Engineering topics

Mckinnon, W. B.

Publications and source records attributed to Mckinnon, W. B..

17 records

NASA NTRS2017

The Geology of Charon as Revealed by New Horizons

Pluto's large moon Charon [radius 606 km; density = 1.70 g cm(exp. -3)] exhibits a striking variety of landscapes. Charon can be divided into two broad provinces separated by a roughly aligned assemblage of ridges and canyons, which span from east to west. North of this tectonic belt is rugged, cratered terrain (Oz Terra); south of it are smoother but geologically complex plains (Vulcan Planum). (All place names here are informal.) Relief exceeding 20 km is seen in limb profiles and stereo topography.

Charon↗

NASA NTRS1994

Some implications of large impact craters and basins on Venus for terrestrial ringed craters and planetary evolution

Approximately 950 impact craters have been identified on the surface of Venus, mainly in Magellan radar images. From a combination of Earth-based Arecibo, Venera 15/1, and Magellan radar images, we have interpreted 72 as unequivocal peak-ring craters and four as multiringed basins. The morphological and structural preservation of these craters is high owing to the low level of geologic activity on the venusian surface (which is in some ways similar to the terrestrial benthic environment). Thus these craters should prove crucial to understanding the mechanics of ringed crater formation. They are also the most direct analogs for craters formed on the Earth in Phanerozoic time, such as Chicxulub. We summarize our findings to date concerning these structures.

Mckinnon, W. B.↗

NASA NTRS1988

The tectonics of Mercury

The probable tectonic history of Mercury and the relative sequence of events are discussed on the basis of data collected by the Mariner-10 spacecraft. Results indicate that Mercury's tectonic activity was confined to its early history; its endogenic activity was principally due to a small change in the shape of its lithosphere, caused by tidal despinning, and a small change in area caused by shrinkage due to cooling. Exogenic processes, in particular the impact activity, have produced more abundant tectonic features. Many features associated with the Caloris basin are due to loading of Mercury's thick lithosphere by extrusive lavas or subsidence due to magma withdrawal. It is emphasized that tectonic features observed on Mercury yield insight into the earliest tectonic events on planets like Mars and, perhaps, the earth, where subsequent events obscured or erased the most ancient tectonic records.

Melosh, H. J.↗

NASA NTRS1985

Dark Halo Craters and the Thickness of Grooved Terrain on Ganymede

Dark halo craters on grooved terrain on Ganymede represent potential probes of the subsurface geology. These craters are surrounded by a broad, diffuse low albedo annulus, or halo and have apparently lost the bright rim and ray deposits associated with very young craters. The recognition of dark halo craters, almost all greater than 12 km in diameter, indicates that material darker and redder than grooved terrain material forms a stratigraphy horizon at about 1 km depth the present surface of grooved terrain in Uruk Sulcus. This material is most likely downdropped cratered terrain material, which only larger craters have excavated into. This is most consistent with extensional tectonic models for grooved terrain formation whereby the lithosphere is stretched and blocks of ancient cratered terrain are downdropped along bounding faults, and subsequently resurfaced by a shallow layer of relatively clean icy material. Evidence for the preservation of cratered terrain material at shallow depth beneath grooved terrain material poses great difficulties for alternative grooved and smoothed terrain formation mechanisms.

Schenk, P. M.↗

NASA NTRS1985

On the Origin of Triton and Pluto

Lyttleton's (1936) hypothesis that Triton and Pluto originated as adjacent prograde satellites of Neptune is evaluated, and it is shown that with the presently accepted masses of Triton and Pluto-Charon, the momentum and energy exchange required to sell Triton on a retrograde orbit is impossible. The Pluto-Charon system could not have acquired its present angular momentum state during an ejection event unless a physical collision was involved, which is quite unlikely. The simplest hypothesis is that Triton and Pluto are independent representatives of large outer solar system planetesimals. Triton is simply captured, with spectacular consequences that include runaway melting of interior ices and release to the surface of clathrated CH4, CO, and N2. Condensed remnants of this protoatmosphere could account for features in Triton's unique spectrum.

Mckinnon, W. B.↗

NASA NTRS1985

Impacts in Ice: Commissions of Solar System Crater Populations

The classical phases of cratering are compression, excavation, and modification. Recent theory, however, supports and alternative sequence of physical events: (1) coupling - energy and momentum are transferred, and the cratering flow field is set up; (2) power-law-growth the flow field expands, with transient cavity dimensions expressible as a simple power law function of time; and (3) late phase - some combination of gravity, strength, and viscosity limits cavity growth, initially causing a deviation from power-law behavior and eventually determining the size and shape of the final (transient) crater. The dominant mechanism that limits cavity growth defines a cratering regime and a single variable, the coupling parameter, determines the scaling laws in the various regimes. The coupling parameter is intermediate in dimensionality between energy and momentum and its exact form is determined. For simple materials with rate and scale independent strength, the transition crater diameter between the strength and gravity regimes is, within reasonable bounds, independent of velocity. Even the smallest craters observed by Voyager appear to be gravity dominated.

