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Thomas, P. C.

Publications and source records attributed to Thomas, P. C..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2013

The Face that Launched a Thousand Slips

Helene, (approximately 17.6 kilometers mean radius) is an L4 Trojan co-orbital of Saturn's moon Dione. Its hemisphere features an unusual morphology consisting of broad depressions and a generally smooth surface patterned with streaks and grooves. The streaks appear to be oriented down-gradient, as are the grooves. This pattern suggests intensive mass-wasting as a dominant process on the leading hemisphere. Kilometer-scale impact craters are very sparse on the leading hemisphere other than the degraded kilometer-scale basins defining the overall satellite shape, and many small craters have a diffuse appearance suggesting ongoing mass wasting. Thus mass wasting must dominate surface-modifying processes at present. In fact, the mass wasting appears to have been sufficient in magnitude to narrow the divides between adjacent basins to narrow septa, similar, but in lower relief, to the honeycomb pattern of Hyperion. The prominent groves occur primarily near topographic divides and appear have cut into a broad, slightly lower albedo surface largely conforming to the present topography but elevated a few meters above the smooth surfaces undergoing mass wasting flow. Low ridges and albedo markings on the surface suggest surface flow of materials traveling up to several kilometers. Diffusive mass wasting produces smooth surfaces - such a pattern characterizes most of the low-lying surfaces. The grooves, however, imply that the transport process is advective at those locations where they occur, that is, erosion tends to concentrate along linear pathways separated by divides. In fact, in many places grooves have a fairly regular spacing of 125-160 meters, defining a characteristic erosional scale. Several questions are prompted by the unusual morphology of Helene: 1) What is the nature of the surface materials? 2) Are the transport processes gradual or catastrophic motion from one or a few events? 3) What mechanisms drive mass wasting and groove development? 4) Have the formative processes been active in the recent past? 5) Finally, is the surface accreting or eroding? The smooth character of the leading edge hemisphere of Helene and the dominance of mass wasting suggest that the surface is composed of fine-grained debris, probably dominated by dust-size to small gravel particles. The Lagrangian points of saturnian satellites are locations where planetesimals might have accreted to form co-orbital satellites such as and they may capture ejecta from their master moon. Published models suggest that Helene is a site of net accretion, but we find no extant explanation for the dominance of fine grain sizes for the surface (and probable subsurface) composition of Helene and the other Lagrangian satellites. Observation of the mass wasting tracks on Helene suggests the presence of well-defined streams of debris with low bordering levees that may be depositional features or remnants of the dissected higher surface. Some flows in grazing illumination appear to have a convex cross-section. This mass-flow morphology might be consistent with the occurrence of large-scale flow events, but which might have occurred through rapid emplacement or slow glacier-like creep. On the other hand, small craters appear to have been "softened" by creep-like processes, indicating ongoing mass wasting.

honeycomb pattern of Hyperion↗
NASA NTRS2011

Evolution of Planetesimals Accreted in the Early Solar System

The purpose of this presentation is to point out that the origins and abundances of short-lived nu-clides in the early solar system had important conse-quences for "icy planetesimals". It is believed that these planetesimals, composed of ice and rock, were once very abundant in the early, outer solar system. Today, spacecraft can visit remnants of that popula-tion and measure their properties. Cassini's flyby of Saturn's satellite Phoebe may have been the first visit to an object related to this population.

Saturn satellites↗
NASA NTRS2007

Iapetus' Geophysics: Rotation Rate, Shape, and Equatorial Ridge

Iapetus has preserved evidence that constrains the modeling of its geophysical history from the time of its accretion until now. The evidence is (a) its present 79.33-day rotation or spin rate, (b) its shape that corresponds to the equilibrium figure for a hydrostatic body rotating with a period of approximately 16 h, and (c) its high, equatorial ridge, which is unique in the Solar System. This paper reports the results of an investigation into the coupling between Iapetus' thermal and orbital evolution for a wide range of conditions including the spatial distributions with time of composition, porosity, short-lived radioactive isotopes (SLRI), and temperature. The thermal model uses conductive heat transfer with temperature-dependent conductivity. Only models with a thick lithosphere and an interior viscosity in the range of about the water ice melting point can explain the observed shape. Short-lived radioactive isotopes provide the heat needed to decrease porosity in Iapetus? early history. This increases thermal conductivity and allows the development of the strong lithosphere that is required to preserve the 16-h rotational shape and the high vertical relief of the topography. Long-lived radioactive isotopes and SLRI raise internal temperatures high enough that significant tidal dissipation can start, and despin Iapetus to synchronous rotation. This occurred several hundred million years after Iapetus formed. The models also constrain the time when Iapetus formed because the successful models are critically dependent upon having just the right amount of heat added by SLRI decay in this early period. The amount of heat available from short-lived radioactivity is not a free parameter but is fixed by the time when Iapetus accreted, by the canonical concentration of Al-26, and, to a lesser extent, by the concentration of Fe-60. The needed amount of heat is available only if Iapetus accreted between 2.5 and 5.0Myr after the formation of the calcium aluminum inclusions as found in meteorites. Models with these features allow us to explain Iapetus? present synchronous rotation, its fossil 16-h shape, and the context within which the equatorial ridge arose.

