Search

The paper derives and exemplifies the stabilizing significance of Euler's number in particle physics, biophysics, geophysics, astrophysics and cosmology., Introduction Natural systems are highly complex and at the same time they impress us with their lasting stability. For instance, the solar system hosts at least 800 thousand orbiting each [...]

In the early Solar System, space rocks that grazed Earth likely pummeled Venus, which may explain why the planetary neighbors are so different from each other. (See 'Unveiling Earth's Wayward [...]

The solar neighbourhood is the closest and most easily studied sample of the Galactic interstellar medium, an understanding of which is essential for models of star formation and galaxy evolution. Observations of an unexpectedly intense diffuse flux of easily absorbed 1/4-kiloelectronvolt X-rays (1,2), coupled with the discovery that interstellar space within about a hundred parsecs of the Sun is almost completely devoid of cool absorbing gas (3), led to a picture of a 'local cavity' filled with X-ray-emitting hot gas, dubbed the local hot bubble (4-6). This model was recently challenged by suggestions that the emission could instead be readily produced within the Solar System by heavy solar-wind ions exchanging electrons with neutral H and He in interplanetary space (7-11), potentially removing the major piece of evidence for the local existence of million-degree gas within the Galactic disk (12-15). Here we report observations showing that the total solarwind charge-exchange contribution is approximately 40 per cent of the 1/4-keV flux in the Galactic plane. The fact that the measured flux is not dominated by charge exchange supports the notion of a million-degree hot bubble extending about a hundred parsecs from the Sun., When the highly ionized solar wind interacts with neutral gas, an electron may hop from a neutral to an outer orbital of an ion, in what is known as charge [...]

The trickle of discoveries has become a torrent. Little more than two decades after the first planets were found orbiting other stars, improved instruments on the ground and in space [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.07.015 Byline: Vishnu Reddy (a), Michael J. Gaffey (b), Michael S. Kelley (c), Andreas Nathues (d), Jian-Yang Li (e), Robert Yarbrough (c) Keywords: Asteroid Vesta; Spectroscopy; Asteroids, Composition; Asteroids; Planetary formation Abstract: High signal-to-noise, rotationally-resolved spectra of Asteroid 4 Vesta's southern hemisphere from the 2007 opposition were used to constrain its compositional and mineralogical variations. The spectra were rotationally-phased using closely timed HST observations of Vesta by Li et al. (Li, J.-Y., McFadden, L.A., Thomas, P.C., Mutchler, M.J., Parker, J.Wm., Young, E.F., Russell, C.T., Sykes, M.V., Schmidt, B.E. [2010]. Icarus 208, 238-251). The average surface of Vesta's southern hemisphere is analogous to a howardite or polymict eucrite assemblage similar to the northern hemisphere, although the band parameters are distinctly shifted towards the diogenite zone on the Band-Band plot. A few distinct compositional units were detected and they might be related to albedo features detected by Hubble Space Telescope (Li et al., 2010). We have identified two compositionally distinct regions overlaying the background surface. The first unit is a polymict eucrite and/or low-Ca eucrite compositional unit at 143[degrees] longitude that border the eucrite zone on the Band-Band plot and the second is a diogenite unit at 159[degrees]. While we did not detect any distinct olivine units as suggested by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130-157), we cannot rule out the possibility of smaller olivine-rich units that are below the detection limit of the instrumentation we used. Based on the analysis and the limitations of the data, we do not suggest that Vesta's surface is olivine-free. Mean pyroxene chemistry estimates for both hemispheres broadly agree with one another (to within one-sigma) with the northern hemisphere ferrosilite (Fs) and wollastonite (Wo) values being slightly higher than southern hemisphere. Author Affiliation: (a) Department of Space Studies, Room 520, Box 9008, University of North Dakota, Grand Forks, ND 58202, USA (b) Department of Space Studies, Room 518, Box 9008, University of North Dakota, Grand Forks, ND 58202, USA (c) Department of Geology and Geography, Box 8149, Georgia Southern University, Statesboro, GA 30460, USA (d) Max-Planck Institute for Solar System Research, 37191 Katlenburg-Lindau, Germany (e) Department of Astronomy, University of Maryland, College Park, MD 20742, USA