Engineering topics

Herbert, F.

Publications and source records attributed to Herbert, F..

25 records · Page 2

NASA NTRS1980

Time-dependent lunar density models

Simple models of the geochemical and geophysical evolution of the moon were constructed with the particular aim of investigating the mass density distribution within the moon as a function of time. The strongly inverted (densest material on top) density distribution resulting from fractional crystallization of the lunar magma ocean was found to rearrange itself into a highly stable stratification. Density stratification due to compositional variation dominated gravitational stability; thus, thermally driven convection was found to be suppressed in regions with relaxed (rearranged so as to minimize the gravitational potential energy) compositional density variation. The suppression of convection as a heat transfer process results in considerable modification of thermal evolution. The models produced sufficient density segregation from magma oceans of a few hundred km thickness, such that relaxation to the most stable rearrangement yielded a polar moment of inertia approximating that measured for the present-day moon. Thus, lunar models matching moment-of-inertia constraints but lacking a metallic core are motivated.

Herbert, F.↗

NASA NTRS1979

Electromagnetic heating of minor planets in the early solar system

Electromagnetic processes occurring in the primordial solar system are likely to have significantly affected planetary evolution. In particular, electrical coupling of the kinetic energy of a dense T-Tauri-like solar wind into the interior of the smaller planets could have been a major driver of thermal metamorphism. Accordingly a grid of asteroid models of various sizes and solar distances was constructed using dc transverse magnetic induction theory. Plausible parameterizations with no requirement for a high environmental temperature led to complete melting for Vesta with no melting for Pallas and Ceres. High temperatures were reached in the Pallas model, perhaps implying nonmelting thermal metamorphosis as a cause of its anomalous spectrum. A reversal of this temperature sequence seems implausible, suggesting that the Ceres-Pallas-Vesta dichotomy is a natural outcome of the induction mechanism. Highly localized heating is expected to arise due to an instability in the temperature-controlled current distribution. Localized metamorphosis resulting from this effect may be relevant to the production and evolution of pallasites, the large presumed metal component of S object spectra, and the formation of the lunar magma ocean.

Herbert, F.↗

NASA NTRS1978

Primordial metamorphism of asteroids via electrical induction in a T Tauri-like solar wind

Possible heating mechanisms for an intense thermal cycle in the early solar system are considered, and inductive asteroid heating in a T Tauri environment is examined. Calculations of thermal models based on recent measurements of the electrical conductivity of carbonaceous meteorites are reported which are presumably more representative of actual conditions in the early solar nebula. Asteroidal thermal evolution is computed using a highly simplified code that restricts induction to the dc limit of the TM mode, approximates the complete unipolar-generation differential equation by a simple series-resistance model, and restricts the modeling of thermal conduction to a calculation of heat transfer across a thin planar shell of given thickness increasing with time, which blankets a body assumed to have an isothermal core. A grid of results is obtained for 20 combinations of asteroid radius and solar distance representing the populaton of asteroids. It is found that there is a relative heating maximum with respect to body radius, that this effect is due to the operation of two distinct heating-rate restrictions which have opposite radius dependences, and that the maximum current density is roughly proportional to the inverse of interior radius.

Herbert, F.↗

NASA NTRS1978

Geophysical and geochemical evolution of the lunar magma ocean

There is increasing evidence that at least the outer few hundred kilometers of the moon were melted immediately following accretion. This paper studies the evolution of this lunar magma ocean. The long time scale for solidification leads to the inference that the plagioclase-rich (ANT) lunar crust began forming, perhaps preceded by local accumulations termed 'rockbergs', at the very beginning of the magma ocean epoch. In this view the cooling and solidification of the magma ocean was primarily controlled by the rate at which heat could be conducted across the floating ANT crust. Thus the thickness of the crust was the factor controlling the lunar solidification time. Heat arising from enthalpy of crystallization was transported in the magma by convection. Mixing length theory is used to deduce the principal flow velocity (typically several cm/s) during convection. The magma ocean is deduced to have been turbulent down to a characteristic length scale of the order of 100 m, and to have overturned on a time scale of the order of 1 yr for most of the magma ocean epoch.

Herbert, F.↗

NASA NTRS1977

Model 'zero-age' lunar thermal profiles resulting from electrical induction

Thermal profiles for the moon are calculated under the assumption that a pre-main-sequence T-Tauri-like solar wind excites both transverse magnetic and transverse electric induction while the moon is accreting. A substantial initial temperature rise occurs, possibly of sufficient magnitude to cause subsequent early extensive melting throughout the moon in conjunction with nominal long-lived radioactives. In these models, accretion is an unimportant direct source of thermal energy but is important because even small temperature rises from accretion cause significant changes in bulk electrical conductivity. Induction depends upon the radius of the moon, which we take to be accumulating while it is being heated electrically. The 'zero-age' profiles calculated in this paper are proposed as initial conditions for long-term thermal evolution of the moon.

Herbert, F.↗

NASA NTRS1977

Some constraints on the thermal history of the lunar magma ocean

If the accumulating evidence is accepted that the outer portion of the moon was molten for 100-200 million years, it is clear that a permanent insulating surface layer existed over nearly all of that epoch. Considerations of crustal stability against break-up and foundering lead to the view that this insulating blanket must have been an early-forming plagioclase-rich layer light enough to float on the hot magma. It is found that radiometric age-dating evidence implies a fairly specific history for the solidification of the lunar magma ocean. The possibility is anticipated that geochronological and petrological constraints will be sufficient to narrow the range of allowed geophysical and geochemical models. It is hoped that such a study will make it possible to deduce the original depth, and hence, the composition of the lunar magma ocean. If the moon accreted homogeneously, the composition of the magma ocean will also be that of the whole moon, and hence such models should allow estimation of the bulk lunar composition.

Herbert, F.↗

NASA NTRS1976

Solar wind induction in Mercury - Constraints on the formation of a magnetosphere

A model is outlined in which the origin of Mercury's magnetic field is attributed to electromagnetic induction from the interplanetary magnetic field. Both transverse magnetic (TM) and transverse electric (TE) induction are considered. It is found that neither mode can produce a totally detached magnetopause, so the highly nonlinear dynamics of magnetopausal flux deflection is investigated as a potential inhibitor of the decaying tendency of linear induction. No mechanism is discovered which can account for the large distance of the magnetopause, its temporal stability, and the orientation of the magnetic field. It is shown that the stochastic trapped-field TE model comes close to fulfilling these requirements, but cannot explain the Mariner 10 observations of Mercury's field. Noting the potential difficulties of the convective magnetic dynamo model, it is concluded that the source of Mercury's field is still poorly understood.

Herbert, F.↗