geophysics↗
NASA NTRS2007

Cassini Observes the Active South Pole of Enceladus

Cassini has identified a geologically active province a the south pole of Saturn's moon Enceladus. The shape of Enceladus suggests a possible intense heating epoch in the past by capture into a 1:4 secondary spin/orbit resonance.

Porco, C. C.↗
NASA NTRS2005

The First Six Months of Iapetus Observations by the Cassini ISS Camera

Since Saturn arrival in June 2004, Iapetus has been studied intensively by the Cassini ISS camera [1] at various ranges. The first of two relatively close flybys in the primary mission occurred on Dec 31, 2004 at an altitude of approx.123,400 km over the northern leading hemisphere, resulting in images with a minimum pixel scale of 740 m. Detailed results of this flyby are given in [2], while this abstract covers the observations obtained earlier. Among the most important discoveries are: (a) Four giant impact basins with diameters between 390 and 550 km were detected, three of them are located in the dark terrain [3]. (b) Data revealed a >1300 km long ridge that marks exactly Iapetus' equator within the dark terrain. Individual mountains within the western part of the ridge reach heights of approx.20 km over surrounding terrain [3]. (c) Impact craters were confirmed to be the main geological feature within the dark terrain and at high southern latitudes. (d) There are numerous craters with dark walls roughly facing towards the central parts of the dark hemisphere [3]. (e) Almost all parts of Iapetus have been imaged at least at low resolution (< 60 km/pxl).

Denk, T.↗
NASA NTRS2005

First Imaging Results from the Iapetus B/C Flyby of the Cassini Spacecraft

The first of two relatively close Iapetus flybys in Cassini's primary mission occured on Dec 31, 2004 18:49 UTC near apoapsis from orbit "B" to "C" at an altitude of approximately 123,400 km over the northern leading hemisphere, resulting in a minimum pixel scale of 740 m for the ISS narrow angle camera (NAC). Data revealed details of a greater than 1300-km-long ridge that had been discovered just one week earlier in optical navigation images. Individual mountains within the western part of the ridge reach heights of approximately 20 km over surrounding terrain. The data set provides constraints on the origin of the albedo dichotomy. It appears very likely that the dark material is overlying an ice crust, but no evidence for emplacement of dark material via surface flows is apparent. Instead, signs for dark-material emplacement through processes that included ballistic transportation are visible. No bright-floor ("punch-through") craters have been found on the dark hemisphere. The ridge discovery may revive the idea of an endogenic origin of the dark side.

Denk, T.↗
NASA NTRS2005

Seismic Shaking Removal of Craters 0.2-0.5 km in Diameter on Asteroid 433 Eros

Impact cratering acts in a variety of ways to create a surprising range of scenery on small satellites and asteroids. The visible crater population is a self-modifying characteristic of these airless objects, and determining the various ways younger craters can add or subtract from the population is an important aspect of small body "geology." Asteroid 433 Eros, the most closely studied of any small body, has two aspects of its crater population that have attracted attention: a fall-off of crater densities below approx.100 m diameter relative to an expected equilibrium population [1] and regions of substantially lower large crater densities [2, 3, 4]. In this work we examine the global variation of the density of craters on Eros larger than 0.177 km, a size range above that involved in small crater depletion hypotheses [1, 5]. We counted all craters on Eros to a size range somewhat below 0.177 km diameter (and different from data used in [3]). The primary metric for this study is the number of craters between 0.177 and 1.0 km within a set radius of each grid point on the 2deg x 2deg shape model of Eros. This number can be expressed as an R-value [6], provided that it is remembered that the large bin size makes individual R values slightly different from those obtained in the usual root-2 bins.

