134 research outputs found
A New Look at Neon-C and SEP-Neon
Studies of the isotopic composition of neon in lunar soils, meteorites, and interplanetary dust particles have revealed several distinct components. In addition to implanted solar wind, which has a ^(20)Ne/^(22)Ne-abundance ratio of 13.7, there is an additional component with ^(20)Ne/^(22)Ne≈11.2, originally attributed to higher-energy solar energetic particles. Using data from the Advanced Composition Explorer, we have measured the fluence of solar wind, suprathermal particles, solar energetic particles and cosmic rays from ~0.3 keV/nucleon to ~300 MeV/nucleon over an extended time period. We use these measured spectra to simulate the present-day depth distribution of Ne isotopes implanted in the lunar soil. We find that the suprathermal tail of the solar wind, extending from a few keV/nucleon to several MeV/nucleon with a power law spectrum, can produce ^(20)Ne/^(22)Ne abundance ratios in the lunar soil that are similar to the measured composition, although there remain significant questions about the extent to which the present-day intensity of suprathermal ions is sufficient to explain the lunar observations
Comparison of the Oxidation State of Fe in Comet 81P/Wild 2 and Chondritic-Porous Interplanetary Dust Particles
The fragile structure of chondritic-porous interplanetary dust particles (CP-
IDPs) and their minimal parent-body alteration have led researchers to believe
these particles originate in comets rather than asteroids where aqueous and
thermal alteration have occurred. The solar elemental abundances and
atmospheric entry speed of CP-IDPs also suggest a cometary origin. With the
return of the Stardust samples from Jupiter-family comet 81P/Wild 2, this
hypothesis can be tested. We have measured the Fe oxidation state of 15 CP-IDPs
and 194 Stardust fragments using a synchrotron-based x-ray microprobe. We
analyzed ~300 nanograms of Wild 2 material - three orders of magnitude more
material than other analyses comparing Wild 2 and CP-IDPs. The Fe oxidation
state of these two samples of material are >2{\sigma} different: the CP-IDPs
are more oxidized than the Wild 2 grains. We conclude that comet Wild 2
contains material that formed at a lower oxygen fugacity than the parent body,
or parent bodies, of CP-IDPs. If all Jupiter-family comets are similar, they do
not appear to be consistent with the origin of CP-IDPs. However, comets that
formed from a different mix of nebular material and are more oxidized than Wild
2 could be the source of CP-IDPs.Comment: Earth and Planetary Science Letters, in pres
The Sulfur, Argon, and Calcium Isotopic Composition of the Galactic Cosmic Ray Source
Galactic cosmic ray measurements of the sulfur, argon, and calcium isotopes made by the Cosmic Ray Isotope Spectrometer on the NASA Advanced Composition Explorer are reported over the energy range from 100 to 400 MeV/nucleon. The propagation of cosmic rays through the Galaxy and heliosphere is modeled with observational constraints imposed by measurements. Source abundance ratios of the sulfur, argon, and calcium isotopes are deduced from this model. Cosmic rays are thought to originate in the cores of superbubbles which contain stellar ejecta mixed with the surrounding interstellar medium. The composition of the superbubble core should reflect the composition of the cosmic rays at their source. Based on the derived isotopic source ratios of sulfur, argon, and calcium, the superbubble material at the cosmic ray source is constrained to be 18%+26%-14% supernova and wind ejecta, with the remainder interstellar medium material. This mix of metal-rich ejecta and interstellar medium in the superbubble core corresponds to a cosmic ray source metallicity of 2.7+3.9-2.1 times solar metallicity
Heterogeneous distribution of Al-26 at the birth of the Solar System
It is believed that Al-26, a short-lived (t1/2 = 0.73 Ma) and now extinct
radionuclide, was uniformly distributed in the nascent Solar System with the
initial Al-26/Al-27 ratio of ~5.2\times10-5, suggesting its external stellar
origin. However, the stellar source of Al-26 and the manner in which it was
injected into the solar system remain controversial: the Al-26 could have been
produced by an asymptotic giant branch star, a supernova, or a Wolf-Rayet star
and injected either into the protosolar molecular cloud or protoplanetary disk.
Corundum (Al2O3) is thermodynamically predicted to be the first condensate from
a cooling gas of solar composition. Here we show that micron-sized corundum
condensates from O-16-rich gas (Big Delta O-17 ~ -25%) of solar composition
recorded heterogeneous distribution of Al-26 at the birth of the solar system:
the inferred initial Al-26/Al-27 ratio ranges from ~6.5x10-5 to <2x10-6; ~50%
of the corundum grains measured are Al-26-poor. Other Al-26-poor, O-16-rich
refractory objects include grossite (CaAl4O7)- and hibonite(CaAl12O19)-rich
calcium-aluminum-rich inclusions (CAIs) in CH chondrites, platy hibonite
crystals in CM chondrites, and FUN (fractionation and unidentified nuclear
isotopic anomalies) CAIs in CV, CO, and CR chondrites. Considering the
apparently early and short duration (<0.3 Ma) of condensation of refractory
O-16-rich solids in the solar system, we infer that Al-26 was injected into the
collapsing protosolar molecular cloud and later homogenized in the
protoplanetary disk. The apparent lack of correlation between Al-26 abundance
and O-isotope compositions of corundum grains put important constraints on the
stellar source of Al-26 in the solar system.Comment: Astrophysical Journal Letters 733, L3
A Direct Measurement of the Geomagnetic Cutoff for Cosmic Rays at Space Station Latitudes
We report new measurements of the vertical geomagnetic cutoff for cosmic rays with rigidities from ~500 to 1700 MV, made using data from the MAST instrument on SAMPEX. A total of ~10,000 nuclei were used to measure the latitude cutoff in nineteen separate rigidity intervals. These results show that cosmic rays and solar particles can penetrate several degrees lower in latitude than would be estimated from commonly used relations for the geomagnetic cutoff, which has implications for the radiation exposure expected on the Space Station. An excellent fit to our measured cutoffs is given by the relation Rc = 15.062cos4 (Λ) - 0.363 GV, where Rc is the geomagnetic cutoff in rigidity, and λ is the invariant latitude. We suggest that this relation is useful over invariant latitudes from Λ = 0° to 64°, corresponding to rigidity cutoffs from ~0.2 to 15 GV
The Phosphorus, Sulfur, Argon, and Calcium Isotopic Composition of the Galactic Cosmic Ray Source
Galactic cosmic ray (GCR) measurements of the phosphorus, sulfur, argon, and calcium isotopes made by the Cosmic Ray Isotope Spectrometer aboard the Advanced Composition Explorer are reported over the energy range from ~100 to ~400 MeV nucleon^(–1). The propagation of cosmic rays through the Galaxy and heliosphere is modeled with constraints imposed by measurements. Isotopic source abundance ratios ^(31)P/^(32)S, ^(34)S/^(32)S, ^(38)Ar/^(36)Ar, and ^(44)Ca/^(40)Ca are deduced. The derived ^(31)P/^(32)S ratio is 2.34 ± 0.34 times larger than the solar system value, lending further credence to the suggestion that refractory elements are enriched in the GCRs due to the sputtering of ions off grains in the cores of superbubbles. By determining the GCR source abundances of argon (a noble gas) and calcium (a refractory), it is determined that material in grains is accelerated to GCR energies a factor of 6.4 ± 0.3 more efficiently than gas-phase material in this charge range. With this information, the dust fraction of phosphorus and sulfur in the interstellar material that is mixed with stellar ejecta to form the GCR seed material is found to be consistent with astronomical observations
- …