Deliquescence-induced formation and habitability of chloride and perchlorate brines

M. Sc. Chem.

Jacob Heinz

ORCID: 0000-0002-8237-4713

von der Fakultät II - Mathematik und Naturwissenschaften

der Technischen Universität Berlin

zur Erlangung des akademischen Grades

Doktor der Naturwissenschaften

Preface

II

Preface

This thesis was written at the Center for Astronomy and Astrophysics at the Technical

University of Berlin and was embedded in the “Habitability of Martian Environments”

(HOME) project (ERC grant no. 339231). Most laboratory work was carried out at the

laboratory of the Astrobiology Group housed at the Institute for Chemistry, and at the

Center for Electron Microscopy (ZELMI) at the TU Berlin.

Acknowledgements

III

Acknowledgements

I would like to express my special thanks to my first supervisor, Prof. Dr. Dirk Schulze-

Makuch, for giving me the opportunity to conduct this interesting research work in the field

of astrobiology and for his invaluable guidance, continuous support and encouragement.

Furthermore, I would like to specially thank Prof. PhD Sam Kounaves for additional

guidance, Prof. PhD Kai Finster for evaluating my doctoral thesis, and Prof. PhD Lyle Whyte

for providing input on the bacterial strain Planococcus halocryophilus used in this study.

This thesis could not have been achieved without the help and support of numerous

excellent co-authors and members of the Astrobiology Research Group at the TU Berlin,

namely Dr. Janosch Schirmack, Dr. Alessandro Airo, Deborah Maus, Annemiek Waajen,

Abstract

IV

The availability of liquid water on Mars is one of the key factors for its habitability.

While there is strong morphological and geochemical evidence for the existence of large

water bodies on the surface of Early Mars, at the present time, the planet is dry, cold and

hostile. However, liquid water might still be found in niches like cold brines. These are

especially relevant for Mars since several hygroscopic salts have been detected in the

Martian regolith which cause a significant freezing point depression of water and, hence,

enhance the habitability range of Mars to subzero temperatures. Formation of these brines

could occur through deliquescence, i.e. water absorption by these salts followed by

dissolution of the salts in the absorbed water. This process might has already been observed

on Mars, namely by the formation of Recurring Slope Lineae (RSL) which are dark, flow-like

features extending downslope from bedrock outcrops.

4-

) and chloride (Cl

(25°C, 70–85% RH), and requires only small amounts of intergranular water. In contrast, the

formation of larger amounts of bulk water requires substantially longer, e.g. 17 days for soil

containing magnesium perchlorate (Mg(ClO

) under the same experimental conditions.

This suggests that RSL on Mars do actually not represent flows of briny water but a

rewetting of salt-cemented soils generated by the evaporation of water tracks that flowed

down the hills at a time when Mars had a warmer and wetter climate.

The brines that formed via deliquescence were too concentrated to enable growth of

Abstract

V

growth in these salty samples a dilution of the brines is necessary. Hence, the maximum salt

concentration suitable for growth at 25°C and 4°C was determined for six Cl

and ClO

4-

salts.

The results showed an increased CaCl

tolerance of P. halocryophilus at 4°C compared to

25°C, while the tolerances to other salts were similar or lower at 4°C compared to 25°C. The

highest ClO

formation and habitability of brines as they might occur on Mars. The results of a large set

of experiments give an impression on how life on Mars could have adapted to its cold and

salty environmental conditions and what influence different salt species and variations in

temperature and salt concentration might have.

