MODELLING UV DEGRADATION OF AIRFALL ORGANIC MATERIAL
AT GALE CRATER
A. Das, J. E. Moores, H. M. Sapers, Centre for Research in Earth and Space Science, York University, 4700
Keele Street, Toronto, ON M3J 1P3 Canada (ankita99@yorku.ca), C. L. Smith, Oberlin College, Oberlin, OH,
USA.

Introduction:
UV radiation is poorly attenuated by the Martian
atmosphere and will cause any organic material on the
surface to degrade. Damaging the bonds of the organic molecule can create radicals which further damage the molecule causing faster degradation (Ehrenfreund et al., 2001). UV radiation received at the surface is dictated by the time of the year and latitude.
The origin of organic material on Mars in the modern
era is accretion of Interplanetary Dust Particles (IDPs)
(Flynn, 1996). According to Flynn 1996, Mars receives almost 8.6 x 106 kg/m2 of unmelted meteoritic
material every year, and of the order 106 kg/year of
this organic material is unaltered. IDPs survive for
years on the surface, with smaller particles having
lifetimes of up to 4900 years for larger sized IDPs
(Moores et al., 2017; Moores & Schuerger, 2012) because most photons incident on the surface do not interact with the individual IDPs, resulting in a of ~ 106
kg/m2 of organic carbon on the surface at equatorial
latitudes (Moores & Schuerger, 2012). This work
aims to probe mechanisms that can preserve organic
carbon on Mars and whether the amount will be detectable by future spacecrafts.
Rhythmic sedimentary rocks have been discovered through remote sensing and in-situ observations
in the mid-latitude and equatorial regions of Mars.
The formations discovered in Gale crater Lewis and
Aharonson (2014) propose a depositional timescale of
the order Myr for these formations and hypothesize
the cause for cyclic bedding to be changes in the
obliquity of Mars (Lewis & Aharonson, 2014). The
sediments are hypothesized to have formed due to accumulation of falling aeolian dust which changes in
atmospheric concentration over variation in orbital
oscillations due to climate changes caused by Mars’
varying obliquity (Laskar, 2004; Lewis & Aharonson,
2014). IDPs being accreted on Mars can become deposited in these sedimentary rocks along with the
dust. Fig 1 demonstrates this process.
IDPs embedded in sedimentary rhythmites will
then be shielded from UV radiation and this process
might result in enhanced preservation of organic carbon. Martian regolith attenuates the UV rays penetrating the rocks. Studies have shown that depth of 2 – 10
mm are required to shield radioresistant microorganisms from penetrating UV rays for minimum survivability (Moore, 2019). Earlier works have indicated at

a covering of 1-2 mm for complete shielding (Mancinelli & Klovstad, 2000; Marschall et al., 2012).
As such, this work aims to address the question of
whether rhythmic sedimentary rocks represent an environment that can preserve modern-day infalling organic material (~ 106 kg/m2/year). If so, these sedimentary rhythmites will provide an interesting lithology in which the amount of organic in-fall to Mars
can be measured by future spacecraft which will be
informed by this work. Gale crater was chosen for this
project since the UV environment is well studied and
discoveries of sedimentary rhythmites have been
made at this site as well.

Fig 1: Journey taken by IDPs on Mars
Modeling:
This work makes use of the outputs produced by a
1.5-dimension Mars Doubling and Adding Radiative
Transfer model (C. L. Smith & Moores, 2020). The
model is similar in functionality to the one developed
by Griffith et al. (2012) for Titan and has been adapted
for Martian conditions (Griffith et al., 2012).
The model considers the surface of Mars to be a
Hapke surface with the radiative response being informed by the observations by the Rover Environmental Monitoring Station (REMS) instrument on
Mars Science Laboratory (M. D. Smith et al., 2016).
The Doubling and Adding Radiative Transfer model
models the atmosphere as consisting of two levels and
takes into account gaseous and aerosol absorption,
Mie scattering by aerosols, and Rayleigh scattering
for the species found in the Martian atmosphere
(Moores et al., 2017).
This work develops an additional numerical
model bringing the following pieces of information
together: a) accretion of IDPs on Mars, b) UV

degradation of organic carbon, and c) burial of IDPs
in rhythmites and consequent UV shielding. The
model devised will explore a parametric space of the
range of IDP particle sizes (ranging from 60 to 270
μm Flynn,1996), the amount of carbon in IDPs, deposition rates of the beds, and thickness of sedimentary
beds. The parameter space will be informed by existing literature.
Initially the IDPs would be subject to the unshielded UV radiation received at surface, but over
time more dust would accumulate on top of the IDPs
and the received radiation and degradation from UV
would decline until the particles are completely protected. The depositional timescales of the sediments
were found to be around 100 kyr per bed with each
bed at Gale crater around 40-60 m (Lewis & Aharonson, 2014).

Fig 2: Diagram depicting working of the model
The model will simulate the following environments the IDP passes through and what affect it can
have on its Carbon content (summarized in Fig 2):
1. IDP accretes on surface and is exposed to UV
radiation without regolith shielding: organic carbon
faces maximum degradation. The rate of degradation
is dependent on solar longitude (Ls), latitude, amount
of dust present in the atmosphere, and other factors
which have been incorporated in the Doubling and
adding Radiative Transfer model.
2. A layer of dust is accreted on top of the IDPs,
embedding them. Organic Carbon is shielded by one
layer of regolith. The layer is formed due to gradual
accumulation of infalling dust at the rate of 10s of microns per year (Lewis & Aharonson, 2014).
3. More layers are accreted on top, resulting in
lesser UV radiation hitting the IDPs as time progresses
4. Eventually, the UV radiation penetrating the top
layers of the dust stone becomes negligible and degradation of the organic carbon present in the IDP
stops.

Other factors affecting the degradation of IDPs
embedded in sedimentary rhythmites:
Radiation from galactic and solar cosmic rays
(GCRs and SCRs) are additional radiation sources
that degrade organic molecules. SCRs are capable of
destroying organic molecules in the first 2 cm of Martian regolith and GCRs penetrate further (Pavlov et
al., 2012). The discovery of perchlorate at the Phoenix
landing site implies possible oxidization of organics
by oxychlorine species (Hecht et al., 2009). Decomposition of perchlorates produces reactive species
which degrades organic material (Quinn et al., 2013).
This work focuses only on the UV degradation of organics and doesn’t consider additional sources of degradation like GCRs, SCRs, and perchlorates.
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