Long-term Stability of Glycine, Alanine, and Phenylalanine on Titan's
Surface Subject to Cosmic Ray Flux
Abstract
The potential commonality of prebiotic chemical processes on Titan and
the primitive Earth makes Titan a prime body of astrobiological
interest. Amino acid synthesis can occur if the abundant simple organics
on Titan’s surface can mix with liquid water. Because events that melt
surface ice, such as impacts, are rare, it is essential to recognize how
long the synthesized molecules remain intact on Titan’s surface. The
degradation of biomolecules in extraterrestrial environments can be
estimated by combining theoretical work about energy deposition on the
surface with experimental results from irradiation of organic molecules.
We modelled the destruction of amino acids on the surface of Titan,
something absent in current literature. We chose Glycine, Alanine, and
Phenylalanine as our molecules of interest due to relevant experimental
results for their radiation stability at Titan temperatures. Titan’s
thick atmosphere prevents solar radiation and energetic particles
trapped in Saturn’s magnetosphere from reaching the surface. The
dominant source of energetic radiation at the surface of Titan is the
diminished flux of Galactic Cosmic Rays (GCR’s) that penetrate the
atmosphere. Sittler Jr et al. (Icarus, 2019) modeled surface GCR flux to
be ~10^-9 ergs/cm^3/s. Using the GCR flux, in
conjunction with the half-life doses at T=100 K from Gerakines et al.
(Icarus, 2012), we estimate the half-lives to be 7.69 x 10^12;, 5.07
x 10^12, and 5.82 x 10^12 years for Glycine, Alanine and
Phenylalanine, respectively. These extraordinarily long half-lives on
Titan’s surface, as compared to similar calculations for amino acids on
Mars, Europa, or Pluto, are directly the result of reduced energy
deposition due to the atmosphere. We thus conclude that the degradation
of these three amino acids by GCR flux is insignificant over geological
time, and will not be an essential factor in interpreting the chemistry
from Titan’s surface samples from future missions, such as Dragonfly.