HOT H ESCAPE AT MARS: AN ASSESSMENT OF THE IMPORTANCE OF
PHOTOCHEMICAL H LOSS MECHANISMS
B. S. Gregory1, M. S. Chaffin1, R. D. Elliott1, H. Gröller2, J. Deighan1, E. M. Cangi1, 1Laboratory for
Atmospheric and Space Physics, University of Colorado Boulder, USA (bethan.gregory@lasp.colorado.edu),
2
Lunar and Planetary Laboratory, University of Arizona, USA.

Introduction:
The loss of hydrogen to space has played an
important role in the desiccation of Mars, which has
lost most of its initial water. Hydrogen escape is
therefore important for understanding the planet’s
atmospheric evolution. This process has been largely
attributed to thermal escape, in which the high-energy
tail of H atoms in a Maxwell-Bolzmann velocity
distribution may have sufficient energy to be
irreversibly removed from the upper atmosphere.
However, a smaller population of higher-temperature
H atoms is also expected to be present, indicated by
modelling studies (e.g. Nagy et al., 1990) and inferred
by observations (e.g. Chaufray et al., 2008;
Bhattacharyya et al., 2015; Chaffin et al., 2018).
These “hot” or nonthermal hydrogen atoms are those
that are either produced by photochemical reactions
which may give them a large initial kinetic energy, or
those that gain additional energy through charge or
momentum transfer processes with other particles.
Here, we present escape estimates for 45
photochemical mechanisms that produce hot H, in
order to compare the importance of these mechanisms
for nonthermal H loss from Mars and direct future
lines of enquiry.
Escape probability profiles:
We use a new Monte Carlo model based on those
of Deighan (e.g. Lillis et al., 2017) and Gröller
(Gröller et al., 2010, 2012, 2014) to produce an escape
probability profile. The model simulates a distribution
of hot H particles and tracks them as they move
through the atmosphere and collide with background
species. The simulation ends when the particles a)
undergo sufficient collisions such that their energies
drop below the escape energy (~0.05-0.13 eV,
dependent on altitude), b) hit the lower boundary of
the model (~80 km) or c) move with more than escape
energy beyond the upper boundary (~5000 km). We
count the number of test particles in category (c) in
the simulation in order to calculate an escape rate.
For two different energies (5 eV and 0.2 eV), we
choose a series of altitudes between 80 and 400 km
and produce 1000 test particles at each altitude. We
count the number of test particles that escape, and
repeat the simulation three further times for each
altitude in order to take the mean. We do this for both
low and high solar activity models (as defined in Fox
(2015)), using background species density profiles

from Fox (2015).
We find that escape probabilities are very small
below ~130-140 and ~140-150 km for low and high
solar activity model atmospheres, respectively, and as
large as 0.92 and 0.84 at 400 km (our highest
production altitude; Figures 1c and 1g). The latter are
much larger than the maximum escape probabilities
for hot oxygen atoms on Mars (Lillis et al., 2017).
Even though the directions of the initial velocities of
the H atoms are uniformly-distributed, with 50% of
the population moving upward initially and the other
half moving downward toward the planet, the
comparatively low mass of H atoms means that the
number of collisions required to turn the downwarddirected particles around is less than that required to
thermalise the particles.
Estimates of nonthermal H escape:
We calculate the total background column density
above each chosen altitude (N), and fit curves of the
form p = Ae-aσN, where p is the escape probability and
σ is the cross-section. The constants a and A are both
between 0 and 1: a is a transparency coefficient to
account for the insufficiency of single collisions to
prevent escape, while A is the escape probability at
which N is zero.
Using reactant density and temperature profiles
(Fox, 2015) and rate coefficients (Rodriguez et al.,
1984; Fox, 2015 and references therein), we calculate
production rate profiles. The escaping H production
rate is equal to the production rate multiplied by the
escape probability at each altitude. We integrate the
escaping H production rate profile over altitude to
obtain a hot H escape flux for each mechanism, for
each of the two example energies. This method is
summarised in Figure 1.
We find that for both the low and high solar
activity cases, for both energies, the dissociative
recombination of HCO+ (HCO+ DR) is the most
important nonthermal mechanism producing escaping
H. For low solar activity conditions, HCO+ DR
contributes 52% and 42% of the total nonthermal H
escape for the 5 eV and 0.2 eV distributions,
respectively. For both low and high solar activity,
other important mechanisms include CO2+ + H2 →
OCOH+ + H, O+ + H2 → OH+ + H, and OH+ + O →
O2+ + H. We find that CO2 + H+ → CO2+ + H is also
within the top five most important mechanisms for

Figure 1: Illustration of escaping H production rate calculation method and preliminary results. Panels (a)
and (e) show number density profiles for the reactants of the top 5 photochemical mechanisms in both the
low (LSA) and high (HSA) solar activity models in our study. Panels (b) and (f) show the hot H production
rate profiles for the top 5 mechanisms, for low and high solar activity, respectively. Panels (c) and (g) show
the escape probability profiles for two kinetic energies for low and high solar activity, respectively. Panels (d)
and (h) show the calculated escaping H production rate profiles, which are the product of the production rate
and escape probability at each altitude. Thicker lines show escape rate results for 5 eV particles, while thinner
lines show escape rate results for 0.2 eV particles.

