Normal Optical Depth Research Articles

Observations of the 22 April 1982 stellar occultation by Uranus and the rings

Abstract The 22 April 1982 stellar occultation of KME 14 by Uranus was observed from Tenerife, Canary Islands, using the Teide Observatory 1.5-m telescope. From model fits to the immersion and emersion ring profiles, we obtained (1) midtimes of the ring events with a typical uncertainty of 0.01 sec; (2) ring widths for rings 4, α, β, γ, δ, and ϵ with a typical uncertainty of a few tenths of a kilometer; and (3) equivalent widths and normal optical depths of all nine rings. The immersion planetary occultation was clouded out, but emersion was successfully observed, and the stratospheric temperature profile was obtained by numerical inversion. The profile shows a temperature maximum near the 8-μbar pressure level, characterized by T(8 μbar) = 141°K and T(8 μbar)–T(13 μbar) = 5°K for the sampled suboccultation latitude of −11°.9. Both the mean temperature and the temperature variations are consistent with the latitude-dependent atmospheric structure found by B. Sicardy et al. (1985, Icarus 64, 88–106) from widely separated observations of the same event.

Voyager 2 photopolarimeter experiment: Evidence for tenuous outer ring material at Saturn

A statistical analysis of the stellar occultation data from the Voyager 2 photopolarimeter indicates significant amounts of tenuous material in two regions exterior to Saturn's F ring. The first region has a radial width of approximately 200 km starting at a Saturn-centered radial distance of 141,650 km (1500 km outside of the F ring) and its normal optical depth is 0.012 ± 0.004. The second is a very broad region of enhanced opacity, at least 1000 km in radial width, beginning at approximately 144,090 km, with a normal optical depth of approximately 0.009 ± 0.004 (more than 100 times greater than the Saturn E ring).

The 1983 June 15 occultation by Neptune. I - Limits on a possible ring system

Observations on 15 June 1983 of an occultation of a star by Neptune from Mauna Kea, Mount Stromlo, Siding Spring, and the Kuiper Airborne Observatory show no evidence for equatorial rings between 25,300 and 200,000 km (R/N/ = 25,269 km). Within most of this region, the upper limit on the optical depth along the line of sight, for rings broader than 6 km , is 0.04, which corresponds to a normal optical depth of 0.016. These results rule out a Neptunian ring system similar to that of Saturn or Uranus, but not a system of low optical depth similar to the Jovian rings. The data show no features that appear likely to have been caused by material in the equatorial plane of Neptune near the Roche limit.

Evidence for material between Saturn's A and F rings from the Voyager 2 photopolarimeter experiment

The region in the Saturn system between the F ring and the outer edge of the A ring is an area that appears, in images from the imaging experiment, to be virtually devoid of material except for three small satellites. Near the orbit of 1980S28, Atlas—the innermost satellite—the Voyager Photopolarimeter Stellar Occultation data show a discontinuity in count rate which marks a boundary between the tenuous materials near the outer edge of the A ring and the orbit of Atlas. The data pertaining to this region have been examined with the aid of statistics and models generated from other similar ring structures. It is concluded that the discontinuity is real, implying the existence of tenuous material of normal optical depth of 0.01 to 0.006 in this region.

The eccentric Saturnian ringlets at 1.29 Rs and 1.45 Rs

The shapes and kinematics of the two major eccentric ringlets in Saturn's C ring are studied in data acquired by four Voyager experiments: imaging science (ISS), radio science (RSS), ultraviolet spectrometer (UVS), and photopolarimeter (PPS). It is found that the ringlets have mean widths of ∼25 km (Titan, 1.29 R s) and ∼64 km (Maxwell, 1.45 R s), eccentricities of order 10 −4, sharp edges on a scale of ∼1 km, normal optical depths τ ∼ 1−2, and are embedded in essentially empty gaps ( τ < 0.05). In addition, they exhibit positive linear width-radius relations, suggesting that differential precession across the ringlets is being prevented by the self-gravity of the ring particles. The kinematics of the Maxwell ringlet are determined solely by Saturn's nonspherical gravity field; the kinematics of the Titan ringlet are apparently determined by its interaction with Titan. Masses, mean surface mass densities, and mass extinction coefficients have been calculated. The comparatively large optical depths and mass extinction coefficients in these features suggest an environment and particle size distribution different from the remainder of the C ring and presumably caused by the mechanism responsible for ring confinement.