Article History: Received 12 March 2010; Revised 10 July 2010; Accepted 14 July 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.06.011 Byline: C.W. Ormel (a)(b), C.P. Dullemond (a), M. Spaans (c) Keywords: Asteroids; Origin, Solar System; Planetary formation; Planetesimals; Solar nebula Abstract: When preplanetary bodies reach proportions of [approximately equal to]1km or larger in size, their accretion rate is enhanced due to gravitational focusing (GF). We have developed a new numerical model to calculate the collisional evolution of the gravitationally-enhanced growth stage. The numerical model is novel as it attempts to preserve the individual particle nature of the bodies (like N-body codes); yet it is statistical in nature since it must incorporate the very large number of planetesimals. We validate our approach against existing N-body and statistical codes. Using the numerical model, we explore the characteristics of the runaway growth and the oligarchic growth accretion phases starting from an initial population of single planetesimal radius R.sub.0. In models where the initial random velocity dispersion (as derived from their eccentricity) starts out below the escape speed of the planetesimal bodies, the system experiences runaway growth. We associate the initial runaway growth phase with increasing GF-factors for the largest body. We find that during the runaway growth phase the size distribution remains continuous but evolves into a power-law at the high-mass end, consistent with previous studies. Furthermore, we find that the largest body accretes from all mass bins; a simple two-component approximation is inapplicable during this stage. However, with growth the runaway body stirs up the random motions of the planetesimal population from which it is accreting. Ultimately, this feedback stops the fast growth and the system passes into oligarchy, where competitor bodies from neighboring zones catch up in terms of mass. We identify the peak of GF with the transition between the runaway growth and oligarchy accretion stages. Compared to previous estimates, we find that the system leaves the runaway growth phase at a somewhat larger radius, especially at the outer disk. Furthermore, we assess the relevance of small, single-size fragments on the growth process. In classical models, where the initial velocity dispersion of bodies is small, these do not play a critical role during the runaway growth; however, in models that are characterized by large initial relative velocities due to external stirring of their random motions, a situation can emerge where fragments dominate the accretion, which could lead to a very fast growth. Author Affiliation: (a) Max-Planck-Institute for Astronomy, Konigstuhl 17, 69117 Heidelberg, Germany (b) Astronomisches Rechen-Institut, Zentrum fur Astronomie der Universitat Heidelberg, Monchhofstr. 12-14, 69120 Heidelberg, Germany (c) Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV, Groningen, The Netherlands Article History: Received 7 December 2009; Revised 10 June 2010; Accepted 10 June 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.04.021 Byline: Hiroshi Kobayashi (a), Hidekazu Tanaka (b), Alexander V. Krivov (a), Satoshi Inaba (c) Keywords: Planetary formation; Planetesimals; Collisional physics; Origin, Solar System; Jovian planets Abstract: As planetary embryos grow, gravitational stirring of planetesimals by embryos strongly enhances random velocities of planetesimals and makes collisions between planetesimals destructive. The resulting fragments are ground down by successive collisions. Eventually the smallest fragments are removed by the inward drift due to gas drag. Therefore, the collisional disruption depletes the planetesimal disk and inhibits embryo growth. We provide analytical formulae for the final masses of planetary embryos, taking into account planetesimal depletion due to collisional disruption. Furthermore, we perform the statistical simulations for embryo growth (which excellently reproduce results of direct N-body simulations if disruption is neglected). These analytical formulae are consistent with the outcome of our statistical simulations. Our results indicate that the final embryo mass at several AU in the minimum-mass solar nebula can reach about [approximately equal to]0.1 Earth mass within 10.sup.7 years. This brings another difficulty in formation of gas giant planets, which requires cores with [approximately equal to]10 Earth masses for gas accretion. However, if the nebular disk is 10 times more massive than the minimum-mass solar nebula and the initial planetesimal size is larger than 100km, as suggested by some models of