For the nineteenth season, the LPI Summer Intern Program offers selected under-graduate students an opportunity to participate in lunar and planetary science research at the Institute and the NASA Johnson Space Center. The 1995 program begins on June 13 and ends August 19. For more information or to apply for internship next year, contact LPI Summer Intern Program, 3600 Bay Area Boulevard, Houston TX 77058-1113. Participants in this summer's program will work on the following projects under the guidance of their advisors at LPI and JSC.

CHRISTOPHER D. ADKINS, Virginia State University

Advisor: Paul Spudis, Lunar and Planetary Institute

The intern will use laser altimetry and a global topographic map produced by the Clementine lunar mission to study processes in the formation of lunar basins. Basin volumes and the topographic relief of the rim crest and ring segments of multiring basins are constraints on the volumes of material excavated and the amounts of structural uplift in basin formation. The quantities are difficult to determine because many basins are flooded with basalt, and, until now, we have not had the data necessary to measure basin volumes and ring and rim relief accurately. The Clementine laser altimeter has provided topographic profiles with 50-meter vertical precision along longitude and the data has been gridded into a global topographic map of 100-kilometer resolution. We will use this data for relatively fresh, unflooded basins, such as Orientale and Humboldtianum, to determine basin volume and ring relief. Measured values will be compared to predicted values to test models of basin formation and evolution. The work will involve computer image manipulation, photogeology, and the creation of digital image models.

BRIAN E. BREWINGTON, California Institute of Technology

Advisor: Stephen Clifford, Lunar and Planetary Institute

A critical feature of most attempts to model the diffusive transport and storage of H2O within the martian regolith is the assumption that its diffusive properties can be reasonably approximated by a soil of uniform pore size. However, this simplification is only valid when the diffusing species is a noncondensable gas. When the gas can condense and obstruct the pore network, the effect of pore structure cannot be ignored, because the condensation of ice preferentially occurs in the smallest (and most numerous) pores, which will impede the rate and extent of diffusive transport and choke off much of the pore network well before the larger pores are filled with ice. The net effect is that, when compared against soils that have geologically reasonable pore size distributions, calculations based on the assumption of a uniform pore size can over-predict vapor transport and the rate of ice accumulation by as much as several orders of magnitude. To calculate the diffusive flux of a condensable gas through a geologically reasonable soil, an accurate model of both the pore-size distribution and pore structure must be used. Unfortunately, even some of the best models for describing diffusion through soils with broad pore-size distributions fail miserably when applied to diffusion problems that involve condensation. Because of the important constraint that pore structure and ice condensation place on diffusion in soil_and because of the implications that such transport has for regolith/atmosphere exchange, the stability of ground ice, and the evolution of the martian climate_our efforts will focus on developing a computer model of pore structure and diffusive transport that more accurately reflects the complexity of this process. We will attempt to test the model against "ground truth" by comparing its predictions with the results of various experimental studies that have been reported in the terrestrial soils literature.

CATHERINE M. CORRIGAN, Michigan State University

Advisor: Mike Zolensky, NASA Johnson Space Center

By exploring the nature of interplanetary dust particles (IDPs) and carbonaceous and ordinary chondrites, we hope to learn a great deal about the original state of the solar nebula, as well as the subsequent processes that have shaped it. One particular area of interest is aqueous alteration of materials, which has played a large role in the development of some asteroids and possibly comets as well. Determining the porosity of a sample is important in studying aqueous alteration. There are already a number of time-proven methods for determining porosity in a bulk sample, and actual, physical liquid/gas flow measurements work quite well in samples large enough to test in this manner. However, these methods are not applicable to small samples of IDPs and most meteorites. By utilizing scanning electron microscope (SEM) images and computer image processing (in the VDAS lab), last summer we developed a method for determining sample porosity efficiently and accurately for nanogram-sized samples. Much work remains to be done. Relatively few IDPs have been examined, and there is a paucity of data for carbonaceous chondrites. This summer we will continue our work on IDPs and extend it to carbonaceous and ordinary chondrites on samples now in hand. With the results of this summer's work in hand, we should be able to proceed with three-dimensional numerical modeling of the fluid flow and attendant alteration within hydrous asteroids.

