Engineering topics

Plescia, J. B.

Publications and source records attributed to Plescia, J. B..

At least 55 records · Page 3

NASA NTRS1988

Cratering history of Miranda - Implications for geologic processes

The terrain of Miranda, which is the least cratered and therefore pressumably youngest of the Uranian moons, encompasses more heavily cratered and coronae regions, of which the former are, again pressumably, the oldest. The coronae are also variably cratered; of these, Elsinore and Arden Coronae have similar, intermediate crater frequencies, and may therefore have formed simultaneously. The Inverness Corona is the youngest of the major terrains. Periods of global expansion are implied by grabben formation that may have occurred both before and after the formation of the coronae.

Plescia, J. B.↗

NASA NTRS1987

Cratering history of the Uranian satellites - Umbriel, Titania, and Oberon

Crater size-frequency data for Umbriel, Titania, and Oberon are presented, and the implications of those data are discussed in terms of the geologic histories of these bodies and the populations of objects that have cratered them. The surfaces of Oberon and Umbriel are old and are inferred to date to a period early in their histories when the cratering rate was significantly higher than at present. No significant endogenic resurfacing appears to have occurred on either body after that inferred period of intense cratering. Titania exhibits the youngest surface of these three and appears to have undergone almost complete endogenic resurfacing. Among the Uranian satellites the surfaces of Oberon and Umbriel are interpreted to be the oldest, that of Titania intermediate, and those of Ariel and parts of Miranda the youngest. The size-frequency distributions for these satellites have an average slope of about -3, indicative of a steep crater production function. The cumulative size-frequency data for these Uranian satellites may be interpreted to indicate that parts of their surfaces are saturated with craters at small diameters.

Plescia, J. B.↗

NASA NTRS1987

Cratering history of Miranda

The surface of the southern hemisphere of Miranda imaged by Voyager 2 is divisible into two general types of terrain: cratered terrain, characterized by numerous craters and undulating intercrater plains; and basins, circular to rectangular areas of complex morphology having large-scale albedo markings. To determine the relative ages of the terrains and the length of geological activity, crater-frequency data were compiled for various parts of the cratered terrain and basins. Crater-frequency data indicate that the cratered terrain is the oldest terrain on Miranda and that it was locally resurfaced.

Plescia, J. B.↗

NASA NTRS1987

Geology and cratering history of Ariel

The surface of Ariel imaged by Voyager 2 can be divided into several types of terrain on the basis of morphology: cratered terrain, subdued terrain, ridged terrain, and plains. Crater statistics were compiled for each of the terrain types. Despite differing morphology, the various terrains on Ariel do not exhibit large variations in crater frequency. None of the observed surfaces on Ariel record the period of accretion. It seems that conditions appropriate for resurfacing could have occurred during the early history of Ariel.

Plescia, J. B.↗

NASA NTRS1987

Orbital dynamics of the Uranian satellites based on Voyager data

The satellites of Uranus all have significant non-zero eccentricities and in the case of Miranda a significant inclination as well. The Voyager 2 encounter with Uranus in January 1986, provided more accurate estimates of the masses and sizes of the satellites. Orbital history and possible tidal heating were reexamined by using the Voyager data. The relevant orbital dynamics equations are described.

Plescia, J. B.↗

NASA NTRS1987

Late-stage flood lavas in the Elysium region, Mars

In the southeastern part of the Elysium region is a unit that exhibits little texture and a generally low albedo and that has a very low crater frequency. This unit has been mapped as smooth plains material and previously interpreted as an eolian deposit on the basis of Mariner 9 images. More recently, the unit was mapped as material deposited during a channeling episode. The author interprets the smooth plains unit as being a volcanic deposit composed of low viscosity lava flows: both flood lavas and individual flows. The reasons for these conclusions are given and briefly discussed.

Plescia, J. B.↗

NASA NTRS1985

Terrestrial Analogs for Planetary Wrinkle Ridges

Wrinkle ridges are common physiographic features on the terrestrial planets. Their origin has remained enigmatic, although two different types of models, volcanic and tectonic, have been proposed. The major impediment to deciphering the origin of wrinkle ridges has been the lack of a terrestrial analog. Seven terrestrial analogs were discussed, two in detail. Their implications for the origin for planetary wrinkle ridges were considered. All of the terrestrial analogs were formed in compressional environments and are the surface breaks of thrust faults.

