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.

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 plume4-9. 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 elements12-14, 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.

Micrometeoroid infall onto Saturn's rings constrains their age to no more than a few hundred million years.

There is ongoing debate as to whether Saturn's main rings are relatively young or ancient- having been formed shortly after Saturn or during the Late Heavy Bombardment. The rings are mostly water-ice but are polluted by non-icy material with a volume fraction ranging from ∼0.1 to 2%. Continuous bombardment by micrometeoroids exogenic to the Saturnian system is a source of this non-icy material. Knowledge of the incoming mass flux of these pollutants allows estimation of the rings' exposure time, providing a limit on their age. Here we report the final measurements by Cassini's Cosmic Dust Analyzer of the micrometeoroid flux into the Saturnian system. Several populations are present, but the flux is dominated by low-relative velocity objects such as from the Kuiper belt. We find a mass flux between 6.9 · 10-17 and 2.7 · 10-16 kg m-2s-1 from which we infer a ring exposure time ≲100 to 400 million years in support of recent ring formation scenarios.

Discriminating Abiotic and Biotic Fingerprints of Amino Acids and Fatty Acids in Ice Grains Relevant to Ocean Worlds.

Identifying and distinguishing between abiotic and biotic signatures of organic molecules such as amino acids and fatty acids is key to the search for life on extraterrestrial ocean worlds. Impact ionization mass spectrometers can potentially achieve this by sampling water ice grains formed from ocean water and ejected by moons such as Enceladus and Europa, thereby exploring the habitability of their subsurface oceans in spacecraft flybys. Here, we extend previous high-sensitivity laser-based analog experiments of biomolecules in pure water to investigate the mass spectra of amino acids and fatty acids at simulated abiotic and biotic relative abundances. To account for the complex background matrix expected to emerge from a salty Enceladean ocean that has been in extensive chemical exchange with a carbonaceous rocky core, other organic and inorganic constituents are added to the biosignature mixtures. We find that both amino acids and fatty acids produce sodiated molecular peaks in salty solutions. Under the soft ionization conditions expected for low-velocity (2-6 km/s) encounters of an orbiting spacecraft with ice grains, the unfragmented molecular spectral signatures of amino acids and fatty acids accurately reflect the original relative abundances of the parent molecules within the source solution, enabling characteristic abiotic and biotic relative abundance patterns to be identified. No critical interferences with other abiotic organic compounds were observed. Detection limits of the investigated biosignatures under Enceladus-like conditions are salinity dependent (decreasing sensitivity with increasing salinity), at the µM or nM level. The survivability and ionization efficiency of large organic molecules during impact ionization appear to be significantly improved when they are protected by a frozen water matrix. We infer from our experimental results that encounter velocities of 4-6 km/s are most appropriate for impact ionization mass spectrometers to detect and discriminate between abiotic and biotic signatures.

Analogue spectra for impact ionization mass spectra of water ice grains obtained at different impact speeds in space.

RATIONALE: Detecting ice grains with impact ionization mass spectrometers in space provides information about the compositions of ice grains and their sources. Depending on the impact speeds of the ice grains onto the metal target of a mass spectrometer, ionization conditions can vary substantially, resulting in changes to the appearance of the resulting mass spectra. METHODS: Here we accurately reproduce mass spectra of water ice grains, recorded with the Cosmic Dust Analyzer (CDA) on board the Cassini spacecraft at typical impact speeds ranging between 4 km/s to 21 km/s, with a laboratory analogue experiment. In this Laser-Induced Liquid Beam Ion Desorption (LILBID) approach, a µm-sized liquid water beam is irradiated with a pulsed infrared laser, desorbing charged analyte and solvent aggregates and isolated ions, which are subsequently analyzed in a time-of-flight (TOF) mass spectrometer. RESULTS: We show that our analogue experiment can reproduce impact ionization mass spectra of ice grains obtained over a wide range of impact speeds, aiding the quantitative analyses of mass spectra from space. CONCLUSIONS: Spectra libraries created with the LILBID experiment will be a vital tool for inferring the composition of ice grains from mass spectra recorded by both past and future impact ionization mass spectrometers (e.g. the SUrface Dust Analyzer (SUDA) onboard NASA's Europa Clipper Mission or detectors on a future Enceladus Mission).

In situ collection of dust grains falling from Saturn's rings into its atmosphere.

Saturn's main rings are composed of >95% water ice, and the nature of the remaining few percent has remained unclear. The Cassini spacecraft's traversals between Saturn and its innermost D ring allowed its cosmic dust analyzer (CDA) to collect material released from the main rings and to characterize the ring material infall into Saturn. We report the direct in situ detection of material from Saturn's dense rings by the CDA impact mass spectrometer. Most detected grains are a few tens of nanometers in size and dynamically associated with the previously inferred "ring rain." Silicate and water-ice grains were identified, in proportions that vary with latitude. Silicate grains constitute up to 30% of infalling grains, a higher percentage than the bulk silicate content of the rings.

