The pole orientation of asteroid 433 Eros determined by photometric astrometry

Refinements to the pole-determination method photometric astrometry (PA) were completed in 1983 (R. C. Taylor and E. F. Tedesco, 1983, Icarus54, 13–22). A goal is to redo the pole analysis for every asteroid whose pole had been determined from earlier versions of PA: Previous PA poles are reviewed in this paper. Asteroid 433 Eros is in that collection and has redone. The result are prograde rotation; a sidereal period of 0.219588 ± 0.000005 day; and a north pole at 22° longitude, +9° latitude. The uncertainty of the pole is 10°. The pole position of Eros determined by C.D. Vesely (1971, In Physical Studies of Minor Planets (T. Gehrels, Ed.), pp. 133–140, NASA SP-267) and Dunlap (1976, Icarus28, 69–78), using earlier versions of photometric astrometry, were within 21 and 7°, respectively, of the present result.     

Nitrate in the Greenland ice sheet in the years following the 1908 tunguska event

The Tunguska event on 30 June 1908 has been subjected to much speculation within different fields of research. Publication of the results of the 1961 expedition to the Tunguska area (Florensky, 1963) supports that a cometary impact caused the event. Based on this interpretation, calculations of the impactor energy release and explosion height have been reported by Ben-Menahem (1975), and velocity, mass, and density of the impactor by Petrov and Stulov (1975). Park (1978) and Turco et al., 1981, Turco et al., 1982, used these numbers to calculate a production of ca. 30 × 10⁶ tons of NO during atmospheric transit. This paper presents a high-resolution study of nitrate concentration in the Greenland ice sheet in ca. 10 years covering the Tunguska event. No signs of excess nitrate are found in three ice cores from two different sites in Greenland in the years following the Tunguska event. By comparing these results with results for other aerosols generally found in the ice, the lack of excess NO₃^? following the Tunguska event can be interpreted as indicating that the impactor nitrate production calculated by Park (1978) and Turco et al., 1981, Turco et al., 1982 are 1–2 orders of magnitude too high. To explain this it is suggested, from other lines of reasoning, that the impactor density determined by Petrov and Stulov (1975) probably is too low.     

Near-infrared studies of the satellites of Saturn and Uranus

New JHK photometry and spectrometry (1.4–2.6 μm) are presented for Enceladus, Hyperion, Phoebe, Umbriel, Titania, and Oberon. From spectral signatures, mainly in the 2-μm region, water ice is verified on Enceladus and identified on Hyperion and the three Uranian satellites. The JHK photometry shows that Phoebe is different from all other satellites and asteroids observed thus far. The new photometry corroborates the earlier conclusion by Cruikshank et al. (1977) Astrophys. J217, 1006–1010] that the Uranian satellites, as a class, have overall surface reflectances different from other water-ice-covered satellites, and the reason for the difference remains unclear. The diameters and the masses of the Uranian satellites are reviewed in light of the probable high albedo representative of ice-covered surfaces and the new dynamical studies by Greenberg, 1975, Greenberg, 1976, Greenberg, 1978.     

Near-infrared colorimetry of J6 Himalia and S9 Phoebe: A summary of 0.3- to 2.2-μm reflectances

VJHK measurements of J6 Himalia and S9 Phoebe, using the new NASA IRTF telescope, show that these objects have carbonaceous chondritic type colors in the 0.5- to 2.2-μm region. For Phoebe, this is in contrast to the JHK colors published by Cruikshank (1980), which indicated that the satellite's surface was unlike the material found on asteroids and on the dark side of Iapetus. J6 is known to have a low albedo from thermal infrared studies (Cruikshank, 1977), and the new VJHK observations of S9 imply that it also has a low albedo. The H and K reflectances of S9 are slightly lower than those of J6, suggesting some slight difference in surface composition or a contamination by foreign material. The conjectured low albedo of S9 can be tested with measurements in the thermal infrared.     

