Verlag des Forschungszentrums Jülich
JUEL-3002
El-Sagheer Mansour, Aida M.; Abd Elkhalk, Hefny; Roessler, Kurt
Cosmic and solar radiation induced suprathermal processes in titans atmosphere
144 S., 1995
Summary
A theoretical study was made on the interaction of energetic H, He, C, and
Fe ions from solar wind and cosmic rays with the atmosphere of Titan, a satellite
of planet Saturn. It was based on computer simulation of binary collisions with
the program MARLOWEin a kind of Monte Carlo calculation.
Two main issues were treated:
1. The production of energetic secondary atoms with energies exceeding
0.5 eV by knock on processes and the range of the primary ions in a model
lattice containing the atmospheric components N2, Ar, and CH4 in three
mixtures: I (98%, 0%, 2%), II (82%, 12%, 6%), and III (65%, 25%, 10%) at
1.6 bar.
2. The application of this model calculation to Titan's atmosphere using the
range of primaries and the barometric formula in a 10 km bin arrangement.
A distribution profile of secondary N, C, and H atoms resulted, depending
on the kind and energy of primary ions. It was folded with the average solar
and cosmic rays flux at Titan resulting in a profile of suprathermal atoms.
This is important to evaluate the chance for the extremely fast hot or
suprathermal chemical reactions (i.e. in thermal non-equilibrium) and their
competition with classical ion-molecule reactions in thermal equilibrium.
MARLOWE had to be modified for the application to gas phase and high
energy collisions (E :::: 106 eV). The latter was achieved by introducing the
Bethe-Bloch formalism and relativistic corrections to inelastic loss calculation.
The study showed that heavy primary particles (C, Fe) produce very high
numbers of secondaries (e.g. 108 Fe in target I : 69774; 108 H : 306 only). This
counterbalances their low abundance in space radiation in view of chemical
effects. A very interesting result was that light primaries with 108 eV and heavy
ones even up to 1010 eV dissipate their energy mainly within Titan's atmosphere
and do not reach the solid surface. For a 98% N2 + 2% CH4 mixture the column
density of all hot atoms generated by space particle radiation from H to Fe was
calculated to about 2.109 cm-2 s-1 An important contribution of these hot atoms
can be expected locally in the actual atmosphere. On a global scale in the change
from a primordial to the existing atmosphere during the lifetime of the solar
system, the number density of these suprathermal atoms amounted to about
2.1026 cm-2, i.e. the total column density of the atmosphere which implies that
each atom once had been a suprathermal atom.
The study was complemented by an experimental approach. Hot chemical
reactions of secondary atoms were studied by 32 MeV 3He2+ ions from Jiilich
compact cyclotron impinging into a gas target with pure CH4 and mixtures of
90% N2 + 10% CH4, and 98% N2 + 2% CH4 at 1.6 bar. The nuclear reaction
12C(3He,4He)llC created radioactive llC recoils which labeled typical hot
products. These could be analyzed by radiogaschromatography. Formation of
unsaturated compounds C2H2 and C2H4 and nitrogen containing molecules such
as HCN, CH3CN and CH3NH2 was strong at low doses (D> 10-2 eV per target
molecule), whereas at higher doses the formation of CH4 and C2H6
predominated. The abundance of nitrogen compounds labeled by llC was
however, relatively very low. Carbon seems to react preferentially with carbon
compounds (and oxygen), nitrogen with nitrogen compounds and in particular
with N2. Thus, the formation of carbon-nitrogen compounds is not favored, a
result which is reflected in the atmospheric composition of Titan.
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