Thomas, P. C.↗
NASA NTRS2001

The Porosity of Eros and Implications for Its Internal Structure

We estimate the porosity of Eros to be between 21-33%. Based on this porosity range and morphologic observations of the surface, we suggest that the asteroid is heavily fractured (but not a rubble pile). Additional information is contained in the original extended abstract.

Wilkison, S. L.↗
NASA NTRS2000

The Residual Polar Caps of Mars: Geological Differences and Possible Consequences

The Martian polar regions have been known to have thick layered sequences (presumed to consist of silicates and ice), CO2 seasonal frost, and residual frosts that remain through the summer: H2O in the north, largely CO2 in the south. The relationship of the residual frosts to the underlying layered deposits could not be determined from Viking images. The Mars Orbiter Camera on Mars Global Surveyor has provided a 50-fold increase in resolution that shows more differences between the two poles. The north residual cap surface has rough topography of pits, cracks, and knobs, suggestive of ablational forms. This topography is less than a few meters in height, and grades in to surfaces exposing the layers underneath. In contrast, the south residual cap has distinctive collapse and possibly ablational topography emplaced in four or more layers, each approx. two meters thick. The top surface has polygonal depressions suggestive of thermal contraction cracks. The collapse and erosional forms include circular and cycloidal depressions, long sinuous troughs, and nearly parallel sets of troughs. The distinctive topography occurs throughout the residual cap area, but not outside it. Unconformities exposed in polar layers, or other layered materials, do not approximate the topography seen on the south residual cap. The coincidence of a distinct geologic feature, several layers modified by collapse, ablation, and mass movement with the residual cap indicates a distinct composition and/or climate compared to both the remainder of the south polar layered units and those in the north.

Thomas, P. C.↗
NASA NTRS1996

MASTER: An Orbiter for the Detailed Study of Vesta

MASTER (Mainbelt Asteroid Exploration/Rendezvous), a Discovery-class orbiter, will carry out a global geological and geochemical survey of Vesta. Primary goals include determining Vesta's mean density and interior structure through detailed mapping of the gravity field, imaging surface morphology at 3-m resolution, mapping mineralogy between 0.4 and 2.5 microns, and determining abundances of key elements through X-ray and gamma-ray spectroscopy. Spectroscopic evidence indicates the presence of basaltic lava flows on Vesta and suggests that impact basins may have exposed mantle materials. These possibilities, combined with the likelihood that Vesta is the ultimate source of HED meteorites, makes this asteroid an important target for a comprehensive orbiter mission. MASTER's global survey, combined with ongoing studies of HED meteorites, will be a major step in understanding the chemical, thermal, and geological evolution of Vesta. A particularly attractive opportunity involves a launch in June 2003 with arrival at Vesta in November 2009. The orbital phase of the mission is scheduled to last one year. A flyby of another mainbelt asteroid enroute to Vesta may be possible.

Veverka, J.↗
NASA NTRS1995

Analysis of Gaspra lightcurves using Galileo shape and photometric models

Galileo-based models for the shape of 951 Gaspra and the global-average photometric behavior of its surface have been used to model a representative subset of the asteroid's telescopic lightcurves. Fitting the synthetic lightcurves to the observed timing of lightcurve extrema, and knowing the orientation of Gaspra's axes at the time of the Galileo flyby, leads to a sidereal rotation period for the asteroid of 7.042024 +/- 0.000020 hr, a slight change from the period reported by Magnusson et al. (1992). Initially, the shapes, amplitudes, and absolute photometry of the synthetic and observed lightcurves agree with each other to within 0.05-0.1 mag. Small modifications to the Gaspra shape model on sides of the asteroid poorly imaged by Galileo (changes of 700 m or less in the southern hemisphere at longitudes 90 deg-270 deg W) reduce the typical discrepancies to approximately 0.05 mag in lightcurve shape and less than 0.03 mag in absolute photometry. The result demonstrates that Earth-based lightcurves can be used to refine the shape of a spacecraft-imaged irregular object in areas that are poorly constrained by the spacecraft observations. The consistency and phase-angle dependence of the Galileo-based model for Gaspra photometry, supports the accuracy of the absolute calibration of the Galileo SSI camera, and confirms the Earth-based determination of the V-filter geometric albedo of the asteroid (0.22 +/- 0.03; Tholen et al., submitted for publication). Remaining discrepancies between the synthetic and observed lightcurves show no indication of systematic latitudinal variations in albedo and also cannot be explained entirely by isolated albedo spots. These discrepancies are most likely caused by (1) small, remaining, hard-to-constrain errors in the Gaspra shape model and/or (2) moderate variations in macroscopic roughness across the asteroid's surface, in particular making longitudes 130 deg to 300 deg W moderately rougher than the opposite hemisphere.

