Low-Temperature Specific Heat Capacity of Water-Ammonia Mixtures Down to the Eutectic.

Robust thermodynamic data are essential for the development of geodynamic and geochemical models of ocean worlds. The water-ammonia system is of interest in the study of ocean worlds due to its purported abundance in the outer solar system, geological implications, and potential importance for origins of life. In support of developing new equations of state, we conducted 1 bar specific heat capacity measurements (Cp) using a differential scanning calorimeter (DSC) at low temperatures (184-314 K) and low mass fractions of ammonia (5.2-26.9 wt %) to provide novel data in the parameter space most relevant for planetary studies. This is the first known set of data with sufficient fidelity to investigate the trend of specific heat capacity with respect to temperature. The obtained Cp in the liquid phase domain above the liquidus generally increases with temperature. Deviations of our data from the currently adopted equation of state by Tillner-Roth and Friend[Tillner-Roth R.; Friend D. G.J. Phys. Chem. Ref. Data1998, 27, 63-96]. are generally negative (ranging from +1 to -10%) and larger at lower temperatures. This result suggests that suppression of the critical behavior of supercooled water (rapid increase in specific heat with decreasing temperature) by ammonia starts at a smaller concentration than that set by Tillner-Roth and Friend.[Tillner-Roth R.; Friend D. G.J. Phys. Chem. Ref. Data1998, 27, 63-96]. Cp measurements of the liquid were also obtained in the partial melting domain between the eutectic and liquidus. This novel data set will be useful in future investigations of conditions where such partial melt may exist, such as the ice shell-ocean boundary or the interiors of ocean worlds that may contain relatively large proportions of dissolved ammonia.

Compositions and Interior Structures of the Large Moons of Uranus and Implications for Future Spacecraft Observations.

The five large moons of Uranus are important targets for future spacecraft missions. To motivate and inform the exploration of these moons, we model their internal evolution, present-day physical structures, and geochemical and geophysical signatures that may be measured by spacecraft. We predict that if the moons preserved liquid until present, it is likely in the form of residual oceans less than 30 km thick in Ariel, Umbriel, and less than 50 km in Titania, and Oberon. The preservation of liquid strongly depends on material properties and, potentially, on dynamical circumstances that are presently unknown. Miranda is unlikely to host liquid at present unless it experienced tidal heating a few tens of million years ago. We find that since the thin residual layers may be hypersaline, their induced magnetic fields could be detectable by future spacecraft-based magnetometers. However, if the ocean is maintained primarily by ammonia, and thus well below the water freezing point, then its electrical conductivity may be too small to be detectable by spacecraft. Lastly, our calculated tidal Love number (k 2) and dissipation factor (Q) are consistent with the Q/k 2 values previously inferred from dynamical evolution models. In particular, we find that the low Q/k 2 estimated for Titania supports the hypothesis that Titania currently holds an ocean.

A Proposed Geobiology-Driven Nomenclature for Astrobiological In Situ Observations and Sample Analyses.

As the exploration of Mars and other worlds for signs of life has increased, the need for a common nomenclature and consensus has become significantly important for proper identification of nonterrestrial/non-Earth biology, biogenic structures, and chemical processes generated from biological processes. The fact that Earth is our single data point for all life, diversity, and evolution means that there is an inherent bias toward life as we know it through our own planet's history. The search for life "as we don't know it" then brings this bias forward to decision-making regarding mission instruments and payloads. Understandably, this leads to several top-level scientific, theoretical, and philosophical questions regarding the definition of life and what it means for future life detection missions. How can we decide on how and where to detect known and unknown signs of life with a single biased data point? What features could act as universal biosignatures that support Darwinian evolution in the geological context of nonterrestrial time lines? The purpose of this article is to generate an improved nomenclature for terrestrial features that have mineral/microbial interactions within structures and to confirm which features can only exist from life (biotic), features that are modified by biological processes (biogenic), features that life does not affect (abiotic), and properties that can exist or not regardless of the presence of biology (abiogenic). These four categories are critical in understanding and deciphering future returned samples from Mars, signs of potential extinct/ancient and extant life on Mars, and in situ analyses from ocean worlds to distinguish and separate what physical structures and chemical patterns are due to life and which are not. Moreover, we discuss hypothetical detection and preservation environments for extant and extinct life, respectively. These proposed environments will take into account independent active and ancient in situ detection prospects by using previous planetary exploration studies and discuss the geobiological implications within an astrobiological context.

A Metamorphic Origin for Europa's Ocean.

Europa likely contains an iron-rich metal core. For it to have formed, temperatures within Europa reached ≳ 1250 K. Going up to that temperature, accreted chondritic minerals - for example, carbonates and phyllosilicates - would partially devolatilize. Here, we compute the amounts and compositions of exsolved volatiles. We find that volatiles released from the interior would have carried solutes, redox-sensitive species, and could have generated a carbonic ocean in excess of Europa's present-day hydrosphere, and potentially an early CO 2 atmosphere. No late delivery of cometary water was necessary. Contrasting with prior work, CO 2 could be the most abundant solute in the ocean, followed by Ca 2 + , SO 4 2 - , and HCO 3 - . However, gypsum precipitation going from the seafloor to the ice shell decreases the dissolved S/Cl ratio, such that Cl > S at the shallowest depths, consistent with recently inferred endogenous chlorides at Europa's surface. Gypsum would form a 3-10 km thick sedimentary layer at the seafloor.