planetesimal formation, the final embryo mass reaches about 10 Earth masses at 3-4AU. The enhancement of embryos' collisional cross sections by their atmosphere could further increase their final mass to form gas giant planets at 5-10AU in the Solar System. Author Affiliation: (a) Astrophysical Institute and University Observatory, Friedrich Schiller University, Schillergaesschen 2-3, 07745 Jena, Germany (b) Institute of Low Temperature Science, Hokkaido University, Kita-Ku Kita 19, Nishi 8, Sapporo 060-0819, Japan (c) School of International Liberal Studies, Waseda University, 1-6-1 Nishi-Waseda, Shinjuku-ku, Tokyo 169-8050, Japan Article History: Received 25 February 2010; Revised 23 April 2010; Accepted 23 April 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.05.030 Byline: J. Salmon (a), S. Charnoz (a), A. Crida (b), A. Brahic (a) Keywords: Disks; Planetary rings; Saturn, Rings Abstract: Planetary rings are common in the outer Solar System but their origin and long-term evolution is still a matter of debate. It is well known that viscous spreading is a major evolutionary process for rings, as it globally redistributes the disk's mass and angular momentum, and can lead to the disk's loosing mass by infall onto the planet or through the Roche limit. However, describing this process is highly dependent on the model used for the viscosity. In this paper we investigate the global and long-term viscous evolution of a circumplanetary disk. We have developed a simple 1D numerical code, but we use a physically realistic viscosity model derived from N-body simulations (), and dependent on the disk's local properties (surface mass density, particle size, distance to the planet). Particularly, we include the effects of gravitational instabilities (wakes) that importantly enhance the disk's viscosity. This method allows to study the global evolution of the disk over the age of the Solar System. Common estimates of the disk's spreading time-scales with constant viscosity significantly underestimate the rings' lifetime. We show that, with a realistic viscosity model, an initially narrow ring undergoes two successive evolutionary stages: (1) a transient rapid spreading when the disk is self-gravitating, with the formation of a density peak inward and an outer region marginally gravitationally stable, and with an emptying time-scale proportional to 1/ M.sub.0.sup.2 (where M.sub.0 is the disk's initial mass), (2) an asymptotic regime where the spreading rate continuously slows down as larger parts of the disk become non-self-gravitating due to the decrease of the surface density, until the disk becomes completely non-self-gravitating. At this point its evolution dramatically slows down, with an emptying time-scale proportional to 1/M.sub.0, which significantly increases the disk's lifetime compared to the case with constant viscosity. We show also that the disk's width scales like t.sup.1/4 with the realistic viscosity model, while it scales like t.sup.1/2 in the case of constant viscosity, resulting in much larger evolutionary time-scales in our model. We find however that the present shape of Saturn's rings looks like a 100 million-years old disk in our simulations. Concerning Jupiter's, Uranus' and Neptune's rings that are faint today, it is not likely that they were much more massive in the past and lost most of their mass due to viscous spreading alone. Author Affiliation: (a) UMR AIM, Universite Paris Diderot/CEA/CNRS, CEA/SAp Orme des Merisiers, bat. 709, 91191 Gif-Sur-Yvette Cedex, France (b) Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WA, UK Article History: Received 8 December 2009; Revised 25 May 2010; Accepted 31 May 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.endeavour.2010.07.001 Byline: Roger D. Launius Abstract: Throughout the history of the space age the dominant vision for the future has been great spaceships plying the solar system, and perhaps beyond, moving living beings from one planet to another. Spacesuited astronauts would carry out exploration, colonization, and settlement as part of a relentlessly forward looking movement of humanity beyond Earth. As time has progressed this image has not changed appreciably even as the full magnitude of the challenges it represents have become more and more apparent. This essay explores the issues associated with the human movement beyond Earth and raises questions about whether humanity will ever be able to survive in the extreme environment of space and the other bodies of the solar system. This paper deals with important historical episodes as well as wider conceptual issues about life in space. Two models of expansion beyond Earth are discussed: (1) the movement of microbes and other types of life on Earth that can survive the space environment and (2) the modification of humans into cyborgs for greater capability to survive in the extreme environments encountered beyond this planet. Author Affiliation: Smithsonian Institution, Space History, National Air and Space Museum, P.O. Box 37012, NASM Room 3556, MRC 311, Washington, DC 20013-7012, United States

