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At least 289 records · Page 16

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NASA NTRS1981

The detection of X-ray variability in O stars

Seven O stars known to have strong, and sometimes variable, stellar winds have been observed repeatedly with the Imaging Proportional Counter on the Einstein Observatory, in a program designed to determine whether the X-ray fluxes from these stars are variable. In three cases, definite changes were seen, either on a time scale of a year (Iota Ori and Delta Ori) or five days (15 Mon). In two of these cases, the X-ray spectrum was harder when the overall flux was higher, indicating that some of the fluctuations may take place in a hot (approximately 10 to the 7th K) emitting region at the bottom of the winds.

Snow, T. P., Jr.↗

NASA NTRS1981

Temperatures of galactic molecular clouds showing CO self-absorption

The CO J = 2-1 line has been observed and, in most cases, mapped in 10 star-forming molecular clouds (W3, NGC 1333, NGC 2071, Mon R2, CRL 961, Rho Oph, W49N, W51A, DR 21, and Cep A). The CO J = 3-2 line has been observed in W3 and DR 21. The CO lines from all these sources are strongly self-absorbed. By comparing the present results with published CO(1-0) line profiles, it is found that large corrections to the temperatures of the cloud cores, as measured by the CO(1-0) lines, are required. The corrections for self-absorption bring the CO brightness temperatures into closer agreement with the grain temperatures inferred from far-IR photometry.

Phillips, T. G.↗

NASA NTRS1981

Ground ice on Mars - Inventory, distribution, and resulting landforms

A model is presented for the distribution of ground ice on Mars, and model predictions are compared with Viking Orbiter photographs of landforms possibly related to ground ice. Estimates of the amount of water originally outgassed on Mars and the amount of water presently on Mars are presented which show that approximately 90% of the estimated outgassed volume may be stored in the Martian subsurface as ground ice. The extent of the Martian cryosphere, the zone in which the temperature is below the freezing point of water and in which ground ice can exist, is examined, and it is shown that, in the presence of a protective surface layer approximately 10 m thick, ground ice may occur beneath the entire Martian surface. Observed features on the Martian surface considered most likely to reflect the presence of ground ice are discussed, including thermokarst-like pits and debris flows, and possibly polygonally patterned ground, curvilinear features and pingo-like mounds, and the geographic distributions of these features as seen in Viking photographs are examined. It is found that the possibly ice-related features are concentrated in the northern midlatitudes, the equatorial zone near Olympus Mons, and the Southern Hemisphere near the edge of the plains, indicating that subsurface ice may be present over the entire planet. The origin of the ice-related landforms is explained by a combination of volcanic heating and variations in insolation.

Rossbacher, L. A.↗

NASA NTRS1982

Ignimbrites of Amazonis Planitia region of Mars

The terms ignimbrite and ash flow tuff are used to describe relatively thin but extensive sheets of material interpreted to be of pyroclastic origin and to consist of volcanic ash or pumice fragments. Criteria for distinguishing ignimbrites from other geologic materials are considered, and characteristics observed for rock units interpreted as ignimbrites in Amazonis Planitia are discussed. A series of rock units which are thought to represent ignimbrites have been mapped along an east-west border zone between the highland plateau and lowland plains of Amazonis Planitia. The area is roughly bounded by a triangle whose apices are the volcanoes Apollinaris Patera, Biblis Patera, and Olympus Mons. Attention is given to aspects of general geology, morphology and stratigraphy, and thickness and volume estimates. Correlations with other data are considered and some theoretical aspects of Martian ignimbrite emplacement are discussed.

Scott, D. H.↗

NASA NTRS1982

Speckle interferometry of molecular cloud sources at 4.8 microns

The spatial structure at 4.8 microns has been observed in W3-IRS 5, Mon R2-IRS 3, and S 140-IRS 1, the first two of which are clearly double. In addition, for four compact sources, low upper limits can be set to the angular diameters which are near those for blackbodies having the same fluxes and color temperatures observed for those sources. With a factor-of-2 higher-resolution, one may be able to distinguish between thermal emission or reddening as the source of the observed spectral shape.

