life Brief Report Complex Brines and Their Implications for Habitability Nilton O. Renno 1,*, Erik Fischer 1, Germán Martínez 2 and Jennifer Hanley 3 1 Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109, USA; erikfi[email protected] 2 Lunar and Planetary Institute, Universities Space Research Association, Houston, TX 75835, USA; [email protected] 3 Lowell Observatory, Flagstaff, AZ 86001, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-734-936-0488 Abstract: There is evidence that life on Earth originated in cold saline waters around scorching hydrothermal vents, and that similar conditions might exist or have existed on Mars, Europa, Ganymede, Enceladus, and other worlds. Could potentially habitable complex brines with extremely low freezing temperatures exist in the shallow subsurface of these frigid worlds? Earth, Mars, and carbonaceous chondrites have similar bulk elemental abundances, but while the Earth is depleted in the most volatile elements, the Icy Worlds of the outer solar system are expected to be rich in them. The cooling of ionic solutions containing substances that likely exist in the Icy Worlds could form complex brines with the lowest eutectic temperature possible for the compounds available in them. Indeed, here, we show observational and theoretical evidence that even elements present in trace amounts in nature are concentrated by freeze–thaw cycles, and therefore contribute significantly to the formation of brine reservoirs that remain liquid throughout the year in some of the coldest places on Earth. This is interesting because the eutectic temperature of water–ammonia solutions can be as low as ~160 K, and significant fractions of the mass of the Icy Worlds are estimated to be water substance and ammonia. Thus, briny solutions with eutectic temperature of at least ~160 K could have formed where, historically, temperature have oscillated above and below ~160 K. We Citation: Renno, N.O.; Fischer, E.; conclude that complex brines must exist in the shallow subsurface of Mars and the Icy Worlds, and Martínez, G.; Hanley, J. Complex that liquid saline water should be present where ice has existed, the temperature is above ~160 K, Brines and Their Implications for and evaporation and sublimation have been inhibited. Habitability. Life 2021, 11, 847. https://doi.org/10.3390/life11080847 Keywords: brines; complex brines; liquid water; habitability; habitable; life; icy worlds; Mars Academic Editor: Jay Nadeau Received: 23 July 2021 1. Introduction Accepted: 11 August 2021 Published: 19 August 2021 One of the top goals of NASA’s space science program is to use knowledge of the history of the Earth, and the life on it, as a guide for determining the processes and Publisher’s Note: MDPI stays neutral conditions that create and maintain habitable environments beyond Earth [1]. Habitability with regard to jurisdictional claims in or astrobiological potential are defined as the potential for an environment to support published maps and institutional affil- life, on scales ranging from microscopic to planetary-wide. In the past, to a large extent, iations. assessments of astrobiological potential have focused on determining whether liquid water has been present on other worlds [1–8]. However, more specifically, the habitability of an environment depends on the presence of three main ingredients: (i) a solvent capable of supporting complex biochemistry [2,3], (ii) a source of energy to maintain the complex molecules, structures, and pathways on which life depends on [4], and (iii) nutrients and Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. raw materials for biosynthesis [5]. Moreover, these ingredients must be available within This article is an open access article environmental conditions amenable to the assembly, stability, and interaction of complex distributed under the terms and structures and molecules [1,6–8]. This article consists of a brief review of the literature conditions of the Creative Commons on brines, the interpretation of observations, and predictions of the formation of complex Attribution (CC BY) license (https:// brines based on thermodynamics. creativecommons.org/licenses/by/ As illustrated in Figure1, brine inclusions produce habitable channels in terrestrial 4.0/). sea ice [9]. Similar features might have existed or might exist on Mars and in the Icy Worlds Life 2021, 11, 847. https://doi.org/10.3390/life11080847 https://www.mdpi.com/journal/life Life 2021, 11, x FOR PEER REVIEW 2 of 10 Life 2021, 11, 847 2 of 10 As illustrated in Figure 1, brine inclusions produce habitable channels in terrestrial sea ice [9]. Similar features might have existed or might exist on Mars and in the Icy becauseWorlds brine because inclusions brine always inclusions form when always saline form water when freezes. saline As the water temperature freezes. of aAs the tempera- salineture of solution a saline decreases, solution ice precipitates, decreases, increasing ice precipitates, the salt concentration increasing of the the salt remaining concentration of the liquidremaining [10]. This liquid process [10]. drives This the concentrationprocess drives of saline the solutionsconcentration towards of the saline value with solutions towards thethe lowest value freezing with the temperature lowest freezing possible, i.e.,temperature their eutectic possible, values [11 i.e.,]. their eutectic values [11]. FigureFigure 1. 