The Uranian satellites: Water ice on Ariel and Umbriel

New near-infrared reflectance spectra are presented for Ariel and Umbriel. Water ice on the surface of Ariel and Umbriel is verified from spectral signatures in the 2-μm region. However, the weaker bands in Umbriel's spectrum indicate that its surface is significantly different from Ariel either in degree of contamination with dark material or in microstate. Umbriel may have a lower albedo than Ariel, Titania, and Oberon and, therefore, may have a diameter comparable to that of Titania.     

Narrow-band spectrophotometry of Ariel,Umbriel, Titania,Oberon, and Triton

The spectral reflectances of Ariel, Umbriel, Titania, Oberon, and Triton were measured in 28 bandpasses between λ326 and λ976 nm on the night of 28/29 June 1974. These observations were made with the 200-in. Hale telescope and multichannel spectrometer. Bandpasses of 8 nm from λ326 to λ566 nm and 16 nm from λ592 to λ976 nm were employed. The spectral reflectances of Ariel, Oberon, and Titania increase from λ342 to λ534 nm and are relatively flat from λ550 to λ976 nm. Umbriel's reflectance decreases monotonically with increasing wavelength through the entire range of measured wavelengths. Triton is found to have a constant spectral reflectance.     

Distributions of H2O and CO2 ices on Ariel, Umbriel, Titania, and Oberon from IRTF/SpeX observations

We present 0.8-2.4 μm spectral observations of uranian satellites, obtained at IRTF/SpeX on 17 nights during 2001-2005. The spectra reveal for the first time the presence of CO₂ ice on the surfaces of Umbriel and Titania, by means of 3 narrow absorption bands near 2 μm. Several additional, weaker CO₂ ice absorptions have also been detected. No CO₂ absorption is seen in Oberon spectra, and the strengths of the CO₂ ice bands decline with planetocentric distance from Ariel through Titania. We use the CO₂ absorptions to map the longitudinal distribution of CO₂ ice on Ariel, Umbriel, and Titania, showing that it is most abundant on their trailing hemispheres. We also examine H₂O ice absorptions in the spectra, finding deeper H₂O bands on the leading hemispheres of Ariel, Umbriel, and Titania, but the opposite pattern on Oberon. Potential mechanisms to produce the observed longitudinal and planetocentric distributions of the two ices are considered.     

A simple two-layer model for the Uranus satellites

A two-layer model of a satellite interior with a rocky core with a density 3–3.4 g cm^-3 and with a H₂O mantle with a density 0.94–1.2 g cm^-3 is applied for the icy satellites. The case of Mimas is discussed separately. A comparison of the results with these obtained for more complicated models as applied for Jupiter and Saturn icy satellites has been carried out. This comparison shows that the two-layer model offers a reasonable approximation and, therefore, it can be applied for the satellites of Uranus. We obtained the dimensionless core radii 0.55–0.74, 0.45–0.68, 0.59–0.67, 0.55–0.65, and dimensionless core masses 0.42–0.72, 0.26–0.63, 0.47–0.61, 0.41–0.57, for Ariel, Umbriel, Titania, and Oberon, respectively.Institute of Geophysics of Warsaw University, Warszawa, Poland.     

The control networks of the satellites of Uranus

Control networks of the five large satellites of Uranus have been established photogrammetrically from pictures taken by the Voyager 2 spacecraft. The control networks cover the illuminated southern hemisphere of each satellite. Coordinates are listed for 103 points on Miranda, 52 points on Ariel, 43 points on Umbriel, 46 points on Titania, and 34 points on Oberon; some points are identified on the U.S. Geological Survey maps of these satellites. Miranda is ellipsoidal in shape with radii of 241, 235 and 232 km. Mean radii are 579 km for Ariel, 586 km for Umbriel, 790 km for Titania, and 762 km for Oberon.     

Resolving dynamic parameters of the August 2007 Titania and Ariel occultations by Umbriel

We observed the 15 August 2007 occultation of the uranian satellite Titania and the 19 August 2007 occultation of Ariel by Umbriel using the Agile high-speed photometer on the APO 3.5 m telescope. We find that the Titania event midpoint occurred at 09:16:39.20 UT and the Ariel event midpoint at 07:59:49.4 UT, which was 26.2 s and 37.4 s later than predicted, respectively. Our best fit impact parameter was 71.0 km for the Titania occultation and 476.9 km for the Ariel event, both of which were less than predicted.     

