Photochemical escape of O and C from Mars: the impact of collisional cross
sections
M. Gacesa, Space and Planetary Science Center and Department of Physics, Khalifa University, Abu Dhabi,
UAE (marko.gacesa@ku.ac.ae), Y. Lee, University of Maryland Baltimore County, MD, USA; Center for Research and Exploration in Space Science and Technology II, MD, USA; NASA Goddard Space Flight Center,
Planetary Environments Laboratory, MD, USA, B. M. Krishna, Department of Physics, Khalifa University,
Abu Dhabi, UAE

Introduction:
Recent estimates based on Ar isotope fractionation in the upper atmosphere suggest that Mars has
lost at least 66% of its original atmosphere to space,
including most of its initial water inventory. In the
current epoch, the photochemical escape is considered responsible for much of the loss of atomic oxygen and carbon, while Jeans escape remains the dominant escape mechanism of hydrogen. The classical
picture is that H and O escape from Mars in 2:1 ratio
when averaged over time periods longer than about
105 years, with H escape responding to O loss. Thus,
the physical processes governing the O escape effectively determine the overall escape rate, while the
variations due to seasonal effects and climate cycles
average out.
More recently, NASA’s Mars Atmosphere and
Volatile EvolutioN (MAVEN) mission detected seasonal variations of hydrogen escape in excess of an
order of magnitude, in contrast to the predictions of
the classical picture. Here, the leading hypothesis is
that these variations occur due to the enhanced vertical transport of water to higher altitudes than previously thought possible because of stronger coupling
of the lower and middle atmospheres during the Martian summers3. The dust storms may also play a role
in enhancing the transport: the atmospheric water
vapor on Mars at altitudes as high as 80 km has been
observed during global and regional dust storms1,2.
During the regional 2019 C-storm, the hydrogen corona reconstructed from measurements of the hydrogen Ly α line brightness implied that enhanced water
transport from mid-altitudes (60-80 km) took place.
The ExoMars Trace Gas Orbiter (TGO) observation
of HDO during the 2018 global dust storm arrived at
the same conclusion.
Quantitative characterization of these loss processes and their role in the past epochs requires an
accurate description of the physical mechanisms involved, including collisional cross sections between
the escaping energetic atoms and atmospheric atomic
and molecular species, as well as accurate global
models of the atmospheric escape to guide the interpretation of the observations.
Results and Discussion:
Here, we present new calculations of scattering
cross sections for C-CO2 elastic and inelastic colli-

sions, as well as preliminary full-dimensional O-CO2
cross sections. These two scattering processes play a
major role in the atmospheric escape due to the
abundance of CO2 in the Martian atmosphere. The
C-CO2 state-to-state velocity-dependent cross sections were computed using a modified ArthursDalgarno quantum-mechanical rigid rotor model,
whereas the O-CO2 cross sections were constructed
using the Multi Configuration Time Dependent
Hartree (MCTDH) formalism on a new fulldimensional potential energy surface capable of correctly describing the vibrational and bending modes
of CO2.
We estimated the impact of the new cross sections on the atmospheric escape rates using a 1-D
column escape model and compared them to the results obtained using a global 3-D model4 (3D Mars
Global Ionosphere Thermosphere Model coupled
with the Adaptive Mesh Particle Simulator; MGITM+AMPS; see Y. Lee et al.5 for a detailed description of the model setup and parameters). The
resulting difference coronae for three scenarios5 considered are given in Fig. 1 while the total O escape
rates are summarized in Table 1.

Figure 1: Difference hot O corona of Mars
(meridional plane view) calculated for three scenarios: Case 2 includes O-CO2 cross sections6 at a single energy (2.5 eV), Case 3 includes O-CO2 cross

sections with the full energy dependence6, and Case
4 includes both O-CO2 and newly calculated energy-dependent O-CO cross sections. All cross sections are taken as total (elastic + inelastic) cross
sections. The color indicates hot O density difference
between the reference Case 1 (previous study) and
three considered scenarios.

Table 1: Total escape rates for Cases 1 through 4
calculated using the global 3D model5.

The most realistic scenarios (Case 3 and Case 4)
predict about 15% and 30% greater O escape rate
than previously used Case 1, respectively.

Figure 2: Hot C density at Mars modeled using the
new C-CO2 cross sections in comparison to hard
sphere cross sections.
In contrast, our preliminary calculation of hot C
escape rate5, shows about 60-70% reduction when
the new C-CO2 cross sections are used in place of
the hard sphere cross sections (Fig. 2). Another notable result is that O-CO cross sections significantly
affect the thermalization of hot O and the total escape rate (Case 4).
The studied scenarios clearly illustrate the importance of using accurate collision cross sections, as
well as their angular (differential cross sections) and
velocity dependence, in a global 3D simulation for
quantitative characterization of the escape of neutral
O and C from Mars.

Acknowledgments:
The authors acknowledge partial support by the
internal grants 8474000362-KU-FSU (BMK, MG)
and 8474000336-KU-SPSC (MG).

References:
1
A. C. Vandaele et al., Nature, 568, 521 (2019)
2
L. Maltagliati et al., Science, 333, 1868 (2011)
3
M. S. Chaffin et al., Geophys. Res. Lett., 41, 8013
(2014)
4
S. Bougher et al., J. Geophys. Res. Planets,
doi:10.1002/2014JE004715 (2015)
5
Y.Lee et al., "Mars’ Hot Oxygen and Carbon
Coronae and Photochemical Escape: Updated Model
Parameters", AGU Fall Meeting 2021
6
M. Gacesa et al., MNRAS 491, 5650 (2020)