“…In contrast, the same measurement technique applied to Saturn's captured moon Phoebe yields a 13 C/ 12 C ratio enhanced by a factor of ∼5 relative to terrestrial values (Clark et al 2019). This enhancement might arise because Phoebe, which likely formed in the primordial Kuiper Belt and was captured by Saturn (e.g., Johnson & Lunine 2005), accreted material from a region of the protoplanetary disk where 12 C-bearing gas was shielded from UV photon processing, allowing preferential accretion of 13 C-rich solids formed from photolysis of 13 C-bearing gas (Neveu et al 2020).…”
Section: A Localized and Patchy Co 2 Atmospherementioning
confidence: 99%
“…In such regions, the disk CO (its main carbon source) would have been dense enough for 12 CO to shield itself from photolytic UV radiation from the early Sun, thereby staying in the gas phase, whereas more tenuous 13 CO was photolyzed to products that eventually resulted in 13 C-rich material (Woods & Willacy 2009) that condensed into solids. Self-shielding of CO has similarly been invoked to explain unusual 1:1 17 O/ 16 O and 18 O/ 16 O correlations in meteorite mineral phases (Lyons & Young 2005), the Sun's light C isotope composition relative to the Earth (Lyons et al 2018), and Phoebe's large enrichment in 13 C (Neveu et al 2020). In each case, it has been assumed that the isotopically heavy material condensed into solids (ices or dust) that accreted onto, or coated, local planetesimals.…”
We analyzed spectral cubes of Callisto’s leading and trailing hemispheres, collected with the NIRSpec Integrated Field Unit (G395H) on the James Webb Space Telescope. These spatially resolved data show strong 4.25 μm absorption bands resulting from solid-state 12CO2, with the strongest spectral features at low latitudes near the center of its trailing hemisphere, consistent with radiolytic production spurred by magnetospheric plasma interacting with native H2O mixed with carbonaceous compounds. We detected CO2 rovibrational emission lines between 4.2 and 4.3 μm over both hemispheres, confirming the global presence of CO2 gas in Callisto’s tenuous atmosphere. These results represent the first detection of CO2 gas over Callisto’s trailing side. The distribution of CO2 gas is offset from the subsolar region on either hemisphere, suggesting that sputtering, radiolysis, and geologic processes help sustain Callisto’s atmosphere. We detected a 4.38 μm absorption band that likely results from solid-state 13CO2. A prominent 4.57 μm absorption band that might result from CN-bearing organics is present and significantly stronger on Callisto’s leading hemisphere, unlike 12CO2, suggesting these two spectral features are spatially antiassociated. The distribution of the 4.57 μm band is more consistent with a native origin and/or accumulation of dust from Jupiter’s irregular satellites. Other, more subtle absorption features could result from CH-bearing organics, CO, carbonyl sulfide, and Na-bearing minerals. These results highlight the need for preparatory laboratory work and improved surface–atmosphere interaction models to better understand carbon chemistry on the icy Galilean moons before the arrival of NASA’s Europa Clipper and ESA’s JUICE spacecraft.
“…In contrast, the same measurement technique applied to Saturn's captured moon Phoebe yields a 13 C/ 12 C ratio enhanced by a factor of ∼5 relative to terrestrial values (Clark et al 2019). This enhancement might arise because Phoebe, which likely formed in the primordial Kuiper Belt and was captured by Saturn (e.g., Johnson & Lunine 2005), accreted material from a region of the protoplanetary disk where 12 C-bearing gas was shielded from UV photon processing, allowing preferential accretion of 13 C-rich solids formed from photolysis of 13 C-bearing gas (Neveu et al 2020).…”
Section: A Localized and Patchy Co 2 Atmospherementioning
confidence: 99%
“…In such regions, the disk CO (its main carbon source) would have been dense enough for 12 CO to shield itself from photolytic UV radiation from the early Sun, thereby staying in the gas phase, whereas more tenuous 13 CO was photolyzed to products that eventually resulted in 13 C-rich material (Woods & Willacy 2009) that condensed into solids. Self-shielding of CO has similarly been invoked to explain unusual 1:1 17 O/ 16 O and 18 O/ 16 O correlations in meteorite mineral phases (Lyons & Young 2005), the Sun's light C isotope composition relative to the Earth (Lyons et al 2018), and Phoebe's large enrichment in 13 C (Neveu et al 2020). In each case, it has been assumed that the isotopically heavy material condensed into solids (ices or dust) that accreted onto, or coated, local planetesimals.…”
We analyzed spectral cubes of Callisto’s leading and trailing hemispheres, collected with the NIRSpec Integrated Field Unit (G395H) on the James Webb Space Telescope. These spatially resolved data show strong 4.25 μm absorption bands resulting from solid-state 12CO2, with the strongest spectral features at low latitudes near the center of its trailing hemisphere, consistent with radiolytic production spurred by magnetospheric plasma interacting with native H2O mixed with carbonaceous compounds. We detected CO2 rovibrational emission lines between 4.2 and 4.3 μm over both hemispheres, confirming the global presence of CO2 gas in Callisto’s tenuous atmosphere. These results represent the first detection of CO2 gas over Callisto’s trailing side. The distribution of CO2 gas is offset from the subsolar region on either hemisphere, suggesting that sputtering, radiolysis, and geologic processes help sustain Callisto’s atmosphere. We detected a 4.38 μm absorption band that likely results from solid-state 13CO2. A prominent 4.57 μm absorption band that might result from CN-bearing organics is present and significantly stronger on Callisto’s leading hemisphere, unlike 12CO2, suggesting these two spectral features are spatially antiassociated. The distribution of the 4.57 μm band is more consistent with a native origin and/or accumulation of dust from Jupiter’s irregular satellites. Other, more subtle absorption features could result from CH-bearing organics, CO, carbonyl sulfide, and Na-bearing minerals. These results highlight the need for preparatory laboratory work and improved surface–atmosphere interaction models to better understand carbon chemistry on the icy Galilean moons before the arrival of NASA’s Europa Clipper and ESA’s JUICE spacecraft.
