[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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