Mckinnon, W. B.↗

NASA NTRS1985

Three-layer Generic Ganymedes: Structure and Evolution

Accurate, three layered structural models were generated for Ganymede and Callisto. Three layered satellites consist of a rock core, a region of mixed rock and ice, and an outer shell of pure ice. This structure would result from either accretional melting of the outermost region of the satellite or differentiation subsequent to localized melting. A completely undifferentiated ice rock satellite possesses only the single, top boundary layer and, in this case, melting is also relatively difficult to initiate; this suggests that, if accretional melting results in a small degree of differentiation initially, subsequent melting may readily occur as the satellite warms due to radiogenic heating. Alternatively, if accretion is not accompanied by a small degree of differentiation, it may prove too difficult to initiate nonaccretional melting and the satellite might remain undifferentiated indefinitely.

Mueller, S. W.↗

NASA NTRS1985

Dark halo craters and the thickness of grooved terrain on Ganymede

On the basis of a morphological analysis of recent Voyager images of the dark halo craters and grooved terrain in the Uruk Sulcus region on Ganymede, it is proposed that dark halos result from the incorporation of cratered terrain material into crater ejecta. In Uruk Sulcus, only craters greater than about 12 km in diameter have halos, indicating that the cratered material there forms a stratigraphic horizon buried in the grooved terrain. A depth of excavation of cratered terrains of 1.0 to 1.6 km is calculated. It is shown that the estimated depth is consistent with current theoretical models of grooved terrain, where relatively clean ice is emplaced over downdropped blocks of cratered terrain. Several Voyager 2 images of the Uruk Sulcus region on Ganymede are provided.

Schenk, P. M.↗

NASA NTRS1985

Geology of icy satellites

The geology of the major icy satellites of Jupiter, Saturn, Uranus, and Neptune is discussed in terms of the four major processes that shape icy satellite surfaces: impact cratering, volcanism, tectonism, and interactions with planetary magnetospheres and solar radiation. The role of these processes in creating the differences that exist among the satellites, in particular the orderly progression of geological properties in the Jovian satellites, is emphasized. Important questions left open after the Voyager missions are summarized.

Mckinnon, W. B.↗

NASA NTRS1984

On the origin of Triton and Pluto

Lyttleton's (1936) hypothesis that Triton and Pluto originated as adjacent prograde satellites of Neptune is evaluated, and it is shown that with the presently accepted masses of Triton and Pluto-Charon, the momentum and energy exchange required to set Triton on a retrograde orbit is impossible. The Pluto-Charon system could not have acquired its present angular momentum state during an ejection event unless a physical collision was involved, which is quite unlikely. The simplest hypothesis is that Triton and Pluto are independent representatives of large outer solar system planetesimals. Triton is simply captured, with spectacular consequences that include runaway melting of interior ices and release to the surface of clathrated CH4, CO, and N2. Condensed remnants of this protoatmosphere could account for features in Triton's unique spectrum.

Mckinnon, W. B.↗

NASA NTRS1984

A reappraisal of Darwin's fission hypothesis and a possible limit to the primordial angular momentum of the Earth

G. H. Darwin proposed that the primordial Earth may have rotated fast enough that the solar tidal period was nearly resonant with the fundamental free oscillation period of a fluid Earth and that a large and unstable tidal oscillation split off to become the moon. Jeffreys argumented that dissipation during resonance would be sufficient to prevent such an unstable oscillation greater than the tidal frequency (period - 2.68 hr). It is considered that solar tides have extracted angular momentum from the Earth-Moon system over 4.5 b.y. The correspondence of the primordial tidal and resonant frequencies is nearly exact. (The effect of central condensation of the proto earth is to increase both frequencies by a similar amount, though the resonance is not precisely known. This result, was unknown to Darwin or Jeffreys. The effects of resonance were evaluated. The resonance is likely to be too damped for fission. This argument is more general than Jeffreys', who considered friction between the oscillating mantle and a rigid core. It is argued that the fact that Q must be so great for fission that equilibrium can not be maintained; the fluid proto Earth passes so quickly through resonance that maximum amplitude is not reached. It is suggested that solar resonant tides acted as a brake on the spin of the primordial partially molten Earth. Certain proposed origins for the Moon do not necessarily involve addition of substantial amounts of angular momentum to the Earth-Moon system. The primordial Earth-Moon system may have had nearly the same angular momentum as it has today.