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.04.007 Byline: David E. Smith (a), Maria T. Zuber (a), Roger J. Phillips (b), Sean C. Solomon (c), Gregory A. Neumann (d), Frank G. Lemoine (d), Stanton J. Peale (e), Jean-Luc Margot (f), Mark H. Torrence (g), Matthieu J. Talpe (a), James W. Head (h), Steven A. Hauck (i), Catherine L. Johnson (j), Mark E. Perry (k), Olivier S. Barnouin (k), Ralph L. McNutt (k), Jurgen Oberst (l) Keywords: Mercury; Mercury, Surface; Mercury, Interior; Geophysics Abstract: On 14 January and 6 October 2008 the MESSENGER spacecraft passed within 200km of the surface of Mercury. These flybys by MESSENGER provided the first observations of Mercury from a spacecraft since the Mariner 10 flybys in 1974 and 1975. Data from the Mercury Laser Altimeter (MLA) provided new information on the equatorial shape of Mercury, and Doppler tracking of the spacecraft through the flybys provided new data on the planet's gravity field. The MLA passes were on opposite hemispheres of the planet and span collectively [approximately equal to]40% of the equatorial circumference. The mean elevation of topography observed during flyby 1, in the longitude range 0-90[degrees]E, is greater than that seen during flyby 2 in the longitude range 180-270[degrees]E, indicating an offset between centers of mass and figure having a magnitude and phase in general agreement with topography determined by Earth-based radar. Both MLA profiles are characterized by slopes of [approximately equal to]0.015[degrees] downward to the east, which is consistent with a long-wavelength equatorial shape defined by a best-fitting ellipse. The Doppler tracking data show sensitivity to the gravitational structure of Mercury. The equatorial ellipticity of the gravitational field, C.sub.2,2, is well determined and correlates with the equatorial shape. The S.sub.2,2 coefficient is [approximately equal to]0, as would be expected if Mercury's coordinate system, defined by its rotational state, is aligned along its principal axes of inertia. The recovered value of the polar flattening of the gravitational potential, J.sub.2, is considerably lower in magnitude than the value obtained from Mariner 10 tracking, a result that is problematic for internal structure models. This parameter is not as well constrained as the equatorial ellipticity because the flyby trajectories were nearly in the planet's equatorial plane. The residuals from the Doppler tracking data suggest the possibility of mascons on Mercury, but flyby observations are of insufficient resolution for confident recovery. For a range of assumptions on degree of compensation and crustal and mantle densities, the allowable crustal thickness is consistent with the upper limit of about 100km estimated from the inferred depth of faulting beneath a prominent lobate scarp, an assumed ductile flow law for crustal material, and the condition that temperature at the base of the crust does not exceed the solidus temperature. The MESSENGER value of C.sub.2,2 has allowed an improved estimate of the ratio of the polar moment of inertia of the mantle and crust to the full polar moment (C.sub.m/C), a refinement that strengthens the conclusion that Mercury has at present a fluid outer core. Author Affiliation: (a) Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA (b) Planetary Science Directorate, Southwest Research Institute, Boulder, CO 80302, USA (c) Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA (d) Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA (e) Department of Physics, University of California, Santa Barbara, CA 93106, USA (f) Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095, USA (g) Stinger Ghaffarian Technologies, Inc., 7701 Greenbelt Road, Suite 400, Greenbelt, Maryland 20770, USA (h) Department of Geological Sciences, Brown University, Providence, RI 02912, USA (i) Department of Geological Sciences, Case Western Reserve University, Cleveland, OH 44106, USA (j) Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC, Canada V6T 1Z4 (k) Space Department, Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723-6099, USA (l) German Aerospace Center, Institute of Planetary Research, D-12489 Berlin, Germany Article History: Received 16 September 2009; Revised 