Keck NIRC2 photometry of Uranus, uranian satellites, and Triton in August 2004

On August 11, 2004, we made adaptive optics observations of the Uranus and Neptune systems with the Keck II Near Infrared Camera. Uranus and Triton were observed in three broadband filters (J, H, and K-prime) and four narrowband filters [Hcont, FeII, He1_B, and H2(v=1-0)]. Miranda, Ariel, Umbriel, and Oberon were observed in the four narrowband filters only. To achieve the highest possible photometric accuracy, and thus the tightest possible constraints on atmospheric aerosol models and gas mixing ratios, we used aperture photometry that accounted for the dependence of point-spread functions and growth curves on the adaptive optics reference object, and accounted for recently determined phase curves of each object. The satellite albedos we determined were compared with published relative spectra to verify the relative consistency of our observations, which were subsequently used to convert published relative spectra to absolute spectra. We used these absolute spectra and synthetic photometry methods to compare published values in dissimilar photometric systems to each other and to our observations. We find our satellite albedos in best agreement with photometry from Karkoschka [Karkoschka, E., 2001. Icarus 151, 51-68], which is the best characterized and most contemporaneous data set. Our estimated absolute accuracy of 5% to 8% is consistent with these intercomparisons. We obtained the following ring-subtracted and discrete feature-free albedos of Uranus: J: (1.66±0.07)×¹⁰⁻², H: (1.09±0.05)×¹⁰⁻², K^′: (2.08±0.15)×¹⁰⁻⁴, Hcont: (3.71±0.23)×¹⁰⁻², FeII: (1.14±0.07)×¹⁰⁻³, He1_B: (2.06±0.16)×¹⁰⁻⁴, and H2: (1.27±0.10)×¹⁰⁻⁴.     

Two-micron spectrophotometry of Uranus and its rings

Multiaperture K photometry and 2.0- to 2.5-μm spectrophotometry of Uranus and its ring system are presented. The photometric results are used, together with a previously published measurement, to set limits on the geometric albedos of Uranus and the rings at ～2.2 μm: (0.74 ± 0.02) × 10(su?4) ≤ p_K (Uranus) ≤ (1.5 ± 0.3) × 10^?4, and (2.7 ± 0.6) × 10^?2 ≤ p_K (rings) ≤ (3.4 ± 0.1) × 10^?2. Reflectance spectra of Uranus and Uranus plus rings show features in the planet's spectrum which are attributed to gaseous CH₄ absorption, and a 2.20-μm feature in the combined spectrum which may be due to the rings. This feature is tentatively identified with either the 2.26-μm absorption feature of NH₃ frost, or the 2.2-μm OH band exhibited by certain silicate minerals. The results of JHK photometry of Uranus' satellite, Ariel (U1), indicate that the infrared colors of this object are very similar to those of the satellites U2, U3, and U4.     

The Uranian satellites: Surface compositions and opposition brightness surges

High-precision spectrophotometry at 5% resolution has been obtained for the Uranian satellites Ariel, Umbriel, Titania, and Oberon. These spectra cover the wavelength region 1.43–2.57 μm and represent a substantial improvement in precision or wavelength coverage over previous studies. The presence of a spectrally dominant water-ice component in the surface of Ariel, Umbriel, Titania, and Oberon is confirmed. The 1.5- and 2.0-μm water absorption band depths and the continuum reflectance (as defined by the reflectance at 1.78 and 2.25 μm) indicate significant differences among the surface compositional properties of the four satellites. Comparisons of the new spectra to those of other solar system bodies, and to laboratory spectra of water ice of various degrees of purity, indicate that these satellites have a significant non-water-ice component on/in their surfaces. The lack of spectral absorptions at 5% resolution attributable to anything other than water ice suggests that the non-water-ice component is a roughly neutral reflector in the 1.5- to 2.5- μm region. The nature of the non-water-ice component cannot be uniquely determined from these data, but it is relatively dark and has spectral similarities to substances such as carbon black, the dark substance covering one face of Iapetus, or other neutral-color, low-reflectance materials. Finally, preliminary measurements of the near-infrared opposition brightness surges of Ariel, Titania, and Oberon show them to be among the largest in the solar system.     

