Jupiter and Saturn formed in a few million years from a gas-dominated protoplanetary disk, and were susceptible to gas-driven migration of their orbits on timescales of only approximately 100,000 years. Hydrodynamic simulations show that these giant planets can undergo a two-stage, inward-then-outward, migration. The terrestrial planets finished accreting much later and their characteristics, including Mars' small mass, are best reproduced by starting from a planetesimal disk with an outer edge at about one astronomical unit from the Sun (1 AU is the Earth-Sun distance). Here we report simulations of the early Solar System that show how the inward migration of Jupiter to 1.5 AU, and its subsequent outward migration, lead to a planetesimal disk truncated at 1 AU; the terrestrial planets then form from this disk over the next 30-50 million years, with an Earth/Mars mass ratio consistent with observations. Scattering by Jupiter initially empties but then repopulates the asteroid belt, with inner-belt bodies originating between 1 and 3 AU and outer-belt bodies originating between and beyond the giant planets. This explains the significant compositional differences across the asteroid belt. The key aspect missing from previous models of terrestrial planet formation is the substantial radial migration of the giant planets, which suggests that their behaviour is more similar to that inferred for extrasolar planets than previously thought.

A low mass for Mars from Jupiter's early gas―driven migration

The formation of planetesimals, the kilometer-sized planetary precursors, is still a puzzling process. Considerable progress has been made over the past years in the physical description of the first stages of planetesimal formation, owing to extensive laboratory work. This review examines the experimental achievements and puts them into the context of the dust processes in protoplanetary disks. It has become clear that planetesimal formation starts with the growth of fractal dust aggregates, followed by compaction processes. As the dust-aggregate sizes increase, the mean collision velocity also increases, leading to the stalling of the growth and possibly to fragmentation, once the dust aggregates have reached decimeter sizes. A multitude of hypotheses for the further growth have been proposed, such as very sticky materials, secondary collision processes, enhanced growth at the snow line, or cumulative dust effects with gravitational instability. We will also critically review these ideas.

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The Growth Mechanisms of Macroscopic Bodies in Protoplanetary Disks

The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.

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Comet 81P/Wild 2 under a microscope.

Cometary nuclei contain the least modified material from the formative epoch of our planetary system, and their compositions reflect a range of processes experienced by material prior to its incorporation in the cometary nucleus. Dynamical models suggest that icy bodies in the main cometary reservoirs (Kuiper Belt, Oort Cloud) formed in a range of environments in the protoplanetary disk, and (for the Oort Cloud) even in disks surrounding neighboring stars of the Sun's birth cluster. Photometric and spectroscopic surveys of more than 100 comets have enabled taxonomic groupings based on free radical species and on crystallinity of rocky grains. Since 1985, new surveys have provided emerging taxonomies based on the abundance ratios of primary volatiles. More than 20 primary chemical species are now detected in bright comets. Measurements of nuclear spin ratios (in water, ammonia, and methane) and of isotopic ratios (D/H in water and HCN; 14N/15N in CN and HCN) have provided critical insights on factors affec...

The Chemical Composition of Comets—Emerging Taxonomies and Natal Heritage

They range in size from microscopic particles to masses of many tons. The geologic diversity of asteroids and other rocky bodies of the solar system are displayed in the enormous variety of textures and mineralogies observed in meteorites. The composition, chemistry, and mineralogy of primitive meteorites collectively provide evidence for a wide variety of chemical and physical processes. This book synthesizes our current understanding of the early solar system, summarizing information about processes that occurred before its formation. It will be valuable as a textbook for graduate education in planetary science and as a reference for meteoriticists and researchers in allied fields worldwide.

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Meteorites and the early solar system II

Ultramicrotomy of samples embedded in epoxy resin is a standard method for preparing ultra-thin sections for electron microscopy. In this report we describe a new embedding technique that uses acrylic resin instead of epoxy. This method offers several important advantages for sectioning small extraterrestrial samples. One is that the acrylic resin is soluble and can be removed after ultramicrotomy to leave a sample that is free of the mounting media. This is important for studying carbon and insoluble organic components. A second major advantage of acrylic is that, when combined with pre-embedding compression, it provides a very effective method of mounting samples collected in silica aerogel. Acrylic embedding is currently being used to mount comet particles collected by NASA's Stardust mission. Combined with a flattening process, the acrylic embedding and sectioning preserves all pieces of collected samples in their collection matrix. In addition to Stardust, acrylic may be applied to other samples collected in aerogel such as those from the Russian Mir space station (Horz et al. 2000) and future missions such as Sample Collection for Investigation of Mars (SCIM) (Leshin 2003), a proposed mission to collect atmospheric dust particles from Mars.

