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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS2012

Bright and Dark Polar Deposits on Mercury: Evidence for Surface Volatiles

Measurements of surface reflectance of permanently shadowed areas near Mercury's north pole reveal regions of anomalously dark and bright deposits at 1064-nanometer wavelength. These reflectance anomalies are concentrated on poleward-facing slopes and are spatially collocated with areas of high radar backscatter postulated to be the result of near-surface water ice. Correlation of observed reflectance with modeled temperatures indicates that the optically bright regions are consistent with surface water ice, whereas dark regions are consistent with a surface layer of complex organic material that likely overlies buried ice and provides thermal insulation. Impacts of comets or volatile-rich asteroids could have provided both dark and bright deposits.

Polar Deposits↗
NASA NTRS2018

Direct Probe of Dark Energy Interactions with a Solar System Laboratory: NASA Innovative Advanced Concepts (NIAC) - Phase 1 Final Report

In this NIAC (NASA Innovative Advanced Concepts) study, we embrace the challenge of direct detection of the galileon dark energy field in the Vainshtein model. We developed a mission concept to directly measure the galileon field using the solar system as a laboratory. The experiment scheme involves precise measurements of the trace of the total scalar force gradient tensor. A tetrahedral constellation off our spacecraft measures the "local" traces while orbiting about 1 AU (Astronomical Unit) away from the Sun and faraway from planets (Figure 1). The trace measurement is insensitive to the much stronger gravity field which satisfies the inverse square law and thus is traceless. Atomic test masses and atom interferometer measurement techniques are used as precise drag-free inertial references while laser ranging interferometers are employed to connect among atom interferometer pairs in spacecraft for the differential gradient force measurements. We conclude that such a mission is scientifically and technologically feasible. We show that a mission of 3-year measurement time would be able to provide high confidence statements (over 3 standard deviations) about the existence and strength of the cubic galileon field of the Sun. In addition, such a mission would also provide rich and diverse scientific data for testing any gravitational theory in general beyond the Newtonian gravity, hunting for ultra-light fields of dark matter, and detecting gravitational waves in the mid-frequency band between those of LIGO (Laser Interferometer Gravitational-Wave Observatory) and LISA (Laser Interferometer Space Antenna). For these reasons, we will term the mission concept Gravity Observation and Dark energy Detection Explorer in the Solar System (GODDESS).

Dark Energy↗
NASA NTRS2022

Constraining Warm Dark Matter and Population III Stars with the Global 21 cm Signal

Upcoming ground- and space-based experiments may have sufficient accuracy to place significant constraints upon high-redshift star formation, Reionization, and dark matter (DM) using the global 21 cm signal of the intergalactic medium. In the early universe, when the relative abundance of low-mass DM halos was important, measuring the global signal would place constraints on the damping of structure formation caused by DM having a higher relic velocity (warm dark matter, or WDM) than in cold dark matter. Such damping, however, can be mimicked by altering the star formation efficiency (SFE) and can be difficult to detect because of the presence of Population III stars with unknown properties. We study these various cases and their degeneracies with the WDM mass parameter mX using a Fisher matrix analysis. We study the mX = 7 keV case and a star formation model that parameterizes the SFE as a strong function of halo mass and include several variations of this model along with three different input noise levels for the likelihood; we also use a minimum halo virial temperature for collapse near the molecular cooling threshold. We find that when the likelihood includes only Population II stars, mX is constrained to an uncertainty of ∼0.4 keV for all models and noise levels at the 68% confidence level. When the likelihood includes weak Population III stars, mX ∼ 0.3 keV, and if Population III star formation is relatively efficient, mX ∼ 0.1 keV uncertainty, with tight Population III star formation parameter constraints. Our results show that the global 21 cm signal is a promising test-bed for WDM models, even in the presence of strong degeneracies with astrophysical parameters.

