Richard W. Shorthill

The "Soil" of Mars (Viking 1)

The location of the Viking 1 lander is most ideal for the study of soil properties because it has one footpad in soft material and one on hard material. As each soil sample was acquired, information on soil properties was obtained. Although analysis is still under way, early results on bulk density, particle size, angle of internal friction, cohesion, adhesion, and penetration resistance of the soil of Mars are presented.

The upper Jovian atmosphere aerosol content determined from a satellite eclipse observation

The Galilean satellite eclipse technique for measuring the aerosol distribution in the upper Jovian atmosphere is described and applied using 30 color observations of the 13 May 1972 eclipse of Ganymede obtained with the 5-m Hale telescope. This event probes the South Temperate Zone. The observed aerosol lies above the visible cloud tops, is very tenuous and varies with altitude, increasing rapidly with downward passage through the tropopause. The aerosol extinction coefficient, κa (λ1.05 μm), is ∼1.1 × 10−9 cm−1 in the lower stratosphere and ∼1.1 × 10−8 cm−1 at the tropopause. The 1σ uncertainty in these values does not exceed 50% The observations require some aerosol above the tropopause but do not clearly determine its structure. The present analysis emphasizes an extended haze distribution, but the alternate possibility is not excluded that the stratospheric aerosol resides in a thin layer. The aerosol extinction increases with decreasing wavelength and indicates the particle radius to be ⩽0.2 μm. Larger radii are impossible. These overall results confirm Axels (1972) suggestion of a small quantity of dust above the Jovian cloud tops and the optical depths are consistent with those required to explain the low uv albedo.

The Environs of Viking 2 Lander

Forty-six days after Viking 1 landed, Viking 2 landed in Utopia Planitia, about 6500 kilometers away from the landing site of Viking 1. Images show that in the immediate vicinity of the Viking 2 landing site the surface is covered with rocks, some of which are partially buried, and fine-grained materials. The surface sampler, the lander cameras, engineering sensors, and some data from the other lander experiments were used to investigate the properties of the surface. Lander 2 has a more homogeneous surface, more coarse-grained material, an extensive crust, small rocks or clods which seem to be difficult to collect, and more extensive erosion by the retro-engine exhaust gases than lander 1. A report on the physical properties of the martian surface based on data obtained through sol 58 on Viking 2 and a brief description of activities on Viking 1 after sol 36 are given.

Modeling the non-grey-body thermal emission from the full moon

Abstract In an attempt to understand the non-grey-body brightness temperature spectra of the Moon and Galilean satellites we have developed a series of thermophysical computer models of solid-surfaced planetary bodies. Surface roughness is modeled as paraboloidal holes (craters) of a specified depth-to-diameter ratio. Previous investigations involving surface roughness have not quantitatively modeled any spectral behavior nor have they explored directional radiation properties. Nondiffuse radiation properties and surface roughness are included in a rigorous analysis of scattered and reemitted radiation within a depression. The model explains, to first order, the behavior of the published limbscans and disk-integrated infrared brightness temperature spectra for the full moon. It is concluded that negative surface relief is required to explain deviation of lunar thermal emission from that expected from a smooth, Lambert surface. This may allow for the quantitative measurement of surface roughness.

Spectrophotometry of J8, J9, and four Trojan asteroids from 0.32 to 1.05 μm

Abstract New 30-channel narrowband photometry from 0.32 to 1.05 μm of the retrograde Jovian satellites J9 (to 0.7 μm) and J8 and the trailing Trojan asteroids 617, 884, 1172, and 1173 is presented. The data confirm previous measurements of J8, 617, 884, and 1172 at λ μ m, but the extension into the infrared shows that the normalized spectral reflectance of these objects rises steadily from ∼0.8 at 0.4 μ m to ∼ 1.4 at 1.05 μ m, suggesting they are too bright in the near infrared to be C-type asteroids. The C classification of 1173 is confirmed. J9 is markedly redder than J8 at visible wavelengths. The results indicate a greater taxonomic contrast between these distant objects and main-belt asteroids than previously thought.

Surface erosion caused on Mars from Viking descent engine plume

During the Martian landings the descent engine plumes on Viking Lander 1 (VL-1) and Viking Lander 2 (VL-2) eroded the Martian surface materials. This had been anticipated and investigated both analytically and experimentally during the design phase of the Viking spacecraft. This paper presents data on erosion obtained during the tests of the Viking descent engine and the evidence for erosion by the descent engines of VL-1 and VL-2 on Mars. From these and other results, it is concluded that there are four distinct surface materials on Mars: (1) drift material, (2) crusty to cloddy material, (3) blocky material, and (4) rock.

Narrow-band spectrophotometry of Ariel, Umbriel, Titania, Oberon, and Triton

Abstract The spectral reflectances of Ariel, Umbriel, Titania, Oberon, and Triton were measured in 28 bandpasses between λ326 and λ976 nm on the night of 28/29 June 1974. These observations were made with the 200-in. Hale telescope and multichannel spectrometer. Bandpasses of 8 nm from λ326 to λ566 nm and 16 nm from λ592 to λ976 nm were employed. The spectral reflectances of Ariel, Oberon, and Titania increase from λ342 to λ534 nm and are relatively flat from λ550 to λ976 nm. Umbriels reflectance decreases monotonically with increasing wavelength through the entire range of measured wavelengths. Triton is found to have a constant spectral reflectance.

Measurement of stratospheric aerosol on Saturn using an eclipse of Titan

Abstract An eclipse of Titan by Saturn was observed on December 20, 1979, to measure the aerosol content in the atmosphere of Saturn. The measurements were made with the 74-in. telescope of the Helwan Observatory, Egypt, in the bandpass 6300–7300 A and extend to ∼5 magnitudes of eclipse darkening. The faint portion of the lightcurve unambiguously requires the presence of aerosol in the lower stratosphere of Saturn. The aerosol extends to at least 20 km above the tropopause and has a one-way stratospheric vertical optical depth of 0.4−0.02+0.04 at 6700 A. The results apply to the sunset terminator at a cronographic latitude of 23°S.

Galilean satellite eclipse studies I. Observations and satellite characteristics

Abstract Spectrophotometric light curves of 12 Galilean satellite eclipses are reported. The observations were made in 20 to 30 channels over the wavelength range 3240 to 10,500 A using the 200-in. telescope. The initial data processing is described. These data measure the Jovian aerosol content in the lower stratosphere and uppermost troposhere and the methane abundance in the lower stratosphere. The data are consistent with a lack of limb darkening on the Galilean satellites. The orbit of Callisto is shown to be inclined 0.08 ± 0.02° to the equatorial plane of Jupiter.

JHK photometry of selected Trojan and Hilda asteroids

Abstract JHK photometry of selected Trojan and Hilda asteroids is presented and compared with that of main belt asteroids and laboratory samples, none of which yields a completely satisfactory match. Leading Trojans and Hildas show similar and homogenous JHK colors, while the trailing Trojans appear more heterogeneous.