980 research outputs found
Closed-form expressions for particle relative velocities induced by turbulence
In this note we present complete, closed-form expressions for random relative
velocities between colliding particles of arbitrary size in nebula turbulence.
These results are exact for very small particles (those with stopping times
much shorter than the large eddy overturn time) and are also surprisingly
accurate in complete generality (that is, also apply for particles with
stopping times comparable to, or much longer than, the large eddy overturn
time). We note that some previous studies may have adopted previous simple
expressions, which we find to be in error regarding the size dependence in the
large particle regime.Comment: 8 pages, accepted as Research Note by A&
The Spherically Symmetric Gravitational Collapse of a Clump of Solids in a Gas
Several mechanisms have been identified that create dense particle clumps in
the solar nebula. The present work is concerned with the gravitational collapse
of such clumps, idealized as being spherically symmetric. Calculations using
the two-fluid model are performed (almost) up to the time when a central
density singularity forms. The end result of the study is a parametrization for
this time, in order that it may be compared with timescales for various
disruptive effects to which clumps may be subject. An important effect is that
as the clump compresses, it also compresses the gas due to drag. This increases
gas pressure which retards particle collapse and leads to oscillation in the
size and density of the clump. The ratio of gravitational force to gas pressure
gives a two-phase Jeans parameter, , which is the classical Jeans
parameter with the sound speed replaced by an the wave speed in a coupled
two-fluid medium. Its use makes the results insensitive to the initial density
ratio of particles to gas as a separate parameter. An ordinary differential
equation model is developed which takes the form of two coupled non-linear
oscillators and reproduces key features of the simulations. Finally, a
parametric study of the time to collapse is performed and a formula (fit to the
simulations) is developed. In the incompressible limit , collapse
time equals sedimentation time. As increases, the collapse time decreases
roughly linearly with until when it becomes
approximately equal to the dynamical time
The rings of Saturn: State of current knowledge and some suggestions for future studies
The state of our current knowledge of the properties of the ring system as a whole, and of the particles individually, is assessed. Attention is primarily devoted to recent results and possibilities for exploration of the ring system by a Saturn orbiter. In particular, the infrared and microwave properties of the ring system are discussed. The behavior of the ring brightness is not well understood in the critical transition spectral region from approximately 100 micrometers to approximately 1 cm. Also, the dynamical behavior of the ring system is discussed. Recent theoretical studies show that ongoing dynamical effects continually affect the ring structure in azimuth (possibly producing the A ring brightness asymmetry) and in the vertical direction. Orbital spacecraft-based studies of the rings will offer several unique advantages and impact important cosmogonical questions. Bistatic radar studies and millimeter-wavelength spectrometer/radiometry will give particle sizes and composition limits needed to resolve the question of the density of the rings, and provide important boundary conditions on the state of Saturn's protoplanetary nebula near the time of planetary formation
The Runaway Greenhouse: A History of Water on Venus
Radiative-convective equilibrium models of planetary atmospheres are discussed for the case when the infrared opacity is due to a vapor in equilibrium with its liquid or solid phase. For a grey gas, or for a gas which absorbs at all infrared wavelengths, equilibrium is impossible when the solar constant exceeds a critical value. Equilibrium therefore requires that the condensed phase evaporates into the atmosphere.
Moist adiabatic and pseudoadiabatic atmospheres in which the condensing vapor is a major atmospheric constituent are considered. This situation would apply if the solar constant were supercritical with respect to an abundant substance such as water. It is shown that the condensing gas would be a major constituent at all levels in such an atmosphere. Photodissociation of water in the primordial Venus atmosphere is discussed in this context
The Evolution of the Water Distribution in a Viscous Protoplanetary Disk
(Abridged) Astronomical observations have shown that protoplanetary disks are
dynamic objects through which mass is transported and accreted by the central
star. Age dating of meteorite constituents shows that their creation,
evolution, and accumulation occupied several Myr, and over this time disk
properties would evolve significantly. Moreover, on this timescale, solid
particles decouple from the gas in the disk and their evolution follows a
different path. Here we present a model which tracks how the distribution of
water changes in an evolving disk as the water-bearing species experience
condensation, accretion, transport, collisional destruction, and vaporization.