Cosmology↗
NASA NTRS2022

X-Ray Morphology of Cluster-Mass Haloes in Self-Interacting Dark Matter

We perform cosmological zoom-in simulations of 19 relaxed cluster-mass haloes with the inclusion of adiabatic gas in the cold dark matter (CDM) and self-interacting dark matter (SIDM) models. These clusters are selected as dynamically relaxed clusters from a parent simulation with M200(1-3) × 10 15 M. Both the dark matter and the intracluster gas distributions in SIDM appear more spherical than their CDM counterparts. Mock X-ray images are generated based on the simulations and are compared to the real X-ray images of 84 relaxed clusters selected from the Chandra and ROSAT archives. We perform ellipse fitting for the isophotes of mock and real X-ray images and obtain the ellipticities at cluster-centric radii of r0.1–0.2R 200 .The X-ray isophotes in SIDM models with increasing cross-sections are rounder than their CDM counterparts, which manifests as asystematic shift in the distribution function of ellipticities. Unexpectedly, the X-ray morphology of the observed non-cool-coreclusters agrees better with SIDM model swith cross-section (σ/m)=0.5–1cm2g−1 than CDM and SIDM with (σ/m)=0.1cm2g−1. Our statistical analysis indicates that the latter two models are disfavoured at the 68 percent confidence level (as conservative estimates). This conclusion is not altered by shifting the radial range of measurements or applying a temperature selection criterion. However, the primary uncertainty originates from the lack of baryonic physics in the adiabatic model, such as cooling, star formation and feedback effects, which still have the potential to reconcile CDM simulations with observations.

dark matter – cosmology↗
NASA NTRS1972

Size frequency distribution of Martian craters and relative age of light and dark terrains.

Light and dark terrains in and around Meridiani Sinus, mapped on the imagery from Mariner 6 and Mariner 7, were found to have significantly different cumulative size frequency distributions of craters. The light terrain on a mosaic of frames 6N11, 6N13, and 6N19 has a greater proportion of large craters and a lesser proportion of smaller craters than the dark terrain on the same frames. The light terrain is interpreted to be generally older than the dark terrain. The filling or partial filling of the smaller craters on the light terrain by surface detritus is suggested. Several wide-angle frames have remarkably similar cumulative crater size frequency distributions that may be representative of a large portion of the Martian surface.

Woronow, A.↗
NASA NTRS1973

ERTS-1 anomalous dark patches

The author has identified the following significant results. Through combined use of imagery from ERTS-1 and NOAA-2 satellites was found that when the sun elevation exceeds 55 degrees, the ERTS-1 imagery is subject to considerable contamination by sunlight even though the actual specular point is nearly 300 nautical miles from nadir. Based on sea surface wave slope information, a wind speed of 10 knots will theoretically provide approximately 0.5 percent incident solar reflectance under observed ERTS multispectral scanner detectors. This reflectance nearly doubles under the influence of a 20 knot wind. The most pronounced effect occurs in areas of calm water where anomalous dark patches are observed. Calm water at distances from the specular point found in ERTS scenes will reflect no solar energy to the multispectral scanner, making these regions stand out as dark areas in all bands in an ocean scene otherwise comprosed by a general diffuse sunlight from rougher ocean surfaces. Anomalous dark patches in the outer parts of the glitter zones may explain the unusual appearance of some scenes.

Strong, A. E.↗
NASA NTRS1974

Lunar surface - Identification of the dark mantling material in the Apollo 17 soil samples

Evidence indicates that Apollo 17 sample 74001, a soil consisting of very dark spheres, is composed almost entirely of the dark mantling material that covers a large region of the southeastern boundary of Mare Serenitatis. Other Apollo 17 samples contain only a component of this material. The underlying basalt in the Taurus-Littrow valley appears to be an extension of material forming the low-albedo ring around part of Mare Serenitatis and much of the surface of Mare Tranquillitatis. The surface of this basalt region is spectrally distinct from areas with dark mantling material. These results are derived from telescopic and laboratory measurements of the optical properties of lunar soil. Digital vidicon color images are used to map the extent of these material units in the Taurus-Littrow region.

