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, $J_t$, 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 $J_t \to 0$, collapse time equals sedimentation time. As $J_t$ increases, the collapse time decreases roughly linearly with $J_t$ until $J_t \gtrsim 0.4$ when it becomes approximately equal to the dynamical time

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

Detached from their homeland: the Latter-day Saints of Chihuahua, Mexico

Over the past few decades, the homeland concept has received an ever-increasing amount of attention by cultural geographers. While the debate surrounding the necessity and applicability of the concept continues, it is more than apparent that no other geographic term (including culture areas or culture regions) captures the essence of peoples’ attachment to place better than homeland. The literature, however, provides few examples of the deep-seated loyalty people have for a homeland despite being physically detached from that space. Employing land use mapping and informal interviews, this paper seeks to help fill that gap by exemplifying how the daily lives of Mormons living in Chihuahua, Mexico reflect their connection to the United States and the Mormon Homeland. Our research revealed that, among other things, the Anglo residents perpetuate their cultural identity through their unique self-reference, exhibit territoriality links reflected in their built environment, and demonstrate unconditional bonding to their homeland through certain holiday celebrations. It is clear to us, as the Anglo-Mormon experience illustrates, that the homeland concept deserves a place within the geographic lexicon

Arresting Children: Examining Recent Trends in Preteen Crime

Are juvenile offenders getting younger? The American public often hears policymakers and justice practitioners assert that young people are committing crimes at younger and younger ages. Is this true? This analysis explores this question by examining data collected by law enforcement agencies across the country. It tracks juvenile crime patterns from 1980 through 2006 and finds that the age profile of juvenile offenders has not changed substantially in 25 years. Crime rates among children under age 13 have generally followed the same crime patterns exhibited among older youth. In a few offense categories, however, increases in preteen crime have outpaced increases among older juveniles, particularly sexual offenses, assaults, and weapons possession (not necessarily firearms). The fact that school authorities and family members often report these offenses suggests a possible hypothesis to explain increases in some preteen crimes: The juvenile justice system today may be dealing with child behavior problems that were once the responsibility of social welfare agencies, schools, and families