Portrait of a doctoral student : Pauline Lévêque

A purely Nantes route

Credit : Pauline Lévêque

It was during secondary school, thanks to her life and earth sciences teacher and a meeting with two LPG researchers, Laetitia Le Deit and Marion Massé, that Pauline’s attention was drawn to planetology. She quickly decided to study for a university degree, which better suited her profile. The field of exobiology can be accessed via a number of routes, including biology, chemistry, astrophysics and geology,” she admits. Hesitant with biology, but more familiar with geology, she began a degree in the field at Nantes Université, after obtaining her scientific baccalauréat in 2016. With her sights set on a Master’s degree in Planetology, she chose the options of Planetology and Astrophysics in its second year and Fluid Mechanics in its third year. During this third year, Pauline did an experimental placement at the LPG in Nantes and the Open University in the UK with Susan Conway, CNRS Research Fellow, Marion Massé, Design engineer at the LPG and Sabrina Carpy, Senior Lecturer at Nantes Université. The aim of the course was to understand the processes involved in the formation of certain Martian flow geomorphologies.

Pauline joined the Planetology Master’s course as a Master 1 student, working on the modelling of the interiors and thermal evolution of Venus, with Caroline Dumoulin, Senior Lecturer at Nantes Université. The Master 2 course focused on the evolution of organic matter during the formation of ice moons, with Olivier Bollengier, Senior Lecturer at Nantes Université, Erwan Le Menn, Research Engineer at the LPG, Gabriel Tobie, CNRS Research Director at the LPG and Christophe Sotin, University Professor at Nantes. A subject on which she will continue to work during her thesis.

Thesis: Experimental approach to the role and fate of primordial organic matter in the ice satellites of giant planets.

This thesis is under the joint supervision of the CEISAM chemistry institute and the Planetology and Geosciences laboratory. Pauline admits that she appreciates this dual supervision: “It makes for really enriching interactions and allows us to mix two types of habits, in terms of experiments and ways of thinking. For me, one of the strong points of the thesis is this bi-disciplinary approach. During her thesis, Pauline is studying organic matter as a starting material present during the formation of ice moons: how does it evolve during the accretion of icy bodies, what volatiles are produced by its degradation and how is primordial organic matter an endogenous source of these volatiles in these bodies.

For this study, the PROMISES ERC team needs organic matter. This is synthesised at the Centre de Recherches Pétrographiques et Géochimiques at Nancy (CRPG) in what is known as a Nebulotron. This instrument enables us to reconstruct the conditions present in the solar system’s primordial nebula: a cluster of gas, irradiated by a very young star. This irradiation will form a plasma that will condense the matter present in the instrument. These condensates will form the organic matter used as the starting material for Pauline’s research.

This organic material will then be subjected to high pressure and temperature. Most of the experiments will be carried out using diamond anvil cells. Since July 2023, the teams have also been using autoclaves, in collaboration with the Orléans geology laboratory (ISTO, UMR 7327), to react larger quantities of sample (ml). Two types of autoclave were used: the first, at a lower pressure (1.35 kbar), was used for in situ Raman analysis of one sample at a time; the second, at a higher pressure (4 kbar), was used for the reaction of several hermetically sealed gold capsules. In both systems, the degradation of organic matter by water will form different molecules, particularly in the form of gases, which will need to be characterised, in addition to the characterisation of the insoluble organic residue.

Several methods can be used to characterise the products of the experiments. For the analysis of volatiles, there is Raman spectroscopy, or gas chromatography (GC-MS) for metal capsules reacted at 4kbar. To do this, Erwan Le Menn has set up a system that involves placing the capsule in a hermetically sealed environment continuously swept by helium to prevent pollution of the sample by gases from the earth’s atmosphere. Within this environment, the capsule will be pierced and the gases escaping will be identified. In addition to these GC-MS analyses, the characterisation of volatile elements is carried out in situ on diamond anvil cells in order to check the reproducibility of the results with the two types of experimental device. Pauline tells us that this characterisation will be carried out at a synchrotron, in Grenoble (ESRF, line ID15B), or in Hamburg (DESY-Petra III, line P02.2). High pressure will be applied to the material so that it is in the form of ice, while remaining at temperature. The X-ray diffraction method is used on solid samples, which will enable the volatiles crystallised in the sample to be analysed. The next step is to characterise the organic material itself. The material produced in the autoclave will be recovered and characterised using the Fourier-transform ion cyclotron resonance (FT-ICR) at the Laboratoire de Chimie Organique, Biomolécule, Réactivité et Analyse (COBRA) in Rouen and the Laboratoire de Chimie et de Physique Approche Multi-échelles des Milieux Complexes (LCP-A2MC) in Metz. The device will ionise the sample using a laser. This ionisation will be just powerful enough to isolate the molecules in the sample without breaking them. The ions will then be sent, by magnetic field, into an ion trap (paracell) coupled to a mass spectrometer, enabling their mass to be determined very accurately (to the nearest electron). Once all the masses have been assigned, Pauline’s team has access to the composition of the sample.

Work in stages

Pauline’s role within the PROMISES ERC is to explore the impact and evolution of organic matter in ice moons at an experimental level. She tells us that she shares this experimental side with Camille Delarue, and that their work is complementary to that of Mathis Pinceloup, who is working on the modelling aspect of these issues. When we asked her to tell us a little about the next stage of her thesis work, Pauline explained: “For the moment, I’m studying the early stages of moon formation using a simple system, namely organic matter and water under accretion conditions. Later on, we can add silicates, iron and a little sulfur to the samples, to get a little closer to the real conditions and the next stages of evolution (differentiation of the moons)”.