VI

Flüssiges Wasser ist eine der wichtigsten Voraussetzung für die Existenz von Leben auf

dem Mars. Während Oberflächenstrukturen und geologische Funde darauf hindeuten, dass

auf dem Mars einst größere Gewässer existierten, präsentiert sich der Planet heute als

trocken, kalt und lebensunfreundlich. Es ist jedoch wahrscheinlich, dass flüssiges Wasser

nicht gänzlich von der Mars-Oberfläche verschwunden ist. So gibt es Hinweise auf flüssige

Salzlösungen, sogenannte „Brines“, die zumindest temporär auf der Planetenoberfläche

stabil sind. Die Wahrscheinlichkeit ihrer Existenz ist durch den Nachweis hygroskopischer

Salze auf dem Mars noch einmal gestiegen, denn diese können in einem „Deliqueszenz“

genannten Prozess Wasser aus der dünnen Mars-Atmosphäre anziehen und sich in diesem

Wasser auflösen. Es gibt Hinweise darauf, dass dieser Prozess bereits in Form von jährlich

wiederkehrenden, dunklen Ablaufrinnen (Recurring Slope Lineae, RSL) beobachtet wurde.

untersuchen sollen. Es hat sich gezeigt, dass die bei der Bildung der RSL beteiligte

Wasserabsorption hervorragend mit der elektrischen Leitfähigkeit des Bodens korreliert.

Die Ergebnisse der Experimente haben verdeutlicht, dass die für RSL typische Verdunklung

des Bodens sehr schnell einsetzen kann. So treten zum Beispiel im Falle von Ca(ClO

haltigen Böden bereits 2,5 Stunden nach dem Start des Experiments (25°C, 70–85%

Luftfeuchte) Verdunklungen auf. Dafür werden nur geringe Mengen intergranularen

Wassers benötigt. Im Gegensatz dazu bedarf es deutlich mehr Zeit bis sich größere

Flüssigkeitsmengen in Form von Tropfen herausbilden, im Fall von Mg(ClO

-haltigen

Böden 17 Tage. Das lässt vermuten, dass RSL auf dem Mars keine Ströme flüssigen Wassers

darstellen, sondern vielmehr nur das temporäre Feuchtwerden salz-zementierter Böden,

sich heraus, dass Brines, die sich durch Deliqueszenz bilden, zu stark konzentriert sind, um

Wachstum von P. halocryophilus zu ermöglichen. Es konnte jedoch gezeigt werden, dass

dessen Überlebensrate deutlich ansteigt, wenn die Temperatur der Brines gesenkt wird.

-haltigen Lösungen ausgeprägt. Außerdem

stellte sich heraus, dass sich die Anwesenheit von NaCl positiv auf die Überlebensfähigkeit

von P. halocryophilus während des wiederholten Einfrierens und Auftauens der Brines

auswirkt. Um jedoch Zellwachstum in diesen Lösungen beobachten zu können, muss die

Salzkonzentration reduziert werden. Daher wurde für sechs unterschiedliche Cl

- und

ClO

4-

bakterielles Wachstum nachgewiesen werden kann. Die Ergebnisse zeigten unter anderem,

dass die CaCl

diesen salzigen Bedingungen führte zu interessanten Stressreaktionen, wie zum Beispiel zur

Ausbildung von Zellclustern mit Nanofilamenten, Krustenbildung um einzelne Zellen oder

die Herausbildung neuer Zellkolonie-Morphologien.

All diese Funde und experimentellen Ergebnisse liefern einen entscheidenden Einblick

in die Entstehung und die potentielle Habitabilität von Brines auf dem Mars. Sie vermitteln

einen Eindruck wie sich Leben an die kalten und salzigen Bedingungen angepasst haben

könnte und welchen Einfluss die unterschiedlichen Salztypen und Schwankungen in

Temperatur und Salzkonzentration haben können.

Contents

VIII

Contents

Preface ......................................................................................................................... II

Abbreviations and chemical formulas ......................................................................... XIV

1. Introduction ........................................................................................................... 1

1.1 Environmental conditions on Mars and the Atacama Desert, Chile, as Mars-

analogue field site ................................................................................................... 1

1.2 Salts on Mars and deliquescence-induced brine formation .................................... 3

1.3 Recurring Slope Lineae (RSL) on Mars ..................................................................... 6

1.4 Microbial habitability of RSL-analogue brines ......................................................... 8

IX

However, the environmental conditions on today´s Mars (Fig. 1-1B) are even more

challenging. There, microbes would have to struggle with low temperatures (-138°C – +30°C

