Engineering topics

Greeley, Ronald

Publications and source records attributed to Greeley, Ronald.

At least 73 records · Page 4

NASA NTRS1994

Assessment of antipodal-impact terrains on Mars

The regions anitpodal to Mars' three largest impact basins, Hellas, Isidis, and Argyre, were assessed for evidence of impact-induced disrupted terrains. Photogeology and computer modeling using the Simplified Arbitrary Lagrangian Eulerian (SALE) finite element code suggest that such terrains could have been found by the Hellas impact. Maximum antipodal pressures are 1100 MPa for Hellas, 520 MPa for Isidis, and 150 MPa for Argyre. The results suggest that if antipodal fracturing were associated with later volcanism, then Alba Patera may be related to the Hellas event, as proposed by Peterson (1978). Alba Patera is a unique volcano in the solar system, being a shield volcano which emitted large volume lava flows. This volcanism could be the result of the focusing of seismic energy which created a fractured region that served as a volcanic conduit for the future release of large volumes of magma. No disrupted terrain features are observed antipodal to the Isidis or Argyre basins, although some of the old fractures in Noctis Labyrinthus could have originated in response to the Isidis impact, and later have been reactivated by the Tharsis tectonics assumed to have produced Noctis. If the lower calculated antipodal pressures for Argyre were capable of producing disrupted terrains, then the terrains have been covered subsequently by volcanic or aeolian material, or modified beyond recognition.

Williams, David A.↗

NASA NTRS1994

Dust deposits on Mars: The 'parna' analog

Parna is an Autralian aboriginal word meaning 'sandy dust'. It has been applied to deposits of clay, silt, and sand which were initially transported by the wind as aggregates, or pellets, of sand size. Parna is distinguished by its silt and clay content, which in some cases exceeds 85% of the total volume of the deposit. Much of the fine-grained playa silt and clay is incorporated into the parna as sand-sized aggregates, which greatly facilitate their transportation and reworking by the wind. Rain following aggregate emplacement can cause their disintegration, rendering the parna immobile by the wind, yet some pellets can survive several wetting/drying episodes. Parna deposits on Earth occur both as dune forms and as sheet deposits which mantle older terrains. In both cases the deposits are typically derived from lacustrine (lake) beds, such as playas. There is substantial evidence to suggest that bodies of water existed on Mars in the past. Thus, the potential is high for lacustrine deposits and the formation of parna on Mars. Although no parna dunes have been identified, it is suggested that the deposits derived from White Rock (-8 deg, 335 deg W), near Mamers Valles (34 deg, 343 deg W), and elsewhere on Mars may represent sheet parna. Data obtained from Mars-94/96 missions and potential landed spacecraft may provide additional evidence for the existence of parna on Mars.

Greeley, Ronald↗

NASA NTRS1994

Fluvial processes in Ma'adim Vallis and the potential of Gusev crater as a high priority site

According to exobiology site selection criteria for Mars, the search for potential extinct/extant water dependent life should focus on sites were water flowed and ponded. The Ma'adim Vallis/Gusev crater system is of high priority for exobiology research, because it appears to have involved long term flooding, different periods and rates of sedimentation, and probable episodic ponding. The topics covered include the following: evidence of nonuniform fluvial processes and early overflooding of the plateau and ponding.

Cabrol, Nathalie↗

NASA NTRS1994

First Galileo image of asteroid 243 Ida

The second spacecraft encounter with an asteroid has yielded an unprecedentedly high resolution portrait of 243 Ida. On 28 Aug. 1993, Galileo obtained an extensive data set on this small member of the Koronis family. Most of the data recorded on the tape recorder will be returned to Earth in spring 1994. A five-frame mosaic of Ida was acquired with good illumination geometry a few minutes before closest approach; it has a resolution of 31 to 38 m/pixel amd was played back during Sept. 1993. Preliminary analyses of this single view of Ida are summarized.

Chapman, C. R.↗

NASA NTRS1994

Strategy for selecting Mars Pathfinder landing sites

A strategy for Pathfinder site selection must be developed that is fundamentally different from most previous considerations. At least two approaches can be identified. In one approach, the objective is to select a site representing a key geologic unit on Mars, i.e., a unit that is widespread, easily recognized, and used frequently as a datum in various investigations. The second approach is to select a site that potentially affords access to a wide variety of rock types. Because rover range is limited, rocks from a variety of sources must be assembled in a small area for sampling. Regardless of the approach taken in site selection, the Pathfinder site should include eolian deposits and provisions should be made to obtain measurements on soils. A recommended approach for selecting the Mars Pathfinder landing site is to identify a deltaic deposit, composed of sediments derived from sources of various ages and geologic units that shows evidence of eolian activity. The site should be located as close as possible to the part of the outwash where rapid deposition occurred because the likelihood of 'sorting' by size and composition increases with distance, decreasing the probability of heterogeneity. In addition, it is recommended that field operation tests be conducted to gain experience and insight into conducting science with Pathfinder.

