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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

The stratigraphy of Mars

A global stratigraphy of Mars was developed from a global geologic map series derived from Viking images; the stratigraphy is composed of three maps. A new chronostratigraphic classification system which consists of lower, middle, and upper Noachian, Hesperian, and Amazonian systems is described. The crater-density boundaries of the chronostratigraphic units and the absolute ages of the Martian epochs aer estimated. The relative ages of major geologic units and featues are calculated and analyzed. The geologic history of Mars is summarized on the maps in terms of epochs.

Tanaka, Kenneth L.↗
NASA NTRS1988

LMC clusters - Age calibration and age distribution revisited

The empirical age relation for star clusters in the Large Magellanic Cloud presented by Elson and Fall (1985) are reexamined using ages based only on main-sequence turnoffs. The present sample includes 57 clusters, 24 of which have color-magnitude diagrams published since 1985. The new calibration is very similar to that found previously, and the scatter in the relation corresponds to uncertainties of about a factor of 2 in age. The age distribution derived from the new calibration does not differ significantly from that derived in earlier work. It is compared with age distributions estimated by other authors for different samples of clusters, and the results are discussed.

Elson, Rebecca A.↗
NASA NTRS1988

The red envelope and the age of the universe

The photometric evolution of old stellar populations is slow in the optical/IR. Consequently, observational error is a strongly limiting factor in the accuracy possible for determining galaxy ages or estimating q0 at high redshifts from indices such as the 4000-A break. Existing observations of the red envelope at z above about 0.3 are consistent with a wide range of evolutionary histories. The middle-UV spectral region (rest wavelengths = 2000-3300 A) appears to promise less ambiguous results.

O'Connell, Robert W.↗
NASA NTRS1988

Relative dating of Hawaiian lava flows using multispectral thermal infrared images - A new tool for geologic mapping of young volcanic terranes

The weathering of Hawaiian basalts in arid and semiarid environments is accompanied by changes in their thermal infrared emittance spectra. The spectral differences can be measured and mapped with multispectral imaging systems. The differences appear to be related to the degree of development, preservation, and alteration of glassy crusts; the oxidation of iron; and the accretion of silica-rich surface veneers. Because the measurements are quantitative and in image format, they are useful for estimating relative ages in geologic mapping of lava flows. In Hawaii this technique is most diagnostic for distinguishing among sparsely vegetated flows less than 1.5 ka in age.

Kahle, Anne B.↗
NASA NTRS1991

An infrared analysis of Puppis A

A quantitative analysis of the infrared emission of the supernova remnant (SNR) Puppis A as seen by the Infrared Astronomical Satellite is presented. Physical parameters of the remnant (and the surrounding interstellar medium), including density, mass, temperature, and age are estimated. The infrared spectrum of Puppis A is fit by models of thermal emission from interstellar dust grains which have been swept up into the SNR where they are collisionally heated by the shocked gas. Evidence of an enhanced abundance of very small grains in the interstellar medium near Puppis A, and evidence of their destruction by the advancing blast wave, are found.

Arendt, Richard G.↗
NASA NTRS1991

951 Gaspra - A pre-Galileo estimate of its surface evolution

An effort is made to quantitatively model the collisional environment and surface evolution of asteroid 951 Gaspra prior to the flyby of the Galileo spacecraft later this year. Gaspra's average intrinsic collisional probability is calculated to be 5.4 x 10 to the -18th/sq km per year with a root mean square impact velocity of 5.0 km/sec. The projectile population and impact strength for Gaspra are highly uncertain and a range of values is considered. Based on these models, Gaspra is unlikely to be a primordial object surviving from the beginning of the solar system. It is most likely a second or multiple generation fragment forming about 100 million yr ago. For a low impact flux, the crater size distribution directly reveals the size distribution of the impacting population. For a nonequilibrium surface, model-dependent isochrons for estimating the age of the surface are given. Lower limits are set by the model for Gaspra's age (about 1 billion yr) and flux for the case of a high impact flux producing an equilibrium surface.

