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NASA NTRS1980

Lunar and Planetary Science Conference, 11th, Houston, TX, March 17-21, 1980, Proceedings. Volume 3 - Physical processes

Geophysical investigations are discussed, taking into account laboratory measurements, planetary measurements, and structural implications and models. Impact processes are also examined. Experimental studies are considered along with aspects of crater morphology and frequency, and models theory. Volcanic-tectonic processes are investigated and topics related to the study of planetary atmospheres are examined. Attention is given to shallow moonquakes, the focal mechanism of deep moonquakes, lunar polar wandering, the search for an intrinsic magnetic field of Venus, the early global melting of the terrestrial planets, the first few hundred years of evolution of a moon of fission origin, the control of crater morphology by gravity and target type, crater peaks in Mercurian craters, lunar cold traps and their influence on argon-40, and solar wind sputtering effects in the atmospheres of Mars and Venus.

Merrill, R. B.↗

NASA NTRS1980

Time-dependent lunar density models

Simple models of the geochemical and geophysical evolution of the moon were constructed with the particular aim of investigating the mass density distribution within the moon as a function of time. The strongly inverted (densest material on top) density distribution resulting from fractional crystallization of the lunar magma ocean was found to rearrange itself into a highly stable stratification. Density stratification due to compositional variation dominated gravitational stability; thus, thermally driven convection was found to be suppressed in regions with relaxed (rearranged so as to minimize the gravitational potential energy) compositional density variation. The suppression of convection as a heat transfer process results in considerable modification of thermal evolution. The models produced sufficient density segregation from magma oceans of a few hundred km thickness, such that relaxation to the most stable rearrangement yielded a polar moment of inertia approximating that measured for the present-day moon. Thus, lunar models matching moment-of-inertia constraints but lacking a metallic core are motivated.

Herbert, F.↗

NASA NTRS1982

Lava flooding of ancient planetary crusts - Geometry, thickness, and volumes of flooded lunar impact basins

The present study is concerned with the determination of the geometry, thickness, volumes, and style of emplacement of lava extruded onto an ancient planetary crust, giving particular attention to crustal regions characterized by the topography of impact basins. Attempts to establish thicknesses and volumes are based on four approaches, related to crater morphometry, crater penetration, stratigraphy, and geophysical techniques. The considered technique is applied to two areas related to lunar impact areas. The Orientale basin is investigated, taking into account the style of flooding, the geometry of deposits and implications for loading of the lithosphere, and aspects of lava thickness and vertical mixing. An analysis of the Archimedes-Apennine Bench region is also conducted, giving attention to the number of data points, rim heights of fresh lunar craters, and the premare nature of impact crater data points.

Head, J. W.↗

NASA NTRS1980

The Seasat low rate data processing system

The development of the algorithms for data processing and distribution, the circuitry, and the performance of the Seasat low rate data processing system are reviewed. The system controls data from the radar altimeter, scatterometer, microwave radiometer, and the visible and IR radiometer for independent transmission of each instrument's readings. The downlink operates at 25 kb/sec, and a yearlong program of geophysical evaluation proceeded shortly after launch, allowing on-line engineering evaluation and alteration of the control algorithms in the system. Some data is preformatted for immediate distribution and storage in archival quality. A catalog and abstracts are provided to users allowing a RAM search from remote terminals for historical conditions. Procedures for verifying and altering the algorithms are detailed.

Brown, J. W.↗

NASA NTRS1981

The physics of creep and attenuation in the mantle

The objective is to investigate the role of dislocations in both creep and attenuation. Attenuation in the mantle at seismic frequencies is thought to be caused by the glide of dislocations in the subgrains. Both kink and impurity drag can contribute to the glide time constant. The kink-formation, or Peierls barrier, model for dislocation glide is seen as a low-temperature, high-frequency mechanism most appropriate for pure systems. It is noted that a small amount of impurity drag brings the dislocation glide characteristic time into the seismic band at upper-mantle temperatures. The attenuation and creep behavior of the mantle are related by way of the dislocation structure. Analysis of the various possible mechanisms is facilitated by casting them and the geophysical data in terms of a pre-exponential characteristic time and an activation energy.

Anderson, D. L.↗

NASA NTRS1981

The evolution of the lunar Nectaris multiring basin

The paper begins by discussing the pre-Nectaris crust, the formation of the basin, its age and its geophysics. The pre-mare basalt deposits are categorized into smooth plains deposits, cratered and patterned plains, knobby terrain, and massifs, and the structure, chemical composition, and probable origin of these are discussed. Alternative scenarios for the formation of the plains are that they consist of impact melt ejecta, ejecta overlying smoothed-out preexisting topographic material, or ejecta overlying extrusive deposits. The mare deposits are assessed in terms of their composition, basalt ages and thickness, and eruption styles. A probable age of 3.6 billion years for the basalts is given. The tectonics of mare ridges and fractures and floor-fractured craters are gone into. Post-mare cratering is also discussed. It is concluded that the morphology of multiring basins is strongly dependent on lithosphere thickness.