Dyck, H. M.↗

NASA NTRS1982

Ultraviolet observations of four symbiotic stars

Observations were obtained with the International Ultraviolet Explorer (IUE) of four symbiotic stars. The UV spectra of YY Her, SY Mus, CL Sco, and BX Mon are characterized by varying degrees of thermal excitation. These low resolution spectra have been analyzed in terms of line-blanketed model atmospheres of early A, B, and F type stars in order to identify the nature of the hot companion in these systems. The expected emission from early main sequence stars does not fully explain the observed distribution of UV continuum energy over the entire IUE spectral range (1200-3200 A). More likely the observed continuum may be originating from an accretion disk and/or hot subdwarf that photoionizes circumstellar material, and gives rise to the high excitation lines that have been detected. The Bowen fluorescent excited lines of O III in SY Mus exhibit slightly broadened profiles that suggest possible turbulent motions in an extended circumstellar cloud with characteristic velocities of approximately 300 km/s.

Michalitsianos, A. G.↗

NASA NTRS1982

Late-stage summit activity of Martian shield volcanoes

The preservation of morphologically fresh lava flows which pre-date the most recent episodes of caldera collapse at the summits of Ascraeus, Arsia and Olympus Montes indicates that explosive eruptions were not associated with this stage of Tharsis shield volcanism. The existence of resurfaced floor segments, complex wrinkle ridges, and lava terraces within the summit craters suggests that lava lakes comprised the dominant form of the intra-caldera activity. Multiple collapse episodes on Ascraeus and Olympus Montes are indicated by the nested summit craters. The most plausible cause of caldera collapse appears to be large-scale sub-terminal effusive activity, which is corroborated by the previously recognized existence of large lava flows on the flanks of these volcanoes. Due to the implied sequence of large-scale explosive (silicic) volcanism followed by effusive (basaltic) activity, it appears highly unlikely that ignimbrites or other forms of pyroclastic flows (previously proposed as possible deposits within the Olympus Mons aureole material) were ever erupted from the Tharsis Montes.

Mouginis-Mark, P. J.↗

NASA NTRS1982

Mars - A large highland volcanic province revealed by Viking images

Many of the mountains in the rugged highland terrain of the Phaethontis and Thaumasia quadrangles are believed to be of volcanic origin. Those provisionally mapped as volcanoes have diagnostic characteristics such as lobate flow fronts around their bases, depressed central areas, or have massive, bulbous accumulations of material of no determinable origin other than volcanic. Most of the volcanoes are younger than materials forming the highlands but are older than early lava flows from Arsia Mons. Many are aligned along older fault and ridge systems that are transverse to the more recent and prominent faults transecting the region. The older faults are generally buried by plains lava flows but their traces are visible in several places in the highlands. These faults are relatively short in length.

Scott, D. H.↗

NASA NTRS1982

Long term compatibility of nitrogen tetroxide with metals - Results at the ten-year milestone

Analyses and results are reported of a test program to establish the effects of long-term (10 years or more) contact of materials with nitrogen textroxide MON-1 for the purpose of designing chemical propulsion system components which can be used for current as well as future planetary spacecraft. The test materials included aluminum alloy, corrosion-resistant steel, titanium alloy, and nickel. The weight and appearance of each specimen were observed, including the presence of deposits, etching, pitting, and cracking. Aluminum alloy and steel were rated satisfactory with minor reservations, titanium alloy was satisfactory, and nickel was unsatisfactory. Criteria for ratings are shown.

Moran, C. M.↗

NASA NTRS1982

Venus - Global surface radar reflectivity

Observations of the surface of Venus, carried out by the Pioneer Venus radar mapper at a wavelength of 17 centimeters, reveal a global mean reflectivity at normal incidence of 0.13 + or - 0.03. Over the surface, variations from a low of 0.03 + or - 0.01 to a high of 0.4 + or - 0.1 are found, with Theia Mons, previously identified as possibly volcanic, showing a value of 0.28 + or - 0.07. Regions of high reflectivity may consist of rocks with substantial inclusions of highly conductive sulfides.

Pettengill, G. H.↗

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

Wind streaks in Tharsis and Elysium - Implications for sediment transport by slope winds

Detailed maps of wind streaks in Tharsis and Elysium have been compiled from Viking Orbiter observations spanning one complete Martian year. The streak pattern is controlled by slope winds on the central volcanoes and on the flanks of the Tharsis bulge, while the global circulation dominates in Elysium. Dust erosion by downslope winds occurs over much of Tharsis and in the vicinity of Elysium Mons; this process is effective even at the low atmospheric pressures found near the summits of the large volcanoes. Erosional streaks are largely absent in Elysium Planitia; net deposition of dust might have occurred during the period of the observations. Surface properties such as slope, thermal inertia, and roughness may influence the efficiency of slope wind production sufficiently to account for the pronounced differences in streak types and patterns present in these two regions.

