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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 415 records · Page 23
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NASA NTRS1979

Dynamics of the accumulation process of the Earth group of planets: Formation of the reverse rotation of Venus

A numerical simulation of the process of formation of the terrestrial planets is carried within the framework of a new theory for the accumulation of planetary and satellite systems. The numerical simulation permitted determining the parameters of the protoplanetary disk from which Mercury, Venus and the Earth were formed as result of the evolution. The acquisition of a slow retrograde rotation for Venus was discovered during the course of the investigation, whereas Mercury and the Earth acquired direct rotation about their axes. Deviations of the semimajor axes of these three planets as well as the masses of the Earth and Venus from the true values are small as a rule (l 10%). It is shown that during the accumulation of the terrestrial planets, there existed a profound relationship between the process of formation of the orbits and masses of the planet and the process of formation of their rotation about their axes. Estimates are presented for the radii of the initial effective bodies and the time of evolution for the terrestrial accumulation zone.

Koslov, N. N.↗
NASA NTRS1980

Intrinsic luminosities of the Jovian planets

Available data and theories on the size and nature of interior power sources in the four Jovian planets are reviewed. These four planets are Jupiter, Saturn, Uranus and Neptune. Various models, ranging from simple cooling to gravitational layering to radioactivity, are discussed. The evidence and interpretations presented in the discussion seem to indicate that (1) all four Jovian planets were once more luminous and are presently cooling to a state of equilibrium with sunlight; and (2) the thermal evolution of the Jovian planets is predominantly controlled by their photospheres and does not depend on interior conductivity. Both conclusions are consistent with either the simple adiabatic cooling model or with more complicated gravitational unmixing models of the type discussed by Stevenson and Salpeter (1977).

Hubbard, W. B.↗
NASA NTRS1980

Formation of complex impact craters - Evidence from Mars and other planets

An analysis of the depth vs diameter data of Arthur (1980), is given along with geomorphic data for 73 Martian craters. The implications for the formation of complex impact craters on solid planets is discussed. The analysis integrates detailed morphological observations on planetary craters with geologic data from terrestrial meteorite and explosion craters. The simple to complex transition for impact craters on Mars appears at diameters in the range of 3 to 8 km. Five features appear sequentially with increasing crater size, flat floors, central peaks and shallower depths, scalloped rims, and terraced walls. This order suggests that a shallow depth of excavation and a rebound mechanism have produced the central peaks, not centripetal collapse and deep sliding. Simple craters are relatively uniform in shape from planet to planet, but complex craters vary considerably. Both the average onset diameter for complex impact craters on Mars and the average depth of complex craters vary inversely with gravitational acceleration on four planets.

Pike, R. J.↗
NASA NTRS1981

Earthlike planets: Surfaces of Mercury, Venus, earth, moon, Mars

The surfaces of the earth and the other terrestrial planets of the inner solar system are reviewed in light of the results of recent planetary explorations. Past and current views of the origin of the earth, moon, Mercury, Venus and Mars are discussed, and the surface features characteristic of the moon, Mercury, Mars and Venus are outlined. Mechanisms for the modification of planetary surfaces by external factors and from within the planet are examined, including surface cycles, meteoritic impact, gravity, wind, plate tectonics, volcanism and crustal deformation. The origin and evolution of the moon are discussed on the basis of the Apollo results, and current knowledge of Mercury and Mars is examined in detail. Finally, the middle periods in the history of the terrestrial planets are compared, and future prospects for the exploration of the inner planets as well as other rocky bodies in the solar system are discussed.

Murray, B.↗
NASA NTRS1981

Interiors of the giant planets

Unlike the terrestrial planets, the giant planets - Jupiter, Saturn, Uranus, and Neptune - have retained large amounts of the carbon, nitrogen, and oxygen compounds that were present in their zone of formation. A smaller fraction of the available hydrogen and helium was retained. The distribution and relative amounts of these components in the interiors of the Jovian planets can be inferred from theoretical and experimental data on equations of state and from the planets' hydrostatic equilibrium response to rotation.

