Toward a Better Understanding of Ice Grain Formation from Enceladus’s Salty Ocean

Introduction: Analysis of micrometer-sized ice grains emitted from Saturn’s moon Enceladus revealed that the moon’s subsurface ocean represents a potentially habitable place in the Solar System. The ocean is salty, with comparable salinity to Earth’s oceans [1]. However, salt concentrations in individual grains sourced from the ocean can vary drastically [2], probably due to the segregation of individual salts upon freezing [3]. The ocean contains a diverse complement of organic compounds [4,5] and likely interacts with the moon’s rocky core through hydrothermal reactions at the seafloor [6]. Recent work demonstrates that traces of bacterial life, if entrapped and preserved in the ice grains, would be detectable [7].

One important aspect for the preservation of organic structures, or even cells, is the phase state (crystalline, glassy, or some mixture of both) of the ice grains. Although organic structures and cells tolerate tiny micrometer-sized crystals in their vicinity [8], glassy phases can favor preservation of these compounds [9].

While the largest amount of glassy phases on Enceladus are found in the active regions around the moon’s south pole [10], freshly emitted material appears to be predominantly crystalline [11]. These phase states of the grains are ultimately linked to their formation conditions, i.e. liquid-solid phase transitions.

Salt-rich ice grains on Enceladus are believed to form via flash-freezing of ocean droplets [1], with fast freezing rates being favorable for the formation of glassy phases [12]. The freezing rates of these grains are not well constrained, but calculations indicate that the grains may freeze within 1 ms after droplet formation [13], meaning they would freeze within the first cm above the water table [14].

Methods: To better understand the formation of crystalline and/or glassy phases upon freezing of ice grains from Enceladus’s ocean, we performed Differential Scanning Calorimetry (DSC) experiments with aqueous solutions of NaCl, KCl, Na₂CO₃, NaHCO₃, NH₄OH, Na₂HPO₄, K₂HPO₄, as well as mixtures of these compounds. The same technique was recently applied to Mars by studying brines of single salt perchlorate compositions [15]. Measured salt concentrations in our experiments covered the range of estimated concentrations of these compounds in Enceladus’s ocean [1,14,16]. We determined the degree of supercooling and the degree of vitrification of the samples (volumes varied from 4 to 40 μL) over a wide range of cooling rates, from as low as 10 K/min up to ~1000 K/min via drop-quenching into liquid nitrogen (flash freezing). We then modeled the freezing process of these solutions and associated mineral formation using the thermodynamic aqueous chemistry software PHREEQC [17] to support our DSC experiments.

Results and Conclusions: Between 0.3 and ~10 percent of the total ice grain volume should form a glassy state upon freezing from Enceladus’s subsurface ocean, strongly correlated with the freezing rate and the salt concentration of individual grains. Organic structures, or potentially cells, are more likely to be preserved in rapidly freezing grains with high concentrations of salts because these grains have a higher degree of vitrification than salt-poor grains.

Significant supercooling is expected to occur during flash-freezing of ice grains from a salty ocean on Enceladus. Upon freezing, the crystallization of minerals appears to follow a particular sequence.

Our experiments and models are an important step toward understanding the formation and structure of ice grains and their capability for preserving organics and cells. Newly derived data will inform future Enceladus models and is relevant to other icy worlds with subsurface water reservoirs, such as Jupiter’s moon Europa or dwarf planet Ceres.

 

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