Macromolecular organic compounds from the depths of Enceladus.

Saturn's moon Enceladus harbours a global water ocean 1 , which lies under an ice crust and above a rocky core 2 . Through warm cracks in the crust 3 a cryo-volcanic plume ejects ice grains and vapour into space4-7 that contain materials originating from the ocean8,9. Hydrothermal activity is suspected to occur deep inside the porous core10-12, powered by tidal dissipation 13 . So far, only simple organic compounds with molecular masses mostly below 50 atomic mass units have been observed in plume material6,14,15. Here we report observations of emitted ice grains containing concentrated and complex macromolecular organic material with molecular masses above 200 atomic mass units. The data constrain the macromolecular structure of organics detected in the ice grains and suggest the presence of a thin organic-rich film on top of the oceanic water table, where organic nucleation cores generated by the bursting of bubbles allow the probing of Enceladus' organic inventory in enhanced concentrations.

New experimental capability to investigate the hypervelocity micrometeoroid bombardment of cryogenic surfaces.

Ice is prevalent throughout the solar system and beyond. Though the evolution of many of these icy surfaces is highly dependent on associated micrometeoroid impact phenomena, experimental investigation of these impacts has been extremely limited, especially at the impactor speeds encountered in space. The dust accelerator facility at the Institute for Modeling Plasmas, Atmospheres, and Cosmic Dust (IMPACT) of NASA's Solar System Exploration Research Virtual Institute has developed a novel cryogenic system that will facilitate future study of hypervelocity impacts into ice and icy regolith. The target consists of a copper block, cooled by liquid nitrogen, upon which layers of vapor-deposited ice, pre-frozen ice, or icy regolith can be built in a controlled and quantifiable environment. This ice can be grown from a variety of materials, including H2O, CH3OH, NH3, and slurries containing nanophase iron. Ice temperatures can be varied between 96 K and 150 K and ice thickness greater than 150 nm can be accurately measured. Importantly, the composition of ion plumes created during micrometeoroid impacts onto these icy layers can be measured even in trace amounts by in situ time-of-flight mass spectroscopy. In this paper, we present the fundamental design components of the cryogenic target chamber at IMPACT and proof-of-concept results from target development and from first impacts into thick layers of water ice.

Ongoing hydrothermal activities within Enceladus.

Detection of sodium-salt-rich ice grains emitted from the plume of the Saturnian moon Enceladus suggests that the grains formed as frozen droplets from a liquid water reservoir that is, or has been, in contact with rock. Gravitational field measurements suggest a regional south polar subsurface ocean of about 10 kilometres thickness located beneath an ice crust 30 to 40 kilometres thick. These findings imply rock-water interactions in regions surrounding the core of Enceladus. The resulting chemical 'footprints' are expected to be preserved in the liquid and subsequently transported upwards to the near-surface plume sources, where they eventually would be ejected and could be measured by a spacecraft. Here we report an analysis of silicon-rich, nanometre-sized dust particles (so-called stream particles) that stand out from the water-ice-dominated objects characteristic of Saturn. We interpret these grains as nanometre-sized SiO2 (silica) particles, initially embedded in icy grains emitted from Enceladus' subsurface waters and released by sputter erosion in Saturn's E ring. The composition and the limited size range (2 to 8 nanometres in radius) of stream particles indicate ongoing high-temperature (>90 °C) hydrothermal reactions associated with global-scale geothermal activity that quickly transports hydrothermal products from the ocean floor at a depth of at least 40 kilometres up to the plume of Enceladus.

3 MV hypervelocity dust accelerator at the Colorado Center for Lunar Dust and Atmospheric Studies.

A hypervelocity dust accelerator for studying micrometeorite impacts has been constructed at the Colorado Center for Lunar Dust and Atmospheric Studies (CCLDAS) at the University of Colorado. Based on the Max-Planck-Institüt für Kernphysik (MPI-K) accelerator, this accelerator is capable of emitting single particles of a specific mass and velocity selected by the user. The accelerator consists of a 3 MV Pelletron generator with a dust source, four image charge pickup detectors, and two interchangeable target chambers: a large high-vacuum test bed and an ultra-high vacuum impact study chamber. The large test bed is a 1.2 m diameter, 1.5 m long cylindrical vacuum chamber capable of pressures as low as 10(-7) torr while the ultra-high vacuum chamber is a 0.75 m diameter, 1.1 m long chamber capable of pressures as low as 10(-10) torr. Using iron dust of up to 2 microns in diameter, final velocities have been measured up to 52 km/s. The spread of the dust particles and the effect of electrostatic focusing have been measured using a long exposure CCD and a quartz target. Furthermore, a new technique of particle selection is being developed using real time digital filtering techniques. Signals are digitized and then cross-correlated with a shaped filter, resulting in a suppressed noise floor. Improvements over the MPI-K design, which include a higher operating voltage and digital filtering for detection, increase the available parameter space of dust emitted by the accelerator. The CCLDAS dust facility is a user facility open to the scientific community to assist with instrument calibrations and experiments.

