552 research outputs found
Infrared (2.08-14 micron) spectra of powered stony meteorites
Infrared biconical reflectance spectra of 60 powdered meteorite samples, representing 50 different stony meteorites, were measured as analogues of asteroidal regolith. Representative samples were measured in directional hemispherical reflectance to assure that Kirchhoff's Law can be used to predict relative emissivity from the reflectance spectra. These spectral data confirm that the O-H fundamental absorption band near 2.9 microns is an extremely sensitive indicator of incipient alteration, which often has taken place in powdered meteorite samples exposed only to water vapor in the air. Such non-carbonaceous samples typically contain less than 1 percent water by weight. Likewise, the C-H fundamental absorption bands near 3.4 and 3.5 microns are equally sensitive indicators of contamination with volatile hydrocarbons, which can also be absorbed from the air. The heavy, macromolecular hydrocarbons native to chondrites do not display such heavy bands, making detection of these bands in remote sensing of asteroids unlikely. Despite the spectral artifacts introduced by alteration and hydrocarbon contamination, powdered stony meteorites display a wide variety of real spectral features that can be used for their identification, including residual reststrahlen bands, absorption bands, and the Christiansen feature. Researchers found that the wavelengths of the peaks or troughs of each of these spectral features can be used independently to infer meteorite composition, but the best results are obtained when the entire spectral curve is used, or at least the portion of it encompassed by the 8 to 14 micron atmospheric window, in a digital search library
On the chemical composition of L-chondrites
Radiochemical neutron activation analysis of Ag, As, Au, Bi, Co, Cs, Ga, In, Rb, Sb, Te, Tl, and Zn and major element data in 14 L4-6 and 3 LL5 chondrites indicates that the L group is unusually variable and may represent at least 2 subgroups differing in formation history. Chemical trends in the S/Fe rich subgroup support textural evidence indicating late loss of a shock formed Fe-Ni-S melt; the S/Fe poor subgroup seemingly reflects nebular fractionation only. Highly mobile In and Zn apparently reflect shock induced loss from L chondrites. However, contrasting chemical trends in several L chondrite sample sets indicate that these meteorites constitute a more irregular sampling of, or more heterogeneous parent material than do carbonaceous or enstatite chondrites. Data for 15 chondrites suggest higher formation temperatures and/or degrees of shock than for LL5 chondrites
Ranges of Atmospheric Mass and Composition of Super Earth Exoplanets
Terrestrial-like exoplanets may obtain atmospheres from three primary
sources: Capture of nebular gases, degassing during accretion, and degassing
from subsequent tectonic activity. Here we model degassing during accretion to
estimate the range of atmospheric mass and composition on exoplanets ranging
from 1 to 30 Earth masses. We use bulk compositions drawn from primitive and
differentiated meteorite compositions. Degassing alone can create a wide range
of masses of planetary atmospheres, ranging from less than a percent of the
planet's total mass up to ~6 mass% of hydrogen, ~20 mass% of water, and/or ~5
mass% of carbon compounds. Hydrogen-rich atmospheres can be outgassed as a
result of oxidizing metallic iron with water, and excess water and carbon can
produce atmospheres through simple degassing. As a byproduct of our atmospheric
outgassing models we find that modest initial water contents (10 mass% of the
planet and above) create planets with deep surface liquid water oceans soon
after accretion is complete.Comment: ApJ, in press. 32 pages, 6 figure
Coreless Terrestrial Exoplanets
Differentiation in terrestrial planets is expected to include the formation
of a metallic iron core. We predict the existence of terrestrial planets that
have differentiated but have no metallic core--planets that are effectively a
giant silicate mantle. We discuss two paths to forming a coreless terrestrial
planet, whereby the oxidation state during planetary accretion and
solidification will determine the size or existence of any metallic core. Under
this hypothesis, any metallic iron in the bulk accreting material is oxidized
by water, binding the iron in the form of iron oxide into the silicate minerals
of the planetary mantle. The existence of such silicate planets has
consequences for interpreting the compositions and interior density structures
of exoplanets based on their mass and radius measurements.Comment: ApJ, in press. 22 pages, 5 figure
Deriving asteroid mineralogies from reflectance spectra: Implications for the MUSES-C target asteroid
In an effort to both bolster the spectral database on ordinary chondrites and constrain our ability to deconvolve modal, mineral chemistry and bulk chemical composition information from ordinary chondrites, we have initiated a spectral study of samples with known bulk compositions from the Smithsonian Institution\u27s Analyzed Meteorite Powder collection. In this paper, we focus on deriving a better formula for determining asteroid mineralogies from reflectance spectra. The MUSES-C mission to asteroid 25143 1998 SF36 will allow any derived mineralogies to be tested with a returned sample
Modal abundances of CAIs: Implications for bulk chondrite element abundances and fractionations
Modal abundances of Ca,Al-rich inclusions (CAIs) are poorly known and
reported data scatter across large ranges. We combine reported CAI modal
abundances and our own set, and present a complete list of CAI modal abundances
in carbonaceous chondrites. This includes (in area%): CV: 2.98, CM: 1.21, Acfer
094: 1.12, CO: 0.99, CK/CV (Ningqiang & DaG 055): 0.77, CK: 0.2, CR: 0.12 and
CB: 0.1. CAIs are Poisson distributed and if only small areas (<1000 mm2) are
studied, the data are probably not representative of the true CAI modal
abundances, explaining their reported large scatter in a single chondrite
group. Carbonaceous chondrites have excess bulk Al concentrations when compared
to the CI-chondritic value. We find a correlation between this excess and CAI
