Mantle dynamics and geodesy

Both completed work and work that is still in progress are presented. The completed work presented includes: (1) core-mantle boundary topography; (2) absolute value for mantle viscosity; (3) code development; (4) lateral heterogeneity of subduction zone rheology; and (5) planning for the Coolfront meeting. The work presented that is still in progress includes: (1) geoid anomalies for a chemically stratified mantle; and (2) geoid anomalies with lateral variations in viscosity

The Mars Observer database

Mars Observer will study the surface, atmosphere, and climate of Mars in a systematic way over an entire Martian year. The observations of the surface will provide a database that will be invaluable to the planning of a future Mars sample return mission. Mars Observer is planned for a September 1992 launch from the Space Shuttle, using an upper-stage. After the one year transit the spacecraft is injected into orbit about Mars and the orbit adjusted to a near-circular, sun-synchronous low-altitude, polar orbit. During the Martian year in this mapping orbit the instruments gather both geoscience data and climatological data by repetitive global mapping. The scientific objectives of the mission are to: (1) determine the global elemental and mineralogical character of the surface material; (2) define globally the topography and gravitational field; (3) establish the nature of the magnetic field; (4) determine the time and space distribution, abundance, sources, and sinks of volatile material and dust over a seasonal cycle; and (5) explore the structure and aspects of the circulation of the atmosphere. The science investigations and instruments for Mars Observer have been chosen with these objectives in mind. These instruments, the principal investigator or team leader and the objectives are discussed

Analytical study of comet nucleus samples

Analytical procedures for studying and handling frozen (130 K) core samples of comet nuclei are discussed. These methods include neutron activation analysis, x ray fluorescent analysis and high resolution mass spectroscopy

Mars 2000

Twenty years after the Viking Mission, Mars is again being scrutinized in the light of a flood of information from spacecraft missions to Mars, the Hubble Space Telescope, and the SNC meteorites. This review provides an overview of the current understanding of Mars, especially in light of the data being returned from the Mars Global Surveyor Mission. Mars does not now have a global magnetic field, but the presence of crustal anomalies indicates that a global field existed early in Martian history. The topography, geodetic figure, and gravitational field are known to high precision. The northern hemisphere is lower and has a thinner and stronger crust than the southern hemisphere. The global weather and the thermal structure of the atmosphere have been monitored for more than a year. Surface-atmosphere interaction has been investigated by observations of surface features, polar caps, atmospheric dust, and condensate clouds. The surface has been imaged at very high resolution and spectral measures have been obtained to quantify surface characteristics and geologic processes. Many questions remain unanswered, especially about the earliest period of Mars' history

Intraplate deformation, stress in the lithosphere and the driving mechanism for plate motions

The initial research proposed was to use the predictions of geodynamical models of mantle flow, combined with geodetic observations of intraplate strain and stress, to better constrain mantle convection and the driving mechanism for plate motions and deformation. It is only now that geodetic observations of intraplate strain are becoming sufficiently well resolved to make them useful for substantial geodynamical inference to be made. A model of flow in the mantle that explains almost 90 percent of the variance in the observed longwavelength nonhydrostatic geoid was developed

Petrogenesis of lunar rocks: Rb-Sr constraints and lack of H2O

Rb and Sr isotopic data and other chemical data indicate major lunar differentiation at about 4.6 AE and very limited subsequent differentiation. The constraints of limited differentiation post 4.6 AE and the apparent lack of H2O on the moon, when applied to the derivation and petrogenesis of lunar samples, suggest the following: (1) soil samples, breccias, metaclastic rocks, and feldspathic basalts represent mixtures of repeatedly-modified clastic material, which was ultimately derived from materials formed during the about 4.6 AE differentiation; and (2) mare basalts crystallized from melts which formed by partial melting and, which developed without equilibration between the melt and crystalline residuum

Petrogenesis of lunar rocks: Rb-Sr constraints and lack of H2O

Rb and Sr isotopic data and other chemical data indicate major lunar differentiation at about 4.6 AE (AE = 10 to the 9th power years) and very limited subsequent differentiation. The constraints of limited differentiation after 4.6 AE and the apparent lack of H2O on the moon, when applied to the derivation and petrogenesis of lunar samples, suggest the following: (1) soil samples, breccias, metaclastic rocks, and feldspathic basalts represent mixtures of repeatedly modified clastic material, which was utimately derived from materials formed during the 4.6 AE differentiation; and (2) mare basalts crystallized from melts which formed by partial melting, and which developed without equilibrium between the melt and crystalline residuum

Introduction to the special section: The Mars Global Surveyor mission

Since the launch of Mars Global Surveyor (MGS) in November 1996, it has returned more information about Mars than all previous missions combined. The scientific impact of MGS has been extraordinary. In many ways we now know Mars to be a very different planet than when MGS arrived in 1997. MGS has provided daily global and high resolution images, a global topographic model better than for Earth, a corresponding gravity model, and a magnetic field model, has mapped the surface composition, and has monitored the atmosphere

Intact capture of hypervelocity particles

Knowledge of the phase, structure, and crystallography of cosmic particles, as well as their elemental and isotopic compositions, would be very valuable information toward understanding the nature of our solar system. This information can be obtained from the intact capture of large mineral grains of cosmic particles from hypervelocity impacts. Hypervelocity experiments of intact capture in underdense media have indicated realistic potential in this endeaver. The recovery of the thermal blankets and louvers from the Solar Max spacecraft have independently verified this potential in the unintended capture of cosmic materials from hypervelocity impacts. Passive underdense media will permit relatively simple and inexpensive missions to capture cosmic particles intact, either by going to a planetary body or by waiting for the particles to come to the Shuttle or the Space Station. Experiments to explore the potential of using various underdense media for an intact comet sample capture up to 6.7 km/s were performed at NASA Ames Research Center Vertical Gun Range. Explorative hypervelocity experiments up to 7.9 km/s were also made at the Ernst Mach Institute. These experiments have proven that capturing intact particles at hypervelocity impacts is definitely possible. Further research is being conducted to achieve higher capture ratios at even higher hypervelocities for even smaller projectiles

A review of lunar sample studies and their application to studies of the terrestrial planets

During the last half decade, hundreds of scientists from many countries have been studying the samples, photographs, and instrumental data returned from the moon by the Apollo and Luna programs. These studies have placed significant limits on chemical, petrologic, and physical parameters, on the time of many events, and on the rate of many processes and are giving greater insight into the natural processes that formed the moon and shaped its surface. Increasingly, it is being recognized that very similar processes governed the origin and evolution of planetary bodies throughout the solar system. Spacecraft have extended our sensors to all the terrestrial planets, and the insights gained from Apollo dominate our interpretation of the photographic and instrumental data returned from these bodies