(Jones et al., 2011)), low pressures (6 – 8 mbar (Catling, D.C., Leovy C. 2006)), and high doses

precipitable microns (pr μm) in the northern summer (Trokhimovskiy et al., 2015), while the

In general, the intensity of the freezing point depression in a given salt-water system is

dependent on the type of salt and its concentration and can be visualized by a water-salt

phase diagram (reviewed e.g. in Hennings, 2014). The maximum freezing point depression

eutectic temperature (T

distributed

Cl

4-

MgCl

33 [20]

e

NaClO

-34

in the

Martian soil

with probably

Ca(ClO

salt

Ca(ClO

al., 2014), i(Keller et al., 2007), j(Hecht et al., 2009), k(Kounaves et al., 2014b), l(Toner et al., 2015),

m(Clark and Kounaves, 2016)

particles on the Phoenix lander struts were interpret as deliquescence of saline mud and

down-dripping of the resulting brine (Rennó et al., 2009). One of the most promising

evidence for the temporal occurrence of briny surface water is the observation of RSL

identified by the Mars Reconnaissance Orbiter (McEwen et al., 2011a).

RSL are relatively dark flow-like features on Mars that extend downslope from bedrock

outcrops and occur annually during spring and summer, especially on steep, equator-facing,

southern slopes (Fig. 1-2) (Runyon and Ojha, 2014). Although the role of liquid water on the

formation of RSL has been questioned (Edwards and Piqueux, 2016) and dry granular flow

mechanisms have been proposed (Dundas et al., 2017) there is spectral evidence for

hydrated ClO

thesis lab experiments have been conducted investigating the deliquescence process as

potential trigger of RSL. These experiments are described in the first publication of this

thesis (chapter 2) and some follow-up experiments are shortly summarized in chapter 7.1.

Furthermore, during the Atacama Desert field trip in 2018 RSL-simulating field experiments

were carried out (unpublished data, chapter 7.2). The results from the field experiments

showed that a saturated solution of NaCl poured down a hill forms a RSL-like darkened track

that dries out slowly during several days (Fig. 7-2). Nevertheless, when the RH increased in

the night and morning hours the briny RSL-like track absorbed additional water correlating

with an increase in EC (Fig. 7-3). However, the water amount evaporating during daytime

exceeded the water amount absorbed through deliquescence in the night and morning.

Hence, no growth but only fading of the RSL-like track could be observed. The more

hygroscopic and therefore more promising ClO

PIA17933).

1.4.1 Water activity and other habitability-limiting factors

The habitability of salt solutions is determined by two main factors: the type of salt and

its concentration. When talking about cold brines (cryobrines) also the effect of

temperature has to be considered as will be discussed more detailed in the Publications II

and III of this thesis (chapters 3 and 4, respectively). At high salt concentration the limiting

factor for habitability at a specific temperature is usually given by the water activity (a

)

which describes the amount of free biologically available water molecules. In pure water all

the lowest a

supporting life is approx. 0.61 (Stevenson et al., 2015).

However, additional lethal effects of the solutes can lead to a reduced halotolerance.

These toxic effects include, among others, salt-induced pH changes (Yaganza et al., 2009),

membrane destabilization (Xie and Yang, 2016), molecular mimicry (Cianchetta et al., 2010),

and chaotropicity of ions causing a destabilization of macromolecules like proteins

(Hallsworth et al., 2003; Hallsworth et al., 2007). Additionally, for brines with a high

divalent:monovalent ion ratio ionic strength might be the limiting factor for life rather than

water activity (Fox-Powell et al., 2016). Results from recent experiments conducted in our

1.4.2 Habitability of Cl

-containing brines

25°C) (Oren, 2008) and, thus, inhabit entrapped salt rocks (Kunte et al., 2002). Endolithic

bacteria in the hyperarid core of the Atacama Desert, Chile, can utilize water condensed

from the atmosphere via deliquescence of NaCl deposits (Davila et al., 2008). Experiments

carried out within our research group showed that halophilic methanogenic archaea can