Ultraviolet Properties of Planetary Ices

Hendrix, Amanda; Domingue, D. L.; Noll, K. S.

doi:10.1007/978-1-4614-3076-6_3

Cited by 11 publications

(10 citation statements)

References 103 publications

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“…There are three ways in which condensation can affect 𝛼: (1) By changing the refractive index of the haze particles, (2) by increasing the mass of particles such that their sedimentation velocities change, which in turn affects the equilibrium number density, and (3) by directly changing the cross sections of the aggregates and/or their monomers. The refractive indices of hydrocarbon and nitrile ices in the FUV are not well known (de Bergh et al, 2008;Hendrix et al, 2013) and therefore we cannot speak to the validity of (1), though 𝛼 is roughly linear with the imaginary part of the refractive index, and so altering its value by a factor of a few could improve the agreement between our model and the data. Options ( 2) and ( 3) can be evaluated using scaling relations.…”

Section: Effects Of Condensationmentioning

confidence: 91%

Constraints on the microphysics of Pluto's photochemical haze from New Horizons observations

Fan

Wong

Liang

et al. 2017

Icarus

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The New Horizons flyby of Pluto confirmed the existence of hazes in its atmosphere.Observations of a large high-to low-phase brightness ratio, combined with the blue color of the haze (indicative of Rayleigh scattering), suggest that the haze particles are fractal aggregates, perhaps analogous to the photochemical hazes on Titan. Therefore, studying the Pluto hazes can shed light on the similarities and differences between the Pluto and Titan atmospheres. We model the haze distribution using the Community Aerosol and Radiation Model for Atmospheres assuming that the distribution is shaped by downward transport and coagulation of particles originating from photochemistry. Hazes composed of both purely spherical and purely fractal aggregate particles are considered. General agreement between model results and solar occultation observations is obtained with aggregate particles when the downward mass flux of photochemical products is equal to the column-integrated methane destruction rate ~1.2 × 10 -14 g cm -2 s -1 , while for spherical particles the mass flux must be 2-3 times greater. This flux is nearly identical to the haze production flux of Titan previously obtained by comparing microphysical model results to Cassini observations. The aggregate particle radius is sensitive to particle charging effects, and a particle charge to radius ratio of 30 ! / is necessary to produce ~0.1-0.2 µm aggregates near Pluto's surface, in accordance with forward scattering measurements.Such a particle charge to radius ratio is 2-4 times higher than those previously obtained for Titan.Hazes composed of spheres with the same particle charge to radius ratio have particles that are 4 times smaller at Pluto's surface. These results further suggest that the haze particles are fractal aggregates. We also consider the effect of condensation of HCN, C 2 H 2 , C 2 H 4 , and C 2 H 6 on the haze particles, which may play an important role in shaping their altitude and size distributions.

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Section: Effects Of Condensationmentioning

confidence: 91%

Constraints on the microphysics of Pluto's photochemical haze from New Horizons observations

Fan

Wong

Liang

et al. 2017

Icarus

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“…The absorption cross sections we used for the O 2 and SO 2 models were derived from measurements of solid samples, since absorption features previously detected at Ganymede have been attributed to O 2 -O 2 dimers (e.g., condensed oxygen) and SO 2 frost. We were unable to find ultraviolet absorption cross sections for solid H 2 O 2 , but substituted cross sections obtained in the gaseous phase, since Hendrix, Domingue, et al (2012) show that the two states are spectrally very similar in the near UV. This means that the contaminant concentrations for our H 2 O 2 models may be inaccurate, depending on the actual state and density of any H 2 O 2 contained within Ganymede's surface ice, but the spectral shapes should be representative.…”

Section: Journal Of Geophysical Research: Planetsmentioning

confidence: 98%

Ganymede's Far‐Ultraviolet Reflectance: Constraining Impurities in the Surface Ice

et al. 2020

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We present reflectance spectra of Ganymede's leading and trailing hemispheres in the wavelength range 138–215 nm, obtained by the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS) in 2014. The most notable feature of both spectra is the absence of a sharp water absorption edge at ~165 nm, seen in laboratory measurements of ice reflectivity and in previous observations of Saturn's icy moons and rings. Rather than displaying a sharp change in the reflectivity at the wavelength of the water ice absorption edge, Ganymede's reflectance gradually increases with wavelength at λ > 165 nm. We show that the observed shape of Ganymede's UV reflectance is inconsistent with intimate mixture models of pure ice with UV‐dark materials including tholins, amorphous carbon, graphite, and silicates. However, we find that intraparticle models, in which a small proportion of a UV‐absorbing contaminant is trapped as inclusions within the ice matrix, are able to suppress the 165 nm feature at contaminant concentrations of <1%. We show that models of ice with inclusions of silicates, Triton‐type tholin, or H2O2 are able to produce the observed gradual increase in reflectivity at λ > 165 nm, but additional absorbing surface materials are required to produce a good fit to Ganymede's previously observed near‐UV and visible reflectance.

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“…Although the surface of Europa is predominantly water ice, its UV spectra reveal important differences between it and the icy satellites in the Saturnian system. Notably, Saturn's moons, including Mimas, Enceladus, Tethys, Dione, and Rhea, are brighter than any of the Galilean moons, indicating important differences in either the exogenic processing within the two systems, the overall composition of the satellites, or both (Hendrix et al, 2013). UV spectra of Iapetus (Hendrix & Hansen, 2008), Tethys (Royer & Hendrix, 2014), Mimas (Hendrix et al, 2012), Enceladus (Hendrix et al, 2010), and Dione and Rhea (Hendrix et al, 2018) all display a relatively sharp absorption edge at 165 nm; shortward of this wavelength, the reflectance is very low.…”

Section: Water Ice Absorption Edgementioning

confidence: 99%

The Far‐UV Albedo of Europa From HST Observations

et al. 2018

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We present an analysis of Europa's far‐UV spectral albedo using observations during the 1999–2015 time period made by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Disk‐integrated observations show that the far‐UV spectrum in the ~130 to 170‐nm range is relatively flat or slightly blue (increasing albedo with decreasing wavelength) for the studied hemispheres: the leading, trailing, and anti‐Jovian hemispheres. At Lyman‐α (121.6 nm), the albedo of the trailing hemisphere continues the blue trend, but it reddens for the leading hemisphere. Also at this wavelength, the albedo of the leading hemisphere, which is higher than the trailing hemisphere at near‐UV and visible wavelengths, is lower than the trailing hemisphere, exhibiting spectral inversion. We find no evidence of a sharp water‐ice absorption edge at 165 nm on any hemisphere of Europa, which is intriguing since such an absorption feature has been observed on the icy Saturnian satellites.

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Ultraviolet Properties of Planetary Ices

Cited by 11 publications

References 103 publications

Constraints on the microphysics of Pluto's photochemical haze from New Horizons observations

Constraints on the microphysics of Pluto's photochemical haze from New Horizons observations

Ganymede's Far‐Ultraviolet Reflectance: Constraining Impurities in the Surface Ice

The Far‐UV Albedo of Europa From HST Observations

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