T. V. Johnson

Asteroid Vesta: Spectral Reflectivity and Compositional Implications

The spectral reflectivity (0.30 to 1.10 microns) of several asteroids has been measured for the first time. The reflection spectrum for Vesta contains a strong absorption band centered near 0.9 micron and a weaker absorption feature between 0.5 and 0.6 micron. The reflectivity decreases strongly in the ultraviolet. The reflection spectrum for the asteroid Pallas and probably for Ceres does not contain the 0.9-micron band. Vesta shows the strongest and best-defined absorption bands yet seen in the reflection spectrum for the solid surface of an object in the solar system. The strong 0.9-micron band arises from electronic absorptions in ferrous iron on the M2 site of a magnesian pyroxene. Comparison with laboratory measurements on meteorites and Apollo 11 samples indicates that the surface of Vesta has a composition very similar to that of certain basaltic achondrites.

Topography on satellite surfaces and the shape of asteroids

Abstract Calculations of the topography and shape of planetary bodies are presented for two sets of models. One set of models deals with the effects of static loading on bodies, taking into account strengths of materials, density, and size. The other set considers the effects of creep deformation on model bodies of differing composition, size and temperature. Application of these models to asteroids and satellites of the major planets indicates that model, even the largest asteroids could retain highly nonspherical shapes, and the four large satellites of Jupiter could sustain statically loaded topography on the order of 10 km. (2) If silicate asteroids have not been heated to near the melting temperature of silicates, initial topography should survive for at least 109 yr under creep deformation. Topography on an insulated icy asteroid will be rapidly reduced if it is of larger scale than the insulating layer, no matter what the thermal history. (3) Of the Galilean satellites of Jupiter, J1 and J2 should retain topography created on silicate surfaces since their formation (or since the surfaces were near the silicate melting temperature. If ice layers of any significant thickness exist, topography on a scale smaller than the layers thickness will be reduced rapidly. (4) J4 and J3 probably fit an icy model throughout and topography of all scales may be reduced with relaxation times 6 yr. These satellites are thus likely to preserve only very recent features on their surfaces, in contrast to the other Galilean satellites. If melting has taken place since formation, these conclusions become even stronger. (5) Of the satellites of the other planets, only Titan appears likely to have undergone topographic reduction by creep, under the models presented. However, if ices other than water are present in large proportion on these satellites relaxation times for topography may be shorter than calculated from the water ice models.

Lunar spectral reflectivity (0.30 to 2.50 microns) and implications for remote mineralogical analysis.

The spectral reflectivity (0.30 to 2.50 microns) of several lunar areas was measured with ground-based telescopes. A narrow absorption band centered at 0.95 micron was revealed for the first time. No other absorption bands appear in the spectrum. The reflectivity continues to rise at longer wavelengths throughout the spectral region studied. A comparison of the telescope measurements of an area 15 kilometers in diameter that includes Tranquillity Base with laboratory measurements of Apollo 11 soil samples reveals remarkable agreement, an indication that properties determined for fairly large lunar areas are relevant to local conditions. The spectra are interpretable in terms of surface mineralogy. The absorption band varies in both depth and shape and the overall slope of the curve changes with lunar area, an indication of differences in the composition and opacity of surface material. However, the lack of variety in the band position suggests there are no major differences (say, from mostly pyroxenes to mostly olivines) in the mineralogy at those sites studied.

Spectrophotometry /0.3 to 1.1 micron/ of visited and proposed Apollo lunar landing sites.

The study uses information gained by analysis of the spectral properties of lunar samples in the laboratory, and telescope spectra of over 100 lunar areas, to provide information regarding the composition and mineralogy of each proposed lunar landing site. Attention is given to (1) the presence of pyroxenes which cause an absorption band near 0.95 micron in the lunar reflection spectrum, (2) the proportion of crystalline to glassy material present in the soil which is derived from the slope of the reflectivity curve between 0.4 and 0.7 micron and the strength of the 0.95 micron absorption band, and (3) the presence of Ti(3+) ions in the glasses on the lunar surface, which affects the reflection spectrum at blue and ultraviolet wavelengths.

Mare Humorum - An integrated study of spectral reflectivity.

Abstract A detailed study was made of the spectral reflectivity (0.3–1.1 μm) of 31 areas (10–20 km in diam) in the Humorum basin region. The results are: (1) There are at least two units in the mare portion of Humorum which are distinguishable by spectral properties. One of these units, called T-type, has a spectral reflectivity resembling that of the Apollo 11 site and also some areas in Oceanus Procellarum. The other unit in southwest Mare Humorum, resembles Mare Serenitatis in spectral character (S-type). An additional unit in the central area (I-type) with intermediate spectral properties is possible. (2) These mare units do not correlate with obvious morphological or albedo changes but agree well with shadings distinguishable on color difference photographs. (3) On the basis of studies of previously sampled sites it is suggested that the T-type unit may be higher in Ti content (similar to Apollo 11) than the S-type material (similar to Apollo 12). (4) The continuity of T-type material through the break in the northeast wall of Mare Humorum and its spectral similarity to areas in Procellarum suggest that the T-type material may result from an event that flooded parts of Mare Procellarum at a period later than the original Humorum basin filling (S-type). Relative ages derived from crater morphology studies support this sequence.

Galileo infrared imaging spectrometry measurements at the Moon

Imaging spectrometer observations were made of the surface of the Moon during the December 1990 flyby of the Earth-Moon system by the Galileo spacecraft. This article documents this data set and presents analyses of some of the data. The near infrared mapping spectrometer (NIMS) investigation obtained 17 separate mosaics of the Moon in 408 spectral channels between about 0.7 and 5.2 μm. The instrument was originally designed to operate in orbit about Jupiter and therefore saturates at many spectral channels for most measurement situations at 1 AU. However, sufficient measurements were made of the Moon to verify the proper operation of the instrument and to demonstrate its capabilities. Analysis of these data show that the NIMS worked as expected and produced measurements consistent with previous ground-based telescopic studies. These are the first imaging spectrometer measurements of this type from space for the Moon, and they illustrate several major points concerning this type of observation and about the NIMS capabilities specifically. Of major importance are the difference between framing and scanning instruments and the effects of the spacecraft and the scan platform on the performance of such an experiment. The science return of subsequent NIMS and other investigation measurements will be significantly enhanced by the experience and results gained.