Origin and evolution of planetary atmospheres
Planetary atmospheres are tenuous reservoirs but exert a strong control on the pressure and temperature at the surface of the planet. The chemical composition of an atmosphere is also a key parameter for evaluating the habitability of a planet. This composition changed with time due to internal (geodynamics, magmatism) and external (volatile delivery by impacts, atmospheric escape) processes. Atmospheres are thus essential records of the entire geological history of a planet.
My research focuses on following the evolution of the composition of the Earth’s atmosphere with time, from the Archean to present time. I study rocks and minerals, which may have sampled atmospheric gases when they formed. I measure the elemental and isotopic composition of noble gases and nitrogen in those samples.
Image: Fluid inclusions in a quartz sample from Barberton (South Africa, 3.3 Ga old).
A major result of paleo-atmospheric studies is that the isotopic composition of atmospheric xenon evolved with time until about 2 Ga ago (Pujol et al. 2011; Avice et al. 2017, 2018). This isotopic evolution could be linked to the escape and fractionation of xenon ions from the atmosphere to the outer space. This escape is possible only if hydrogen escape, a potential strong driver of Earth’s oxidation, was active until 2 Ga ago (Zahnle et al. 2019).
Studying paleo-atmospheres trapped in geological samples also allows to put constraints on the partial pressure of nitrogen (pN2) in the ancient atmosphere. Argon-nitrogen mixing relationships show that the pN2 in the Archean was similar to or lower than the modern one but further investigations are required to better understand the evolution of the pN2 over geological eons.
Space technologies applied to atmospheric sampling on Venus
Venus shares similarities with Earth and is often considered as Earth’s sister planet. Yet, the two planets evolved very differently and only Earth is considered habitable today. The habitability of Venus in the past remains debated. Similarly to the case of Earth, the chemical and isotopic composition of the Venus atmosphere holds clues about the entire evolution of the planet. Unfortunately, existing data are scarce and do not allow to draw any firm conclusion on the origin of volatile elements on Venus and on the evolution of its atmosphere.
I am taking part to the development of a mission concept named “Cupid’s Arrow” and led by C. Sotin (ex JPL, now Univ. of Nantes), which aims at sampling and measuring the elemental and isotopic composition of noble gases in the Venus atmosphere.
During my stay at Caltech as a postdoctoral scholar (2016-2018), I collaborated with engineers at the Jet Propulsion Laboratory (JPL/NASA). We developed a miniaturized quadrupole ion trap mass spectrometer (QITMS). This instrument is based on the invention by W. Paul (Nobel prize in 1989), holds in one hand and is able to measure the relative proportions of all noble gas isotopes. High levels of cleanliness allow to measure the same finite aliquot of gas in static mode during hours or even days, which permits to achieve very good internal reproducibility (Avice et al., 2020). Such a compact, versatile instrument could be the ideal payload of a small satellite mission to Venus.
Sampling the atmosphere of Venus presents interesting challenges. In the scenario of a skimmer probe, sampling takes place at 13km/s and behind a shock wave. These conditions are likely to modify the composition of the sample. The Cupid’s Arrow team is running numerical simulations reproducing atmospheric sampling at high velocity. (see this presentation given by A. Borner) We also develop an analog experiment, which aims at understanding the effect of gas expansion from the Venus atmosphere, through a miniaturized valve and to the sampling cylinders.
Guillaume Avice 