Returning humans to the Moon presents an unprecedented opportunity to determine the origin of volatiles stored in the permanently shaded regions (PSRs), which trace the history of lunar volcanic ...activity, solar wind surface chemistry, and volatile delivery to the Earth and Moon through impacts of comets, asteroids, and micrometeoroids. So far, the source of the volatiles sampled by the Lunar Crater Observation and Sensing Satellite (LCROSS) plume has remained undetermined. We show here that the source could not be volcanic outgassing and the composition is best explained by cometary impacts. Ruling out a volcanic source means that volatiles in the top 1-3 meters of the Cabeus PSR regolith may be younger than the latest volcanic outgassing event (~1 billion years ago; Gya).
Context. Measurements of isotopic abundances in cometary ices are key to understanding and reconstructing the history and origin of material in the solar system. Comets are considered the most ...pristine material in the solar system. Isotopic fractionation (enrichment of an isotope in a molecule compared to the initial abundance) is sensitive to environmental conditions at the time of comet formation. Therefore, measurements of cometary isotope ratios can provide information on the composition, density, temperature, and radiation during formation of the molecules, during the chemical evolution from the presolar cloud to the protosolar nebula, and the protoplanetary disk before accretion in solid bodies. Most isotopic abundances of 12C/13C and 16O/18O in comets to date are in agreement with terrestrial abundances. Prior to the Rosetta mission, measurements of 12C/13C in comets were only available for HCN, CN, and C2 and for 16O/18O in H2O. Measurements of 12C/13C in comets were only available from ground based observations and remote sensing, while 16O/18O in H2O had also been measured in-situ. To date, no measurements of the CO2 isotopologues in comets were available. Aims. This paper presents the first measurements of the CO2 isotopologues in the coma of 67P/Churyumov-Gerasimenko (67P). Methods. We analyzed measurements taken by the Double Focusing Mass Spectrometer (DFMS) of the ROSINA experiment on board the ESA spacecraft Rosetta in the coma of 67P. Results. The CO2 isotopologues results for 67P are: 12C/13C = 84 ± 4, 16O/18O = 494 ± 8, and 13C16O2/12C18O16O = 5.87 ± 0.07. The oxygen isotopic ratio is within error bars compatible with terrestrial abundances but not with solar wind measurements. Conclusions. The carbon isotopic ratio and the combined carbon and oxygen isotopic ratio are slightly (14%) enriched in 13C, within 1σ uncertainty, compared to solar wind abundances and solar abundances. The small fractionation of 12C/13C in CO2 is probably compatible with an origin of the material in comets from the native cloud.
Context. Early measurements of Rosetta's target comet, 67P/Churyumov-Gerasimenko (67P), showed a strongly heterogeneous coma in H sub(2)O, CO, and CO sub(2). Aims. The purpose of this work is to ...further investigate the coma heterogeneity of 67P, and to provide predictions for the near-perihelion outgassing profile based on the proposed explanations. Methods. Measurements of various minor volatile species by ROSINA/DFMS on board Rosetta are examined. The analysis focuses on the currently poorly illuminated winter (southern) hemisphere of 67P. Results. Coma heterogeneity is not limited to the major outgassing species. Minor species show better correlation with either H sub(2)O or CO sub(2). The molecule CH sub(4) shows a different diurnal pattern from all other analyzed species. Such features have implications for nucleus heterogeneity and thermal processing. Conclusions. Future analysis of additional volatiles and modeling the heterogeneity are required to better understand the observed coma profile.
Context. Pre-equinox measurements of comet 67P/Churyumov-Gerasimenko with the mass spectrometer ROSINA/DFMS on board the Rosetta spacecraft revealed a strongly heterogeneous coma. The abundances of ...major and various minor volatile species were found to depend on the latitude and longitude of the nadir point of the spacecraft. The observed time variability of coma species remained consistent for about three months up to equinox. The chemical variability could be generally interpreted in terms of surface temperature and seasonal effects superposed on some kind of chemical heterogeneity of the nucleus. Aims. We compare here pre-equinox (inbound) ROSINA/DFMS measurements from 2014 to measurements taken after the outbound equinox in 2016, both at heliocentric distances larger than 3 AU. For a direct comparison we limit our observations to the southern hemisphere. Methods. We report the similarities and differences in the concentrations and time variability of neutral species under similar insolation conditions (heliocentric distance and season) pre- and post-equinox, and interpret them in light of the previously published observations. In addition, we extend both the pre- and post-equinox analysis by comparing species concentrations with a mixture of CO2 and H2O. Results. Our results show significant changes in the abundances of neutral species in the coma from pre- to post-equinox that are indicative of seasonally driven nucleus heterogeneity. Conclusions. The observed pre- and post-equinox patterns can generally be explained by the strong erosion in the southern hemisphere that moves volatile-rich layers near the surface.
