Thermogravimetric technique for volatiles detection in planetary and space environments

Introduction. The PhD work has been performed at Institute of Astrophysics and Space Planetology (IAPS-INAF) in the framework of the two projects VISTA (Volatile In-Situ Thermogravimeter Analyser) and CAM (Contamination Assessment Microbalance), funded by Italian Space Agency and European Space Agency, respectively, both aiming at developing a microbalance sensor for space mission applications, i.e. to study the minor bodies of Solar System (i.e., ESA-M5 Missions Call, MarcoPolo-M5, Akon, JEM and Castalia) by measuring in-situ volatiles material of scientific interest (VISTA project) and to assess the contamination issue (CAM project). VISTA is a miniaturized thermogravimeter (composed by Piezoelectric Crystal Microbalance and the related Proximity Electronics), based on Thermogravimetric Analysis (TGA), i.e. a widely used technique to monitor the processes involving compounds, i.e. absorption/desorption and evaporation/sublimation. Thanks to the variation in the microbalance oscillation frequency it is possible to estimate the sample mass loss/deposited from thermal cycles. VISTA is composed of two sensor heads, i.e. the Sensor Head 1 (SH1) for in-orbit contamination measurements from outgassing processes and Sensor Head 2 (SH2) for planetary in-situ measurements, respectively. The breadboard and the Engineering Model of VISTA SH1 have been developed for ESA Project, i.e. CAM, an Invitation to Tender of European Space Agency (EMITS-ESA) aiming at developing a thermogravimeter for contamination measurements in space, leaded by IAPS-INAF and developed by a consortium of three Italian institutes and one Industry. The VISTA SH2 breadboard has been developed in the framework of MarcoPolo-R Mission, where VISTA was part of the scientific payload. Objectives. In this work, the VISTA capability to monitor the contamination processes in space environment and for the study of planetary surfaces and atmospheres has been demonstrated as well as the good capability of sensor heads to monitoring and to characterizing a contaminant source and organic compounds by realizing TGA cycles and Effusion Method (EM) to obtain the vapor pressures and enthalpy of sublimation. Material and Method. The first phase of the work was based on the study of Volatile Organic Compounds (VOCs): 1) in planetary atmospheres including their physical-chemical properties and their connections with the atmospheric aerosol sources (biogenic and anthropogenic); 2) in space, coming from outgassing processes of materials exposed to space environment, and the related instrumentation issues. Thus, organic compounds (found in Carbonaceous Chondrite meteorites and in Earth's VOCs) have been selected to perform deposition processes and TGA cycles obtaining a complete characterization with SH1 and SH2. The vapor pressures and enthalpy of sublimation were identified as those thermochemical parameters able to characterize a kinetics process regarding VOCs in planetary atmosphere and in space. Thus, a laboratory activity was planned and divided in a first design and development phase of two laboratory setup and in a second calibration phase of VISTA sensor heads. A third phase was devoted at performing different tests for contamination study in space (using a contaminant source and SH1 breadboard) and for VOCs characterization in atmosphere (using five dicarboxylic acids and SH2 breadboard). Results. The breadboards of VISTA instrument SH1 and SH2 have been developed to monitor the contamination in space (SH1) and to characterize organic compounds (SH2). The main results reached in the PhD work with VISTA SH1 have been: 1) to monitor contamination processes in vacuum chamber simulating the space environment (between 5x10-9 to 7x10-4 g/cm2); 2) the contaminant source characterization by means of TGA cycles (ΔTmax~60°C) and retrieval of vapour pressure of compounds (Pi) and the enthalpy of sublimation (ΔHsub) by using the Langmuir and Clausius-Clapeyron relations; 3) the sensor regeneration by means of thermal cycles by using the integrated heaters on crystal surface (with an accuracy within 0.1°C). On the other hand, the main scientific objectives reached with VISTA SH2 have been: 1) the volatiles material measurement deposited on the sensor surface at different temperatures by using the Effusion Method simulating the asteroidal/cometary environment; 2) the characterization of VOCs, i.e. dicarboxylic acids, by calculating the enthalpy of sublimation (ΔHsub) with Van't Hoff relation. Conclusion. In this work, the VISTA SH1 and SH2 Breadboards have been designed and developed as well as two different laboratory set-up to verify the capability of SH1 and SH2 to monitor a contamination process and to characterize a pure organic compound, respectively, using TGA cycles and EM. The enthalpies of sublimation results obtained with SH1 from one contaminant source (adipic acid) using TGA and EM, are in agreement within 3.5% while the enthalpies of sublimation obtained for five dicarboxylic acids and using EM, are in agreement within 6% (oxalic, succinic and adipic acids) and 11% (azelaic and suberic acids) with previous works. These results demonstrate the capability of SH1 and SH2 Breadboards to detect organic contaminant and to characterize different organic compounds presents in VOCs terrestrial atmosphere obtaining a good characterization for a pure compound.