Experimental and simulation efforts in the astrobiological exploration of exooceans

The icy satellites of Jupiter and Saturn are perhaps the most promising places in the Solar System regarding habitability. However, the potential habitable environments are hidden underneath km-thick ice shells. The discovery of Enceladus’ plume by the Cassini mission has provided vital clues in our understanding of the processes occurring within the interior of exooceans. To interpret these data and to help configure instruments for future missions, controlled laboratory experiments and simulations are needed. This review aims to bring together studies and experimental designs from various scientific fields currently investigating the icy moons, including planetary sciences, chemistry, (micro-)biology, geology, glaciology, etc. This chapter provides an overview of successful in situ, in silico, and in vitro experiments, which explore different regions of interest on icy moons, i.e. a potential plume, surface, icy shell, water and brines, hydrothermal vents, and the rocky core

Complementary Mass Spectral Analysis of Isomeric O-bearing Organic Compounds and Fragmentation Differences through Analog Techniques for Spaceborne Mass Spectrometers

Mass spectrometers on board spacecraft typically use either impact ionization or electron ionization (EI) as ion sources. Understanding the similarities and differences in the spectral signatures and fragmentation patterns produced by different techniques in mass spectrometry could elucidate the composition of organic compounds. Here we present a comparison between the mass spectra obtained through laser-induced liquid beam ion desorption (LILBID; proven to simulate the impact ionization mass spectra of ice grains) and EI mass spectra of pairs of low-mass, isomeric aldehydes and ketones. Our comparison confirms that EI produces more fragmentation of carbonyl compounds, particularly aldehydes, than LILBID. We find protonated molecular ions [M+H]+ in LILBID but molecular ions [M]+ in EI spectra. From the evaluated species, LILBID generally produces oxygen-carrying fragment ions (e.g., [CHO]+ and [C2H3O]+) in the mass ranges 26–30 and 39–44 u, while in EI, most ions in these ranges correspond to hydrocarbon fragments. The LILBID spectra additionally show mostly protonated oxygen-bearing fragments [CH3O]+ and [C2H5O]+ at m/z 31 and 45, less commonly observed in EI spectra. We observe a decrease in the relative intensities of cation fragment mass lines between m/z 26 and 33 and an increase between m/z 39 and 45, with an increasing carbon number for ketones and aldehydes with LILBID and EI, respectively. Our study provides a basis for complementary compositional analysis to identify the structural properties of organic species in a space environment using different spaceborne mass spectrometers (e.g., SUrface Dust Analyzer and MAss Spectrometer for Planetary EXploration) on board NASA’s future Europa Clipper space mission

Detection of phosphates originating from Enceladus’s ocean

Saturn’s moon Enceladus harbours a global1 ice-covered water ocean2,3. The Cassini spacecraft investigated the composition of the ocean by analysis of material ejected into space by the moon’s cryovolcanic plume4,5,6,7,8,9. The analysis of salt-rich ice grains by Cassini’s Cosmic Dust Analyzer10 enabled inference of major solutes in the ocean water (Na+, K+, Cl–, HCO3–, CO32–) and its alkaline pH3,11. Phosphorus, the least abundant of the bio-essential elements12,13,14, has not yet been detected in an ocean beyond Earth. Earlier geochemical modelling studies suggest that phosphate might be scarce in the ocean of Enceladus and other icy ocean worlds15,16. However, more recent modelling of mineral solubilities in Enceladus’s ocean indicates that phosphate could be relatively abundant17. Here we present Cassini’s Cosmic Dust Analyzer mass spectra of ice grains emitted by Enceladus that show the presence of sodium phosphates. Our observational results, together with laboratory analogue experiments, suggest that phosphorus is readily available in Enceladus’s ocean in the form of orthophosphates, with phosphorus concentrations at least 100-fold higher in the moon’s plume-forming ocean waters than in Earth’s oceans. Furthermore, geochemical experiments and modelling demonstrate that such high phosphate abundances could be achieved in Enceladus and possibly in other icy ocean worlds beyond the primordial CO2 snowline, either at the cold seafloor or in hydrothermal environments with moderate temperatures. In both cases the main driver is probably the higher solubility of calcium phosphate minerals compared with calcium carbonate in moderately alkaline solutions rich in carbonate or bicarbonate ions

Is There Such a Thing as a Biosignature?

