Protosolar Nebula Research Articles

The Growth of Planetary Embryos: Orderly, Runaway, or Oligarchic?

We consider the growth of a protoplanetary embryo embedded in a planetesimal disk. We take into account the dynamical evolution of the disk caused by (1) planetesimal-planetesimal interactions, which increase random motions and smooth gradients in the disk, and (2) gravitational scattering of planetesimals by the embryo, which tends to heat up the disk locally and repels planetesimals away. The embryo's growth is self-consistently coupled to the planetesimal disk dynamics. We demonstrate that the details of the evolution depend on only two dimensionless parameters incorporating all the physical characteristics of the problem: the ratio of the physical radius to the Hill radius of any solid body in the disk (which is usually a small number), and the number of planetesimals inside an annulus of the disk with width equal to the planetesimal Hill radius (which is usually large). We derive simple scaling laws describing embryo-disk evolution for different sets of these parameters. The results of exploration in the framework of our model of several situations typical for the protosolar nebula can be summarized as follows: Initially, the planetesimal disk dynamics is not affected by the presence of the embryo, and the growth of the embryo's mass proceeds very rapidly in the runaway regime. Later, when the embryo starts being dynamically important its accretion slows down, similar to the growth picture. The scenario of orderly growth originally suggested by Safronov is never realized in our calculations; the scenario of runaway growth suggested by Wetherill & Stewart is only realized for a limited range in mass. The slow character of the planetesimal accretion in the oligarchic stage of the embryo's accumulation leads to a considerable increase of the protoplanetary formation timescale compared with that following from a simple runaway accretion picture valid for homogeneous planetesimal disks.

Atomic Deuterium/Hydrogen in the Galaxy

An accurate value of the deuterium/hydrogen (D/H) ratio in the local interstellar medium (LISM) and a better understanding of the D/H variations with position in the Galactic disk can provide essential information on the primordial D/H ratio in the Galaxy at the time of the protosolar nebula, and the amount of astration and mixing in the Galaxy over time. Recent measurements have been obtained with UV spectrographs on FUSE, HST, and IMAPS using hot white dwarfs, OB stars, and late-type stars as background light sources against which to measure absorption by D and H in the interstellar medium along the lines of sight. Recent analyses of FUSE observations of seven white dwarfs and subdwarfs provide a weighted mean value of D/H = (1.52 +/- 0.08) 10(-5) (15.2 +/- 0.8 ppm), consistent with the value of (1.50 +/- 0.10) 10(-5) (15.0 +/- 1.0 ppm) obtained from analysis of lines of sight toward nearby late-type stars. Both numbers refer to the ISM within about 100 pc of the Sun, which samples warm clouds located within the Local Bubble. Outside of the Local Bubble at distances of 200 to 500 pc, analyses of far-UV spectra obtained with IMAPS indicate a much wider range of D/H ratios between 0.8 to 2.2 ppm, providing information on inhomogeneous astration in the Galactic disk.

Radiation pressure on bacterial clumps in the solar vicinity and their survival between interstellar transits

Radiation pressure cross-sections for clumps of hollow bacterial grains with thin coatings of graphite are calculated using rigorousGuttler formulae. The carbonized skins are expected to form through exposure to solar ultraviolet radiation, but a limiting thickness of about 0.03 μm is determined by opacity effects. The ratios of radiation pressure to gravity P/G are calculated for varying sizes of the clumps and for varying thickness of the graphite coatings. Bacterial clumps and individual desiccated bacteria without coatings of radii in the range 0.3–8 μm have P/G ratios less than unity, whereas particles with coatings of 0.02μmthickness have ratios in excess of unity. Such coatings also provide protection from damaging ultraviolet radiation. Putative cometary bacteria, such as have beenrecently collected in the stratosphere, are thus not gravitationally bound in the solar system provided they possess carbonised exterior coatings. They are rapidly expelled from the solar system reaching nearby protosolar nebulae in timescales of a few million years. Even with the most pessimistic assumptions galactic cosmic rays are unable to diminish viability to an extent that vitiates the continuity of panspermia.

