摘要:

The Voyager Uranus/Interstellar Mission is the continuation of the NASA program of exploration of the outer solar system. The first phase of the Voyager program included encounters with Jupiter and Saturn as summarized in theJournal of Geophysical Research(volume 86, pages 8123–8841, 1981, and volume 88, pages 8639–9018, 1983). With the successful completion of this first phase a second phase was undertaken with the objectives of exploring the Uranus system and investigating the interplanetary and interstellar media. Additional objectives included preserving the capability for extending the investigations to include an encounter with the Neptune system and a search for the heliopa

ISSN:0148-0227    

DOI:10.1029/JA092iA13p14873

年代:1987

数据来源: WILEY

摘要:

We have analyzed radio Doppler data and star‐satellite imaging data from Voyager 2 at Uranus, along with 8 years of ground‐based astrometric data, and have obtained improved masses and densities for the satellites of Uranus as well as a new ratio of the mass of the Sun to the mass of the Uranian system of 22902.94 ± 0.04. The mean density of Uranus is 1.285 ± 0.001 g cm−3. The satellite densities are 1.25 ± 0.33 for Miranda, 1.55 ± 0.22 for Ariel, 1.58 ± 0.23 for Umbriel, 1.685 ± 0.068 for Titania, and 1.635 ± 0.060 for Oberon, all expressed in units of grams per cubic centimeter. The mean uncompressed density of all five satellites is 1.48 ± 0.06 g cm−3. This is 0.10 g cm−3higher than the value expected for a homogeneous solar mix consisting of 34% anhydrous rock, 51% water ice, 7% ammonia ice, and 8% methane, present as clathrate hydrate. In order to reconcile this difference, we suggest that the Uranian moons contain roughly 15% by mass of pure graphite, in addition to a normal solar component of rocks and ices. If so, at least 50% of the carbon within the nebular gases from which Uranus and its satellites condensed was in the form of graphite, the remaining being in CH4. The high thermal conductivity of graphite ensures that the Uranian moons have remained cold and undifferentiated since the time of their formation, despite heating caused by the decay of radioactive nuclides. Apparently, an alternative cometary origin for the satell

ISSN:0148-0227    

DOI:10.1029/JA092iA13p14877

年代:1987

数据来源: WILEY

摘要:

We have analyzed the densities of the Uranian satellites derived from Voyager data to place constraints on their uncompressed densities and probable silicate mass fractions. Two bounding models were used: (1) a completely differentiated satellite with the current temperature/depth profile set by conduction of heat from the silicate core, and (2) a homogeneous model with the temperature profile also set by conduction. Results from such models for the largest satellites, Titania and Oberon, show that within the errors the silicate mass fraction of these two bodies should lie within the range 0.42–0.65, with the most probable values being 0.50–0.58. These values are significantly higher than similar model results for Saturn's small satellites (Rhea's silicate fraction is ∼0.35–0.42) but similar to the system averages for Jupiter and Saturn (including Titan). The derived silicate fractions are also higher than predicted for solar nebular models having CH4as the dominant carbon‐bearing species. Examining current models of solar nebular chemistry and the three systems visited by Voyager, we suggest that the satellites accreted from material in the outer envelopes of their respective primaries, with the abundance of CO and solid organic material in the solar nebula and satellite‐forming nebulae being important determinants of the rock/water ice ratios in these systems, and thus the satellite mean densities. Preferential dissolution of the water component of planetesimals in the envelopes of the giant planets may have also contributed to determining the water/rock fractions incorporated into the planets'

ISSN:0148-0227    

DOI:10.1029/JA092iA13p14884

年代:1987

数据来源: WILEY

摘要:

Voyager 2 imaging observations over a wide range of phase angles are used to determine the fundamental photometric parameters for the five largest satellites of Uranus. Over the spectral range covered by Voyager cameras (approximately 0.35–0.60 µm) the disk‐averaged colors are moderately gray (no redder than the spectrum of Saturn's satellite Phoebe). Geometric albedos range from 0.19 for Umbriel to 0.40 for Ariel. Phase coefficients determined generally between phase angles of 10° and 60° vary from 0.021 magnitude per degree (mag/deg) for Ariel to 0.028 mag/deg for Miranda. Phase integrals lie in the range of 0.5–0.65. We estimate the following Bond albedos: about 0.1 for Umbriel and about 0.2 for the other satellites. The most complete phase angle coverage occurred in the case of Titania, for which data are available between 0.8° and 152.6°. The Titania phase curve shows a pronounced opposition surge, which can be modeled in detail by using Hapke's theory. A regolith texture more porous than that of the lunar surface is indicated. Observations at high phase angles (>139°) can be used to constrain the large‐scale roughness of the satellites. For Titania, Umbriel, and Oberon we find values of (the large‐scale roughness parameter in Hapke's model) comparable to those of the Moon. For Ariel a significantly rougher surfa

