PhD Defense “Hydrated silica formation conditions on Earth and Mars, and interactions with organic matter” – Simon Gouzy

Abstract : Opal (SiO₂ · nH₂O, amorphous) is a ubiquitous mineral found on planetary surfaces that have hosted or still host liquid water, such as Earth and Mars. It is formed abiotically by the aqueous alteration of silicate rocks on the surface and sub-surface in environments where the conditions are particularly favorable for the emergence, development and presence of life. In this thesis we demonstrate that natural opal is capable of trapping the organic matter (OM) present in its environment when it precipitates, but also preserving it from alteration processes over geological time. Using non-destructive techniques (IR, AFM-IR) we were able to reveal the precise location of the OM in natural opal and obtain information about its nature. Thanks to a UV irradiation campaign (190-400 nm) on four mineral matrices (opal, chalcedony, sepiolite, silicified limestone), naturally colored by organic pigments, we have demonstrated that opal has the greatest capacity to preserve its OM.

We have developed a method for the synthesis of opal under conditions as close as possible to weathering conditions (<50°C) with properties characteristic of natural common opal-AG formed by weathering. The study of these synthetic opals enabled us to show that: (1) the ratio between the nanograins (and derived structures) and the hydrogel significantly influences the properties of the opal (porosity, water content, etc.); (2) the opal is capable of trapping OM during its formation. Considering all the research presented in this manuscript, we show that opal represents a target of major exobiological interest, especially for Mars.

Jury members :
President: Brigitte Zanda
Reporter: Olivier Beyssac and Isabelle Basile-Doelsch
Examiners : Bénédicte Ménez and Julien Alléon