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Pettengill, Gordon H.

Publications and source records attributed to Pettengill, Gordon H..

12 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

Research Participation in the Mars Orbiter Laser Altimeter Experiment

This report describes the tasks that have been completed by the Principal Investigator, Gordon Pettengill, and his team during the first year of this grant. Dr. Pettengill was assisted by Dr. Peter Ford and Ms. Joan Quigley. Our main task has been to analyze the polar clouds detected by MOLA (Mars Orbiter Laser Altimeter) during the nominal mission of the Mars Global Surveyor (MGS) in 1999-2001 and to correlate the results with other data sets, in particular that from TES, the MGS thermal emission spectrometer. Starting with the Martian cloud database that we constructed prior to the start of this grant, we have examined all TES footprints that overlap MOLA clouds in time and space, correlating the thermal signature against specific categories that we assign to MOLA clouds on the basis of visual inspection. We are particularly interested in clouds in the region of "cold spots", areas of anomalously low thermal brightness temperature that have been detected in the polar winter by several instruments beginning with IRIS on Mariner 9. They are thought to indicate regions of active CO2 sublimation or snowfall, and it is hoped that MOLA measurements may tell us more about these regions.

Pettengill, Gordon H.↗
NASA NTRS2002

Participation in the Mars Orbiting Laser Altimeter Experiment

This NASA Grant, 5-4434, has covered the active participation of the Principal Investigator, Prof. Gordon Pettengill, and his Co-Investigator, Peter Ford, in the Mars Orbiting Laser Altimeter (MOLA) Experiment, over a period of five years. This participation has included attending team meetings, planning observing operations, developing data-reduction software algorithms, and processing data, as well as presenting a number of oral reports at scientific meetings and published papers in refereed journals. This research has concentrated on the various types of Martian clouds that were detected by the laser altimeter.

Pettengill, Gordon H.↗
NASA NTRS2000

Latest Results from the Mars Orbiter Laser Altimeter (MOLA)

The first MOLA data were acquired in Sept. 1997 shortly after the arrival of Mars Global Surveyor (MGS) at Mars and before the start of aerobraking. Subsequently, data were obtained during several months in the spring and summer of 1998. All these data were obtained over the northern hemisphere. At the beginning of March 1999 the mapping phase of the MGS mission began in the designed 400 km near circular, near polar orbit and MOLA began collecting continuous global data. By the end of 1999 MOLA had acquired over 250 million altimeter measurements of the radius of the planet at accuracies of the order of a few meters radially and about 100 meters horizontally. These observations revealed the detailed structure of the Martian surface, the form of the polar caps, as well as the shape of the planet. The early results provided a unique view of the north polar cap and subsequently of the south polar cap from which an estimate of the total volume of present-day surface water ice of 3.2 to 4.7 million cubic km was obtained. To obtain full coverage of the polar caps it was necessary for MOLA to be pointed off nadir approximately 20 degrees and on several occasions MGS has performed a roll maneuver to make these observations. It is hoped that these off-nadir observations of the central region of the cap (MGS only reaches to latitudes plus or minus 86.5) will assist in the detection of the seasonal deposition of CO2 since it is believed that these high latitudes regions may be the accumulation zone.

Smith, David E.↗
NASA NTRS1992

Venus surface radiothermal emission as observed by Magellan

Radiothermal emission results obtained by the Magellan spacecraft during its first 8-month cycle of data taking, corresponding to one full rotation of Venus, are reported. The new Magellan data confirm the existence of regions with extremely low values of radiothermal emission. Possible mechanisms to explain the anomalous phenomena are an interaction with a single interface between the atmosphere and a high-dielectric permittivity medium versus volume scattering from the interior of a low-loss medium. The discrimination between these mechanisms requires careful observation of the polarized state of the radiothermal emission and that of the radar scattering.

Pettengill, Gordon H.↗
NASA NTRS1992

Venus topography and kilometer-scale slopes

During the first 8 months of the Magellan mission, the radar altimeter has made some three million measurements of the surface of Venus covering the latitude range from 85 deg N to 80 deg S. Methods involving range correlation, Doppler filtering, multiburst summation, and range migration are used to focus the observations and to achieve high surface resolution. Results are presented as maps of the global distribution of topography, meter-scale roughness, and power reflection coefficient. The results are similar to those reported in previous experiments (surface heights exhibit a unimodal distribution with more than 80 percent of the surface lying within 1 km of the 6051.84-km mean radius) but the higher resolution of the Magellan altimeter has disclosed several surprisingly steep features, e.g., the southwest face of the Maxwell Montes, the southern face of the Danu Montes, and the chasmata to the east of Thetis Regio, where average kilometer-scale slopes of greater than 30 deg are not uncommon. This conclusion is corroborated by close inspection of synthetic aperture radar imagery.