The role of sticky interstellar organic material in the formation of asteroids

Abstract— Collision experiments and measurements of viscoelastic properties were performed involving an interstellar organic material analogue to investigate the growth of organic grains in the protosolar nebula. The organic material was found to be stickiest at a radius of between 2.3 and 3.0 AU, with a maximum sticking velocity of 5 m s−1 for millimeter‐size organic grains. This stickiness is considered to have resulted in the very rapid coagulation of organic grain aggregates and subsequent formation of planetesimals in the early stage of the turbulent accretion disk. The planetesimals formed in this region appear to be represent achondrite parent bodies. In contrast, the formation of planetesimals at <2.1 and >3.0 AU begins with the establishment of a passive disk because silicate and ice grains are not as sticky as organic grains.

Formation of giant planets by fragmentation of protoplanetary disks.

The evolution of gravitationally unstable protoplanetary gaseous disks has been studied with the use of three-dimensional smoothed particle hydrodynamics simulations with unprecedented resolution. We have considered disks with initial masses and temperature profiles consistent with those inferred for the protosolar nebula and for other protoplanetary disks. We show that long-lasting, self-gravitating protoplanets arise after a few disk orbital periods if cooling is efficient enough to maintain the temperature close to 50 K. The resulting bodies have masses and orbital eccentricities similar to those of detected extrasolar planets.

Towards simulation of high temperature methane spectra

Methane plays a central role in gas layers of temperatures up to around 3000 K in a number of astrophysical objects ranging from giant planets to brown dwarfs, over proto-solar nebulae, to several classes of cool stars. In order to model and analyse these objects correctly, an accurate and complete list of spectral lines at high temperature is demanded. Predicting high temperature spectra implies, however, predicting hot bands and thus modelling highly excited vibrational states. This is a real challenge in the case of methane. We report the preliminary results of a theoretical study combining the global effective Hamiltonian approach and its computational implementation (STDS package: http://www.u-bourgogne.fr/LPUB/shTDS.html) with semi-quantitative statistical considerations.

Water and organic matter D/H ratios in the solar system: a record of an early irradiation of the nebula?

Water and organic molecules are intimately mixed in the primitive planetary objects of the solar system such as Meteorites and Comets. In few rare examples, the D/H ratios of both water and organic matter (OM) have been measured independently. A critical review of these data are presented. Based on the comparison between the water and OM D/H ratios, it seems possible that the origin of the deuterium enrichment in the solar system is linked to an early X-ray irradiation of the protosolar nebula, where ion–molecule reactions took place at the very surface of the disk.

Molybdenum Nucleosynthetic Dichotomy Revealed in Primitive Meteorites

The collapse of the presolar cloud gave rise to a global homogenization of material available to form the Sun and planets. In the resulting, presumably homogeneous solar system, the nuclides are present in proportions referred to as their cosmic abundances. Here we report molybdenum isotopic compositions of bulk samples and leachate fractions of the primitive meteorites Orgueil and Allende. Two complementary nucleosynthetic components are revealed in Orgueil. One (Mo-m) is enriched in s-process nuclides and may be hosted in presolar grains while the other (Mo-w) is probably distributed in various phases and is depleted in s-process nuclides. The most likely carrier of Mo-m is silicon carbide, although we cannot exclude graphite or an unidentified presolar phase. Excesses in Mo-w are also detected in the bulk sample and all leachate fractions of Allende. These results illustrate that the apparent cosmic abundance pattern of the nuclides, in fact, reflects a mixture of various nucleosynthetic components that survived planetary-scale homogenization in the protosolar nebula.

The End of Interstellar Chemistry as the Origin of Nitrogen in Comets and Meteorites

We describe a mechanism for enhanced nitrogen isotope fractionation in dense molecular gas where most of the molecules containing carbon and oxygen have condensed on grains but where N2 remains in the gas. The lack of hydroxl molecules prevents the recycling of N atoms into N2, and the nitrogen eventually becomes atomic. Ammonia is formed efficiently under these conditions and rapidly accretes as ice. We find that a significant fraction of the total nitrogen is ultimately present as solid NH3. This interstellar ammonia is enhanced in 15N with 15NH3/14NH3 almost 80% higher than the cosmic 15N/14N ratio. It is possible that a large part of the nitrogen available to the early solar system was highly fractionated ammonia ice and hence that the 15N enhancements of primitive solar system material and the depletion of N2 in comets are concomitant. Other implications of this theory for observations of dense molecular material and the nitrogen inventory available to the protosolar nebula are briefly discussed.

