[meteorite-list] Ancient Earth Hammered by Double Space Impact

From: Greg Redfern <gredfern029_at_meteoritecentral.com>
Date: Tue, 18 Mar 2014 19:44:17 -0400
Message-ID: <CAOU4k=wsn_0HkbE_3cCtbT-XPz0xCJfu3TjiZp92OeqDEx76og_at_mail.gmail.com>

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Greg Redfern
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On Tue, Mar 18, 2014 at 6:49 PM, Ron Baalke <baalke at zagami.jpl.nasa.gov> wrote:
>
>
> http://www.bbc.com/news/science-environment-26172181
>
> Ancient Earth hammered by double space impact
> By Paul Rincon
> BBC News
> 18 March 2014
>
> We've all seen the films where an asteroid hurtles towards our planet,
> threatening civilisation.
>
> What's less well known is that menacing space rocks sometimes come in
> twos.
>
> Researchers have outlined some of the best evidence yet for a double space
> impact, where an asteroid and its moon apparently struck Earth in tandem.
>
> Using tiny, plankton-like fossils, they established that neighbouring
> craters in Sweden are the same age - 458 million years old.
>
> Details of the work were presented at the 45th Lunar and Planetary Science
> Conference in The Woodlands, Texas, and the findings are to be published
> in the Meteoritics and Planetary Science journal.
>
> However, other scientists cautioned that seemingly contemporary craters
> could have landed weeks, months or even years apart.
>
> A handful of possible double impacts (or doublets) are already known on
> Earth, but Dr Jens Ormo says there are disputes over the precision of
> dates assigned to these craters.
>
> "Double impact craters must be of the same age, otherwise they could just
> be two craters right next to each other," the researcher from the Centre
> for Astrobiology in Madrid, Spain, told BBC News.
>
> Dr Ormo and his colleagues studied two craters called Lockne and Malingen,
> which lie about 16km apart in northern Sweden. Measuring about 7.5km wide,
> Lockne is the bigger of the two structures; Malingen, which lies to the
> south-west, is about 10 times smaller.
>
> Binary asteroids are thought to form when a so-called "rubble pile" asteroid
> begins to spin so fast under the influence of sunlight that loose rock
> is thrown out from the object's equator to form a small moon.
>
> Telescope observations suggest that about 15% of near-Earth asteroids
> are binaries, but the percentage of impact craters on Earth is likely
> to be smaller.
>
> Only a fraction of the binaries that strike the Earth will have the necessary
> separation between the asteroid and its moon to produce separate craters
> (those that are very close together will carve out overlapping structures).
>
> Calculations suggest around 3% of impact craters on Earth should be doublets
> - a figure that agrees with the number of candidates already identified
> by researchers.
>
> The unusual geological characteristics of both Lockne and Malingen have
> been recognised since the first half of the 20th Century. But it took
> until the mid-1990s for Lockne to be formalised as a terrestrial impact
> crater.
>
> In the last few years, Dr Ormo has drilled about 145m down into the Malingen
> structure, through the sediment that fills it, down to crushed rocks known
> as breccias and deeper, reaching the intact basement rock.
>
> Lab analysis of the breccias revealed the presence of shocked quartz,
> a form of the quartz mineral that is created under intense pressures and
> is associated with asteroid strikes.
>
> This area was covered by a shallow sea at the time of the Lockne impact,
> so marine sediments would have begun to fill in any impact craters immediately
> after they were created.
>
> One-two punch
>
> Dr Ormo's team set out to date the Malingen structure using tiny fossilised
> sea creatures called chitinozoans, which are found in sedimentary rocks
> at the site.
>
> Their method, known as biostratigraphy, allows geologists to assign relative
> ages to rocks based on the types of fossil creatures found within them.
>
> The results revealed the Malingen structure to be the same age as Lockne
> - about 458 million years old. This seems to confirm that the area was
> rocked by a double asteroid strike during the Ordovician Period.
>
> Dr Gareth Collins, who studies impact cratering at Imperial College London,
> and was not involved with the research, told BBC News: "Short of witnessing
> the impacts, it is impossible to prove that two closely separated craters
> were formed simultaneously.
>
> "But the evidence in this case is very compelling. Their proximity in
> space and consistent age estimates makes a binary-impact cause likely."
>
> Simulations suggest the asteroid that created Lockne was some 600m in
> diameter, while the one that carved out Malingen was about 250m. These
> measurements are somewhat larger than might be suggested by their craters
> because of the mechanics of impacts into marine environments.
>
> Dr Ormo added that Malingen and Lockne were just the right distance apart
> to have been created by a binary. As mentioned, if two space rocks are
> too close, their craters will overlap. But to qualify as a doublet, the
> craters can't be too far apart, because they will exceed the maximum distance
> at which an asteroid and its moon can stay bound by gravitational forces.
>
> "The Lockne impactor was big enough to generate what's known as an atmospheric
> blow-out, where you blow away the atmosphere above the impact site," said
> Dr Ormo.
>
> This can cause material from the asteroid strike to spread around the
> globe, as happened during the huge Chicxulub impact thought to have killed
> off the dinosaurs 66 million years ago.
>
> The Ordovician event wasn't powerful enough for that material to be traced,
> as it would have been very dilute in the atmosphere. But the impact would
> have had regional effects; for example, any sea creatures unlucky enough
> to be swimming nearby would have been instantly vaporised.
>
> Other candidate double impact craters include Clearwater East and West
> in Quebec, Canada; Kamensk and Gusev in southern Russia; and Ries and
> Stenheim in southern Germany.
>
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Received on Tue 18 Mar 2014 07:44:17 PM PDT