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Acrylic embedding of Stardust particles encased in aerogel

In this study, we have performed pulse-heating experiments at different temperatures for three organic molecules (a polycyclic aromatic hydrocarbon [PAH], a ketone, and an amino acid) (absorbed into microporous aluminum oxide (Al2O3) in order to imitate the heating of the organic molecules in interplanetary dust particles (IDPs) and micrometeorites (MMs) during atmospheric entry and to investigate their survival. We have shown that modest amounts (a few percent) of these organic molecules survive pulse-heating at temperatures in the 700 to 900 C range. This suggests that the porosity in IDPs and MMs, combined with a sublimable phase (organic material, water), produces an ablative cooling effect, which permits the survival of organic molecules that would otherwise be lost either by thermal degradation or evaporation during atmospheric entry.

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Survival of organic phases in porous IDPs during atmospheric entry: A pulse‐heating study

We analyzed 2 ultra-carbonaceous interplanetary dust particles and 2 cometary Wild 2 particles with infrared spectroscopy. We characterized the carrier of the 3.4 micron band in these samples and compared its profile and the CH2/CH3 ratios to the 3.4 micron band in the diffuse interstellar medium (DISM), in the insoluble organic matter (IOM) from 3 primitive meteorites, in asteroid 24 Themis and in the coma of comet 103P/Hartley 2. We found that the 3.4 micron band in both Wild 2 and IDPs is similar, but different from all the other astrophysical environments that we compared to. The 3.4 micron band in IDPs and Wild 2 particles is dominated by CH2 groups, the peaks are narrower and stronger than in the meteorites, asteroid Themis, and the DISM. Also, the presence of the carbonyl group C=O at 1700 cm-1 (5.8 micron) in most of the spectra of our samples, indicates that these aliphatic chains have O bonded to them, which is quite different from astronomical spectra of the DISM. Based on all these observations we conclude that the origin of the carrier of the 3.4 micron band in IDPs and Wild 2 samples is not interstellar, instead, we suggest that the origin lies in the outermost parts of the solar nebula.

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The origin of the 3.4 micron feature in Wild 2 cometary particles and in ultracarbonaceous interplanetary dust particles

Introduction: Micrometeorites are terrestrially collected extraterrestrial dust particles smaller than about a millimeter. The accretion rate, size distribution and composition of micrometeorites bears on numerous studies including: deducing the compositions of parent bodies; calibrating terrestrial sedimentation rates; interpreting the isotopic record of seawater; linking influx to global climate change; and assessing the role of ET materials in life processes. In 1995 Taylor et al. [1] retrieved ~ 200g of material from the bottom of the South Pole water well (SPWW) of which about 0.1% were cosmic spherules (melted micrometeorites). Using the particle size distribution, area suctioned and age of ice melted (1100–1500 AD) they computed a terrestrial accretion rate for cosmic spherules 50–700 μm in diameter of 1600 ± 300 tons/yr [1] or 4 ± 2 percent of the flux measured above the atmosphere [2]. We are repeating this analysis using samples collected in 2000 from the SPWW. Compared with 1995 collection, the 2000 samples have less iron-oxide contamination allowing us to find unmelted micrometeorites. We are analyzing the deployment dedicated to the central plateau. Because this area was vacuumed in 1995 the 2000 sample should contain only those particles derived from the older, 700– 1100 AD, ice [3] and allow us to calculate a flux for a second time period. We have found and mounted 3272 micrometeorites from the central plateau sample. We present a preliminary size distribution (as not all micrometeorites have yet been measured) and an estimate of unmelted to melted ratio for the different size fractions. Methods: The samples studied were collected in 2000 from the SPWW, a 4,000-m reservoir melting pre-industrial ice. The well’s central plateau was vacuumed and yielded ~ 10 g of material. We sieved this sample into >425, 250–425, 150– 250, 106–150 and 53–106μm size fractions. Using a binocular microscope we sorted 100% of the >150-μm fractions, 29% of the106-150 fraction and 9% of the 53–106μm fraction and removed all potential ET grains. We mounted and sectioned over 4000 particles. Using a SEM/EDAX we checked each particle for composition and found that 3272 of particles mounted were micrometeorites. Optical microscopy was used to size the particles and to classify them based on their cross-sectional textures. Results: The size distributions for the 1995 and for the 2000 plateau samples are shown in Figure 1. For the 2000 collection we show individual data points for micrometeorites >250μm, as these have all been sized, and plot the cumulative number of micrometeorites for the three smallest size fractions. The numbers of micrometeorites in the smallest two size fractions have been increased to account for the fact that only a portion of each size fraction was sorted. The best-fit line to the tail of the 2000 plateau sample has a slope of -4.5 slightly less steep than the -5.2 tail-slope calculated for the 1995 plateau sample [2]. Nevertheless the two curves are quite similar indicating that the size distributions from the two collections are similar. The 2000 collection has more micrometeorites than the 1995 collection. The addition of the unmelted component cannot explain the increased number as the unmelted micrometeorites would predominantly change the number of micrometeorites in the smaller size fractions. The fact that we found an order of magnitude more >250μm micrometeorites in the 2000 collection may be due to an increase in the plateau area as the well deepens or to an increase in the flux rate. Although we have not yet measured the plateau area our video records do not show an order of magnitude increase in the plateau area.

