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NASA NTRS1982

Amalthea

Voyager images have revealed Amalthea to be an irregular object 270 x 165 x 150 km in size. The spin period is probably synchronous with the orbital period of 11.9 hr, with the long axis pointing toward Jupiter. The satellite's surface is heavily scarred by impact craters, the largest of which has a diameter of 90 km (comparable to the mean radius of the satellite). Amalthea is very dark (reflectance about 5-6%) and very red, but isolated bright spots (reflectance up to 20%) occur. The spectrum of these bright spots is less red and may show an absorption feature near 0.6 micron. It is likely that the surface of Amalthea has been severely altered by its environment and by contamination from Io (especially by sulfur). It may, therefore, be very difficult to obtain definitive information on the composition of the intrinsic Amalthea material from remote sensing measurements.

Thomas, P.↗
NASA NTRS1982

Volcanic eruptions on Io - Implications for surface evolution and mass loss

Active volcanism on Io results in a continual resurfacing of the satellite. Analysis of required burial rates to erase impact craters, the mass production in the observed plumes, and the energy requirements for the volcanic activity suggest resurfacing rates of 0.001 to 10 cm/yr in recent geologic time. If this rate is typical of the last 4.5 Gyr, then extensive recycling of the upper crust and mantle must have occurred. The currently estimated loss rate of S, O, and Na from Io into the magnetosphere corresponds to only a small fraction of the resurfacing rate and should not have resulted in either extensive erosion or total depletion of any of the escaping species.

Johnson, T. V.↗
NASA NTRS1983

Possible lunar source areas of meteorite ALHA 81005 Geochemical remote sensing information

Antarctic meteorite ALHA 81005 is a regolith breccia apparently sent to earth by an impact event in the lunar highlands. Laboratory studies of this sample provide information that is used to understand the source region on the moon using remote sensing data. The meteorites low thorium content is inconsistent with thorium values measured for the central lunar nearside from orbit with Apollo gamma-ray spectrometers. Similarly, the mineral assemblages inferred from near-IR spectra of small impact craters on the lunar nearside do not exhibit the significant component of olivine and Fe-bearing feldspar that is observed in the meteorite spectra. The existing remote sensing data suggest the most probable source region for ALHA 81005 is the nearside limb or the lunar farside and that the composition of ALHA 81005 represents a surface unit that has not previously been extensively sampled.

Pieters, C. M.↗
NASA NTRS1983

Galilean satellite geomorphology

Research on this task consisted of the development and initial application of photometric and photoclinometric models using interactive computer image processing and graphics. New programs were developed to compute viewing and illumination angles for every picture element in a Voyager image using C-matrices and final Voyager ephemerides. These values were then used to transform each pixel to an illumination-oriented coordinate system. An iterative integration routine permits slope displacements to be computed from brightness variations, and correlated in the cross-sun direction, resulting in two dimensional topographic data. Figure 1 shows a 'wire-mesh' view of an impact crater on Ganymede, shown with a 10-fold vertical exaggeration. The crater, about 20 km in diameter, has a central mound and raised interior floor suggestive of viscous relaxation and rebound of the crater's topography. In addition to photoclinometry, the computer models that have been developed permit an examination on non-topographically-derived variations in surface brightness.

Malin, M. C.↗
NASA NTRS1984

Aeolian processes on Venus

Many of the questions regarding aeolian processes on Venus and the subsequent implications for surface history involve understanding the physics of particle motion in the venusian environment. The surface environment of Venus is simulated as closely as practicable using the Venus Wind Tunnel and to determine threshold wind speeds, particle flux, particle velocities, and the characteristics of various aeolian bedforms. Despite the relatively low wind speeds on Venus, the flux of windblown material on Venus is potentially high. A high fraction of material is transported as surface creep by rolling, estimates yield rates up to 100 kg per cm lane width per year depending upon the availability of material and wind frequency, suggesting that the formation of lowland plains by aeolian processes and the burial of various landforms such as impact craters could occur on short geological time-scales. Wind tunnel simulations demonstrate that aeolian processes may be very effective in modifying the surface through erosion and deposited and may have an important influence on the composition of the atmosphere.

Greeley, R.↗
NASA NTRS1984

A bibliography of planetary geology principal investigators and their associates, 1982 - 1983

This bibliography cites recent publications by principal investigators and their associates, supported through NASA's Office of Space Science and Applications, Earth and Planetary Exploration Division, Planetary Geology Program. It serves as a companion piece to NASA TM-85127, ""Reports of Planetary Programs, 1982". Entries are listed under the following subject areas: solar system, comets, asteroids, meteorites and small bodies; geologic mapping, geomorphology, and stratigraphy; structure, tectonics, and planetary and satellite evolutions; impact craters; volcanism; fluvial, mass wasting, glacial and preglacial studies; Eolian and Arid climate studies; regolith, volatiles, atmosphere, and climate, radar; remote sensing and photometric studies; and cartography, photogrammetry, geodesy, and altimetry. An author index is provided.