Cosmic dust and our origins

The small solid particles in the space between the stars provide the surfaces for the production of many simple and complex molecules. Processes involving the effects of ultraviolet irradiation of the thin (hundredth micron) mantles are shown to produce a wide range of molecules and ions also seen in comets. Some of the more complex ones inferred from laboratory experiments are expected to play an important role in the origin of life. An outline of the chemical evolution of interstellar dust as observed and as studied in the laboratory is presented. Observations of comets are shown to provide substantial evidence for their being fluffy aggregates of interstellar dust as it was in the protosolar nebula, i.e. the interstellar cloud which collapsed to form the solar system. The theory that comets may have brought the progenitors of life to the earth is summarized.

Molybdenum Evidence for Inherited Planetary Scale Isotope Heterogeneity of the Protosolar Nebula

Isotope anomalies provide important information about early solar system evolution. Here we report molybdenum isotope abundances determined in samples of various meteorite classes. There is no fractionation of molybdenum isotopes in our sample set within 0.1 permil and no contribution from the extinct radionuclide 97Tc at mass 97 (97Tc/92Mo<3E-6). Instead, we observe clear anomalies in bulk iron meteorites, mesosiderites, pallasites, and chondrites characterized by a coupled excess in p- and r- or a mirror deficit in s-process nuclides (Mo-HL). This large scale isotope heterogeneity of the solar system observed for molybdenum must have been inherited from the interstellar environment where the sun was born, illustrating the concept of ``cosmic chemical memory''. The presence of molybdenum anomalies is used to discuss the filiation between planetesimals.

Composition of Comets and Interstellar Dust

AbstractRecent observational results from ground and space-based observatories in combination with laboratory simulations allow us to monitor the evolution, survival, transport and transformation of cosmic dust from molecular clouds and the diffuse interstellar medium, until their incorporation into Solar System material. The composition of comets encodes information on their origin and can be used as a tracer for processes which were predominant in the protosolar nebula. A comparison of the composition and abundances of interstellar molecules and cometary volatiles reveals a general correspondence, but discrepancies are also apparent. Detailed studies indicate that cometary ices are a mixture of original interstellar material with material that has been moderately to heavily processed in the pre-solar nebula. The inventory and distribution of ice and organic molecules in interstellar clouds and in comets and their common link are briefly described.

Major element fractionation in chondrites by distillation in the accretion disk of a T Tauri Sun?

Abstract— Redistribution or loss of batches of condensate from a cooling protosolar nebula is generally thought to have led to the formation of the chemical groups of chondrites. This demands a nebula hot enough for silicate vaporization over 1–3 AU, the region where chondrites formed. Alternatively, heating of a protosolar accretion disk may have been confined to an annular zone at its inner edge, ˜0.06 AU from the protosun. Most infalling matter was accreted by the protosun, but a proportion was heated and carried outwards by an x‐wind. Shu et al. (1996, 1997) proposed that larger objects such as chondrules and calcium‐aluminum‐rich inclusions (CAIs) were returned to the disk at asteroidal distances by sedimentation from the x‐wind. Fine dust and gas were lost to space. The model implies that solids were not lost from the cold disk. The chemical compositions of the chondrite groups were produced by mixing different proportions of CAIs and chondrules with disk solids of CI composition. Heating at the inner edge of the disk was accompanied by particle irradiation, which synthesized nuclides including 26Al.The x‐wind model can produce CAIs, not chondrules, and allows survival of presolar grains &gt;0.06 AU from the protosun. Normalization to Al and CI indicates that non‐carbonaceous chondrites may be disk material that gained a Si‐ and Mg‐enriched fraction. Carbonaceous chondrites are different; they appear to be CI that lost lithophile elements more volatile than Ca. Five carbonaceous chondrite groups also lost Ni and Fe but the CH group gained siderophiles. Elemental loss from CI is incompatible with the x‐wind model. Silicon and CI normalization confirms that the CM, CO, CK and CV groups may be CI that gained refractories as CAIs. The Si‐, Mg‐rich fraction may have formed by selective vaporization followed by precipitation on grains in the x‐wind. This fractional distillation mechanism can account for lithophile element abundances in non‐carbonaceous chondrite groups, but an additional process is required for the loss of Ca and Mn in the EL group and for fractionated siderophile abundances in the H, L and LL groups. Heated and recycled fractions were not homogenized across the disk so the chondrite groups were established in a single cycle of enhanced protosolar activity in lt;104 years, the time for a millimeter‐sized particle to drift into the Sun from 2 to 3 AU, due to gas‐drag.

