Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “planetesimal”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Impact constraints on the environment for chemical evolution and the continuity of life

The moon and the earth were bombarded heavily by planetesimals and asteroids that were capable of interfering with chemical evolution and the origin of life. This paper explores the frequency of giant terrestrial impacts able to stop prebiotic chemistry in the probable regions of chemical evolution. The limited time available between impacts disruptive to prebiotic chemistry at the time of the oldest evidence of life suggests the need for a rapid process for chemical evolution of life. On the other hand, rapid chemical evolution in cloud systems and lakes or other shallow evaporating water bodies would have been possible because reactants could have been concentrated and polymerized rapidly in this environment. Thus life probably could have originated near the surface between frequent surface-sterilizing impacts. There may not have been continuity of life depending on sunlight because there is evidence that life, existing as early as 3.8 Gyr ago, may have been destroyed by giant impacts. The first such organisms on earth were probably not the ancestors of present life.

Oberbeck, Verne R.↗
NASA NTRS1991

Variations in magmatic processes among igneous asteroids

Six asteroid classes (types V, E, A, R, M, S) are composed primarily of differentiated assemblages produced by igneous processes within their parent planetesimals. These are identified by surface materials which deviate from a chondritic composition to a degree that require igneous chemical fractionation processes. There are large variations among these igneous asteroids in the peak temperatures attained, in the efficiency of magmatic phase separation, and in the depth within the original parent body exposed at the present surface. These variations provide important constraints on the nature of asteroidal heating events, on the differentiation processes within small planetary bodies, and on the disruption of those parent bodies. Variations due to depth within the parent body and due to degree of magmatic differentiation are detailed.

Gaffey, M. J.↗
NASA NTRS1991

A CCD comparison of outer Jovian satellites and Trojan asteroids

The eight small outer Jovian satellites are not as well known as the brighter, more illustrious Galilean satellites. They are divided into two groups, each containing four satellites; the inner group travels in prograde orbits while the outer group travels in retrograde orbits. From the distinct orbital characteristics of the two groups, most of the theories of their origin involve the capture and breakup of two planetesimals upon entry into the atmosphere of proto-Jupiter. Their proximity to the Trojans asteroids has led to conjectures of a link between them and the Trojans. However, Tholen and Zellner (1984) found no red spectrum among six of the satellites and postulated that they were all C-type objects; therefore, they were unlikely to be derivatives of the Trojan population. Charge-coupled device (CCD) photometry and spectroscopy of the eight outer Jovian satellites obtained from 1987 to 1989 and a comparison between these eight satellites and the Trojan asteroids are presented.

Luu, Jane X.↗
NASA NTRS1990

The formation and structure of fluffy cometary nuclei from random accumulation of grains

It is now generally accepted that solid bodies in the solar system are formed by accumulation of solid grains. The mechanism of formation of smaller bodies, less than some tens of kilometers in radius, is investigated. These include planetesimals and comets. Results indicate that essentially complete agglomeration occurs for relative velocities of less than about 1000 cm/sec and fairly efficient mass build-up occurs up to 5000/cm. These velocities are consistent with current models of the primordial solar nebula. The temperature produced in a collision will be under 20 K. For ice/dust grains, the resultant aggregates will be of low density with their volatiles preserved. Cometary nuclei with radii of a few km will be irregularly shaped and fragile with low mean density and significant interanl density variations. The characteristics of such a structure could account for the observed properties of comets.

Donn, B. D.↗
NASA NTRS1990

Disentangling nebular and asteroidal features of CO3 carbonaceous chondrite meteorites

Ten carbonaceous CO3 chondrites (including four chondrites from Antarctica and the Colony, Isna, Kainsaz, Lance , Ornans, and Warrenton chondrites) were analyzed with respect to mean compositions of olivines and low-Ca pyroxenes in order to distinguish the primary nebular features of these chondrites from the secondary (asteroidal or nebular) features. In three of the Antarctic chondrites, matrices and metal grains were analyzed in order to investigate the origin of mineralogical trends in the CO3 sequence. Based on these results, the CO3 chondrites were classified into subtypes 3.0-3.7. In the silicates of chondrites ALH A77307 and Colony, classified as type 3.0, metamorphic effects appear to be absent. Chemical and mineralogical studies suggest that the type 3.1 to 3.7 CO chondrites represent a metamorphic sequence that formed from material closely resembling type 3.0 CO chondrites by metamorphism in one or more planetesimals or asteroids, not by interactions between chondritic ingredients in the solar nebula.

Scott, Edward R. D.↗
NASA NTRS1991

Mapping the stability field of Jupiter Trojans

Jupiter Trojans are a remnant of outer solar system planetesimals captured into stable or quasistable libration about the 1:1 resonance with the mean motion of Jupiter. The observed swarms of Trojans may provide insight into the original mass of condensed solids in the zone from which the Jovian planets accumulated, provided that the mechanisms of capture can be understood. As the first step toward this understanding, the stability field of Trojans were mapped in the coordinate proper eccentricity, e(sub p), and libration amplitude, D. To accomplish this mapping, the orbits of 100 particles with e(sub p) in the range of 0 to 0.8 and D in the range 0 to 140 deg were numerically integrated. Orbits of the Sun, the four Jovian planets, and the massless particles were integrated as a full N-body system, in a barycentric frame using fourth order symplectic scheme.

