DOI: 10.48577/jpl.fbogvm
title: Cryogenic Differential Calorimetry: Exothermicity of Amorphous-to-Crystalline Phase Transitions (ACPT) in Astrophysical and Cometary Ice Analogs

description: AbstractAmorphous ice is understood to be predominant phase of water in cometary nuclei. A significant number of other volatiles can be trapped in amorphous H2O ice and released during the amorphous-to-crystalline phase transition (ACPT). This phase transition is an exothermic and is considered a potential cause of cometary outbursts, such as those observed in Comet 1P/Halley and Comet Hale-Bopp. However, only a single experimental study reported in the literature suggests that the presence of impurities in amounts &gt;2% in amorphous H2O ice suppresses the exothermic effect and results in an endothermic phase transition, which contradicts the hypothesis of exothermic-phase-transition-driven comet outbursts. To further explore this phenomenon, we conducted experiments on pure H2O, CO:H2O, and CO2:H2O ice mixtures with varying mixing fractions (11%, 50% and 80% of CO with respect to H2O (100%) and 11%, 25% and 50% of CO2 with respect to H2O (100%)). Our experimental setup is a highly sensitive cryogenic differential scanning calorimeter with a K noise floor and fifth decimal resolution in temperature ratio of reference and ice. Our calorimetric data has been internally calibrated to ice sublimation endotherm to derive quantitative calorimetric data. We find that ACPT is exothermic in CO:H2O ice mixtures at all CO mixing ratios studied as well as in CO2:H2O ice mixtures with lower CO2 mixing ratios. In mixtures with the highest CO2 content (50% with respect to H2O (100%)) examined, the ACPT exotherm is weakened. Our results demonstrate that ACPT exothermicity persists throughout CO and CO2 mixing ratios observed in majority of comets and should play an important role in comet outbursts, when CO and CO2 are the major volatiles trapped in amorphous H2O ice.IntroductionWater (H2O) is one of the most common and ubiquitous molecules detected in the interstellar medium (ISM), on solar system bodies, and on the nuclei and comae of comets (M. E. Brown et al., 2012; R. H. Brown et al., 2006; Chiar et al., 2011; Chyba & Phillips, 2002; Davies et al., 1997; Honda et al., 2009; Paige et al., 1992; Sunshine et al., 2006; Vincendon et al., 2010). At the low temperatures and pressures typical of the ISM, H2O exists in an amorphous form and, depending on temperature, a comet’s nucleus could potentially retain this amorphous form of water-ice (Jenniskens et al. 1995; Prialnik 2002). Impurities such as CO, CH4, CO2, and other molecules can be trapped in amorphous H2O ice (Jenniskens & Blake 1996); these molecules have also been observed in cometary outgassing (Gasc et al. 2017). Experimental work has shown that amorphous H2O ice is porous and capable of trapping significant amounts of volatile molecules (CO, CH4, CO2, etc.) (Bar-Nun et al. 1985; Bar-Nun et al. 1987; Collings et al. 2003; Gudipati et al. 2023; Kipfer et al. 2024). Consequently, amorphous ice plays a critical role in the chemical and physical changes of the ISM, the surfaces of icy bodies, and cometary nuclei. Pure ices of various atomic and molecular species, of importance in astrophysical, planetary, and cometary environments, sublimate in a wide temperature range, from 20 K to &gt;200 K (see for example: Rubin et al. 2023, Gudipati et al. 2023; Fray and Schmitt 2009). In this work we focus on only volatiles that sublimate below H2O ice sublimation, which starts at 170 K under high vacuum conditions similar to those in space. Highly volatile species such as CO and O2 normally sublime at very low temperatures (&lt;60 K) but can be trapped in amorphous H2O ice and released during the amorphous-to-crystalline phase transition (ACPT), which occurs between 135 K and 150 K, depending on the initial amorphous ice’s thickness (Gudipati et al. 2023; Prialnik & Bar-Nun 1992). The phase transition of pure amorphous H2O ice is exothermic, and the released heat energy is experimentally found