Evolution of aerosols in Titan’s ionospheric plasma: An experimental simulation
Résumé
Observatio
ns by the space mission Cassini
have revealed the formation of complex organic molecules in Titan
’s
ionosphere.
To better understand this
complex chemistry, several experiments were conducted to reproduce analogs of Titan
’s
aerosols in laboratories, named ‘tholins’.
These tholins appear
to be polymeric and nitrogenous molecules
.
Aerosols stay many years in Titan
’s
upper atmosphere, a dusty
plasma where molecules are continuously bombarded by charged
molecules. Consequently, they are likely to evolve during their stay.
Here we
address this question by experimental simulation: we
analyze the effect of harsh plasma environment on tholins alread
y formed.
o separate the processes of formation and evolution,
aerosols samples used are first formed with the
experiment PAMPRE at LATMOS
[
T Gautier
Icarus
2016
].
T
hen tholins are
prepared in
the
shape of thin pellets and positi
oned at the
center of a plasma reactor, where they are exposed during several hours
in a DC N
2
-
H
2
glow discharge, partially representative of
Titan’s ionosphere. Methane is purposefully removed from this second phase to prevent the formation of new aerosol
s.
Morphological changes.
Pellets become rougher during the exposure. Surface structure is observed through a Scanning
Electron Microscope before and after an exposure of four hours. It shows that plasma sputteri
ng attacks the surface and removes some
material.
Chemical modifications.
We perform in situ infrared transmission spectroscopy during the exposure. Characteristic absorption
bands of tholins are distorted, witnessing changes in the samples chemical struc
ture. Especially, we suspect C
-
H bond environment to
be complexed. Nitrile functions are also modified.