The concept of a biosignature is widely used in astrobiology to suggest a link between some observation and a biological cause, given some context. The term itself has been defined and used in several ways in different parts of the scientific community involved in the search for past or present life on Earth and beyond. With the ongoing acceleration in the search for life in distant time and/or deep space, there is a need for clarity and accuracy in the formulation and reporting of claims. Here, we critically review the biosignature concept(s) and the associated nomenclature in light of several problems and ambiguities emphasized by recent works. One worry is that these terms and concepts may imply greater certainty than is usually justified by a rational interpretation of the data. A related worry is that terms such as “biosignature” may be inherently misleading, for example, because the divide between life and non-life—and their observable effects—is fuzzy. Another worry is that different parts of the multidisciplinary community may use non-equivalent or conflicting definitions and conceptions, leading to avoidable confusion. This review leads us to identify a number of pitfalls and to suggest how they can be circumvented. In general, we conclude that astrobiologists should exercise particular caution in deciding whether and how to use the concept of biosignature when thinking and communicating about habitability or life. Concepts and terms should be selected carefully and defined explicitly where appropriate. This would improve clarity and accuracy in the formulation of claims and subsequent technical and public communication about some of the most profound and important questions in science and society. With this objective in mind, we provide a checklist of questions that scientists and other interested parties should ask when assessing any reported detection of a “biosignature” to better understand exactly what is being claimed

Developing a Laser Induced Liquid Beam Ion Desorption Spectral Database as Reference for Spaceborne Mass Spectrometers

Spaceborne impact ionization mass spectrometers, such as the Cosmic Dust Analyzer on board the past Cassini spacecraft or the SUrface Dust Analyzer being built for NASA's upcoming Europa Clipper mission, are of crucial importance for the exploration of icy moons in the Solar System, such as Saturn's moon Enceladus or Jupiter's moon Europa. For the interpretation of data produced by these instruments, analogue experiments on Earth are essential. To date, thousands of laboratory mass spectra have been recorded with an analogue experiment for impact ionization mass spectrometers. Simulation of mass spectra of ice grains in space is achieved by a Laser Induced Liquid Beam Ion Desorption (LILBID) approach. The desorbed cations or anions are analyzed in a time-of-flight mass spectrometer. The amount of unstructured raw data is increasingly challenging to sort, process, interpret and compare with data from space. Thus far this has been achieved manually for individual mass spectra because no database containing the recorded reference spectra was available. Here we describe the development of a comprehensive, extendable database containing cation and anion mass spectra from the laboratory LILBID facility. The database is based on a Relational Database Management System with a web server interface and enables filtering of the laboratory data using a wide range of parameters. The mass spectra can be compared not only with data from past and future space missions but also mass spectral data generated by other, terrestrial, techniques. The validated and approved subset of the database is available for general public (https://lilbid-db.planet.fu-berlin.de)

Developing a Laser Induced Liquid Beam Ion Desorption Spectral Database as Reference for Spaceborne Mass Spectrometers

Spaceborne impact ionization mass spectrometers, such as the Cosmic Dust Analyzer on board the past Cassini spacecraft or the SUrface Dust Analyzer being built for NASA's upcoming Europa Clipper mission, are of crucial importance for the exploration of icy moons in the Solar System, such as Saturn's moon Enceladus or Jupiter's moon Europa. For the interpretation of data produced by these instruments, analogue experiments on Earth are essential. To date, thousands of laboratory mass spectra have been recorded with an analogue experiment for impact ionization mass spectrometers. Simulation of mass spectra of ice grains in space is achieved by a Laser Induced Liquid Beam Ion Desorption (LILBID) approach. The desorbed cations or anions are analyzed in a time-of-flight mass spectrometer. The amount of unstructured raw data is increasingly challenging to sort, process, interpret and compare with data from space. Thus far this has been achieved manually for individual mass spectra because no database containing the recorded reference spectra was available. Here we describe the development of a comprehensive, extendable database containing cation and anion mass spectra from the laboratory LILBID facility. The database is based on a Relational Database Management System with a web server interface and enables filtering of the laboratory data using a wide range of parameters. The mass spectra can be compared not only with data from past and future space missions but also mass spectral data generated by other, terrestrial, techniques. The validated and approved subset of the database is available for general public (https://lilbid-db.planet.fu-berlin.de)

How to identify cell material in a single ice grain emitted from Enceladus or Europa