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Size Distribution of Antarctic Micrometeorites

Introduction The preliminary examination of Stardust samples has yielded a number of surprising findings, such as the presence of large refractory mineral grains, including materials resembling CAIs, chondrules, and AOA fragments [1-3]. Furthermore, intact organics and fine grained materials are unexpectedly rare in Stardust tracks [4,5]. The fact that intact fine grained materials are uncommon may be related to the apparent rarity of presolar grains in Stardust samples [1,6]. So far, 3 presolar grains have been found in Stardust samples – all identified by O isotopic imaging of crater residues [6]. Although the mineralogical identity of these presolar grains is uncertain, their estimated abundance (11 ppm) is strikingly lower than silicate stardust grains in primitive meteorites (100 – 300 ppm; 7,8) or interplanetary dust particles (300 – 10,000 ppm; 9-11). Furthermore, O isotopic measurements of all silicate minerals extracted from tracks have fallen within the range of meteoritic materials [1-3]. These observations suggest that either [1] comet Wild-2 materials are less pristine than meteorites and IDPs, [2] fine grained materials and presolar grains were preferentially destroyed during capture, or [3] have been segregated from the typically studied ‘terminal particles’ e.g. in bulbous track walls. Searches for presolar grains have focused on silicate stardust because this is the most abundant presolar grain phase in meteorites and IDPs. Here we report C, N, and Si isotopic measurements of a SiC grain identified within a melt-dominated particle from the wall of track #141 (hereafter Coki). Experimental: Coki is a 2 mm type B track [12] having a very large, bulbous cavity. The track was prepared by flattening a keystone extracted from aerogel block C2061, and embedding the entire flattened track in acrylic resin. Orthogonal strips were then cut from the track and microtomed into < 100 nm thick slices and placed onto Cu TEM grids with 10 nm thick carbon thin-films. The microtomed slices were examined with a 200 kV Tecnai F20 FEG TEM/STEM using standard techniques including brightand darkfield imaging, electron diffraction, high resolution imaging and EDX compositional analysis. Isotopic measurements were performed with the JSC NanoSIMS 50L ion microprobe in two separate sessions. In the first analysis, C and Si isotopic images were taken simultaneously, acquiring images of 12 C , 13 C , 28 Si , 29 Si , 30 Si , and 12 C 28 Si in multidetection with electron multipliers. The sample was subsequently analyzed for C and N isotopes by measuring 12 C , 13 C , 12 C 14 N , 12 C 15 N , 28 Si , and 12 C 28 Si The images were obtained by rastering a <1 pA, <100 nm Cs +

Graciela Matrajt

Papers

Acrylic embedding of Stardust particles encased in aerogel

Survival of organic phases in porous IDPs during atmospheric entry: A pulse‐heating study

The origin of the 3.4 micron feature in Wild 2 cometary particles and in ultracarbonaceous interplanetary dust particles

Size Distribution of Antarctic Micrometeorites

Discovery of Presolar SiC from Comet Wild-2