[meteorite-list] Meteorites Are Cool When They Land

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:46:22 2004
Message-ID: <200105031712.KAA00874_at_zagami.jpl.nasa.gov>

http://www.guardian.co.uk/science/story/0,3605,482065,00.html

As cool as the cosmos
The Guardian (United Kingdom)
May 3, 2001

People expect meteorites to land in a blaze of heat, but they are in fact
icy cold, says Duncan Steel

Recently a woman walking her dog in York heard a whiz and a bang, and then
spotted a smoking hole in the ground. Assuming it was a meteorite impact,
the police cordoned off the area and a curator from the York Museum was
called in to dig up the supposed cosmic rock.

However, it was obvious, even from the sparse initial report, that this was
not a visitor from space. Meteorites are cold when they land, and generally
remain intact on the surface. It proved to be an exploding underground power
cable.

The chance of an extraterrestrial rock reaching the ground depends
critically on its dimensions, different things happening in distinct size
regimes. Every year the Earth accumulates about 40,000 tons of cosmic
detritus, mostly as billions of tiny flecks ranging in size from sand grains
to peas. Travelling at typically 20 miles per second, these are heated and
then evaporated by friction with the upper atmosphere, and in their death
throes they appear to us as meteors, or shooting stars.

Smaller particles - less than a tenth of a millimetre in size - decelerate
without being destroyed. This is possible because their relatively high
surface areas enable them to lose their energy by radiating it away, whereas
larger bodies cannot do this quickly enough, and so are melted and
vaporised. The small grains slow to a halt, and then gradually fall from
altitudes of some tens of miles until they reach the Earth's surface. Such
minute dust particles are termed micrometeorites.

This drop from altitude can take a surprisingly long time: years, in the
case of the tiniest interplanetary dust. Flying research jets in the
stratosphere, Nasa scientists ave collected samples by employing sticky
plates that project from the wings. Another way such cosmic dust has been
gathered for analysis is by melting and sieving large volumes of snow and
ice in the Antarctic and Greenland, where it has accumulated over millennia.

Actually, there are micrometeorites all around. In a typical house, about
one in a thousand dust particles on the mantlepiece may be of
extraterrestrial origin , depending of course on how often it is cleaned,
and how polluted your neighbourhood might be. A good repository of metallic
micrometeorites is the gutter on your roof, because they are gradually swept
down by the rain, and their high density makes them more likely to remain in
the sludge that builds up there.

Much larger meteoroids - say between the size of an orange and a basketball
- produce extremely bright meteors, called fireballs. These can light up the
night sky, producing daylight conditions for a second or two. Then the
object is gone, its atoms added to the atmosphere. In a small fraction of
cases, though, it is possible that some solid lump may reach the ground
intact. That is what we call a meteorite.

For it to survive in this way, the incoming lump needs to be lucky. First,
it is more likely to escape destruction if it is strong, made of metal or
solid rock as opposed to a loose, fragile structure.

Next, there is a greater chance of avoiding vaporisation if the object has
an entry speed close to the minimum possible: that is about seven miles per
second, identical with the Earth's escape velocity.

Finally, the survival probability of a meteoroid is much enhanced if it
happens to arrive at a fairly oblique angle. Under that circumstance it
decelerates slowly, over a prolonged period rather than just a couple of
seconds. The same applies to re-entering space probes such as the Shuttle,
or the Apollo lunar capsules. Come in at too steep an angle and you burn up;
but if the approach is too shallow, you bounce off the top of the atmosphere
like a flat stone skimmed across a pond.

Meteorites, then, are rare. But what is their temperature? Arriving on a
suitable trajectory, they are slowed from their phenomenal cosmic velocity
to essentially zero in 10 or 20 seconds. As they do so, their outer layers
are intensely heated, and ablate away. The melted exterior of a meteorite,
called a fusion crust, is obvious. But the interior is unchanged. During its
fiery plunge there is simply not enough time for the heat to be conducted
into its centre.

As a result, when meteorites reach the ground their interiors are still at
the temperature they had in space, around minus 30C. If the Earth had no
atmosphere, that would be the planet's temperature at our distance from the
Sun. It is only the natural greenhouse effect that gives our home its
pleasing ambient temperature. (The scientific debate over global warming
concerns whether manmade emissions are enhancing the greenhouse effect by a
small but significant proportion.)

One can think of the effect of atmospheric entry on a meteorite as being
similar to throwing a deep-frozen joint of meat into a fire. Five minutes
later the outside may be charred, but the middle is still icy.

Just-fallen meteorites, then, are cold. It is not unusual for an observed
fall, someone reaching the object within a few minutes, to produce a report
of ice on its exterior. Water vapour in the air first condenses on to the
meteorite and then freezes.

Do meteorites make craters? The multitude of scars on the Moon is obvious,
but there are also about 250 recognised impact craters on the Earth. The
best known is Meteor Crater in Arizona. That massive hole, 1,300 yards
across, was formed 49,000 years ago when a nickel-iron projectile about 40
yards in size slammed into the ground, releasing on impact energy equivalent
to about 20 megatons of TNT. The atmosphere does not brake such a body by
much.

Little boulders from space are different, though. The atmosphere slows them
down in one way or another. They are either totally destroyed, or else
brought to a halt. An orange-sized rock would be decelerated by the time it
reaches a height of 20 miles. From there it gets to the ground under free
fall: it plummets at the same rate as an object dropped from a plane,
reaching a terminal velocity of about 200mph.

Compared to the hypervelocity impact of a very large body like an asteroid
or a comet, that is a low speed. As a result, small meteorites do not form
craters. If one were to land on soft ground, it might bury itself a bit, but
that is all. In 1947 a large metallic body broke up on atmospheric entry
over the east of Siberia, and many of the fragments weighing over a ton
managed to penetrate the earth by a metre or so. The 60-ton meteorite that
landed in Namibia in prehistoric times - the largest known intact meteorite
- sits on the surface, although it did cause a bit of a dent.

If you should hear a loud bang and find a smoking crater, suspect that
someone has been playing with dynamite. To excavate an impact crater a
projectile from space has got to be large, and would make a hole far too big
for the police to cordon off. Mind you, they would have other things to
worry about.

* Duncan Steel works at the University of Salford. His most recent book is
Target Earth, published by Time Life
Received on Thu 03 May 2001 01:12:55 PM PDT