D/H Ratios on Saturn and Jupiter from Cassini CIRS

Pierel, Justin; Nixon, C. A.; Lellouch, E.; Fletcher, Leigh N.; Bjoraker, G. L.; Achterberg, R. K.; Bézard, Bruno; Hesman, B. E.; Irwin, Patrick; Flasar, F. M.

doi:10.3847/1538-3881/aa899d

Cited by 19 publications

(24 citation statements)

References 38 publications

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“…Our D/H measurement is in agreement with the methane D/H of 1.6(±0.2) × 10 −5 derived from the Cassini/CIRS observations of CH 3 D by Fletcher et al (2009b), but not with the estimate of 2.85(±0.2) × 10 −5 that was estimated by converting the D/H derived from measurements of HD by Pierel et al (2017). Our D/H in methane is within the uncertainties of the result of 1.7(±1.1) × 10 −5 determined from high-resolution 5 µm spectroscopy by Noll and Larson (1991), and is at the lower end of the ISO/SWS determination of 2.0 +1.4 −0.7 × 10 −5 by Lellouch et al (2001).…”

Section: Discussionsupporting

confidence: 71%

“…Working in the opposite direction, if we apply this fractionation factor to our (D/H) CH 4 we estimate a hydrogen D/H ratio of 1.23 +0.27 −0.23 ×10 −5 which is significantly below the uncertainty ranges of the estimated proto-solar values 2.1(±0.5) × 10 −5 from Geiss and Gloeckler (1998). Surprisingly, our (D/H) H 2 estimated from our methane analysis is not within the uncertainty range of the CIRS analysis from Pierel et al (2017) which yielded a hydrogen-derived D/H value of 2.10(±0.13)×10 −5 . Indeed, if we compare our D/H in methane with the CIRS-derived D/H in hydrogen, we would need an entirely different fractionation factor of approximately 0.79.…”

Section: Deuterium Fractionationcontrasting

confidence: 62%

“…It is notable that CIRS estimates of Saturn's D/H in methane, inferred from HD lines in the far-infrared by Pierel et al (2017) and measured in CH 3 D by Fletcher et al (2009b), are both in disagreement with each other. Taken at face value, the Lellouch et al (2001), Pierel et al (2017) and this work. The values quoted for the Methane column assume a fractionation factor of 1.34 ± 0.19 (Lellouch et al 2001).…”

Section: Introductionmentioning

confidence: 91%

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Refining Saturn’s deuterium-hydrogen ratio via IRTF/TEXES spectroscopy

Blake

Fletcher²,

Greathouse³

et al. 2021

A&A

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The abundance of deuterium in giant planet atmospheres provides constraints on the reservoirs of ices incorporated into these worlds during their formation and evolution. Motivated by discrepancies in the measured deuterium-hydrogen (D/H) ratio on Jupiter and Saturn, we present a new measurement of the D/H ratio in methane for Saturn from ground-based measurements. We analysed a spectral cube (covering 1151–1160 cm−1 from 6 February 2013) from the Texas Echelon Cross Echelle Spectrograph (TEXES) on NASA’s Infrared Telescope Facility (IRTF) where emission lines from both methane and deuterated methane are well resolved. Our estimate of the D/H ratio in stratospheric methane, 1.65−0.21+0.27 × 10−5 is in agreement with results derived from Cassini CIRS and ISO/SWS observations, confirming the unexpectedly low CH3D abundance. Assuming a fractionation factor of 1.34 ± 0.19 we derive a hydrogen D/H of 1.23−0.23+0.27 × 10−5. This value remains lower than previous tropospheric hydrogen D/H measurements of (i) Saturn 2.10(±0.13) × 10−5, (ii) Jupiter 2.6(±0.7) × 10−5 and (iii) the proto-solar hydrogen D/H of 2.1(±0.5) × 10−5, suggesting that the fractionation factor may not be appropriate for stratospheric methane, or that the D/H ratio in Saturn’s stratosphere is not representative of the bulk of the planet.

