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Title Author Abstract Title Authors Abstract Title Authors Abstract Our analyses of these minerals provide constraints on heat sources in the comet-forming region, aqueous activity on cometary bodies, and the extent and mechanisms of radial mixing of material in the early nebula. The sulfides in the Wild 2 collection are most likely the products of low-temperature aqueous alteration. They provide evidence of radial mixing of material from the inner solar system to the comet-forming region and possible secondary aqueous processing on the cometary body. Title Authors Abstract Title Authors Abstract Title Authors Abstract Title Author Abstract We present new solid state astrochemical laboratory results in which one of these routes is tested. Title Authors Abstract
1. Native (parent) CO2 ice in comets C/2007 N3 (Lulin) (Ootsubo et al. 2010), 103P/Hartley (A'Hearn et al., in prep.), and others It seems clear that several Solar System objects that can now be studied in detail preserve compositional traces of various early stages in the processing and agglomeration of solar nebula materials to form planetesimals. Some asteroids inside Jupiter's orbit appear to have retained a fraction of their original organics and H2O inventories, although their bulk composition is silicate-rich. Objects having very low (≤1 g/cm3) densities, including some comets and TNOs, and Saturn's satellite Hyperion, either accreted as porous planetesimals or have lost the more volatile ices from their interiors since accretion. Hyperion may be a special case of a large, unmelted planetesimal that is primarily made of H2O, and probably CO2 ice, laced with hydrocarbon-bearing dust and small amounts of other, unidentified volatile molecules (Dalton et al. 2011). With a bulk density of 0.6 g/cm3 and an effective diameter of ~180 km, Hyperion's surface morphology, and perhaps its shape, indicates that is has been modified by a combination of impacts and sublimation (Thomas et al. 2007, Cruikshank et al. 2007, Howard et al. 2011, Dalton et al. 2011). Sublimation depressions often have accumulations of dark material on their floors, while lumps of the same or similar dark material lay at random places in the surrounding icy landscape. Hartmann (1980) and subsequent authors have described this process of vertical segregation of dust and ice by sublimation on icy bodies in the outer Solar System. A study of Cassini data for Hyperion, including images with spatial resolution ~40-80 meters and VIMS compositional maps with resolution ~1-4 km, establishes local and regional trends in the distribution of H2O, CO2, aliphatic and aromatic hydrocarbons, and adsorbed H2. Complexed (wavelength-shifted) CO2 and adsorbed H2 are concentrated in the dark floor deposits, while the hydrocarbons appear to be broadly distributed within the moderate-albedo (~0.6) icy terrain. H2 is viewed as a photoproduct of H2O ice, while CO2 may be formed locally from H2O ice and carbon from carbonaceous grains, or may represent trapped molecules leaching out from the interior. The long-term loss of CO2 (which is unstable as a surface ice at Hyperion's heliocentric distance) from the interior may contribute to Hyperion's low bulk density. The material from which Hyperion formed is expected to be some combination of native interstellar ices and solid organic matter, plus an unknown fraction of the same material processed in the solar nebula. The dominant form of carbon in interstellar ice depends primarily on competition between CO hydrogenation (CO + H → CHO), and CO oxidation (CO + O → CO2) on grain surfaces (Tielens & Whittet 1997). The HCO radical produced in the first reaction readily undergoes further reactions to the organic molecules H2CO, CH3OH, and others. The second reaction produces CO2, in which the carbon is sequestered in a tightly bound molecule that tends to inhibit further chemical changes. The apparent high abundance of CO2 in the composition of Hyperion, and the absence of H2CO and CH3OH, thus discriminates between two paths of chemical evolution of the materials from which it accreted. In addition to ices, interstellar dust carries hydrogenated amorphous carbon (Pendleton & Allamandola 2002) and relatively refractory polycyclic aromatic hydrocarbons (PAHs); together these are the dominant carriers of carbon. In the solar nebula, PAHs were destroyed inward of ~2 AU (Kress