[meteorite-list] The wonderful wizards of Osmium CHICXULUB I

[Michael L Blood]

Hi Sterling and all,
        I saw an educational video that stated they had discovered
An "impact crater" (based on shocked quartz - shattercones)
That was 500 MILES in diameter.
        Michael

on 4/10/08 4:35 PM, Sterling K. Webb at sterling_k_webb at sbcglobal.net wrote:

> Hi, List,
> 
>     There's a little bit of "straw-manning" going on here
> (caution: science journalism at work -- theirs not mine).
> They say the accepted size estimate of the Chicxulub
> impactor is 15 km to 19 km. That's wrong. The most
> commonly accepted estimate is 10 km (although
> some favor 12 or 13 km).
> 
>     Their size estimate is based on the idea that all the
> osmium they found was ALL the osmium from the impactor.
> I doubt that the transport mechanism from impactor to ocean
> muck was 100% efficient.
> 
>     Two-thirds of the planet is ocean, one third land. If what
> what they found in the muck was two-thirds of the osmium?
> The impactor would be 5 km across instead of 4.4 km, not an
> astounding increase.
> 
>     All we know from the Chicxulub crater is the kinetic
> energy of the impact: not the size, not the speed, but the
> product of the two: mass times ( speed squared ). The Bang
> at Chicxulub was 100 TeraTons of TNT. (That's 500 Zetta-
> Joules, zetta being 10^21), or 100,000,000 MegaTons of TNT!
> 
>     A 5 km impactor weighs 1/8th of what a 10 km impactor
> of the same material would and so it would have to go 2.8
> times faster when it hit (2.8 squared = 8). Interestingly,
> while we know the energy well, estimates of velocity are
> a little shy. Those that offer up big impactors keep the
> speed down and those that talk of smaller impactors boost
> the speed estimate appropriately.
> 
>     But if a 5 km stoney impactor did all that damage, we are
> talking about velocities in the neighborhood of 35 to 45 km/sec.
> A highly eccentric orbit is required to achieve those kinds of
> encounter velocities with the Earth.
> 
>     The most recent theory (I like it) of where the Chicxuluber
> came from is the breakup of the parent body of the Baptistina
> family of asteroids about 160 million years ago (the biggest
> survivor of which is 298 Baptistina).
> 
>     The high encounter velocity also encourages proponents
> of the comet impact theory. True, the press release says:
> "chemical traces of the impactors left behind in rocks...
> suggest otherwise," but you can forget that. The "traces"
> are of a carbonaceous chondrite, a likely composition for
> a "comet," which is afterall just an asteroid with extra frosting.
> 
> 
> 
> Sterling K. Webb
> ---------------------------------------------------------------------------
> ----- Original Message -----
> From: "Darren Garrison" <cynapse at charter.net>
> To: <Meteorite-list at meteoritecentral.com>
> Sent: Thursday, April 10, 2008 4:52 PM
> Subject: [meteorite-list] The wonderful wizards of Osmium
> 
> 
> http://space.newscientist.com/article/dn13649-ocean-mud-yields-secrets-of-past
> -earth-impacts.html
> 
> Ocean mud yields secrets of past Earth impacts
> 20:28 10 April 2008
> NewScientist.com news service
> David Shiga
> 
> Mud at the bottom of the ocean holds precious clues about asteroids that
> struck
> Earth in the past, a new study reveals.
> 
> Scientists would love to have a better record of asteroid and comet impacts
> to
> understand how these catastrophic events have affected life and Earth's
> climate.
> But most impactors that made it through the atmosphere either gouged out a
> crater that was subsequently erased or splashed into the ocean.
> Now, scientists have developed a new tool to uncover these events, based on
> concentrations of the metal osmium found in mud at the bottom of the ocean.
> The
> technique was developed by François Paquay of the University of Hawaii in
> Honolulu, US, and his colleagues.
> 
> Osmium atoms come in two varieties, or isotopes, one of which is slightly
> heavier than the other. Crucially, the osmium in meteorites is much richer
> in
> the lighter form than the stuff native to Earth. As a result, scientists can
> determine how much of the otherworldly stuff is present in any given deposit
> of
> the metal they find.
> 
> Paquay's team has been looking for the metal in samples of ocean sediment
> obtained by drilling into the ocean floor. The sediment was laid down in
> layers
> over time, allowing scientists to date when they were deposited.
> 
> Multiple strikes
> In 1995, members of Paquay's team pointed out high levels of the lighter
> osmium
> isotope - associated with extraterrestrial material - in ocean sediment laid
> down around the time of the impact that killed off the dinosaurs 65 million
> years ago.
> 
> Since then, they have found another big spike in extraterrestrial osmium
> laid
> down at the time of another known impact event that happened 35 million
> years
> ago. At that time, multiple impacts shook the Earth in what is known as the
> Late
> Eocene impacts.
> 
> The team estimates that 80,000 tonnes of osmium from the object that wiped
> out
> the dinosaurs was vaporised by the heat of the impact. It then dissolved
> into
> seawater and eventually accumulated on the ocean floor. The Late Eocene
> impacts
> 35 million years ago laid down an estimated 20,000 tonnes.
> 
> Smaller impacts
> Based on these amounts, the team estimates that the dinosaur-killing object
> was
> 4.1 to 4.4 kilometres across, while the largest of the Late Eocene impactors
> would have been 2.8 to 3 km across.
> 
> These are much lower than previous estimates based on the size of the
> craters
> associated with these events. These have given impactor size estimates of 15
> to
> 19 km for the one that killed off the dinosaurs, and 8 km for the larger of
> two
> impactors involved in the Late Eocene impacts.
> 
> What accounts for the difference? For one thing, the calculations by
> Paquay's
> team assume that 100% of the osmium from the impactors was vaporised and
> dissolved into seawater. If a smaller percentage actually ended up on the
> ocean
> floor, then the impactors could have been bigger.
> 
> Comet impacts?
> But even after taking this into account, Paquay thinks the impactors were
> smaller than the crater-based calculations suggest. If the impactors were as
> large as these calculations imply, then 90% of the osmium from the impactors
> is
> hiding somewhere other than in ocean sediment. "We think that this is
> unlikely,
> but we can't rule this possibility out without additional work," he says.
> 
> Another possibility is that the impacting objects were comets rather than
> asteroids, and contained much less osmium to begin with. But chemical traces
> of
> the impactors left behind in rocks and reported in previous studies suggest
> otherwise.
> 
> Kenneth Farley of Caltech in Pasadena, US, who has studied other traces of
> impacts in sediment, but is not a member of Paquay's team, is impressed with
> the
> new method.
> 
> "I am hoping that this technique will allow the detection of previously
> unknown
> impacts so we can get a better handle on impact frequency and assess
> whether -
> and how - impacts affect life and climate," he told New Scientist.
> 
> Unique signature
> Although impacts are also known to contribute unusually large amounts of an
> element called iridium to sediment, the iridium concentrations are much
> harder
> to translate into impactor sizes, Farley says.
> 
> Unlike osmium, extraterrestrial iridium does not have a unique isotope
> signature, so is harder to distinguish from iridium native to Earth.
> 
> And while samples show osmium is laid down evenly across the planet, the
> distribution of iridium is very patchy, making it hard to draw conclusions
> without a large number of samples from different parts of the planet.
> 
> 
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