Infrared observations of the Uranian rings

ABSTRACTWe present maps of the Uranian rings at an effective wavelength of 2.2 µm, and spectrophotometric observations covering the ranges 2.0 – 2.4 µm and 2.9 – 3.9 µm. The maps, constructed from data obtained in May 1978 and July 1979, reveal a 2:1 azimuthal brightness variation whose amplitude and phase are consistent with the known width variation and apsidal precession of the eccentric e ring. A detailed comparison of the maps with models based on stellar occultation data leads to an upper limit of ~0.01 on the normal optical depth of any broad (5000 km wide), diffuse component of the ring system. An unexplained feature of the 1978 map is a significant east-west asymmetry, corresponding to a brightening of the near side of the rings relative to the far side.

Maps of the rings of Uranus at a wavelength of 2.2 microns

Maps of the ring system of Uranus, as seen in reflected sunlight at a wavelength of 2.2 μm, are presented for May 1978 and July 1979. Large azimuthal brightness variations revealed by these maps are consistent with the variable width established for the ϵ ring from occultation studies and support the existing elliptical model for this ring. An upper limit of ∼0.010 is placed on the normal optical depth of any broad (∼5000 km wide) axisymmetric ring component. There exists, however, an unexplained east-west asymmetry in the May 1978 map. In addition, multiaperture broadband infrared photometry of Uranus is reported, from which the geometric albedo of the planet at wavelengths of 1.25, 1.65, and 2.2 μm and the geometric albedo of the rings at 2.2 μm are derived.

Limit on possible narrow rings around Jupiter

An upper limit to the optical depth of the Jovian ring at high spatial resolution, determined from stellar occultation data, is reported. The spatial resolution of the observation is limited to about 13 km in Jupiter's equatorial plane by the projection of the Fresnel zone on the equatorial plane in the radial direction. At this resolution, the normal optical depth limit is about 0.008. This limit applies to a strip in the Jovian equatorial plane that crosses the orbits of Amalthea, 1979J1, 1979J3, and the ring. An upper limit on the number density of kilometer-size boulders has been set at one per 11.000 sq km in the equatorial plane.

VLA observations of Saturn at 1.3, 2, and 6 cm

Radio maps with a resolution of 1″.5 were made of Saturn at 1.3, 2, and 6 cm. The inclination of the ring plane was −5°.4. A fraction of 0.49 ± 0.08 of Saturn's emitted light is transmitted through the rings at λ2 and λ6 cm. This value converts to an effective head-on or normal optical depth of 0.07 ± 0.02. The transparency at this small inclination angle must be provided entirely by the regions of very low optical depth, e.g., the Cassini and Encke's Divisions, and to achieve our number this requires either a relatively large fraction of gaps or a strong forward scattering by the ring particles. The planetary disk appears to be much less limb darkened in the N-S than the E-W direction, while cuts across the planet, averaged over all directions, agree with the theoretical limb-darkening curves for a planet with a uniform atmosphere and solar abundances for all chemical elements.

Radial widths, optical depths, and eccentricities of the Uranian rings

Observations of the stellar occultation by the Uranian rings of 15/16 August 1980 are used to estimate radial widths and normal optical depths for segments of rings 6, 5, 4, ɑ, β, η, y, and δ. Synthetic occultation profiles are generated to match the observed light curves. A review of published data confirms the existence of width-radius relations for rings ɑ and β, and indicates that the optical depths of these two rings vary inversely with their radial widths. Masses are obtained for rings ɑ and β, on the assumption that differential precession is prevented by their self-gravity. A quantitative comparison of seven є-ring occultation profiles obtained over a period of 3.4 yr reveals a consistent structure, which may reflect the presence of unresolved gaps and subrings. Elliptical models for rings 6, 5, 4, ɑ, β, and є are presented for comparison with the results of previous studies, particularly that of Elliot et al. (1981a).

The Jovian ring

The results of further measurements of the Jovian ring system are presented. The system has three major components: the bright ring, the faint sheet, and the out‐of‐plane halo. The bright ring has an outer radius of 1.81±0.01 RJ, an inner radius 1.72±0.01 RJ, an eccentricity not greater than 0.003 and a normal optical depth 3 × 10−5. The faint sheet extends from the inner edge of the bright ring to the surface of Jupiter. Its optical depth is approximately 7 × 10−6. Three arguments are presented to show that a halo of material envelops the above two rings and extends 104 km above the ring plane. A simple model is invoked to account for the halo by means of interactions between the Jovian magnetic field and charged ring particles less than 0.5 µm in diameter. The source of small particles is probably within the bright ring itself and may be due to micrometeorite impact into larger ring bodies. Small particles evolve in towards Jupiter under Poynting Robertson and other drag forces. The outer edge of the ring system is defined by the satellite 1979J1.

