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RESEARCH IN PLANETARY GEOLOGICAL SCIENCES

Brown University

Professors:
James W. Head, Paul C. Hess, John F. Mustard, E. Marc Parmentier, Carle M. Pieters, Malcolm J. Rutherford, Peter H. Schultz
Visiting Professors:
Lionel Wilson (Lancaster University)
Visiting Senior Researcher:
Alexander Basilevsky (Vernadsky Institute, Moscow)
Senior Research Scientist:
Robert Pappalardo
INTRODUCTION
Active involvement in planetary research at Brown dates back to the mid-1960's near the beginning of the modern era of planetary exploration. This tradition continues today with involvement in a wide variety of research programs and continued participation in planetary missions. A major emphasis that links diverse approaches within the Department of Geological Sciences is the study of surface processes and their role in the overall evolution of planetary geologic history and internal evolution. A variety of perspectives is provided by the Brown faculty: planetary processes, remote sensing, geophysics, petrology, and terrestrial analog studies. As a consequence, students involved in Planetary Geological Sciences at Brown commonly develop cross-disciplinary backgrounds with the graduate program tailored to emphasize the strengths of individual students while maintaining a solid foundation in the geological sciences.

RESEARCH THEMES

Three broad areas provide the foundation of interdisciplinary studies of the planets and satellites including the Earth: Research efforts within these three areas involve an integration of experimental, theoretical, and observational analyses.

RECENT RESEARCH RESULTS

The following discussion highlights particularly exciting results in the context of the research themes identified above.

The Planetary Geological Processes theme focuses on the mechanisms and effects of forces that shape planetary surfaces and their relation to the interior. The infrequency of major collisions on the Earth may seem to lessen the imperative for understanding the impact process, but both the planetary record and signatures of past catastrophic events in Earth history establish this process as fundamental in importance. The near-collision with an asteroid in March 1989 and the Tunguska collision in 1908 underscore the relevance of this process, even on human time scales. Members of the Planetary Group study this process through field studies, laboratory experiments, analysis of the contrasting planetary record, theoretical models, and spectroscopic signatures. Examples of recent research include analysis of oblique impacts as a cause for prolonged environmental stress on the Earth due to re-entry of high-velocity ricocheted debris ("skipping stones"). Laboratory experiments have permitted exploring the effect of an atmosphere on the size of impact craters as well as the stages of emplacement of ejecta, thereby helping to understand not only geologic time scales on planets under very different environments (atmospheric pressures ranging over a factor of 10,000) but also the significance of contrasting morphologies (Venus, Mars, the Moon). This approach has led to the identification of a unique series of oblique impacts in Argentina and better understanding of the spectacular impact craters on Venus. Understanding of the cratering process through such efforts has permitted us to begin characterization of lunar crustal stratigraphy using craters as probes of the interior.

In another area, theoretical models of the ascent and eruption of magma have been developed and tested by comparing predictions with observations of features on Earth and on bodies with low gravity and no atmosphere (Moon) to the extreme of high gravity and dense atmosphere (Venus). We have derived a basic model of the physics of ascent and eruption on the planets and applied this to predictions of neutral buoyancy zones on Venus and their relation to the unusually low volcanic edifices observed there, density traps and dike emplacement on the Moon and an explanation of why no major shield volcanoes are observed there, and to the eruption of gas-containing magmas on Mars and the Moon and assessment of pyroclastic deposits produced there. These analyses have also given us new insight into the emplacement of plume-related volcanic deposits on the Earth and Venus. Internal processes shape planetary surfaces over a much longer time frame in response to variations in temperature, composition, and phase changes in the interior. Correlation of surface patterns of volcanism, tectonism, and impact cratering with models of processes and evolution of planetary interiors leads to a general understanding of planetary evolution.

Much of the of the information we have about any planet comes from measuring its physical and chemical properties, either directly through laboratory analysis of selected samples or indirectly via sophisticated remote sensors. Laboratory methods that establish the foundation for interpretation continue to improve in measurement capabilities and breadth (visible through mid-infrared spectroscopy). Information extraction techniques are becoming more powerful and sophisticated in parallel with the vast increase in the amount of available laboratory and observational data. Advanced sensors are being developed and utilized in terrestrial and extraterrestrial applications and are powerful new tools for exploration, especially when data are examined in a regional or global context. The information derived is used to set essential boundary conditions to understand the broader questions concerning Formation and Evolution of Planetary Crusts and Interiors.