An explication of the radiometric method for size and albedo determination

A new radiometric model for disk-integrated photometry of asteroids is presented. With empirical support from photometry of Mercury and the Moon, the model assumes that observed sunward beaming of the infrared emission is due to craters. In contrast to earlier theoretical studies of the lunar emission, the observable flux ratio between a cratered sphere and a smooth sphere is calculated for large ranges in wavelength, temperature, and phase angle. Revised diameters and albedos based on the crater model are given for 84 asteroids. The revised values are in good agreement with Morrison's (1977) radiometric results. It is shown that the systematic discrepancy between radiometric and polarimetric albedos (Zellner and Gradie, 1976) is probably a double-valued function of albedo. Some typical geometric albedos from this paper, Morrison (1977), and Zellner and Gradie (1976), respectively, are: Ceres (0.050 ± 0.005, 0.053 ± 0.004, 0.068), Vesta (0.235 ± 0.032, 0235 ± 0.11, 0.271), mean C type (0.031 ± 0.009, 0.035 ± 0.009, 0.061 ± 0.005), mean S type (0.117 ± 0.030, 0.136 ± 0.032, 0.181 ± 0.23), and mean M type (0.105 ± 0.037, 0.115 ± 0.033, 0.157 ± 0.079). Areas of disagreement between radiometry and polarimetry are underscored, and research to resolve them is suggested.     

Ultraviolet observations of small bodies in the solar system by OAO-2

Broadband filter photometry from 2100 to 4300 Å has been obtained by OAO-2 for the following objects: The Galilean satellites; Titan; the rings of Saturn; and three asteroids. Agreement with independent ground-based photometry in the region of overlap is good. The previously known decrease in reflectivity from visual to ground-based ultraviolet wavelengths continues to 2590 Å for all these objects. Europa's reflectivity continues to decline towards 2110 Å, and the rings' reflectivity levels off from 2590 to 2110 Å. Other targets were too faint at 2110 Å to be measured reliably by OAO-2.The low ultraviolet albedo of Titan has important implications for that planet's atmospheric structure (Caldwell, Larach, and Danielson, 1973; Danielson, Caldwell, and Larach, 1973; Caldwell, 1974b). The ultraviolet reflectivity of Saturn's rings is suggestive of a two-component system, one being pure H₂O particles. The ultraviolet albedos of the Galilean satellites are consistent with existing upper limits for atmospheric abundances, but require either that former estimates of the fractional coverage of H₂O frost are too high, an unlikely circumstance, or that the frost has been darkened by some external agent in the space environment.     

The occulation of β Scorpii by Jupiter. IV. Diurnal temperature variations and the methane mixing ratio in the Jovian upper atmosphere

The nightime cooling of the Jovian atmosphere near the occulation level of 10¹⁴cm^?3 is calculated using the models of Strobel (1973) and Strobel and Smith (1973). The amount of cooling is found to depend on χ, the methane mixing ratio; μ the mean molecular weight; and the sunrise temperature. Using the range of sunrise (emersion) temperatures observed by Veverka et al. (1974), the overnight cooling is calculated to be 1.5–5.5°K, if reasonable assumptions are made for χ and μ. The argument may be reversed to show that the agreement in measured sunrise and sunset temperatures obtained by other observers of the β Sco occulation implies that χ cannot be significantly greater than the generally accepted value of 7 ×10^?4.     

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.     

Application of a radiative transfer model to bright icy satellites

A radiative transfer model, derived largely from the work of B.W. Hapke (1981, J. Geophys. Res.86, 3039–3054) and J.D. Goguen (1981, Ph.D. thesis, Cornell University, Ithaca, N.Y.), is fit to Voyager imaging observations of Europa, Mimas, Enceladus, and Rhea. It is possible to place constraints on the single-scattering albedo, the porosity of the optically active upper regolith, the single-particle phase functions, and, in the cases of Europa and Mimas, the mean slope angle of macroscopic surface features. The texture of the surfaces of the Saturnian satellites appears to be similar to the Earth's moon. However, Europa is found to have a distinctly more compact regolith and a more forward-scattering single-particle phase function.     

Approximate axial alignment times for spinning bodies

Using a different approximate scheme for each of two separate body shapes, we compute for torque-free rotating quasi-rigid bodies the rate of damping of the nutation angle due to internal energy dissipation. The first method calculates the alignment rate as well as the free precession period of a Maxwell body; it modifies, and thereby makes clearer, a similar calculation by Gerstenkorn (1967b). The second method deals with an elongated body and models it by symmetrically placing masses along a massless rod, where all energy dissipation occurs through the hysteresis loss of the strain energy stored during the precession motion. For both cases, the damping time is proportional to the material strength of the body and to its damping properties, and inversely proportional to the square of the body's radius and the cube of the rotation frequency.The results agree well with those of another approximate solution (Burns and Safronov, 1973) and numerically check that solution for the alignment times of asteroids. They show that essentially all observed asteroids align quite rapidly and thus explain why asteroids are observed in pure spin despite numerous misaligning collisions.     

