main interests are the long term evolution of terrestrial
planets and their habitability. Comparing well-known
terrestrial planets and studying how they evolved can
improve knowledge of both how these complex systems work and
what determines their habitability, in particular when it
comes to the Earth.
Habitability is normally defined as the existence of liquid
water at or near the surface.The "fluid envelope" of a
terrestrial planet (i.e. atmosphere and ocean, if present)
evolves greatly with time due to coupling with planet's
interior (mantle and crust) and escapes to space.
is habitable while sustaining a plate tectonics convection
regime. On the other hand, Mars and Venus do not possess
plate tectonics and are not currently habitable. The
increased exchanges permitted by plate tectonics could be an
important factor for habitability. From another point of
view, it can be noted that in the solar system plate
tectonics only exists on the planets where liquid water is
found. Plate tectonics might therefore require liquid water
to be present. This illustrates how habitability and surface
conditions are clearly linked to the inner dynamics of the
planets and the exchanges of volatiles between solid and
is a logical start to studies about habitability, even
though liquid water is not stable on the surface at
present-day. Many signs point at very different conditions
earlier in the history of the planet. I investigated how
surface conditions could have changed over the last 4-4.5
billion years of martian evolution.
Figure 1: Possible mechanisms contributing to atmosphere
formation and erosion [from Gillmann, 2012].
This work was
supported by French PNP funding (National Planetology
Programm). It led to three publications (Gillmann et al.,
2009a; Gillmann et al., 2011; Leblanc et al., 2012). I used
modelling of non-thermal escape (sputtering, dissociative
recombination, ion pick up, ionospheric outflow) powered by
Extreme UV flux. It was constrained by ASPERA-3 (Analyzer of
Space Plasma and EneRgetic Atoms) measurements (i.e. Lundin
et al., 2009) and numerical modelling (Fox, 2004; Leblanc
and Johnson, 2002; Ma et al., 2004; Lammer et al., 2003).
Degassing from the mantle replenished the volatile content
of the atmosphere. It was estimated through three different
sources. First published results from numerical models
(O’Neill et al., 2007; Manga et al., 2006; Breuer and Spohn,
2006), secondly observation of the surface (Greeley and
Schneid, 1991) and finally self-consistent modelling using
the stagYY code (i.e. Tackley, 2008).
Figure 2: (left) Evolution of CO² partial pressure during
the history of Mars for different CO² contents of the lavas.
(right) Corresponding part of the atmosphere coming from
I studied the evolution of water at the surface, CO2 in the
atmosphere and used Carbon, Nitrogen and Argon isotopic
ratios as constraints. Our approach uses the present-day as
starting point and reconstructs the history of Mars backward
in time, which had not been done quantitatively at the time.
Our results show that the martian atmosphere is quite young
(2 billion years old), as opposed to the remnants of a
primordial atmosphere. It also mainly originates from
volcanism. Additionally, we showed that the martian
atmosphere was thin except during the first 500 Myr and that
it was cold and dry with water under solid form or in the
subsurface, Higher partial pressure and water content in the
atmosphere could only be reconcilied with late evolution and
present conditions if one takes into account different
mechanisms, such as the effects of impact delivery of
Figure 3: Global evolution of martian CO² pressure during
the last 4 Gyr, including the effect of LHB.
As we became
aware of the huge influence of early history of the planets
on their long term evolution, we started to study this phase
in greater depth. Venus was a good place to start as its
thick atmosphere retains traces of events that occured early
on. We started with the modelling of the first 500 Myr of
the evolution of its atmosphere.
It also led to a publication
(Gillmann et al., 2009b). We modelled the intense linked
hydrodynamic escape of hydrogen and oxygen and its effect on
noble gas isotopic ratios (Zahnle and Kasting 1986; Kasting
and Pollack, 1983; Hunten et al., 1987; Chassefière,
Figure 4: Shematic view of the hydrodynamic escape process,
leading to an important depletion in light species, but also
affecting noble gases and other heavy components (water,
self consistent scenario of the evolution of Venus
detailing the interactions and coupling between atmosphere,
hydrodynamic escape and the “solid” planet.
By analyzing the different parameters governing the escape,
we were able to reproduce the present-day isotopic ratios
for Neon and Argon with limited hydrodynamic escape for
Venus and an Earth-like situation. We were also able to
propose a self-consistent scenario for the early evolution
of Venus and its differences from the Earth. The study
predicts that Venus should have received less water than the
Earth during its accretion and that the hydrodynamic escape
controlled the rate of the solidification of the magma ocean
(which froze after 70-100 Ma) by pumping volatiles out of it
(fig. 6). The intense escape would also have led to a very
dry state by efficiently removing most of the primitive
water from the atmosphere and magma ocean. On the contrary,
Earth would have received more water early on (Morbidelli et
al., 2000; Raymond et al., 2006) and would have remained wet
until after the freezing of the magma ocean, leaving
volatiles both in the mantle and at the surface. In that
case, one of the differences between Earth and Venus could
have occurred as early as the first few hundred million
Figure 5: self consistent scenario of the evolution of Venus
detailing the interactions and coupling between atmosphere,
hydrodynamic escape and the “solid” planet.