The nature and distribution of surface compositions of planetary bodies detected with spectroscopic techniques has provided new information that can be used to address issues of crustal evolution. For example, using lunar craters as probes of the interior has allowed the probable detection of mafic layered plutons on the Moon (associated with 50-100 km diameter craters) and possibly the detection of exposed deep crust or mantle in deposits of a 2000 km diameter basin on the lunar farside. These are windows into the first 500 million years of lunar crustal evolution. Similarly, from a range of remotely acquired information it is now known that the variety, extent, and timing of lunar volcanism are substantially greater than that recognized from the returned lunar samples. The composition of the Martian crust and its spatial diversity has been enigmatic, largely because much of the surface is thought to be heavily altered or covered by dust. However, through the use of advanced sensor data and new algorithms we have seen through this altered surface layer and found relatively pristine areas of distinct volcanic compositions. Analyses of compositional diversity in the altered surface layer itself reveals that deposits of different ages were formed under very different conditions than the present. Complementary to this is the development of a conceptual framework for the formation and evolution of planetary crusts. We are assessing this from the point of view of primary crusts (e.g., impact related; the lunar highland crust), secondary crusts (e.g., mantle partial melting; the lunar maria, the laterally accreting oceanic crust of Earth, and the vertically accreting crust of Venus), and tertiary crust (e.g., reworked primary and secondary crust; the terrestrial continents, and possibly the mountain belts and tesserae of Venus). Meteorites provide an additional dimension, yielding clues to the earliest periods of planetary formation. Spectroscopic analyses of meteorites and their potential parent bodies in the asteroid population are one of the few means to evaluate the products of primordial condensation.

Venus is the most Earth-like of the planets and satellites. Radar images of Venus from the recent Magellan Mission have revealed a geologically young, highly volcanically modified, folded and faulted surface remarkably similar in some ways to the Earth, but quite different in others. This information has opened a wide range of issues concerning the interior of Venus, its evolution, the way it transfers heat from its interior to the surface, modes of crustal formation, the possibility that the planet periodically catastrophically overturns and resurfaces, and the implications for understanding terrestrial geologic and tectonic history. In dramatic contrast, Mars exhibits little evidence for extensive tectonics; instead, the entire planet or its rigid outer shell may have moved in response to external (e.g., impact) and internal (e.g., upwelling) events. Such a process, once proposed to explain the apparent shifting latitudes during Earth history, now seems more applicable to one-plate planets such as Mars. This perspective affects not only our view of the martian interior but also our view of the exchange of water between subsurface reservoirs and the atmosphere. The evolution of a planet as a multiple plate planet (e.g., Earth), a single-plate planet (e.g., the Moon, Mars, and Mercury), or a planet undergoing pulses of plate tectonics and lithospheric instability (e.g., Venus?) provides a signature of the history of the interior, uniquely revealed by Planetary Comparisons.

NEW RESEARCH OPPORTUNITIES
IN THE NEXT DECADE

Marcel Proust described voyages of discovery as consisting of both seeking new landscapes and having new eyes. New planetary missions, advancing technologies, unexpected discoveries, and new perspectives all will stimulate scientific opportunities over the next decade. An abundance of new data about the surface of Venus, the Moon, Mars, satellites of Jupiter, and primitive bodies is anticipated as a result of recent and forthcoming activities. New techniques for remotely sensing diverse surface compositions, new environments for laboratory studies (e.g., low-g earth orbit), advanced Earth and space-based telescopes, and electromagnetic rail guns will parallel this advance in mission data. The past demonstrates, however, that unexpected discoveries (e.g., meteorites from Mars, a cosmic signature and major impact structure at a geologic boundary correlated with life extinctions) or new perspectives (e.g., the role of impacts in climate changes and in the formation of the Earth-Moon system) play an equally important role in creating new opportunities and establishing new objectives. The following examples provide a philosophical overprint that places such opportunities into a strategy of the approach of our group.

MISSION TO PLANET EARTH:
THE PLANETARY PERSPECTIVE

The age of space exploration has caused us to see the Earth as one member of the Solar System that cannot be viewed in isolation and is subject to the same mechanisms and processes as other planets. We now view the Earth as a set of globally interconnected surface-hydrosphere-atmosphere systems, but a planet finite in size, capacity for change, and resources. Geological scientists will be playing an important role in understanding and monitoring the Earth as a system, from providing a planetary perspective and chronicle of the past, to observing the present and future with advanced sensors as part of the Mission to Planet Earth. We plan to actively apply our expertise in planetary problems and advanced information extraction to Earth Systems Science issues in these areas.

