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NASA NTRS1977

The constitution of cometary nuclei

The nongravitational term in the expression for the total force acting on a comet is calculated, and an upper limit is obtained for the product of the radial nongravitational term times the radius times the square root of the albedo. This condition is satisfied for ten periodic comets with q no greater than 1.5 AU, and the activity of these comets is consistent with control by H2O ice. Some of the comets must be spotty to account for their low albedo values. The effect of cosmic rays on comets, leading to frosting of their surface, is discussed.

Whipple, F. L.↗

NASA NTRS1978

Lunar occultation of Saturn. III - How big is Iapetus

The orbital light curve of Iapetus is analyzed, and the bright material on the trailing face of the satellite is found to have an effective limb-darkening parameter of 0.62 (+0.10, -0.12). This result is combined with observations of a lunar occultation of Iapetus to derive a radius of 718 (+87, -78) km for the satellite. The specific patchy albedo model of Morrison et al. (1975) is shown to fit the observed orbital light curve precisely and to yield an occultation radius of 724 + or - 60 km. It is noted that the values determined for the limb-darkening parameter and for the normal reflectance (in V) of the material at the center of the bright face are consistent with a partial water-frost cover on the bright side of Iapetus. It is concluded that if the albedo model of Morrison et al. reliably represents the surface of Iapetus, the radius of that satellite is 724 + or - 60 km, which is consistent with the radiometric radius of 835 (+50, -75) km derived by Morrison et al. (1975).

Veverka, J.↗

NASA NTRS1978

Measurement of tropospheric 300 nm solar ultraviolet flux for determination of O/1D/ photoproduction rate

The tropospheric importance of the OH radical, and the reaction scheme that leads to its formation, are now being widely investigated. Ozone photolysis at wavelengths no greater than 318 nm produces O(1D), a small fraction of which then reacts with water vapor to yield OH. Although experimental data are available for the O(1D) quantum yield, as well as the O3 absorption cross section, all previous tropospheric photochemical models have had to use theoretical calculations to determine the UV flux. Discussed in this paper are aircraft spectral measurements of the solar UV flux at two altitudes - 2 and 6 km. These results have been compared with three theoretical approaches. The measured experimental fluxes have been combined here with recent quantum yield data to calculate the O(1D) photoproduction rate for various albedo values. This rate is larger than that used in models by about a factor of 2 for reasonable values of assumed albedo.

Sellers, B.↗

NASA NTRS1978

The moon in Heiligenschein

An analysis of 25 photometric digital images of the moon has been carried out to obtain a single image in a new mapping parameter, the Heiligenschein exponent. The data necessarily represent a range of lunar phases, but all are within 10 hours of full moon. The new parameter characterizes the rate at which lunar features brighten as their local phase angles approach zero. Although considerable contrast is present in this parameter, there is only a small corelation with normal albedo. In particular, the large albedo difference between maria and highlands is not simply reflected in Heiligenschein differences, which are larger within each category of terrain than the difference between the Heiligenschein averages of each. A correlation with age may be present in both the maria and the highlands, but its determination will require separation into distinct geochemical provinces.

Wildey, R. L.↗

NASA NTRS1978

Global color variations on the Martian surface

Spectral reflectance data from the Viking-II Orbiter vidicon cameras have provided a means of mapping and classifying Martian surface materials over about 65% of the planet. The imagery, acquired at three wavelengths, was processed to remove an average Martian photometric function; albedo maps at the three wavelengths were created. Analysis of the imagery showed that the dark region between the equator and about 30 deg S in the Martian highlands consists of ancient ridges, crater rims and rugged plateaus which are very red, and younger intercrater volcanic plains exhibiting mare ridges which are much less red. In addition probable eolian deposits were found in the intermediate and high albedo regions.

Soderblom, L. A.↗

NASA NTRS1979

Geologic applications of thermal-inertia mapping from satellite

The author has identified the following significant results. After digitization, a noise rejection filter was applied to data obtained by USGS aircraft. An albedo image was formed by combining three bands of visible data. Along with the day and nighttime thermal data, the albedo image was used to construct a relative thermal-inertia image. This image, registered to a topographic base, shows there are thermal property differences in the vicinity of the contact between the Fort Union and Wasatch formations in the Powder River Basin, Wyoming.

Offield, T. W.↗

NASA NTRS1978

Calculation of the relative influence of cloud layers on received ultraviolet and integrated solar radiation

Discrete ordinate radiative transfer calculations are used to examine the relative influence of cloud layers on the received ultraviolet flux and the received solar energy for a plane parallel atmosphere. The wavelength dependence for the cloud influence on the integrated solar radiation is primarily a function of the interaction of the cloud layer with the surrounding atmosphere and the underlying surface. It is found that for uv wavelengths greater than 300 nm, the ratio of uv to solar energy flux transmission through the atmosphere is insensitive to changes of cloud height, cloud scattering parameters and surface albedo (for realistic values), but is dependent on cloud thickness. For wavelengths approaching and less than 300 nm, absorption by tropospheric ozone results in sensitivity to cloud height and surface albedo.