modal abundances and conclude that the excess Al was delivered by CAIs. The
excess Al is only a minor fraction (usually ~10 rel%, but 25 rel% in case of
CVs) of the bulk chondrite Al and cannot have contributed much 26Al to heat the
chondrite parent body. Ordinary, enstatite, R- and K-chondrites have an Al
deficit relative to CI chondrites and only very low CAI modal abundances, if
any are present at all. Carbonaceous chondrites also had an initial Al deficit
if the contribution of Al delivered by CAIs is subtracted. Therefore all
chondrites probably lost a refractory rich high-T component. Only minor amounts
of CAIs are present in the matrix or have been present in the chondrule
precursor aggregates. Most CAI size distributions contain more than one size
population, indicating that CAIs from within a single meteorite group had
different origins.Comment: Meteoritics & Planetary Sciences (in press
Bulk element compositions of meteorites: A guide for interpreting remote-sensing geochemical measurements of planets and asteroids
We report a large database of bulk meteorite elemental abundances, compiled to aid in the interpretation of elemental abundance data determined by remote-sensing instrumentation on planetary missions. A custom user interface was developed for easy access and manipulation of the abundance data. The database contains almost 3000 individual analyses of more than 1000 individual meteorites. Most major and minor elements are included, as well as small number of trace elements measurable by remote-sensing gamma-ray spectroscopy (notably Th and U). All meteorite classes show variability in bulk compositions between individual analyses. Some of this spread is intrinsic to the parent bodies of the meteorites. However, some variability is undoubtedly due to systematic uncertainties, caused by inter-laboratory bias, misclassification, effect of weathering, and unrepresentative sampling. We use the database here to investigate both how well different meteorite groups can be distinguished on the basis of bulk compositions and how bulk compositions can be related to the cosmochemical and geological processes that produced them. The major elements measurable by X-ray and gamma-ray remote-sensing-oxygen, magnesium, aluminum, silicon, sulfur, calcium and iron-reflect to differing degrees nebular elemental fractionations and parent-body igneous processes and can be used together to distinguish most classes and sub-classes of meteorites. Potassium is potentially useful as a tracer of thermal processes in the early solar system. Thorium and uranium abundances could be used to trace igneous processes on differentiated asteroids
Formation and Structure of Low Density Exo-Neptunes
Kepler has found hundreds of Neptune-size (2-6 R_Earth) planet candidates
within 0.5 AU of their stars. The nature of the vast majority of these planets
is not known because their masses have not been measured. Using theoretical
models of planet formation, evolution and structure, we explore the range of
minimum plausible masses for low-density exo-Neptunes. We focus on highly
irradiated planets with T_eq>=500K. We consider two separate formation pathways
for low-mass planets with voluminous atmospheres of light gases: core nucleated
accretion and outgassing of hydrogen from dissociated ices. We show that
Neptune-size planets at T_eq=500K with masses as small as a few times that of
Earth can plausibly be formed core nucleated accretion coupled with subsequent
inward migration. We also derive a limiting low-density mass-radius relation
for rocky planets with outgassed hydrogen envelopes but no surface water. Rocky
planets with outgassed hydrogen envelopes typically have computed radii well
below 3 R_Earth. For both planets with H/He envelopes from core nucleated
accretion and planets with outgassed hydrogen envelopes, we employ planet
interior models to map the range of planet mass--envelope mass--equilibrium
temperature parameter space that is consistent with Neptune-size planet radii.
Atmospheric mass loss mediates which corners of this parameter space are
populated by actual planets and ultimately governs the minimum plausible mass
at a specified transit radius. We find that Kepler's 2-6 R_Earth planet
candidates at T_eq=500--1000K could potentially have masses less than ~4
M_Earth. Although our quantitative results depend on several assumptions, our
qualitative finding that warm Neptune-size planets can have masses
substantially smaller than those given by interpolating the masses and radii of
planets within our Solar System is robust.Comment: 17 pages, 9 figures, accepted for publication in Ap
Thermal evolution and sintering of chondritic planetesimals III. Modelling the heat conductivity of porous chondrite material
The construction of models for the internal constitution and the temporal
evolution of large planetesimals, the parent bodies of chondrites, requires
information on the heat conductivity of the complex mixture of minerals and
iron metal found in chondrites. It is attempted to evaluate the heat
conductivity of a multi-component mineral mixture and granular medium from the
heat conductivities of its mixture components. Random mixtures of solids with
chondritic composition and packings of spheres are numerically generated. The
heat conduction equation is solved in high spatial resolution for a test cube
filled with such matter. From the heat flux through the cube the heat
conductivity of the mixture is derived. The model results for porous material
are consistent with data for compacted sandstone, but are at odds with
measurements for H and L chondrites. The discrepancy is traced back to shock
modification of the currently available meteoritic material by impacts on the
parent body over the last 4.5 Ga. This causes numerous micro-cracks that act as
additional barriers for heat transfer. The void structure in meteorites is
different from that which probably existed in the pristine material of the
parent bodies. The results obtained for the heat conductivity of the pristine
material are used for calculating models for the evolution of the H chondrite
parent body which are fitted to the cooling data of a number of H chondrites.
The fit to the data good.Comment: 19 pages, 8 figures, accepted by Astronomy & Astrophysic
- ā¦