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Monica Grady: Visitors from outer space (1 of 4)

Ron wrote:

> Dr. Monica Grady is one of the world's top exobiologists - meaning
> she studies life outside Earth. She is also the meteor curator at
> London's Natural History Museum. In this interview, she tells
> EcoWatch reporter Chris Jeavens the biggest finds are sometimes
> right under our noses.


Focus: Visitors From Outer Space (Astronomy Now, November 1997, pp.
45-47):

Around four particles of extraterrestrial material lands on Earth per
hour, per square kilometre. Much can be learned from these meteorites
(by Monica Grady).

Many people have suffered from visitors who just drop in, often at the
most awkward times. But very few have experienced at first hand a
visitor from space, even though calculations indicate that meteorite
falls are not rare events: approximately 40,000 tonnes of
extraterrestrial material land on the Earth each year - this is around
four particles per hour per square kilometre of the Earth's surface. So,
what can be learned by studying these uninvited guests?

Different types

First of all, it is important to discriminate between different types of
extraterrestrial visitor. Meteorites are natural objects that survive
their fall to Earth from space, and are recovered. They are not
radioactive, and are almost always cold when they land.
Meteorites are distinguished from meteors, or 'shooting stars', which
are pieces of dust that burn up high in the atmosphere. No material is
recovered on Earth from a shooting star, but a 'meteoroid' is a small
body travelling through space, that may, or may not, land on Earth as a
meteorite.
Meteorites were formed at the birth of the Solar System, approximately
4,560 million years ago. Although the Earth, along with the other
planets, was also formed at this time, none of the original material
remains: it has been removed by bombardment or otherwise eroded, or
recycled through geological activity. It is only by studying meteorites
that we can learn about the processes and materials that shaped the
Solar System and our planet.
Almost all of the extraterrestrial material arrives on the Earth as
dust, (known as micrometeorites or cosmic dust), small fragments (less
than one millimetre across) that are captured by the Earth and come from
asteroids and comets. A small proportion of micrometeorites might also
be from outside the Solar System, from interstellar space.
Larger meteorites fall much less frequently than small ones. Meteor
Crater in Arizona was produced by the impact of an iron meteorite
approximately 50,000 years ago. The crater is about 1.2 km across; the
original meteorite was estimated to weigh between 15,000 and 25,000
tonnes, and would have been around 35 to 40 m in diameter, but most of
it was vaporised by the impact. Over 30 tonnes of iron meteorite, named
Canon Diablo, have been recovered from the vicinity of Meteor Crater.


Best regards,

Bernd

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