36 research outputs found
Probing the Oxidation State of Ocean Worlds with SUDA: Fe (ii) and Fe (iii) in Ice Grains
Characterizing the geochemistry of Europa and Enceladus is a key step for astrobiology investigations looking for evidence of life in their subsurface oceans. Transition metals with several oxidation states, such as iron, may be tracers of the oxidation state of icy ocean moon interiors. Their detection, as well as the characterization of their oxidation states, on the moons' (plume) ice grains would bring valuable new information about the geochemistry of both the subsurface oceans and surface processes. Impact ionization mass spectrometers such as the SUDA instrument on board Europa Clipper can analyze ice grains ejected from icy moons' surfaces and detect ocean-derived salts therein. Here we record mass spectra analogs for SUDA using the Laser Induced Liquid Beam Ion Desorption technique for Fe2+ and Fe3+ salts (both sulfates and chlorides). We show that impact ionization mass spectrometers have the capability to detect and differentiate ferrous (Fe2+) from ferric (Fe3+) ions in both cation and anion modes owing to their tendency to form distinct ionic complexes with characteristic spectral features. Peaks bearing Fe3+, such as [Fe3+ (OH)2]+ and [Fe3+ (OH)a Clb]â, are particularly important to discriminate between the two oxidation states of iron in the sample. The recorded analog spectra may allow the characterization of the oxidation state of the oceans of Europa and Enceladus with implications for hydrothermal processes and potential metabolic pathways for life forms in their subsurface oceans
Abundant phosphorus expected for possible life in Enceladusâs ocean
Saturnâs moon Enceladus has a potentially habitable subsurface water ocean that contains canonical building blocks of life (organic and inorganic carbon, ammonia, possibly hydrogen sulfide) and chemical energy (disequilibria for methanogenesis). However, its habitability could be strongly affected by the unknown availability of phosphorus (P). Here, we perform thermodynamic and kinetic modeling that simulates P geochemistry based on recent insights into the geochemistry of the oceanâseafloor system on Enceladus. We find that aqueous P should predominantly exist as orthophosphate (e.g., HPO42â), and total dissolved inorganic P could reach 10â7 to 10â2 mol/kg H2O, generally increasing with lower pH and higher dissolved CO2, but also depending upon dissolved ammonia and silica. Levels are much higher than <10â10 mol/kg H2O from previous estimates and close to or higher than âŒ10â6 mol/kg H2O in modern Earth seawater. The high P concentration is primarily ascribed to a high (bi)carbonate concentration, which decreases the concentrations of multivalent cations via carbonate mineral formation, allowing phosphate to accumulate. Kinetic modeling of phosphate mineral dissolution suggests that geologically rapid release of P from seafloor weathering of a chondritic rocky core could supply millimoles of total dissolved P per kilogram of H2O within 105 y, much less than the likely age of Enceladusâs ocean (108 to 109 y). These results provide further evidence of habitable ocean conditions and show that any oceanic life would not be inhibited by low P availability
Detection of phosphates originating from Enceladusâs ocean
Saturnâs 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 moonâs cryovolcanic plume4,5,6,7,8,9. The analysis of salt-rich ice grains by Cassiniâs Cosmic Dust Analyzer10 enabled inference of major solutes in the ocean water (Na+, K+, Clâ, HCO3â, CO32â) 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 Enceladusâs ocean indicates that phosphate could be relatively abundant17. Here we present Cassiniâs 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 Enceladusâs ocean in the form of orthophosphates, with phosphorus concentrations at least 100-fold higher in the moonâs plume-forming ocean waters than in Earthâs 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
Moderate D/H Ratios in Methane Ice on Eris and Makemake as Evidence of Hydrothermal or Metamorphic Processes in Their Interiors: Geochemical Analysis
Dwarf planets Eris and Makemake have surfaces bearing methane ice of unknown
origin. D/H ratios were recently determined from James Webb Space Telescope
(JWST) observations of Eris and Makemake (Grundy et al., submitted), giving us
new clues to decipher the origin of methane. Here, we develop geochemical
models to test if the origin of methane could be primordial, derived from
CO or CO ("abiotic"), or sourced by organics ("thermogenic"). We find that
primordial methane is inconsistent with the observational data, whereas both
abiotic and thermogenic methane can have D/H ratios that overlap the observed
ranges. This suggests that Eris and Makemake either never acquired a
significant amount of methane during their formation, or their original
inventories were removed and then replaced by a source of internally produced
methane. Because producing abiotic or thermogenic methane likely requires
temperatures in excess of ~150{\deg}C, we infer that Eris and Makemake have
rocky cores that underwent substantial radiogenic heating. Their cores may
still be warm/hot enough to produce methane. This heating could have driven
hydrothermal circulation at the bottom of an ice-covered ocean to generate
abiotic methane, and/or metamorphic reactions involving accreted organic matter
could have occurred in response to heating in the deeper interior, generating
thermogenic methane. Additional analyses of thermal evolution model results and
predictions from modeling of D-H exchange in the solar nebula support our
findings of elevated subsurface temperatures and a lack of primordial methane
on Eris and Makemake. It remains an open question whether their D/H ratios may
have evolved subsequent to methane outgassing. Recommendations are given for
future activities to further test proposed scenarios of abiotic and thermogenic
methane production on Eris and Makemake, and to explore these worlds up close.Comment: Submitted to Icarus, 29 pages, 5 figures, 1 tabl
The Science Case for a Return to Enceladus
The plume of Enceladus is unique in the solar system in providing direct access to fresh material from an
extraterrestrial subsurface ocean. The Cassini Mission, though not specifically designed for it, was able to take
advantage of the plume to conduct the best characterization to date of an extraterrestrial ocean. Evidence gathered
from multiple instruments points to a global, subsurface liquid water ocean rich in salts and organic compounds,
with water-rock interactions occurring presumably in hydrothermal systems at or below the moonâs sea floor.
