ON THE EFFECT OF THE OBLIQUITY OF MARS TO THE HYDROGEN ESCAPE AND THE FATE OF WATER IN THE LAST MILLIONS OF YEARS.
G. Gilli, Instituto de Astrofisica de Andalucia (IAA-CSIC), Granada, Spain (gilli@iaa.es), F. Gonzalez-Galindo,
IAA-CSIC, Granada, Spain, F. Forget, J. Naar, E. Millour, Laboratoire de Meteorologie Dynamique (LMD)-IPSL,
Paris, France, J-Y. Chaufray, Laboratoire Atmospheres, Observations Spatiales (LATMOS)-IPSL, Paris, France.

Introduction
The climate of a planet depends on its orbital and rotation
parameters, and in particular on the obliquity (i.e. inclination of its axis of rotation to its orbit plane). While
for the Earth the oscillations of those parameters are
small (± 1.3º for obliquity), in the past 250 Myrs Mars’s
axis inclination covered a large range of variations, between 0 and 66º, with a mean obliquity of about 35º
[Laskar et al., 2004]. Different orbital configurations
induce significant modifications in fundamental aspects
of the Martian climate, mainly due to the differences in
the distribution of the insolation, such as the CO2 cycle
(and thus the surface pressure),the dust and water cycle,
or the global circulation [Forget et al., 2017]. The north
pole insolation is a key parameter controlling the stability of water ice of the northern polar cap. As shown in
Figure 1 it could have varied up to 350 W m−2 in the
past 20 million years.
Mars was not always as dry as it is today, as several
geologic and mineralogical observations indicate the evidence for past liquid water: valley networks and lakes
are still visible on the surface [Bibring et al., 2004]. Loss
to space appears to explain why the Mars atmosphere
evolved from an early, warmer climate to the cold, dry
climate that we see today. Substantial amounts of water
could have escaped into the interplanetary medium in
the form of atomic hydrogen [Jakosky et al., 2018]. Furthermore, recent observations indicate that the amount
of exosphere hydrogen at Mars has important seasonal
variations, with significant increases of both the water abundance in the mesosphere and the H escape rate
during dust storms [Chaffin et al., 2014, Clarke et al.,
2014]. By analysing observations by SPICAM on board
Mars Express and simulations with the LMD Mars General Circulation Model (LMD-MGCM), Chaufray et al.
[2021] suggested that episodic dust storm and associated enhancement at high altitude near the perihelion,
averaged over one Martian year or longer period, are a
major factor in the H escape estimates. Nevertheless,
the accumulated water lost at the estimated rate for 4
billion years is much lower than the amount of water
needed to form the flow channels observed on Mars.
Both the dust content and the water content of the atmosphere are expected to vary with the obliquity of the
planet, thus, the loss rate definitely is not expected to
have been constant with time and may vary significantly
during Martian history.

Figure 1: Evolution of the obliquity (a), eccentricity (b) and
the north pole insolation (c) at the summer equinox (Ls = 90°)
from -20 to +10 million years, provided by Laskar et al. [2004]
using the most recent data for the rotational state of Mars, and
a new numerical integration of the Solar System)

Previous measurements by different space missions
put in evidence the strong coupling between the escape
and the water cycle, but many unknowns remain, including the role of the different processes involved in transporting water from the lower to the upper atmosphere
and in converting the water molecules into Hydrogen
atoms. Also, the relative importance of the different neutral and ionospheric chemical reactions in the production
of thermospheric H or the effects of global dust storms
compared to the regular seasonal variability have to be
taken into account (see abstract by Gonzalez-Galindo et

H escape during Mars high obliquity Epochs
al. 2022, this issue)