Absolute spectrophotometry of Neptune: 3390 to 7800 Å

Absolute spectrophotometry of Neptune from 3390 to 7800 Å, with spectral resolution of 10 Å in the interval 3390–6055 and 20 Å in the interval 6055–7800 Å, is reported. The results are compared with filter photometry (Appleby, 1973; Wamsteker, 1973; Savage et al., 1980) and with synthetic spectra computed on the basis of a parameterization proposed by Podolak and Danielson (1977) for aerosol scattering and absorption. A CH₄/H₂ ratio of is derived for the convectively mixed part of Neptune's atmosphere, and constrains optical properties of hypothetical aerosol layers.     

A laboratory atlas of the 5ν1 NH3 absorption band at 6475 Å with applications to Jupiter and Saturn

The 5ν₁ absorption band of NH₃ is displayed from 6418 to 6550 Å. The total band intensity has been measured: S_B = 0.66 cm^?1m^?1amagat^?1. Line intensities and self-broadening coefficients have been measured for some of the prominent lines. Our line intensities are in good agreement with those of Rank et al. (1966), but are about 2 times greater than those of Mason (1970). The spectrum displayed was obtained photoelectrically at a pressure of 0.061 atm, and shows many more lines than the spectrum obtained by McBride and Nicholls (1972a) at a pressure of 0.39 atm. Therefore, our new measurements can provide the basis for making a more complete rotational analysis than those of McBride and Nicholls (1972a).Since the total band absorption has previously been measured by others on moderate resolution photoelectric scans of the spectra of Jupiter and Saturn, we can use the band intensity to derive the NH₃ abundance in the atmospheres of these two planets. The NH₃ abundances in a single vertical path obtained by this method are about 10m amagat for Jupiter and 2m amagat for Saturn. These results are in agreement with previous results obtained from higher resolution photographic spectra.     

Asteroidal collisions

Taff (1973) has concluded that asteroidal collisions rates are much lower than those found by previous authors. His calculations are found to be in error as a consequence of inclusion of an extraneous and incorrect factor of ～10^?5. The assumption of molecular chaos in the asteroid belt, while not strictly correct, is not an important source of error in calculations of asteroidal collision rates.     

Spatially resolved absolute spectrophotometry of Saturn: 3390 to 8080 Å

Absolute spectrophotometry of four regions on the visible disk of Saturn (north and south polar regions, equatorial band, south “temperate” region) from 3390 to 8080 Å is reported. Spectral resolution is 10 Å in the interval 3390–6055 Å, and 20 Å; aperture size is 1.92 arcsec. The explicit purpose of our observations was to provide ground-based photometric calibration for the Pioneer Saturn Imaging Photopolarimeter (IPP). We also compare our data with earlier spectrophotometric measurements of Saturn (R.L. Younkin and G. Munch, 1963,Mem. Soc. Roy. Sci. Liege7, 123–136; W.M. Irvine and A.P. Lane, 1971,Icarus16, 10–26; T.B. McCord, T.V. Johnson, and J.H. Elias, 1971,Astrophys. J.165, 413–424) and with the M. Podolak and R.EE. Danielson (1977)Icarus30, 479–492) parameterization of “Axel Dust.” The latter reproduces the broad features but not the details of the observed spectral reflectivity (I/F). We find that large depths of clear molecular hydrogen (>14 km-am in the temperate regions) are needed to match the observed upturn in reflectivity shortward of 3800 Å.     

Towards a more habitable Mars — or — the coming martian spring

Two schemes are presented in which matter is moved closer to Mars so as to increase the equinoctal precession period of Mars. In such a way the Martian springs, during which Mars is habitable in the long winter model of Sagan (1971), could be extended and, in fact, prolonged indefinitely. The schemes involve using solar energy to move the satellite Phobos and/or material from the asteroid belt. Difficulties exist with the proposals but they are presented to stimulate discussion.     

