MORE THAN BEFORE: INCREASE IN ESTIMATED OXYGEN PHOTOCHEMICAL ESCAPE RATES FROM EMM DATA AND UPDATED MODELING.
K. Chirakkil, Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO,
USA (Krishnaprasad.Chirakkil@lasp.colorado.edu); Space and Planetary Science Center, Khalifa University, Abu
Dhabi, UAE, J. Deighan, Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder,
CO, USA, R. Lillis, Space Sciences Laboratory, University of California, Berkeley, CA, USA, R. Elliott, M. Chaffin,
S. Jain, Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO, USA, M.
Gacesa, Space and Planetary Science Center, and Department of Physics, Khalifa University, Abu Dhabi, UAE, M.
Fillingim, Space Sciences Laboratory, University of California, Berkeley, CA, USA, G. Holsclaw, Laboratory for
Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO, USA, H. AlMatroushi, Mohammed
Bin Rashid Space Center, Dubai, UAE, D. Brain, Laboratory for Atmospheric and Space Physics, University of
Colorado Boulder, Boulder, CO, USA, S. England, Aerospace and Ocean Engineering, Virginia Polytechnic Institute
and State University, Blacksburg, VA, USA, S. Raghuram, Laboratory for Atmospheric and Space Physics, University
of Colorado Boulder, Boulder, CO, USA; Space and Planetary Science Center, Khalifa University, Abu Dhabi, UAE,
F. Lootah, Mohammed Bin Rashid Space Center, Dubai, UAE, H. AlMazmi, UAE Space Agency, Abu Dhabi, UAE.

Introduction
Atmospheric escape plays a major role in the climate
history and evolution of Mars. Quantitative characterization of the current atmospheric loss rate is thus crucial to understanding the evolution of water and CO2
inventories at Mars. During the present epoch, the photochemical escape of neutral atoms such as O, N, C and
H is understood to be a major non-thermal escape mechanism. One of the critical but less certain parameter in
computing the density of hot atoms in the corona (where
most of the escape to space happens) and the resulting escape rate is the collision cross sections between nascent
hot O atoms (that are primarily produced by the dissociative recombination of O+
2 ions with electrons) and
the major thermospheric neutral species, which is CO2 .
The previous studies initially assumed an isotropic hard
sphere approximation for all hot O collisions. Later, a
more realistic quantum mechanical calculation of O +
O collisions (Kharchenko et al. 2000) was applied to O
collisions with different thermospheric species.

Estimation of hot oxygen escape flux
Recently, the new quantum mechanical calculations of
doubly differential cross sections have been performed
for O + CO2 by Gacesa et al. (2020). The present study
updates the existing three-dimensional Monte Carlo hot
O transport simulations (Lillis et al. 2017) by adopting
these new cross sections and their angular dependencies. In-situ observations from the Mars Atmosphere
and Volatile EvolutioN (MAVEN) spacecraft during its
Deep Dip campaign (when the periapsis altitude was
lowered to ∼ 130 km) are used as input to the model.
For each set of neutral density and electron and ion
temperature profiles, we run the Monte Carlo model to
calculate escape probabilities as a function of altitude.

We assume that the atmosphere is spherically symmetric
with the same altitude profile everywhere. Hot O atoms
start at a set altitude and are given random directions and
an initial energy drawn at random from the initial energy
distribution for that altitude (Lillis et al. 2017). They
are propagated through collisions with thermal neutrals,
using the above mentioned collision cross sections.
In order to determine photochemical escape fluxes
of hot oxygen from in-situ measurements, we must separately calculate two different quantities as a function
of altitude: (1) the production rate of hot oxygen atoms
from O+
2 DR (Figure 1a), and (2) the probability that,
once produced, a hot O atom will escape the atmosphere
(Figure 1b). The hot O production rate, in number per
cubic centimeter per second, is given by twice the DR
rate, since each reaction produces two oppositely directed O atoms. The escaping production rate is then
calculated by multiplying the total production rate with
the ecsape probability (Figure 1c). The escaping flux
is calculated by integrating the ecsaping production rate
with altitude. The inclusion of new O + CO2 cross sections is giving a factor of ∼ 6 enhancement in the oxygen
escape flux as compared to the approach by Lillis et al.
(2017) (Figure 1c).

