Engineering topics

Dobrovolskis, A. R.

Publications and source records attributed to Dobrovolskis, A. R..

17 records

NASA NTRS2013

Saturn's F Ring Core: Calm in the Midst of Chaos

The long-term stability of the narrow F Ring core has been hard to understand. Instead of acting as "shepherds", Prometheus and Pandora together stir the vast preponderance of the region into a chaotic state, consistent with the orbits of newly discovered objects like S/2004S6. We show how a comb of very narrow radial locations of high stability in semimajor axis is embedded within this otherwise chaotic region. The stability of these semimajor axes relies fundamentally on the unusual combination of rapid apse precession and long synodic period which characterizes the region. This situation allows stable "antiresonances" to fall on or very close to traditional Lindblad resonances which, under more common circumstances, are destabilizing. We present numerical integrations of tens of thousands of test particles over tens of thousands of Prometheus orbits that map out the effect. The stable antiresonance zones are most stable in a subset of the region where Prometheus first-order resonances are least cluttered by Pandora resonances. This region of optimum stability is paradoxically closer to Prometheus than a location more representative of "torque balance", helping explain a longstanding paradox. One stable zone corresponds closely to the currently observed semimajor axis of the F Ring core. While the model helps explain the stability of the narrow F Ring core, it does not explain why the F Ring material all shares a common apse longitude; we speculate that collisional damping at the preferred semimajor axis (not included in the current simulations) may provide that final step. Essentially, we find that the F Ring core is not confined by a combination of Prometheus and Pandora, but a combination of Prometheus and precession.

Planetary Rings↗

NASA NTRS2003

Creation and Distribution of CAIs in the Protoplanetary Nebula

CaAl rich refractory mineral inclusions (CAIs) found at 1 - 10% mass fraction in primitive chondrites appear to be several million years older than the dominant (chondrule) components in the same parent bodies. A prevalent concern is that it is difficult to retain CAIs for this long against gas-drag-induced radial drift into the sun. We assess a hot inner (turbulent) nebula context for CAI formation, using analytical models of nebula evolution and particle diffusion. We show that outward radial diffusion in a weakly turbulent nebula can prevent significant numbers of CAI-size particles from being lost into the sun for times of 1 - 3 x 10(exp 6) years. To match the CAI abundances quantitatively, we advocate an enhancement of the inner hot nebula in silicate-forming material, due to rapid inward migration of very primitive, silicate and carbon rich, meter-sized objects. 'Combustion' of the carbon into CO would make the CAI formation environment more reduced than solar, as certain observations imply. Abundant CO might also play a role in mass-independent chemical fractionation of oxygen isotopes as seen in CAIs and associated primitive, high-temperature condensates.

Cuzzi, J. N.↗

NASA NTRS1998

Chondrule Rimming by Sweepup of Dust in the Protoplanetary Nebula: Constraints on Primary Accretion

Chondrite constituents display signs of aerodynamic sorting. In prior work we have shown that plausible conditions of nebula turbulence aerodynamically select chondrule-sized particles for strong concentration into zones 5-6 orders of magnitude denser than average. We refer to this process as Turbulent Concentration (TC). The TC hypothesis also predicts a particle size distribution within densely concentrated zones which is in very good agreement with that of chondrules; disaggregated from CV chondrites by Paque and Cuzzi and, from an LL UOC, by Hughes. Here we present results from the TC hypothesis relating to other aspects of the properties of chondrules and chondrites specifically, the thickness of fine-grained rims, which am often referred to as 'accretion rims'. Additional information is contained in the original extended abstract.

Cuzzi, J. N.↗

NASA NTRS1996

Interaction of Particles and Turbulence in the Solar Nebula

The most widely accepted theories for the formation of the Solar system claim that small solid particles continue to settle into a thin layer at the midplane of the Solar nebula until it becomes gravitationally unstable and collapses directly into km-sized planetesimals. This scenario has been challenged on at least two grounds: (1) due to turbulence, the particles may not settle into a thin layer, and (2) a thin layer may not be unstable. The Solar nebula contains at least three sources of turbulence: radial shear, vertical shear, and thermal convection. The first of these is small and probably negligible, while the last is poorly understood. However, the second contribution is likely to be substantial. The particle-rich layer rotates at nearly the Keplerian speed, but the surrounding gaseous nebula rotates slower because it is partly supported by pressure. The resulting shear generates a turbulent boundary layer which stirs the particles away from the midplane, and forestalls gravitational instability. Our previous work used a 'zero-equation' (Prandtl) model to predict the intensity of shear-generated turbulence, and enabled us to demonstrate numerically that settling of particles to the midplane is self-limiting. However, we neglected the possibility that mass loading by particles might damp the turbulence. To explore this, we have developed a more sophisticated 'one-equation' model which incorporates local generation, transport, and dissipation of turbulence, as well as explicit damping of turbulence by particles. We also include a background level of global turbulence to represent other sources. Our results indicate that damping flattens the distribution of particles somewhat, but that background turbulence thickens the particle layer.