Neptune's rotation period: A correction and a speculation on the difference between photometric and spectroscopic results

An error in the Hayes and Belton (1977), Icarus32, 383–401) estimate of the rotation period of Neptune is corrected. If Neptune exhibits the same degree of limb darkening as Uranus near 4900 Å, the rotation period is 15.4 ± 3 hr. This value is compatible with a recent spectroscopic determination of Munch and Hippelein (1979) who find a period of 11.2_?1.2^+1.8 hr. However, if, as indirect evidence suggests, the law of darkening on Neptune at these wavelengths is less pronounced than on Uranus, then the above estimates may need to be lengthened by several hours. Recent photometric data are independently analyzed and are found to admit several possible periods, none of which can be confidently assumed to be correct. The period of Neptune most probably falls somewhere in the range 15–20 hr. The Hayes-Belton estimate of the period of Uranus is essentially unaffected by the above-mentioned error and remains at 24 ± 4 hr. All observers agree that the rotation period of Uranus is longer than that of Neptune.     

Chaotic behavior and the origin of the 31 Kirkwood gap

The sudden eccentricity increases discovered by J. Wisdom (Astron J.87, 577–593, 1982) are reproduced in numerical integrations of the planar-elliptic restricted three-body problem, verifying that this phenomenon is real. Maximum Lyapunov characteristic exponents for trajectories near the $\text{3}\text{1}$ commensurability are computed both with the mappings presented in Wisdom (1982) and by numerical integration of the planar-elliptic problem. In all cases the agreement is excellent, indicating that the mappings accurately reflect whether trajectories are chaotic or quasiperiodic. The mappings are used to trace out the chaotic zone near the $\text{3}\text{1}$ commensurability, both in the planar-elliptic problem and to a more limited extent in the three-dimensional elliptic problem. The outer boundary of the chaotic zone coincides with the boundary of the $\text{3}\text{1}$ Kirkwood gap in the actual distribution of asteroids within the errors of the asteroid orbital elements.     

Scattering of light from particulate surfaces: I. A laboratory assessment of multiple-scattering effects

A convenient photometric function for many particulate surfaces is the generalization of the Lommel-Seeliger law derived by Hapke (1963) and Irvine (1966). This generalization accounts for the effects of mutual shadowing among particles, but still assumes that multiple scattering within the surface layer can be neglected—an assumption which is evidently valid for dark surfaces. We describe a series of laboratory measurements which test the range of validity of this basic assumption, and the applicability of the Hapke-Irvine photometric function, for particulate surfaces whose normal reflectances ranges from 0.04 to 1.04. We find that multiple-scattering effects can be neglected, and that the Hapke-Irvine function can be used, for particulate surfaces whose normal reflectance is about 0.3, or less. The function is definitely inapplicable to surfaces whose normal reflectance exceeds 0.4.     

Some consequences of meteoroid impacts on Saturn's rings

High-velocity impacts of interplanetary meteoroids on Saturn's rings are discussed. It is shown that the neutral gas emitted by impact vaporization may be responsible, to a large part, for the observed neutral ring atmosphere. Both the predicted neutral gas injection rate and the gas temperature (or kinetic energy) are compatible with the measurements (see Broadfoot, A. L., B. R. Sandel, D. E. Shemansky, J. B. Holberg, G. R. Smith, D. F. Strobel, J. C. McConnell, S. Kumar, D. M. Hunten, S. K. Atreya, T. M. Dohnahne, H. W. Moos, J. L. Bertaux, J. E. Blamont, R. B. Pomphrey, and S. Linik, Science212, 206–211, 1981). Heavy ejecta particles produce a particulate ring “halo”. The physical properties of this halo are calculated, and it appears to be identical with the tenous particle population discussed by Baum and Kreidl (1982). Erosion of Saturn's ring particles, the resulting mass balance, and regolith formation are estimated. This provides some constraints on surface properties and optical albedo.     

A possible rotation period for the minor planet 164 Eva

The minor planet 164 Eva passed through opposition on December 1, 1975 with a magnitude B_opp = 11.3 mag. Photoelectric observations at the Observatory of Torino, Italy, were carried out in two nights on Oct. 27/28 and Nov. 11, each with a run of about 3 hr. Two further successful photoelectric observations were carried out at the OHP, France, each with a run of about 6 hr. From all observed parts of the lightcurve a resulting synodic period of rotation of about 27.3 hr can be deduced, with a range of the total amplitude of at least Δm = 0.07 mag. With this period of 27.3 hr the minor planet 164 Eva is one more long period object, falling now between 654 Zelinda (H. J. Schober, 1975, Astron. Astrophys.44, 85–89) and 139 Juewa (J. Goguen et al., 1976, Icarus29, 137–142), at the high end in the histogram of the distribution of minor planet rotation periods.