet revealed an anomalous platinum layer, a marker for the YDB. A study of tree rings in Germany also showed evidence of the YDB, as did freshwater and marine varves, the annual laminations that occur in bodies of water. Even stalagmites in China displayed signs of abrupt climate change around the time of the Younger Dryas cooling event.

"The important takeaway is that these proxy records suggest a causal connection between the YDB cosmic impact (http://m.phys.org/tags/cosmic+impact/) event and the Younger Dryas cooling event," Kennett said. "In other words, the impact event triggered this abrupt cooling.

"The chronology is very important because there's been a long history of trying to figure out what caused this anomalous and enigmatic cooling," he added.

"We suggest that this paper goes a long way to answering that question and hope that this study will inspire others to use Bayesian statistical analysis in similar kinds of studies because it's such a powerful tool." Explore further: Study supports theory of extraterrestrial impact (http://phys.org/news/2012-03-theory-extraterrestrial-impact.html)

More information: Bayesian chronological analyses consistent with synchronous age of 12,835–12,735 Cal B.P. for Younger Dryas boundary on four continents www.pnas.org/cgi/doi/10.1073/pnas.1507146112 (http://www.pnas.org/cgi/doi/10.1073/pnas.1507146112)