1.(a) ( Brinea) Brine inclusions inclusions in sea icein containingsea ice containing two bacteria two inside bacteria a triple inside junction a betweentriple junction ice between ice crystals.crystals. The The scale scale bar at bar the at bottom the bottom of the image of the is 10 imageµm long. is (10b) Epifluorescentμm long. (b) image Epifluorescent highlighting image highlight- theing presence the presence of the two of bacteriathe two in bacteria the junction. in the After junction. Junge et al.,After 2001 Junge [9]. et al., 2001 [9]. The eutectic temperature (TEut) of binary aqueous solutions of salts already found on Mars isThe as low eutectic as ~199 temperature K [11–16]. Eutectic (TEut) solutions of binary have aqueous salt mass solutions fractions c ofEut ~0.3–0.5salts already found on forMars salts is such as low as NaCl as ~199 and Ca(ClO K [114–)162 [17]. ].Eutectic The freezing solutions point depressionhave salt ofmass saline fractions solu- χEut~0.3–0.5 tions with respect to the value for pure water, DT , depends on their composition, but for salts such as NaCl and Ca(ClO4)2 [17]. TheEut freezing point depression of saline solutions even ubiquitous natural terrestrial solutions, such as seawater, remain partially liquid at with respect to the value for pure water, TEut, depends on their composition, but even temperatures as low as 230 K (DTEut ≈ 43 K) [18]. Moreover, these solutions can remain liquidubiquitous (metastable) natural at lower terrestrial temperatures, solutions while, such complex as seawater brines can, remain have significantly partially liquid at tem- lowerperatures eutectic as temperatures low as 230 than K ( binaryTEut ≈ brines 43 K) [ 19[18]. ]. Moreover, these solutions can remain liquid (metastable)Earth, Mars, at and lower carbonaceous temperatures, chondrites while have similarcomplex elemental brines abundances can have [20 significantly,21], lower buteutectic the Earth temperatures is depleted in thethan most binary volatile brines elements [19]. such as ammonia. The Icy Worlds, on the other hand, are expected to be richer in more volatile elements than the Earth because they formedEarth, further Mars, away and from carbonaceous the sun [22–24 ].chondrites Indeed, ammonia have has similar been observed elemental in abundances Enceladus’[20,21], but water the plumesEarth [is25 depleted]. As our knowledgein the most of thevolatile salts present elements on Mars such and as theammonia. The Icy IcyWorlds Worlds, on is very the limited other [14ha,22nd,24, are–42], expected we hypothesize to be that richer they havein more salts derivedvolatile from elements than the elementsEarth because found in they carbonaceous formed further chondrites away [20], from at least the in sun trace [22 amounts,–24]. Indeed, and use ammonia this has been hypothesis together with our knowledge of the salts already discovered on Mars and the Icyobserved Worlds [ 43in– Enceladus’45] to study the water complex plumes brines [25]. that mightAs our exist knowledge on them. of the salts present on Mars andThe the motivation Icy World fors ouris very approach limited to study [14,22,24 complex–42 brines], we on hypothesize Mars and the Icy that Worlds they have salts de- isrived that, asfrom an ionic elements solution found starts toin freeze, carbonaceous ice and the chondrites least soluble compounds[20], at least precipitate in trace amounts, and first,use whilethis hypothesis the brine with together the lowest with possible our eutectic knowledge temperature of the freezes salts last. already This process discovered on Mars drivesand the the Icy composition Worlds of[43 aqueous–45] to solutionsstudy the towards complex that brines of the brines that might with the exist lowest on them. eutectic temperatures possible for the compounds available in the solutions [11]. Indeed, The motivation for our approach to study complex brines on Mars and the Icy Worlds we postulate that this is the process that concentrates CaCl2 in Don Juan Pond in Antarctica (Figureis that2,)[ as46 an,47 ]ionic and other