Whitford-Stark, J. L.↗

NASA NTRS1982

Seasat measurement system evaluation - Achievements and limitations

The Seasat project, which demonstrated the feasibility of microwave oceanographic remote sensing, was sponsored by the National Aeronautics and Space Administration and managed by the Jet Propulsion Laboratory. The evaluation of the measurement system (elements of the satellite, the sensors, the data handling, and data processing subsystems) was a key activity of the Seasat project. This paper summarizes the primary achievements and highlights the generic limitations of the evaluation process. Other papers in this issue present details of the geophysical evaluation process carried out under the auspices of the Seasat project.

Lame, D. B.↗

NASA NTRS1982

Description of Seasat radiometer status and results

Improved geophysical algorithms are shown to be able to determine sea surface temperature (SST) to an accuracy of 1 C (1 sigma) under favorable surface and atmospheric conditions over an SST range of 10-30 C. Under similar conditions, the radiometer wind retrievals track the scatterometer winds within a scatter of 2 m/s (1 sigma) over a wind speed range of 0-25 m/s. These conditions require that contamination from land, rain, sun glint, and radio frequency interference be avoided. What is more, water vapor determinations in the midlatitudes and in the tropical Pacific are found to agree with estimates of precipitable water from radiosondes to within 10%.

Lipes, R. G.↗

NASA NTRS1982

The Balmer basin - Regional geology and geochemistry of an ancient lunar impact basin

Photogeologic, geochemical and geophysical information is cited to support the contention that an ancient multi-ringed basin exists in the east limb region of the moon, centered at 15 deg S and 70 deg E. The inner ring of the basin, with a diameter of 225 km, is composed of isolated rugged mountains of pre-Nectarian terra; the less distinct outer ring, whose diameter is approximately 450 km, is made up of irregular segments of surrounding large craters. It is noted that two units of light plains material occur in this area and that they are confined for the most part to the region within the proposed outer basin ring. According to orbital geochemical data, the younger unit (Imbrian age plains) consists of a mare basalt not unlike others of the nearside. This unit possesses high Mg/Al concentration ratios as determined from X-ray fluorescence data; it is also relatively high in Th and Fe when compared with the surrounding highlands. It is thought that the relatively high albedo of the Balmer plains may derive from either a reworking by numerous secondary craters from the surrounding impacts or a basaltic composition with higher albedo and lower Fe than the nearside maria.

Maxwell, T. A.↗

NASA NTRS1982

Constraints on the moon's origin from the partitioning behaviour of tungsten

An evaluation is conducted of the parameters which control the partitioning of W between metal and silicate in the moon, taking into account information on the possible magnitudes of the lunar W depletion and the possible lunar metal content. A major question regarding depletion of W during partial melting is found to be related to the very homogeneous W/La ratios in lunar samples. It is concluded that a metal content consistent with present geophysical constraints on the size of a metallic lunar core could account for the depletion of W observed in lunar rocks. The low W/La ratio of the moon cannot be used as evidence for the formation of the moon from the earth's mantle by fission following terrestrial core formation. The approximate coincidence of W/La ratios in the earth, moon, and eucrite parent body may only reflect the operation of the same depletion mechanism in similar conditions on several parent bodies.

Newsom, H. E.↗

NASA NTRS1982

Mars - Gravity data analysis of the crater Antoniadi

Topography and gravity information for this 370-km crater are analyzed to determine a depth of compensation with an Airy isostatic model. A least squares fit to the gravity profile gives an estimate of 115 km for the depth of compensation. It is noted that Antoniadi is the only large Martian crater for which both topographic and gravity data are available for analysis. The goal here is to reduce these geophysical data for additional information on the internal structure of Mars. The results show that if Antoniadi had fully isostatically adjusted, the additional mass material would have been about 100 km below the surface. This is regarded as another data point for geophysicists developing the internal structure of Mars.

Sjogren, W. L.↗

NASA NTRS1981

The excavation stage of basin formation - A qualitative model

One of the most complex problems in planetary geology and geophysics is the determination of the nature of the impact cratering processes at scales of tens to thousands of kilometers that produce the complex morphological structures of multiring basins. The cratering process is frequently considered to be divided into three stages, including a short high-pressure stage of initial contact between the projectile and the planetary crust, a longer excavation or cratering flow stage culminating in the formation of a transient crater, and a still longer modification stage during which the transient crater is modified into the observed final geologic form. The transient crater may be considered as the initial boundary condition of the modification stage. In the present investigation, the nature of the transient crater is indicated by the cratering flow field determined from numerical simulations of the excavation stage. Attention is given to empirical and theoretical scaling.