Levison, H. F.↗
NASA NTRS1991

Impact and collisional processes in the solar system

As impact cratered terrains have been successively recognized on certain planets and planetary satellites, it has become clear that impact processes are important to the understanding of the accretion and evolution of all solid planets. The noble gases in the normalized atmospheric inventories of the planets and the normalized gas content of meteorites are grossly similar, but demonstrate differences from each other which are not understood. In order to study shock devolatilization of the candidate carrier phases which are principally thought to be carbonaceous or hydrocarbons in planetesimals, experiments were conducted on noble gase implantation in various carbons: carbon black, activated charcoal, graphite, and carbon glass. These were candidate starting materials for impact devolatilization experiments. Initial experiments were conducted on vitreous amorphous carbon samples which were synthesized under vapor saturated conditions using argon as the pressurizing medium. An amino acid and surface analysis by laser ionization analyses were performed on three samples of shocked Murchison meteorite. A first study was completed in which a series of shock loading experiments on a porous limestone and on a non-porous gabbro in one and three dimensions were performed. Also a series of recovery experiments were conducted in which shocked molten basalt a 1700 C is encapsulated in molybdenum containers and shock recovered from up to 6 GPa pressures.

Ahrens, Thomas J.↗
NASA NTRS1991

Infrared spectral studies of asteroids

The research objective was to improve the understanding of the surface mineralogy of asteroids and to link the vast existing body of meteorite geochemical data with specific astronomical objects which may be the targets of future NASA missions. The methodology used is as follow: (1) to use advanced astronomical instrumentation to obtain reflection spectra in the 0.3 to 5.2 micron wavelength range of selected asteroids; (2) to compare the asteroid data with similar data on simulated asteroid regoliths of various compositions to determine the surface mineralogy and meteoritic affinities of asteroid spectral classes and specific asteroids; (3) to integrate the mineralogical information with other astronomical data, orbital dynamics studies, and meteoritic geochemistry data to reconstruct the condensational, thermal, and collisional history of the present asteroids and their parent planetesimals; and (4) to use the information obtained to assist planning of future NASA asteroid missions such as Galileo and CRAF.

Bell, Jeffrey F.↗
NASA NTRS1992

The early Martian atmosphere

Atmospheric noble gases on Mars, Earth, and Venus are severely and variably depleted with respect to solar abundances, and are characterized by isotopic signatures that differ from planet to planet and from meteoritic and inferred solar compositions. Earlier theories based on simple accretion of meteoritic veneers as planetary volatile sources are unable to replicate these distributions, and so attention is now focusing on evolutionary processes that can fractionate both elements and isotopes, operating either on the early planets themselves or in large preplanetary planetesimals.

Pepin, Robert O.↗
NASA NTRS1992

Thermal evolution and chemical differentiation of the terrestrial magma ocean

The release of gravitational energy resulted in global melting and formation of a magma ocean during accretion of the Earth. Although it is believed that the formation of the magma ocean resulted in gravitational differentiation of melt and solid, the differentiation might be disturbed by the following processes: (1) convective mixing; (2) cooling and solidification; and (3) growth of the earth, which results in secular increase of pressure, and stirring by planetesimal impacts. The purpose of this study is to investigate the differentiation processes of the terrestrial magma ocean by taking into account various disturbing processes.

Abe, Y.↗
NASA NTRS1992

Evidence for magma oceans on asteroids, the moon, and Earth

There are sound theoretical reasons to suspect that the terrestrial planets melted when they formed. For Earth, the reasons stem largely from the hypothesis that the moon formed as a result of the impact of a Mars-sized planetesimal with the still accreting Earth. Such a monumental event would have led to widespread heating of the Earth and the materials from which the moon was made. In addition, formation of a dense atmosphere on the Earth (and possibly the Moon) would have led to retention of accretional heat and, thus, widespread melting. In other words, contemporary theory suggests that the primitive Moon and terrestrial planets had magma oceans.

Taylor, G. Jeffrey↗
NASA NTRS1992

Impact erosion of terrestrial planetary atmospheres

I review current ideas about the nature of the planetesimals - composition, size distribution, and the planetary encounter velocity. Previous papers on accretion and erosion of planetary atmospheres as a result of multiple impacts are reviewed. Finally, the effects of blowing off a substantial fraction of the atmosphere from a terrestrial planet due to a single giant body impact are discussed.