The current composition of giant planet atmospheres provides information on how such planets formed, and on the origin of the solid building blocks that contributed to their formation. Noble gas ...abundances and their isotope ratios are among the most valuable pieces of evidence for tracing the origin of the materials from which the giant planets formed. In this review we first outline the current state of knowledge for heavy element abundances in the giant planets and explain what is currently understood about the reservoirs of icy building blocks that could have contributed to the formation of the Ice Giants. We then outline how noble gas isotope ratios have provided details on the original sources of noble gases in various materials throughout the solar system. We follow this with a discussion on how noble gases are trapped in ice and rock that later became the building blocks for the giant planets and how the heavy element abundances could have been locally enriched in the protosolar nebula. We then provide a review of the current state of knowledge of noble gas abundances and isotope ratios in various solar system reservoirs, and discuss measurements needed to understand the origin of the ice giants. Finally, we outline how formation and interior evolution will influence the noble gas abundances and isotope ratios observed in the ice giants today. Measurements that a future atmospheric probe will need to make include (1) the
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He/
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He isotope ratio to help constrain the protosolar D/H and
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He/
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He; (2) the
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Ne and
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Ne to separate primordial noble gas reservoirs similar to the approach used in studying meteorites; (3) the Kr/Ar and Xe/Ar to determine if the building blocks were Jupiter-like or similar to 67P/C-G and Chondrites; (4) the krypton isotope ratios for the first giant planet observations of these isotopes; and (5) the xenon isotopes for comparison with the wide range of values represented by solar system reservoirs.
Characterizing Europa’s subsurface ocean is essential for assessing Europa’s habitability. The suite of instruments on the Europa Clipper spacecraft will, among others, magnetically sound Europa’s ...interior by measuring the ocean’s induced magnetic field. This magnetic field is generated in response to the Jovian time-varying magnetic environment in which Europa is immersed. However, the dynamic magnetized plasma flow of the Jovian magnetosphere creates electrical currents that give rise to magnetic perturbations near Europa. These perturbations complicate the interpretation of the induction signal, and hence the characterization and inferences on potential habitability. Thus, characterization of the ocean by magnetic sounding requires an accurate characterization of the plasma as it flows across Europa.
We present the Plasma Instrument for Magnetic Sounding (PIMS), the instrument for the Europa Clipper mission that will measure the plasma contribution to the magnetic field perturbations sensed by the Europa Clipper Magnetometer. PIMS is composed of four Faraday Cup plasma spectrometers that use voltage-biased gridded apertures to dissect the space plasmas that they encounter. The instrument uses sensitive preamplifiers and processing electronics to measure the current that results when charged particles strike the instrument’s metal collector plates, thus enabling a measure of the plasma characteristics near Europa to produce a more accurate magnetic sounding of Europa’s subsurface ocean. PIMS consists of two sensors: one placed near the top of the Europa Clipper spacecraft and one near the bottom. Each sensor contains two Faraday Cups with a 90° full-width field-of-view. The sensors were specifically designed to withstand the Europa environment, measure both ions and electrons, and have two separate voltage ranges intended to analyze the magnetospheric and ionospheric environments, respectively. In this paper, we describe the scientific motivation for this experiment, the design considerations for the PIMS instrument, the details of the ground calibration, and other details pertinent to understanding the scientific data retrieved by PIMS.
The origin of cometary volatiles remains a major open question in planetary science. Comets may have either agglomerated from crystalline ices condensed in the protosolar nebula (PSN) or from ...amorphous ice originating from the molecular cloud and interstellar medium. Here, based on the recent argon, krypton, and xenon measurements performed by the ROSINA mass spectrometer on board the European Space Agency's Rosetta spacecraft in the coma of 67P/Churyumov-Gerasimenko, we show that these noble gas relative abundances can be explained if the comet's building blocks formed from a mixture of gas and H2O grains resulting from the annealing of pristine amorphous ice (i.e., originating from the presolar cloud) in the PSN. In this scenario, the different volatiles released during the amorphous-to-crystalline ice phase transition would have been subsequently trapped at lower temperatures in stoichiometric hydrate or clathrate hydrate forms by the crystalline water ice generated by the transition. Once crystalline water was completely consumed by clathration in the ∼25-80 K temperature range, the volatile species remaining in the gas phase would have formed pure condensates at lower temperatures. The formation of clathrates hydrates and pure condensates to explain the noble gas relative abundances is consistent with a proposed interstellar origin of molecular oxygen detected in 67P/Churyumov-Gerasimenko, and with the measured molecular nitrogen depletion in comets.
