The photochemistry of Pluto's atmosphere as illuminated by New Horizons

Wong, Michael L.; Fan, Siteng; Gao, Peter; Liang, Mao‐Chang; Shia, Run‐Lie; Yung, Yuk L.; Kammer, Joshua A.; Summers, M. E.; Gladstone, G. R.; Young, L. A.; Ennico, Kimberly; Weaver, Harold A.; Stern, S. A.

doi:10.1016/j.icarus.2016.09.028

Cited by 82 publications

(101 citation statements)

References 13 publications

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“…The surface temperature is high enough to allow thermal desorption of CH 4 , explaining its presence in the gas-phase, while other species require the presence of alternative pathways. Our experiments show that photoninduced processes, as discussed in Wong et al (2017), can contribute to the processes enriching the gas-phase abundance of C 2 H 6 and C 2 H 2 species, while C 2 H 4 was not detected in the gas-phase. These results qualitatively agree with Pluto observations reported by Young et al (2018), which measured C 2 H 6 to be the most abundant hydrocarbon (apart from CH 4 ), followed by C 2 H 2 and C 2 H 4 .…”

Section: Astrophysical Implicationsmentioning

confidence: 63%

Photon-induced desorption of larger species in UV-irradiated methane ice

Carrascosa

Cruz-Díaz

Muñoz

et al. 2020

Monthly Notices of the Royal Astronomical Society

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At the low temperatures found in the interior of dense clouds and circumstellar regions, along with H 2 O and smaller amounts of species such as CO, CO 2 , or CH 3 OH, the infrared features of CH 4 have been observed on icy dust grains. Ultraviolet (UV) photons induce different processes in ice mantles, affecting the molecular abundances detected in the gas-phase. This work aims to understand the processes that occur in a pure CH 4 ice mantle submitted to UV irradiation. We studied photon-induced processes for the different photoproducts arising in the ice upon UV irradiation. Experiments were carried out in ISAC, an ultra-high vacuum chamber equipped with a cryostat and an F-type UV-lamp reproducing the secondary UV-field induced by cosmic rays in dense clouds. Infrared spectroscopy and quadrupole mass spectrometry were used to monitor the solid and gas-phase, respectively, during the formation, irradiation, and warm-up of the ice. Direct photodesorption of pure CH 4 was not observed. UV photons form CH x · and H· radicals, leading to photoproducts such as H 2 , C 2 H 2 , C 2 H 6 , and C 3 H 8 . Evidence for the photodesorption of C 2 H 2 and photochemidesorption of C 2 H 6 and C 3 H 8 was found, the latter species is so far the largest molecule found to photochemidesorb. 13 CH 4 experiments were also carried out to confirm the reliability of these results. Boogert et al. 1996;Öberg et al. 2008). Methane ice can be formed from successive hydrogenation of carbon atoms over a dust grain surface, from photoprocessing of CH 3 OH ice, or even from gas-phase reactions and subsequent freeze out over dust grains (Öberg et al. 2008, and references therein). CH 4 constitutes a source of carbon atoms in ice mantles, with abundances around 5% and 2% of the water ice in low-mass and high-mass protostars, respectively (Dartois 2005;Öberg et al. 2011;Boogert et al. 2015). Complex organic molecules (COMs), containing six or more atoms and at least one carbon, can be formed from methane processing in the interstellar and circumstellar medium, or comets. These systems contain variable quantities, up to 4% relative to water, of solid methane, which is exposed to vacuum ultraviolet (UV) photons with a spectral energy distribution as simulated in our experiments (Gerakines

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Section: Astrophysical Implicationsmentioning

confidence: 63%

Photon-induced desorption of larger species in UV-irradiated methane ice

Carrascosa

Cruz-Díaz

Muñoz

et al. 2020

Monthly Notices of the Royal Astronomical Society

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“…The chemical models of Wong et al (2017) also showed clear minima and maxima in density for C 2 H 4 and C 2 H 2 . In Wong et al (2017), the altitude of the C 2 H 4 maximum is above that of the C 2 H 2 maximum, as seen in the Alice solar occultation, although the observed altitude of both maxima is above what is modeled. The modeled minima for both C 2 H 4 and C 2 H 2 are near 200 km, with the dominant loss mechanism being condensation.…”

Section: Middle Atmosphere (~30 To 400 Km Altitude)mentioning

confidence: 92%

“…Other species were predicted to be present, from Pluto photochemical models (e.g., Summers et al 1997;Krasnopolsky & Cruikshank 1999;Wong et al 2017), by analogy with Titan and Triton photochemisty (e.g., Yung et al 1984;Strobel et al 1990), by cosmochemical arguments (e.g., 29 N 2 , Ar), from the influx of H 2 O at the top of Pluto's atmosphere from incoming Kuiper Belt material, which is on the order of 50 kg day -1 , or 2 x 10 22 molecules s -1 if this all H 2 O (Horanyi et al 2016), and minor species that might accompany the H 2 O (Despois et al 2005). We discussed here that no definitive signal of HCN or CO absorption was seen.…”

Section: Future Workmentioning

confidence: 99%

Structure and composition of Pluto's atmosphere from the New Horizons solar ultraviolet occultation

