P31C-01 08:00h
Ceres Observations with HST: Size, Shape, Pole and Rocky Core
Our team has obtained 259 images of Ceres using the High Resolution Channel of the Advanced Camera for Surveys on the Hubble Space Telescope. These images were obtained at high temporal sampling in three filter bands (V, U, and Mid-UV) over the entire rotational phase of Ceres. The resolution was 30 km/pixel, allowing us to detect and follow the motions of surface features on Ceres for the first time. Light and dark patches are seen on the surface, with high contrast and differences among the three filters. A strong feature near the equator provided a control point to definitively determine the rotational pole of Ceres: RA=291 deg and Dec=59 deg. Based on limb fits to an accuracy of nearly 0.1 pixel, our shape measurements show that Ceres is rotationally symmetric with an equatorial radius of 487.3 km and a polar radius of 454.7 km with roughly 5 km uncertainties. This flattening is significantly less than expected for a relaxed body of Ceres' mean density and rotation period, but it is consistent with such a body if Ceres has a central rocky core surrounded by water ice. Support for this project is through STScI grant HST-GO-09748.
P31C-02 08:15h
A Primitive Achondrite With Oxygen Isotopic Affinities to CV Chondrites: Implications for Differentiation and Size of the CV Parent Body
NWA 3133 found in Northwest Africa (as several stones totaling $\sim$ 4 kg) has a metamorphic texture with $\sim$120$\deg$ triple grain junctions (mean grainsize = 0.28 mm); no chondrules are present. Olivine (46 vol.$%$; Fa$_{22}$, FeO/MnO = 64) and orthopyroxene (28 vol.$%$; Fs$_{18.9}$Wo$_{2.3}$, FeO/MnO = 42) are the most abundant phases, with less intermediate plagioclase (An$_{53.5}$Or$_{2.3}$), Cr-diopside (Fs$_{7.5}$Wo$_{48.3}$, Cr$_{2}$O$_{3}$ = 0.71 wt.$%$), Al-Ti-bearing chromite (Cr/(Cr+Al) = 0.73, TiO$_{2}$ = 2.6 wt.$%$), Na-Mg-bearing merrillite, troilite (1-5 wt.$%$ Ni) and Fe-Ni metal (15-20 wt.$%$ Ni). Clinopyroxene, chromite and merrillite are inhomogeneously distributed as relatively large grains. Metal is partially altered to limonite (W1-2), and minor limonite and calcite occur along grain boundaries. Oxygen isotopic compositions ($\delta$$^{17}$O, $\delta$$^{18}$O) determined in two laboratories by laser fluorination on handpicked olivine (-3.67, +0.94; -3.62, +0.89; -2.91, +1.78 per mil) and on acid-washed whole rock fragments (-2.25, +2.46; -1.75, +3.06 per mil) plot on the mixing line for Allende and other CV3 chondrites. Thus, NWA 3133 could be regarded as the first known "CV7 chondrite", and may have been formed by metamorphic recrystallization (or perhaps igneous processes) in the CV parent body. Other meteorites with oxygen and/or Cr isotopic affinities to the CV3 chondrites are the three members of the Eagle Station pallasite grouplet and the silicated irons Bocaiuva and NWA 176 (Clayton and Mayeda., 1996; Liu et al, 2001; Shukolyukov and Lugmair, 2001). Our inference from these data is that the CV parent body was an at least partially differentiated (and relatively large) object consisting of a metal+silicate core region surrounded by a presumably silicate-rich mantle and a chondrule-CAI-rich regolith, the deepest portions of which were metamorphosed and/or partially melted to form primitive achondritic lithologies.
P31C-03 08:30h
Timescales for Accretion of the First Planetesimals
With new precise chronology becoming available for CAIs and chondrules our understanding of early disc evolution is improving dramatically. However, the timescales for first planetesimal accretion are more problematic. It long has been theorized that the first 1000 km sized planetesimals or planetary embryos accreted by runaway growth over timescales of roughly 10${^5}$ years. However, isotopic evidence for the existence of such early objects has been lacking. Chondrites usually contain CAIs as well as chondrules and the isotopic ages for these objects provide evidence that the parent bodies formed $ > $ 2 million years after the start of the solar system. Similarly, the strontium isotope data for basaltic achondrites as well as other lines of isotopic evidence for eucrites and angrites provide evidence that some asteroidal sized differentiated objects did not form until similar, relatively late times. The most likely candidates for samples of very early-formed differentiated planetesimals are certain iron meteorites. Tungsten isotopes provide the opportunity to define the timescales for accretion and metal segregation recorded in these objects. New high precision W isotope data for $ > $30 iron meteorites show that the timescales for accretion, metal segregation and re-equilibration are variable for all magmatic iron groups but extend over periods of 10${^6}$ years. Some groups include iron meteorites with very unradiogenic W and these could be vestiges of the earliest primary planetesimals. These objects not only provide constraints on early accretion, they also help define the initial ${^1^8^2}$Hf/${^1^8^0}$Hf of the solar system and the accretion rates and age of later objects, including the Moon.
