4,146 research outputs found
X-ray Amorphous Components of Antarctica Dry Valley Soils: Weathering Implications for Mars
The Antarctic Dry Valleys (ADV) comprise the largest ice-free region of Antarctica. Precipitation usually occurs as snow, relative humidity is frequently low, and mean annual temperatures are about -20C [1]. Substantial work has focused on soil formation in the ADVs [2], however, little work has focused on the mineralogy of secondary alteration phases. The dominant weathering process in the ADV region is physical weathering, however, chemical weathering has been well documented [3]. The occurrence of chemical weathering processes are suggested by the presence of clay minerals and iron and titanium oxides in soil. Previously we have investigated soils from two sites in the ADVs and have shown evidence of chemical weathering by the presence of clay minerals (vermiculite, smectite), short-range ordered (SRO) and/or X-ray amorphous materials, and Fe- and Tioxides as well as the presence of discrete calcite crystals [4, 5]. The Chemistry and Mineralogy (CheMin) instrument onboard the Mars Curiosity rover has detected abundant amounts (approx. 25-30 wt. %) of X-ray amorphous materials in a windblown deposit or soil (Rocknest) and in a sedimentary rocks [6,7,8]. The occurrence of large amounts of X-ray amorphous materials in Mars sediments is surprising because these materials are usually present in small quantities in terrestrial environments. The objective of this study is to further characterize the chemistry and mineralogy, specifically the secondary alteration mineralogy and the presence of X-ray amorphous material, of soils from two sites we have previously studied, a subxerous soil in Taylor Valley, and an ultraxerous soil in University Valley. While the chemical alteration processes and mineralogy of the ADV has been documented previously, there has been limited discussion on the occurrence and formation of X-ray amorphous and SRO materials in Antarctica soils. The process of aqueous alteration in the ADVs may have implications for pedogenic processes on Mars, and may lead to a better understanding to the abundance of amorphous material found in sediments in Gale crater
Chemical Abundance Constraints on White Dwarfs as Halo Dark Matter
We examine the chemical abundance constraints on a population of white dwarfs
in the Halo of our Galaxy. We are motivated by microlensing evidence for
massive compact halo objects (Machos) in the Galactic Halo, but our work
constrains white dwarfs in the Halo regardless of what the Machos are. We focus
on the composition of the material that would be ejected as the white dwarfs
are formed; abundance patterns in the ejecta strongly constrain white dwarf
production scenarios. Using both analytical and numerical chemical evolution
models, we confirm that very strong constraints come from Galactic Pop II and
extragalactic carbon abundances. We also point out that depending on the
stellar model, significant nitrogen is produced rather than carbon. The
combined constraints from C and N give from
comparison with the low C and N abundances in the Ly forest. We note,
however, that these results are subject to uncertainties regarding the
nucleosynthesis of low-metallicity stars. We thus investigate additional
constraints from D and He, finding that these light elements can be kept
within observational limits only for \Omega_{WD} \la 0.003 and for a white
dwarf progenitor initial mass function sharply peaked at low mass (2).
Finally, we consider a Galactic wind, which is required to remove the ejecta
accompanying white dwarf production from the galaxy. We show that such a wind
can be driven by Type Ia supernovae arising from the white dwarfs themselves,
but these supernovae also lead to unacceptably large abundances of iron. We
conclude that abundance constraints exclude white dwarfs as Machos. (abridged)Comment: Written in AASTeX, 26 pages plus 4 ps figure
A Theoretical Model for the Relation for Supermassive Black Holes in Galaxies
We construct a model for the formation of black holes within galactic bulges.
The initial state is a slowly rotating isothermal sphere, characterized by
effective transport speed \aeff and rotation rate . The black hole
mass is determined when the centrifugal radius of the collapse flow exceeds the
capture radius of the central black hole. This model reproduces the observed
correlation between black hole masses and galactic velocity dispersions, \mbh
\approx 10^8 M_\odot (\sigma/200 \kms)^4, where \sigma = \sqrt{2} \aeff.
