What is it about?

A rare form of a highly hydrated sodium-magnesium sulphate may be a common mineral on Jupiter’s moon Europa as well as other possible icy-ocean worlds elsewhere in the universe. The mineral was identified in a set of laboratory experiments aimed at investigating how salt minerals might form when ocean waters on Europa encounter freezing conditions. Europa has long been known to possess a global saline ocean ~100 km deep, that is completely encased beneath a ~25 km thick ice crust. The presence of a salty liquid water ocean means that Europa has long been considered a likely place to find life beyond Earth. Periodically, liquid ocean water reaches the surface where it encounters freezing conditions. However, when salty water freezes, the dissolved salts are excluded from the solid ice and become concentrated into pockets of liquid brine. As temperatures continue to fall, eventually the salts themselves begin to precipitate out of solution. We investigated this process for a liquid solution formulated to match what is already known about Europa’s likely ocean composition. One experiment involved subjecting the solution, termed MEOS for model Europan ocean solution, to very slow freezing, at a rate of 0.3 K per day, for almost a year. A second experiment saw the MEOS being cooled very quickly at 360 K per hour. In both experiments we monitored the formation of salts, as a function of temperature, using in situ synchrotron X-ray powder diffraction. The two extremes of cooling were intended to investigate how Europa’s ocean behaved under equilibrium thermodynamic and non-equilibrium conditions and the timescales of different surface delivery processes. Although previous studies based on equilibrium thermodynamics modelling predicted the sequence of precipitating salts, which we observed in the early stages of the slow freeze experiment, they did not predict the formation of an unusual and highly hydrated sodium-magnesium sulphate phase containing 16 molecules of water for every two molecules of sulphate. Moreover, the phase only formed in the slow-freeze experiment 175 days from the start, and some 50 days after the MEOS had been held at a base temperature of 245 K. We also observed the salt to be present in the assemblage of multiple salts that formed almost simultaneously in the rapid freeze experiment. However, what was unexpected was that the amount of this new phase increased as the cooling continued, eventually reaching temperatures close to Europa’s surface temperature. The two results combined, led us to conclude that this salt phase likely has relatively slow formation kinetics, but is thermodynamically stable at low temperature. Because it forms at lower temperatures than other salts, the increased viscosity of the brine impairs the rate at which it can sink, meaning that its abundance may be enhanced in the near-surface regions of Europa’s ice crust. Because Europa’s ocean gained its salinity from the leaching of elements from the rocky material at its core, which would have ultimately derived from the refractory dust present at the moon’s formation in the early solar nebula, the similarity in composition of these materials across the galaxy means that the sodium-magnesium salt may well be present on other icy-ocean worlds and exoplanets. Solid particles of dust are produced in the atmospheres of ageing stars, and a combination of the different rates of stellar ageing and the outward radial migration of stars as they orbit around the galactic centre ensures that the dust from different stars is well mixed. For example, the analysis of meteorites suggests that the materials from which our sun and solar system derived came from 35-40 different stars, while models suggest the Sun has migrated some 2000 light years away from its birthplace. Around ~26 % of known exoplanets are believed to fall into the ocean-world category. The discovery of the highly hydrated sodium-magnesium salt phase in relation to Europa may have significant astrobiological implications. The osmotic conditions inside brine pockets within the polar sea ice on Earth that is home to a wide range of cold-loving microorganisms, are controlled by the low temperature precipitation of salts. The low temperature stability of the sodium-magnesium sulphate phase could potentially play a similar role in Europa’s ice crust. Indeed, there is a school of thought within origin of life circles that cold temperatures and icy conditions could have played an important role in the development of life. The confinement of biological precursor molecules within cold brines should overcome the barriers that would otherwise prevent the molecules from linking together to form more complex molecular species, such as RNA. In this context, sodium and magnesium salts have been shown to influence these initial polymerisation processes and could also potentially provide active surfaces for other complex biogenic reactions. Earth experienced a number of freezing episodes 630–720 million years ago, during the Neoproterozoic, where the oceans were largely covered by extensive glaciation. This Cryogenian era (also referred to as Snowball Earth) was the result of runaway ice–albedo feedback. The Earth’s albedo - a measure of how reflective of light and heat the surface is - was further increased by an extensive salt crust in tropical regions left behind by the sublimation of sea ice. It was during the Cryogenian that eukaryote cells first acquired the ability to biosynthesize certain sterols (an organic compound containing rings of carbon atoms), some of which extended the temperature range for biological membrane processes. This provided the cells with an evolutionary advantage against large variations in temperature and is believed to have resulted in the global dominance of green algae within the marine ecosystem. The end of the Cryogenian coincided with the rise of predation by heterotrophic planktonic microbes, which obtained their energy by consuming other organisms, such as the green algae. The rise of predation established a food chain leading, ultimately, to the Cambrian explosion and the development of intricate life forms, including those from which all animals – and humans – evolved. Given the likely high proportion of ocean worlds among the exoplanet population, the likely close similarities in their original building block materials, and that the evolution of life on Earth has likely been influenced by global ice ages, the role of low-temperature salts in the development of life could prove to be universal.

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Why is it important?

Salts formed at low temperature from freezing oceans are important for stabilizing ecological conditions in polar ice on Earth, which plays host to a wide range of cold-adapted organisms, and may even have been important for the origin of life itself. Jupiter's moon Europa has maintained an ice covered liquid ocean throughout it's entire history, and since conditions within Europa's ocean not too dissimilar to Earth's (salinity, temperature and pressure), Europa is a prime target in the search for Life beyond Earth. We have identified a new low-temperature salt that may be widespread on Europa and which, due to the origin of Europa's building block materials, may be common on other ocean-bearing planets throughout the universe

Perspectives

The questions of where we came from and whether we alone in the universe are fundamental to our own sense of what it actually means to exist. The ultimate answer to these questions will in turn depends on whether life on Earth is just a one-off lucky occurrence, or an inevitable outcome of the laws of physics, chemistry and biology. In this context our experiments reported in the paper, although primarily focused on Europa, will hopefully contribute to the search for the greater universal truth surrounding the origin and possibility of life elsewhere and how inanimate matter transformed into living organisms, creatures and ultimately conscious beings

Dr Stephen P. Thompson
Diamond Light Source

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This page is a summary of: Laboratory exploration of mineral precipitates from Europa's subsurface ocean, Journal of Applied Crystallography, September 2021, International Union of Crystallography, DOI: 10.1107/s1600576721008554.
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