A 2 MV Van de Graaff accelerator as a tool for planetary and impact physics research.

Investigating the dynamical and physical properties of cosmic dust can reveal a great deal of information about both the dust and its many sources. Over recent years, several spacecraft (e.g., Cassini, Stardust, Galileo, and Ulysses) have successfully characterised interstellar, interplanetary, and circumplanetary dust using a variety of techniques, including in situ analyses and sample return. Charge, mass, and velocity measurements of the dust are performed either directly (induced charge signals) or indirectly (mass and velocity from impact ionisation signals or crater morphology) and constrain the dynamical parameters of the dust grains. Dust compositional information may be obtained via either time-of-flight mass spectrometry of the impact plasma or direct sample return. The accurate and reliable interpretation of collected spacecraft data requires a comprehensive programme of terrestrial instrument calibration. This process involves accelerating suitable solar system analogue dust particles to hypervelocity speeds in the laboratory, an activity performed at the Max Planck Institut für Kernphysik in Heidelberg, Germany. Here, a 2 MV Van de Graaff accelerator electrostatically accelerates charged micron and submicron-sized dust particles to speeds up to 80 km s(-1). Recent advances in dust production and processing have allowed solar system analogue dust particles (silicates and other minerals) to be coated with a thin conductive shell, enabling them to be charged and accelerated. Refinements and upgrades to the beam line instrumentation and electronics now allow for the reliable selection of particles at velocities of 1-80 km s(-1) and with diameters of between 0.05 µm and 5 µm. This ability to select particles for subsequent impact studies based on their charges, masses, or velocities is provided by a particle selection unit (PSU). The PSU contains a field programmable gate array, capable of monitoring in real time the particles' speeds and charges, and is controlled remotely by a custom, platform independent, software package. The new control instrumentation and electronics, together with the wide range of accelerable particle types, allow the controlled investigation of hypervelocity impact phenomena across a hitherto unobtainable range of impact parameters.

Mass spectrometry of hyper-velocity impacts of organic micrograins.

The study of hyper-velocity impacts of micrometeoroids is important for the calibration of dust sensors in space applications. For this purpose, submicron-sized synthetic dust grains comprising either polystyrene or poly[bis(4-vinylthiophenyl)sulfide] were coated with an ultrathin overlayer of an electrically conductive organic polymer (either polypyrrole or polyaniline) and were accelerated to speeds between 3 and 35 km s(-1) using the Heidelberg Dust Accelerator facility. Time-of-flight mass spectrometry was applied to analyse the resulting ionic impact plasma using a newly developed Large Area Mass Analyser (LAMA). Depending on the projectile type and the impact speed, both aliphatic and aromatic molecular ions and cluster species were identified in the mass spectra with masses up to 400 u. Clusters resulting from the target material (silver) and mixed clusters of target and projectile species were also observed. Impact velocities of between 10 and 35 km s(-1) are suitable for a principal identification of organic materials in micrometeoroids, whereas impact speeds below approximately 10 km s(-1) allow for an even more detailed analysis. Molecular ions and fragments reflect components of the parent molecule, providing determination of even complex organic molecules embedded in a dust grain. In contrast to previous measurements with the Cosmic Dust Analyser instrument, the employed LAMA instrument has a seven times higher mass resolution--approximately 200--which allowed for a detailed analysis of the complex mass spectra. These fundamental studies are expected to enhance our understanding of cometary, interplanetary and interstellar dust grains, which travel at similar hyper-velocities and are known to contain both aliphatic and aromatic organic compounds.

Characteristics of a dust trajectory sensor.

