The magnetosphere of uranus: hot plasma and radiation environment

The low-energy charged-particle (LECP) instrument on Voyager 2 measured lowenergy electrons and ions near and within the magnetosphere of Uranus. Initial analysis of the LECP measurements has revealed the following. (i) The magnetospheric particle population consists principally of protons and electrons having energies to at least 4 and 1.2 megaelectron volts, respectively, with electron intensities substantially excceding proton intensities at a given energy. (ii) The intensity profile for both particle species shows evidence that the particles were swept by planetry satellites out to at least the orbit of Titania. (iii) The ion and electron spectra may be described by a Maxwellian core at low energies (less than about 200 kiloelectron volts) and a power law at high energies (greater than about 590 kiloelectron volts; exponentmicro, 3 to 10) except inside the orbit of Miranda, where power-law spectra (micro approximately 1.1 and 3.1 for electrons and protons, respectively) are observed. (iv) At ion energies between 0.6 and 1 megaelectron volt per nucleon, the composition is dominated by protons with a minor fraction (about 10(-3)) of molecular hydrogen; the lower limit for the ratio of hydrogen to helium is greater than 10(4). (v) The proton population is sufficiently intense that fluences greater than 10(16) per square centimeter can accumulate in 10(4) to 10(') years; such fluences are sufficient to polymerize carbon monoxide and methane ice surfaces. The overall morphology of Uranus' magnetosphere resembles that of Jupiter, as evidenced by the fact that the spacecraft crossed the plasma sheet through the dawn magnetosheath twice per planetary rotation period (17.3 hours). Uranus' magnetosphere differs from that of Jupiter and of Saturn in that the plasma 1 is at most 0.1 rather than 1. Therefore, little distortion ofthe field is expected from particle loading at distances less than about 15 Uranus radii.

Coloration and darkening of methane clathrate and other ices by charged particle irradiation: applications to the outer solar system

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+NH3 in 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+CH4 clathrate, H2O+C2H6, H2O+CH4+NH3, H2O+C2H6+NH3, and H2O+C2H2 are all initially white ices, and all produce yellowish to brownish organic products upon charged particles irradiation. Significant coloration occurs with doses of 10(9) erg cm-2, corresponding to short interplanetary irradiation times. Uranian magnetospheric electrons penetrate to approximately 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 solids G approximately equal to 1 molecule keV-1, the upper limit for the time for total destruction of CH4 and other simple hydrocarbons in the upper 1 mm is 5 x 10(4) years (Miranda) to 3 x 10(6) years (Oberon). Remote detection of CH4 is possible only when its replenishment rate exceeds the destruction rate at the depth probed by spectroscopy. Reflection spectroscopy or irradiated H2O+CH4 frost 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 CH4 ice bands on Triton and Pluto suggests ongoing surface activity and/or atmospheric haze. Over 4 x 10(9) year time scales, > or = 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, exposure age, and color/albedo.

Fast Ion Bombardment of Ices and Its Astrophysical Implications

Ices such as water, carbon dioxide, and methane are now known to be pervasive constituents of the solar system and probably of the interstellar medium as well. Many of these ices and ice-covered surfaces are exposed to bombardment by the energetic ions of space. Laboratory experiments have been carried out to study the effects of such bombardment. Surprisingly efficient erosion of ice layers is associated with electronic excitation of the ices by the ions. These results are a challenge to an understanding of the physical processes involved and have implications for a number of astrophysical problems of current interest.

Ultraviolet Photometer Observations of the Saturnian System

Several interesting cloud and atmospheric features of the Saturn system have been observed by the long-wavelength channel of the two-channel ultraviolet photometer aboard the Pioneer Saturn spacecraft. Reported are observations of the most obvious features, including a Titan-associated cloud, a ring cloud, and the variation of atmospheric emission across Saturn's disk. The long-wavelength data for Titan suggest that a cloud of atomic hydrogen extends at least 5 Saturn radii along its orbit and about 1.5 Saturn radii vertically. A ring cloud, thought to be atomic hydrogen, has also been observed by the long-wavelength channel of the photometer; it shows significant enhancement in the vicinity of the B ring. Finally, spatially resolved observations of Saturn's disk show significant latitudinal variation. Possible explanations of the variation include aurora or limb brightening.

Amorphous Ice on Saturnian Rings and on Icy Satellites: Its Formation, Stability, and Observability

Saturnian rings and icy satellites may be covered with amorphous rather than crystalline ice. Its likely source is water molecules sputtered by particles in the radiation belts, it may be stable, and its presence could be deduced from the rate of temperature drop in a shadow. Observation of this effect is, however, difficult, especially for the rings. A possible relation to the brightness anisotropy of ring A is pointed out.