6 April 2010; Accepted 8 April 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.05.007 Byline: Matthew H. Burger (a), Rosemary M. Killen (a), Ronald J. Vervack (b), E. Todd Bradley (c), William E. McClintock (d), Menelaos Sarantos (e), Mehdi Benna (a), Nelly Mouawad (f) Keywords: Mercury, Atmosphere; Mercury, Surface; Atmospheres, Structure; Magnetospheres Abstract: We present a Monte Carlo model of the distribution of neutral sodium in Mercury's exosphere and tail using data from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during the first two flybys of the planet in January and September 2008. We show that the dominant source mechanism for ejecting sodium from the surface is photon-stimulated desorption (PSD) and that the desorption rate is limited by the diffusion rate of sodium from the interior of grains in the regolith to the topmost few monolayers where PSD is effective. In the absence of ion precipitation, we find that the sodium source rate is limited to [approximately equal to]10.sup.6-10.sup.7 cm.sup.-2 s.sup.-1, depending on the sticking efficiency of exospheric sodium that returns to the surface. The diffusion rate must be at least a factor of 5 higher in regions of ion precipitation to explain the MASCS observations during the second MESSENGER flyby. We estimate that impact vaporization of micrometeoroids may provide up to 15% of the total sodium source rate in the regions observed. Although sputtering by precipitating ions was found not to be a significant source of sodium during the MESSENGER flybys, ion precipitation is responsible for increasing the source rate at high latitudes through ion-enhanced diffusion. Author Affiliation: (a) Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA (b) The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA (c) Department of Physics, University of Central Florida, Orlando, FL 32816, USA (d) Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309, USA (e) Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA (f) Astronomy Department, University of Maryland, College Park, MD 20742, USA

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.11.036 Byline: Mehdi Benna (a), Brian J. Anderson (b), Daniel N. Baker (c), Scott A. Boardsen (d), George Gloeckler (e), Robert E. Gold (b), George C. Ho (b), Rosemary M. Killen (a), Haje Korth (b), Stamatios M. Krimigis (b), Michael E. Purucker (a), Ralph L. McNutt (b), Jim M. Raines (e), William E. McClintock (c), Menelaos Sarantos (d), James A. Slavin (d), Sean C. Solomon (f), Thomas H. Zurbuchen (e) Keywords: Mercury; Magnetospheres; Magnetic fields Abstract: The MESSENGER spacecraft flyby of Mercury on 14 January 2008 provided a new opportunity to study the intrinsic magnetic field of the innermost planet and its interaction with the solar wind. The model presented in this paper is based on the solution of the three-dimensional, bi-fluid equations for solar wind protons and electrons in the absence of mass loading. In this study we provide new estimates of Mercury's intrinsic magnetic field and the solar wind conditions that prevailed at the time of the flyby. We show that the location of the boundary layers and the strength of the magnetic field along the spacecraft trajectory can be reproduced with a solar wind ram pressure P.sub.sw =6.8nPa and a planetary magnetic dipole having a magnitude of 210 R.sub.M.sup.3 -nT and an offset of 0.18 R.sub.M to the north of the equator, where R.sub.M is Mercury's radius. Analysis of the plasma flow reveals the existence of a stable drift belt around the planet; such a belt can account for the locations of diamagnetic decreases observed by the MESSENGER Magnetometer. Moreover, we determine that the ion impact rate at the northern cusp was four times higher than at the southern cusp, a result that provides a possible explanation for the observed north-south asymmetry in exospheric sodium in the neutral tail. Author Affiliation: (a) Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA (b) The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA (c) Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309, USA (d) Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA (e) Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI 48109, USA (f) Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA Article History: Received 7 July 2009; Revised 18 November 2009; Accepted 19 November 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.03.014 Byline: F. Merlin (a)(b), M.A. Barucci (a), C. de Bergh (a), S. Fornasier (a), A. Doressoundiram (a), D. Perna (a)(c), S. Protopapa (d) Keywords: Kuiper Belt; Ices; Spectroscopy; Trans-neptunian objects; Centaurs Abstract: Several