Douglas B. Nash

Io's surface composition based on reflectance spectra of sulfur/salt mixtures and proton-irradiation experiments

Abstract The available full-disk reflectance spectra of Io in the range 0.3 to 2.5 μm have been interpreted by comparison with new laboratory spectra of a wide variety of natural and synthetic mineral phases in order to determine a surface compositional model for Io that is consistent with Ios other known chemical and physical properties. Our results indicate that the dominant mineral phases are sulfates and free sulfur derived from them, which points toward a low temperature and initially water-rich surface assemblage. Our current preferred mineral phase mixture that best matches the Io data and is simultaneously most consistent with other constraints, consists of a fine-grained particulate mixture of free sulfur (55 vol%), dehydrated bloedite [Na 2 Mg(SO 4 ) 2 · x H 2 O] (30 vol%) ferric sulfate [Fe 2 (SO 4 ) 3 · x H 2 O] (15 vol%), and trace amounts of hematite [Fe 2 O 3 ]. Other salts may be present, such as halite and sodium nitrate, as well as clay minerals. Such a model is consistent with a probable pre- and post-accretion thermal history of Io-forming material and Ios observed Na emission and other properties. These results further support the evaporite surface hypothesis of Fanale et al ; while not precluding the presence of certain silicate phases such as montmorillonite. The average surface of Ios leading hemisphere appears to contain less free sulfur and more salts and to be finer grained than that of the trailing hemisphere. Since Io is immersed in Jupiters magnetosphere, irradiation damage effects from low-energy proton bombardment were studied. Irradiation damage of lattices is estimated to be a relatively minor but operative process on the surface of Io; irradiation darkening by sulfate reduction to free sulfur and by F-center production in salts may be partly responsible for the differences in albedo of leading and trailing hemispheres and equatorial and polar regions of Io, but slight regional differences in relative intrinsic phase concentration on the surface may likewise account for these global variations in albedo. Possible unusual surface properties predicted by this model include: posteclipse darkening in certain wavelenghts, limb brightening in certain wavelengths, and unusual surface electrical properties. Further refinement of Ios surface composition model and better understanding of surface irradiation effects will be possible when observational data in the range 0.20 to 0.30 μm are obtained and when improved spectra in the range 0.30 to 5.0 μm are obtained having increased spectral, spatial, and temporal resolution.

Mid-infrared reflectance spectra (2.3–22 μm) of sulfur, gold, KBr, MgO, and halon

Biconical diffuse reflectance spectra in the mid-infrared are presented for powder and other solid forms of sulfur, gold, potassium bromide, magnesium oxide, and Halon. Comparisons are made with previously published results of other investigators, and recommendations are made regarding the relative usefulness of these materials as reflectance standards in the mid-IR. Sulfur has strong intrinsic bands at wavelengths >7 μm that must be taken into account for its use as a reflectance standard. Some sulfur samples have hydrocarbon contaminants and in powder form may have adsorbed water, both of which produce bands in the 3–4-μm region. Potassium bromide has several weak intrinsic bands and is very sensitive to adsorbed water contamination; otherwise it is a good IR reference material. Magnesium oxide and Halon have major band structure and low reflectivity at wavelengths >2.6 μm and thus are unsuitable as reference materials in the mid-IR. Vapor-deposited gold on fine sandpaper (600 grit) is very bright, spectrally flat, and fairly diffuse, so it is the superior material (of those examined) for reflectance reference material throughout the IR. Fine gold powder, on the other hand, is much less bright than evaporated gold, and its reflectivity at wavelengths greater than its particle size is highly sensitive to particle packing density.

High-resolution infrared spectroscopy of Io and possible surface materials

Abstract We have obtained new spectra of Io in the 3.5- to 4.2- and 4.5- to 5.4-μm regions with a resolution (λ/Δλ) of roughly 500. The Io spectra cover a range of longitudes, and times from 1983 through 1985. Laboratory spectra of various materials have also been obtained. In this wavelength region are located several strong bands of SO 2 , known to be a major component of Ios surface, as well as the bands of several potential surface materials. In the Io spectra we identify several new features attributable to SO 2 , and have obtained strengths of the 33 S, 34 S, and 18 O isotopic bands, which appear normal. We place limits on the amounts of Na 2 SO 3 , and Na 2 SO 4 present. Finally, we use the data to place limits on the SO 2 gas abundance in Ios atmosphere.