Icy moons like Enceladus, and perhaps Europa, emit material sourced from their subsurface oceans into space via plumes of ice grains and gas. Both moons are prime targets for astrobiology investigations. Cassini measurements revealed a large compositional diversity of emitted ice grains with only 1 to 4% of Enceladus's plume ice grains containing organic material in high concentrations. Here, we report experiments simulating mass spectra of ice grains containing one bacterial cell, or fractions thereof, as encountered by advanced instruments on board future space missions to Enceladus or Europa, such as the SUrface Dust Analyzer onboard NASA's upcoming Europa Clipper mission at flyby speeds of 4 to 6 kilometers per second. Mass spectral signals characteristic of the bacteria are shown to be clearly identifiable by future missions, even if an ice grain contains much less than one cell. Our results demonstrate the advantage of analyses of individual ice grains compared to a diluted bulk sample in a heterogeneous plume

Experimental and Simulation Efforts in the Astrobiological Exploration of Exooceans

The icy satellites of Jupiter and Saturn are perhaps the most promising places in the Solar System regarding habitability. However, the potential habitable environments are hidden underneath km-thick ice shells. The discovery of Enceladus’ plume by the Cassini mission has provided vital clues in our understanding of the processes occurring within the interior of exooceans. To interpret these data and to help configure instruments for future missions, controlled laboratory experiments and simulations are needed. This review aims to bring together studies and experimental designs from various scientific fields currently investigating the icy moons, including planetary sciences, chemistry, (micro-)biology, geology, glaciology, etc. This chapter provides an overview of successful in situ, in silico, and in vitro experiments, which explore different regions of interest on icy moons, i.e. a potential plume, surface, icy shell, water and brines, hydrothermal vents, and the rocky core

On biosignatures and tracers of life

editorial reviewe

Experimente für das Aufspüren von Biosignaturen in Eisteilchen mit Raumfahrtmissionen zu Enceladus und Europa