et al. 2010); aliphatic hydrocarbons are less stable than PAHs, and are more readily destroyed. The presence of both aromatic and aliphatic hydrocarbons in the ice of Hyperion supports the view that it accreted from outer solar nebula materials. The low-albedo dust interspersed in the ices consists of these relatively small hydrocarbons plus other macromolecular carbonaceous material consisting of the more refractory kerogen-like organic solids. These astronomical kerogens are thought to be produced in interstellar space by the irradiation of ices on (silicate) grains, a process simulated in the laboratory, and then incorporated into the solar nebula during accretion. Outside the terrestrial planet formation zone, the kerogens are preserved (Chick and Cassen 1997) and together with silicates are the refractory part of the feedstock of comets, carbonaceous meteorites, and icy bodies. Some of these bodies are now recognized as planetary satellites. The astronomical kerogen bears a structural and optical similarity to some tholins (also characterized as carbon nitrides) synthesized in the laboratory, and have pronounced colors ranging from brown to red (e.g., Imanaka et al., Quirico et al. 2008). They impart the generally reddish colors to many outer Solar System bodies (e.g., Cruikshank et al. 2005), but when exposed to the space environment these materials become blacker and more neutral in color as they become dehydrogenated and undergo increasing graphitization. The organic molecules and low-albedo dust found in Hyperion's ices may represent original interstellar material that was largely unaltered in the solar nebula. As the ices slowly evaporate and leave concentrations of the carbonaceous dust exposed to the space environment, that dust is dehydrogenated and driven to neutral-colored and spectroscopically featureless graphite. References:
A'Hearn, M., et al. In preparation. Title Authors Abstract By means of thermal models, I estimated the temperature of meteoroids. These temperatures strongly depend on the orbit of the bodies before they enter the Earth's atmosphere. I will present a work in progress aimed at studying which are the orbits that allow the meteoroids to stay cold for a time long enough to bring the organic matter to Earth. These temperatures-orbit constraints might help in the identification of the source regions of organic rich meteorites. TITLE Hydrated minerals on asteroids in the Main Belt AUTHORS J. de León (1), R. Duffard
(1), Z. Lin (1), J. L. Ortiz (1), & L. M. Lara(1) (1) Instituto de Astrofísica
de Andalucía – CSIC. Glorieta de la Astronomía, s/n, 18008 Granada, Spain ABSTRACT Knowledge of the hydrated mineral inventory on the asteroids is important
for deducing the origin of Earth's water, interpreting the meteorite record and
unravelling the processes occurring during the
earliest times of our Solar System history. Observations show that hydrated
minerals are common in the mid/outer main asteroid belt. The mechanisms
responsible for such hydration are not clear, and the rotational variations
observed in several diagnostic absorption features suggest that hydration was
uneven. The presence of hydrated minerals on a body can be explained if that
body had water ice and a source of heat to melt that ice. Heating sources could
be both the 26Al (if the heat is present in the early formation times) and the
heat generated by a collision. In the meteorite collection, hydrated minerals
are found mostly among the carbonaceous chondrites, in particular the CI, CM,
and CR groups. These meteorites have mineralogies> indicative
of low levels of metamorphism (1200 °C) and evidence for aqueous alteration
[1][2]. The CM and CI carbonaceous chondrites typically contain 5 – 15%
H2O/OH by weight, some of them containing even more than 20%. The CI chondrites
are composed almost entirely (>=90 vol %) of
fine-grained phyllosilicates, though other hydrous and hydroxylated
minerals are also present. The phyllosilicates, or sheet silicates, are an
important group of hydrated minerals that includes the micas, chlorite,
serpentine, talc, and the clay minerals. Clay minerals are one of the primary
products of chemical weathering and one of the more abundant constituents of
sedimentary rocks.
Figure 1. Three different spectra showing the absorption
band at 0.7 μm. The
asteroid (19) Fortuna, a phyllosilicate antigorite (from [4]), and a CM2 carbonaceous chondrite.