different trans-neptunian objects have been studied in order to investigate their physical and chemical properties. New observations in the 1.1-1.4[mu]m range, obtained with the ISAAC instrument, are presented in order to complete previous observations carried out with FORS1 in the visible and SINFONI in the near infrared. All of the observations have been performed at the ESO/Very Large Telescope. We analyze the spectra of six different objects (2003 AZ.sub.83, Echeclus, Ixion, 2002 AW.sub.197, 1999 DE.sub.9 and 2003 FY.sub.128) in the 0.45-2.3[mu]m range with the model of Hapke (Hapke, B. [1981]. J. Geophys. Res. 86, 4571-4586) and the method of Shkuratov et al. (Shkuratov, Y., Starukhina, L., Hoffmann, H., Arnold, G. [1999]. Icarus 137, 235-246). Water ice is found on two objects, and in particular it is confirmed in its amorphous and crystalline states on 2003 AZ.sub.84 surface. Upper limits on the water ice content are given for the other four TNOs investigated, confirming previous results (Barkume, K.M., Brown, M.E., Schaller, E.L. [2008]. Astron. J. 135, 55-67; Guilbert, A., Alvarez-Candal, A., Merlin, F., Barucci, M.A., Dumas, C., de Bergh, C., Delsanti, A. [2009]. Icarus 201, 272-283). Whatever the absorption features in the near infrared, all objects but one exhibit a moderate red slope in the visible, as most TNOs and Centaurs. We discuss the implications of the presence of water ice and the probable sources of the red slope. Author Affiliation: (a) LESIA/Observatoire de Paris, 5 Place Jules Janssen, 92195 Meudon Cedex, France (b) Department of Astronomy, University of Maryland, College Park, MD 20742, USA (c) INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy (d) Max-Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany Article History: Received 19 March 2009; Revised 3 March 2010; Accepted 7 March 2010 Article Note: (footnote) [star] Based on observations made with ESO Telescopes at the Paranal Observatories under programme ID 178.C-0036.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.03.004 Byline: J.E. Chambers Keywords: Origin, Solar System; Planetary formation; Planetesimals; Solar nebula Abstract: The formation of 1-1000km diameter planetesimals from dust grains in a protoplanetary disk is a key step in planet formation. Conventional models for planetesimal formation involve pairwise sticking of dust grains, or the sedimentation of dust grains to a thin layer at the disk midplane followed by gravitational instability. Each of these mechanisms is likely to be frustrated if the disk is turbulent. Particles with stopping times comparable to the turnover time of the smallest eddies in a turbulent disk can become concentrated into dense clumps that may be the precursors of planetesimals. Such particles are roughly millimeter-sized for a typical protoplanetary disk. To survive to become planetesimals, clumps need to form in regions of low vorticity to avoid rotational breakup. In addition, clumps must have sufficient self gravity to avoid break up due to the ram pressure of the surrounding gas. Given these constraints, the rate of planetesimal formation can be estimated using a cascade model for the distribution of particle concentration and vorticity within eddies of various sizes in a turbulent disk. We estimate planetesimal formation rates and planetesimal diameters as a function of distance from a star for a range of protoplanetary disk parameters. For material with a solar composition, the dust-to-gas ratio is too low to allow efficient planetesimal formation, and most solid material will remain in small particles. Enhancement of the dust-to-gas ratio by 1-2 orders of magnitude, either vertically or radially, allows most solid material to be converted into planetesimals within the typical lifetime of a disk. Such dust-to-gas ratios may occur near the disk midplane as a result of vertical settling of short-lived clumps prior to clump breakup. Planetesimal formation rates are sensitive to the assumed size and rotational speed of the largest eddies in the disk, and formation rates increase substantially if the largest eddies rotate more slowly than the disk itself. Planetesimal formation becomes more efficient with increasing distance from the star unless the disk surface density profile has a slope of -1.5 or steeper as a function of distance. Planetesimal formation rates typically increase by an order-of-magnitude or more moving outward across the snow line for a solid surface density increase of a factor of 2. In all cases considered, the modal planetesimal size increases with roughly the square root of distance from the star. Typical modal diameters are 100km and 400km in the