“…Neveu et al. ( 2020 ) interpreted the high 13 C enrichment in CO 2 ice from Phoebe as a result of self-shielding of CO from photodissociation in the protosolar nebula. Fig.…”
Comets are considered the most primitive planetary bodies in our Solar System. ESA’s Rosetta mission to Jupiter family comet 67P/Churyumov-Gerasimenko (67P/CG) has provided a wealth of isotope data which expanded the existing data sets on isotopic compositions of comets considerably. In a previous paper (Hoppe et al. in Space Sci. Rev. 214:106, 2018) we reviewed the results for comet 67P/CG from the first four years of data reduction after arrival of Rosetta at the comet in August 2014 and discussed them in the context of respective meteorite data. Since then important new isotope data of several elements, among them the biogenic elements H, C, N, and O, for comet 67P/CG, the Tagish Lake meteorite, and C-type asteroid Ryugu became available which provide new insights into the formation conditions of small planetary bodies in the Solar System’s earliest history. To complement the picture on comet 67P/CG and its context to other primitive Solar System materials, especially meteorites, that emerged from our previous paper, we review here the isotopic compositions of H, C, and N in various volatile molecules, of O in water and a suite of other molecules, of the halogens Cl and Br, and of the noble gas Kr in comet 67P/CG. Furthermore, we also review the H isotope data obtained in the refractory organics of the dust grains collected in the coma of 67P/CG. These data are compared with the respective meteoritic and Ryugu data and spectroscopic observations of other comets and extra-solar environments; Cl, Br, and Kr data are also evaluated in the context of a potential late supernova contribution, as suggested by the Si- and S-isotopic data of 67P/CG.
“…ab On Earth and in many solar system materials, 13 C/ 12 C ≈ 1/90, 15 N/ 14 N ≈ 1/270, and 34 S/ 32 S ≈ 1/20 so expected amounts of the compounds with the rarer isotope are about as many times lower than their bulk abundances. For a given instrument limit of detection, O and H isotopic measurements require more sample since 18 O/ 16 O ≈ 1/500, 17 O/ 16 O ≈ 1/2500, and D/H ≥ 1/5000 (see, e.g., data compilation in Neveu et al, 2020). ac Selliez et al (2019).…”
Evidence suggests that Saturn's icy moon Enceladus has a subsurface ocean that sources plumes of water vapor and ice vented to space from its south pole. In situ analyses of this material by the Cassini spacecraft have shown that the ocean contains key ingredients for life (elements H, C, N, O and possibly S; simple and complex organic compounds; chemical disequilibria at water-rock interfaces; clement temperature, pressure, and pH). The Cassini discoveries make Enceladus' interior a prime locale for life detection beyond Earth. Scant material exchange with the inner Solar System makes it likely that such life would have emerged independently of life on Earth. Thus, its discovery would illuminate life's universal characteristics. The alternative result of an upper bound on a detectable biosphere in an otherwise habitable environment would likewise considerably advance our understanding of the prevalence of life beyond Earth. Here we outline the rationale for returning vented ocean samples, accessible from Enceladus' surface or low altitudes, to Earth for life detection. Returning samples allows analyses using laboratory instruments that cannot be flown, with decades or more to adapt and repeat analyses. We describe an example set of measurements to estimate the amount of sample to be returned and discuss possible mission architectures and collection approaches. We then turn to the challenges of preserving sample integrity and implementing planetary protection policy. We conclude by placing such a mission in the broader context of Solar System exploration.
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