Plescia, J. B.↗

NASA NTRS1985

Impact cratering history of the Saturnian satellites

The impact flux history of the Saturn system has been modelled, and absolute ages estimated. The model assumes that the general history of impact flux was similar to that of the inner solar system and that the dominant source of material in the postaccretionary phases was external to the Saturn system. Population I and II craters represent the synheavy and postheavy bombardment periods, respectively. Observed crater densities are representative of the actual ages and do not appear to be saturation-equilibrium surfaces. It is concluded that some of the satellites have geologic histories which extend to about 3.5 b.y. Significant endogenic resurfacing has occurred on several satellite surfaces, which indicates that thermal conditions and material compositions are more complicated than might have been anticipated prior to Voyager encounters.

Plescia, J. B.↗

NASA NTRS1985

Chronology of surface units on the icy satellites of Saturn

Because the impact flux history in the Saturnian system has been similar in its general form to that for the inner solar system and Jupiter, it is possible to develop an absolute calibration model for the relative ages derived from the density of impact craters. This has been done by identifying the density at which the crater populations change characteristics from that during post-accretion heavy bombardment to a later population of low but continuous flux. The transition between these two populations occurred at a known time throughout the solar system, 3.9 by, and provides an absolute calibration point on which to construct a calibration curve. Results based on calibrating relative age data to this absolute flux model suggest that most surfaces are ancient, greater than 3.9 by (or within several 100 million years). Only Enceladus and Dione have had prolonged activity that has resulted in resurfacing. The activity on Enceladus was probably driven by tidal forces, whereas the mechanism for driving resurfacing on Dione has yet to be identified.

Boyce, J. M.↗

NASA NTRS1984

A bibliography of planetary geology principal investigators and their associates, 1982 - 1983

This bibliography cites recent publications by principal investigators and their associates, supported through NASA's Office of Space Science and Applications, Earth and Planetary Exploration Division, Planetary Geology Program. It serves as a companion piece to NASA TM-85127, ""Reports of Planetary Programs, 1982". Entries are listed under the following subject areas: solar system, comets, asteroids, meteorites and small bodies; geologic mapping, geomorphology, and stratigraphy; structure, tectonics, and planetary and satellite evolutions; impact craters; volcanism; fluvial, mass wasting, glacial and preglacial studies; Eolian and Arid climate studies; regolith, volatiles, atmosphere, and climate, radar; remote sensing and photometric studies; and cartography, photogrammetry, geodesy, and altimetry. An author index is provided.

Plescia, J. B.↗

NASA NTRS1984

Geology of Dione

Dione is one of the most geologically complex of the Saturnian satellites. Crater counts and surface morphology indicates the geologic units observed are of variable age and origin. In an attempt to understand the processes which have affected Dione, a geologic map was prepared. Several geologic units were identified; ancient heavily cratered terrain, two plains units: cratered plains and lightly cratered plains, lobate deposits, crater rim deposits and bright wispy materials.

Plescia, J. B.↗

NASA NTRS1984

Cratering history of the Saturnian satellites

In an attempt to relate the observed crater frequencies on the saturnian satellites with absolute time, the cratering history of the Saturn system was modeled. The model is based upon several assumptions. First, the cumulative flux history at Saturn resembles that of the inner solar system. That is to say, a high cratering rate which rapidly (exponentially) decayed over the first few hundred million years followed by a significatly lower but more constant flux to the present. Second, the end of the heavy bombardment occured about 3.7-3.8 b.y. ago. Third, Population I craters, dominated by large diameters and having a shallow size-frequency distribution slope, were formed during the period of heavy bombardment. Population II craters, characterized by small craters and a steep distribution slope, have formed since the heavy bombardment ended.

Plescia, J. B.↗

NASA NTRS1983

The geology of dione

Dione is one of the more geologically complex of Saturn's satellites. Several geologic units have been identified including ancient heavily cratered terrain; two plains units: cratered plains and lightly cratered plains; lobate deposits; crater rim deposits; and bright wispy material. The only structural features observed are a series of troughs which cross portions of th surface and subtle northeast and northwest trending lineaments. The troughs are associated with volcanic deposits and are interpreted to be the vents through which material was erupted. Correlations exist between telescopically observed albedo patterns and the distribution of geologic units.