Occurrence of central peak craters on the Uranian satellites: Implications for surface structure and comparison with the Jovian and Saturnian systems

Craters with central peaks occur on the Uranian satellites Ariel, Umbriel, Titania, and Oberon; but do not occur on Miranda. The inelastic surface of Miranda is apparently due to the heavy tectonic reworking of its surface. A theory of expansion/contraction is proposed to explain the tectonic history of Miranda. The existence of central peak craters on the four largest satellites of Uranus implies that they have surface strengths similar to those of the Saturnian satellites and silicate bodies of the inner solar system which all have central peak craters. The absence of central peak craters on Miranda implies that it has an inelastic surface similar to those of the Jovian ice satellites Ganymede and Callisto whose surfaces do not contain central peak craters.     

Surface of Miranda: Identification of water ice

Near-infrared spectrophotometry at 5% resolution shows Miranda to have a water-ice surface. Estimates of Miranda's albedo made from the depth of its 2.0-μm absorption band suggest that its visual geometric albedo is likely to be between 10 and 70%, which when combined with the satellite's visual magnitude, yields a diameter of 500 ± 225km. There is some evidence that suggests the visual geometric albedo of Miranda may be ≥0.3, which implies that its diameter may lie near the lower end of the estimated range. With these results all the Uranian satellites are now known to have water-ice surfaces.     

Near-infrared spectrophotometry of the satellites and rings of Uranus

New spectrophotometry from 1.5 to 2.5 μm is reported for the Uranian satellites Titania, Oberon, and Umbriel. A spectrum of the rings of Uranus from 2.0 to 2.4 μm is also reported. No evidence is found for frost covering the surface of the ring material, consistent with the low albedo of the rings (P_K = 0.03) previously reported by Nicholson and Jones (1980). The surfaces of the satellites are found to be covered by dirty water frost. Assuming albedos of the frost and gray components covering the Uranian satellites to be the same as the light and dark faces of Iapetus, radii are derived that are roughly twice those inferred from the assumption of a visual albedo of 0.5.     

How to compare the surface of Io to laboratory samples

Since one does not know the photometric functions of various parts of Io, one cannot convert the observed geometric albedo of the satellite to a parameter more directly measurable in the laboratory. One must therefore convert laboratory reflectances to geometric albedos before quantitative comparisons between Io's surface and a laboratory sample are made. This procedure involves determining the wavelength dependence of the sample's photometric function. For substances such as sulfur, whose reflectance varies strongly with wavelength, it is incorrect to assume that the photometric function, and hence the ratio (laboratory reflectance/geometric albedo) is independent of wavelength. To illustrate this point, measurements of the color dependence of this ratio for sulfur are presented for the specific case in which the measured laboratory reflectance is the sample's normal reflectance. In general, unless the laboratory reflectance is precisely the geometric albedo, a wavelength-dependent correction factor must be determined before the laboratory sample can be compared quantitatively with Io's surface.     

Photometric study of the major satellites of Uranus

In this paper, we analyze the results of ground-based and space-born photometric observations of the major satellites of Uranus—Miranda, Ariel, Umbriel, Titania, and Oberon. All sets of photometric observations of the satellites available in the literature were examined for uniformity and systematic differences and summarized to a unified set by wavelength ranging from 0.25 to 2.4 μm. This set covers the interval of phase angles from 0.034° to 35°. The compound phase curves of brightness of the satellites in the spectral bands at 0.25, 0.41, 0.48, 0.56, 0.75, 0.91, 1.4, and 1.8 μm, which include a pronounced opposition surge and linear part, were constructed. For each satellite, the geometric albedo was found in different spectral bands taking into account the brightness opposition effect, and its spectral dependence was studied. It has been shown that the reflectance of the satellites linearly depends on the wavelength at different phase angles, but has different spectral gradients. The parameters of the phase functions of brightness, including the amplitude and the angular width of the brightness opposition surge, the phase coefficient, and the phase angle at which the nonlinear increase in brightness starts, were determined and their dependences on wavelength and geometric albedo were analyzed. Our investigations show that, in their optical properties, the satellites Miranda and Ariel, Titania and Oberon, and Umbriel present three types of surfaces. The observed parameters of the brightness opposition effect for the Uranian satellites, some ice satellites of Jupiter and Saturn, and the E-and S-type asteroids are analyzed and compared within the framework of the coherent backscattering and mutual shadowing mechanisms.     

