Geology Reference
In-Depth Information
Analysis of meteorites
(a) spectral analysis of the light received by telescopes
from stars, including the Sun, and from other radiant
bodies such as nebulae (gas clouds);
(b) laboratory chemical analysis of meteorites, which
represent the solid constituents of the Solar System.
As anyone knows who has slept in the open on a clear
starry night, shooting stars are a common phenome-
non. They are the visible manifestation of many tons of
Solar-System debris that fall to Earth each day.
3
Smaller
infalling bodies may be vaporized completely in the
atmosphere by frictional heating, but about 1% of them
are large enough to survive and reach the Earth's
surface as recoverable
meteorites
.
A simplified classification of meteorites is shown in
Box 11.1.
Spectral analysis
Stars are intensely hot nuclear fusion reactors. They
derive their high temperatures and radiant output from
the energy released when light nuclei (such as
1
H and
2
H) fuse together into more stable, heavier nuclei (such
as
3
He). The theory of this
thermonuclear
process is
outlined in Box 11.2.
In common with any very hot body - such as a red-
hot poker or the filament of an incandescent light
bulb - the hot surface of a star radiates light consisting
of a continuum of wavelengths (the 'white light' we
receive from the Sun). Superimposed on this smooth
electromagnetic spectrum are dark lines that constitute
the
absorption spectra
(see Chapter 6) of elements
present in the cooler outer atmosphere of the star
(see Plate 4). Each line is characteristic of a specific
transition in a specific chemical element. From the
wavelengths and intensities of these absorption lines
(called
Fraunhofer lines
),
2
an astronomer can establish
the identity and abundance of most elements present
in the star. The 'calibration' factors used to translate
absorption line intensity into element abundance have
to be estimated theoretically, yet astronomers are
confident that the abundance data available today, for
about 70 elements in a great many stars, are mostly
accurate to within a factor of two. As element abun-
dances vary by as much as 10
12
times (Figure 11.2),
such uncertainties are tolerably small. Stars progres-
sively alter their internal element abundances through
the nucleosynthesis taking place deep inside them, but
the cool outer envelope of a star (where the Fraunhofer
lines originate) is thought to remain representative of
the material from which the star originally accreted.
In discussing the Solar System, we shall be concerned
with abundances in our nearby sun rather than stars in
general.
Primitive meteorites
The commonest meteorite type (see Box 11.1) is the
chondrites
, so called because most of them contain
chondrules
(millimetre-sized spheroidal assemblages of
crystals and glass - see Plate 5). Chondrules
4
are con-
sidered to be solidified droplets of melt formed by
impact melting of dust in the early protoplanetary
disk. Despite the high temperatures implied by chon-
drule melting (typically 1500 °C), it is commonly the
case that minerals in the silicate-metal-sulfide matrix
surrounding
the chondrules (Plate 5) have highly
variable chemical compositions, indicating that they
have never achieved chemical equilibrium with each
other or with the chondrule minerals. The lack of equi-
librium suggests low ambient temperatures during and
after accretion of some chondrites. Such 'unequilibrated
chondrites' preserve various chemical, mineralogical,
and structural signatures inherited from the protoplan-
etary disk.
Chondrites of one particular class, known as
carbo-
naceous chondrites,
also contain a complex, tarry organic
component and various hydrous silicate minerals. The
very limited thermal stability of these components
suggests that carbonaceous chondrites have suffered the
least thermal and chemical processing of any chondrite
group. They are described as being chemically
primi-
tive
; one particular subgroup known as
CI chondrites
5
The infall rate of meterorites to the Earth's surface is about
40,000 tons per year.
3
From the Greek
chondros
for 'grain', a reference to their
rounded shape.
4
An abbreviation of 'carbonaceous Ivuna' named after the local-
ity in Tanzania from which the type example was recovered.
Paradoxically, despite being classified as chondrites on chemi-
cal grounds, CI chondrites actually contain no chondrules.
5
After Joseph von Fraunhofer, a German physicist who made
the first systematic study of them.
2
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