All the spectra are normalized to unity at 0.55 μm. Two spectral regions have been the focus for hydrated mineral studies on
asteroids: the 3 μm region
spanning 2.4 - 3.6 μm, and the visible region
between 0.4 - 0.9 μm. Spectra of some main belt
asteroids show an absorption feature centred near 0.7 μm with a
width of roughly 0.25 μm, attributed to a Fe2+ --> Fe3+ charge transfer
transition in oxidized Fe found in phyllosilicates [3]. Figure 1 shows examples
of this feature in the spectra of a main beltasteroid,
a carbonaceous chondrite and a phyllosilicate. In
order to determine how significant and homogeneous is the presence of hydrated
minerals in the surface of asteroids we have started a long-term program to
obtain visible reflectance spectra of main belt asteroids with the 0.7 μm absorption band previously detected. Our
observations are done using CAFOS at the 2.2m telescope
in the Calar Alto Astronomical Observatory. To search
for any variation in the position, with, and depth of this absorption band, and
correlations with other physical/dynamical parameters of the object, we obtain
a spectrum every quarter of its full rotational period. Here we present the
results for a preliminary set of 60 asteroids. References 1. Sears D. W. G. and Dodd R. T. (1988) Overview and classification of
meteorites. In Meteorites and the Early Solar System (J. F. Kerridge and M. S. Matthews, eds.), pp. 1-31. Univ. of Arizona, Tucson. 2. Rubin A. E. (1996) Mineralogy of meteorite groups. Meteoritics
& Planet. Sci., 32, 231-247. 3. Vilas, F.; Gaffey, M. J. (1989) Phyllosilicate absorption features in main-belt and
outer-belt asteroid
reflectance spectra. Science, 246, 790-792. 4. King, T. V. V. & Clark, R. N. (1989), JGR, 94, 13997-14008. Title Author Abstract Title Author Abstract Title Author Abstract Title Authors ABSTRACT Title Author Abstract Title Author Abstract Title Author Abstract Title Author Abstract Title Authors Abstract TITLE Organics
as a Fingerprint of Nature and Extent of Hydration in Asteroids AUTHORS E. Quirico (1), P. Beck (1), & F-R Orthous-Daunay
(1). (1) IPAG
CNRS/University Joseph Fourier 38041 Grenoble Cedex 9
France (eric.quirico@obs.ujf-grenoble.fr). ABSTRACT Chondrites
are rocks originating from undifferentiated asteroids. They have experienced
post-accretional processes on their parent body, as
aqueous alteration, thermal or shock metarmopshim,
that more or less extensively modified the initial mineralogical assemblage and
organics. The impact of these processes is generally rated by the so-called
petrologic classification, assigning a number between 3 and 6 for an increasing
thermal metamorphism, and from 3 to 1 for an increasing aqueous alteration [1].
The properties of hydration is generally linked to the extent and nature of the
post-accretional events, in the sense that the
thermally processed chondrites are generally water-free. However, for a certain
number of objects, thermal processes and aqueous alteration both play a role in
a complex manner that is difficult to unravel. Thermal and hydration processes are
not independent. Organics are sensitive to thermal procesess, either induced by
schocks or ragiogenic
processes [2]. In type 3 chondrites, and heavily schock-processed
MCMs, they lost their alkyl component. In types
½ chondrites, their CH2/CH3 ratio provides clues on the chemical class
(e.g. CR against CM), and then indirectly testifies on different hydration
states (e.g. molecular water against hydroxyles).
More interestingly, organics could testifies of the
past action of thermal processes, that were contemporaneous or followed by
aqueous alteration. This is for instance the case of some type 3 carbonaceous
chondrites (e.g. CV [3]), short-duration metamorphosed CM [4-5] or CR [6].
