Astronomy/iridium

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Question
Hello,

it's been pointed out that iridium is much more common in meteors than in the Earth's crust. How much more frequent?  Also, do the other planets also contain higher amounts of iridium than the Earth does? Lastly, do comets, asteroids and meteors contain higher levels of radioactive materials(per square metre, obviously) than usually found on Earth per sq m? Thanks.

Answer
Hi Geoff,
After researching iridium on the internet, I didn't know that it was the 2nd densest element
(Osmium is top dog) and one of the least corrosive of the metals... even aqua regia won't touch it.
That being said, I found that the crustal abundance in parts per million (ppm) is stated as
0.0004 whereas the iridium concentration found at the K-T boundary was 100 times more, or I assume 0.04 ppm if my arithmetic is correct.  That's quite a difference, even though the numbers are quite small.  While the entire earth has a higher abundance, it's thought that during the molten stage(s)...most of the dense elements including iridium, sank to the core along with most of the iron and nickel.  And BTW, iridium is not radioactive.
So while I've never made any tests on meteorites, nor have I read about it's abundance in comets and asteroids, I think that comets, being composed of mostly water ice, frozen gases, and dust... contain very little iridium. Asteroids and meteors, probably not a result of ever being molten, probably run at that 0.04 ppm figure as an educated guess, but I don't think anyone really knows with no testing of that nature ever being done.
Nor would I expect much in the radioactive elements in asteroids and comets either due to their low density and gravelly appearance.
I don't know about the other planets and their iridium content.  If they were molten like the Earth early on (~ 4 billion years ago), then the iridium probably migrated to their cores too,
just like the Earth, so one would probably not find much in their crustal regions, except like the Earth, in the meteorite impact basins.  But I don't think the Viking, Mars rovers, or the Russian Venera missions to Venus, ever tested for that element, so we just don't know.
Oh, check out website
http://en.wikipedia.org/wiki/Iridium_anomaly
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA USA
 
FOLLOW UP:
Since your question involves a single element of the Periodic table (and the radioactivity portion of the question too), perhaps you might want to search out the allexperts.com chemistry and physics departments, as they may be far more knowledgeable than I am about individual elements and their properties and abundances on planet Earth.  (Just like extra-terrestrial life is really more a biology question, and not exclusively an astronomy question.)

Oh, and as far as radioactivity of iridium, see
http://answers.yahoo.com/question/index?qid=20091022170822AAQLTN8
therefore most naturally occurring iridium is not radioactive.
tom  

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Tom Whiting

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Astronomy has been my hobby/pasttime for over 50 years.  Currently own 3 telescopes, the largest of which is a 30 inch Newtonian truss Dob that is portable.I taught Astronomy/Meteorology at the University Level for 13 years before retiring in 1995. Being retired and home most of the time, I am able to answer all questions relatively quickly, unless it's a new moon weekend with good observing conditions.  No astrology questions please, or questions about alleged UFO picture identifications.

Experience

Experience: Astronomy has been my hobby and study for over 50 years. We currently now own a 30 inch portable telescope (Updated - Pennsylvania`s largest portable telescope). It can be seen on our website at:http://www.velocity.net/~bwhiting and also attend several regional starparties during the year, and have been on 5 total solar eclipse expeditions.

Organizations: President, Erie County Mobile Observers Group for over 15 years.

Publications: Wrote the "Over Erie Skies" newspaper article in our local newspaper for 11 years (1975-86).

Education: Masters Degree- Taught at the University level for 13 years. Retired 20 years -USAF Pilot - KC-135 with 180 combat missions;  Also Eagle Scout, Philmont staff 2 Yrs, Order of Arrow Lodge Chief, Ham Radio (inactive).

Awards: two discoveries: The mini-coathanger asterism in Ursa Minor (the little dipper) And the mini-ladle- another asterism in the bowl of Ursa Minor. Clients: Currently President of the ECMOG as mentioned above.

Education/Credentials
BS  Metallurgical Engineering Grove City College, PAMaster's Degree, Gannon University, Erie, PA Also retired USAF pilot, 20 years.

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