Some unsolved problems in evolutionary dynamics in the solar system

Several unsolved problems in the evolutionary histories leading to current dynamical configurations of the planets and their systems of satellites are discussed. These include the possibilities of rather tight constraints on the primordial rotation states of Mercury and Venus and the stabilizing mechanism for the latter's retrograde spin, a brief mention of the problem of origin of the moons of Earth and Mars, the excessive heat flow from Jupiter's satellite lo which is not compatible with an otherwise self-consistent model of origin of the Laplace three-body libration, the mechanism for the long history of resurfacing of Saturn's satellite Enceladus and the possibly short lifetime of the A ring and the mechanisms for resurfacing the satellites of Uranus, especially Ariel, if the high stability of the mean motion orbital resonances at the 2/1 commensurability involving Ariel and Umbriel precludes a long term occupancy of the resonance. Finally, excessive times of accumulation of the outer planets in current models may possibly be reducible from the effects of nebular gas drag.     

On matching the spectrum of Io: Variations in the photometric properties of sulfur-containing mixtures

The problem of comparing laboratory spectra of sulfur-containing binary mixtures with the spectrum of Io is discussed. For the satellite, the observable is the geometric albedo as a function of wavelength, whereas in the laboratory one often measures some other type of albedo. In a previous paper we demonstrated that for pure sulfur the multiplicative factor which converts the laboratory albedos to geometric albedos can be strongly wavelength dependent. The present paper demonstrates that this is also true for binary sulfur-containing mixtures. Furthermore, there is no universal conversion factor applicable to all binary mixtures, nor can the factor be interpolated for a particular mixture from the conversion factors of the two end members. The conversion factor is a function not only of the specific composition of a binary mixture, but of the relative particle size distributions of the two components, and must be measured specifically for each individual sample if a quantitative comparison between a laboratory sample and Io's surface is desired.     

Near-infrared spectra of the Galilean satellites: Observations and compositional implications

We have obtained reflectivity spectra of the trailing and leading sides of all four Galilean satellites with circular variable filter wheel spectrometers operating in the 0.7- to 5.5-μm spectral interval. These observations were obtained at an altitude of 41,000 ft from the Kuiper Airborne Observatory. Features seen in these data include a 2.9-μm band present in the spectra of both sides of Callisto; the well-known 1.5-μm and 2.0-μm combination bands and the previously more poorly defined 3.1-μm fundamental of water ice observed in the spectra of both sides of Europa and Ganymede; and features centered at 1.35 ± 0.1, 2.55 ± 0.1, and 4.05 ± 0.05 μm noted in the spectra of both sides of Io. In an effort to interpret these data, we have compared them with laboratory spectra as well as synthetic spectra constructed with a simple multiple-scattering theory. We attribute the 2.9-μm feature of Callisto's spectra primarily to bound water, with the product of fractional abundance of bound water and mean grain radius in micrometers equaling approximately 3.5 × 10^?1 for both sides of the satellite. The fractional amounts of water ice cover on the trailing side of Ganymede, its leading side, and the leading side of Europa were found to be 50 ± 15, 65 ± 15, and 85% or greater, respectively. The bare ground areas on Ganymede have reflectivity properties in the 0.7- to 2.5-μm spectral region comparable to those of Callisto's surface and also have significant quantities of bound water, as does Callisto. Interpretation of the spectrum for the trailing side of Europa is complicated by magnetospheric particle bombardment which causes a perceptible broadening of strong bands, but the ice cover on this side is probably comparable to that on the leading side. These irradiation effects may be responsible for much of the difference in the visual geometric albedos of the two sides of Europa. Minor, but significant, amounts of ferrous-bearing material (either ferrous salts or alkali feldspars but not olivines or pyroxenes) account for the 1.35-μm feature of Io. The two longer wavelength bands are most likely attributable to nitrate salts. Ferrous salts and nitrates can jointly also account for much of the spectral variation in Io's visible reflectivity, thereby eliminating the need to postulate large quantities of sulfur. The absence of noticeable features near 3-μm wavelength in Io's spectra leads to upper bounds of 10% on the fractional cover of water and ammonia ice and 10^?3 on the relative abundance of bound water and hydroxylated material on Io. The two sides of Io have similar compositions. We suggest that the systematic increase in fractional water ice cover from Callisto to Ganymede to Europa is bought about by variations in efficiencies of recoating the satellite's surface by interior water brought to the surface, and by the deposition of extrinsic dust. The most important component of the latter is debris, derived from the outer irregular satellites of Jupiter, which impacts the Galilean satellites at relatively low velocities. Europa has the largest water ice cover because its crust is thinnest and thus the frequency of water recoating is the greatest, and because it is farthest from the sources of low-velocity dust. While models which depict Io's surface as consisting primarily of very fine-grained ice are no longer viable, we are unable to definitively distinguish between the salt assemblage and alkali feldspar models. The salt model can better account for Io's reflectivity spectrum from 0.3 to 5 μm, but the absence of appreciable quantities of bound water and hydroxylated material may not be readily understood within the context of that model.     

