[meteorite-list] The Hunt For Meteorites Is Focusing On Railway Stations

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:55:50 2004
Message-ID: <200201241648.IAA27547_at_zagami.jpl.nasa.gov>

http://www.guardian.co.uk/science/story/0,3605,638054,00.html

Hidden treasures

The hunt for meteorites is focusing on railway stations, writes
Matthew Genge
The Guardian (United Kingdom)
January 24, 2002

If you were in Paddington station on a particular night, you would have seen
something very unusual. A man in his late thirties, smart but casually dressed,
was behaving quite strangely, seeming more than a little interested in the
station's polished floor. Walking slowly across the atrium, he stared at the
ground with a look of intense concentration, occasionally apologising to yet
another commuter into whose path he had strayed.

Watch some more and occasionally he would crouch, with his nose inches from
the stone. Then he would shake his head, stand up and pace around some more.
You'd probably conclude he was just another of the poor deluded souls that
amble about the streets of London living in their own world.

However, you'd be wrong. You had witnessed history in the making. The man
was Dr Birger Schmitz, from the Earth Science Centre in Gothenburg, and he
was looking for pieces of rock from space. That evening was the first meteorite
hunting expedition in London.

Among all the places in the world you are likely to find meteorites, Paddington
station would seem the least likely. Dr Schmitz, however, was not looking for
meteorites lying on the floor but meteorites in the floor.

Paddington station is paved with polished blocks of limestone and in one
limestone quarry in Sweden, Schmitz and colleages have found 40 meteorites
completely encased in the rocks. It is an occupational hazard to take more than a
passing interest in limestone floors whether they are in stations, airports or
hotels. But you never know when you might get lucky. Dr Schmitz has already
found a number in buildings and one that has spent the past 40 years in a
limestone slab paving a Swedish driveway.

Meteorites are difficult to find and, except where they have been seen to fall,
cannot be collected in temperate regions such as the UK. The majority of the
21,000 known meteorites are in the Antarctic or in hot deserts because here
they survive for long periods of time and can accumulate. The meteorites with
the oldest terrestrial residence ages are from the Antarctic and fell on Earth as
much as 2.5m years ago.

In terms of geology and the 4.5bn-year history of the Earth, this is very recent.
What makes the meteorites that Schmitz and colleagues have found in Sweden
unusual is that they are "fossil meteorites" contained in limestones 480m years
old. These are meteorites that fell during a period of time known as the
Ordovician, when the Earth's land surface was still a barren wasteland devoid of
even plant life.

These Ordovician fossil meteorites reveal events that happened in space
hundreds of millions of kilometres from Earth 480m years ago. The fossil
meteorites have been found in a 1.75 metre-thick layer of limestone deposited
on the bottom of a shallow sea. Mixed in with and surrounding the meteorites
are fossils of straight nautoloids, which are the shells of strange squid-like
creatures. One meteorite reputably sits directly on top of a trilobite fossil and
could represent the smallest meteoritic extinction event in history.

What is particularly puzzling is that so many meteorites are found in the 6000
square metres of the quarry. The limestone was deposited very slowly, at around
one millimetre per thousand years, so it represents 1.75m years of history. But
even so, there are a hundred times as many meteorites embedded in the
limestone than would be expected from the rate they fall on Earth today. It
appears that meteorites may have fallen much more frequently in the Ordovician
than now.

The fossil meteorites are a type of meteorite known as L ordinary chondrites and
are still the most common to fall. Trapped argon gas within these meteorites,
produced by the decay of radioactive potassium, provides a clue as to why so
many meteorites fell in the Ordovician. Argon in L chondrites was lost 480m
years ago because of a giant collision that probably broke the L chondrite
asteroid to pieces. Could the debris from this collision have showered the Earth
with meteorites? Dr Schmitz suggests it did and points out that more craters
formed on Earth at this time due to the impact of larger pieces of the asteroid.

These giant asteroid collisions have occurred many times in the history of the
solar system. There are numerous families of asteroids in the main asteroid belt
between Mars and Jupiter, formed by the break-up of a larger parent asteroid,
and these could have showered the Earth with meteorites and larger
crater-forming objects. Fossil meteorites are, however, difficult to find in rocks.
Only where thin slabs are cut for building are they likely to be recovered. Cosmic
dust may provide a more reliable way of identifying these events since enormous
amounts are produced in asteroid collisions and these can be recovered from
rocks deposited in the deep ocean.

Looking for meteorites in polished limestone slabs is, however, worthwhile. Next
time you are waiting for yet another delayed train, join in the great meteorite
hunt.

· Matthew Genge is a meteorite scientist at the Natural History Museum.
Received on Thu 24 Jan 2002 11:48:54 AM PST