Formation and early evolution of the atmosphere

Abstract The tectonic activity of the Earth allowed exchange of volatile elements (H, C, N, rare gases) between the surface of the Earth (atmosphere, crust, sediments, oceans) and the mantle. However, some of these elements still present elemental and isotopic heterogeneities that allow us to reconstruct the volatile composition of the terrestrial mantle. The protosolar nebula supplied a significant fraction of helium and neon, which were presumably trapped during the major phase of the Earth’s accretion and were possibly hosted by accreting dust and/or small porous planetesimals. Surprisingly, volatile elements are in chondritic proportion despite their drastic (10 −3 ) depletion in the mantle relative to chondrites, in a way that recalls the case of highly siderophile elements. From stable isotope systematics, we find that the contribution of comets to the volatile inventory of the Earth was very limited. The integrated flux of chondritic-like material necessary to provide water, carbon and nitrogen is consistent with that required for the formation of the lunar craters as well as that necessary to account for the inventory of siderophile elements in the mantle. A consequence of this scenario is that the Earth’s surface was oxidized very early. Alternatively, volatile and siderophile elements of the mantle could be the remnant of small patches of chondritic material that did not equilibrate with the core nor drastically degas.

Iodine-Xenon dating of chondrules from the Qingzhen and Kota Kota enstatite chondrites

Initial 129I/ 127I values (I-Xe ages) have been obtained for individual mineralogically characterized chondrules and interchondrule matrix from the enstatite chondrites Qingzhen (EH3) and Kota Kota (EH3). In view of the absence of aqueous alteration and the low-peak metamorphic temperatures experienced by these meteorites, we suggest that the I-Xe ages for the chondrules record the event in which they were formed. These ages are within the range recorded for chondrules from ordinary chondrites, demonstrating that chondrules formed during the same time interval in the source regions of both ordinary chondrites and enstatite chondrites. The timing of this chondrule-forming episode or episodes brackets the I-Xe closure age of planetesimal bodies such as the Shallowater aubrite parent body. Although chondrule formation need not have occurred close to planetesimals, the existence of planetesimals at the same time as chondrule formation provides constraints on models of this process. Whichever mechanisms are proposed to form and transport chondrules, they must be compatible with models of the protosolar nebula which predict the formation of differentiated bodies on the same timescale at the same heliocentric distance.

Clues to the origin of interplanetary dust particles from the isotopic study of their hydrogen-bearing phases

Ion microprobe quantitative imaging was performed for H, D, 12,13C, 16,18O, 27Al, and 28,29,30Si in five stratospheric particles with a 1.5 × 1.5 μm spatial resolution to determine the carriers of high D/H ratios and to give new clues about the parent bodies of interplanetary dust particles (IDPs). Among these particles, four appear to be of extraterrestrial origin. Using imaging, the large variations of D/H ratios can be correlated at the micrometer scale with chemical composition so that endmembers can be identified. From the systematics of variation of C/H and D/H ratios, water present as hydroxyls in phyllosilicates (at C/H = 0) and three different types of organic matter (OM1, 2, and 3) were identified. Water exhibits D/H ratios lying in the chondritic domain (D/H ∼150 × 10−6), whereas OMs are enriched in deuterium. OM1 is similar to the macromolecular organic matter of carbonaceous chondrites (D/H = 250 × 10−6 and C/H = 1.5). OM2 (D/H = 1500 × 10−6 and C/H = 1.0) is close to cometary HCN. OM3 (D/H = 2000 × 10−6 and C/H = 3.0) is a highly condensed carbonaceous phase with no counterpart in known extraterrestrial objects. The C, O, and Si isotopic compositions are solar within ± 10%.The identification of these phases allows a better understanding of the origin of D/H variations inferred for the protosolar nebula. The only mechanism that can explain such high D/H ratios is interstellar chemistry. However, D/H ratios in interplanetary dust organic matter remain lower than those measured in molecules from cold molecular clouds. This difference can be accounted for by an isotopic exchange with liquid water in IDP parent bodies. In addition, the close association of the chondritic component OM1 with the likely cometary component OM2 is evidence for a link between carbonaceous chondrites and comets. Although IDPs contain cometary organic matter, the water-D/H ratio is lower than that measured in comets (310 × 10−6). IDPs seem thus constituted of various materials formed over a large range of heliocentric distances.