A ground-to-exosphere Mars GCM with an update
water cycle parameterization
In this study we use an improved version of the groundto-exosphere Mars General Circulation Model (MGCM)
developed at the Laboratoire de Meteorologie Dynamique
(LMD) with the water cloud microphysics as in Navarro
et al. [2014] plus an update photochemical model which
includes water-derived ions (H2 O+, H3 O+, OH+) as in
González-Galindo et al. [2021]
Modeling the water cycle on Mars is challenging
because of destabilizing feedback, such as the strong
coupling with atmospheric temperature, through clouds
and global circulation. The water vapour mixing ratio
in the mesosphere also depends on the supersaturation
of the upper atmosphere, which is not well known. Furthermore, the effect of the microphysical processes at
high altitude depend on model parameters not well constrained by observations [Naar et al., 2021].
The LMD-MGCM was successfully applied to simulate the past martian climate at different epochs, and for
"recent" geological time (million of years ago) simulations showed that higher planetary obliquity conditions
produce a significant increase of the water content in
the atmosphere and accumulation of ice in the flanks of
the prominent martian volcanoes, where geological observations indicated past glacial activity [Forget et al.,
2006]. See abstract by Lange et al. (this issue) for simulations of the climate of Mars at low obliquity.
However, those studies are focused on the lower atmosphere and the variability of the escape rate with the past
martian obliquity was never explored before.

Figure 2: LMD-MGCM Simulated Loss/Escape rate in
atoms/s for the reference year (with current martian obliquity
= 25º) and with higher obliquity (obliq. = 35º). Model outputs have been averaged binning solar Longitudes Ls every
15º degree. The vertical lines on the right of the panel indicate
minimum/maximum values of the loss escape over one Martian Year, for current obliquity (blue) and for higher obliquity
(red) simulations.

Simulations of H Escape in "recent" Mars epochs
We aim to provide an estimation of the variation of water
escape in past epochs characterized by different orbital
parameters. In particular, the sensitivity of the hydrogen
escape rate to changes in the obliquity during last 10-20
millions years of Mars history will be presented, and the
fate of water will be discussed. As shown in Figure 1
the obliquity varied between 15º and 45º in the past 20
million year, with a mean value around 35º.
Long simulations are needed before the modified
water cycle forced by different orbital conditions becomes stable. Furthermore, ground-to-exosphere simulations are particularly computational expensive, and
this is one of the main limitation of this work. Figure
2 show simulations after two consecutive martian years,
initialized with MGCM outputs after 20 martian years,
which simulate stable water cycle conditions in the low
atmospheres.

Figure 3: Estimated H loss rate from observations made
from different spacecraft, together with simulated H loss rate
(min/max values over 1 Martian Year) as shown in Figure 2,
for comparison. (Adapted from Jakosky et al. [2018] )

Preliminary results and future work
Present-day loss rates of H by Jeans’ (i.e. thermal)
escape was determined by MAVEN, with values variable during the season between about 1-11· 1026 at/s−1
[Jakosky et al., 2018]. Similar results was found by observations made from Mariner 6, 7, and 9, Mars Express,
Hubble space Telescope and Rosetta flyby of Mars, as
shown in Figure 3. Our simulations with the LMD-