Ford, Peter G.↗
NASA NTRS1992

Pancakelike domes on Venus

The shape of seven large domes on the plains of Venus, with volumes between 100 and 1000 cu km, is compared with that of an axisymmetric gravity current spreading over a rigid horizontal surface. Both the altimetric profiles and the horizontal projection of the line of intersection of domes on the SAR images agree well with the theoretical similarity solution for a newtonian fluid, but not with the shape calculated for a rigid-plastic rheology, nor with that for a static model with a strong skin. As a viscous current spreads, it generates an isotropic strain rate tensor whose magnitude is independent of radius. Such a flow can account for the randomly oriented cracks that are uniformly distributed on the surface of the domes. The stress induced by the flow in the plains material below is obtained, and is probably large enough to produce the short radial cracks in the surface of the plains beyond the domes. The viscosity of the domes can be estimated from their thermal time constants if spreading is possible only when the fluid is hot, and lies between 10(exp 14) and 10(exp 17) Pa s. Laboratory experiments show that such viscosities correspond to temperatures of 610 - 690 C in dry rhyolitic magmas. These temperatures agree with laboratory measurements of the solidus temperature of wet rhyolite. These results show that the development of the domes can be understood using simple fluid dynamical ideas, and that the magmas involved can be produced by wet melting at depths below 10 km, followed by eruption and degassing.

Mckenzie, Dan↗
NASA NTRS1991

Alphonsus crater - Floor fracture and dark-mantle deposit distribution from new 3.0-cm radar images

The lunar crater Alphonsus is characterized by numerous fractures or graben, and by endogenic dark-halo craters. Existing maps of fractures from analysis of lunar photography may be biased by the east-west solar illumination. This paper presents new high-resolution, dual-polarization 3.0-cm wavelength radar images of Alphonsus with radar illumination from northerly directions, and uses these data to better map the locations of both the graben and a variety of dark-mantle deposits. The distribution of fractures, and several graben which cut the crater floor and central ridge, are cited as possible evidence for simultaneous, post-Imbrium uplift of both structures. Some of the endogenic dark halo deposits are more extensive in depolarized radar images than in photographs; these extensions are attributed in some cases to more distant emplacement of pyroclastic material, and in others to fortuitous connections with smoother, less cratered portions of the Alphonsus floor.

Zisk, Stanley H.↗
NASA NTRS1991

Magellan - Mission summary

The Magellan radar mapping mission is in the process of producing a global, high-resolution image and altimetry data set of Venus. Despite initial communications problems, few data gaps have occurred. Analysis of Magellan data is in the initial stages. The radar system data are of high quality, and the planned performance is being achieved in terms of spatial resolution and geometric and radiometric accuracy. Image performance exceeds expectations, and the image quality and mosaickability are extremely good. Future plans for the mission include obtaining gravity data, filling gaps in the initial map, and conducting special studies with the radar.

Saunders, R. Stephen↗
NASA NTRS1991

Magellan - Radar performance and data products

The Magellan Venus orbiter carries only one scientific instrument: a 12.6-centimeter-wavelength radar system shared among three data-taking modes. The synthetic-aperture mode images radar echoes from the Venus surface at a resolution of between 120 and 300 meters, depending on spacecraft altitude. In the altimetric mode, relative height measurement accuracies may approach 5 meters, depending on the terrain's roughness, although orbital uncertainties place a floor of about 50 meters on the absolute uncertainty. In areas of extremely rough topography, accuracy is limited by the inherent line-of-sight radar resolution of about 88 meters. The maximum elevation observed to date, corresponding to a planetary radius of 6062 kilometers, lies within Maxwell Mons. When used as a thermal emission radiometer, the system can determine surface emissivities to an absolute accuracy of about 0.02. Mosaicked and archival digital data products will be released in compact disk (CDROM) format.

Pettengill, Gordon H.↗
NASA NTRS1991

Magellan - Electrical and physical properties of Venus' surface

Magellan probes Venus'surface by 12.6-cm-wavelength vertical and oblique radar scattering and measures microwave thermal emission. Emissivity and root-mean-square slope maps between 330 deg and 30 deg E and 90 deg N and 80 deg S are dissimilar, although some local features are exceptions. Inferred surface emissivities typically are 0.85, but vary from 0.35 at Maxwell to 0.95 northeast of Gula Mons and other locations. Lowest emissivities appear in topographically high areas; this relation suggests that a phase change or differences in chemical weathering occur at about 6055-kilometer radius. Initial results indicate that there are significant variations in the surface scattering function.

Tyler, G. Leonard↗
NASA NTRS1988

Venus - Surface electromagnetic properties

Pioneer Venus high-angle radar imaging data are used here to correct values of the specular power reflection coefficient for the effects of diffusely scattering small-scale surface roughness. A value of 5.0 + or - 0.9 is found for the average dielectric permittivity of the rolling plains and lowlands, suggesting that most of the Venus surface is overlain by at most only a few cm of soil or dust. The unexpectedly high values of power reflection coefficient are confirmed by observations of correspondingly low values of radiothermal emission. Thus there can be little doubt that most of the highlands contain significant amounts of a conducting mineral near the surface.

Pettengill, Gordon H.↗