Therefore, the asteroidal setting of hydration can be
discussed more thoroughly, by considering the presence of surface and bulk
thermal processes. In this
presentation, I will review the spectral properties of organic matter in the
different class of chondrites along with their hydration state, including the
uncommon shocked CM and CR recently investigated, and that may be more representative
of asteroidal surfaces. I will show that organics are
a valuable tracer of post-accretional processes in
conditions of combined actions of aqueous alteration and thermal processes. References:
[1] Beck et al GCA 74, 4881 [2] Orthous-Daunay 2011. PhD Thesis [3] Bonal et al. 2006. GCA
70, 1849-1863 [4] Quirico et al. 2009 MPS 72:5208 [5]
Quirico et al. 2011 LPSC Meeting Abstract [6] Briani et al. 2010 MPS TITLE The
Fraction of Ch Asteroids in the C Complex from Sloan Digital Sky Survey Data AUTHOR A. S. Rivkin (1). (1) JHU/APL,
Laurel MD (andy.rivkin@jhuapl.edu). ABSTRACT Motivation:
Asteroids represent leftover building blocks from the formation of the inner
planets. While some are fragments of melted and differentiated objects (or are
intact melted and differentiated objects, like Vesta),
the majority experienced minimal heat and pressure. Understanding the distribution of asteroid types provides
insight into the earliest times in solar system history, and the strength and
extent of the processes occurring then. Spectroscopy and spectrophotometry
have been mainstays of asteroid studies for decades. A long- standing aim of asteroid spectroscopy has been to establish
and identify linkages or analogs between asteroids and meteorites, allowing the
vast body of meteoritical literature to be leveraged
by asteroid researchers and also providing context for meteoriticists
and an understanding of the formation locations for their samples. Classification
of asteroid spectra has likewise been done for decades, often spurred by the
availability of a new dataset. It was established relatively early on that the
most-common objects in the asteroid belt had spectra reminiscent of carbonaceous
chondrites [1]. Because carbonaceous chondrites can have water- or hydroxyl-bearing
minerals (hereafter ``hydrated minerals'' for simplicity) and organic
compounds, their parent bodies (usually identified as C-class or similar) are
of potential interest not only for insights into the formation of the solar
system but also for a possible role in aiding the start of life on Earth. Hydrated
minerals have deep absorptions in the 3-μm spectral region [2-4] with
absorptions due to organic matter at slightly longer wavelengths. Because these
wavelengths can be difficult to observe from Earth,
there have been attempts to identify "proxy bands" that are easier to observe
and can be used to obtain the information contained in the 3-μm region. The most
commonly-used proxies are the U-B magnitude [2,5],
which correlates with 3-μm band depth, and the existence of a band centered
near 0.7 μm [6,7] whose presence correlates with the presence of the 3-μm band
(though its absence is not diagnostic, as detailed below). The 0.7-μm
band has been incorporated into the Bus taxonomy [8], with C-complex asteroids
showing the band classified in the Ch or Cgh taxa, and those without classified as C, Cb,
Cg, or B. The SDSS data have a
spectral resolution too low to distinguish between most of these classes, other
than broadly between objects with and without the 0.7-μm band.
Therefore,
in this work I will use "Ch" as a catch-all name for the Ch and Cgh classes, with "C" a catch-all for the other members of
the C complex. "Ctot" will represent the whole set of C complex asteroids,
both C and Ch. Dataset:
For this work, the third release of the Sloan Digital Sky Survey Moving Object
Catalog (SDSSMOC) was used [9]. This release included over 200,000 moving
objects observed through June 2004. Those 200,000 objects were cut down to a
smaller sample size by, in turn, only considering those objects with a* <
0 (a* is defined in [9] as a
function of asteroidal colors, and a* < 0 for "C-like"
objects, a sample of nearly 45,000 objects), removing observations which were
not associated with known objects (leaving over 16,000 objects), and finally
using the estimated relative colors of X and D vs. C asteroids to further
exclude interlopers. The final,
restricted sample size is 3593 observations of 3104 objects. For comparison the SMASS survey included
405 C-complex objects, and the S3OS2 survey included 193 C-complex objects
(with roughly 85 objects in common between those two surveys). Thus, the SDSS
sample studied here represents a roughly six to eight-fold increase in the
sample size available from previous studies. While there has been an additional update to the Moving Object
Catalog since the Third Release, its inclusion of objects observed on
non-photometric nights was deemed a large enough disincentive to offset the
larger sample size it offered.
