“Experimental study of the retention of volatiles by CO2 ice: astrophysical relevance”

This thesis is a study of CO2 ice, particularly its ability to retain volatiles even above their effective sublimation temperature. Various experiments were carried out in the Experimental Astrophysics Laboratory (LAE) that the “Caracterizaciones de Interés Astrofísico” research group has in the Alcoy campus of the Polytechnic University of Valencia.
The basic components of our experimental set-up used to carry out these experiments are high vacuum and low temperature equipment, a quartz microbalance (QCMB), two lasers and a quadrupole mass spectrometer (QMS). The main element is a high vacuum chamber (P minor 10-7 mbar), whose pressure conditions are obtained using a a turbomolecular pump aided by rotary pump (~10-3 mbar).
The first phase of a closed-cycle cryostat of He (40 K) connected to a heat shield acts as a cryopump, achieving a chamber pressure less than 10-7 mbar measured with a ITR IoniVac transmitter sensor (accurate to within 5 %).
The cryostat second stage is called the cold finger and is able to reach a temperature of 10 K. Below this is the substrate with a QCMB (formed by a flat gold surface), placed in thermal contact with the cold finger. The temperature in the sample (QCMB) is controlled by the Intelligent Temperature Controller ITC 503S (Oxford Instruments) using a silicon diode sensor (Scientific Instruments), located just behind the sample to measure the temperature, which allows to vary it between 10 and 300 K with 1 % of accuracy. Another sensor is located at the end of the second stage of the cryostat in order to check the behavior of the system.
Pure gases or mixtures are prepared in a pre-chamber by their partial pressures measured by a ceramic Ceravac CTR 90 sensor (Leybold Vacuum), accurate to within 0,2 %, which is not influenced by gas type. Gases enter the chamber through a needle valve (Leybold D50968), which regulates the flow of gas through the QMS (AccuQuad RGA 100, with a resolution ~0,5 uma) which allows us to verify the proportion of gases inside the vacuum chamber.
Experiments consist mainly of deposits of different molecules under high vacuum conditions (minor or equal to 10-7 mbar) in order to prevent undesirable contamination and low temperatures (T more than 10 K) to simulate astrophysical conditions. Thus, to study the capacity of CO2 ice to retain volatiles within their structure, we performed a series of thermal desorption experiments for pure CO2, CH4 and N2 and for CO2:N2 and CO2:CH4 binary mixtures deposited at a proportion of 95:5. After deposition, the sample was heated controlled (1 K min-1), monitoring the partial pressure of each gas with the QMS and the frequency variation with QCMB.
The astrophysical importance of CO2 ice is evident because it has been detected in many astrophysical environments both in the interstellar medium (ISM) and the Solar System (SS):
· ISM: The presence of CO2 in interstellar ice was by data obtained with the ISO satellite (Infrared Space Observatory) which confirmed that: firstly, CO2 ice is in the line of observation of various stars, and secondly, that it is their high abundance in relation to other molecules (CO, CH4, N2…). Often it is the second most abundant ice detected (H2O is usually the dominant molecule).
· SS: IR reflection spectra of different objects in the outer SS show that there is CO2 ice on solid surfaces of various bodies in the SS, for example, in the cometary nuclei and on planets and satellites such as Triton, the satellites of Uranus: Ariel, Umbriel, and Titania, on the surfaces of the icy satellites of Jupiter: Europa, Ganymede and Callisto, and, principally on the surface of Mars where ice is more abundant and therefore it is possible that this is the molecule to mark the temperatures of desorption of volatile compounds instead of water.
Therefore, the choice of this molecule for this thesis id justified for an experimental study whose findings can contribute to the understanding of the physical/chemical mechanisms that occur in different astrophysical environments. Studies conducted by our group have revealed that CO2 ice is able to absorb and retain other molecules in its structure, which will be injected into the ambient at certain temperatures and conditions. In this thesis we show that this behavior should be taken into account in the same manner as in the case of water, when developing models of chemical evolution in different astrophysical environments where CO2 ice can play an important role.
This thesis presents results for N2 and CH4 in a matrix of CO2 ice and shows different mechanisms that explain the presence of volatile molecules on ice surfaces, through retention processes. These results help explain the presence of certain volatile compounds in some astrophysical scenarios through the ice retention structure of CO2 ice. Volatiles and hypervolatiles (differentiation depends on the group of molecules, in our case, N2 and CH4 are hypervolatiles) even after the occurrence of phenomena such as impacts, in which temperatures higher than the normal sublimation temperatures of these compounds are reached.
In the laboratory it was found that CO2 ice efficiently retains CH4 and N2 at temperatures higher than their characteristic sublimation temperature. When these hypervolatiles are trapped, they mainly sublimate at three different temperatures around 50, 75 and 90 K. These correspond to the crystallization, end compression of the ice and the sublimation of CO2 respectively.
In addition, we show possible astrophysical applications of the results, focusing on the kind of destructive events (known as catastrophic disruptions) paying particular attention to the case of comet 9P/Tempel 1 in Deep Impact mission. From our results, we propose two different distributions for the ice in the outer layers of the comet 9P/Tempel 1. In the first, no hypervolatiles exist just below the surface. While in the second, we propose a progressive enrichment of hypervolatiles as we move into the comet, although the outside may contain a small fraction of hypervolatiles.
The fact that volatile molecules remain trapped in the structure of CO2 ice could be important for chemical evolution models if they play an important role in these processes.