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Author Topic: Why is iridium much more common in asteroids than in the Earth's crust?  (Read 17558 times)

Offline chris

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Scientists use iridium as a marker of ancient asteroid impacts, but why should this element be more common in the impactors than Earth itself?

Chris


 

Offline RD

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Iridium is one of the rarest non-radioactive, non-noble gas elements in the Earth's crust, but it is relatively common in meteorites. Iridium and osmium are the densest elements, and both are believed to have dropped below the Earth's crust toward the core when the Earth was young and molten. The concentration of iridium in meteorites matches the concentration of iridium in the Earth as a whole.
http://en.wikipedia.org/wiki/Iridium
 
« Last Edit: 08/05/2008 14:44:31 by RD »
 

Offline chris

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Ah, right. So iridium is present but it's distribution is not uniform; it's largely absent form the crust because it has been incorporated into the core. Asteroids, however, coming from the same proto-planetary soup as we did, still contain their iridium uniformly.

Brilliant; thanks.

Chris
 

Offline JimBob

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There is also the fact that the asteroid belt in this universe - exclusive of asteroids coming in from outside the solar system -  is consider by many to have been a shattered planet that had formed between Earth and Mars but was subsequently hit by a comet larger than most and came apart. This theory has been around for a while.
 

Offline LeeE

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A number of bodies in the solar system display very large impact craters and the fact that these impacts did not destroy the bodies implies that these impacts are not capable of doing so.

Having said that, the impact that caused the Odysseus crater on Tethys may have come close to splitting it if the Ithaca Chasma, on the opposite side of the moon, was created by the same impact that caused the crater.

The estimated total mass of the asteroid belt is actually very small - equivilent to about 4% of the Moon's mass, so if it had been a single body at an earlier point in time, it would have been a very small one.  That is unless you also assume that only a small proportion of the original body has been left in it's original orbit and the main part has been distributed elsewhere.

I believe that analysis of the asteroid sourced meteorites indicate that they were never  a single body.

I'm not sure if any extra-solar system asteroids have ever been identified.  Bearing in mind the distances between the stars in our region of the galaxy, the likelyhood of an asteroid from another star system intersecting ours is tiny.  Perhaps someone has worked out the probability of this happening and if so, I'd be interested to know if the Earth is old enough for this to have happened yet.
 

Offline JimBob

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I believe that analysis of the asteroid sourced meteorites indicate that they were never  a single body.


Where can I find the info on that - I had not thought from my reading that the asteroid belt was anything other than core material. Can you direct me to a source for this, please? And the, along with that, is the fact there is accretion of stone meteoritic material and water from comets and other things that hit the various planets in our system. There are enough water meteorites hitting and becoming part of the earth each year to add almost an inch of water to the earth's oceans each year.  (NASA)

SO if the collision that broke apart the proto-planet in the orbit the asteroids hang out in occurred 4 billion years ago as it is thought by some to have happened, then there has been plenty of time for this material to fall under the influence of other gravitational bodies in the solar system - esp. Saturn and Jupiter - and be incorporated into the.

This brings us to another question. What is the origin of Saturn's rings? Part of these rings, not all, are considered to be made of stony material.
 
 

Offline LeeE

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It was something I remembered reading somewhere but a quick look at the Wikipedia article gives this reference:

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980810a.html

There are quite a few Google hits on "chemical analysis of asteroids" but I haven't gone through them.

However, the Wiki article also reckons that it was likely that the asteroid belt was actually formed formed from the breakup of a number of planetesimals that had too much energy to accrete into a planet.  So arguably, while the asteroid belt may not have been formed from a single body that was destroyed by a large asteroid impact, it could be said to have been formed by the destruction of several large bodies impacting each other.  In addition, the article says that most (no value given) of the mass has indeed been lost since formation.

http://en.wikipedia.org/wiki/Rings_of_Saturn says it's likely that Saturn's rings did form from the breakup of a single moon.  Incidentally, that page links to a very nice (enhanced) mosaic pic from the Cassini orbiter as Saturn eclipses the sun.
 

Offline JimBob

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I thought I had heard stories to this effect

... or something
 ;D
 

Offline roj2003

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If you visit the "arXiv" site, "Earth and Planetary Astrophysics", here newbielink:http://uk.arxiv.org/list/astro-ph.EP/recent [nonactive]  you can search back for the latest ideas, which suggest that less than 0.01% is left of the mass of the original meteorite band between Mars and Jupiter. Planetary migration threw some into the sun and some into the Oort Cloud.
 

Offline OokieWonderslug

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Any impact large enough to disrupt a planet will fling the majority of it's mass far into space and would not be left behind to form any asteroid belt. This would have created a spherical shell of debris expanding away from the impact point.

This would leave very little mass in orbit (which we see) and would make intense cratering on the sides of any planets exposed to it. That is why most objects in the Solar system have more craters on one side versus the other.
 

Offline dlr

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"most objects in the Solar system have more craters on one side versus the other"   !!!

I had never heard that before.      Do you have any references where I could go and find out more?  I found references to one side of the Moon being more heavily cratered than the other, and one side of Mars being more heavily cratered than the other, but both situations were attributed to the effects of resurfacing by volcanism -- thinner crust in those regions making them more liable to massive lava flows which covered up older craters.  I couldn't find any reference to there being more craters on one side of Mercury than on the other.  The discussion on the case of the moon was very interesting.  I tried to link to it in my reply but I wasn't allowed to.  If you'd like to read it, go to  sservi.nasa.gov and search for question #  3318     A very interesting discussion.   
 

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