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Monica Grady: Visitors from outer space (3 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).

Carbonaceous chondrites

Carbonaceous chondrites have chemical compositions (apart from hydrogen
and helium) that are close to that of the Sun. The parent bodies of the
carbonaceous chondrites probably formed towards the outer edges of the
asteroid belt, where it was cooler, allowing some ices to condense.
The carbonaceous chondrites can be sub-divided into several groups, the
most primitive of which is that of the CI chondrites, very rich in
water, sulphur and organic compounds. There are only five of these rare
meteorites(*), and it is possible that this small group might be from
the remains of burnt-out comets, rather than from the asteroid belt.
Each time a comet approaches the Sun, it loses ice. It is possible to
envisage an episode when the last vestiges of ice are evaporated from a
cometary nucleus, leaving behind just the stony material. This, then,
having no 'fuel' to drive it, falls towards the Sun, and may be captured
by the Earth as an unusual meteorite.
There are several other groups of chondritic meteorites, also with
compositions unfractionated since their aggregation, but with lower
volatile contents. Although these meteorites have not melted since their
formation, they do contain materials that were once molten. These
spherical silicate assemblages (chondrules) were produced by rapid
cooling of droplets of molten stone. The droplets came from collisions
between clumps of dust grains in the early stages of the formation of
the Solar System, so meteorites such as these represent the materials
from which the Solar System grew.
In addition to chondrules, chondrites contain organic compounds in
varying quantities. Some groups contain amino and carboxylic acids and
complex hydrocarbons, whilst others contain elemental carbon. It is
meteorites like these, together with the ice- and volatile-rich comets,
which probably brought volatile materials to the newly-formed Earth, and
helped establish our planet's atmosphere and oceans. Without them, there
would be no life on Earth.
Also buried within chondrites are tiny grains of dust that came from
stars other than our own Sun. These grains are diamonds (invisible to
the naked eye) and silicon carbide (or carborundum). The diamond grains
are too small to be seen individually. The interstellar origins of the
diamonds have been inferred from the isotopic compositions of nitrogen
and the noble gas, xenon, trapped within the diamond lattice, and
released when the diamond is burnt in the laboratory. The diamonds were
blown from the surfaces of neighbouring stars, and carried on the
stellar wind into the collapsing dust cloud that formed our Solar
System. From these grains, we learn that our Sun did not grow in
isolation, but had neighbours.

(*) Alais - Ivuna - Orgueil - Revelstoke - Tonk

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