Processing of silicate dust grains in Herbig Ae/Be systems

We have analysed the 10 μm spectral region of a sample of Herbig Ae/Be (HAEBE) stars. The spectra are dominated by a broad emission feature caused by warm amorphous silicates, and by polycyclic aromatic hydrocarbons. In HD 163296 we find aliphatic carbonaceous dust, the first detection of this material in a HAEBE star. The silicate band shows a large variation in shape, due to variable contributions of three components: (i) a broad shoulder at 8.6 μm; (ii) a broad maximum at 9.8 μm; and (iii) a narrow feature with a broad underlying continuum at 11.3 μm. From detailed modeling these features can be identified with silica (SiO2), sub-micrometer sized amorphous olivine grains and micrometer sized amorphous olivine grains in combination with forsterite (Mg2SiO4), respectively. Typical mass fractions are 5 to 10 per cent of crystalline over amorphous olivine, and a few per cent of silica compared to the olivines. The detection of silica in emission implies that this material is heated by thermal contact with other solids that have a high absorptivity at optical to near-IR wavelengths. The observed change in peak position of the silicate band in HAEBE stars from 9.7 μm to 11.3 μm is dominated by an increase in average grain size, while changes in composition play only a minor rôle. The HAEBE stars, β Pic and the solar system comet Halley form a sequence of increasing crystallinity. We find that the abundance of SiO2 tends to increase with increasing crystallinity. This is consistent with the compositional changes expected from thermal annealing of amorphous grains in the inner regions of the disk. We confirm earlier studies that the timescale for crystallisation of silicates in disks is longer than that of coagulation. Our results indicate that the processes that governed grain processing in the proto-solar nebula, are also at work in HAEBE stars.

Isotope geochemistry. The origin of water on earth.

The protosolar nebula is believed to have had a homogeneous isotopic composition. Yet the isotopic composition of water on Earth differs widely from that of the primitive Sun. In his Perspective, Robert charts recent progress towards solving this mystery. It seems likely that the water came from a meteoritic source, whose water was derived in turn from the interstellar medium.

Rapid collisional evolution of comets during the formation of the Oort cloud

The Oort cloud of comets was formed by the ejection of icy planetesimals from the region of giant planets--Jupiter, Saturn, Uranus and Neptune--during their formation. Dynamical simulations have previously shown that comets reach the Oort cloud only after being perturbed into eccentric orbits that result in close encounters with the giant planets, which then eject them to distant orbits about 10(4) to 10(5) AU from the Sun (1 AU is the average Earth-Sun distance). All of the Oort cloud models constructed until now simulate its formation using only gravitational effects; these include the influence of the Sun, the planets and external perturbers such as passing stars and Galactic tides. Here we show that physical collisions between comets and small debris play a fundamental and hitherto unexplored role throughout most of the ejection process. For standard models of the protosolar nebula (starting with a minimum-mass nebula) we find that collisional evolution of comets is so severe that their erosional lifetimes are much shorter than the timescale for dynamical ejection. It therefore appears that collisions will prevent most comets escaping from most locations in the region of the giant planets until the disk mass there declines sufficiently that the dynamical ejection timescale is shorter than the collisional lifetime. One consequence is that the total mass of comets in the Oort cloud may be less than currently believed.

Solar Wind Record on the Moon: Deciphering Presolar from Planetary Nitrogen

Ion microprobe analyses show that solar wind nitrogen associated with solar wind hydrogen implanted in the first tens of nanometers of lunar regolith grains is depleted in 15N by at least 24% relative to terrestrial atmosphere, whereas a nonsolar component associated with deuterium-rich hydrogen, detected in silicon-bearing coatings at the surface of some ilmenite grains, is enriched in 15N. Systematic enrichment of 15N in terrestrial planets and bulk meteorites relative to the protosolar gas cannot be explained by isotopic fractionation in nebular or planetary environments but requires the contribution of 15N-rich compounds to the total nitrogen in planetary materials. Most of these compounds are possibly of an interstellar origin and never equilibrated with the 15N-depleted protosolar nebula.