Namiki, Noriyuki↗
NASA NTRS1991

Equilibrium wave spectrum and sea state bias in satellite altimetry

For a well-developed sea at equilibrium with a constant wind, the energy-containing range of the wavenumber spectrum for wind-generated gravity waves is approximated by a generalized power law involving the angular spread function and mu, interpreted as a fractal codimension of a small surface patch. Dependence of mu on the wave age is estimated, and the 'Phillips constant', beta, along with the low-wavenumber boundary, k0, of the inertial subrange are analyzed on the basis of the wave action and energy conservation principles. The resulting expressions are employed to evaluate various non-Gaussian statistics of a weakly nonlinear sea surface, which determine the sea state bias in satellite altimetry. The locally accelerated decay of the spectral density function in a high-wavenumber dissipation subrange is pointed out as an important factor of wave dynamics and the geometrical optics treatment of the sea state bias. The analysis is carried out in the approximation of a unidirectional wave field and confined to the case of a well-developed sea.

Glazman, Roman E.↗
NASA NTRS1998

HD 98800: A Unique Stellar System of Post-T Tauri Stars

HD 98800 is a system of four stars, and it has a large infrared excess that is thought to be due to a dust disk within the system. In this paper we present new astrometric observations made with Hipparcos, as well as photometry from Hubble Space Telescope WFPC2 images. Combining these observations and reanalyzing previous work allow us to estimate the age and masses of the stars in the system. Uncertainty in these ages and masses results from uncertainty in the temperatures of the stars and any reddening they may have. We find that HD 98800 is most probably about 10 Myr old, although it may be as young as 5 Myr or as old as 20 Myr old. The stars in HD 98800 appear to have metallicities that are about solar. An age of 10 Myr means that HD 98800 is a member of the post T Tauri class of objects, and we argue that the stars in HD 98800 can help us understand why post T Tauris have been so elusive, HD 98800 may have formed in the Centaurus star-forming region, but it is extraordinary in being so young and yet so far from where it was born.

Soderblom, David R.↗
NASA NTRS2001

A Young Stellar Cluster in the Nucleus of NGC 4449

We have obtained 1-2 A resolution optical Echellette spectra of the nuclear star cluster in the nearby starburst galaxy NGC4449. The light is clearly dominated by a very young (6 - 10 Myr) population of stars. For our age dating, we have used recent population synthesis models to interpret the observed equivalent width of stellar absorption features such as the H I Balmer series and the Ca II triplet around 8500 A. We also compare the observed spectrum of the nuclear cluster to synthesized spectra for stellar populations of varying ages. All these approaches yield a consistent cluster age. Metallicity estimates based on the relative intensities of various ionization lines yield no evidence for significant enrichment in the center of this low mass galaxy: the metallicity of the nuclear cluster is about one fourth of the solar value, in agreement with independent estimates for the disk material of NGC4449.

