C. W. Hord

Photopolarimetry from voyager 2; preliminary results on saturn, titan, and the rings.

The Voyager 2 photopolarimeter was reprogrammed prior to the August 1981 Saturn encounter to perform orthogonal-polarization, two-color measurements on Saturn, Titan, and the rings. Saturns atmosphere has ultraviolet limb brightening in the mid-latitudes and pronounced polar darkening north of 65�N. Titans opaque atmosphere shows strong positive polarization at all phase angles (2.7� to 154�), and no single-size spherical particle model appears to fit the data. A single radial stellar occultation of the darkened, shadowed rings indicated a ring thickness of less than 200 meters at several locations and clear evidence for density waves caused by satellite resonances. Multiple, very narrow strands of material were found in the Encke division and within the brightest single strand of the F ring.

Mariner 9 Ultraviolet Spectrometer Experiment: Mars Airglow Spectroscopy and Variations in Lyman Alpha

Abstract Mariner 9 ultraviolet spectrometer observations show the Mars airglow consists principally of emissions that arise from the interaction of solar ultraviolet radiation with carbon dioxide, the principal constituent of the Mars atmosphere. Two minor constituents, atomic hydrogen and atomic oxygen, also produce airglow emissions. The airglow measurements show that ionized carbon dioxide is only a minor constituent of the ionosphere. Using the airglow measurements of atomic oxygen, it is possible to infer that the major ion is ionized molecular oxygen. The escape rate of atomic hydrogen measured by Mariner 9 is approximately the same as that measured two years earlier by Mariner 6 and 7. If the current escape rate has been operating for 4.5 billion years and if water vapor is the ultimate source, an amount of oxygen has been generated that is far in excess of that observed at present. Mariner 9 observations of Mars Lyman alpha emission over a period of 120 days show variations of 20%.

Mariner Ultraviolet Spectrometer: Topography and Polar Cap

Mars, the red planet, reflects sunlight in the ultraviolet, but it is the atmosphere, not the surface, that is responsible for the reflected light. Even though there are atmospheric scatterers in addition to the molecular scatterers, it is possible to relate the intensity of the scattered radiation with the atmospheric pressure. The variation of pressure over the planet reveals the topography to vary over 7 kilometers in height and to be correlated with visible features. The carbon dioxide polar cap, in addition to being a cold trap for volatile gases in the atmosphere, may alsobe a very efficient adsorption trap for nonvolatiles. This last property may make the cap a repository for gases produced by geological or biological activity (15).

Mariner 9 ultraviolet spectrometer experiment: Structure of Mars' upper atmosphere

Analysis of 18 observations of the limb intensity profile of the CO Cameron bands in the Martian airglow shows that the equivalent subsolar zenith intensity, ICAM, is related to the Ottawa 10.7 cm radio flux index, F10.7, by the expression ICAM = 0.062(74 + F10.7)kR, with a correlation coefficient of 0.80. Comparison of averaged limb intensities of the CO2+ doublet and the Cameron bands on four favorable occasions is consistent with the intensities being directly proportional, in the ratio 0.24:1. The mean of 18 Cameron band topside scale heights is 17.8 km, corresponding to an exospheric temperature of 325°K, and the largest and smallest values observed differ by 9.5 km. These observations are in accord with theoretical predictions within the uncertainties in the latter. However, the solar EUV flux used in these predictions is a factor of at least two too weak to produce the electron densities measured by the S-band occultation experiment.

Mariner 9 ultraviolet spectrometer experiment - Initial results.

The ultraviolet airglow spectrum of Mars has been measured from an orbiting spacecraft during a 30-day period in November-December 1971. The emission rates of the carbon monoxide Cameron and fourth positive bands, the atomic oxygen 1304-angstrom line and the atomic hydrogen 1216-angstrom line have been measured as a function of altitude. Significant variations in the scale height of the CO Cameron band airglow have been observed during a period of variable solar activity; however, the atomic oxygen and hydrogen airglow lines are present during all the observations. Measurements of the reflectance of the lower atmosphere of Mars show the spectral characteristics of particle scattering and a magnitude that is about 50 percent of that measured during the Mariner 6 and 7 experiments in 1969. The variation of reflectance across the planet may be represented by a model in which the dominant scatterer is dust that absorbs in the ultraviolet and has an optical depth greater than 1. The atmosphere above the polar region is clearer than over the rest of the planet.