Tests showed that the inclusion, averaging, or exclusion of multiple
observations of the same object had no appreciable effect on the results below,
which are quoted below using the full 3593-observation sample. Approach:
The C asteroids, that is those without a 0.7-μm band,
could be expected to have band depths (BD) of zero (basically by
definition). When taking
observational uncertainties into account, however, we might expect some of them
to show non-zero band depths. Taking
the entire C class population, we might expect the distribution of band depths
to be normally distributed around zero.
Figure 1 shows the histogram of band depths for all 3593 C-complex
observations, that is Ctot. This distribution is not centered at zero, and has a
somewhat non-Gaussian appearance.
How- ever, this distribution includes both C and Ch asteroids. It can
be argued that while objects with positive band depths may be either Ch or C,
the vast majority of objects with negative band depths will be C rather than
Ch. By making the assumption that all objects with negative band depths are C
rather than Ch, one can estimate the number of C asteroids with positive band
depths and their distribution, and estimate the number of Ch asteroids as the
"leftover" objects.
Results:
There are 1015 objects with BD < -0.01 and 529 with band depths between
-0.01 and 0. The implied
distribution of C asteroids is also shown in Figure 1. This is well-fit
by a Gaussian centered at zero band depth with σ of 0.021. This distribution, scaled by a
factor of 26 to match the actual asteroid distribution, is within ~7% of the
observed BD<0 distribution except for BD< -0.05, where 8 objects are
expected but none are seen. The
symmetry of this distribution includes an additional ~1015 C asteroids in the
dataset with band depths greater than zero. Thus, with 2049 objects with band depths greater than zero,
a Ch fraction of 29 ± 0.7% is suggested.
These objects have a distribution that itself can be fit with a Gaussian
with σ = 0.02 and a mean band depth of ~0.034. Unlike the C population, however, it is not obvious that the
Ch band depth should have a Gaussian distribution, which could be interpreted
as a preferred band depth for Ch material. Having said that, it is interesting to note that most CM meteorites
measured by [10] and Hiroi (unpublished, in RELAB
database) have 0.7-μm bands of roughly 3-4%, while non-CM meteorites (including
other hydrated carbonaceous chondrites like the CI group) do not have the band.
The distribution of band depths in the C-complex, then, is consistent with a
group with no band (and associated scatter) and a group with a ~3-4% band
(perhaps CM-like, and again with associated scatter). References: [1] Johnson, T. V. and F. P. Fanale (1973), JGR
78 8507. [2] Feierberg, M. A. et al. (1985), Icarus 63 63. [3] Jones, T. D. et al.
(1990), Icarus 88 172. [4] Rivkin,
A. S. (2003), Met. Plan. Sci. 38, 1383. [5] Vilas, F. (1995), Icarus 115 217.
[6] Vilas, F. and M. J. Gaffey (1989),
Science 246 790. [7] Vilas, F. Icarus
111 456. [8] Bus, S. J. and R. P. Binzel (2002), Icarus 158 146. [9] Ivezić, Ž et al. (2001), Astron
J. 122 2749. [10] Hiroi, T. et al. (1996), Met.
Plan. Sci. 31 321. This work was supported by the NASA Planetary Geology and Geophysics
Program. Thanks to Dave Trilling, Ellen Howell, and Cristina Thomas
for useful discussions.