Boeker, Torsten↗
NASA NTRS1996

Compositional Evolution of Saturn's Ring: Ice, Tholin, and 'CHIRON'-Dust

We address compositional evolution in planetary ring systems subsequent to meteoroid bombardment. The huge surface area to mass ratio of planetary rings ensures the importance of this process, given currently expected values of meteoroid flux. We developed a model which includes both direct deposition of extrinsic meteoritic 'pollutants', and ballistic transport of the increasingly polluted ring material as impact ejecta. Certain aspects of the observed regional variations in ring color and albedo can be understood in terms of such a process. We conclude that the regional scale color and albedo differences between the C ring and B ring can be understood if all ring material began with the same composition (primarily water ice, based on other data, but colored by tiny amounts of non-icy, reddish absorber) and then evolved entirely by addition and mixing of extrinsic, neutrally colored, highly absorbing material. This conclusion is readily extended to the Cassini Division and its surroundings as well. Typical silicates are unable to satisfy the ring color, spectroscopic, and microwave absorption constraints either as intrinsic or extrinsic non-icy constituents. However, 'Titan Tholin' provides a satisfactory match for the inferred refractive indices of the 'pre-pollution' nonicy ring material. The extrinsic bombarding material is compatible with the properties of Halley or Chiron, but not with the properties of other 'red' primitive objects such as Pholus. We further demonstrate that the detailed radial profile of color across the abrupt B ring - C ring boundary is quite compatible with such a 'pollution transport' process, and that the shape of the profile can constrain key parameters in the model. We use the model to estimate the 'exposure age' of Saturn's rings to extrinsic meteoroid flux. We obtain a geologically young 'age' which is compatible with timescales estimated independently based on the evolution of ring structure due to ballistic transport, and also with other 'short timescales' estimated on the grounds of gravitational torques.

Cuzzi, Jeffrey N.↗
NASA NTRS2005

Pulsar Wind Nebulae, Space Velocities and Supernova Remnant

The original proposal for this LTSA grant was for X-ray studies of pulsars, and especially pulsar wind nebulae and what they could tell us about pulsar properties, especially their space velocities. By any metric, this program has been very successful. No fewer than 14 papers on directly related topics (and several dozen more on related topics) have been published in refereed journals with the PI as lead or co-author, all observational results that have had significant impact on the field. These include the first X-ray detection of the "Duck" pulsar, a clear demonstration that estimated pulsar ages can be off by over an order of magnitude (via observations of the young supernova remnant G11.2-0.3) and the detection of the first pulsar wind nebula around a millisecond pulsar. These publications have also resulted in 4 press releases. Moreover, they also represent the thesis work of two PhD students at MIT (Froney Crawford and Mike Pivovaroff) and one postdoctoral fellow, Bryan Gaensler, now Assistant Professor at Harvard.

Source record↗
NASA NTRS2005

Measurements of the Coefficient of Restitution of Quartz Sand on Basalt: Implications for Abrasion Rates on Earth and Mars

Knowledge of the rates at which rocks abrade from the impact of saltating sand provides important input into estimating the age and degree of modification of arid surfaces on Earth and Mars. Previous work has relied on measuring mass loss rates in the field and the laboratory. The susceptibility of rocks and other natural materials has been quantified on a relative scale from laboratory studies.

Banks, M.↗
NASA NTRS2007

Simulation of Malaria Transmission among Households in a Thai Village using Remotely Sensed Parameters

We have used discrete-event simulation to model the malaria transmission in a Thailand village with approximately 700 residents. Specifically, we model the detailed interactions among the vector life cycle, sporogonic cycle and human infection cycle under the explicit influences of selected extrinsic and intrinsic factors. Some of the meteorological and environmental parameters used in the simulation are derived from Tropical Rainfall Measuring Mission and the Ikonos satellite data. Parameters used in the simulations reflect the realistic condition of the village, including the locations and sizes of the households, ages and estimated immunity of the residents, presence of farm animals, and locations of larval habitats. Larval habitats include the actual locations where larvae were collected and the probable locations based on satellite data. The output of the simulation includes the individual infection status and the quantities normally observed in field studies, such as mosquito biting rates, sporozoite infection rates, gametocyte prevalence and incidence. Simulated transmission under homogeneous environmental condition was compared with that predicted by a SEIR model. Sensitivity of the output with respect to some extrinsic and intrinsic factors was investigated. Results were compared with mosquito vector and human malaria data acquired over 4.5 years (June 1999 - January 2004) in Kong Mong Tha, a remote village in Kanchanaburi Province, western Thailand. The simulation method is useful for testing transmission hypotheses, estimating the efficacy of insecticide applications, assessing the impacts of nonimmune immigrants, and predicting the effects of socioeconomic, environmental and climatic changes.