Luminescence and reflectance of tranquillity samples: effects of irradiation and vitrification.

Luminescence measurements of Tranquillity samples indicate that energy efficiencies for excitation by protons and ultraviolet are in the range 10-6 or below; natural and induced thermoluminescence is even weaker. If these samples are typical, lunar surface luminescence cannot occur at reported levels. Comparison of proton luminescence spectra from the exterior and interior of rocks and fine fragments provides evidence of solar wind impingement on the moons surface. Spectral reflectance and albedo measurements of fresh rock powders before and after both laboratory proton irradiation and fusion indicate that vitrification may be an important mechanism of lunar darkening.

The reflection spectrum of liquid sulfur: Implications for Io

Abstract The spectral reflectance from 0.38 to 0.75 μm of a column of liquid sulfur has been measured at several temperatures between the melting point (∼118°C) and 173°C. Below 160°C the spectral reflectance was observed to vary reversibly as a function of temperature, independent of the previous thermal history of the column. Once the temperature exceeded 160°C, the spectrum would not change given a subsequent decrease in temperature. The spectral reflectance of the liquid-sulfur column at all temperatures was very low (10–19%). Combining this information with Voyager spectrophotometry of Jupiters satellite Io, it is concluded that liquid sulfur at any temperature on Ios surface would be classified as a “black area” according to the standards used by the Voyager imaging team in their spectrophotometric analysis ( L. Soderblom, T. V. Johnson, D. Morrison, E. Danielson, B. L. Smith, J. Veverka, A. Cook, C. Sagan, P. Kupferman, D. Pieri, J. Mosher, C. Avis, J. Gradie, and T. Clancy (1980) . Geophys. Res. Lett. 7, 963–966).

Spectral reflectance change and luminescence of selected salts during 2–10 KeV proton bombardment: Implications for Io

Abstract Radiation damage and luminescence, caused by magnetospheric charged particles, have been suggested by several authors as mechanisms for explaining some of the peculiar spectral/albedo features of Io. We have pursued this possibility by measuring the uv-visual spectral reflectance and luminescent efficiency of several proposed Io surface constituents during 2 to 10-keV proton irradiation at room temperature and at low temperature (120 T F -center absorption bands at 4580 and 5560 A. Na 2 SO 4 shows a generalized darkening which increases toward longer wavelengths. NaNO 3 shows a spectral reflectance change indicative of the partial alteration of NaNo 3 to NaNo 2 . NaNO 2 shows no change. The luminescent efficiencies of NaCl and KCl are ∼10 −4 at 300°K and increase by one-half order of magnitude at ∼130°K. The efficiencies of K 2 CO 3 , Na 2 CO 3 , Na 2 SO 4 , and NaNO 3 are 10 −4 , 10 −4 , 10 −5 and 10 −6 , respectively, at 300°K and they all decrease by one-half order of magnitude at ∼130°K. These results indicate that magnetospheric proton irradiation of Io could cause spectral features in its observed ultraviolet and visible reflection spectrum if salts such as those studied here are present on its surface. However, because the magnitude of these spectral effects is dependent on competing factors such as surface temperature, incident particle energy flux, solar bleaching effects, and trace element abundance, we are unable at this time to make a quantitative estimate of the strength of these spectral effects on Io. The luminescent efficiencies of pure samples that we have studied in the laboratory suggest that charged-particle induced luminescence from Ios surface might be observable by a spacecraft such as Voyager when viewing Ios dark side.

ON THE DISTRIBUTION OF LUNAR MARIA AND THE SYNCHRONOUS ROTATION OF THE MOON

Presentation of a hypothesis to explain the predominance of maria on the near side of the lunar surface