2.1. Deuterium
Deuterium is particularly interesting because stellar processing and
the recycling of gas through stars (astration) generally cause it to
be destroyed and not created, and its very existence is an argument
for the Hot Big Bang as was originally stressed by Gamow at a time
when its presence had only been established in terrestrial and
meteoritic water where it is enhanced by a factor of about 6 owing to
fractionation. That this is so was established from studies of the
solar wind in meteorites and lunar foils and soils from 1970 onwards
(Black 1971,
1972;
Geiss & Reeves 1972;
Boesgaard & Steigman
1985).
The Solar wind contains 3He inherited from the interstellar
medium (ISM) when the Solar System was formed 4.6 Gyr ago; 3He
resulting from destruction of proto-solar deuterium; and (perhaps)
3He
dredged up by turbulent mixing from nuclear-processed material deep
inside the Sun
(Schatzman 1987).
The result is 3He / 4He = (4.0
± 0.2
(s.e.)) × 10-4 or, assuming He/H = 0.1 in the Sun,
y23 
3He / H = (4.0 ±
0.2) × 10-5. On the other hand, gas released by heating
carbonaceous
chondrites, believed to represent solar wind particles implanted near
the time of the birth of the Solar System, contains a smaller
proportion of 3He, 3He / 4He = (1.52
± .05) × 10-4, corresponding to the
proto-solar abundance y3 of 3He on its
own. If fresh production of 3He
by dredge-up is ignored, the proto-solar D / H ratio is
simply the difference
y23 - y3 = (2.5±0.2) ×
10-5, with which ground-based and
Voyager infrared observations of deuterated molecules (HD,
CH3D) in
the atmospheres of Jupiter, Saturn and Uranus, carried out since 1972,
are in fair agreement (see
Boesgaard & Steigman
1985;
Pagel 1987a).
Interstellar deuterium was discovered in 1973, in molecular
form from radio observations of molecular clouds
(Jefferts, Penzias &
Wilson 1973)
and as HD and DI (Lyman bands and Lyman series) from
ultra-violet spectroscopy of diffuse clouds in front of hot stars
using the Copernicus satellite
(Spitzer et al. 1973;
Rogerson & York 1973).
A relatively low abundance of DCO+ and DCN at the Galactic
centre
(Penzias 1979)
supports the purely (or at least mainly)
destructive effect of astration on deuterium, but unknown
fractionation effects make it difficult to infer the interstellar
D / H ratio from molecules except in the case of
DCO+
(Dalgarno & Lepp 1984)
which agrees with atomic lines in giving a ratio
~ 10-5. A hyperfine
transition of DI, at 91.6 cm wavelength, has been searched for several
times, but without a definite detection
(Pasachoff &
Vidal-Madjar 1989).
Most determinations of the interstellar D/H ratio come from
observations of Ly 
,
,
in absorption on lines
of sight to hot
stars at distances up to 1 kpc and a few from IUE observations of Ly
emission lines, with interstellar absorption superposed, from very
nearby stars, the deuterium line appearing as a weak component
displaced to the violet by 81 km s-1. Difficulties arise from
appropriate modelling of the velocities and velocity dispersions of
the intervening clouds (done with the help of optical observations of
NaI) and from the possibility of spurious signals arising from
hydrogen clouds expelled at about 80 km s-1 from the target
star, for which there is direct evidence in some cases
(Vidal-Madjar et al. 1983;
Gry, Lamers &
Vidal-Madjar 1984).
Estimates of the interstellar D / H ratio thus cover quite
a wide range, from 2.5 × 10-5,
the same as the proto-solar value, to 6 × 10-6
(Boesgaard & Steigman
1985;
Pasachoff & Vidal-Madjar
1989).