Hubbard, W. B.↗
NASA NTRS1980

On the early global melting of the terrestrial planets

Attention is given to all the mechanisms currently known which might have been responsible for the melting of the moon. A comparison is conducted of the terrestrial planets and the moon, taking into account the conditions in the early solar system as a function of time. The assumption is made that the terrestrial planets and the moon all reached sizes comparable to their present sizes at the same time, so that the same heating mechanisms were operating on all of the bodies when their early crusts were formed. A chronology of the early solar system is established in order to evaluate the time periods during which each of the mechanisms might have been active. If the moon and planets formed within 2,000,000 years after the formation of the protosun, the conclusion appears inescapable that all of the terrestrial planets melted.

Hostetler, C. J.↗
NASA NTRS1982

The accretional heating of the terrestrial planets - A review

Accretional heating in forming planets results from the transfer of kinetic energy of objects striking the proto-planet surface. By accounting for all energy transfer for every cratering event it is theoretically possible to determine the thermal state of newly-formed planets. Various models of the thermodynamics of planetary formation are presented, ranging from the simple to the complex. Few definite conclusions can be drawn from the theoretical models, except that larger terrestrial planets were melted by their formation, cores being formed during formation. Mars may have been extensively heated, core formation in that case being contemporaneous with accretion. Mercury was unlikely to have been subject to much bombardment by planetesimals from other zones, and its core may not have its origin in accretional heat. The moon may have been completely or slightly melted, depending on the nature of the late formation of the earth and Venus.

Ransford, G. A.↗
NASA NTRS1985

Use of the synoptic view: Examples from Earth and other planets

Space technology has added the synoptic view to other techniques used in geomorphology. Synoptic views are provided by spacecraft images or by application of space technology to time-honored information systems. Examples of spacecraft images of Earth are LANDSAT, SEASAT, and the SIR (Shuttle Imaging Radar) series. Examples of applied space technologies include the digital conversion of topographic maps to shaded relief maps and digital correlation methods. From the study of other planets we have learned that synoptic views enable the deciphering of a planet's history: large features are identified and mapped before small ones; studies proceed from the general to the specific. On Earth, we generally recognize smaller features and study specific processes first, then extrapolate toward larger features and a general synthesis. With the advent of space images of Earth, perhaps the time is ripe to employ the methods used for other planets to the study of terrestrial geology and geomorphology. The following examples illustrate the use of regional-scale studies on Earth: the application of synoptic-view images in Antarctica, the use of digital methods and correlations of multiple data sets in regional studies, and some benefits to our understanding of terrestrial geology that have been obtained from analyses of other planets.

Lucchitta, B. K.↗
NASA NTRS1985

Models of the millimeter-centimeter spectra of the giant planets

In the present comparison of various model atmosphere calculations with data for all four giant planets, the absorption profile of ammonia at centimeter wavelengths was assumed to be represented by a Ben Reuven (1966) line profile; this permitted the derivation of information on the vertical distribution of ammonia in these planets' atmospheres. It appeared that ammonia must be depleted in the upper atmospheres of all four planets by a factor of 4-5 with respect to the solar abundance for Jupiter and Saturn, and by a factor of 100-200 for Uranus and Neptune. Given the vertical ammonia distribution in the atmospheres, as derived from the cm-data, a best fit to mm-data on all four planets was found by changing the high frequency tail of the ammonium lineshape profile.

De Pater, I.↗
NASA NTRS1987

Accumulation of the planets

In modeling the accumulation of planetesimals into planets, it is appropriate to distinguish between two stages: an early stage, during which approximately 10 km diameter planetesimals accumulate locally to form bodies approximate 10 to the 25th g in mass; and a later stage in which the approximately 10 to the 25th g planetesimals accumulate into the final planets. In the terrestrial planet region, an initial planetesimal swarm corresponding to the critical mass of dust layer gravitational instabilities is considered. In order to better understand the accumulation history of Mercury-sized bodies, 19 Monte-Carlo simulations of terrestrial planet growth were calculated. A Monte Carlo technique was used to investigate the orbital evolution of asteroidal collision debris produced interior to 2.6 AU. It was found that there are two regions primarily responsible for production of Earth-crossing meteoritic material and Apollo objects. The same techniques were extended to include the origin of Earth-approaching asteroidal bodies. It is found that these same two resonant mechanisms predict a steady-state number of Apollo-Amor about 1/2 that estimated based on astronomical observations.