JAMES T. DAVENPORT, South Dakota School of Mines and Technology

Advisor: Paul Schenk, Lunar and Planetary Institute

Io is the most volcanically active object in the solar system, yet we know little about even the gross composition of the lavas that cover the vast majority of its surface. Numerous volcanic pits, or calderas, dot the surface. Geologic mapping and dimensional analysis of volcanic calderas will allow us to constrain the physical strength of Io's tortured crust and may ultimately provide clues to the nature and evolution of magma beneath the crust. The intern will learn stereo (not previously used for Io) and multispectral imaging techniques as well as develop geologic mapping techniques using remote sensing data.

MICHELE C. DODGE, University of Hawai at Hilo

Advisor: Walter S. Kiefer, Lunar and Planetary Institute

Lunar mascon basins are regions of above-average gravitational acceleration, implying the presence of excess mass. This situation is unexpected for topographic lowlands. The Clementine spacecraft recently made near-global gravity and topography observations of the Moon, with higher spatial resolution and more uniform coverage than previous datasets. The intern will analyze data for one or two mascon basins to develop improved models for the mass distributions and compensation mechanisms responsible for these gravity anomalies.

DAVID W. GWYNN, Rutgers University

Advisor: Fredrich Hörz, NASA Johnson Space Center

We are participating in the development of flight instruments to collect hypervelocity cosmic dust particles in space and return them to Earth for compositional analysis. Such instruments use thin foils that will be penetrated by most particles. A substantial fraction of particles will fragment at these impact conditions. The size-frequency distribution and radial dispersion of these fragments determines how much material per unit surface area can be collected on the collector-substrate to the rear of the penetrated foil. This spatial density of impactor residue in turn dictates which analysis methods may be suitable (or unsuitable) for studying composition. We study this fragmentation process using powder propellant and light-gas guns that launch small glass spheres from 1 to 7 kilometers per second against aluminum foils of variable thickness. A massive witness plate behind the penetrated foil simulates the collector substrate and intercepts the debris cloud exiting the target foil. This cloud, however, consists of both projectile fragments and debris dislodged from the target foil itself. There are no reliable morphologic criteria to distinguish among the secondary craters on the witness plate those that were produced by projectile fragments from those caused by target debris. Scanning Electron Microscope (SEM) methods, combined with Energy Dispersive X-Ray Spectroscopy (EDS), are necessary to reveal the actual fragment composition responsible for each witness plate crater. The intern will analyze three or four representative witness plates and determine the spatial distribution of projectile fragments (silicate) vs. foil debris (aluminum) using SEM-EDS methods in the elemental mapping mode. These element maps will then be used to deduce the spatial distribution of both projectile and target-derived debris and associated size distributions, the latter extracted from a crater diameter measurement.

BETH N. HARTMAN, Smith College

Advisor: Deborah Domingue, Lunar and Planetary Institute

Hapke's photometric model is used to describe textures of different planetary surfaces and compare their reflective properties to one another by using a mathematical description of how individual particles scatter light. Understanding how the particles or grains of a planet's regolith scatter light is important in interpreting planetary images and in understanding geologic processes that have formed and modified planetary surfaces. Currently, the best method or mathematical expression to use for describing the particle-scattering behavior is controversial. Helfenstein et al. (1994) have submitted a paper to Icarus that examines albedo dependence on particle phase functions. In this project we will examine three different expressions for particle phase function to see which best describes the laboratory work of McGuire and Hapke (1994). The selected function will then be applied to snow reflectance data, supplied by Anne Verbiscer, to help interpret results of Hapke modeling of icy outer planet satellite surfaces.