P31C-04 08:45h
A new chronology for asteroid formation in the early solar system based on $^{182}$W systematics
Chondrites are generally considered to represent the chemically least processed material of the solar system. They contain Ca-Al-rich inclusions (CAIs), which probably condensed at high temperatures from a gas of approximately solar composition. CAIs are widely considered to be the first solids formed in the solar nebula, such that their most precise U-Pb age of $4567.2 \pm 0.6$ Ma is commonly taken as the age of the solar system. Differentiated meteorites such as iron meteorites derive from asteroids that underwent large-scale chemical differentiation, most notably core formation. Chondrites are widely considered to represent the precursor materials from which asteroids accreted and then differentiated. If this succession of events is correct, the accretion of chondrite parent bodies predates core formation in asteroids. The relative chronology of these processes, however, has not yet been determined. We obtained precise Hf-W ages for CAIs, pristine chondrites, and iron meteorites with the aim of understanding the genetic and temporal relationship between chondrites and iron meteorites. The W isotope data reveal that type IIIAB, IVA, IVB, and IC iron meteorites predate the last major thermal overprint of CAIs and the formation of chondrite parent bodies. These irons are remnants of first-generation planetesimals and represent the oldest yet dated material formed in the solar system. They constrain the minimum age of the solar system to $4570.5 \pm 2.0$ Ma. Assuming a homogeneous distribution of $^{26}Al$ the initial $^{26}Al/^{27}Al$ of the solar system must have been higher than $\sim 8 \times 10^{-5}$. These high amounts of $^{26}Al$ can have provided an efficient means to promote rapid differentiation of early-formed asteroids. We suggest that chondrite parent bodies formed by re-accretion of debris produced during collisional disruption of first-generation planetesimals, and that chondrules formed in the vicinity of first-generation planetesimals.
P31C-05 09:00h
Planet Formation and Early Solar System heating: Recent advancements
In a seminal review in 1991, Wood and Pellas laid out the basic characteristics of protoplanetary heating as preserved in the meteorite record and pointed out the strengths and drawbacks of the two plausible heat sources: $^{26}$Al and electromagnetic induction. A decade and a half later, considerable advances have been made, in our theoretical understanding of planet formation and its effect on protoplanetary heating. In addition, remarkable progress has been made in meteoritic studies, including geochronological measurements and the search for fossil remains of $^{26}$Al in various chondrites and achondrites. The case for $^{26}$Al heating of asteroids has become increasingly robust. Its decay product has been found in most classes of chondrites and some achondrites. Reasons as to why evidence for $^{26}$Al might be obscured in other achondrites have been suggested. The heat source appears capable of explaining the full range of temperature excursions of asteroids within the main belt. The process of accretion has been shown to be intrinsically related to the thermal evolution of a protoplanetary body, since the timescale of accretion is comparable to the half life of the heat source. Thus, discerning characteristics of the parent body like volume proportion of various petrologic types, thermal-model cooling rates and closure ages, have been shown to be a function of accretion rate. A study of $^{244}$Pu fission tracks and $^{40}$Ar-$^{39}$Ar thermochronology of unshocked H chondrites seems to provide support to a layered onion shell parent body, where the outer layers cooled faster than the asteroidal interior. This fits the pattern of heating generated by $^{26}$Al, where the heat source is distributed uniformly per unit mass. Nebular homogeneity and detectable excesses of fossil $^{26}$Mg are necessary but not sufficient conditions for $^{26}$Al heating to be viable. It is critical for the heat source to be live at the time, and after, the formation of the planetary building blocks. Thus, the time, relative to CAI formation, at which accretion initiates, and the duration of the accretionary process strongly influence the efficacy of $^{26}$Al as a heat source. The oldest chondrule Pb-Pb dates measured are 4566.6 +/- 1.0 Myrs for Allende: this denotes a time interval of 0.6 +/- 1.2 Myrs between CAI and chondrule formation. If initiation of accretion is assumed to postdate formation of the oldest chondrules, time of initiation of accretion relative to CAI formation can be interpreted as the time difference between CAI and chondrule formation: thus, accretion can initiate between 0 to 1.8 Myrs after CAI formation. Recent accretional models produce Mars-sized ($\sim$10$^{27}$ g) planetary embryos in $ < $ 1 Myr. Runaway growth generates a bimodal size distribution with numerous $\sim$100 km bodies that accrete on a comparable timescale. Melting and metarmorphism in the asteroid belt is likely to have occurred if accretion initiates between 0 to 1.8 Myrs and if duration of accretion is of the order of a few Myrs. The issue of possible nebular heterogeneity of $^{26}$Al clouds its efficacy as a heat source. Recent work, on electromagnetic induction, suggests that electromagnetic induction could cause melting in asteroids: however, the input parameter sets for the models remain largely unconstrained. Thus, the issue, of whether electromagnetic induction could have been the protoplanetary heat source in the early Solar System, remains unresolved.