Data–model comparison
Observations from the Emirates Mars Ultraviolet Spectrometer (EMUS; Holsclaw et al. 2021) instrument onboard Emirates Mars Mission (EMM; Amiri et al. 2022,
AlMatroushi et al. 2021) spacecraft are used for validating the modeled coronal oxygen densities. EMUS
is a high-sensitivity UV spectrometer, which measures
emissions in the spectral range 100–170 nm. The FUV
solar resonant fluorescence emission line of atomic oxygen at 130.4 nm in the corona is observed by EMUS up

(a)

400

(b)

400
present work
Lillis et al. (2017)

300

Altitude (km)

Altitude (km)

present work
Lillis et al. (2017)

200

100
-4

300

200

-3

-2

-1

0

1

2

100
0.0001

3

3

0.001

0.01

1

(c)

400

present work
Lillis et al. (2017)

(d)

8000

EMUS O I 130.4 nm
average O I 130.4 nm
model

7000
6000

300

Altitude (km)

Altitude (km)

0.1

Hot O Escape Probability

Log Hot O Total Production Rate (#/cm /s)

Escaping flux = 7.18e+07 #/cm 2 /s
Escaping flux = 1.43e+07 #/cm 2 /s

200

5000
4000
3000
2000
1000

100
-4

-3

-2

-1

0

1

2

3

0

1

10

3

Log Hot O Escaping Production Rate (#/cm /s)

100
Counts

1000

10000

Figure 1: (a) Total production rate of hot oxygen, (b) escape probability of hot oxygen, (c) escaping production rate and escape
flux of hot oxygen, and (d) comparison of EMM/EMUS hot oxygen corona observations with the Monte Carlo model.

to several Mars radii (> 6 RM ). Observing this tenuous
hot oxygen corona is challenging (Deighan et al. 2015),
and such high-altitude observations are made for the first
time. The oxygen coronal brightnesses are obtained using the long exposure time EMUS observation strategy,
termed as U-OS4.
The U-OS4 provides long exposure times for the mid
and outer exosphere and will occur while the spacecraft
is charging in a near-inertial orientation (Holsclaw et al.
2021). There are two scenarios for this observation: UOS4a and U-OS4b. The U-OS4a is a cross-exosphere
observation that is targeting the coronal oxygen emission
at 130.4 nm with the instrument boresight vector in the
plane of the spacecraft orbit, perpendicular to both the
Mars-Sun line and orbit normal, while U-OS4b targets
the interplanetary background. A typical such U-OS4a
observation taken on February 13, 2022 (EMM orbit
number 172) is compared with the Monte Carlo hot O
model (Figure 1d). The comparison shows a gross match

between the modeled and observed profiles.

References
Almatroushi, H., AlMazmi, H., AlMheiri, N. et
al. (2021), Emirates Mars Mission Characterization of Mars Atmosphere Dynamics and Processes, Space Sci. Rev. 217, 89, https://doi.
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Gacesa, M., Lillis, R. J., Zahnle, K. J. (2020),
O(3 P) + CO2 scattering cross-sections at superthermal collision energies for planetary aeronomy,
Monthly Notices of the Royal Astronomical Society, 491, 4, 5650–5659, https://doi.org/

Kharchenko, V., Dalgarno, A., Zygelman, B.,
and Yee, J.-H. (2000), Energy transfer in collisions of oxygen atoms in the terrestrial atmosphere, J. Geophys. Res., 105(A11), 24899–
24906, https://doi.org/10.1029/2000JA000085

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Lillis, R. J., Deighan, J., Fox, J. L. et al. (2017),
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