Mckinnon, W. B.↗

NASA NTRS1982

Tectonic deformation of Galileo Regio and limits to the planetary expansion of Ganymede

Galileo Regio is the largest and most prominent unit of dark, ancient, heavily cratered terrain on Ganymede. Its major tectonic feature, an arcuate system of rimmed furrows, formed very early, as nearly the entire cratering record post-dates it. Galileo Regio has undergone very little subsequent structural alteration, as opposed to the rest of the planet (i.e., replacement by grooved and smooth terrain). Its survival as an intact lithospheric unit during the era of grooved terrain formation constrains the concomitant expansion of Ganymede, if any, to be less than one percent in radius. This limit is derived from an analysis which considers Galileo Regio to be a thin, freely floating elastic spherical shell on an expanding planet. A comparison of tectonic features (both endogenic and impact produced) on Ganymede and Callisto suggests that the ultimate source that powered the creation of grooved terrain lies in the Ganymedean core.

Mckinnon, W. B.↗

NASA NTRS1982

Impact into the earth's ocean floor - Preliminary experiments, a planetary model, and possibilities for detection

Impact processes and plate tectonics are invoked in an experimental study of craters larger than 100 km in diameter on the ocean floor. Although the results obtained from 22-caliber (383 m/sec) ammunition experiments using dense, saturated sand as a target medium cannot be directly scaled to large events, the phenomenology exhibited is that expected of actual craters on the ocean floor: steep, mixed ejecta plume, gravitational adjustment of the crater to form a shallow basin, and extensive reworking of the ejecta, rim, and floor materials by violent collapse of the transient water cavity. Excavation into the mantle is predicted, although asthenospheric influence on outer ring formation is not. The clearest geophysical signature of such a crater is not topography; detection should instead be based on gravity and geoid anomalies due to uplift of the Moho, magnetic anomalies, and seismic resolution of the Moho uplift and crater formation fault planes.

Mckinnon, W. B.↗

NASA NTRS1981

Application of ring tectonic theory to Mercury and other solar system bodies

It is pointed out that multiringed structures, by their presence or absence, provide a powerful tool for deciphering the thermal histories of the solid planets. The theory of ring tectonics considered by Melosh and McKinnon (1978) and Melosh et al. (1980) establishes the framework of that undertaking. The present investigation has the objective to apply this conceptualization in detail to the multiringed basins on Mercury, taking into account also a brief review concerning the current state of understanding of ring tectonics on the moon, Mars, earth, Ganymede, and Callisto. The small, icy satellites of Saturn are also discussed. The mechanics of multiple ring formation are related to the collapse of the transient basin cavity when the excavation depth and lithosphere thickness are comparable. Attention is given to the Caloris Basin on Mercury, the peak ring basins on Mercury, and the Argyre Basin on Mars.

Mckinnon, W. B.↗

NASA NTRS1980

Evolution of planetary lithospheres - Evidence from multiringed structures on Ganymede and Callisto

The thickness and viscosity of a planetary lithosphere increase with time as the mantle cools, with a thicker lithosphere leading to the formation of one (or very few) irregular normal faults concentric to the crater. Since a gravity wave or tsunami induced by impact into a liquid mantle would result in both radial and concentric extension features, which are not observed in the case of the large impact structures on Ganymede and Callisto, an alternative mechanism is proposed in which the varying ice/silicate ratios, tectonic histories, and erosional mechanisms of the two bodies are considered to explain the subtle differences in thin lithosphere ring morphology between Ganymede and Callisto. It is concluded that the present lithosphere thickness of Ganymede is too great to permit the development of any rings.

Mckinnon, W. B.↗

NASA NTRS1978

The mechanics of ringed basin formation

The study investigates the mechanics of ring formation in lunar basins by collapse or slumping. Two classes of models are presented in order to broaden the range of plausible mechanisms involved. Extrusion flow models which assume a weak layer underlying a strong layer as well as plastic flow models which assume a continuous decrease of cohesion strength with depth are considered. In both classes, strength or resistance to deformation decreases with depth. If this decrease is rapid enough, in either set of models, at least one concentric fault scarp forms. The ratio between the scarp radius and the crater radius varies between 1.2 and about 2.7 depending on the model, and is thus in the range of observed radius ratios.

Melosh, H. J.↗

NASA NTRS1978

An investigation into the role of plastic failure in crater modification

The mechanical stability of an impact crater is analyzed in this paper via a quasi-static, axisymmetric slip line theory of plasticity. The yield model incorporated is Mohr-Coulomb and a simplified rectangular profile is used for the transient cavity. The degree of stability (or instability) is described as a function of internal friction angle, depth/diameter ratio, and a dimensionless parameter. To match the observed slumping of large lunar craters the cohesion strength of the lunar surface material must be low (less than 20 bars) and the angle of internal friction must be less than 2 deg. It is not implausible that these failure strength characteristics are realized by freshly shocked rock.

Mckinnon, W. B.↗