ISSN:0148-0227    

DOI:10.1029/JA092iA13p14895

年代:1987

数据来源: WILEY

摘要:

We report the ultraviolet (0.25 µm) and infrared (0.75 µm) geometric albedos, phase curves, and phase coefficients for the large Uranian satellites using full disk photometric observations obtained by the photopolarimeter subsystem during the Voyager 2 encounter with the Uranian system. The phase coefficients and phase curves of the Uranian satellites are consistent with surfaces that have a loosely packed regolith with a heavily cratered terrain. Ariel, Titania, and Oberon exhibit a reddening with increasing phase angle of observation. We do not observe this effect for Umbriel. We have determined preliminary phase integrals for Ariel, Umbriel, Titania, and Oberon, and we have used this information and the geometric albedos reported herein and previously reported by the Voyager imaging subsystem to derive Bond albedos for the satellites. These are 0.22 ± 0.1, 0.07 ± 0.05, 0.16 ± 0.12, and 0.19 ± 0.22 for Ariel, Umbriel, Titania, and Oberon, respectively. We have compared the geometric albedos and color ratios of the Uranian satellites with comparable sets of data from the literature on other large satellites in the solar system, and we find that the Uranian satellites as a class are separate and distinct from the large Jovian and Saturnian satellites. This indicates a compositional difference between the three classes of large icy satellites. This may be due to different conditions at the time of formation and/or differences in the subsequent evolution of the surfaces of these objects. This finding is consistent with the hypothesis that a common surficial modification process exists for all the satellites in the Uranian system which is different from the processes which modify the surfaces of the Jovian and Saturnian satel

ISSN:0148-0227    

DOI:10.1029/JA092iA13p14905

年代:1987

数据来源: WILEY

摘要:

Voyager 2 obtained images of Titania over phase angles ranging from 0.8° to 150° and at sufficient resolution to investigate the photometric behavior of different surface units. The large, relatively narrow opposition surge detected from Earth was confirmed and can be successfully modeled with Hapke's (1986) photometric theory. Opposition effects do not vary greatly between bright areas (craters and ejecta) and dark areas. The primary difference in phase behavior is that the brightness of the brighter areas decreases more slowly with increasing phase angle than does the brightness of the dark areas. Thus, contrary to the situation on the Moon the fresher craters and their ejecta become less prominent in relation to the background as opposition is approached. This effect is best explained by a modest difference in single‐scattering albedo. There is evidence that the angular width of the opposition surge is slightly narrower for bright areas, a situation consistent with the interpretation that the ejecta regolith is somewhat less compacted than that in the background dark terr

ISSN:0148-0227    

DOI:10.1029/JA092iA13p14911

年代:1987

数据来源: WILEY

摘要:

Crater size‐frequency data for Umbriel, Titania, and Oberon are presented, and the implications of those data are discussed in terms of the geologic histories of these bodies and the populations of objects that have cratered them. The surfaces of Oberon and Umbriel are old and are inferred to date to a period early in their histories when the cratering rate was significantly higher than at present. No significant endogenic resurfacing appears to have occurred on either body after that inferred period of intense cratering. Titania exhibits the youngest surface of these three and appears to have undergone almost complete endogenic resurfacing. Among the Uranian satellites the surfaces of Oberon and Umbriel are interpreted to be the oldest, that of Titania intermediate, and those of Ariel and parts of Miranda the youngest. The size‐frequency distributions for these satellites have an average slope of about −3, indicative of a steep crater production function. The cumulative size‐frequency data for these Uranian satellites may be interpreted to indicate that parts of their surfaces are saturated with craters at small di

ISSN:0148-0227    

DOI:10.1029/JA092iA13p14918

年代:1987

数据来源: WILEY

摘要:

