We should have
considered this obvious a long time ago when just about every significant
fireball has a sister blasting in often a few hours later. We also do have coincident craters on Earth
that are surely part of the same event.
Comets also
happen to be prolific producers of subsidiary bombardment as well.
The fact is that
any significant body will hold a slew of smaller objects in its train that it
has simply collected over time even without disintegration going its work as is
likely true in the asteroid belt which is driven by Jupiter. It happens to be far too easy to
underestimate the dominant effect of Jupiter on every stray object in the solar
system.
Ancient Earth
hammered by double space impact
By Paul Rincon
Science editor, BBC News website, The Woodlands,
Texas
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.
Proposed double
impact craters
Clearwater East and West (Canada): 26km/36km
diameter, 290 million years old
Kamensk and Gusev (Russia): 25km/3km diameter, 49
million years old
Ries and Steinheim (Germany): 25km/3.8km, 14.5 or 15
million years old
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).
“Start Quote
Short of witnessing the impacts, it is impossible to
prove that two closely separated craters were formed simultaneously”
Dr Gareth Collins Imperial College London
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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