Cryovolcanically active ocean worlds, such as Saturn's moon Enceladus and potentially Jupiter's moon Europa, eject water ice grains formed from subsurface water into space. The ejected ice grains can be analyzed by impact ionization mass spectrometers on-board spacecraft, thereby exploring the habitability of the subsurface oceans during flybys. An archetype of such mass spectrometers, the Cosmic Dust Analyzer (CDA) on-board the Cassini spacecraft, sampled individual ice grains from Enceladus in the Enceladean plume and Saturn's E ring. The SUrface Dust Analyzer (SUDA) instrument, being built for the upcoming Europa Clipper mission, will analyze ice grains in Europa's vicinity. Interpreting the spaceborne measurements requires terrestrial calibration and this PhD thesis therefore deals with terrestrial analogue experiments and comprises two conjoint projects. The appearance of recorded impact ionization mass spectra is a function of not only composition but also impact speed (i.e. kinetic energy) of the ice grains onto the mass spectrometer's metal target. In the first project, mass spectra of water ice grains as recorded by the CDA at typical impact speeds ranging between 4 and 21 km/s are simulated using a laboratory analogue experiment which is capable of reproducing compositional variations of the ice grains. In this Laser-Induced Liquid Beam Ion Desorption (LILBID) process, a µm-sized liquid water beam is irradiated by a pulsed infrared laser at suitable energies and wavelengths. The created ions are subsequently analyzed in a Time-of-Flight mass spectrometer (ToF-MS). Categorizing the ice grain mass spectra into five different speed regimes, the significantly varying spectral appearances can be accurately reproduced by tuning the laser energy and the delay time of the mass spectrometer's gating system. The LILBID facility is capable of quantitatively reproducing CDA spectra of ice grains at impact speeds up to 15 km/s. Above that speed a qualitative match is achieved. The experimental parameters used for this "speed calibration" can now be applied to ice grains carrying a wide variety of non-water compounds as observed in the Enceladean plume and Saturn's E ring. More than 10,000 laboratory analogue spectra of over 200 different organic and inorganic compounds dissolved or suspended in water have been recorded with the LILBID facility. The enormous amount of data are increasingly challenging to sort, process, interpret and eventually compare to the data from space. Thus far, manual comparison of ice grain and LILBID mass spectra has been required. As part of the research presented here, a comprehensive spectral reference library containing all recorded data from the LILBID facility has been developed. This relational database is based on Structured Query Language (SQL) and enables filtering the laboratory data for scores of experimental parameters, such as laser energy and delay time, as well as mass lines in the spectra. The LILBID mass spectra in the reference library can be compared not only to data from space missions but also to any kind of available mass spectral data. The CDA has proven to be very successful in analyzing inorganic and organic ice grain constituents to characterize the habitability of Enceladus' ocean. Hitherto biosignatures have not been identified in extraterrestrial ocean environments. In the second project, the mass spectral appearances of amino acids, fatty acids, and peptides in water ice grains have been simulated using the LILBID facility. The investigated organic molecules and their fragments are clearly identifiable in the mass spectra and their detection limits are determined to be at the ppm or ppb level, depending on the molecular species and instrument polarity. By comparing the laboratory spectra with e.g. SUDA spectra, these key organic molecules can be recognized in ice grains from extraterrestrial ocean worlds. While the detection of peptides would strongly indicate extant biological processes, amino acids and fatty acids can be either produced abiotically or biotically. Discriminating between abiotic and biotic signatures of amino acids and fatty acids on ocean worlds is crucial for the search for life and its emergence on these bodies. Therefore, the mass spectral appearances and detection limits of amino acids and fatty acids, in proportions representative of either abiotic or biotic formation processes, have been investigated in matrices realistic for extraterrestrial subsurface oceans. The analytes are mixed with numerous additional organic and inorganic background compounds suitable for ice grains formed from Enceladean ocean water which has interacted with a rocky core. Differing abiotic and biotic mass spectral fingerprints of amino acids and fatty acids can be reliably identified and distinguished from each other, even under these demanding matrix conditions. In a salty matrix, the organics form characteristic sodiated molecular cations. Detection limits of the organic biosignatures are at the ppm or ppb level, strongly dependent on the pKa values of the organics and the salinity of the ice grains. The conducted experiments suggest that the survivability and ionization efficiency of large organic molecules during impact ionization of an ice grain is significantly improved when the molecules are protected by a frozen water matrix. Applying the "speed calibration" of the first project to these measurements shows that ice grain encounter velocities of 3 - 8 km/s, with an optimal window at 4 - 6 km/s, are most appropriate to detect encased amino acids, fatty acids, and peptides with a spaceborne mass spectrometer, and in turn discriminate between abiotic and biotic signatures in the resulting mass spectra.Der Saturnmond Enceladus stößt Partikel aus Wassereis, die sich aus unterirdischem Ozeanwasser gebildet haben, in das Weltall. Eine ähnliche kryovulkanische Aktivität findet vermutlich auch auf dem Jupitermond Europa statt. Die ausgetoßenen Eispartikel können mit vorbeifliegenden Raumsonden mit Einschlagsionisations-Massenspektrometern analysiert werden, um die Habitabilität