فراوانی ِ دوتریوم
Fr.: abondance de deutérium
The number of → deuterium (D) atoms with respect to
→ hydrogen (H) in an astrophysical object.
Deuterium is a primordial product of → Big Bang nucleosynthesis.
According to theoretical models, the primordial D/H ratio is estimated to be
(2.61 ± 0.15) x 10-5 (Steigman et al. 2007, MNRAS 378, 576).
Nuclear reactions in stars convert D into
He tending to a lower D/H ratio in the → interstellar medium
over time (→ deuterium burning).
However, chemical and physical → fractionation
processes can produce local → enhancements in the D/H ratio.
For example, low-temperature ion-molecule reactions in
→ molecular cloud cores can enhance
the D/H ratio in icy grains by as much as two orders of magnitude
above that observed in the interstellar medium.
The D/H ratio in the → solar nebula, estimated from
observations of CH4 in → Jupiter and
→ Saturn, is 2.1 ± 0.4 x 10-5,
assuming that these gaseous planets obtained most of
their hydrogen directly from solar nebula gas. This estimate is
consistent with → protosolar D/H value inferred from the
→ solar wind
implanted into lunar soils. Moreover, the D/H ratio derived from
the interstellar Dα line (which is displaced from the
→ Lyman alpha line
of 1H at 1216 Å by
-0.33 Å) is 1.6 x 10-5 (Linsky et al. 1995, ApJ 451, 335).
High D/H ratios (relative to Earth's water) are measured spectroscopically from water
in three comets (all from the → Oort cloud):
→ Halley (3.2 ± 0.1 x 10-4),
→ Hyakutake (2.9 ± 1.0 x 10-4), and
→ Hale-Bopp (3.3 ± 0.8 x 10-4).
These are all about twice the D/H ratio for terrestrial
water (1.49 x 10-4) and about 15 times the
value for the above-mentioned solar nebula gas. Note that
→ carbonaceous chondrites have the highest
water abundance of all → meteorites.
Their D/H ratios range from 1.20 x 10-4 to 3.2 x10-4 with a
case at (7.3 ± 1.2) x10-4.
Different authors interpret the high comet ratios in very different ways.
Some consider the high D/H ratio as evidence against a
cometary origin of most of the terrestrial water. Others, on the contrary,
argue that comets are the main
reservoir of deuterium-rich water that raised the terrestrial D/H a
factor of six above the protosolar value.
For more details see
"Sources of Terrestrial and Martian Water" by Campins, H. and
Drake, M. (2010) in "Water & life: the unique properties of
H20" Eds. R. Lynden-Bell et al. CRC Press, pp. 221- 234.
→ deuterium; → abundance.