Simonelli, Damon P.↗
NASA NTRS1993

The Clementine Mission science return at the Moon and Geographos

The Clementine Mission is being built and flown by the Naval Research Laboratory under the sponsorship of the Strategic Defense Initiative Organization of the United States Department of Defense in joint-cooperation with NASA, and will explore the Moon and the near-Earth asteroid (NEA) 1620 Geographos with lightweight sensors developed by the Lawrence Livermore National Laboratory. A NASA Science Team for this mission will be selected by way of a NRA in April 1993. The instrument suite includes imaging cameras that cover a spectral range from the near-ultraviolet to the mid-infrared, a laser ranger, and, potentially, a charged particle telescope. To be launched in early 1994, Clementine will be in lunar orbit from February through May 1994, at which time it will depart the Moon for a flyby of 1620 Geographos in August 1994. This mission represents an outstanding opportunity for scientists interested in the Moon and asteroids. It is anticipated that the data returned from this mission will permit: an assessment of global lunar crustal heterogeneity and a resolution of less than 1 km; an assessment of the lithologic heterogeneity of Geographos at a scale of 100 m or better; and an assessment of surface processes on Geographos on the order of 10 m. The basic mission of Clementine and some of the key scientific questions that will be addressed are described. Additional material on the Clementine mission, its data handling and processing, and its instrument suite is presented elsewhere.

Vorderbruegge, R. W.↗
NASA NTRS1993

Polar sediment accumulation: Role of surface winds at the two poles

The accumulation of the large deposits of volatile and nonvolatile sediments at both Martian poles has occurred through periods of likely climate change. Most data on wind directions near the Martian poles and seasonal activity relate to a very short period of time, at one point in climate cycles. It is still uncertain what the net budgets to the poles are and how this budget (if known) would fit into longer climate/sediment cycles. Pending further data we examined the full suite of Viking high-resolution, high-latitude images for wind markers of all sizes and types. These probably represent timescales of formation from days to several tens of thousands of years. The goal is to estimate the effectiveness, and possible drivers, of wind systems that bring materials near the surface to the regions of polar sediments and that also remove materials from the polar areas.

Thomas, P. C.↗
NASA NTRS1992

Gaspra's shape and surface features: Comparison to small satellites

The several thousand asteroids constitute the second largest population of substantial solid objects in the solar system (there are probably more comets), and have been studied from the Earth for nearly two centuries. Asteroids are important members of the solar system for several reasons, among them: (1) they contain the materials that reflect the processes going on early in the time of planet formation; (2) their impacts have been the major external influences on the surface of some terrestrial planets; and (3) their impacts on Earth have probably had major effects on biologic evolution. This paper examines and describes the asteroid Gaspra, and compares it to small satellites.

Thomas, P. C.↗
NASA NTRS1991

The shape of Tethys

After considering the most important relationships between density, rotation rate, shape, and internal structure on the basis of equilibrium figures, the shape of the Saturn satellite Tethys is established from limb-coordinate data. Attention is given to the shape's significance for Tethys' internal structure. It is alternatively concluded that Tethys is either differentiated, and possesses a small rocky core, or has been interpreted as possessing a mass that is too low by about 15 percent.

Thomas, P. C.↗
NASA NTRS1988

Radii, shapes, and topography of the satellites of Uranus from limb coordinates

The present use of limb coordinates to ascertain radii, shape and local topographic characteristics for the five large Uranian satellites directly measured satellite ellipsoidal shapes, determining radii with subpixel accuracy. While Umbriel, Titania, and Oberon are spherical, Miranda and Ariel are noted to be ellipsoids whose equatorial bulges agree with previously determined mean densities. Miranda's topography exhibits substantial deformation of cratered terrain and the younger coronae, as well as uplift and complex faulting at coronae margins.

Thomas, P. C.↗