BRIDGING DISCIPLINES

Planetary exploration both provides and requires new perspectives as well as new data. Related areas including advanced computer modeling of complex processes, intact capture and return of primitive solar system materials, astronomical investigations of completely inaccessible regions of the solar system(s), probing the materials and processes linked to life's beginnings, etc. Such fields will provide unexpected yet extremely relevant applications to planetary science. Earth System Science is emerging as a critical new direction for interdisciplinary research which will involve a strong earth science component. The approaches pioneered in the planetary program will figure significantly in this new area, largely because of the interdisciplinary nature of planetary geoscience. Much Earth System Science research will involve understanding the interrelationship among the primary systems of the Earth including oceans, atmosphere, and the land surface. The data to be acquired by the Earth Observing System will be global in nature and will be obtained by advanced sensors. Members of the planetary group are planning an active role in the analysis of these data and in bridging disciplines to bring new understanding to the critical area of new research opportunities (e.g., global surface processes, advanced analysis approaches applied to change detection, etc.).

GEOLOGICAL PROCESSES IN CONTRASTING
ENVIRONMENTS AND CONDITIONS

Different planetary settings permit testing models of geologic processes. Examples include volcanic eruptions in high pressure (Venus, sub-oceanic) and low pressure (Mars, Moon) environments; impact cratering under atmospheric pressures ranging seven orders of magnitude and gravity over three orders of magnitude; erosional and weathering styles and rates as a function of gravity and atmospheric conditions. For example, the thermal structure of Venus lithosphere may be comparable to that in the Earth's Archean; hence, Venus may provide clues for crustal formation and continental accretion processes in early Earth history. In contrast, the preserved cratering record on Mars and Venus provides a unique perspective for exogenically driven forces and a unique opportunity for interpolation to the Earth. In addition, the space environment on the Moon and asteroids creates an entirely different weathering regime than that which is commonly encountered in terrestrial situations; paradigms of soil formation will continue to undergo reassessment and development.

CATASTROPHISM AND CHAOS

Awareness of sudden changes in the environment caused by planetary geologic processes has increased due to events such as Mount Pinatubo and evidence for a major impact at the end of the Cretaceous era of Earth history. This trend will continue as different planetary environments reveal additional records of major climatic changes (e.g., Mars) and records of catastrophic processes (e.g., impact cratering on the Moon). The chaotic response to catastrophic processes gives "uniformitarianism" new meaning from earliest stages of planetary formation and accretion to the subsequent geologic response including early differentiation, crustal formation, and climate change. Major strides will be made in the next decade as sophisticated techniques for monitoring, characterizing, and modeling such processes permit testing alternative scenarios from the terrestrial and planetary geologic record.

TESTS FOR THE PLURALITY OF PLANETS

Do similarities or differences in the present geologic state of planets reveal fundamental differences in the starting conditions or evolutionary paths? Detailed information about planetary surfaces permits reassessment of the nature of planetary interiors, processes of planetary heat loss, patterns of convection and thermal evolution, early differentiation and crustal formation, and exchange processes with the atmosphere. Comparative planetology permits testing fundamental issues of planetary formation and evolution through new tools, perspectives, and data.

INTERDISCIPLINARY INTERACTIONS

The breadth of planetary science demands an interdisciplinary approach. Within the department, members of the Planetary Group regularly interact with research by other subgroups: mineralogy, petrology, geochemistry (geochemical and petrological analysis of lunar samples and meteorites, planetary petrogenesis, and experimental studies in planetary environmental conditions); tectonophysics (terrestrial and planetary tectonic and geophysical processes); and climate dynamics (basic parameters affecting climate change on the planets).

Within the University we have collaborations with the Center for Foreign Policy Development (interaction on US/Russian scientific exchange), Engineering Division (detector science, optical design), Computer Sciences ("image cube" analysis, supercomputer problem solving, development of teaching software), Ecology (global change, biodiversity, remote sensing), and Environmental Sciences (global perspective, Earth System Science).

Collaborative and off-campus research activities at other institutions include measurements of planetary surface composition at the University of Hawaii and Mauna Kea Observatory; impact experiments at NASA-Ames Research Center in California; Galileo mission activities at Jet Propulsion Laboratory in California; collaborations with private industry such as California Research and Technology; involvement with National Laboratories such as Los Alamos and Sandia; planetary radar data acquisition at Arecibo Observatory, Puerto Rico; Mars Orbiting Laser Altimeter activities at Goddard Space Flight Center, Maryland; Mars 1996-1998 mission and other scientific studies at the Institute of Space Research, Russian Academy of Sciences, Moscow; and the general scientific cooperation and collaboration including Venera and Luna Sample Return mission data analysis at the Vernadsky Institute, Russian Academy of Sciences, Moscow. Such involvement brings a vitality to both the graduate and undergraduate programs in teaching and research (e.g., Russian scientists talking about the history of the Soviet space program to undergraduate students in Geological Science 5).

Planetary Geology | Brown University