Ahrens, Thomas J.↗
NASA NTRS1992

The first identification of a mainbelt asteroid source for a meteorite Class E-type Apollo asteroid (3103) 1982BB: A link between the Hungaria asteroid zone and the Aubrite meteorites

One of the major goals of asteroid investigators has been to determine the source bodies of the meteorites, and, thus, to provide the needed spatial context. However, despite careful investigation of many of these asteroids, no specific mainbelt asteroidal parent bodies for particular meteorites have yet been identified. Results are reported that represent the first direct link of a set of meteorites (the aubrites or enstatite achondrites) back to a particular source region in the asteroid belt (the Hungaria zone of the innermost belt located at a heliocentric distance of about 1.9 AU (1.9 times the Earth's distance from the Sun)). Since most asteroids in the belt are still located near their original heliocentric formation distances, this link allows the composition and temperature of the solar nebula to be determined at a specific distance from the Sun during the time of planetesimal formation. Orbital considerations suggest that 3103 could be derived from the Hungaria zone. However, the spectral gype of 3103 makes such a source probable. The Hungaria zone is the only region of the asteroid belt where E-type asteroids are common. E-type objects constitute about 55 percent of the classified Hungaria population. Although a more detailed investigation of the dynamical evolution of this object is needed, it appears highly probable that 3103 1982BB was derived from the Hungaria region of the innermost asteroid belt, that it is a fragment of the collisional breakup of an E-object near 1.9 AU, and escaped the mainbelt via the nu sup 16 resonance.

Gaffey, M. J.↗
NASA NTRS1991

Formation of a planet orbiting pulsar 1829 - 10 from the debris of a supernova explosion

How the 10-earth mass planet in a nearly circular 0.7 AU orbit around PSR1829 - 10 might have been created inside the young SNR is described. It is proposed that the planet formed from a rotationally supported disk of about 0.02 solar mass of heavy elements that fell back from the supernova explosion to an initial radius of about 1000 km. Viscous evolution of the disk then concentrated most of its angular momentum into a small amount of material at the disk's outer extremity: 10 earth masses at 10 exp 13 cm. Here, dust grains that had condensed and precipitated toward the midplane grew through cohesive collisions and gravitational instabilities into 100-km planetesimals which coagulated into the planet on a million-yr time scale. The presence of a more massive and more distant second planet is found to be unlikely.

Lin, D. N. C.↗
NASA NTRS1991

The random component of planetary rotation

In the present computation of the rms rotational angular momentum accumulated by a planet as a result of the stochastic off-center impacts of large planetesimals over the course of the accretionary epoch, planets' observed spin periods and obliquities were used to estimate the masses of the largest bodies which impacted each planet during its history. Several planets have likely undergone at least one collisional event with a body having over 5 percent of the planet's ultimate mass, thereby supporting giant-impact theories of the origins of the moon and Charon as well as the explanation of Mercury's excess Fe in terms of the partial collisional stripping of its mantle.

Lissauer, Jack J.↗
NASA NTRS1991

Impact experiments. III - Catastrophic fragmentation of aggregate targets and relation to asteroids

An experimental study is conducted of collisions employing artificial aggregates as targets, in order to ascertain the fragmentation characteristics of weakly bonded bodies that consist of stronger constituent particles, such as loosely aggregated planetesimals. It is established that weakly bonded aggregate bodies may exhibit unexpectedly high impact strengths due to constituent particles' energy dissipation. The cumulative size distributions of fragments that are larger than the constituent grains form power-law segments resembling those formed by the fragmentation of homogeneous targets.

Ryan, Eileen V.↗
NASA NTRS1992

Cometary origin of carbon and water on the terrestrial planets

An early high-temperature phase of the protosolar accretion disk is implied by at least three different telltales in chondrites and confirmed by peculiarities in the dust grains of Comet Halley. The existence of this high-temperature phase implies a large accretion rate hence a massive early disk. This clarifies the origin of the Kuiper Belt and of the Oort cloud, those two cometary populations of different symmetry that subsist today. Later, when the dust sedimented and was removed from the thermal equilibrium with the gas phase, a somewhat lower temperature of the disk explains the future planets' densities as well as the location beyond 2.6 AU of the carbonaceous chondrite chemistry. This lower temperature remains however large enough to require an exogenous origin for all carbon and all water now present in the earth. The later orbital diffusion of planetesimals, which is required by protoplanetary growth, is needed to explain the origin of the terrestrial biosphere (atmosphere, oceans, carbonates and organic compounds) by a veneer mostly made of comets.

Delsemme, A. H.↗
NASA NTRS1992

Apollo 15 impact melts, the age of Imbrium, and the Earth-Moon impact cataclysm

The early impact history of the lunar surface is of critical importance in understanding the evolution of both the primitive Moon and the Earth, as well as the corresponding populations of planetesimals in Earth-crossing orbits. Two endmember hypotheses call for greatly dissimilar impact dynamics. One is a heavy continuous (declining) bombardment from about 4.5 Ga to 3.85 Ga. The other is that an intense but brief bombardment at about 3.85 +/- Ga was responsible for producing the visible lunar landforms and for the common 3.8-3.9 Ga ages of highland rocks. The Apennine Front, the main topographic ring of the Imbrium Basin, was sampled on the Apollo 15 mission. The Apollo 15 impact melts show a diversity of chemical compositions, indicating their origin in at least several different impact events. The few attempts at dating them have generally not produced convincing ages, despite their importance. Thus, we chose to investigate the ages of melt rock samples from the Apennine Front, because of their stratigraphic importance yet lack of previous age definition.

Ryder, Graham↗