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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 18 Mar 2014 15:49:05 -0700 (PDT)
Message-ID: <201403182249.s2IMn5o6006554_at_zagami.jpl.nasa.gov>

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.
Received on Tue 18 Mar 2014 06:49:05 PM PDT