Wetherill, G. W.↗
NASA NTRS1986

Accumulation of the terrestrial planets and implications concerning lunar origin

In order to provide a context for understanding lunar formation, 28 new three-dimensional simulations of terrestrial planet formation from a gas-free planetesimal swarm have been carried out. The natural orbital and collisional evolution of 500 initial planetesimals ranging in mass from 5.7 x 10 to the 24th g to 1.1 x 10 to the 26th g is followed until only final planets in noncrossing orbits remain. The results are in general agreement with the number, size, and orbits of the observed terrestrial planets, but also show considerable variation of stochastic origin. These results are combined with 11 simulations using 500 bodies of equal initial mass presented earlier, as well as with some other numerical studies, to conclude that for a wide range of initial conditions, terrestrial planet accumulation was characterized by giant impacts, ranging in mass up to 3 times the mass of Mars, at typical impact velocities of about 9 km/sec. These large planetesimals and the impacts they produce are sufficient to explain the unexpectedly large angular momentum of the earth-moon system.

Wetherill, G. W.↗
NASA NTRS1987

Evolutionary tracks of the terrestrial planets

On the basis of the model proposed by Matsui and Abe (1986) it is shown that two major factors - distance from the sun and the efficiency of retention of accretional energy - control the early evolution of the terrestrial planets. A diagram of accretional energy versus the optical depth of a protoatmosphere provides a means to follow the evolutionary track of surface temperature of the terrestrial planets and an explanation for why the third planet in our solar system is an 'aqua' planet.

Matsui, Takafumi↗
NASA NTRS1987

Spectral differential imaging detection of planets about nearby stars

Direct ground-based optical imaging of planets in orbit about nearby stars may be accomplished by spectral differential imaging using multiple passband acoustooptic filters with a CCD. This technique provides two essential results. First, it provides a means to modulate the stellar flux reflected from a planet while leaving the flux from the star and other sources in the same field of view unmodulated. Second, spectral differential imaging enables the CCD detector to achieve a sufficiently high dynamic range to locate planets near a star in spite of an integrated brightness differential of 5 x 10 to the 8th. Spectral differential imaging at nearby diffraction limited imaging conditions with telescope apodization can reduce the time to conduct a sensitive planetary search to a few hours in some cases. The feasibility of this idea is discussed here and shown to provide, in principle, the discrimination and sensitivity to detect a Jovian-class planet about stars at distances of about 10 parsecs. The detection of brown dwarfs is shown to be feasible as well.

Smith, W. Hayden↗
NASA NTRS1989

Theories of the origin and evolution of the giant planets

Following the accretion of solids and gases in the solar nebula, the giant planets contracted to their present sizes over the age of the solar system. It is presently hypothesized that this contraction was rapid, but not hydrodynamic; at a later stage, a nebular disk out of which the regular satellites formed may have been spun out of the outer envelope of the contracting giant planets due to a combination of total angular momentum conservation and the outward transfer of specific angular momentum in the envelope. If these hypotheses are true, the composition of the irregular satellites directly reflects the composition of planetesimals from which the giant planets formed, while the composition of the regular satellites is indicative of the composition of the less volatile components of the outer envelopes of the giant planets.

