51 research outputs found
Vapor-Melt Exchange -- Constraints on Chondrite Formation Conditions and Processes
The bulk volatile contents of chondritic meteorites provide clues to their
origins. Matrix and chondrules carry differing abundances of moderately
volatile elements, with chondrules carrying a refractory signature. At the high
temperatures of chondrule formation and the low pressures of the solar nebula,
many elements, including Na and Fe, should have been volatile. Yet the evidence
is that even at peak temperatures, at or near the liquidus, Na and Fe (as FeO
and Fe-metal) were present in about their current abundances in molten
chondrules. This seems to require very high solid densities during chondrule
formation to prevent significant evaporation. Evaporation should also be
accompanied by isotopic mass fractionation. Evidence from a wide range of
isotopic systems indicates only slight isotopic mass fractionations of
moderately vola-tile elements, further supporting high solid densities.
However, olivine-rich, FeO-poor chondrules commonly have pyroxene-dominated
outer zones that have been interpreted as the prod-ucts of late condensation of
SiO2 into chondrule melts. Late condensation of more refractory SiO2 is
inconsistent with the apparent abundances of more volatile Na, FeO and Fe-metal
in many chondrules. Despite significant recent experimental work bearing on
this problem, the conditions under which chondrules behaved as open systems
remain enigmatic
STELLAR ORIGINS OF EXTREMELY C-13- AND N-15-ENRICHED PRESOLAR SIC GRAINS: NOVAE OR SUPERNOVAE?
Extreme excesses of 13C (12C/13C < 10) and 15N (14N/15N < 20) in rare presolar SiC grains have been considered diagnostic of an origin in classical novae, though an origin in core collapse supernovae (CCSNe) has also been proposed. We report C, N, and Si isotope data for 14 submicron- to micron-sized 13C- and 15N-enriched presolar SiC grains (12C/13C < 16 and 14N/15N < ~100) from Murchison, and their correlated Mg–Al, S, and Ca–Ti isotope data when available. These grains are enriched in 13C and 15N, but with quite diverse Si isotopic signatures. Four grains with 29,30Si excesses similar to those of type C SiC grains likely came from CCSNe, which experienced explosive H burning occurred during explosions. The independent coexistence of proton- and neutron-capture isotopic signatures in these grains strongly supports heterogeneous H ingestion into the He shell in pre-supernovae. Two of the seven putative nova grains with 30Si excesses and 29Si depletions show lower-than-solar 34S/32S ratios that cannot be explained by classical nova nucleosynthetic models. We discuss these signatures within the CCSN scenario. For the remaining five putative nova grains, both nova and supernova origins are viable because explosive H burning in the two stellar sites could result in quite similar proton-capture isotopic signatures. Three of the grains are sub-type AB grains that are also 13C enriched, but have a range of higher 14N/15N. We found that 15N-enriched AB grains (~50 < 14N/15N < ~100) have distinctive isotopic signatures compared to putative nova grains, such as higher 14N/15N, lower 26Al/27Al, and lack of 30Si excess, indicating weaker proton-capture nucleosynthetic environments
The Compositional Structure of the Asteroid Belt
The past decade has brought major improvements in large-scale asteroid
discovery and characterization with over half a million known asteroids and
over 100,000 with some measurement of physical characterization. This explosion
of data has allowed us to create a new global picture of the Main Asteroid
Belt. Put in context with meteorite measurements and dynamical models, a new
and more complete picture of Solar System evolution has emerged. The question
has changed from "What was the original compositional gradient of the Asteroid
Belt?" to "What was the original compositional gradient of small bodies across
the entire Solar System?" No longer is the leading theory that two belts of
planetesimals are primordial, but instead those belts were formed and sculpted
through evolutionary processes after Solar System formation. This article
reviews the advancements on the fronts of asteroid compositional
characterization, meteorite measurements, and dynamical theories in the context
of the heliocentric distribution of asteroid compositions seen in the Main Belt
today. This chapter also reviews the major outstanding questions relating to
asteroid compositions and distributions and summarizes the progress and current
state of understanding of these questions to form the big picture of the
formation and evolution of asteroids in the Main Belt. Finally, we briefly
review the relevance of asteroids and their compositions in their greater
context within our Solar System and beyond.Comment: Accepted chapter in Asteroids IV in the Space Science Series to be
published Fall 201
High-temperature Dust Condensation around an AGB Star: Evidence from a Highly Pristine Presolar Corundum
Corundum (-AlO) and amorphous or metastable
AlO are common components of circumstellar dust observed around
O-rich asymptotic giant branch (AGB) stars and found in primitive meteorites.