Because solids are transported in a disk at different rates depending on their
sizes, the motions will lead to water being concentrated in some regions of a
disk and depleted in others. These enhancements and depletions are consistent
with the conditions needed to explain some aspects of the chemistry of
chondritic meteorites and formation of giant planets. The levels of
concentration and depletion, as well as their locations, depend strongly on the
combined effects of the gaseous disk evolution, the formation of rapidly
migrating rubble, and the growth of immobile planetesimals. We present examples
of evolution under a range of plausible assumptions and demonstrate how the
chemical evolution of the inner region of a protoplanetary disk is intimately
connected to the physical processes which occur in the outer regions.Comment: 45 pages, 7 figures, revised for publication in Icaru
Accretion of dust by chondrules in a MHD-turbulent solar nebula
(Abridged) Numerical magnetohydrodynamic (MHD) simulations of a turbulent
solar nebula are used to study the growth of dust mantles swept up by
chondrules. A small neighborhood of the solar nebula is represented by an
orbiting patch of gas at a radius of 3 AU, and includes vertical stratification
of the gas density. The differential rotation of the nebular gas is replaced by
a shear flow. Turbulence is driven by destabilization of the flow as a result
of the magnetorotational instability (MRI), whereby magnetic field lines
anchored to the gas are continuously stretched by the shearing motion. A
passive contaminant mimics small dust grains that are aerodynamically well
coupled to the gas, and chondrules are modeled by Lagrangian particles that
interact with the gas through drag. Whenever a chondrule enters a region
permeated by dust, its radius grows at a rate that depends on the local dust
density and the relative velocity between itself and the dust. The local dust
abundance decreases accordingly. Different chondrule volume densities lead to
varying depletion and rimmed-chondrule size growth times. Most of the dust
sweep-up occurs within 1 gas scale height of the nebula midplane. Chondrules
can reach their asymptotic radius in 10 to 800 years. The vertical variation of
nebula turbulent intensity results in a moderate dependence of mean
rimmed-chondrule size with nebula height. The technique used here could be
combined with Monte Carlo (MC) methods that include the physics of dust
compaction, in a self-consistent MHD-MC model of dust rim growth around
chondrules in the solar nebula.Comment: 33 pages, 9 figures. Icarus, in pres
Scale Dependence of Multiplier Distributions for Particle Concentration, Enstrophy and Dissipation in the Inertial Range of Homogeneous Turbulence
Turbulent flows preferentially concentrate inertial particles depending on
their stopping time or Stokes number, which can lead to significant spatial
variations in the particle concentration. Cascade models are one way to
describe this process in statistical terms. Here, we use a direct numerical
simulation (DNS) dataset of homogeneous, isotropic turbulence to determine
probability distribution functions (PDFs) for cascade multipliers, which
determine the ratio by which a property is partitioned into sub-volumes as an
eddy is envisioned to decay into smaller eddies. We present a technique for
correcting effects of small particle numbers in the statistics. We determine
multiplier PDFs for particle number, flow dissipation, and enstrophy, all of
which are shown to be scale dependent. However, the particle multiplier PDFs
collapse when scaled with an appropriately defined local Stokes number. As
anticipated from earlier works, dissipation and enstrophy multiplier PDFs reach
an asymptote for sufficiently small spatial scales. From the DNS measurements,
we derive a cascade model that is used it to make predictions for the radial
distribution function (RDF) for arbitrarily high Reynolds numbers, ,
finding good agreement with the asymptotic, infinite inertial range theory
of Zaichik and Alipchenkov [New Journal of Physics 11, 103018 (2009)]. We
discuss implications of these results for the statistical modeling of the
turbulent clustering process in the inertial range for high Reynolds numbers
inaccessible to numerical simulations.Comment: 21 pages, 14 figures, accepted for publication in Physical Review
Growth of Dust as the Initial Step Toward Planet Formation
We discuss the results of laboratory measurements and theoretical models
concerning the aggregation of dust in protoplanetary disks, as the initial step
toward planet formation. Small particles easily stick when they collide and
form aggregates with an open, often fractal structure, depending on the growth
process. Larger particles are still expected to grow at collision velocities of
about 1m/s. Experiments also show that, after an intermezzo of destructive
velocities, high collision velocities above 10m/s on porous materials again
lead to net growth of the target. Considerations of dust-gas interactions show
that collision velocities for particles not too different in surface-to-mass
ratio remain limited up to sizes about 1m, and growth seems to be guaranteed to
reach these sizes quickly and easily. For meter sizes, coupling to nebula
turbulence makes destructive processes more likely. Global aggregation models
show that in a turbulent nebula, small particles are swept up too fast to be
consistent with observations of disks. An extended phase may therefore exist in
the nebula during which the small particle component is kept alive through
collisions driven by turbulence which frustrates growth to planetesimals until
conditions are more favorable for one or more reasons.Comment: Protostars and Planets V (PPV) review. 18 pages, 5 figure
Towards Initial Mass Functions for Asteroids and Kuiper Belt Objects
Our goal is to understand primary accretion of the first planetesimals. The
primitive meteorite record suggests that sizeable planetesimals formed in the
asteroid belt over a period longer than a million years, each composed entirely
of an unusual, but homogeneous, mixture of mm-size particles. We sketch a
scenario in which primary accretion of 10-100km size planetesimals proceeds
directly, if sporadically, from aerodynamically-sorted mm-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 assess
the conditions needed to match estimates of both asteroid and Kuiper Belt
Object (KBO) formation rates. For nebula 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 (the model naturally
helps explain the peculiar size distribution of chondrules within such
objects). The scenario also produces 10-100km diameter primary KBOs. 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.Comment: Icarus, in pres
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