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 RESEARCH ACTIVITIES

PAST RESEARCH

As a researcher, I am committed to high standards of quality in my work, which translates into a strong drive towards precision, clarity and rigor, as well as high level publications. This is evidenced by my track-record of publications in peer-reviewed journals, such as in Earth and Planetary Sciences Letters. Likewise, I strive to regularly submit my work for presentations at international meetings and workshop to chair my understanding and gain input by the scientific community. I repeatedly managed to obtain talk-presentations at major conferences like AGU and EGU and got an invited presentation at EPSC (See C.V.). 

During my past research, I investigated three areas that I believe deserve special notice.

The first concerns the late (last four billion years) evolution of the martian atmosphere (fig. 5). 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 5: Possible mechanisms contributing to atmosphere formation and erosion [from Gillmann, 2012].

 

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, a view that has been more widely adopted since then.

 

Our second main finding is linked to the first 500 Myr of the evolution of Venus. 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, 1996a,b).

Figure 6: 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 years.

Our third achievement was our first investigation of the coupling between mantle convection and surface conditions of Venus. It was funded by an ETH Fellowship in Zürich and led to a publication (Gillmann and Tackley, Submitted) and others in preparation. 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.

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