Radio Science Investigations of the Saturn System with Voyager 1: Preliminary Results

Voyager 1 radio occultation measurements of Titan's equatorial atmosphere successfully probed to the surface, which is provisionally placed at a radius of 2570 kilometers. Derived scale heights plus other experimental and theoretical results indicate that molecular nitrogen is the predominant atmospheric constituent. The surface pressure and temperature appear to be about 1.6 bars and 93 K, respectively. The main clouds are probably methane ice, although some condensation of nitrogen cannot be ruled out. Solar abundance arguments suggest and the measurements allow large quantities of surface methane near its triple-point temperature, so that the three phases of methane could play roles in the atmosphere and on the surface of Titan similar to those of water on Earth. Radio occultation measurements of Saturn's atmosphere near 75 degrees south latitude reached a maximum pressure of 1.4 bars, where the temperature is about 156 K. The minimum temperature is about 91 K near the 60-millibar pressure level. The measured part of the polar ionosphere of Saturn has a peak electron concentration of 2.3 x 10(4) per cubic centimeter at an altitude of 2500 kilometers above the 1-bar level in the atmosphere, and a plasma scale height at the top of the ionosphere of 560 kilometers. Attenuation of monochromatic radiation at a wavelength of 3.6 centimeters propagating obliquely through Saturn's rings is consistent with traditional values for the normal optical depth of the rings, but the near-forward scattering of this radiation by the rings indicates effective scattering particles with larger than expected diameters of 10, 8, and 2 meters in the A ring, the outer Cassini division, and the C ring, respectively. Preliminary analysis of the radio tracking data yields new values for the masses of Rhea and Titan of 4.4 +/- 0.3 x 10(-6) and 236.64 +/- 0.08 x 10(-6) times the mass of Saturn. Corresponding values for the mean densities of these objects are 1.33 +/- 0.10 and about 1.89 grams per cubic centimeter. The density of Rhea is consistent with a solar-composition mix of anhydrous rock and volatiles, while Titan is apparently enriched in silicates relative to the solar composition.

Temperatures and optical depths of Saturn's rings and a brightness temperature for Titan

The Pioneer Saturn infrared radiometer viewed Saturn's rings at 20‐ and 45‐µm wavelength under several conditions of illumination. The data are analyzed to infer radial locations of major ring boundaries, temperatures and temperature gradients, and normal optical depths. Error bounds on the above inferred quantities are given. Most ring boundaries are defined to ±0.01 Rs(1 Rs≡6 × 104 km) and are in good agreement with those inferred from the imaging photopolarimeter experiment. Temperatures generally decrease with radial distance from the planet. A significant temperature gradient exists from the colder north (unilluminated) side of the rings to the warmer south side. The gradient appears to be steepest on the south side. Ring optical depths are greater than some previously published values and are approximately 0.1 for the Cassini division and the C ring. In addition, the C ring optical depth decreases towards the planet. The temperature drop during eclipse is ≳10 K, implying low thermal inertia for the ring particles. Titan's 45‐µm brightness temperature is 75±5 K, in good agreement with earth‐based observations.

Photometry and polarimetry of Saturn's rings from Pioneer Saturn

We present a profile of the average normal optical depth for Saturn's rings between 1.22 and 2.35 Rs. In the A and B rings, horizontal inhomogeneities make these values deceptive. A thinner component of the B ring with optical depth less than 0.08 covers ½% to 4% of its surface area. In the A ring the more transparent component has optical depth τs &gt;0.10 and covers more than 7% of its area. These thinner parts of the rings would rarely be apparent from earth‐based observations. The particles in the C ring are larger than 15µ and different from those in the B and A rings. The C ring is either homogeneous with high albedo (in the red) and forward scattering phase functions or shows a gradient in albedo with distance from Saturn. In the latter case the red albedo ranges from at the inner edge of ring C to at the outer edge, and the phase function is both moderately forward and backward scattering. Polarimetry of Saturn's rings provides only an upper limit for the B ring polarization, p &lt;15%, which is consistent with predictions from ground‐based data. In the outer A ring, the polarization is negative p(blue) = −7±2%; p(red) = −1±2%. Azimuthal variations are seen on the darkside of the A ring with a peak‐to‐trough ratio of 1.25. The position angle of the maximum is close to that seen from earth. No variations are seen in the B and C rings with an upper limit of 6%. The measured color of the A ring supports the many particle thick model of Saturn's rings.

Saturn's ring and nearby faint satellites

Observations of Saturn's rings during passage of the Earth through the ring plane, coupled with those of others, suggest a ring thickness of 1.3 ± 0.3 km. The wide disparity in the optical depth of Cassini's division found by other investigators is resolved, and for conservative isotropic single scattering, a normal optical depth for Cassini's division of 0.060 ± 0.006 is obtained. We find the mean normal optical depth of ring C to be 0.074 ± 0.007. Analysis of all available observations of faint objects near Saturn indicates the presence of at least one previously undiscovered satellite of Saturn. The orbit for Janus determined by Dollfus is supported. These satellites may be major members of an extended ring.