Plescia, J. B.↗

NASA NTRS1983

Crater numbers and geological histories of Iapetus, Enceladus, Tethys and Hyperion

The surfaces of the Saturn satellites Tethys, Iapetus and Encedalus display surfaces which indicate active geological processes and therefore suggest a degree of internal evolution. By contrast, the Saturn satellite Hyperion and the coorbitals 1980S1 and 1980S3 show no trace of geological activity and may be fragments of once-larger bodies. Activity on Iapetus appears to have been confined to the dark terrain, and offers no clue as to its timing and extent. The widest terrain type and crater number variations are those of Encedalus, which indicate the most prolonged period of geological activity of any of the satellites studied.

Plescia, J. B.↗

NASA NTRS1982

Tectonic history of the Tharsis region, Mars

The faults of the Tharsis region of Mars have been mapped and delineated on the basis of age. The orientation of each group of faults was analyzed to determine if they were radial to a point. The results indicate that there are at least four discrete centers of faulting within the Tharsis area which are of significantly different age. The four centers proposed are, from oldest to youngest, (1) in the Thaumasia highlands, (2) in northern Syria Planum, and finally, (3) and (4) near Pavonis Mons. While episodes 3 and 4 occupy the same geographic position, they are separated in time by the development of the Tharsis lava plains and shields. Each episode of tectonism was separated from the next by a period of major basaltic volcanism. The tectonic features of the Tharsis area are thus viewed as being the cumulative result of the superposition of several discrete episodes of faulting, rather than the result of one episode about a single point.

Plescia, J. B.↗

NASA NTRS1982

A bibliography of planetary geology principal investigators and their associates, 1981 - 1982

Over 800 publications submitted by researchers supported through NASA's Planetary Geology Program are cited and an author/editor index is provided. Entries are listed under the following subjects: (1) general interest topics; (2) solar system, comets, asteroids, and small bodies; (3) geologic mapping, geomorphology, and stratigraphy; (4) structure, tectonics, geologic and geophysical evolution; (5) impact craters: morphology, density, and geologic studies; (6) volcanism; (7) fluvial, mass wasting, and periglacial processes; (8) Eolian studies; (9) regolith, volatile, atmosphere, and climate; (10) remote sensing, radar, and photometry; and (11) cartography, photogrammetry, geodesy, and altimetry.

Plescia, J. B.↗

NASA NTRS1982

Crater densities and geological histories of Rhea, Dione, Mimas and Tethys

The Saturn icy satellites Rhea, Dione, Mimas and Tethys were imaged by Voyager 1 with sufficiently high resolution to permit crater counts that will determine the character and magnitude of crater density variations and the relative ages of the different terrain types. The data show that the surfaces of these satellites are not uniformly cratered, and therefore not of uniform age. Density variations are found to occur at all diameters, especially in the distribution of large-diameter craters (greater than 50 km). It is concluded that this satellite group, despite their small size and mass and low density, have evolved beyond their original, accretionary state. The alteration processes appear in light of crater density determinations to have been active beyond the period of heavy bombardment. The energy source for the evolution of these bodies has yet to be determined.

Plescia, J. B.↗

NASA NTRS1982

The geology of Ganymede

A broad outline of the geologic history of Ganymede is presented, obtained from a first attempt to map the geology on a global scale and to interpret the characteristics of the observed geologic units. Features of the ancient cratered terrain such as craters and palimpsests, furrows and troughs, are discussed. The grooved terrain is described, including its sulci and cells, and the age relation of these units is considered along with the structure and origin of this terrain. The Gilgamesh Basin and Western Equatorial Basin in the post grooved terrain are treated, as are the bright and dark ray craters and the regolith. The development of all these regions and features is discussed in context. For the regolith, this includes the effect of water migration, sputtering, and thermal annealing. The histories of the ancient cratered terrain, the grooved terrain, and the post grooved terrain are presented.

Shoemaker, E. M.↗