Plescia, J. B.↗
NASA NTRS1984

Meteorite bombardment and dating of planetary surfaces

Dating by measurement of impact crater frequencies developed in the past years primarily on the basis of the data from the missions to the Moon and Mars. The method allows a good relative dating to be obtained and the moons of Jupiter and Saturn through photographic analyses. A cratering chronology was obtained for the period between the oldest Moon crust (4.3 to 4.4 billion years) to the present time which gives a good absolute dating of any areas of the Moon's surface.

Neukum, G.↗
NASA NTRS1982

Planetary science: A lunar perspective

An interpretative synthesis of current knowledge on the moon and the terrestrial planets is presented, emphasizing the impact of recent lunar research (using Apollo data and samples) on theories of planetary morphology and evolution. Chapters are included on the exploration of the solar system; geology and stratigraphy; meteorite impacts, craters, and multiring basins; planetary surfaces; planetary crusts; basaltic volcanism; planetary interiors; the chemical composition of the planets; the origin and evolution of the moon and planets; and the significance of lunar and planetary exploration. Photographs, drawings, graphs, tables of quantitative data, and a glossary are provided.

Taylor, S. R.↗
NASA NTRS1984

Lunar and Planetary Science Conference, 14th, Houston, TX, March 14-18, 1983, Proceedings. Part 2

Various topics on the geology and evolution of the moon, planets, and meteorites are addressed. Some of the subjects considered include: Venusian rocks, impact cratering rate in recent time, ice and debris in Martian fretted terrain, geological evolution of Ganymede's Galileo Regio, and Lu-Hf and Sm-Nd evolution in lunar mare basalts. Also discussed are: ages and cosmic ray exposure history of moon rocks, U-Pb geochronology of zircons from lunar breccia, petrologic comparisons of Cayley and Descartes, chemistry and origin of chondrites and condrules, and the petrogenesis of SNC meteorites.

Boynton, W. V.↗
NASA NTRS1984

Terrestrial mass extinctions, cometary impacts and the sun's motion perpendicular to the galactic plane

An analysis of the data of Raup and Sepkoski (1984) is reported which suggests that the dominant cyclicity in major marine extinctions during at least the past 250 Myr is 30 + or - 1 Myr, with the standard deviation of an individual episode being + or - 9 Myr. This terrestrial cycle is found to be strongly correlated with the time needed for the solar system to oscillate vertically about the galactic plane. It is argued that galactic triggering or forcing of terrestrial biological crises may arise as a result of collisions or close encounters of the solar system with clouds of gas and dust, which would gravitationally perturb the solar system's family of comets and thereby increase the flux of comets and meteorites near the earth, leading to large-body impacts. A dominant cyclicity of 31 + or - 1 Myr is found for the observed age distribution of impact craters on earth, the phase of this cycle agreeing with the shown by the major biological crises.

Rampino, M. R.↗
NASA NTRS1984

Impact ejection, spallation, and the origin of meteorites

A model for the ejection of material from an impact crater which links ejection velocity, fragment size, and shock pressure through a simplified stress-wave propagation and reflection scheme is presented. It is shown that a small amount of material (0.01 to 0.05 projectile mass) may be ejected at high velocity without suffering petrologically detectable shock pressures. The largest fragments ejected at any velocity are spalls that originate from the target planet's surface. The spall size is proportional to the radius of the primary impactor and the target tensile strength and inversely proportional to ejection velocity. The shock level in the spalls is low, typically half of the dynamic crushing strength of the rock. The model also predicts the aspect ratio of the spalled fragments, the angle of ejection, and the sizes and shock level of other fragments originating deeper in the target. Comparison with observational and experimental data shows generally good agreement.

Melosh, H. J.↗
NASA NTRS1984

NASA-Ames vertical gun

A national facility, the NASA-Ames vertical gun range (AVGR) has an excellent reputation for revealing fundamental aspects of impact cratering that provide important constraints for planetary processes. The current logistics in accessing the AVGR, some of the past and ongoing experimental programs and their relevance, and the future role of this facility in planetary studies are reviewed. Publications resulting from experiments with the gun (1979 to 1984) are listed as well as the researchers and subjects studied.