Croft, S. K.↗

NASA NTRS1981

The modification stage of basin formation - Conditions of ring formation

The impact cratering process may be divided into three stages, including a short high pressure phase of initial contact between the projectile and target, a longer cratering flow phase during which material is both ejected and displaced to form the transient crater, and a still longer modification phase during which the transient crater is modified into the final observed form. The present investigation is concerned with the nature of the modification stage as constrained by the initial boundary condition which comprises the transient crater and its surroundings, and the final boundary condition which comprises the structure of observed craters and basins. A brief description is provided of an interpretation of basin ring structure based on geophysical, geological, and dimensional analysis of craters and basins on the moon, Mercury, and Mars, on structural observations of terrestrial impact and explosion craters, and on an empirical crater restoration model. A scenario of basin formation is constructed, and relations of this scenario to observed morphology are discussed.

Croft, S. K.↗

NASA NTRS1982

Description of the dedicated gravitational satellite mission /Gravsat/

A description of the dedicated gravitational satellite (Gravsat) mission is presented. Scientific objectives are to substantially improve knowledge of solid earth geophysics and oceanography by determining a more accurate geopotential. The most stringent requirement is determination of mean gravity anomalies over regions 100-km square to an accuracy of 2.5 mgal (1 gal = 1 cm/s squared). Fundamental geodetic data will be satellite-to-satellite measurement of relative range rate. Orbital characteristics are discussed. An error budget is presented which indicates that the requisite measurement precision can be achieved by closed-loop RF doppler measurements. Implicit in the discussion of the error budget is a description of the spacecraft and systems components.

Pisacane, V. L.↗

NASA NTRS1982

Existence of a component corotating with the earth in high-latitude disturbance magnetic fields

A study of the data from the high-latitude North American IMS network of magnetic stations suggests that there is a component in substorm perturbations that corotates with the earth. It is as yet not certain whether the existence of this component stems from the corotation of a part of the magnetospheric plasma involved in the substorm mechanism or if it is a 'phase change' resulting from the control of the substorm manifestations by the earth's main magnetic field which is not axially symmetric. There are other geophysical phenomena showing a persistence of longitudinal variations corotating with the earth. These phenomena are of significance for a better understanding of ionosphere-magnetosphere coupling.

Suzuki, A.↗

NASA NTRS1981

A survey of the goals and accomplishments of the Seasat mission

The objectives of the Seasat mission, which was dedicated to establishing the utility of microwave sensors for the remote sensing of the earth's oceans, and data collected during some three months of orbital operations are briefly reviewed. In particular, consideration is given to the Seasat sensors and measurement goals, surface and in situ observations, evaluation organization, and current status of geophysical evaluation for each sensor (altimeter, Seasat-A scatterometer system, scanning multichannel microwave radiometer, visible and infrared radiometer, and synthetic aperture radar). Finally, future plans related to the Seasat project are discussed.

Born, G. H.↗

NASA NTRS1981

Microwave scatterometer measurements of oceanic wind vector

Results from Seasat-A scatterometer (SASS) remote sensing of oceanic wind vectors are reviewed, together with the theory of radar backscattering. The SASS was designed to measure the surface wind stress and neutral stability wind vector at 19.5 m altitude. Requirements included windspeeds from 4-24 m/sec to within 2 m/sec or 10%, a 1000 km swath, directions from 0-360 deg to within 20 deg, a 50 km resolution cell, and cross-track and along-track spacing between resolution cells of 100 km. The method was based on Bragg scattering of microwaves from cm-length capillary ocean waves. The strength of the backscatter is proportional to the capillary wave amplitude, which is in equilibrium with the ocean surface wind speed. The measurement grouping, least-squares estimator, and dB deviation model function table for the SASS geophysical algorithm are discussed. Comparisons of SASS, GOASEX, and ground truth wind data are made, and a 0.92 correlation between SASS and ground truth data is determined from regression analysis.

Jones, W. L.↗

NASA NTRS1981

Seasat SMMR observations of ocean surface temperature and wind speed in the North Pacific

The Scanning Multichannel Microwave Radiometer (SMMR), aboard both the SEASAT and Nimbus-7 satellites, was designed primarily to determine global sea surface temperatures and wind speeds over an extended range of atmospheric conditions. The Seasat mission lasted approximately three months from July to October 1978, during which time sufficient data was acquired to enable an assessment of SMMR measurement capabilities under a variety of conditions. This paper reports some results of initial Seasat SMMR assessments in the North Pacific area. A brief description is given of retrieval techniques used to derive the geophysical parameters. SMMR-derived sea surface temperatures and winds are compared with surface truth point measurements from XBT's and buoys, and with surface truth fields derived from point measurements. Accuracies of approximately 1.5 C and 2.5 m/s rms are obtained in these comparisons.

Njoku, E. G.↗