low solar activity conditions and N2+ + H2 → N2H+ +
H is within the top five most important mechanisms
for high solar activity conditions. The top five
mechanisms contribute 90-94% and 88-91% of the
total nonthermal H escape for low and high solar
activity, respectively. For each of the top
mechanisms, the reactant number density, H
production rate, escape probability and escaping H
production rate profiles are shown for low and high
solar activity in Figure 1.
Applications and future work:
Our escape probability curves are independent of
reactant number density, background number density
and temperature profiles, so are flexible for use with
different model or observational output profiles,
including temporally-varying profiles. We have
performed detailed Monte Carlo model calculations
for HCO+ DR (Gregory et al., in prep), and will carry
out more detailed modelling of the subsequent four
most important mechanisms shown here, for both low
and high solar activity conditions, in a future study.
The detailed modelling will allow for better
predictions of nonthermal H escape, as the particle
energies given by each mechanism will be properly
accounted for. Though shown to be less important

than other mechanisms here, we will also examine
resonant charge exchange (H + H+), as it is known to
be important on Venus.
Conclusion:
Here we show new escape probability profiles for
nonthermal H, of up to more than 90%, which is
higher than that for nonthermal oxygen (Lillis et al.,
2017). Since our escape estimates are independent of
density and temperature profiles, they are flexible for
use with different inputs. We produce escape
estimates for 45 photochemical H-producing
mechanisms, and show that the most important, for
both low and high solar activity model atmospheres
and particle energies of 5 eV and 0.2 eV, is the
dissociative recombination of HCO+. Our preliminary
results give a total nonthermal escape flux of 11-13%
and 5% of the canonical thermal H escape flux (2.4
×108 cm-2 s-1; McElroy, 1972) for 5 eV and 0.2 eV
populations, respectively. We are currently working
on detailed Monte Carlo modelling of the five most
important mechanisms, as well as resonant charge
exchange, in order to obtain more comprehensive
escape rates for the most significant nonthermal H
escape sources.

References:
Bhattacharyya, D., Clarke, J.T., Bertaux, J.L.,
Chaufray, J.Y. and Mayyasi, M., 2015. A strong
seasonal dependence in the Martian hydrogen
exosphere. Geophysical Research Letters, 42(20),
pp.8678-8685.
Chaffin, M.S., Chaufray, J.Y., Deighan, J., Schneider,
N.M., Mayyasi, M., Clarke, J.T., Thiemann, E., Jain,
S.K., Crismani, M.M.J., Stiepen, A. and Eparvier,
F.G., 2018. Mars H escape rates derived from
MAVEN/IUVS
Lyman
alpha
brightness
measurements and their dependence on model
assumptions. Journal of Geophysical Research:
Planets, 123(8), pp.2192-2210.
Chaufray, J.Y., Bertaux, J.L., Leblanc, F. and
Quémerais, E., 2008. Observation of the hydrogen
corona
with
SPICAM
on
Mars
Express. Icarus, 195(2), pp.598-613.
Fox, J.L., 2015. The chemistry of protonated species
in the Martian ionosphere. Icarus, 252, pp.366-392.
Gröller, H., Lammer, H., Lichtenegger, H.I.M.,
Pfleger, M., Dutuit, O., Shematovich, V.I., Kulikov,
Y.N. and Biernat, H.K., 2012. Hot oxygen atoms in
the Venus nightside exosphere. Geophysical research
letters, 39(3).
Gröller, H., Lichtenegger, H., Lammer, H. and
Shematovich, V.I., 2014. Hot oxygen and carbon
escape from the Martian atmosphere. Planetary and
Space Science, 98, pp.93-105.
Gröller, H., Shematovich, V.I., Lichtenegger, H.I.M.,
Lammer, H., Pfleger, M., Kulikov, Y.N., Macher, W.,
Amerstorfer, U.V. and Biernat, H.K., 2010. Venus'
atomic hot oxygen environment. Journal of
Geophysical Research: Planets, 115(E12).
Lillis, R.J., Deighan, J., Fox, J.L., Bougher, S.W.,
Lee, Y., Combi, M.R., Cravens, T.E., Rahmati, A.,
Mahaffy, P.R., Benna, M. and Elrod, M.K., 2017.
Photochemical escape of oxygen from Mars: First
results from MAVEN in situ data. Journal of
Geophysical Research: Space Physics, 122(3),
pp.3815-3836.
Nagy, A.F., Kim, J. and Cravens, T.E., 1990, April.
Hot hydrogen and oxygen atoms in the upper
atmospheres of Venus and Mars. In Annales
Geophysicae (Vol. 8, pp. 251-256).
Rodriguez, J.M., Prather, M.J. and McElroy, M.B.,
1984. Hydrogen on Venus: Exospheric distribution
and escape. Planetary and space science, 32(10),
pp.1235-1255.