Meeting the criteria of âextended regions of liquid water, conditions favorable for the assembly of complex organic
molecules, and energy source(s) to sustain metabolism,â the ocean of Enceladus can therefore be considered
habitable. It is also the only confirmed place beyond the Earth where we can easily sample fresh material from a
demonstrably habitable environment without the complications of digging or drilling. The next step is to
investigate whether Enceladusâ ocean is actually inhabited. Here, we summarize the evidence for Enceladusâ ocean
and its habitability, identify constraints and outstanding questions on the detectability of life within its ocean, and
recommend a return to Enceladus with a dedicated search-for-life mission (or missions)
Investigating Europaâs habitability with the Europa Clipper
The habitability of Europa is a property within a system, which is driven by a multitude of physical and chemical processes and is defined by many interdependent parameters, so that its full characterization requires collaborative investigation. To explore Europa as an integrated system to yield a complete picture of its habitability, the Europa Clipper mission has three primary science objectives: (1) characterize the ice shell and ocean including their heterogeneity, properties, and the nature of surfaceâiceâocean exchange; (2) characterize Europaâs composition including any non-ice materials on the surface and in the atmosphere, and any carbon-containing compounds; and (3) characterize Europaâs geology including surface features and localities of high science interest. The mission will also address several cross-cutting science topics including the search for any current or recent activity in the form of thermal anomalies and plumes, performing geodetic and radiation measurements, and assessing high-resolution, co-located observations at select sites to provide reconnaissance for a potential future landed mission. Synthesizing the missionâs science measurements, as well as incorporating remote observations by Earth-based observatories, the James Webb Space Telescope, and other space-based resources, to constrain Europaâs habitability, is a complex task and is guided by the missionâs Habitability Assessment Board (HAB)
The Geochemical Potential for Metabolic Processes on the Sub-Neptune Exoplanet K2-18b
Quantifying disequilibria is important to understand whether an environment
could be habitable. It has been proposed that the exoplanet K2-18b has a
hydrogen-rich atmosphere and a water ocean, making it a "hycean world". The
James Webb Space Telescope recently made measurements of methane, CO, and
possibly dimethyl sulfide (DMS) in the atmosphere of this planet. The initial
interpretation of these data is that they may support the occurrence of hycean
conditions. Here, I attempt to take a next step in exploring the prospects for
habitability. I use constraints on the abundances of atmospheric gases to
calculate how much chemical disequilibrium there could be, assuming K2-18b is a
hycean world. I find that the presence of oxidized carbon species coexisting
with abundant H (1-1000 bar) at cool to warm (25-120{\deg}C) conditions
creates a strong thermodynamic drive for methanogenesis. More than ~75 kJ (mol
C) of free energy can be released from CO hydrogenation. Partially
oxidized carbon compounds such as DMS (if present) also have potential to
provide metabolic energy, albeit in smaller quantities. Because of the
thermodynamic instability of CO under hycean conditions, other reductive
reactions of CO are likely to be favored, including the synthesis of amino
acids. Glycine and alanine synthesis can be energy-releasing or at least much
less costly on K2-18b than in Earth's ocean, even when NH is scarce but not
totally absent. These first bioenergetic calculations for a proposed
ocean-bearing exoplanet lay new groundwork for assessing exoplanetary
habitability.Comment: To be published in The Astrophysical Journal Letter