Schultz, P. H.↗
NASA NTRS1984

Asteroid-meteorite connection: Regolith effects implied by lunar reflectance spectra

The effect of soil formation on the spectral characteristics of asteroids were estimated in situ properties of materials affected by the space environment other than those associated with agglutinates. In his initial survey of the reflectance properties of lunar samples, J.B. Adams measured a few chips from the exterior and interior of Apollo rocks. For Apollo 12 basalts 12053-12063 , the exterior and interior spectra exhibited strong absorption bands which were very similar in nature; the exterior fragment was a bit brighter, which was attributed to microbrecciation. Mature lunar soils measured undisturbed surface areas at the landing sites. It was hypothesized that freshly exposed material at fresh impact craters should be comparable to powdered rock samples in the laboratory. This expected trend seemed to be true with the early reflectance data, but as the more complete near-infrared data has become available it is clear that there is no simple correspondence of rock type (laboratory powder or chip measurement) to freshly exposed surface material (telescopic measurements of fresh crater classes).

Pieters, C. M.↗
NASA NTRS1984

Geological, structural, and geomorphological analyses from SIR-B

A Shuttle Imaging Radar-B (SIR-B) study designed to develop a better understanding of the application of radar in geological studies is described. The specific objectives for deltaic environments include the examination of delta morphology and the intertidal zone, the surface expression of shallow bathymetry, the characterization of vegetation cover, and the water balance of the delta. In impact crater environments, the goals include the establishment of the radar characteristics of exposed craters and the application of this knowledge to test for the detectability of very poorly exposed craters.

Head, J. W., III↗
NASA NTRS1985

A Statistical Treatment of the Regional Slope Characteristics of Venus and Earth

Regional slope is a scale dependent parameter that describes the planar gradient over some (relative broad) area of topography. As such, it has a fundamental relevance to the geological evolution of a planetary surface. It is controlled by the interaction of those processes which tend to increase or reduce surface relief (e.g., tectonism, volcanism, impact cratering, weathering, viscous relaxation) and in turn, exerts control on the erosion, transportation and deposition of surface materials. The regional slope values, measured over 3 by 3 regions for Venus and Earth are calculated. The regional slope frequency distribution information was analyzed for Venus and Earth. The effects on terrestrial regional slope resulting from the removal of the ocean load from sea floor topography were examined.

Shapiro, V. I.↗
NASA NTRS1985

Enigmatic Hills of the Mottled Plains, Mare Acidalium Quadrangle (MC-4), Mars

Interpretations of Mariner 9 and Viking images of the mottled plains in Mare Acidalium are present and discussed. Although the boundaries between all the units are gradational, four subdivisions were recognized within the mottled plains region. One of these subdivisions was designated hummocky mottled plains. The unit is characterized by many small (1 km), irregularly spaced dark hills surrounded by brighter material. One very poor-quality, high-resolution Viking strip (528B01-04) indicates that some of these dark hills have circular summit depressions. Possible interpretations of origin of these hills include: cinder cones, volcanic domes, maar volcanoes, pseudocraters, impact craters, or pingos. The uncratered hills or knobs may be igneous plugs, volcanic necks, or the dark remnants of a hummocky, cratered surface protruding through a relatively bright eolian blanket. The presence, however, of the small dark hills on the flow of the large (130 km diameter) C(2) crater Lomonosov, suggests that similar dark hills elsewhere also may be secondary.

Witbeck, N. E.↗
NASA NTRS1984

Venus - Volcanism and rift formation in Beta Regio

A new high-resolution radar image of Beta Regio, a Venus highland area, confirms the presence of a major tectonic rift system and associated volcanic activity. The lack of identifiable impact craters, together with the apparent superposition of the Theia Mons volcanic structure on the rift system, suggest that at least some of the volcanic activity occurred in relatively recent geologic time. The presence of topographically similar highland areas elsewhere on Venus (Aphrodite Terra, Dali Chasma, and Diana Chasma) suggests that rifting and volcanism are significant processes on Venus.

Campbell, D. B.↗
NASA NTRS1984

Cometary showers and unseen solar companions

The possibility that an invisible solar companion passing through the Oort cloud every 28 Myr precipitates a sufficiently high rate of cometary collisions with the earth to account for periodic mass species extinctions recorded in the fossil record is discussed. A Monte Carlo simulation shows that any hypothesized 'death star' with a 28 Myr orbit would experience an average 10 percent change in period per orbit. Production of an 18-fold increase in cometary impacts would be associated with a 0.055 probability that a 10 km nucleus would hit the earth in a shower once every 510 Myr, longer than the proposed extinction periodicity. However, if the death star orbit has a 0.6 eccentricity and the Oort cloud is sufficiently densely populated, a 2 billion comet shower may be possible. A survey of large terrestrial impact craters indicates that 6-12 craters with diameters over 10 km originated in periodic showers. The extinctions in any case occur at 26 Myr periods and cannot be correlated with the 33 Myr period of recrossing the galactic plane, or with any other known phenomena.

Weissman, P. R.↗