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

Sun Jun 22 14:54:33 EDT 2008

Hi, Michael, List,

    Estimates of Chicxulub crater size range from
170-180 kilometers (or about 115 miles) up to 300
kilometers ( ~180 miles).
http://en.wikipedia.org/wiki/Chicxulub_crater

    The placement of the unique wells of Yucatan,
the cenotes, delimit a crater rim with a diameter of
170-195 km.
http://www.nature.com/nature/journal/v376/n6539/abs/376415a0.html

    Chicxulub is a multi-ring basin (the big ones are).
The question is whether the 170-180 km ring is the
outermost ring. Some see it; some don't. Here's all
the geological (like gravimetric) data:
http://miac.uqac.ca/MIAC/chicxulub.htm

    The 300 km rim, if present, is high degraded. The
chief reason for believing in it is the depressed terrain
outside the 170-195 km rim, because depressed terrain
occurs between rims but not outside the outermost rim
(usually).

    So, you can see that it's highly doubtful that the
crater was 500 miles or 800 km in diameter. You shouldn't
believe everything you see on TV (or video).

    However, it was plenty big enough. Just ask your
local dinosaurs... Whoops!

Sterling K. Webb
-------------------------------------------------------------------
----- Original Message ----- 
From: "Michael L Blood" <mlblood at cox.net>
To: "Sterling K. Webb" <sterling_k_webb at sbcglobal.net>; "Meteorite List" 
<Meteorite-list at meteoritecentral.com>
Sent: Sunday, June 22, 2008 6:41 AM
Subject: Re: [meteorite-list] The wonderful wizards of Osmium CHICXULUB I

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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