to be between 90 kJ/kg and 100 kJ/kg (Gronkowski & Wesołowski 2016; Gudipati et al. 2023; Kouchi & Sirono 2001; Schmitt et al. 1989). The phase transition of amorphous H2O ice is a bulk process that happens within an ice grain, and the temperature range of ACPT depends on the heating rate and thickness of the ice (Gronkowski & Wesołowski 2016), whereas sublimation is a process that is restricted to the surface, though molecular mobility could occur within the ice. Therefore, studies of phase transitions in cometary ice analogs are essential for understanding cometary outgassing and outbursts as well as evolution of ice grains in interstellar and protoplanetary environments. Phase transition studies of H2O ice have been extensively conducted using infrared spectroscopy (Bergren et al. 1978; Bertie & Whalley 1964; Hagen et al. 1981; Hagen & Tielens 1982; Hornig et al. 1958; La Spisa et al. 2001; Whalley 1977).Recent observation of comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer onboard the Rosetta spacecraft revealed a dichotomy in the outgassing of H2O and CO2, with molecules such as NH3 and O2 accompanying H2O, and CO, CH4, H2S, and HCN outgassing with CO2 from different regions of 67P (Rubin et al., 2023; Gasc et al., 2017). This suggests an interesting distribution and trapping of volatiles in ices and thermal gradient in comet nuclei. Several laboratory experiments have investigated the outgassing of volatiles in ice mixtures with respect to change in temperature (Collings et al. 2004; Fayolle et al. 2011; Gudipati et al. 2023). The outgassing of trapped volatiles (CO, CO2, O2, etc.) in amorphous H2O occurs during the phase transition to crystalline ice, with only a small percentage (&lt;5%) of the total remaining volatiles trapped in the crystalline ice and later sublimating alongside H2O (Gudipati et al. 2023). Given that cometary ices consist of H2O mixed with varying ratios of CO, CO2, CH4, and other minor impurities across different comets (A’Hearn et al. 2012; Biver & Bockelée-Morvan 2019; Rubin et al. 2023), it is crucial to understand how these volatiles affect the exothermic transition of H2O ice, and its impact on sublimation of trapped volatiles, of which CO and CO2 are observed to have the highest mixing percents (25%) with respect to H2O (100%) both in the interstellar ice grains and in cometary outgassing.It was proposed by Prialnik and Bar-Nun in 1992 that the exothermic amorphous-to-crystalline ice phase transition results in a runaway internal heating of the nucleus leading to outburst of the comet 1P/Halley (Prialnik and Bar-Nun 1992). However, in 2001, (Kouchi & Sirono 2001) performed experiments on H2O ice with impurities such as CO and CO2 and found that concentration of CO or CO2 above 2% suppress the exothermicity of H2O ice during the ACPT, rendering it endothermic. Based on these experiments, they suggested that the phase transition of H2O ice might not be responsible for the cometary outbursts. To the best of our knowledge, no other calorimetry experiments on cometary ice analogs containing other volatiles trapped in H2O ice exist in the literature to date. This lack of data and the need to understand thermally driven processes in cometary nuclei have motivated us to investigate the sublimation of volatiles during phase transition of pure H2O ice and H2O ice containing trapped volatiles. Here, experiments were conducted on various ratios of CO or CO2 co-deposited with H2O at 11 K to form amorphous H2O cometary ice-analogs to understand the role of trapped CO and CO2 on the latent heat of H2O ice crystallization. Our work expands on the initial calorimetry of pure H2O ice that has been previously reported (Gudipati et al., 2023). In this work we used the same experimental facility and improved the sensitivity by close to an order of magnitude to undertake cryogenic calorimetric measurements and derive the heat released during phase transitions of amorphous H2O ice with trapped CO or CO2 molecules and quantitatively determined the exothermicity of realistic comet ice analogs.