Galileo ultraviolet spectrometer observations of atomic hydrogen in the atmosphere of Ganymede

Atomic hydrogen Lyman alpha radiation (121.6 nm) has been measured in emission from the atmosphere of Ganymede with the Galileo ultraviolet spectrometer. An exospheric model with the following parameters has been fit to the observational data: atomic hydrogen density directly above the surface (radius 2634 km) equal to 1.5 × 104 atoms cm−3 scale height 2634 km, exospheric temperature 450 K. A model calculation of the photodissociation of water vapor from surface ice at 146 K is used to obtain the photodissociation rate necessary to supply the hydrogen atoms that are escaping from the exosphere of Ganymede. The calculated escape flux of atomic hydrogen is 7 × 108 atoms/cm² sec. Two alternate but speculative sources of the atomic hydrogen escaping from Ganymede are photodesorption of water ice by ultraviolet photons in the wavelength range 120.5–186.0 nm and sputtering of water ice by Jupiters magnetospheric ion plasma.

Photometric observations of jupiter at 2400 angstroms.

The photopolarimeter instrument on Voyager 2 was used to obtain a map of Jupiter at an effective wavelength of 2400 angstroms. Analysis of a typical north-south swath used to make this map shows strong absorption at high latitudes by a molecular or particulate constituent in the Jovian atmosphere. At 65� north latitude, the absorbing constituent extends to altitudes above the 50-millibar pressure level.

The Galileo and Pioneer Venus ultraviolet spectrometer experiments - Solar Lyman-alpha latitude variation at solar maximum from interplanetary Lyman-alpha observations

Solar Ly-alpha latitude variation at solar maximum is examined on the basis of interplanetary Ly-alpha observations made during the Galileo and Pioneer Venus UV spectrometer experiments. A comparison is made of the latitude variation of the interplanetary (IP) Ly-alpha signal in 1986 at solar minimum from Pioneer Venus and in 1990 at solar maximum from Galileo. The Galileo EUV spectrometer shows that a large enhancement of the IP Ly-alpha emission occurred over the intervening four years near the solar equator. An IP Ly-alpha model is developed which considers the latitude variation of the solar Ly-alpha flux. The model fit to the data shows a 25-percent decrease of the full disk solar Ly-alpha flux from solar equator to solar pole in 1990. A detailed study of the Galileo IP Ly-alpha observations on day-of-year 190, 193, 197, and 200 in 1990 reveals that large variations occur in response to the 27-d solar variation. Analysis of these data shows that a maximum variation of 20 percent can be expected in the IP Ly-alpha upwind intensity over this 27-d period.

Mariner 6 and 7 ultraviolet spectrometer experiment - Photometry and topography of Mars.

Abstract Reflectance properties of Mars are measures in a 100-A band centered at 3050 A by the ultraviolet spectrometer. The instrument has an angular resolution which is equivalent to an area of 10 × 30 km on the surface of Mars. The transition from dusty conditions, which prevailed at the time of arrival of Mariner 9 on 14 November 1971, began on 1 January 1972, and relatively clear conditions existed after 23 January 1972. As the atmosphere became clearer, the scattering properties began to show a morning enhancement in both terminator and illuminated disk reflectance. A topographic map of Mars based on the scattering of ultraviolet light from the Mars atmosphere is shown. This map is based upon 3050-A data obtained after the Mars atmosphere had cleared in the ultraviolet. Ultraviolet light which is Rayleigh-scattered by the Mars molecular atmosphere, with allowance for uniform turbidity, is proportional to surface pressure independent of atmospheric temperature structure. Comparison with Mariner 9 radio occultation measurements determines the fraction of total reflectance that is due to atmospheric scattering.

Multidimensional radiative transfer: Applications to planetary coronae

Abstract A numerical solution to the integral equation for radiative transfer by resonance reradiation in an isothermal spherical atmosphere is described. The method presented is 100 times more efficient than earlier spherical radiative transfer models. The new model can accommodate density variations in the full three dimensional space and includes effects due to the presence of pure absorbers. Complete frequency redistribution is assumed for photon scattering. Applications of this model to the problem of solar photons scattered by atomic hydrogen in the atmospheres of Venus, Earth and Mars are described, and limb and disk profiles, as well as equivalent mean disk intensities for Venus, Earth and Mars, are presented.