der unterirdischen Ozeane zu untersuchen. Ein Urtyp solcher Massenspektrometer, der Cosmic Dust Analyzer (CDA) auf der Cassini Raumsonde, untersuchte einzelne Eisteilchen in Enceladus' Plume und in Saturn's E Ring. Der SUrface Dust Analyzer (SUDA), der bereits für die anstehende Europa Clipper Mission gebaut wird, wird die Eispartikel in Europa's Umgebung analysieren. Die Interpretation der im Weltall stattfindenden Messungen erfordert erdgebundene Kalibrationen. Diese Dissertation befasst sich deshalb mit erdgebundenen Analogexperimenten und besteht aus zwei miteinander verbundenen Projekten. Das Erscheinungsbild der aufgenommenen Einschlagsionisations-Massenspektren hängt nicht nur von der Zusammensetzung der Eispartikel sondern auch von deren Einschlagsgeschwindigkeit auf das Massenspektrometer ab. Im ersten Projekt werden Massenspektren von Wassereispartikeln mit typischen Einschlagsgeschwindigkeiten zwischen 4 und 21 km/s mit einem Analogexperiment im Labor simuliert, mit dem es zudem möglich ist, Variationen in der Zusammensetzung der Eispartikel nachzustellen. In diesem laserinduzierten Flüssigstrahl-Desorptions (LILBID) Prozess wird ein wenige Mikrometer breiter Wasserstrahl mit einem gepulsten Infrarot-Laser bei geeigneten Energien und Wellenlängen beschossen. Die entstandenen Ionen werden anschließend mit einem Flugzeit-Massenspektrometer (ToF-MS) untersucht. Die drastischen Unterschiede im Erscheinungsbild der Eispartikel-Massenspektren, die zuvor in fünf verschiedene Geschwindigkeitsbereiche eingeteilt wurden, können detailgetreu durch das Variieren der Laserenergie und der Verzögerungszeit des Massenspektrometers nachgestellt werden. Mit dem LILBID Experiment können CDA Spektren von Eispartikeln mit Einschlagsgeschwindigkeiten von bis zu 15 km/s quantitativ nachgestellt werden. Spektren höherer Geschwindigkeiten können qualitativ nachgestellt werden. Die experimentellen Parameter, die für diese "Geschwindigkeitskalibration" verwendet wurden, können nun auf Eispartikel angewendet werden, die neben Wasser zahlreiche anderen Substanzen beinhalten, wie sie bereits in Enceladus' Plume und Saturn's E Ring gefunden wurden. Mit dem LILBID Versuchsaufbau im Labor wurden bereits über 10.000 Analogspektren von mehr als 200 verschiedenen organischen und anorganischen Substanzen, in Wasser gelöst oder suspendiert, aufgenommen. Diese große Datenmenge lässt sich zunehmend umständlicher sortieren, prozessieren und interpretieren, was auch den Vergleich mit den Daten aus dem Weltall erschwert. Das Vergleichen der Massenspektren der Eispartikel mit den LILBID Laborspektren musste bisher manuell für individuelle Spektren durchgeführt werden. Innerhalb dieser Arbeit wird eine umfangreiche spektrale Referenzbibliothek entwickelt, die alle aufgenommenen LILBID Daten beinhaltet. Diese relationale Datenbank, die auf Structured Query Language (SQL) basiert, ermöglicht es, die Labordaten nach zahlreichen experimentellen Parametern zu filtern, wie zum Beispiel Laserenergie, Verzögerungszeit und Massenlinien in den Spektren. Die LILBID Massenspektren der Referenzbibliothek können nicht nur mit Daten von Raumsonden sondern auch mit jeglicher anderer Art verfügbarer Massenspektren verglichen werden. Mit dem CDA wurden erfolgreich anorganische und organische Bestandteile von Eispartikel untersucht und es konnten Rückschlüsse auf die Habitabilität von Enceladus' Ozean gezogen werden. Bisher wurden jedoch keine Biosignaturen in außerirdischen Ozeanwelten identifiziert. Im zweiten Projekt werden die spektralen Erscheinungsbilder von Aminosäuren, Fettsäuren und Peptiden in Wassereispartikeln mit dem LILBID Experiment simuliert. Die untersuchten organischen Moleküle und ihre Fragmente können in den Massenspektren eindeutig identifiziert werden. Die Moleküle und Fragmente können bis in den ppm oder ppb Bereich nachgewiesen werden, abhängig von der individuellen Molekülspezies und der Polarität des Instruments. Durch das Vergleichen der Laborspektren mit denen von beispielsweise SUDA können diese organischen Biomarker zukünftig in Eispartikeln von außerirdischen Ozeanwelten nachgewiesen werden. Während das Aufspüren von Peptiden ein deutlicher Hinweis auf vorhandene biologische Prozesse wäre, können Aminosäuren und Fettsäuren abiotisch oder biotisch entstehen. Die Unterscheidung von abiotischen und biotischen Signaturen von Aminosäuren und Fettsäuren in Ozeanwelten ist für die Suche nach Leben auf diesen Welten entscheidend. Deshalb werden die spektralen Erscheinungsbilder und Nachweisgrenzen von Aminosäuren und Fettsäuren in Proportionen untersucht, wie sie für entweder abiotische oder biotische Prozesse repräsentativ ist. Um ein realistisches Szenario zu simulieren, werden die Analyten mit zahlreichen zusätzlichen organischen und anorganischen Substanzen gemischt, wie sie in Eisteilchen von Enceladus erwartet werden, die sich aus einem Ozean gebildet haben, der mit einem Gesteinskern wechselwirkte. Abiotische und biotische spektrale Fingerabdrücke von Aminosäuren und Fettsäuren können sogar in diesen anspruchsvollen Matrizen zuverlässig identifiziert und voneinander unterschieden werden. In den salzreichen Matrizen bilden die organischen Moleküle charakteristische kationische Natriumkomplexe. Die organischen Biosignaturen können bis in den ppm oder ppb Bereich nachgewiesen werden, abhängig von den pKS Werten der organischen Substanzen und der Salinität der Eispartikel. Die Experimente legen nahe, dass "Überlebenswahrscheinlichkeit" und Ionisierungseffizienz der komplexen organischen Moleküle während der Einschlagsionisation eines Eispartikels signifikant erhöht sind, wenn die Moleküle von einer Wassereismatrix geschützt werden. Anwendung der "Geschwindigkeitskalibration" des ersten Projektes auf diese Messungen zeigt, dass Einschlagsgeschwindigkeiten der Eispartikel von 3 - 8 km/s (optimal sind 4 - 6 km/s) am besten geeignet sind, eingeschlossene Aminosäuren, Fettsäuren und Peptide mit weltraumgestützten Massenspektrometern zu detektieren und dabei zwischen abiotischen und biotischen Signaturen in den aufgenommenen Massenspektren zu unterscheiden