Greeley, Ronald↗

NASA NTRS1994

Potential landing sites for Mars Pathfinder

In addition to a better understanding of the geological evolution of Mars, new techniques for processing available data have emerged, new data have been acquired, and the engineering approaches for placing spacecraft on the surface have evolved. Selection of the Mars Pathfinder landing site must take these issues into account, along with mission constraints. An advantage of Mars Pathfinder is the rover for sampling surface materials over a range of tens of meters. However, engineering constraints and the limited scientific payload of this mission require new approaches for landing site selection. One approach is to select sites exhibiting a wide variety of rocks near the lander. An alternative approach is to select sites in which the regional geology consists of a single rock type representing a key datum for the geological study of Mars, and is uniformly distributed within the landing ellipse.

Kuzmin, Ruslan O.↗

NASA NTRS1994

Wind Streaks on Venus: Clues to Atmospheric Circulation

Magellan images reveal surface features on Venus attributed to wind processes. Sand dunes, wind-sculpted hills, and more than 5830 wind streaks have been identified. The streaks serve as local "wind vanes," representing wind direction at the time of streak formation and allowing the first global mapping of near-surface wind patterns on Venus. Wind streaks are oriented both toward the equator and toward the west. When streaks associated with local transient events, such as impact cratering, are deleted, the westward component is mostly lost but the equatorward component remains. This pattern is consistent with a Hadley circulation of the lower atmosphere.

Greeley, Ronald↗

NASA NTRS1994

Dunes and Microdunes on Venus: Why Were So Few Found in the Magellan Data?

A search through cycle 1, 2, and 3 Magellan radar data covering 98% of the surface of Venus revealed very few dunes. Only two possible dune fields and several areas that may contain microdunes smaller than the resolution of the images (75 m) were identified. The Aglaonice dune field was identified in the cycle I images by the specular returns characteristic of dune faces oriented perpendicular to the radar illumination. Cycle 1 and 2 data of the Fortuna-Meshkenet dune field indicate that there has been no noticeable movement of the dunes over an 8-month period. The dunes, which are oriented both parallel and perpendicular to the radar illumination, appear to be dark features on a brighter substrate. Bright and dark patches that were visible in either cycle 1 or 2 data, but not both, allowed identification of several regions in the southern part of Venus that may contain microdunes. The microdunes are associated with several parabolic crater deposits in the region and are probably similar to those formed in wind tunnel experiments under Venus-like conditions. Bragg scattering and/or subpixel reflections from the near-normal face on asymmetric microdunes may account for these bright and dark patches. Look-angle effects and the lack of sufficient sand-size particles seem to be the most likely reasons so few dunes were identified in Magellan data. Insufficient wind speeds, thinness of sand cover, and difficulty in identifying isolated dunes may also be contributors to the scarcity of dunes.

Weitz, Catherine M.↗

NASA NTRS1993

Laboratory spectra of field samples as a check on two atmospheric correction methods

Atmospheric correction is the first step toward quantitative analysis of imaging spectroscopy data. Two methods, MODTRAN model and the empirical line, were used to convert AVIRIS radiance values to reflectance values. A set of laboratory spectra of field samples corresponding to AVIRIS coverage was used to assess these methods. This will also serve to select bands for future quantative analyses.

Xu, Pung↗

NASA NTRS1993

AIRSAR views of aerolian terrain

The Stovepipe Wells Dune Field provides a unique opportunity to observe several dune forms in one scene. These forms include reverse dunes, star dunes, transverse, and linear dunes. A sand mantle surrounds the dune field and can also be observed in the radar image. Dune types were discriminated best in co-polarized channels. Three major wind directions are responsible for the various dune forms. Winds from the north and south are responsible for the reverse dunes, winds from north for the transverse dunes, and the north - south and westerly winds for the star dunes. The winds also reflect the topographic configuration of this part of Death Valley. Vegetation over the dunes was most pronounced in the cross-polarized images. Lancaster et al. (1992) find that cross-polarized images are most useful in differentiating active from inactive dunes. This is because the vegetation backscatter signature is present over inactive dunes. Future studies should include multiple look and incidence angles to determine if the dune forms can still be seen at other angles.

Blumberg, Dan G.↗

NASA NTRS1993

Formation of Venusian canali - Considerations of lava types and their thermal behaviors

Because liquid water is unstable at present venusian surface conditions, the discovery of channels (termed 'canali') on Venus thousands of kilometers long was not predicted. Low viscosity lavas that remain fluid for several thousand kilometers are considered to be the canali-forming agents; possible compositions of Venusian canali-forming lavas include komatiite and high-Fe-Ti 'lunar'-type basalts. Results of analytical and numerical models of these lavas reveal that total cooling is more efficient on Venus than on Earth, suggesting that Venusian lavas rapidly form insulating crusts, and, thus, that the canali lavas were essentially 'tube-fed.' The models also reveal that thermal erosion should be less efficient on Venus than on Earth, suggesting that Venusian channels are either the product of mechanical (rather than thermal) erosion or constructional processes.