Spinhirne, J. D.↗

NASA NTRS1979

On the nature of the blue and ultraviolet absorption in the Jovian atmosphere

Spatially resolved reflectivities from 3000 to 6600 A of three positions from the center to the limb of the Jovian Equator, North Equatorial Belt, and North Tropical Zone are analyzed to determine the vertical distribution and wavelength dependence of various sources of blue and uv absorption. Six different models of the distribution of absorbing dust particles are examined. In each model, the variation of dust optical depth and cloud single-scattering albedo are determined. Only those models having dust above the upper NH3 cloud layer will fit the data. The high altitude dust distribution is approximately uniform over the three regions examined. The contrast in reflectivity of the belts and zones may be modeled by a different cloud single-scattering albedo in the different regions.

Cochran, W. D.↗

NASA NTRS1979

Scattering of emitted radiation from inhomogeneous and nonisothermal layers

A parametric study is performed for the exiting monochromatic intensities scattered from finite plane-parallel inhomogeneous layers that are driven solely by a distribution of thermal sources. Intensities are obtained by invariantly imbedding the standard and thermal scattering functions. The single-scattering albedo and the Henyey-Greenstein phase-function parameter are varied independently, and both linear and exponential profiles are considered. Linear temperature profiles are used, including temperature inversions. The resulting intensities, as a function of the direction cosine of propagation, are discussed from a remote-sensing point of view. For an isothermal and homogeneous medium, the gross characteristics of the exiting intensity, represented by its overall slope, mean value (magnitude), and an interior maximum value, can be related to the total optical depth, single-scattering albedo, and phase function, respectively. For a homogeneous medium, linearly decreasing (in the line of sight) temperature profiles tend to obscure the phase-function information and decrease the apparent optical depth. On the other hand, linearly increasing temperature profiles tend to retain phase-function information and increase the apparent optical depth. Temperature inversion profiles give intensities very similar to those for purely linear profiles.

Bergstrom, R. W.↗

NASA NTRS1978

Mare basalt types on the front side of the moon - A summary of spectral reflectance data

A unit map of nearside basalt types has been prepared from all telescopic spectral reflectance data currently available for lunar soils. Four parameters were chosen (UV/VIS ratio, albedo, 1 micron band strength, 2 micron band strength) to distinguish and map each of 13 mare basalt types and three additional volcanic groups. Multispectral imagery and albedo maps were used to define unit boundaries while spectra were used to examine the 1 and 2 micron bands and calibrate and quantify the multispectral images. Although the volume of each basalt type is not known, it is clear from the unit map that only 1/3 to 1/2 of the surface basalt types are likely to be represented in the returned lunar samples. For mature lunar soils a single parameter alone does not provide chemical information, but when the four are used together, TiO2 and in some cases FeO can be estimated. Further study of the mineralogy of unsampled lunar basalts requires precise spectra to 2.5 microns with higher spectra and spatial resolution.

Pieters, C. M.↗

NASA NTRS1979

Methane band limb-brightening on Uranus

Area scanner measurements of the brightness distributions of Uranus at 6300 and 7250 A are presented. At the former wavelength, which is characteristic of continuum radiation, the observed limb-darkening is consistent with the results of Stratoscope observations. At the latter wavelength, in the center of a strong methane absorption band, the planet shows substantial limb-brightening as has been reported by other investigators. The limb-brightening and geometric albedo of Uranus in this methane band can be explained in terms of a clear atmospheric model in which the methane partial pressure above the saturation level is equal to its equilibrium vapor pressure. This conclusion is relatively insensitive to the methane mixing ratio below the saturation level. The presence of a high-altitude, conservatively, isotropically scattering haze of optical thickness 0.1, such as that previously proposed as a possible cause of the 7250-A limb-brightening, would produce a geometric albedo at this wavelength that is substantially larger than the value observed for Uranus.

Pilcher, C. B.↗

NASA NTRS1979

A stability theorem for energy-balance climate models

The paper treats the stability of steady-state solutions of some simple, latitude-dependent, energy-balance climate models. For north-south symmetric solutions of models with an ice-cap-type albedo feedback, and for the sum of horizontal transport and infrared radiation given by a linear operator, it is possible to prove a 'slope stability' theorem, i.e., if the local slope of the steady-state iceline latitude versus solar constant curve is positive (negative) the steady-state solution is stable (unstable). Certain rather weak restrictions on the albedo function and on the heat transport are required for the proof, and their physical basis is discussed.