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Section: Discussionsupporting

confidence: 71%

Section: Deuterium Fractionationcontrasting

confidence: 62%

Section: Introductionmentioning

confidence: 91%

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Refining Saturn’s deuterium-hydrogen ratio via IRTF/TEXES spectroscopy

Blake

Fletcher²,

Greathouse³

et al. 2021

A&A

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“…D/H in Neptune’s atmosphere is 4.1 ± 0.4 × 10 −5 , derived from Herschel observations of hydrogen [24]. This is around twice as enriched as the protosolar ratio of ( D / H ) proto = 2.25 ± 0.35 × 10 −5 inferred from observations of Jupiter by ISO and Cassini [73,74]. Present-day ice reservoirs, including icy moons, comets and Kuiper belt objects (KBOs), have variable D/H in the range false(D/Hfalse)ices=15−60×10−5 [26,75–77].…”

Section: Atmospheric Observations and Implicationsmentioning

confidence: 91%

Neptune and Uranus: ice or rock giants?

Teanby

Irwin

Moses

et al. 2020

Phil. Trans. R. Soc. A.

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Existing observations of Uranus and Neptune’s fundamental physical properties can be fitted with a wide range of interior models. A key parameter in these models is the bulk rock:ice ratio and models broadly fall into ice-dominated (ice giant) and rock-dominated (rock giant) categories. Here we consider how observations of Neptune’s atmospheric temperature and composition (H 2 , He, D/H, CO, CH 4 , H 2 O and CS) can provide further constraints. The tropospheric CO profile in particular is highly diagnostic of interior ice content, but is also controversial, with deep values ranging from zero to 0.5 parts per million. Most existing CO profiles imply extreme O/H enrichments of >250 times solar composition, thus favouring an ice giant. However, such high O/H enrichment is not consistent with D/H observations for a fully mixed and equilibrated Neptune. CO and D/H measurements can be reconciled if there is incomplete interior mixing (ice giant) or if tropospheric CO has a solely external source and only exists in the upper troposphere (rock giant). An interior with more rock than ice is also more compatible with likely outer solar system ice sources. We primarily consider Neptune, but similar arguments apply to Uranus, which has comparable C/H and D/H enrichment, but no observed tropospheric CO. While both ice- and rock-dominated models are viable, we suggest a rock giant provides a more consistent match to available atmospheric observations. This article is part of a discussion meeting issue ‘Future exploration of ice giant systems’.

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“…Clathrate hydrates require water, and our lack of knowledge of the oxygen abundance allows for a number of interior models with a range of values for the C/O ratio (Lunine 2011). CIRS measurements suggest that Saturn's D/H ratio is less than that for Jupiter, which mostly reminds us that our understanding planetary formation and evolution is incomplete (Pierel et al 2017). On first look, the profiles do not tell a consistent story.…”

Section: Elemental and Isotopic Abundancesmentioning

confidence: 99%

Cassini Exploration of the Planet Saturn: A Comprehensive Review

Ingersoll

2020

Space Sci Rev

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Before Cassini, scientists viewed Saturn’s unique features only from Earth and from three spacecraft flying by. During more than a decade orbiting the gas giant, Cassini studied the planet from its interior to the top of the atmosphere. It observed the changing seasons, provided up-close observations of Saturn’s exotic storms and jet streams, and heard Saturn’s lightning, which cannot be detected from Earth. During the Grand Finale orbits, it dove through the gap between the planet and its rings and gathered valuable data on Saturn’s interior structure and rotation. Key discoveries and events include: watching the eruption of a planet-encircling storm, which is a 20- or 30-year event, detection of gravity perturbations from winds 9000 km below the tops of the clouds, demonstration that eddies are supplying energy to the zonal jets, which are remarkably steady over the 25-year interval since the Voyager encounters, re-discovery of the north polar hexagon after 25 years, determination of elemental abundance ratios He/H, C/H, N/H, P/H, and As/H, which are clues to planet formation and evolution, characterization of the semiannual oscillation of the equatorial stratosphere, documentation of the mysteriously high temperatures of the thermosphere outside the auroral zone, and seeing the strange intermittency of lightning, which typically ceases to exist on the planet between outbursts every 1–2 years. These results and results from the Jupiter flyby are all discussed in this review.

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D/H Ratios on Saturn and Jupiter from Cassini CIRS

Cited by 19 publications

References 38 publications

Refining Saturn’s deuterium-hydrogen ratio via IRTF/TEXES spectroscopy

Refining Saturn’s deuterium-hydrogen ratio via IRTF/TEXES spectroscopy

Neptune and Uranus: ice or rock giants?

Cassini Exploration of the Planet Saturn: A Comprehensive Review

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