Pieters, C.↗
NASA NTRS1975

The composition of the ultraviolet dark markings on Venus

The ultraviolet dark clouds are an ephemeral phenomenon in the Venus atmosphere, apparently just near the limit of stability. The UV dark material is moderately abundant, perhaps 10%, since the contrasts between light and dark material are some 20%. The material should absorb light between 3000-4000 A, and ideally should also have visible absorptions in the blue, as the overall spectral albedo of Venus indicates. Such a material is bromine dissolved in hydrobromic acid. Solar radiation near 2500 A is sufficient to partially photolyze HBr into Br2. HBr in the Venus atmosphere is inferred to have a mixing ratio of 0.0001 versus CO2. With a water vapor mixing ratio of 0.001, droplets of hydrobromic acid are possible. These droplets would eventually evaporate in the drier upper atmosphere. The refractive index of these hydrobromic acid droplets of 52% (by weight) composition is 1.46, within the limits set by polarization studies of Venus.

Sill, G. T.↗
NASA NTRS1976

A far-infrared map of the Ophiuchus dark cloud region

An extensive area of the Ophiuchus dark cloud has been mapped at far-infrared (40-50 microns) wavelengths with high angular resolution (1 arcmin) using a 102-cm balloon-borne telescope. Three sources were detected and resolved. Two of the sources were identified with strong 2-micron sources that were also associated with radio continuum emission. The far-infrared flux is consistent with radiation from dust heated by an early B star. Previous 2-micron observations have indicated the presence of a large number of early B stars embedded in the dark cloud; the far-infrared observations indicate, however, that although some early B stars are in the dark cloud, most of the 2-micron sources are either behind the cloud or are stars not on the main sequence.

Fazio, G. G.↗
NASA NTRS1976

Molecular hydrogen in interstellar dark clouds

A simplified H2 formation mechanism is proposed in which small interstellar grains furnish the reaction sites. This mechanism results in a maximum value for the rate constant of about 2 by 10 to the -18th power per cu cm/sec for dark clouds at 10 K. Also, the nascent molecules are ejected in excited states, in qualitative agreement with Copernicus observations. A time-dependent treatment of the chemical evolution of a dark cloud with little or no ionizing radiation shows that the clouds require more than 10 million years to achieve chemical equilibrium. The observed residual atomic hydrogen in several dark clouds suggests that the clouds are 1 to 10 million years old. Other consequences of the temporal cloud model are in accord with astronomical observations.

Allen, M.↗
NASA NTRS1978

Phobos: Photometry and origin of dark markings on crater floors

High resolution photographs of Phobos taken during close flybys of Viking Orbiter 1 reveal many dark patches on the floors of several craters. The apparently dark material is only prominent at large phase angles. Analysis of the photometric properties indicates that the dark patches represent areas of unusually rough textures whose reflectance near zero phase is similar to that of the mean surface (approximately 6 percent in the visible), but whose phase curve is much steeper. The contrast of such areas is less than 10 percent zero phase but approaches 100 percent near phase angles of 90 degrees. It is proposed that these intricately textured deposits represent patches of vesticular impact melt.

Goguen, J.↗
NASA NTRS1978

Radio observations of interstellar CN toward diffuse clouds, dark clouds, black clouds, and circumstellar clouds

Emission at 113 GHz from the CN molecule has been searched for in a large number of interstellar regions, primarily dark dust clouds. Lines have been detected in four dark clouds, the first time CN has been observed in this type of object. Comparative CO observations were also performed. The CN/CO abundance ratio varies from cloud to cloud, even among objects which are apparently otherwise similar. This variation suggests that the chemistry of dark clouds may be time-dependent. A previously reported detection of CN emission from a diffuse cloud was not confirmed. Several black clouds and circumstellar clouds were reobserved to obtain better line parameters and to serve as comparative interstellar chemical systems.