Dacles-Mariani, Jennifer S.↗

NASA NTRS1994

What initiated planetesimal formation?

The physical structure of primitive (chondritic) meteorites, even after some geological processing and modification, is thought by most to contain clues as to the first stage of accretion of solid matter into objects that might be called planetesimals. However, theoretical understanding of the processes responsible for this important stage is shaky. We note what we believe are fundamental obstacles for the Goldreich-Ward version of rapid and direct planetesimal formation via gravitational instability in a settled particle layer, and describe an alternative scenario which might lead from grainy nebula gas to primitive planetesimals in a way that has intriguing connections to the meteorite evidence.

Cuzzi, J. N.↗

NASA NTRS1994

Turbulent diffusion and concentration of chondrules in the protoplanetary nebula

Turbulence is known to possess structure on many scales. The largest or integral scale L is usually taken to be the largest dimension of the system. Two different turbulent regimes are likely to be of importance. Early stage convective turbulence in a hot nebula probably extends over the entire nebula scale height, and probably has typical eddy frequency comparable to the orbit frequency. At a later stage, midplane shear layer turbulence can be driven by the presence of a differently rotating, densely settled particle layer, with different length and timescales.

Cuzzi, J. N.↗

NASA NTRS1984

Angular momentum drain - A mechanism for despinning asteroids

The new mechanism of angular momentum drain is proposed to account for the relatively slow rotation rates of intermediate-sized asteroids. Impact ejecta on a spinning body preferentially escape in the direction of rotation, systematically draining away spin angular momentum and leading to the counterintuitive result that collisions can reduce the spin of midsized objects. The existing theory of asteroid rotation is reviewed, and the escape of ejecta from hypervelocity impacts on bodies of different sizes and physical properties is described. The effect of this mass loss on asteroidal rotation is calculated and shown to be a significant brake on the spins of intermediate-sized asteroids. Finally, this new process is incorporated in a revised theory of collisional evolution, its predictions are compared with observational data, and its applications are discussed.

Dobrovolskis, A. R.↗

NASA NTRS1983

Atmospheric tides on Venus. III - The planetary boundary layer

Diurnal solar heating of Venus' surface produces variable temperatures, winds, and pressure gradients within a shallow layer at the bottom of the atmosphere. The corresponding asymmetric mass distribution experiences a tidal torque tending to maintain Venus' slow retrograde rotation. It is shown that including viscosity in the boundary layer does not materially affect the balance of torques. On the other hand, friction between the air and ground can reduce the predicted wind speeds from about 5 to about 1 m/sec in the lower atmosphere, more consistent with the observations from Venus landers and descent probes. Implications for aeolian activity on Venus' surface and for future missions are discussed.

Dobrovolskis, A. R.↗

NASA NTRS1983

The obliquity of Pluto

Pluto's obliquity (the angle between its spin axis and orbit normal) varies between 102 and 126 deg over a period of about 3 million years. These oscillations are nearly sinusoidal and quite stable, leading to only modest changes in the insolation regime. Thus, Pluto's rotation has been slightly retrograde ever since its current orbit and rotation rate were established.

Dobrovolskis, A. R.↗

NASA NTRS1983

Does Venus breathe?

It is speculated that the periodic absorption and desorption of CO2 by the soil of Venus may buffer daily temperature, pressure and wind variations in the lower atmosphere, effectively eliminating the net tidal torque on the atmosphere. The redistribution of mass would still generate a sizable torque, however, which may serve as a balance for that which is caused by the gravitationally induced tide. This novel tidal mechanism represents a somewhat weaker competitor to the atmospheric tides which have previously been studied.