Trajectories of cosmic dust particles are determined by the measurement of the electrical signals that are induced when a charged grain flies through a position-sensitive electrode system. A typical dust trajectory sensor has four sensor planes consisting of about 16 wire electrodes each. Two adjacent planes have orthogonal wire directions. The sensor is highly transparent and mechanically robust, provides a large sensitive area, large field of view, and can, at least in principle, achieve unlimited precision. While a sensor model had already undergone limited testing in the dust laboratory, its response as a function of position and angle of incidence of the trajectory and as a function of sensor dimensions was generally unknown. To better understand its characteristics, the operation of a sensor model consisting of three planes and seven wires per plane was simulated using the COULOMB computer program. We show that the response of the reduced model can be applied to a model with more planes and more wires per plane. The effect of a trajectory's position and angle on the signal strength is discussed as well as the influence of geometrical parameters such as wire diameter, distance between wire planes, and wire length. We found a greater effect of the wire diameter on the signal strength, and a lesser effect of the plane distance, than expected. A set of similarity rules is provided for the design of a larger sensor. Finally, we discuss the optimization of the sensor for different applications.

Slow dust in Enceladus' plume from condensation and wall collisions in tiger stripe fractures.

One of the spectacular discoveries of the Cassini spacecraft was the plume of water vapour and icy particles (dust) originating near the south pole of Saturn's moon Enceladus. The data imply considerably smaller velocities for the grains than for the vapour, which has been difficult to understand. The gas and dust are too dilute in the plume to interact, so the difference must arise below the surface. Here we report a model for grain condensation and growth in channels of variable width. We show that repeated wall collisions of grains, with re-acceleration by the gas, induce an effective friction, offering a natural explanation for the reduced grain velocity. We derive particle speed and size distributions that reproduce the observed and inferred properties of the dust plume. The gas seems to form near the triple point of water; gas densities corresponding to sublimation from ice at temperatures less than 260 K are generally too low to support the measured particle fluxes. This in turn suggests liquid water below Enceladus' south pole.

The source of Saturn's G ring.

The origin of Saturn's narrow G ring has been unclear. We show that it contains a bright arc located 167,495.6 +/- 1.3 km from Saturn's center. This longitudinally localized material is trapped in a 7:6 corotation eccentricity resonance with the satellite Mimas. The cameras aboard the Cassini spacecraft mainly observe small (1 to 10 micrometers) dust grains in this region, but a sharp decrease in the flux of energetic electrons measured near this arc requires that it also contain larger (centimeter- to meter-sized) bodies whose total mass is equivalent to that of a approximately 100-meter-wide ice-rich moonlet. Collisions into these bodies may generate dust, which subsequently drifts outward to populate the rest of the G ring. Thus, the entire G ring could be derived from an arc of debris held in a resonance with Mimas.

Cassini dust measurements at Enceladus and implications for the origin of the E ring.

During Cassini's close flyby of Enceladus on 14 July 2005, the High Rate Detector of the Cosmic Dust Analyzer registered micron-sized dust particles enveloping this satellite. The dust impact rate peaked about 1 minute before the closest approach of the spacecraft to the moon. This asymmetric signature is consistent with a locally enhanced dust production in the south polar region of Enceladus. Other Cassini experiments revealed evidence for geophysical activities near Enceladus' south pole: a high surface temperature and a release of water gas. Production or release of dust particles related to these processes may provide the dominant source of Saturn's E ring.

Composition of saturnian stream particles.

During Cassini's approach to Saturn, the Cosmic Dust Analyser (CDA) discovered streams of tiny (less than 20 nanometers) high-velocity (approximately 100 kilometers per second) dust particles escaping from the saturnian system. A fraction of these impactors originated from the outskirts of Saturn's dense A ring. The CDA time-of-flight mass spectrometer recorded 584 mass spectra from the stream particles. The particles consist predominantly of oxygen, silicon, and iron, with some evidence of water ice, ammonium, and perhaps carbon. The stream particles primarily consist of silicate materials, and this implies that the particles are impurities from the icy ring material rather than the ice particles themselves.

High-velocity streams of dust originating from Saturn.

High-velocity submicrometre-sized dust particles expelled from the jovian system have been identified by dust detectors on board several spacecraft. On the basis of periodicities in the dust impact rate, Jupiter's moon Io was found to be the dominant source of the streams. The grains become positively charged within the plasma environment of Jupiter's magnetosphere, and gain energy from its co-rotational electric field. Outside the magnetosphere, the dynamics of the grains are governed by the interaction with the interplanetary magnetic field that eventually forms the streams. A similar process was suggested for Saturn. Here we report the discovery by the Cassini spacecraft of bursts of high-velocity dust particles (> or = 100 km s(-1)) within approximately 70 million kilometres of Saturn. Most of the particles detected at large distances appear to originate from the outskirts of Saturn's outermost main ring. All bursts of dust impacts detected within 150 Saturn radii are characterized by impact directions markedly different from those measured between the bursts, and they clearly coincide with the spacecraft's traversals through streams of compressed solar wind.