  Organic aerosols accumulated in Titan's orange haze start forming in its
ionosphere. This upper part of the atmosphere is highly reactive and complex
ion chemistry takes place at altitudes from 1200 to 900 km. The ionosphere is a
nitrogen plasma with a few percent of methane and hydrogen. Carbon from methane
enables the formation of macromolecules with long organic chains, finally
leading to the organic aerosols. On the other hand, we suspect that hydrogen
and the protonated ions have a different erosive effect on the aerosols. Here
we experimentally studied the effect of hydrogen and protonated species on
organic aerosols. Analogues of Titan's aerosols were formed in a CCP RF plasma
discharge in 95% N2 and 5% CH4. Thereafter, the aerosols were exposed to a DC
plasma in 99% N2 and 1% H2. Samples were analysed by scanning electron
microscopy and in situ infrared transmission spectroscopy. Two pellet
techniques - KBr pressed pellets and thin metallic grids - were compared to
confirm that modifications seen are not due to the material used to make the
pellet. We observed that the spherical aerosols of ~500 nm in diameter were
eroded under N2-H2 plasma exposure, with the formation of holes of ~10 nm at
their surface. Aerosols were globally removed from the pellet by the plasma. IR
spectra showed a faster disappearance of isonitriles and/or carbo-diimides
compared to the global band of nitriles. The opposite effect was seen with
beta-unsaturated nitriles and/or cyanamides. Double bonds as C=C and C=N were
more affected than amines and C-H bonds. N-H and C-H absorption bands kept a
similar ratio in intensity and their shape did not vary. Therefore, it seems
that carbon and hydrogen play opposite roles in Titan's ionosphere: the carbon
from methane lead to organic growth while hydrogen and protonated species erode
the aerosols and react preferentially with unsaturated chemical functions.
