[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Monica Grady: Visitors from outer space (2 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).

Meteorite origins

In order to understand what meteorites can teach us, their origin and
history must be appreciated. Between Mars and Jupiter, at the hiatus
between rock and gas in the Solar System, lies the asteroid belt, the
place from which most meteorites come. There are several thousand
asteroids, the largest of which, named Ceres, is about 914 km across
(for comparison, Earth has a diameter of around 13,000 km, and the Moon
around 3,500 km).
Asteroids are rocky, metallic or carbonaceous bodies. They are material
remaining after the planets formed: Jupiter's gravitational pull
prevented the bodies from joining together to form a single planet.
Occasionally, influenced by Jupiter, the orbit of an asteroid is altered
such that it might collide with another, and break up. Images of
asteroids obtained recently by the Galileo and NEAR probes show that
asteroids themselves have cratered surfaces indicating that collisions
are frequent within the asteroid belt. Fragments of disrupted asteroids
fall to Earth as meteorites, pieces of ancient material remaining from
the birth of the Solar System.
Just as the parent asteroids from which they came have a range of
compositions, so too can meteorites be divided into several types, each
group representing different stages in planetary formation. There are
three main types of meteorite (stone, iron and stony-iron), reflecting
their overall composition.
Most meteorites are stony (96 percent of all falls), made up of the same
minerals (olivine, pyroxene, plagioclase) as many terrestrial rocks,
minerals which contain silicon, oxygen, magnesium, iron, calcium and
aluminium. The stony meteorites can be sub-divided into chondrites,
those which have remained unmelted since formation (or aggregation) of
the parents. These almost all contain small rounded droplets of
once-molten material, or chondrules. Chondrites retain a chemical
signature close to that of the original material from which they
aggregated.
The other division of stony meteorites is the achondrites. These are
igneous rocks, like basalts, that formed from melts on their parent
bodies. Achondrites do not contain chondrules and, as a consequence of
the melting process, are chemically differentiated, i.e., no longer
exhibit a primordial signature.
The other large division of meteorites, the irons are, as their name
suggests, made dominantly from iron metal typically with five to fifteen
percent by weight, nickel. These meteorites have all been formed during
extensive melting processes on the parent bodies from which the
meteorites originated. The heat source for melting was, in some cases,
the result of impacts, but for many iron meteorites the heat source was
most probably from the decay of short-lived radioactive isotopes, such
as a radioactive isotope of aluminium, 26Al.
The iron meteorite parent asteroids were sufficiently large that this
heat built up and was retained, allowing reduction reactions (similar to
smelting in a blast furnace) to occur within the parents. Iron-nickel
metal, produced from the reduction of silicate minerals, migrated under
gravity to the centre of the parents, forming a core, whilst the less
dense remaining silicates rose to the surface, forming a crust.
Most iron meteorites show a distinctive pattern if a slice through the
meteorite is polished and etched with acid. This pattern, the
Widmanstätten pattern, is produced from the intergrowth of two alloys of
iron metal with nickel (kamacite and taenite), each alloy containing
different nickel concentrations. It is a result of the very slow rate at
which the metal cooled: between one and one hundred degrees Celsius per
million years, allowing nickel atoms to diffuse through the iron
lattice. The final pattern is 'frozen' in when the nickel no longer has
sufficient energy to move.
Iron meteorites are the closest physical analogy we have to the material
which forms the Earth's core. (In contrast, either short-lived
radioactive elements were absent, or heat from radioactive decay of
these elements was dissipated in the smaller chondritic parents, thus
melting did not occur.)
The third meteorite group is that of the stony-irons: a mix, as the name
suggests, of stone and metal. The pallasite sub-group of these very rare
meteorites, composed of almost equal volumes of stone and iron, have one
of the most beautiful of appearances, produced from the intergrowth of
iron-nickel metal with olivine (a magnesium-iron silicate mineral,
common on Earth as the semi-precious gemstone, peridot). Pallasites were
also formed by melting in their parent, and represent an intermediate
stage between iron meteorites and differentiated silicates, a snapshot
of material from the core/mantle boundary of the body.
Although much can be learned about planetary differentiation processes
from iron and stony-iron meteorites, it is from the chondrite group of
stony meteorites that the most information about the origin and
evolution of the Solar System has been derived. There are many
subdivisions within chondrites, and one of the most important is the
class of carbonaceous chondrites.

----------
Archives located at:
http://www.meteoritecentral.com/list_best.html

For help, FAQ's and sub. info. visit:
http://www.meteoritecentral.com/mailing_list.html
----------