Allen, M.↗
NASA NTRS1978

Phobos - Photometry and origin of dark markings on crater floors

Viking Orbiter 1 close encounter pictures of Phobos reveal unusual dark patches on the floors of many craters. Photometry of these features indicates that they have a similar normal reflectance but a phase coefficient 30% larger than the average surface. These facts suggest that the 'dark' material has a similar composition but a much rougher texture than the average surface of Phobos. By analogy with terrestrial impact, explosion and laboratory craters, the dark patches are interpreted as impact generated melt pools which remain visible on Phobos due to the low value of g and consequent small amount of fallback ejecta and slumping. Since blanketing by ejecta from subsequent impacts and micrometeoroid erosion should eventually obliterate such features, they should be more conspicuous in the fresher craters. Similar features should be visible on small asteroids and other low g bodies. However, they are not prominent on the surface of Deimos - possibly because most of the craters imaged at high resolution on the outer satellite appear to have been blanketed by several meters of fine-grained material.

Goguen, J.↗
NASA NTRS1979

Alphonsus-type dark-halo craters - Morphology, morphometry and eruption conditions

The Alphonsus dark-halo craters are sites of volcanic eruptions with a characteristic crater radius of approximately 1 km. They are characterized by dark haloes ranging from 3 to 4 km, by substantial amounts of nonjuvenile material within the deposits, and by a lack of any associated lava flows or prominent constructional features, such as cones or domes. In the present paper, possible eruption styles are analyzed, using these observational constraints. It is concluded that an eruption process analogous to terrestrial volcanian explosive activity provides the most reasonable explanation for the characteristics of the deposits. The dark-halo craters appear to have formed contemporaneously with the emplacement of lavas in the adjacent Mare Nubium.

Head, J. W., III↗
NASA NTRS1980

Observations of SO in dark and molecular clouds

The 1(0)-0(1) transition of SO at 30 GHz has been observed in several sources, including the first detection of sulfur monoxide in cold dark clouds without apparent internal energy sources. The SO transition appears to be an excellent tracer of structure in dark clouds, and the data support suggestions that self-absorption is important in determining emission profiles in such regions for large line-strength transitions. Column densities estimated from a comparison of the results for the two isotopic species indicate a high fractional abundance of SO in dark clouds.

Rydbeck, O. E. H.↗
NASA NTRS1981

Accretionary dark rims in unequilibrated chondrites

Textural and qualitative EDX investigations of dark-rimmed particles in six low petrologic type chondrites indicate that the rims accreted on host particles over a wide range of temperatures prior to initial accumulation and lithification of the meteorites in which the rimmed particles are now contained. Many dark rims are enriched in moderately volatile trace elements such as Na, Cl, P, and K, relative to the host particles and matrix. The range of physical/chemical environments associated with hypervelocity impacts may have offered the setting for the formation of dark-rimmed particles early in solar system history.

King, T. V. V.↗
NASA NTRS1981

High intensity surge and seasonal effects in the dark current of the Nimbus-4 BUV experiment

Seasonal global maps of the dark current produced by corpuscular radiation contributing to the background level of the Nimbus-4 Backscattered Ultraviolet (BUV) instrument were developed, using BUV monochrometer nighttime data in the pulse counting mode during solar and magnetically quiet periods. The existence of high intensity surges has been discovered which occur on a sporadic basis and which cause sufficient enhancements of dark current within the subauroral regions to produce background levels similar to those within the South Atlantic anomaly. Examples are provided of the nominal quiet dark current intensity maps, and the variability and implications of the surge data are discussed.

Stassinopoulos, E. G.↗
NASA NTRS1983

Mineralogy and petrology of the Abee enstatite chondrite breccia and its dark inclusions

A model is proposed for the petrogenesis of the Abee E4 enstatite chondrite breccia, which consists of clasts, dark inclusions and matrix, and whose dark inclusions are an unusual kind of enstatite chondritic material. When the maximum metamorphic temperature of the breccia parent material was greater than 840 C, euhedral enstatite crystals in metallic Fe, Ni, and sulfide-rich areas grew into pliable metal and sulfide. Breccia parent material was impact-excavated, admixed with dark inclusions, and rapidly cooled. During this cooling, the clast and matrix material acquired thermal remanent magnetization. A subsequent ambient magnetic field imparted a uniform net magnetic orientation to the matrix and caused the magnetic orientation of the clasts to be less random. The Abee breccia was later consolidated by shock or by shallow burial and long period, low temperature metamorphism.

Rubin, A. E.↗