Lee, S. W.↗

NASA NTRS1983

The exciting stars of Herbig-Haro objects

Accurate astrometry and infrared photometry are presented for 16 objects that are believed to represent the exciting stars for Herbig-Haro (HH) objects in their vicinities. These identifications are made on the bases of steeply rising energy distributions, proper motions of HH objects, alignment of sources with HH objects, or far-infrared emission. Exciting stars for the following HH objects are newly discovered or recognized: HH 17; HH 31; HH 32; HH 33 and 40; HH 34; HH 38 and 43; HH group 14 in NGC 2264; RNO 40; RNO 43; and GGD 33a, 33b, and 35. Geometrical alignments of some sources with HH objects support the view that young stars are capable of channeling kinetic energy into small solid angles, as does the fact that some sources are slightly displaced from the apices of fan nebulae (HH 17, R Mon, Haro 6-10). These exciting stars span a wide range of bolometric luminosity, although all are likely to be very young.

Cohen, M.↗

NASA NTRS1983

Venus - Global surface radio emissivity

Observations of thermal radio emission from the surface of Venus, made by the Pioneer Venus radar mapper at a wavelength of 17 cm, show variations that are dominated by changes in surface emissivity. The regions of lowest emissivity (0.54 + or - 0.05 for the highland areas of Aphrodite Terra and Theia Mons) correspond closely to regions of high radar reflectivity reported earlier. These results support the inference of inclusions of material with high electrical conductivity in the surface rock of these areas.

Ford, P. G.↗

NASA NTRS1982

Molecular clouds in Orion and Monoceros

Attention is given to a fairly homogeneous and nearly complete inventory of the molecular clouds in Orion and Monoceros, to a peak CO intensity of 0.5 K and a velocity resolution of 0.6 km/sec, from which a contour map of velocity-intergrated CO intensity is derived and presented with a schematic key. It is noted that, with the exception of Mon R2, the main areas of star formation in this survey lie on the forward surface of a large blunt cone or wedge of molecular clouds pointing towards the center of Barnard's Loop and the older subassociations of OB stars in Orion. This is a region which the present negative CO results, 21-cm observations, and galaxy counts, despite the currently low density, show to have once contained the dense molecular gas from which older OB stars formed.

Thaddeus, P.↗

NASA NTRS1984

Volcanism and rift formation in Beta Regio, Venus: New radar results

A high-resolution (approximately 2 km) image of Beta Regio, Venus, obtained with the Arecibo radar system, reveals additional details of its structure which seem to confirm past suggestions that this region consists of a rift system and associated volcanism. Numerous long linear features approximately parallel to the north-south trending trough discovered by the Pioneer-Venus Orbiter are interpreted as being indicative of extensive faulting and the detail for the radar reflectivity anomaly coincident with Theia Mons suggests that it is a major volcano situated on what would be the western bounding fault of the rift system.

Campbell, D. B.↗

NASA NTRS1984

Compressional features in the Tharsis Region of Mars: Orientations and Timing of Deformation

The Tharsis ridge system appears to form a great circle around the major volcanoes. The roughly circumferential orientation of these ridges could be interpreted as the formation of a ridge system in response to a single stress field with a center near Pavonis Mons. Wise, et al. (1979) plotted the perpendiculars to strikes of ridges on the eastern flanks of Tharsis as great circles on the upper hemisphere of an equal area net. In order to further test for circular symmetry and possible other influences on ridge orientations, normals to vector means of ridge orientations were calculated using over 1850 digitized ridge segments sampled in 10 degree boxes. Orientations of individual ridge segments were weighted by their lengths, and the magnitudes of normals to vectors means were weighted by cummulative length of the ridge segments. Normals to vector means with magnitudes less than 100 km are not shown. Assuming a single fold origin for ridges, the resulting plot shows compressive stress trajectories for the ridges in the Tharsis region. The averaged compressive stress orientation around Tharsis confirm the suggestion by Wise, et al. that the ridge system is not concentric to any single point.

Watters, T. R.↗

NASA NTRS1984

Emission lines in the spectra of the RV Tauri stars U Monocerotis and AC Herculis

Strong metallic emission lines have been observed in the spectra of the RV Tauri stars U Mon and AC Her near recent light minima. While these two variables differ in a number of significant respects, the metallic emission spectra are quite similar, resembling the emission that was previously reported as present in the spectrum of R Sct at a deep minimum. The data appear to support Baird's shock-wave model for the spectral behavior of RV Tauri stars.

Bopp, B. W.↗