Saturn's rings: Particle composition and size distribution as constrained by microwave observations: I. Radar observations

We have calculated the radar backscattering characteristics of a variety of compositional and structural models of Saturn's rings and compared them with observations of the absolute value, wavelength dependence, and degree of depolarization of the rings' radar cross section (reflectivity). In the treatment of particles of size comparable to the wavelength of observation, allowance is made for the nonspherical shape of the particles by use of a new semiempirical theory based on laboratory experiments and simple physical principles to describe the particles' single scattering behavior. The doubling method is used to calculate reflectivities for systems that are many particles thick using optical depths derived from observations at visible wavelengths. If the rings are many particles thick, irregular centimeter- to meter-sized particles composed primarily of water ice attain sufficiently high albedos and scattering efficiencies to explain the radar observations. In that case, the wavelength independence of radar reflectivity implies the existence of a broad particle size distribution that is well characterized over the range 1 cm ? r ? m by n(r)dr = n₀r^?3dr. A narrower size distribution with $\text{a}\text{～ 6}\text{cm}$ is also a possibility. Particles of primarily silicate composition are ruled out by the radar observations. Purely metallic particles, either in the above size range and distributed within a many-particle-thick layer or very much larger in size and restricted to a monolayer, may not be ruled out on the basis of existing radar observations. A monolayer of very large ice “particle” that exhibit multiple internal scattering may not yet be ruled out. Observations of the variation of radar reflectivity with the opening angle of the rings will permit further discrimination between ring models that are many particles thick and ring models that are one “particle” thick.     

Coincident observations of ionospheric troughs and the equatorial plasmapause

Explorer 45 traversed the plasmapause (determined approximately via the saturation of the d.c. electric field experiment) at near-equatorial latitudes on field lines which were crossed by Ariel 4 (~600km altitude) near dusk in May 1972 and on field lines which were crossed by Isis II (~1400km altitude) near midnight in December 1971 and January 1972. Many examples were found in which the field line through the near-equatorial plasmapause was traversed by Explorer 45 within one hour local time and one hour universal time of Ariel and Isis crossings of the same L coordinate. For the coincident passes near dusk, the RF electron density probe on Ariel detected electron density depletions near the plasmapause L coordinates when Ariel was in darkness. When the Ariel passes were in sunlight, however, electron depletions were not discernable near the plasmapause field line. On the selected near-midnight passes of Isis II, electron density depressions were typically detected (via the topside sounder) near the plasmapause L coordinate. The dusk Ariel electron density profiles are observed to reflect O⁺ density variations. Even at the high altitude of Isis near midnight, O⁺ is found to be the dominant ion in the trough region whereas H⁺ is dominant at lower latitudes as is evident from the measured electron density scale heights. In neither local time sector was it possible to single out a distinctive topside ionosphere feature as an indicator of the plasmapause field line as identified near the equator. At both local times the equator-determined plasmapause L coordinate showed a tendency to lay equatorward of the trough minimum.     

Mutual phenomena of Jupiter's Galilean satellites 1973–74

Two series of predictions have been published for the 1973–1974 mutual phenomena of Jupiter's satellites, one (June–October, 1973) by Milbourn and Carey, and the other (February 1973–May 1974) by Brinkmann and Millis. The main purpose of this paper is to investigate some significant discrepancies between these two sets of predictions. New predictions are calculated for the period June 1973–May 1974. They agree very nearly with the predictions by Milbourn and Carey, but frequently differ by several minutes (up to 30 min when Jupiter III and IV are involved) from those by Brinkmann and Millis. Unlike the previous predictions, the new ones also give the estimated light decreases during the phenomena. The method of prediction is documented for future applications to Jupiter's and Saturn's satellites. The paper concludes with a brief discussion of the problems involved in extracting information about the positions, radii, and albedos of the satellites from observed light curves.