This model also predicts the ratio \mrat of black hole mass to host mass:
\mrat 0.004 (\sigma/200 \kms).Comment: 9 pages, 2 figures, submitted to Astrophysical Journal Letter
Specificity of Amino Acid Transport in the Tapeworm Hymenolepis Diminuta and its Rat Host
Paper by A. J. MacInnis, D. J. Graff, A. Kilejian, and C. P. Rea
A Geology Sampling System for Small Bodies
Human exploration of Small Bodies is being investigated as a precursor to a Mars surface mission. Asteroids, comets, dwarf planets, and the moons of Mars all fall into this Small Bodies category and some are being discussed as potential mission tar-gets. Obtaining geological samples for return to Earth will be a major objective for any mission to a Small Body. Currently the knowledge base for geology sampling in microgravity is in its infancy. Furthermore, humans interacting with non-engineered surfaces in a microgravity environment poses unique challenges. In preparation for such missions, a team at the National Aeronautics and Space Administration (NASA) John-son Space Center (JSC) has been working to gain experience on how to safely obtain numerous sample types in such an environment. This abstract briefly summarizes the type of samples the science community is interested in, discusses an integrated geology sampling solution, and highlights some of the unique challenges associated with this type of exploration
Mirror Dark Matter and Core Density of Galaxies
We present a particle physics realization of a recent suggestion by Spergel
and Steinhardt that collisional but dissipationless dark matter may resolve the
core density problem in dark matter-dominated galaxies such as the dwarf
galaxies. The realization is the asymmetric mirror universe model introduced to
explain the neutrino puzzles and the microlensing anomaly. The mirror baryons
are the dark matter particles with the desired properties. The time scales are
right for resolution of the core density problem and formation of mirror stars
(MACHOs observed in microlensing experiments). The mass of the region
homogenized by Silk damping is between a dwarf and a large galaxy.Comment: 9 pages, LaTex. The present version shows that atomic scattering
inherent in the mirror model can solve the core density problem without the
need for an extra U(1) discussed in the original version; all conclusions are
unchanged. This version is accepted for publication in Phys. Rev.
Black Hole Binary Formation in the Expanding Universe --- Three Body Problem Approximation ---
We study black hole MACHO binary formation through three-body interactions in
the early universe at s. The probability distribution functions
of the eccentricity and the semimajor axis of binaries as well as of the
coalescence time are obtained assuming that the black holes are randomly formed
in space. We confirm that the previous order-of-magnitude estimate for the
binary parameters is valid within error. We find that the
coalescence rate of the black hole MACHO binaries is events/year/galaxy taking into consideration several possible
factors which may affect this estimate. This suggests that the event rate of
coalescing binary black holes will be at least several events per year within
15 Mpc. The first LIGO/VIRGO interferometers in 2001 will be able to verify
whether the MACHOs are black holes or not.Comment: Revtex, 25 pages, 10 figures, to appear in PR
Gaussian random waves in elastic media
Similar to the Berry conjecture of quantum chaos we consider elastic analogue
which incorporates longitudinal and transverse elastic displacements with
corresponding wave vectors. Based on that we derive the correlation functions
for amplitudes and intensities of elastic displacements. Comparison to numerics
in a quarter Bunimovich stadium demonstrates excellent agreement.Comment: 4 pages, 4 figure
Artemis Curation: Preparing for Sample Return from the Lunar South Pole
Space Policy Directive-1 mandates that the United States will lead the return of humans to the Moon for long-term exploration and utilization, followed by human missions to Mars and other destinations. In addition, the Vice President stated that It is the stated policy of this administration and the United States of America to return American astronauts to the Moon within the next five years, that is, by 2024. These efforts, under the umbrella of the recently formed Artemis Program, include such historic goals as the flight of the first woman to the Moon and the exploration of the lunar south-polar region. Among the top priorities of the Artemis Program is the return of a suite of geologic samples, providing new and significant opportunities for progressing lunar science and human exploration. In particular, successful sample return is necessary for understanding the history of volatiles in the Solar System and the evolution of the Earth-Moon system, fully constraining the hazards of the lunar polar environment for astronauts, and providing the necessary data for constraining the abundance and distribution of resources for in-situ resource utilization (ISRU). Here we summarize the ef-forts of the Astromaterials Acquisition and Curation Office (hereafter referred to as the Curation Office) to ensure the success of Artemis sample return (per NASA Policy Directive (NPD) 7100.10E)
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