Kiang, Richard K.↗
NASA NTRS2009

Luminous Red Galaxies at Z=0.4-0.5

We report on a study of approx.20,000 luminous red galaxies (LRG's) at z=0.4-0.5 observed by the Sloan Digital Sky Survey. In order to differentiate among them, we measured restframe magnitudes, u (3000-3500 A), b (4200-4800 A), and y (5700-6300 A) from the spectra themselves. The galaxies show a significant range in restframe colors and absolute magnitudes. We binned the spectra according to the restframe u-b color and y-band absolute magnitude in order to increase the S/N. We used 3 approaches to estimate the ages and metal content of these binned spectra: via their spectral energy distributions, from spectral-line indices, and by full spectral fitting. The three methods usually produce discordant results

Heap, Sally↗
NASA NTRS2012

A Suzaku Study of Ejecta Structure and Origin of Hard X-ray Emission in the Supernova Remnant G156.2+5.7

We report an X-ray study of the evolved Galactic supernova remnant (SNR) G1S6.2+S.7 based on six pointing observations with Suzaku. The remnant's large extent (100' in diameter) allows us to investigate its radial structure in the northwestern and eastern directions from the apparent center. The X-ray spectra. were well fit with a two-component non-equilibrium ionization model representing the swept-up interstellar medium (ISM) and the metal-rich ejecta. We found prominent central concentrations of Si, S and Fe from the ejecta component; the lighter elements of O, Ne and Mg were distributed more uniformly. The temperature of the ISM component suggests a slow shock (610-960 km/s), hence the remnant's age is estimated to be 7,000-15,000 yr, assuming its distance to be approx. 1.1 kpc. G1S6.2+5.7 has also been thought to emit hard, non-thermal X-rays, despite being considerably older than any other such remnant. In response to a recent discovery of a background cluster of galaxies (2XMM J045637.2+522411), we carefully excluded its contribution, and reexamined the origin of the hard X-ray emission. We found that the residual hard X-ray emission is consistent with the expected level of the cosmic X-ray background. Thus, no robust evidence for the non-thermal emission was obtained from G156.2+5.7. These results are consistent with the picture of an evolved SNR.

Uchida, Hiroyuki↗
NASA NTRS2012

In-Situ Geochronology as a Mission-Enabling Technology

Although there are excellent estimates of ages of terrains on Mars from crater counting, even a few absolute ages would serve to validate the calibration. Results with uncertainties, although much larger than those that could be achieved in labs on Earth, would be extremely valuable. While there are other possibilities for in-situ geochronology instruments, we describe here to alternative technologies, being developed in JPL. There are two common features of both. The first is analysis by means of miniature mass spectrometer. The second is use of laser sampling to reduce or avoid sample handling, preparation and pre-treatment and equally importantly, to allow analysis of individual, textually resolved minerals in coarse-grained rocks. This textural resolved minerals in coarse-grained rocks. This textural resolution will aid in selection of grains more or less enriched in the relevant elements and allow construction of isochrons for more precise dating. Either of these instruments could enable missions to Mars and other planetary bodies.

geochronology↗
NASA NTRS2015

McLaughlin Crater as a Candidate Landing Site for Humans on Mars

McLaughlin Crater is an ancient (Noachian) Martian impact crater located at 337.6 East, 21.9 North, just south of the dichotomy boundary. This site should be considered for future landed exploration because: a) it is located at the boundary of three types of scientifically important terrain that will yield key results about the geological evolution and habitability of Mars; b) it contains surfaces where radiometric dating can be related to age dates estimated from crater counting, c) it contains volatile-rich rocks that will not only yield interesting results regarding ancient atmospheric chemistry, but will also be high quality, accessible targets for in-situ resource utilization (ISRU), and d) the site within the crater provides a flat, low-risk and low-elevation landing zone, which will facilitate landing large payloads on Mars.

Michalski, J. R.↗