Dobrovolskis, A. R.↗

NASA NTRS1983

Preferred orbit planes in the gravitational field of a tumbling spheroidal galaxy

An analytic method is described by means of which the preferred planes into which gas will settle by dissipative differential precession in a slightly nonspherical, nonstatic potential can be determined. Attention is given to the analysis of circular orbit precession in the external potential of a slightly prolate spheroidal mass that is tumbling about a short axis, providing a crude model for barred spiral galaxies as well as for prolate elliptical ones. The results obtained agree with previous work on the existence, orientation, and stability of periodic orbits in triaxial potentials. Depending on the source and injection geometry of the gas and on the nature of dissipation in the gas, the orbital dynamics presented could lead to enhanced, or even catastrophic, radial inflow and to steady state warped structures.

Durisen, R. H.↗

NASA NTRS1982

Internal stresses in Phobos and other triaxial bodies

The unusual dynamical behavior of Phobos, its strange appearance, and its mysterious network of grooves all make it an intriguing object. Geophysical studies, though, have been hampered by the lack of suitable theories applicable to nonspherical bodies. In this paper the Martian satellites are modeled as homogeneous, elastic triaxial ellipsoids subject to tidal, rotational, and self-gravitational stresses. A novel semianalytical treatment then gives the stress and strain fields throughout their interiors. Yield phenomena and their possible surface expressions are also investigated. The results indicate that Phobos and Deimos have always been stable with respect to tidal fracture or disruption, but that Phobos will probably break up before colliding with Mars. Applications of the new formulation to other nonspherical bodies in the solar system are also discussed.

Dobrovolskis, A. R.↗

NASA NTRS1980

Life near the Roche limit - Behavior of ejecta from satellites close to planets

A study of the dynamics of nearby debris from impact craters was made to explain the distinctive features seen on Phobos, Deimis, and Amalthea. The planetary tides and satellite rotation were considered, and the usual pseudo-energy (Jacobi) integral was numerically calculated in the framework of a restricted body problem where satellites are modelled as triaxial ellipsoids rather than point masses. Iso-contours of this integral show that Deimos and Amalthea are entirely closed by Roche lobes, and the surfaces of their model ellipsoids lie nearly along equipotentials. Presently, the surface of Phobos overflows its Roche lobe, except for regions within a few km of the sub-Mars and anti-Mars points. The behavior of crater ejecta from the satellites of Mars were also examined by numerical integration of trajectories for particles leaving their surfaces in the equatorial plane.

Dobrovolskis, A. R.↗

NASA NTRS1980

Atmospheric tides and the rotation of Venus. I - Tidal theory and the balance of torques

Insolation absorbed by the surface of Venus is quickly redeposited at the bottom of the atmosphere. This periodic heating causes mass to flow away from the warm afternoon side of the planet and into the cooler morning region. The Sun's gravitational field exerts a torque on this atmospheric tide tending to accelerate the retrograde zonal circulation. When this torque is transmitted to the crust, it can balance the despinning effect of tides in the body of Venus. The slow retrograde rotation of Venus may be a steady state among tides in the atmosphere, tides in the solid body, and possibly the influence of the Earth.

Dobrovolskis, A. R.↗

NASA NTRS1980

Atmospheric tides and the rotation of Venus. II - Spin evolution

Tides in the atmosphere of Venus may help to stabilize its slow retrograde rotation. The frequency dependence of the body tides also affects its rotational stability. However, the obliquity is probably maintained near 180 deg by friction between the core and mantle of Venus. In any case, it appears most likely that Venus originated with an obliquity greater than 90 deg.

Dobrovolskis, A. R.↗

NASA NTRS1979

Planetary atmospheres

The present paper deals with some of the principal data on extraterrestrial atmospheres obtained during the period 1975-1978. The atmospheres of Venus, Mars, Jupiter, and the Jovian satellites are examined, showing that many first-order questions concerning composition, physical state, and kinematics of these atmospheres have been answered.

Ingersoll, A. P.↗

NASA NTRS1978

Venus' rotation and atmospheric tides

On the basis of a presented theory, it is suggested that Venus' current rotation is a stable balance between atmospheric and solar body tides. The theory is concerned with Venus' atmospheric tides, driven by solar heating, and how these tides could serve as a third torque to balance the effects of solar body torque and to maintain a stable equilibrium resonance with regard to the earth's gravitational effects. In the absence of the atmospheric tidal torque, or some other torque, it would be expected that Venus would be despun until synchronous rotation (one side always facing the sun) is attained, rather than retain the retrograde rotation period of 243 days.

Ingersoll, A. P.↗