regions corresponding to the asteroid belt and Kuiper belt in the Solar System, respectively. Author Affiliation: Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USA Article History: Received 9 October 2009; Revised 3 March 2010; Accepted 5 March 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.03.005 Byline: Jeffrey N. Cuzzi (a), Robert C. Hogan (b), William F. Bottke (c) Keywords: Asteroids; Origin, Solar System; Kuiper Belt; Planetesimals; Disks Abstract: Our goal is to understand primary accretion of the first planetesimals. Some examples are seen today in the asteroid belt, providing the parent bodies for the primitive meteorites. The primitive meteorite record suggests that sizeable planetesimals formed over a period longer than a million years, each of which being composed entirely of an unusual, but homogeneous, mixture of millimeter-size particles. We sketch a scenario that might help explain how this occurred, in which primary accretion of 10-100km size planetesimals proceeds directly, if sporadically, from aerodynamically-sorted millimeter-size particles (generically 'chondrules'). These planetesimal sizes are in general agreement with the currently observed asteroid mass peak near 100km diameter, which has been identified as a 'fossil' property of the pre-erosion, pre-depletion population. We extend our primary accretion theory to make predictions for outer Solar System planetesimals, which may also have a preferred size in the 100km diameter range. We estimate formation rates of planetesimals and explore parameter space to assess the conditions needed to match estimates of both asteroid and Kuiper Belt Object (KBO) formation rates. For parameters that satisfy observed mass accretion rates of Myr-old protoplanetary nebulae, the scenario is roughly consistent with not only the 'fossil' sizes of the asteroids, and their estimated production rates, but also with the observed spread in formation ages of chondrules in a given chondrite, and with a tolerably small radial diffusive mixing during this time between formation and accretion. As previously noted, the model naturally helps explain the peculiar size distribution of chondrules within such objects. The optimum range of parameters, however, represents a higher gas density and fractional abundance of solids, and a smaller difference between Keplerian and pressure-supported orbital velocities, than 'canonical' models of the solar nebula. We discuss several potential explanations for these differences. The scenario also produces 10-100km diameter primary KBOs, and also requires an enhanced abundance of solids to match the mass production rate estimates for KBOs (and presumably the planetesimal precursors of the ice giants themselves). We discuss the advantages and plausibility of the scenario, outstanding issues, and future directions of research. Author Affiliation: (a) Ames Research Center, 245-3 Moffett Field, United States (b) Bay Area Research Institute, Sonoma, CA, United States (c) Southwest Research Institute, Boulder, CO, United States Article History: Received 20 October 2009; Revised 8 February 2010; Accepted 8 March 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.01.034 Byline: T.M. Davison (a), G.S. Collins (a), F.J. Ciesla (b) Keywords: Collisional physics; Impact processes; Planetary formation; Planetesimals Abstract: Collisions between planetesimals at speeds of several kilometres per second were common during the early evolution of our Solar System. However, the collateral effects of these collisions are not well understood. In this paper, we quantify the efficiency of heating during high-velocity collisions between planetesimals using hydrocode modelling. We conducted a series of simulations to test the effect on shock heating of the initial porosity and temperature of the planetesimals, the relative velocity of the collision and the relative size of the two colliding bodies. Our results show that while heating is minor in collisions between non-porous planetesimals at impact velocities below 10kms.sup.-1, in agreement with previous work, much higher temperatures are reached in collisions between porous planetesimals. For example, collisions between nearly equal-sized, porous planetesimals can melt all, or nearly all, of the mass of the bodies at collision velocities below 7kms.sup.-1. For collisions of small bodies into larger ones, such as those with an impactor-to-target mass ratio below 0.1, significant localised heating occurs in the target body. At impact velocities as low as 5kms.sup.