Figure
1: The inner curves show the set of C- complex-like objects in the SDSS sample
that have band depths < 0 and a reflection of that set about the y axis, and
a fit to that population. The
histogram of the total population is marked with triangles, and the difference
between that curve and the inner curve(s) are interpreted as Ch-like objects
(that is, the excess of objects with band depths > 0 above what might be expected
from scatter alone). Also shown is
a fit to that outer curve assuming 29% Ch-like objects with band depth ~3-4%
and 71% non-Ch objects with band depth 0%, and
scatter about both means. TITLE AUTHOR TITLE An
estimate of the flux of primitive bodies in all range of masses during the LHB
and the source of terrestrial water AUTHOR J. M. Trigo-Rodríguez (1). (1)
Institute of Space Sciences (CSIC-IEEC). ABSTRACT O
isotope data of terrestrial materials compared with the values for primitive
chondrites suggests that Earth was formed from high-temperature, highly reduced
materials [1]. It is consistent with the hypothesis of the terrestrial planets
being grown from planetesimals formed in the inner solar system [2]. The grand tack scenario recently
proposed for the final setting of the solar system dynamic configuration
introduces new pathways for the early delivery of volatiles to Earth [3]. It
was a short period, coincident with the called Late Heavy Bombardment (LHB), in
which the dynamic behavior of Jupiter and Saturn allowed a gravitational
pathway for Kuiper Belt Objects (KBOs)
arrived from the outer belt to the inner region. That flux of water-rich bodies
enriched the volatile content of terrestrial planets and the already formed satellites
of giant planets [4]. At that time, gravitational perturbations also allowed
the capture of Trojans and Main Belt Comets (MBCs)
that have been subjected since then to collisional
and thermal processing. Since then, the disruption captured
objects in the main belt could have produced different families with water-rich
members, like the recently described 24 Themis [5]. I
envision a scenario in which the fragile ice-rich bodies visiting the inner
solar system were subjected to thermal processing that, together with close
approaches with planets, caused an important rate of catastrophic disruptions.
Direct collisions of KBOs and their fragments with
terrestrial planets enriched the volatile content of rocky planets. The
particular noble gas pattern currently observed in Earth's atmosphere [6] could
be explained as consequence of solar wind noble gas implantation in the
meteoroid streams produced at that time. I will introduce new calculations in
which the mass of water arrived to Earth from KBO fragments could have been, at
least, one order of magnitude higher (i.e. about 1024 g) than the estimated by
direct collisions with comets. Consequently, a more efficient and complete
scenario for the setting of volatiles on rocky planets needs to take into account
the delivery in all range of sizes as previously noted [7]. References:
[1] Wasson J.T. 2000. Rev. Geoph. 38-4,
491-512. [2] Hansen B.M.S., 2009. ApJ 703, 1131-1140. [3] Walsh et al. 2011.
Nature, in press. [4] Trigo-Rodríguez J.M. y F.J. Martín-Torres. 2011. Plan. Space. Sci. 59, doi:10.1016/j.pss.2011.02.011. [5] Campins
H. et al. 2010. Nature 464, 1320-1321. [6] Marty B.,
and Meibom A. 2007. eEarth 2, 43-49. [7] Anders E. 1989. Nature 342, 255-258. TITLE Depletion
and excitation of the asteroid belt by migrating planets AUTHORS K. J.
Walsh (1,2), Morbidelli, A. (2), Raymond, S. N. (3),
O'Brien, D. P. (4), Mandell, A. M. (5) (1) SwRI, Boulder, CO (2) Obs. de la Côte d'Azur, Nice, France
(3) Lab. d'Astrophysique de Bordeaux, Floirac, France (4) PSI,
Tucson, AZ (5) NASA Goddard, Greenbelt, MD (kwalsh@boulder.swri.edu) ABSTRACT The
excitation and depletion of the asteroid belt has historically been modelled with stranded planetary embryos or resonance
sweeping caused by the dissipation of the solar nebular gas. Both of these
methods rely on the asteroids, with their substantial diversity, being "born"
largely where they are found today. We present a model of early inner solar
system evolution whereby the gas migration of Jupiter and Saturn bring them to
1.5 AU, truncating the disk of planetesimals, before they migrate outward to
their current locations. This
model, dubbed "The Grand Tack", solves some outstanding problems, including the
size of Mars, and has substantial implications for the excitation, depletion
and origin of the asteroid belt. TITLE Hydrogen and Oxygen Isotopic Compositions of Asteroidal
Water: Evidence of Fluid Inclusions from Ordinary Chondrites AUTHORS H. Yurimoto (1, 2),
S. Itoh (1), M. E. Zolensky
(3), M. Kusakabe (4), & A. Karen (5). (1) Natural History Sciences, Hokkaido
University; (2) CRIS, Hokkaido University.