P31C-06 09:15h
Advances in Understanding Planetary Building Blocks: John Wood's Legacy
Chondrites are increasingly understood to be the products of uniquely nebular processes. Refractory inclusions are now thought to be mixtures of high-temperature condensates and evaporation residues. High-resolution chronometers demonstrate that the formation of refractory inclusions predated chondrules. The heating mechanism for chondrules remains controversial, but rapid melting (required for retention of moderately volatile elements) of dust aggregates by nebular shocks or stellar outflows appears plausible. After (commonly incomplete) melting, as revealed by their textures, chondrules solidified during non-linear cooling and were sorted aerodynamically, perhaps in nebular eddies. Accretion of chondrules, inclusions, and other components within several million years of their formation produced planetesimals. These bodies experienced thermal processing driven by decay of short-lived radionuclides. Increasingly sophisticated thermal evolution models of asteroids, now involving incremental accretion, can account for the metamorphic effects, peak temperatures, radio-isotope blocking ages, and cooling rates measured in chondrites. Many onion-shell asteroids were subsequently converted into rubble piles, reassembled after catastrophic collisions and modified by shock. These planetesimals were the building blocks for planets. Many constraints provided by chondrites to the study of nebular processes are attributable to this year's Whipple Award recipient, John Wood. He is responsible for a universally used chondrite classification scheme that first quantified the effects of thermal metamorphism, the first estimates of chondritic asteroid cooling rates (based on Ni diffusion profiles in metal), the proposal of a nebular shock model for chondrule formation, and constraints from chondrites on the accretion process and on planetesimal heat sources. Moreover, he was also the first to recognize the feldspathic composition of the lunar highlands (based on plagioclase grains in Apollo 11 soils) and to propose an explanation through fractional crystallization in a magma ocean. From observations of tiny samples, Wood has evoked astrophysical and geologic hypotheses that have shaped our understanding of the early solar system.
P31C-07 INVITED 09:30h
A Framework for Chondrite Formation in the Nebula
More than a dozen subclasses of chondritic meteorites, the most primitive accessible samples of planetary material, are recognized. It has long been assumed that these subclasses formed at different times and/or places in what is now the asteroid belt. Recently the ages of individual chondrule in particularly primitive chondrites have been determined by radiometric techniques, and these data, though still fragmentary, permit placement of the subclasses in a crude time - radial distance matrix. Distances are related to spectral correspondences between the chondrite subclasses and present families of asteroids, and times to the age ranges found for chondrules in the various subclasses, which are unexpectedly long and which seem to be offset from one another. The variable abundance of refractory inclusions, which formed in a brief period at the beginning of nebular activity, in chondrite subclasses provide another age criterion. The mm-sized chondrules that comprise the bulk of most chondritic material were created by brief energetic events in the solar nebula, probably the action of energetic shock waves on precursor solids (this, at least, is the conventional wisdom). The process was modulated by the presence of carbon (probably presolar organic compounds; Nakano et al., 2003) in the precursor material, which during chondrule melting reduced Fe oxides to varying degrees, providing a basis for metal/silicate fractionation and the variability in bulk Fe/Si that is one of the properties differentiating the chondrite subclasses. Greatly enhanced system C/O was needed at the time and place when the end-member enstatite chondrite subclass were formed. The source of the putative chondrule-forming shocks constitutes an interesting problem. Chondrules were formed over too long a time period (3 Myr or more) for the shocks to have been caused by (early) gravitational instability in the nebula. Tidal disturbances by early stellar companions of the sun (Larson, 2002) are a more plausible mechanism, but they may be inconsistent with the present orderly geometry of the planetary system.