Young

Kammer

Steffl

et al. 2018

Icarus

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108

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The Alice instrument on NASA's New Horizons spacecraft observed an ultraviolet solar occultation by Pluto's atmosphere on 2015 July 14. The transmission vs. altitude was sensitive to the presence of N 2 , CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , and haze. We derived line-of-sight abundances and local number densities for the 5 molecular species, and line-of-sight optical depth and extinction coefficients for the haze. We found the following major conclusions: (1) We confirmed temperatures in Pluto's upper atmosphere that were colder than expected before the New Horizons flyby, with upper atmospheric temperatures near 65-68 K. The inferred enhanced Jeans escape rates were (3-7) x 10 22 N 2 s -1 and (4-8) x 10 25 CH 4 s -1 at the exobase (at a radius of ~ 2900 km, or an altitude of ~1710 km). (2) We measured CH 4 abundances from 80 to 1200 km above the surface. A joint analysis of the Alice CH 4 and Alice and REX N 2 measurements implied a very stable lower atmosphere with a small eddy diffusion coefficient, most likely between 550 and 4000 cm 2 s -1 . Such a small eddy diffusion coefficient placed the homopause within 12 km of the surface, giving Pluto a small planetary boundary layer. The inferred CH 4 surface mixing ratio was ~ 0.28-0.35%. (3) The abundance profiles of the "C 2 H x hydrocarbons" (C 2 H 2 , C 2 H 4 , C 2 H 6 ) were not simply exponential with altitude. We detected local maxima in line-of-sight abundance near 410 km altitude for C 2 H 4 , near 320 km for C 2 H 2 , and an inflection point or the suggestion of a local maximum at 260 km for C 2 H 6 . We also detected local minima near 200 km altitude for C 2 H 4 , near 170 km for C 2 H 2 , and an inflection point or minimum near 170-200 km for C 2 H 6 . These compared favorably with models for hydrocarbon production near 300-400 km and haze condensation near 200 km, especially for C 2 H 2 and C 2 H 4 (Wong et al. 2017). (4) We found haze that had an extinction coefficient approximately proportional to N 2 density. This paper extends the analysis of Gladstone et al. (2016) in the following ways: (i) it uses an improved reduction of the raw observations, and includes more details about the observation and reduction process, (ii) it presents error analysis, including correlations between the measurements of various species, (iii) it includes analysis of extinction by haze at the long-wavelength end of the Alice range, (iv) it improves or extends the density retrievals of N 2 , CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 and haze, and (v) it includes a joint analysis with new results from the New Horizons radio occultation (Hinson et al. 2017). Observations and ReductionWe recap here the salient features of the Alice ultraviolet spectrograph on the New Horizons spacecraft and its observation of Pluto's atmosphere during the solar occultation. Alice (which is a name, not an acronym) is described in more detail in Stern et al. (2008), FIGURE 1. The Alice Solar Occultation Channel (SOCC), Pluto, and the Sun at the time of solar ingress at 2015 Jul 14 12:44 UT (left) an...

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“…Although only a crude approximation, 1D models have succeeded in explaining the observed global-averaged vertical profiles of many chemical species and particles on planets (e.g., Titan by Yung et al 1984 andZhang et al 2010a, Jupiter by , Venus by 10 0 10 2 10 4 10 6 10 8 10 10 Eddy Diffusivity (m 2 s -1 ) -Vertical profiles of eddy diffusivity in typical 1D chemical models of planets in and out of the solar system. Sources: Zhang et al (2012) for Venus, Allen et al (1981) for Earth, Nair et al (1994) for Mars, Li et al (2015) for Titan, Wong et al (2017) for Pluto, for Jupiter, Saturn, Uranus and Neptune, Moses et al (2013) for GJ436b and Moses et al (2011) for HD189733b and HD209458b. For HD209458b, we show eddy diffusivity profiles assumed in a gas chemistry model (dashed, Moses et al 2011) and that derived from a 3D particulate tracer transport model (solid, Parmentier et al 2013).…”

Section: Introductionmentioning

confidence: 99%

Global-mean Vertical Tracer Mixing in Planetary Atmospheres. I. Theory and Fast-rotating Planets

Zhang

Showman

2018

ApJ

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Most chemistry and cloud formation models for planetary atmospheres adopt a one-dimensional (1D) diffusion approach to approximate the global-mean vertical tracer transport. The physical underpinning of the key parameter in this framework, eddy diffusivity K zz , is usually obscure. Here we analytically and numerically investigate vertical tracer transport in a 3D stratified atmosphere and predict K zz as a function of the large-scale circulation strength, horizontal mixing due to eddies and waves and local tracer sources and sinks. We find that K zz increases with tracer chemical lifetime and circulation strength but decreases with horizontal eddy mixing efficiency. We demarcated three K zz regimes in planetary atmospheres. In the first regime where the tracer lifetime is short compared with the transport timescale and horizontal tracer distribution under chemical equilibrium (χ 0 ) is uniformly distributed across the globe, global-mean vertical tracer mixing behaves diffusively. But the traditional assumption in current 1D models that all chemical species are transported via the same eddy diffusivity generally breaks down. We show that different chemical species in a single atmosphere should in principle have different eddy diffusion profiles. In the second regime where tracer is short-lived but χ 0 is non-uniformly distributed, a significant non-diffusive component might lead to a negative K zz under the diffusive assumption. In the third regime where the tracer is long-lived, global-mean vertical tracer transport is also largely influenced by non-diffusive effects. Numerical simulations of 2D tracer transport on fast-rotating zonally symmetric planets validate our analytical K zz theory over a wide parameter space. Subject headings: planetary atmospheres -eddy mixing -tracer transport -methods: analytical and numerical

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The photochemistry of Pluto's atmosphere as illuminated by New Horizons

Cited by 82 publications

References 13 publications

Photon-induced desorption of larger species in UV-irradiated methane ice

Photon-induced desorption of larger species in UV-irradiated methane ice

Structure and composition of Pluto's atmosphere from the New Horizons solar ultraviolet occultation

Global-mean Vertical Tracer Mixing in Planetary Atmospheres. I. Theory and Fast-rotating Planets

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