CIRCUMPOLAR OCEAN STABILITY ON MARS 3 GY AGO
F. Schmidt, F. Costard, S. Bouley, A. Sejourné,, Université Paris-Saclay, CNRS, GEOPS, 91405, Orsay,
France, Institut Universitaire de France (IUF) M. Way, I. Aleinov NASA/Goddard Institute for Space Studies,
2880 Broadway, NY, NY 10025, USA GSFC Sellers Exoplanet Environments Collaboration, Greenbelt, MD,
USA, TheoreticalAstrophysics, Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden
Center for Climate Systems Research, Columbia University, New York, NY 10025, USA
Summary: Was the nature of the Late Hesperian climate “Warm and wet” or “cold and dry?”
Constructed in this manner the question leads to a
paradox since both options seem implausible [1]. A
“Warm and Wet” climate would have generated
extensive fluvial erosion but not many valley networks have been observed in late Hesperian epoch.
A cold climate would keep a northern ocean frozen
most of the time. A moderate cold climate would
have transferred the water from the ocean to the
land in the form of snow and ice, especially to the
Tharsis region and southern highlands. But this
would in turn prevent tsunami formation, as some
evidence suggests. We provide a novel view from
numerical climate simulations in agreement with
surface observations to show that the Martian climate could have been both cold and wet. We use an
advanced General Circulation Model (GCM) to
demonstrate that an ocean can be stable, even if the
mean surface temperature of Mars is lower than
0°C. Rainfall appears moderate near the shorelines
and in the ocean. Ice covers much of the southern
plateau where the mean temperature is below 0°C
and a glacier return flow back to the ocean exists.
This scenario is accomplished with a 1 bar CO2
dominated atmosphere with 10% H2. At 3 Ga, the
geologic evidence of a shoreline and tsunami deposits along the ocean/land dichotomy are compatible
with ice sheets drained by valley glaciers in the
southern highlands.
Introduction: The possibility of a Martian
ocean is a topic of debate with strong implications
on the habitability of the Red Planet. Geomorphological arguments in favor and against an ocean
have been recently reviewed [2]. There is evidence
of Martian paleo-shorelines [3] in Deuteronilus
(sometimes noted contact no 2) in a geometry close
to the current equipotential height [4]. Deuteronilus
shoreline seems to be the latest in the last stage of
Tharsis induced true polar wander [5]. Recent interpretation of tsunami deposits near the paleoshorelines [6,7] has provided new evidence to the
debate. In addition, the potential impact crater at
the origin of the tsunami wave may have been identified [8]. Our investigations suggest a long-term
stable water body 3 Gy ago in the Northern lowland
of Mars.

Various scenarios have been investigated to
maintain an ocean [1]. If the climate is cold, the
ocean should have been entirely frozen shortly after
its formation. It’s higher albedo helping to assist in
keeping the climate cool. If warm, the ice-free
ocean should have produced intense fluvial erosion
of Hesperian terrains. But there is a lack of observation of such extensive valley network [1]. In addition, water should have accumulated in the southern
highlands in form of ice and thus removed from the
ocean. A cold and wet Mars scenario has been theoretically proposed [9] but the long-term stability of
an ocean in such a scenario has never been achieved
in a 3D-GCM.
Model: We utilize a fully coupled
ocean/atmosphere 3-D General Circulation Model
simulations called ROCKE-3D [10], which is based
upon a parent Earth Climate Model known as ModelE2 [11]. This model allows one to estimate the
interaction between atmosphere/ocean circulation
but also encompasses a sophisticated surface hydrological scheme. We also included a glacier return
flow to the ocean.
We assume the solar luminosity to be ∼79% of
its current value (1360.67W.m−2) [12], hence at 3
Gy, the flux at Mars would be 452.8 W.m−2. The
ocean shoreline is set to –3900 meters in all runs.
This gives an ocean surface fraction of ∼16% which
is small in comparison to Earth at ∼71%. The ocean
is also shallower than the mean depth for Earth. For
this reason, the time to bring our ancient Mars model with its fully coupled ocean and atmosphere into
equilibrium is much shorter (∼100s of years) than
would be the case for an Earth like ocean (∼1000s
of years). We assume this equilibrium has been
reached when the net radiative balance (the difference between incoming and outgoing fluxes) is less
than 0.2 W.m−2.
H2 provides a powerful greenhouse component
with CO2 as a background gas. Other gas combinations with CH4 or H2S may have an equivalent radiative effect, but little motivates their use. We run
simulation with 10% and 20% H2 in a CO2 dominated atmosphere with 0°, 20°, 40°, 60° obliquity,
since the latter had large excursions in the past [13,
14].

Results: Figures 1 and 2 show the simulated effect of an ocean circulation on the surface temperature field averaged over 10 Martian years for
H2=10% and CO2 =90% and total pressure of 1 bar.
Despite a global mean surface temperature below
0°C, the ocean is stable due to its low altitude, low
albedo and circulation. Thanks to the ocean gyre,
the net heat is toward the pole and appears to keep
the circumpolar ocean warmer than a slab ocean up
to 4.5°C.

Figure 1: Net outward heat flux transported by
the ocean for 60° obliquity and H2=10%. The positive value (up to 15 W.m-2) near the North pole indicates that heat goes toward the atmosphere due to
ocean circulation. For a non-circulating slab ocean,
this flux is null. Black contour lines represent surface elevation level and the red contour line is the
paleo-shoreline.

called the "wet lowlands", rain, evaporation and
surface runoff are balanced.
Figure 3 represents a scheme with the climatic
regions superposed with the geological evidences.
Valleys network are present mainly near the shoreline. Glacial valleys are present from the icy highland domain to the ocean, as predicted by the GCM.
Discussion and conclusion: Our results [15]
demonstrate that a cold and wet climate could have
been stable 3Ga and is consistent with geomorphological evidence thanks to glacier return flow. Using fully coupled atmosphere / dynamic ocean modeling, we show that the ocean's circulation regionally warms the surface up to 4.5°C. In these conditions the ocean is stable even for a global mean
planetary temperature below 0°C. This Martian climate may be similar to ancient Earth's with an active water cycle around the time of the early stages
of life's appearance. Future work should encompass
careful analysis of this stable ocean domain and its
application to the past Martian climate, especially
in the Noachian epoch.

Figure 3: Proposed scenario of a cold and wet
climate at the Hesperian age (3 Ga).

Figure 2: Latitudinal profile of the temperature
increase (Tcirculating − Tslab) due to the circulating
ocean, as a function of obliquity.
On land, there is a clear boundary at the 0°C
isotherm which corresponds approximately to the
Martian dichotomy. In the high altitude domain,
commonly referred to as the "icy highlands", the
surface is mostly frozen and snow precipitation is
dominant. The extensive accumulation of snow in
the highlands can lead to the formation of significant ice sheets that may flow down to the Northern
and Hellas basin oceans. Our model is not able to
simulate glacier flow details, but only a global mass
flux from land to the ocean. In the lowest altitudes,

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