-1, the mass of melt will be nearly double the mass of the impactor, and the mass of material shock heated by 100K will be nearly 10 times the mass of the impactor. We present a first-order estimate of the cumulative effects of impact heating on a porous planetesimal parent body by simulating the impact of a population of small bodies until a disruptive event occurs. Before disruption, impact heating is volumetrically minor and highly localised; in no case was more than about 3% of the parent body heated by more than 100K. However, heating during the final disruptive collision can be significant; in about 10% of cases, almost all of the parent body is heated to 700K (from an initial temperature of [approximately equal to]300K) and more than a tenth of the parent body mass is melted. Hence, energetic collisions between planetesimals could have had important effects on the thermal evolution of primitive materials in the early Solar System. Author Affiliation: (a) Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, United Kingdom (b) Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60430, USA Article History: Received 1 May 2009; Revised 6 January 2010; Accepted 31 January 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.01.031 Byline: Simon B. Porter (a), Steven J. Desch (a), Jason C. Cook (b) Keywords: Satellites, Surfaces; Ices, IR spectroscopy; Saturn, Satellites; Uranus, Satellites; Kuiper Belt; Interplanetary dust Abstract: The spectra of water ice on the surfaces of icy satellites and Kuiper Belt Objects (KBOs) indicate that the surface ice on these bodies is in a crystalline state. This conflicts with theoretical models, which predict that radiation (galactic cosmic rays and solar ultraviolet) should damage the crystalline structure of ice on geologically short timescales. Temperatures are too low in the outer Solar System for the ice to anneal, and reflectance spectra of these bodies should match those of amorphous solid water (ASW). We assess whether the kinetic energy deposited as heat by micrometeorite impacts on outer Solar System bodies is sufficient to anneal their surface ice down to a near-infrared optical depth (350[mu]m). We calculate the kinetic energy flux from interplanetary micrometeorite impacts, including gravitational focusing. We also calculate the thermal diffusion of impact heat in various surfaces and the rate of annealing of ice. We conclude that the rate of annealing from micrometeorite impacts is sufficient to explain the crystallinity of ice on nearly all the surfaces of the saturnian, uranian and neptunian satellites. We discuss how the model can be used in conjunction with spectra of KBOs to probe dust fluxes in the Kuiper Belt. Author Affiliation: (a) School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA (b) Southwest Research Institute, 1050 Walnut Street, Boulder, CO 80302, USA Article History: Received 12 September 2009; Revised 14 January 2010; Accepted 21 January 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.02.005 Byline: Henry B. Throop (a), John Bally (b) Keywords: Origin, Solar System; Solar nebula; Jupiter; Jupiter, Atmosphere Abstract: If Jupiter and the Sun both formed directly from the same well-mixed proto-solar nebula, then their atmospheric compositions should be similar. However, direct sampling of Jupiter's troposphere indicates that it is enriched in elements such as C, N, S, Ar, Kr, and Xe by 2-6x relative to the Sun (Wong, M.H., Lunine, J.I., Atreya, S.K., Johnson, T., Mahaffy, P.R., Owen, T.C., Encrenaz, T. [2008]. 219-246). Most existing models to explain this enrichment require an extremely cold proto-solar nebula which allows these heavy elements to condense, and cannot easily explain the observed variations between these species. We find that Jupiter's atmospheric composition may be explained if the Solar System's disk heterogeneously accretes small amounts of enriched material such as supernova ejecta from the interstellar medium during Jupiter's formation. Our results are similar to, but substantially larger than, isotopic anomalies in terrestrial material that indicate the Solar System formed from multiple distinct reservoirs of material simultaneously with one or more nearby supernovas (Trinquier, A., Birck, J.-L., Allegre, C.J. [2007]. Astrophys. J. 655, 1179-1185). Such temporal and spatial heterogeneities could have been common at the time of the Solar System's formation, rather than the cloud having a purely well-mixed 'solar nebula' composition. Author Affiliation: (a) Southwest Research Institute, 1050 Walnut St., Ste. 300, Boulder, CO 80302, USA (b) Center for Astrophysics and Space Astronomy, University of Colorado, UCB 389, Boulder, CO 80309-0389, USA Article History: Received 14 October 2008; Revised 3 February 2010; Accepted 7 February 2010