We report a detailed isotopic and microstructural investigation of a unique
presolar corundum grain, QUE060, identified in an acid residue of the Queen
Alexandra Range 97008 (LL3.05) meteorite. Based on its O and Mg isotopic
compositions, this 1.4 m diameter grain formed in a low- or
intermediate-mass AGB star. It has four developed rhombohedral 011
faces of corundum and a rough, rounded face with cavities. High Mg contents
(Mg/Al 0.004) are due to the decay of radioactive Al. No spinel
(MgAlO) inclusions that might have exsolved from the corundum are
observed, but there are several high-Mg domains with modulated structures. The
subhedral shape of grain QUE060 is the first clear evidence that corundum
condenses and grows to micrometer sizes in the extended atmospheres around AGB
stars. The flat faces indicate that grain QUE060 experienced little
modification by gas-grain and grain-grain collisions in the interstellar medium
(ISM) and solar nebula. The Mg distribution in its structure indicates that
grain QUE060 has not experienced any severe heating events since the exhaustion
of Al. However, it underwent at least one very transient heating event
to form the high-Mg domains. A possible mechanism for producing this transient
event, as well as the one rough surface and cavity, is a single grain-grain
collision in the ISM. These results indicate that grain QUE060 is the most
pristine circumstellar corundum studied to date
Stellar Origins of Extremely - and -enriched Presolar SiC Grains: Novae or Supernovae?
Extreme excesses of (/<10) and
(/<20) in rare presolar SiC grains have been considered
diagnostic of an origin in classical novae, though an origin in core collapse
supernovae (CCSNe) has also been proposed. We report C, N, and Si isotope data
for 14 submicron- to micron-sized - and -enriched presolar SiC
grains (/<16 and /<~100) from Murchison, and
their correlated Mg-Al, S, and Ca-Ti isotope data when available. These grains
are enriched in and , but with quite diverse Si isotopic
signatures. Four grains with excesses similar to those of type C
SiC grains likely came from CCSNe, which experienced explosive H burning
occurred during explosions. The independent coexistence of proton- and
neutron-capture isotopic signatures in these grains strongly supports
heterogeneous H ingestion into the He shell in pre-supernovae. Two of the seven
putative nova grains with excesses and depletions show
lower-than-solar / ratios that cannot be explained by classical
nova nucleosynthetic models. We discuss these signatures within the CCSN
scenario. For the remaining five putative nova grains, both nova and supernova
origins are viable because explosive H burning in the two stellar sites could
result in quite similar proton-capture isotopic signatures. Three of the grains
are sub-type AB grains that are also enriched, but have a range of
higher /. We found that -enriched AB grains
(~50</<~100) have distinctive isotopic signatures compared to
putative nova grains, such as higher /, lower
/, and lack of excess, indicating weaker
proton-capture nucleosynthetic environments.Comment: fix typo in one of the authors' name
The ancient heritage of water ice in the solar system
Identifying the source of Earth's water is central to understanding the
origins of life-fostering environments and to assessing the prevalence of such
environments in space. Water throughout the solar system exhibits
deuterium-to-hydrogen enrichments, a fossil relic of low-temperature,
ion-derived chemistry within either (i) the parent molecular cloud or (ii) the
solar nebula protoplanetary disk. Utilizing a comprehensive treatment of disk
ionization, we find that ion-driven deuterium pathways are inefficient,
curtailing the disk's deuterated water formation and its viability as the sole
source for the solar system's water. This finding implies that if the solar
system's formation was typical, abundant interstellar ices are available to all
nascent planetary systems.Comment: 33 pages, 7 figures including main text and supplementary materials.
Published in Scienc
Exploring the Origins of Deuterium Enrichments in Solar Nebular Organics
Deuterium-to-hydrogen (D/H) enrichments in molecular species provide clues
about their original formation environment. The organic materials in primitive
solar system bodies have generally higher D/H ratios and show greater D/H
variation when compared to D/H in solar system water. We propose this
difference arises at least in part due to 1) the availability of additional
chemical fractionation pathways for organics beyond that for water, and 2) the
higher volatility of key carbon reservoirs compared to oxygen. We test this
hypothesis using detailed disk models, including a sophisticated, new disk
ionization treatment with a low cosmic ray ionization rate, and find that disk
chemistry leads to higher deuterium enrichment in organics compared to water,
helped especially by fractionation via the precursors CHD/CH. We
also find that the D/H ratio in individual species varies significantly
depending on their particular formation pathways. For example, from
AU, CH can reach , while D/H in CHOH
remains locally unaltered. Finally, while the global organic D/H in our models
can reproduce intermediately elevated D/H in the bulk hydrocarbon reservoir,
our models are unable to reproduce the most deuterium-enriched organic
materials in the solar system, and thus our model requires some inheritance
from the cold interstellar medium from which the Sun formed.Comment: 11 pages, 7 figures, accepted for publication in Ap
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