Titan's polar lakes are thought to be predominantly composed of liquid ethane and methane; however, little is known on the ratio of these hydrocarbons in the lakes, and the stability and dynamics of ...these mixtures. Here we provide the first experimental constraints under Titan surface conditions of liquid hydrocarbon mixture evaporation. Our results are relevant to Titan's polar temperatures and pressures (∼92 K and 1.5 bar), and cover a wide range of methane–ethane compositions. We show that evaporation is negligible for pure ethane, but increases nearly linearly with increasing methane concentration. Early dissolution of N2 results in ternary mixtures evaporating, which is modeled by a ‘hybrid’ thermodynamic equilibrium approach combining Perturbed-Chain Statistical Associating Fluid Theory with a diffusion and buoyancy-driven mass flux model. The approach follows the experimental evaporation rate measurements presented in this study, and allows for the calculation of the corresponding liquid methane–ethane–nitrogen ratios. Such results along with Cassini inferred lake evaporation rates can be used to estimate the composition of Titan's polar liquids, and may have implications on their origin. Our results suggest that Ontario Lacus is predominantly composed of ethane (>50–80 mol%), indicating it may be a residual lake following extensive seasonal methane evaporation, and/or might be in contact with a subsurface liquid reservoir.
•Evaporation rates of C2H6–CH4 mixtures were experimentally measured.•Experiments performed at simulated Titan surface conditions•Evaporation is negligible for C2H6 and increases linearly with CH4 concentration.•Constraints on the composition of Ontario Lacus are provided.•Our results along with observations can be used to estimate polar lake composition.
Many moons in the solar system are thought to potentially harbor hidden oceans based on the features observed at their surfaces. However, the magnetic induction signatures measured in the vicinity of ...these moons provide the most compelling evidence for the presence of a subsurface ocean, specifically for the Jovian moons Europa and Callisto. Interpretation of these magnetic signatures can be challenging due to the various systematic and random sources of noise that are present in the magnetic field measurement. In this work, a novel magnetometric ocean detection methodology based on Principal Component Analysis is presented and shown to provide enhanced discrimination and geophysical characterization of ocean properties in the presence of noise and error sources. The proposed methodology is robust for a single‐encounter mission or an orbiting mission with multiple flybys. Here, it is applied to the Neptunian moon Triton as a prime example of an active, potential ocean world residing in the requisite time‐varying magnetic field environment that enables magnetic induction investigation of its interior. In addition to the usual noise sources, other confounding factors are addressed, including the presence of an intense conductive ionosphere, the small amplitude of Neptune's driving magnetic field, and the uncertainty of Neptune's magnetic phase at the time‐of‐arrival which can potentially hinder accurate ocean detection and characterization. The proposed methodology is applicable to any moon in the solar system residing in a time‐varying magnetic field environment.
Plain Language Summary
The search for habitable oceans in the solar system motivates the need for advances in analytic techniques to positively determine the presence of subsurface oceans in challenging environments. The Principal Component Analysis (PCA) method described in this article is a new paradigm for processing space‐based magnetic field measurements for definitive detection and constrained characterization of subsurface oceans. Using Neptune's largest moon Triton as an example ocean world, PCA is directly applied to a three‐axis magnetic field data set and shown to be a powerful ocean classification tool for a single or multiple flybys, even in the presence of Triton's highly conducting ionosphere which can mask the magnetic response from the ocean. The method is able to reliably distinguish between the magnetic field signatures associated with the ocean‐plus‐ionosphere and ionosphere‐only model classes and can further determine key characteristics of the hidden ocean in the face of the confounding factors of a conductive ionosphere, local plasma current perturbations, spacecraft timing and position uncertainties, data outages, and various sources of instrument noise. The flexibility and extensibility afforded by the PCA‐based method enhance the existing and future capabilities for ocean detection and characterization at candidate ocean worlds throughout the solar system.
Key Points
A novel sub‐surface ocean detection and characterization method has been developed based on Principal Component Analysis processing of magnetic induction data
Enables differentiation between ocean‐plus‐ionosphere and ionosphere‐only induction responses in the presence of various noise sources
Applied here to the compelling target of Triton, thought to possibly harbor a sub‐surface ocean beneath a highly conducting ionosphere
ABSTRACT Titan's thermospheric photochemistry is primarily driven by solar radiation. Similarly to other planetary atmospheres, such as Mars', Titan's atmospheric structure is also directly affected ...by variations in the solar extreme-UV/UV output in response to the 11-year-long solar cycle. Here, we investigate the influence of nitrogen on the vertical production, loss, and abundance profiles of hydrocarbons as a function of the solar cycle. Our results show that changes in the atmospheric nitrogen atomic density (primarily in its ground state N(4S)) as a result of photon flux variations have important implications for the production of several minor hydrocarbons. The solar minimum enhancement of CH3, C2H6, and C3H8, despite the lower CH4 photodissociation rates compared with solar maximum conditions, is explained by the role of N(4S). N(4S) indirectly controls the altitude of termolecular versus bimolecular chemical regimes through its relationship with CH3. When in higher abundance during solar maximum at lower altitudes, N(4S) increases the importance of bimolecular CH3 + N(4S) reactions producing HCN and H2CN. The subsequent remarkable CH3 loss and decrease in the CH3 abundance at lower altitudes during solar maximum affects the overall hydrocarbon chemistry.
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