Gregg, Tracy K. P.↗

NASA NTRS1993

Martian aeolian features and deposits - Comparisons with general circulation model results

The relationships between near-surface winds and the distribution of wind-related features are investigated by means of a general circulation model of Mars' atmosphere. Predictions of wind surface stress as a function of season and dust optical depth are used to investigate the distribution and orientation of wind streaks, yardangs, and rock abundance on the surface. The global distribution of rocks on the surface correlates well with predicted wind stress, particularly during the dust storm season. The rocky areas are sites of strong winds, suggesting that fine material is swept away by the wind, leaving rocks and coarser material behind.

Greeley, Ronald↗

NASA NTRS1993

Volcanic geology of Hadriaca Patera and the eastern Hellas region of Mars

A detailed examination of the geomorphology of Hadriaca Patera, a low-relief volcano in the southern highlands of Mars northeast of the Hellas basin, is presented, and the surrounding eastern Hellas region is considered in order to assess whether the volcanic geology of Hadriaca Patera is consistent with previous characterizations and highland paterae. The morphologic characteristics of the channels suggest erosion by groundwater sapping and surface runoff. The erosional morphology of the volcano, the lack of lava flow features, and the friable nature of the flank materials indicate that Hadriaca Patera consists predominantly of pyroclastic deposits. From the predominance of hydrovolcanic eruptions in the development of Hadriaca and Tyrrhena patera, it is inferred that the transition in volcanic eruption style can be attributed to a volatile depletion of the crust, whereas magmatic eruptions at the paterae would be indicative of temporal changes in Martian magmas.

Crown, David A.↗

NASA NTRS1993

Mars exobiology landing sites for future exploration

The selection of landing sites for Exobiology is an important issue for planning for future Mars missions. Results of a recent site selection study which focused on potential landing sites described in the Mars Landing Site Catalog are presented. In addition, basic Exobiology science objectives in Mars exploration are reviewed, and the procedures used in site evaluation and prioritization are outlined.

Landheim, Ragnhild↗

NASA NTRS1992

Volatile history of Mangala Valles, Mars

The Mangala Valles channel system differs from other Mars outflow channels as they emanate from a point source determined by the local interplay of tectonism, volcanism, and volatiles that led directly to the release of groundwater. Water discharge rates through the southern reaches of Mangala Valles may have been substantially lower than estimates determined fron the canyon system as a whole, suggesting that the accumulation and release of water may have lasted longer than previously envisioned. The sequence of events determined from mapping of units along the southern reaches of Mangala Valles is closely related to the area structures.

Zimbelman, James R.↗

NASA NTRS1992

Geologic evolution of the east rim of the Hellas basin, Mars

The east rim of the Hellas basin, one of the largest obvious impact features on Mars, is studied. The distribution of geologic materials in the region is documented, and their modes of origin are constrained based on their geomorphologic characteristics. Superposition relationship and crater size-frequency distributions allow the relative ages of the geologic units to be determined and provide information regarding the timing and extent of the geologic processes emplacing and modifying the surface materials. Systematic analysis of the degradation of the Hellas basin is used to illustrate the sequence of events and interplay of geologic processes which control the evolution of many regions on Mars.

Crown, David A.↗

NASA NTRS1992

Small volcanic edifices and volcanism in the plains of Venus

The different types of eruption that have occurred over time in the Venusian plains are considered. The most extensive volcanic units consist of flood lavas, the largest of which have volumes of the order of thousands of cubic kilometers. They are inferred to have erupted at high effusion rates, and they exhibit a range of radar backscatter characteristics indicating different surface textures and ages. Small edifices on the plains occur mainly in clusters associated with fracture belts. The majority are shield volcanos that may be up to a few tens of kilometers across but are generally 10 km or less in diameter. Volcanic domes have diameters up to several tens of kilometers and volumes of the order of 100 cu cm. These are interpreted as being constructed of lava erupted with a relatively high effective viscosity and thus possibly composed of more silicic lava. For many domes, the flanks were unstable during and after eruption and experienced gravity sliding that produced steep scalloped outer margins.

Guest, John E.↗

NASA NTRS1992

Surface modification of Venus as inferred from Magellan observations of plains

Magellan radar and altimetry data are analyzed to understand the extent to which surface processes, such as impact crater ejecta emplacement, aeolian processes, and weathering, have modified Venusian plains. The analysis focuses on three areas: (1) lava flows of different relative ages in Sedna Planitia to determine the degradation history of Venusian volcanic surfaces; (2) the 65-km-diameter impact crater Stuart to evaluate the effects of ejecta emplacement on plains surfaces; and (3) elevated plains in western Ovda Regio to explore the nature and rate of production of highly reflective surfaces that dominate terrains that are greater than 6054 km in radius.

Arvidson, Raymond E.↗