Wavelength Dependence of Polarization. XVI. Atmosphere of Venus

view Abstract Citations (40) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Wavelength Dependence of Polarization. XVI. Atmosphere of Venus Coffeen, David L. Abstract The observations of polarization of sunlight scattered by the Venus atmosphere are compared with cal- culations for various light-scattering mechanisms. Indications of ultraviolet Rayleigh scattering are consist- ent with a normal Rayleigh optical depth of about 0.07, at 3400 A, above the level at which the cloud scattering optical depth is unity. This amount corresponds to an upper limit of 55 mb for the gas pressure at this level. To analyze the cloud polarization, calculations of single scattering from spheres were made with the Mie computer program of Herman and Browning. The percentage polarization of the scattered light was obtained for 300 values of the size parameter (ratio of particle circumference to wavelength) for each of 19 scattering angles and 22 refractive indices. Then the polarizations were smoothed at each size by averaging over a neighboring size distribution [n (x) , from 0.75x, to 1. , thereby removing the dominant interference effects of discrete sizes. The resulting patterns of polarization (plotted as contours on a scatter- ing-angle-mean particle-size map) exhibit a regular variation as a function of refractive index. A comparison of the computed contours of zero polarization with the observations, corrected by subtraction of the Rayleigh polarization contribution, rules out all sizes of water spheres as the dominant aerosol scatterers. The best fit is for spheres having little absorption, with real refractive indices between 1.43 and 1.55, and mean diam- eters of 2.5+0.5 ,i. The observed polarization is at all phases smaller than the theoretical, apparently a consequence of dilution of single-scattering polarization by multiply scattered photons; the average dilution factor varies from 2.5 at 120 phase angle to about 20 at 10 . In an environment of and K, many liquids would have a saturation vapor pressure greater than the ambient pressure and therefore could not condense, while others can be excluded by spectroscopic evidence. The refractive index should in any case fall within the above range, ruling out spheres of H2O, CO2, NH2, SO2, etc. The Mie theory is not precisely applicable to aspherical particles, even for random orientations. Nevertheless, solid aerosols are considered, within the size and refractive index limits defined by the good fit between Mie theory and obser- vation. The thermodynamic (vapor pressure) and spectroscopic requirements are readily satisfied by a solid, and a number of common minerals have appropriate refractive indices, including SiO2 and NaCl. Publication: The Astronomical Journal Pub Date: April 1969 DOI: 10.1086/110823 Bibcode: 1969AJ.....74..446C full text sources ADS | Related Materials (44) Part 1: 1960AJ.....65..466G Part 2: 1960PGLO...40..467G Part 3: 1960AJ.....65..470G Part 4: 1960PGLO...41..470G Part 5: 1964AJ.....69..826G Part 6: 1964PGLO...62..826G Part 7: 1965AJ.....70..447G Part 8: 1965AJ.....70..403C Part 9: 1965AJ.....70..579G Part 10: 1966AJ.....71...62G Part 11: 1966AJ.....71..111G Part 12: 1966AJ.....71..355C Part 13: 1967AJ.....72..631G Part 14: 1967AJ.....72..887C Part 15: 1968AJ.....73...20C Part 16: 1968AJ.....73..677K Part 17: 1969AJ.....74...85S Part 18: 1969AJ.....74..190G Part 19: 1969AJ.....74..433C Part 21: 1969AJ.....74..528C Part 22: 1969AJ.....74.1179C Part 23: 1969AJ.....74.1066P Part 24: 1970AJ.....75...54C Part 25: 1970AJ.....75..182Z Part 26: 1971AJ.....76..645Z Part 27: 1971AJ.....76..651Z Part 28: 1971AJ.....76..576K Part 29: 1974AJ.....79..565C Part 30: 1974AJ.....79..581C Part 31: 1974AJ.....79..590G Part 32: 1975AJ.....80..595S Part 33: 1975AJ.....80..602S Part 34: 1975AJ.....80..702C Part 35: 1976AJ.....81..641K Part 36: 1977AJ.....82..908C Part 37: 1979AJ.....84..356C Part 38: 1979AJ.....84..671G Part 39: 1979AJ.....84.1244S Part 40: 1979AJ.....84..683L Part 41: 1979AJ.....84.1200C Part 42: 1979AJ.....84.1419D Part 43: 1979AJ.....84.1802S Part 44: 1980AJ.....85..564S Part 45: 1980AJ.....85..751S