KEVIN C. A. PETERSON, Acadia University

Advisor: Arch Reid, Lunar and Planetary Institute, and University of Houston, Department of Geosciences

Sample DOM85505 is an LL5 chondrite from the 1993 Antarctic meteorite collection. The meteorite is unusual in that it contains an achondritic clast that cannot readily be matched with the common achondrites. One objective of this study is to study the main meteorite, which is a highly shocked LL chondrite, and to compare DOM85505 with other known chondrites. A more important objective is to provide a description of the achondritic clast and to compare that clast with other achondrite meteorites. Rare achondrite-chondrite mixtures are most likely a consequence of impact of an achondrite onto the chondritic parent. We hope to establish the type of achondrite involved, specifically, whether it represents a new type of achondrite, or a more familiar type subjected to unusual conditions. Furthermore, we wish to explore the significance of this unusual association for aggregation histories of chondrites and achondrites.

KATHERINE RAWLINS, Yale University

Advisor: Julianne I. Moses, Lunar and Planetary Institute

Groundbased radar observations of Mercury have revealed surprisingly strong depolarized echoes from Mercury's north and south poles. These radar signatures are consistent with the presence of water ice in polar crater floors and other permanently shaded regions at high latitudes on the otherwise extremely hot planet. Recent thermal models show that ice in permanently shaded polar regions on Mercury should be stable over long timescales. However, these studies do not consider all the possible loss mechanisms for polar ice and do not examine the details of how the water might have arrived at Mercury in the first place. We will examine the possible sources and sinks of water on Mercury and explore the stability of water ice in Mercury's polar regions.

KAREN SPIKER, New Mexico Institute of Mining and Technology

Advisor: Allan Treiman, Lunar and Planetary Institute

The walls of the Valles Marineris canyons on Mars are steep scarps up to 8 kilometers tall, and provide a unique view of Mars subsurface geology. The plateau above the canyons is cut by many small normal faults that form shallow grabens. Some of these fault lines appear to continue onto the canyon's walls as ridges, more resistant to weathering than the surrounding material. Fault lines usually do not form ridges, as the crushed rock in fault zones weathers more easily than the surrounding solid rock. These ridges in Valles Marineris are almost unstudied. In this research project, the intern will learn the geology and setting of the Valles Marineris, map selected areas of the Valles Marineris in detail, and model the three-dimensional shapes of the fault/ridge surfaces. These data and analogies from the Earth will be used to examine how the ridges may have formed and their implications for the volcanology, tectonics, and hydrology of Mars.

CATHERINE THIBAULT, Denison University

Advisor: Faith Vilas, NASA Johnson Space Center

C-class asteroids probably underwent aqueous alteration during their history in the solar system. Spectral reflectance studies in the visible and near-infrared have identified absorption features similar to those seen in laboratory reflectance spectra of phyllosilicates. This project will involve study of a large number of telescopic narrowband reflectance spectra of asteroids that contain the dominant 0.7- micrometers Fe2+- Fe3+ absorption feature, quantifying the characteristics of these absorption features, and studying any trends found in the context of solar system compositional formation.

JOHN M. WOODELL, North Carolina State University

ADVISORS: Carlton C. Allen, NASA Johnson Space Center and David S. McKay, NASA Johnson Space Center

Violent volcanic eruptions on the Moon have produced large deposits of microscopic glass beads. These glasses have been found in many lunar soil samples, including some composed almost completely of glass. Several of the glasses have been shown to be outstanding "ores" for oxygen production at a future lunar base. We have conducted experiments to obtain oxygen from a wide range of lunar soils, and are now studying the release of oxygen specifically from the volcanic glass beads in these samples. The intern will use optical and electron microscopes to identify the volcanic glass particles in each soil sample and correlate the glass compositions with evidence of oxygen release. In addition, the intern will use remote sensing data from the Clementine lunar mission to identify the compositions of pyroclastic deposits on the lunar surface.