Methane clathrate is expected to be an important carbon‐containing ice in the outer solar system. We investigate the effect of electron irradiation by coronal discharge on several simple hydrocarbons enclathrated in or mixed with H2O or H2O+NH3in simulation of the effects of the solar wind, planetary magnetospheric particles, and cosmic rays on surfaces containing these ices in the outer solar system and interstellar space. H2O+CH4clathrate, H2O+C2H6, H2O+CH4+NH3, H2O+C2H6+NH3, and H2O+C2H2are all initially white ices, and all produce yellowish to brownish organic products upon charged particle irradiation. Significant coloration occurs with doses of 109erg cm−2, corresponding to short interplanetary irradiation times. Uranian magnetospheric electrons penetrate to ∼1 mm depth and deposit this dose in 8, 30, 65, 200, and 500 years into the surfaces of Miranda, Ariel, Umbriel, Titania, and Oberon, respectively. Further irradiation of the laboratory ice surface results in a progressive darkening and a more subdued color. For a conversion efficiency to solidsG≃ 1 molecule keV−1, the upper limit for the time for total destruction of CH4and other simple hydrocarbons in the upper 1 mm is 5 × 104years (Miranda) to 3 × 106years (Oberon). Remote detection of CH4is possible only when its replenishment rate exceeds the destruction rate at the depth probed by spectroscopy. Reflection spectroscopy of irradiated H2O+CH4frost is compared with the spectra of several outer solar system objects and to other relevant organic and inorganic materials. Ultraviolet‐visible and infrared transmission spectroscopy of the postirradiation residues is presented. Persistence of color and of CH4ice bands on Triton and Pluto suggests ongoing surface activity and/or atmospheric haze. Over 4 × 109year time scales, ≥ 10 m of satellite and cometary surface material is processed by cosmic rays to a radiation‐hardened ice‐tholin mixture devoid of CH4. Preaccretional chemistry, exogenous materials, and endogenous organic chemistry all contribute to the spectral properties of icy satellites which accreted simple CH(O) molecules. Radiation darkening traces the deposition of mobilized or impact‐exposed carbon‐bearing volatiles on these satellites. More exhaustive experiments are necessary to work out the detailed relationships between initial composition, exposu

ISSN:0148-0227    

DOI:10.1029/JA092iA13p14933

年代:1987

数据来源: WILEY

摘要:

Measurements of the ion and electron fluxes in the Uranian magnetosphere made by the low‐energy charged particle (LECP) instrument on the Voyager 2 spacecraft are used to discuss possible particle‐induced modifications of the moons and rings of Uranus. The energy spectra of the orbit‐integrated particle dosages expected on the major moons are derived from the LECP measurements of particle intensities and pitch angle distributions. Laboratory‐derived results on charged‐particle‐induced chemical and physical modifications of H2O, CH4, CO, and CO2ions are used. The erosion rates of water ice and the darkening of organic ices on the surfaces of the moons are estimated from the orbit‐integrated fluxes. Significant darkening of fresh organic ices to depths of the order of a micron is expected to occur for times as short as a few thousand years. Darkening at deeper depths will occur at increasingly longer times. The implications of these results for the interpretation of remote sensing data from such objects

ISSN:0148-0227    

DOI:10.1029/JA092iA13p14949

年代:1987

数据来源: WILEY

摘要:

During the Voyager 2 flyby of Uranus the plasma wave and radio astronomy instruments detected a region of impulsive noise near the equatorial plane just inside the orbit of Miranda, at a radial distance of 4.51RU. This noise is believed to be caused by micron‐sized particles hitting the spacecraft. Analysis of various coupling mechanisms shows that when a dust particle hits the spacecraft at a high velocity, the particle is instantly vaporized and ionized, thereby releasing a cloud of charged particles, some of which are collected by the antenna. The resulting voltage pulse is detected by the plasma wave instrument. Based on reasonable assumptions about the charge yield and collection efficiency of the antenna, the number density and mass of the particles can be estimated from the rate and amplitude of the voltage pulses. The analysis shows that the maximum number density of the particles is about 1.6 × 10−3m−3, and the thickness of the impact region, based on a Gaussian fit, is 3480 km. The maximum number density occurs slightly after the ring plane crossing at a distance of about 280 km from the equatorial plane. The mass threshold for detecting the particles is estimated to be about 4.5 × 10−10g, and the rms mass of the particles is about 2.6 × 10−9g. For a density of a few grams per cubic centimeter the particles have radii of the order of a few microns. Possible sources for these particles include the rings, the small satellite 1985U1 discovered outside the ring system, or other unseen small bodies that lie between synchronous orbit (3.15RU) and 4.51RU. If the particles are charged, electromagnetic forces produced by the rotating tilted dipole of Uranus may play a role in their transport a

ISSN:0148-0227    

DOI:10.1029/JA092iA13p14959

年代:1987

数据来源: WILEY