Cahalan, R. F.↗

NASA NTRS1979

Perturbation of the zonal radiation balance by a stratospheric aerosol layer

The effect of stratospheric aerosols on the earth's monthly zonal radiation balance is investigated using a model layer consisting of 75% H2SO4, the primary constituent of the background aerosol layer. The reduction in solar energy absorbed by the earth-atmosphere system is determined through the albedo sensitivity, and the optically thin approximation is used in conjunction with the Henyey-Greenstein phase function for scattering. An infrared radiative transfer model is used to estimate the increased greenhouse effect from the aerosol layer, and the infrared heating compensates for the albedo effect in altering the radiation balance. The results indicate that the dominant influence of the thin model stratospheric aerosol layer is an increased reflection of solar energy all over the globe except for the polar-winter region, but the change in the radiation balance is uniform and small equatorward of 50 deg.

Harshvardhan, MR.↗

NASA NTRS1979

The diameter and reflectance of Triton

The surface reflectance, radius and geometric albedo of Triton are investigated. Image tube spectra of the reflectance of the satellite between 3200 and 7400 A are presented which indicate that the surface is composed mainly of rocky (non-icy) material, with violet absorption suggesting a composition similar to that of lunar fines, terrestrial igneous rocks and chondrites. Photometric/radiometric calculations based on measurements of the 20-micron thermal flux and the known visual magnitude of Triton reveal an upper limit of 5280 km to its diameter, and a lower limit of 0.19 to its geometric albedo. Computations of Rayleigh scattering in a potential inert atmospheric component are also presented which appear to preclude the presence of large quantities of nitrogen and noble gases on Triton.

Cruikshank, D. P.↗

NASA NTRS1979

The performance of a seasonal global climatic model

A seasonal global climatic model developed by Sellers is studied. Several changes are made to correct errors in the original model and to allow its use in a time dependent simulation mode. The major changes are in the infrared radiation formulation and in the size of the time steps. The seasonal cycles of surface temperature, wind, ice, snow, albedo, horizontal heat transport, and vertical flux of solar and infrared radiation are compared to recent observations. The seasonal cycle of temperature is fairly well reproduced, but has too small an amplitude in the Northern Hemisphere high latitudes. An overestimate of albedo at the poles is found, which is related to errors in some of the other variables. The sensitivity of the model to CO2 and solar constant (O) changes is summarized. Doubling the CO2 amount causes a 2.45 K increase in global average surface temperature (T). Increasing Q by 1% increases T by 3.14K. Decreasing 0 by 1% decreases T by 4.65K. An annual average version of the model is more sensitive to changes in Q than the model with the seasonal cycle.

Robock, A.↗

NASA NTRS1979

Free oscillations in a climate model with ice-sheet dynamics

A study of stable periodic solutions to a simple nonlinear model of the ocean-atmosphere-ice system is presented. The model has two dependent variables: ocean-atmosphere temperature and latitudinal extent of the ice cover. No explicit dependence on latitude is considered in the model. Hence all variables depend only on time and the model consists of a coupled set of nonlinear ordinary differential equations. The globally averaged ocean-atmosphere temperature in the model is governed by the radiation balance. The reflectivity to incoming solar radiation, i.e., the planetary albedo, includes separate contributions from sea ice and from continental ice sheets. The major physical mechanisms active in the model are (1) albedo-temperature feedback, (2) continental ice-sheet dynamics and (3) precipitation-rate variations. The model has three-equilibrium solutions, two of which are linearly unstable, while one is linearly stable. For some choices of parameters, the stability picture changes and sustained, finite-amplitude oscillations obtain around the previously stable equilibrium solution. The physical interpretation of these oscillations points to the possibility of internal mechanisms playing a role in glaciation cycles.

Kallen, E.↗

NASA NTRS1979

Evidence for early volcanism in mare Smythii

The major geologic units in and around the Smythii basin are reflected in the Al2O3 and MgO values obtained using improved orbital X-ray fluorescence data from Apollo 15. One of these units is a high albedo plains-forming unit that has Al2O3 and MgO concentrations different from those of similar plains-forming units located in the terra west of Smythii and south of Crisium and also different from those of adjacent terra and mare materials. The chemical composition of the plainsforming unit suggests a volcanic origin; the inferred Al2O3 and MgO-rich nature of this presumed early volcanic material is supported by the fact that even the later, low albedo, lightly cratered mare fill in Mare Smythii has a high MgO value, and its Al2O3 concentration is higher than that of similar basaltic material in other maria.

Conca, J.↗

NASA NTRS1979

Colorimetry and magnitudes of asteroids

In the present paper, 1500 UBV observations are analyzed by a new rather general multiple scattering theory which provided clear insight into previously poorly-recognized optical nature of asteroid surfaces. Thus, phase curves are shown to consist of a surface-texture controlled component, due to singly scattered light, and a component due to multiple scattering. Phase curve shapes can be characterized by a single parameter, the multiple scattering factor, Q. As Q increases, the relative importance of the opposition effect diminishes. Asteroid surfaces are particulate and strikingly similar to texture, being moderately porous and moderately rough on a scale greater than the wavelength of light. In concequence, Q (and also the phase coefficient) correlate well with geometric albedo, and there exists a purely photometric means of determining albedos and diameters.

Bowell, E.↗