VENUS' LONG TERM EVOLUTION:
We then wanted to couple this early evolution to the
long term history of the planet, thus we investigated the
coupling between mantle convection and surface onditions of
Figure 6: Mechanisms and feedbacks between layers in a
terrestrial planet: current state of the model.
It was funded by an ETH Fellowship in Zürich and led to a
publication (Gillmann and Tackley, 2014). We investigated the coupled evolution
of the atmosphere and mantle on Venus, focusing on
mechanisms that deplete or replenish the atmosphere:
atmospheric escape and volcanic degassing of the mantle.
These processes were linked to obtain a coupled model of
mantle convection and atmospheric evolution, including
feedback of the atmosphere on the mantle via the surface
temperature. During early atmospheric evolution hydrodynamic
escape is dominant, while for later evolution we focused on
non-thermal escape. The atmosphere is replenished by
degassing, using mantle convection simulations (Armann and
Tackley, 2012), and includes episodic lithospheric overturn.
The evolving surface temperature was calculated from the
amount of CO2 and water in the atmosphere using a gray
radiative-convective atmosphere model. This surface
temperature in turn acts as a boundary condition for the
mantle convection model. We obtained a Venus-like behavior
for the solid planet and an atmospheric evolution leading to
the present conditions. CO2 pressure is unlikely to vary
much over the history of the planet; a late massive build-up
of the atmosphere over several resurfacing events seems
unlikely. In contrast, water pressure is strongly sensitive
to volcanic activity and varies rapidly, leading to
variations in surface temperatures of up to 200 K, which
have an effect on volcanic activity and mantle convection.
Low surface temperatures trigger a mobile lid regime that
stops once surface temperatures rise again, making way to
stagnant lid convection that insulates the mantle.
Figure 7: Comparative evolution of volcanic production rate,
surface temperature and volume averaged mantle temperature
with time for the reference case. Also indicated are the
different convection regimes. The transition from mobile lid
tio stagnant lid is progressive. Early evolution (before 400
Myr) follows an episodic, but mostly stagnant, lid pattern.
VENUS AND LARGE IMPACTS:
As we already observed in the case of Mars, other mechanisms
could have influence on the evolution of terrestrial
planets. Large impacts by meteorites, for example, are a
prime suspect for large scale modification of surface and
interior modifications. Impacts can indeed erode the
atmposphere of the planet, but also bring volatiles to its
surface. Additionally, those collisions can deposit a large
amount of energy in the solid part of the planet. We
investigated each of these effects.
Impact erosion of the atmosphere has been incorporated by
using results of numerical simulations by the SOVA hydrocode
developed by Shuvalov et al. (2009). We compared those
results to the simple tangent plane model as a constraint.
Both models agree that large impacts are quite inefficient
at removing volatiles from Venus' atmosphere. Large impacts
are not numerous enough to substantially erode Venus'
atmosphere. Single impacts don't have enough eroding power.
Swarms of small bodies (<50km radius) might be a better
candidate for this process. The amount of volatiles brought
by large ordinary chondrite impactors is superior to losses
and comparable to the degassing caused by the impact.
Carbonaceous chondrite impactors are unlikely: they release
too many volatiles, causing surface temperature to stay
above 900K up to present-day.
Mantle dynamics can also be modified by the heating caused
by impacts. Heated material propagates by spreading across
the upper mantle due to its buoyancy. Old crust destroyed or
remixed in the mantle.
Figure 8: Evolution of Venusian mantle temperature and
mantle composition with a 800km radius impact occuring at
150 Ma. For composition, 0 stands for a depleted mantle and
1 for basaltic composition. Atmosphere coupling is taken
A large part of the upper mantle melts, leading to its
depletion and degassing. The timing of the impact has a
small influence on the final surface temperature of the
planet. However, it has a strong influence on the evolution
of the surface temperature between 3.5 and 1.5 Ga, thus
affecting surface conditions AND mantle (feedback). A large
impact could lead to bypassing completely the low surface
temperature period and thus make a strong difference for
mantle convection evolution. Large impacts can affect long
term evolution of terrestrial planets through their
influence on key periods of the history of the body and
trigger changes dur to the atmosphers/mantle feedback.
enough evently distributed high energy impacts, the mantle
can be depleted by more than 90% of its volatiles during
Late Veneer. This drastically cuts down degassing in the
late history of the planet and leads to lower present-day
surface temperatures. Total depletion of the mantle seems
unlikely, meaning either few large impacts (1 to 4) or low
energy (slow, grazing...) collisions. Combined with the lack
of plate tectonics and volatile recycling in the interior of
Venus, Late Veneer collisions could help explain why Venus
seems dry today.
Figure 9: Long term effects of large impacts depending on
their timing. Reference case without impact is in black