REFERENCES

MGCM show that, if the martian obliquity was higher
than current values (e.g. 35º) the escape rate would have
increased up to one order of magnitude (1028 atoms/s),
especially during dust storm seasons. Preliminary results give 5 times larger time-integrated escape rate than
current values. This indicates that a significant H loss
could have taken in the past, producing the evaporation
of the large reservoir of liquid water potentially present
on the surface of Mars in the past million of years.
This theoretical exercise will be performed with
other orbital parameters (e.g. eccentricity and time of
perihelion) in order to cover a set of past Martian conditions, following the variability range given in Laskar
et al. [2004].
Acknowledgment
GG and FGG are funded by the Spanish Ministerio
de Ciencia, Innovación y Universidades, the Agencia
Estatal de Investigación and EC FEDER funds under
project RTI2018100920JI00. The IAA team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the of Excellence
Severo Ochoa" award to the Instituto de Astrofı́sica de
Andalucı́a (SEV20170709)
References
J.-P. Bibring, Y. Langevin, F. Poulet, A. Gendrin,
B. Gondet, M. Berthé, A. Soufflot, P. Drossart,
M. Combes, G. Bellucci, V. Moroz, N. Mangold,
B. Schmitt, OMEGA Team, S. Erard, O. Forni,
N. Manaud, G. Poulleau, T. Encrenaz, T. Fouchet,
R. Melchiorri, F. Altieri, V. Formisano, G. Bonello,
S. Fonti, F. Capaccioni, P. Cerroni, A. Coradini,
V. Kottsov, N. Ignatiev, D. Titov, L. Zasova, P. Pinet,
C. Sotin, E. Hauber, H. Hoffman, R. Jaumann,
U. Keller, R. Arvidson, J. Mustard, T. Duxbury, and
F. Forget. Perennial water ice identified in the south
polar cap of Mars. , 428(6983):627–630, Apr. 2004.
doi: 10.1038/nature02461.
M. S. Chaffin, J.-Y. Chaufray, I. Stewart, F. Montmessin,
N. M. Schneider, and J.-L. Bertaux. Unexpected variability of Martian hydrogen escape. , 41(2):314–320,
Jan. 2014. doi: 10.1002/2013GL058578.
J. Y. Chaufray, F. Gonzalez-Galindo, M. A. LopezValverde, F. Forget, E. Quémerais, J. L. Bertaux,
F. Montmessin, M. Chaffin, N. Schneider, J. T.
Clarke, F. Leblanc, R. Modolo, and R. V. Yelle.
Study of the hydrogen escape rate at Mars during martian years 28 and 29 from comparisons

between SPICAM/Mars express observations and
GCM-LMD simulations. , 353:113498, Jan. 2021.
doi: 10.1016/j.icarus.2019.113498.
J. T. Clarke, J. L. Bertaux, J. Y. Chaufray, G. R. Gladstone, E. Quemerais, J. K. Wilson, and D. Bhattacharyya. A rapid decrease of the hydrogen corona
of Mars. , 41(22):8013–8020, Nov. 2014. doi:
10.1002/2014GL061803.
F. Forget, R. M. Haberle, F. Montmessin, B. Levrard,
and J. W. Head. Formation of Glaciers on Mars by
Atmospheric Precipitation at High Obliquity. Science,
311(5759):368–371, Jan. 2006. doi: 10.1126/science.1120335.
F. Forget, S. Byrne, J. W. Head, M. A. Mischna,
and N. Schörghofer. Recent Climate Variations.
In R. M. Haberle, R. T. Clancy, F. Forget, M. D.
Smith, and R. W. Zurek, editors, Asteroids, Comets,
Meteors - ACM2017, pages 464–496. 2017. doi:
10.1017/9781139060172.016.
F. González-Galindo, J.-Y. Chaufray, F. Lefèvre,
F. Montmessin, M. Vals, L. Rossi, F. Forget, and
E. Millour. Escape of water from Mars: study with a
ground-to-exosphere global model. In European Planetary Science Congress, pages EPSC2021–395, Sept.
2021. doi: 10.5194/espc2021-395.
B. M. Jakosky, D. Brain, M. Chaffin, S. Curry, and et al.
Loss of the Martian atmosphere to space: Present-day
loss rates determined from MAVEN observations and
integrated loss through time. , 315:146–157, Nov.
2018. doi: 10.1016/j.icarus.2018.05.030.
J. Laskar, A. Correia, M. Gastineau, F. Joutel, B. Levrard, and P. Robutel. Long term evolution and
chaotic diffusion of the insolation quantities of mars.
Icarus, 170(2):343–364, 2004. ISSN 0019-1035. doi:
https://doi.org/10.1016/j.icarus.2004.04.005.
J. Naar, F. Forget, M. Vals, E. Millour, A. Bierjon,
F. González-Galindo, and B. Richard. Modeling the
global water cycle on Mars with improved physical parametrization. In European Planetary Science
Congress, pages EPSC2021–559, Sept. 2021. doi:
10.5194/espc2021-559.
T. Navarro, J.-B. Madeleine, F. Forget, A. Spiga, E. Millour, F. Montmessin, and A. Määttänen. Global climate modeling of the Martian water cycle with improved microphysics and radiatively active water ice
clouds. Journal of Geophysical Research (Planets), 119:
1479–1495, July 2014. doi: 10.1002/2013JE004550.