Pollack, J. B.↗
NASA NTRS1990

An assessment of the conditions for critical velocity ionization at the weakly magnetized planets

It has been proposed that critical velocity ionization (CVI) may occur in the magnetosheaths of weakly magnetized planets where the solar wind flows through the planets' upper atmospheres. One can examine this possibility for Venus and Mars by using magnetosheath flow and exosphere models to determine whether the criteria for CVI are met. These criteria include Brenning's empirical condition on the cross-field flow velocity (it must not exceed the local magnetosonic velocity) and the 'Townsend condition' requiring that the integrated probability of impact ionization by a hot electron be greater than 1 along a streamline past the planet. Application of the Spreiter-Stahara gas dynamic flow model and the Nagy and coworkers' models for the exospheres lead to the conclusion that the conditions can be met in a limited region near Venus. However, evidence for CVI has not been identified in in-situ data. Since key details of the planet-solar wind interaction can be explained in terms of photoionization alone, other signatures of the process must be sought in the observations if the presence of CVI is to be positively inferred.

Luhmann, J. G.↗
NASA NTRS1989

Which stars have planets?

The question of predicting which stars have planetary systems is discussed, with special attention given to the work of Nakano (1988), who combined scaling arguments with his theory (Nakano, 1987) of formation of planets in the solar system to estimate planetary growth rates around stars of various masses. It is argued that, in addition to stellar mass, the process of planet formation depends on other parameters, such as angular momentum and collisions. The existence of binary and multiple star systems of varying type demonstrates that angular momentum variations can play a crucial role, while the fact that the solar system has four giant planets and four terrestrial planets is considered to be due to the effects of random impacts and scatterings. It is concluded that, at present, the concept of determining what types of planetary systems are to be expected about stars of varying masses cannot be resolved.

Lissauer, J. J.↗
NASA NTRS1991

Evolution of ore deposits on terrestrial planets

Ore deposits on terrestrial planets materialized after core formation, mantle evolution, crustal development, interactions of surface rocks with the hydrosphere and atmosphere, and, where life exists on a planet, the involvement of biological activity. Core formation removed most of the siderophilic and chalcophilic elements, leaving mantles depleted in many of the strategic and noble metals relative to their chondritic abundances. Basaltic magma derived from partial melting of the mantle transported to the surface several metals contained in immiscible silicate and sulfide melts. Magmatic ore deposits were formed during cooling, fractional crystallization and density stratification from the basaltic melts. Such ore deposits found in earth's Archean rocks were probably generated during early histories of all terrestrial planets and may be the only types of igneous ores on Mars. Where plate tectonic activity was prevalent on a terrestrial planet, temporal evolution of ore deposits took place. Repetitive episodes of subduction modified the chemical compositions of the crust and upper mantles, leading to porphyry copper and molybdenum ores in calc-alkaline igneous rocks and granite-hosted tin and tungsten deposits. Such plate tectonic-induced mineralization in relatively young igneous rocks on earth may also have produced hydrothermal ore deposits on Venus in addition to the massive sulfide and cumulate chromite ores associated with Venusian mafic igneous rock. Sedimentary ore deposits resulting from mechanical and chemical weathering in reducing atmospheres in Archean earth included placer deposits (e.g., uraninite, gold, pyrite ores). Chromite, ilmenite, and other dense unreactive minerals could also be present on channel floors and in valley networks on Mars, while banded iron formations might underlie the Martian northern plains regions. As oxygen evolved in earth's atmosphere, so too did oxide ores. By analogy, gossans above sulfide ores probably occur on Mars, but not submarine ferromanganese nodules and crusts which have precipitated in oxygenated seawater on earth.

Burns, R. G.↗
NASA NTRS1990

Statistics of close approaches between asteroids and planets - Project Spaceguard

A data base of close approaches to the major planets has been generated via numerical integrations for a large number of planet-crossing asteroid orbits over the course of 200,000 yr; these data are then applied to such statistical theories as those of Kessler (1981) and Wetherill (1967). Attention is given to the orbits of the Toro-class asteroids, which violate the assumption of a lack of mean motion resonance locking between target planet and asteroid. A modified form of the Kessler theory is proposed which can address the problem of approaches between orbits that are either nearly coplanar or nearly tangent. A correlation analysis is used to test the assumption that the orbital elements of a planet-crossing orbit change solely due to close approaches.

Milani, A.↗