E-mail: yuri@ep.sci.hokudai.ac.jp; (3) Astromaterials
Research and Exploration Science, NASA Johnson Space Center; (4) Department of
Environmental Biology and Chemistry, University of Toyama; (5) Toray Research
Center, Inc. ABSTRACT Introduction: Over the past three decades we have become increasingly aware of the
fundamental importance of water, and aqueous alteration, on primitive
solar-system bodies. Some carbonaceous and ordinary chondrites have been
altered by interactions with liquid water within the first 10 million years
after formation of their parent asteroids. In fact, millimeter to
centimeter-sized aggregates of purple halite containing aqueous fluid inclusions
were reported in the matrix of two freshly-fallen brecciated
H chondrite falls, Monahans (1998, hereafter simply "Monahans") (H5) and Zag (H3-6)
[1, 2]. If the isotopic compositions of the aqueous fluid are determined, we
can discuss differences of aqueous fluid between Earth and asteroids and origin
of aqueous fluid of planetary bodies. However, no isotope data had been
presented because of small sizes of the fluid inclusions. Recently, preliminary
data have been presented using secondary ion mass spectrometry [3]. Here we
report progress of measurements of hydrogen and oxygen isotopic compositions of
the aqueous inclusion fluids of the ordinary chondrites. Methods: The samples used in this study were fluid inclusion-bearing halite crystals
of 0.1 to 1 mm in size picked from fresh fracture surfaces of the chondrites.
We synthesized fluid inclusions of known isotopic composition in halite
crystals in order to calculate d-values from measurement data. A Cameca ims-1270 equipped with a cryo-sample-stage
of Hokkaido University was prepared for the measurements. The cryo-sample-stage (Techno. I. S. Corp.) was
cooled down to c.a. -190°C using liquid nitrogen at which the aqueous fluid in
inclusions was frozen into ice. We excavated the salt crystal surfaces to
expose the frozen fluids by a 15 keV Cs+ beam and
measured negative secondary ions. A normal incident electron gun was applied to
compensate electrostatic charging for the sputtered regions. The secondary ions
from deep craters of ~10 μm in depth emitted stably but the intensities changed
gradually during measurement cycles because states of charge compensation were
shifted. Results and
Discussion: Reproducibility of multiple
measurements of standard fluid inclusions resulted in ±90‰ (2s) for dD, and ±29‰ (2s) for d18O. The relatively poor reproducibility
is due to variable states of charge compensation on deep sputtered surface
among inclusions. On the other hand, the reproducibility of D17O is ±8‰ (2s)
because the observed variations of isotope ratios follow a mass dependent fractionation
law. Variations of dD of asteroidal
fluid range over -330 (90; 2s) to +1200 (90)‰ for Monahans
and -300 (96)‰ to +90 (98)‰ for Zag. D17O of asteroidal fluids range over -16 (22)‰ to +18 (10)‰ for Monahans and +3 (10)‰ to +27 (11)‰ for Zag.
The variations are larger than the reproducibility of standard analyses and suggest
that isotope equilibria were under way in the asteroidal fluid before trapping into halite. The mean
values of dD and D17O are +290‰ and +9‰, respectively.
The mean values and the variations of the asteroidal
fluids are different from the representative values of ordinary chondrites,
suggesting that the origin of fluid was not indigenous to the H chondrite
parent-asteroid but rather was an exogenous fluid delivered onto the asteroid
from icy objects such as C, P or D asteroids, comets, or icy satellites of outer
planets. The exogenous nature of aqueous fluid suggests that aqueous fluid on
inner planetary bodies results of two components mixing between nebular water
vapor and cometary water ice. References: [1] Zolensky M. E. et al., 1999. Science,
285: 1377-1379. [2] Zolensky M. E. et al., 1999.
MAPS, 34: A124. [3] Yurimoto H. et al., 2010. MAPS,
45: A222. Title Author Abstract |
