Cryogenic Differential Calorimetry: Exothermicity of Amorphous-to-Crystalline Phase Transitions (ACPT) in Astrophysical and Cometary Ice Analogs

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 >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 >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 (<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 (<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.

**摘要** 无定形冰（amorphous ice）被认为是彗核中水的主要存在相。大量其他挥发物可被捕获于无定形H₂O冰中，并在无定形-结晶相变（amorphous-to-crystalline phase transition, ACPT）过程中被释放。该相变属于放热过程，被认为是彗星爆发的潜在诱因之一，例如1P/哈雷彗星和海尔-波普彗星观测到的爆发事件。然而，目前仅见一篇文献报道的实验研究指出：当无定形H₂O冰中杂质含量超过2%时，会抑制其放热效应，使相变变为吸热过程，这与“放热相变驱动彗星爆发”的假说相悖。为进一步探究该现象，我们开展了纯H₂O冰、CO:H₂O及CO₂:H₂O冰混合物的实验，混合比例分别为：以H₂O（100%）计，CO占比11%、50%和80%；CO₂占比11%、25%和50%。本实验采用的装置为高灵敏度低温差示扫描量热仪（differential scanning calorimeter），其噪声基底可达微开尔文量级，参比与冰样的温度比率分辨率达小数点后五位。我们的量热数据以冰升华吸热过程为内标进行校准，以获得定量量热结果。实验发现：所有测试CO占比的CO:H₂O冰混合物，以及CO₂占比较低的CO₂:H₂O冰混合物，其ACPT均为放热过程；而在CO₂占比最高（以H₂O 100%计为50%）的测试样品中，ACPT的放热效应被削弱。我们的结果表明：在大多数彗星中观测到的CO和CO₂混合比例范围内，ACPT的放热特性均得以保持；当CO和CO₂为无定形H₂O冰中捕获的主要挥发物时，该相变应在彗星爆发过程中发挥重要作用。

**引言**
水（H₂O）是星际介质（interstellar medium, ISM）、太阳系天体以及彗星核与彗发中被探测到的最常见、分布最广泛的分子之一（M. E. Brown等，2012；R. H. Brown等，2006；Chiar等，2011；Chyba与Phillips，2002；Davies等，1997；Honda等，2009；Paige等，1992；Sunshine等，2006；Vincendon等，2010）。在星际介质典型的低温低压环境中，水以无定形形式存在；而根据温度条件的不同，彗核有可能保留这种无定形水冰形态（Jenniskens等，1995；Prialnik，2002）。CO、CH₄、CO₂等杂质可被捕获于无定形H₂O冰中（Jenniskens与Blake，1996），这类分子也在彗星排气过程中被观测到（Gasc等，2017）。实验研究表明，无定形H₂O冰具有多孔结构，可捕获大量挥发性分子（如CO、CH₄、CO₂等）（Bar-Nun等，1985；Bar-Nun等，1987；Collings等，2003；Gudipati等，2023；Kipfer等，2024）。因此，无定形冰在星际介质、冰质天体表面以及彗核的化学与物理演化过程中扮演着关键角色。
在天体物理、行星科学与彗星研究中具有重要意义的各类纯原子与分子冰，其升华温度区间跨度极大，覆盖20 K至200 K以上（例如：Rubin等，2023；Gudipati等，2023；Fray与Schmitt，2009）。本研究仅关注升华温度低于H₂O冰的挥发物：在类似太空的高真空环境下，H₂O冰的升华起始温度为170 K。像CO和O₂这类高挥发性物质通常在极低温度（<60 K）下即可升华，但可被捕获于无定形H₂O冰中，并在135 K至150 K之间发生的无定形-结晶相变（ACPT）过程中被释放，该相变的具体温度区间取决于初始无定形冰的厚度（Gudipati等，2023；Prialnik与Bar-Nun，1992）。
纯无定形H₂O冰的相变属于放热过程，实验测得其释放的热能介于90 kJ/kg至100 kJ/kg之间（Gronkowski与Wesołowski，2016；Gudipati等，2023；Kouchi与Sirono，2001；Schmitt等，1989）。无定形H₂O冰的相变是发生在冰颗粒内部的整体过程，ACPT的温度区间取决于升温速率与冰样厚度（Gronkowski与Wesołowski，2016）；而升华过程仅局限于表面，尽管冰内部可能发生分子迁移。因此，对彗星冰类似物的相变开展研究，对于理解彗星排气与爆发过程，以及星际和原行星环境中冰颗粒的演化均至关重要。
目前已有大量研究采用红外光谱法开展H₂O冰的相变研究（Bergren等，1978；Bertie与Whalley，1964；Hagen等，1981；Hagen与Tielens，1982；Hornig等，1958；La Spisa等，2001；Whalley，1977）。罗塞塔探测器搭载的ROSINA质谱仪对67P/楚留莫夫-格拉希门克彗星的最新观测显示，其H₂O与CO₂的排气过程存在二分性：NH₃、O₂等分子伴随H₂O一同排出，而CO、CH₄、H₂S和HCN则与CO₂一同从67P彗星的不同区域释放（Rubin等，2023；Gasc等，2017）。这表明彗星冰中挥发物的分布与捕获模式，以及彗核内部的热梯度均具有值得关注的特性。
已有多项实验室实验针对冰混合物中挥发物随温度变化的排气过程展开研究（Collings等，2004；Fayolle等，2011；Gudipati等，2023）。无定形H₂O冰中捕获的挥发物（CO、CO₂、O₂等）会在向结晶冰的相变过程中被释放，仅约<5%的总挥发物会残留于结晶冰中，并后续与H₂O一同升华（Gudipati等，2023）。鉴于不同彗星的冰均由H₂O与不同比例的CO、CO₂、CH₄及其他微量杂质混合而成（A’Hearn等，2012；Biver与Bockelée-Morvan，2019；Rubin等，2023），阐明这些挥发物如何影响H₂O冰的放热相变，以及该效应对捕获挥发物升华过程的影响，就显得尤为关键。其中，CO和CO₂是在星际冰颗粒与彗星排气中均被观测到的占比最高的挥发物，其相对于H₂O（100%）的混合占比可达25%。
Prialnik与Bar-Nun于1992年提出假说：无定形冰向结晶冰的放热相变将导致彗核内部出现失控式升温，进而引发1P/哈雷彗星的爆发（Prialnik与Bar-Nun，1992）。然而，2001年Kouchi与Sirono（2001）针对含CO、CO₂等杂质的H₂O冰开展实验后发现，当CO或CO₂的浓度超过2%时，会抑制ACPT过程中H₂O冰的放热效应，使相变变为吸热过程。基于该实验结果，他们提出H₂O冰的相变或许并非彗星爆发的诱因。
据我们所知，截至目前文献中尚未见其他针对含捕获挥发物的彗星冰类似物的量热实验研究。这种数据缺失，以及理解彗核内热驱动过程的需求，促使我们开展纯H₂O冰以及含捕获挥发物的H₂O冰在相变过程中挥发物升华行为的研究。本研究中，我们将CO或CO₂与H₂O在11 K下共沉积，制备不同比例的无定形H₂O彗星冰类似物，以探究捕获的CO和CO₂对H₂O冰结晶潜热的影响。我们的工作拓展了此前报道的纯H₂O冰量热研究（Gudipati等，2023）：本研究沿用了相同的实验装置，但将灵敏度提升了近一个数量级，以开展低温量热测量，获取含捕获CO或CO₂分子的无定形H₂O冰在相变过程中释放的热量，并定量确定真实彗星冰类似物的放热特性。