[meteorite-list] The wonderful wizards of Osmium

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Thu, 10 Apr 2008 17:52:07 -0400
Message-ID: <4t2tv31ssk0o6aq6av3dsog69dlgfsapij_at_4ax.com>

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 Fran?ois 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.
Received on Thu 10 Apr 2008 05:52:07 PM PDT