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At least 55 records · Page 3

Comparative studies of lunar, Martian, and Mercurian craters and plains

The spatial distribution of lunar smooth plains is not consistent with experimental simulations of melt rock emplacement during cratering in layered materials. Nor is it consistent with the location of melt rocks (suevite) near the Ries basin. Lunar smooth plains surrounding Imbrium are most extensive in areas where pre-existing craters are most degraded. This observation suggests that plains form by impact of basin and local primary crater ejecta, together with deposition of debris excavated by the resultant secondary cratering events. Craters within the belt of smooth plains surrounding the Caloris basin on Mercury are most degraded nearest the basin; this suggests that Mercurian smooth plains must, at least in part, be emplaced in a manner similar to plains surrounding the Imbrium basin. Mercurian uplands have a primary crater population deficient in small crater diameters (less than approximately 30 km). Lunar uplands far from major basins also have a crater population deficient in small crater sizes. Martian cratered terrain exhibits a similar crater deficiency, which was previously interpreted as due to obliteration of small craters (less than approximately 30 km) by some surface process. A crater size distribution deficient in small sizes (less than approximately 30 km) on the Mercurian, lunar, and Martian uplands has implications for the origin of debris bombarding the inner solar system during the period recorded by these surfaces. It is proposed that during late heavy bombardment, the inner solar system was inundated with bodies that broke up under tidal fission as they approached the planets. Such a mechanism would lend to production of a crater population deficient in small crater sizes, and it would also explain the large degree of spatial clustering of primary craters on Mercury, the moon, and Mars.

Oberbeck, V. R.

Crater populations in the early history of Mercury

Crater populations on two major geologic units of Mercury have been classified into three morphologic types which characterize their state of degradation. The results indicate that one or more processes either prior to or contemporary with the formation of the 1300-km-diameter Caloris Planitia reduced the population of fresh craters smaller than 70 to 80 km in diameter and totally erased the population of fresh craters smaller than 20 to 30 km.

Guest, J. E.

Crew health status and monitoring summary - The second manned Skylab mission

The evaluation of available medical data to determine the health status of the Skylab 3 crew members is discussed. The utilization of raw data from medical experiments during the course of the mission as a source of near-real time criteria for monitoring crew health is described. The percentage change of selected metabolic parameters, including calorie intake, Na intake, K intake, urine output, water intake, and body weight, from their pre-flight values, vectorcardiographic data, and results of the lower body negative pressure experiment provided a core of information upon which clinical judgements could be made. Minor clinical problems occurring during the mission are outlined.

Buchanan, P.

Can internal heat contribute to the high surface temperature of Venus

It is shown that current observational data on conditions at the Venusian surface are adequate to exclude the possibility that thermal energy from a hot interior contributes to the high surface temperature of the planet. The maximum energy flux conducted from the interior is estimated by assigning a maximum thermal conductivity and a minimum thickness to the crust. It is found that the crust must be at least 10 km thick and that the maximum thermal flux for this thickness is about 6 millionths of a calorie per sq cm/sec. The relative importance of this internal energy source is assessed by comparing it with the amount of solar energy deposited at the surface. The result demonstrates that the absorbed solar energy is at least one and probably two orders of magnitude higher than the minimum flux conducted from the interior. It is concluded that a very efficient atmospheric trapping mechanism (the greenhouse effect) is operating on Venus.

Morrison, D.

The effect of direct heating and cooling of heat regulation centers on body temperature

Experiments were done on 28 rabbits in which puncture instruments were left in the brain for 1-2 days until the calori-puncture hyperthermia had passed and the body temperature was again normal. The instrument remaining in the brain was then used as a galvanic electrode and a second fever was produced, this time due to the electrical stimulus. It was concluded that heat is a centrally acting antipyretic and that cold is a centrally acting stimulus which produces hyperpyrexia cold-induced fever.

Barbour, H. G.

Global tectonics of Mercury and the moon

Lobate scarps on Mercury have been studied to determine the nature of the surface stress history and implications for the planet's early tectonic history. Morphologic and transection relations indicate that most Mercurian scarps are tectonic in nature and are due to compressive stresses in the surface layer. The azimuthal distribution of lobate-scarp trends is compatible with an early predominantly compressive global stress field due to thermal shrinkage of the planet. Superposition relations indicate that the contractive phase was largely a pre-Caloris process. The effects of stresses due to planetary despinning were either negligible, predate the scarps, or were largely obscured by cratering or volcanism. The tectonic history of Mercury as recorded in the lobate scarps is different from that which caused the system of lineaments on the moon.

Cordell, B. M.

Rayed craters on the moon and Mercury

Systems of bright rays are exhibited by many fresh craters on the moon and Mercury. Diameter/density distributions suggest that lunar-rayed craters represent the Class 1 craters, and that Mercurian rayed craters represent post-Caloris craters. Photogeological analyses of lunar imagery indicate that the ray systems are composed of finely divided material from the primary crater along with locally derived ejecta from secondary and tertiary craters. The primary ray material probably occurs in moderately thick (0.1-1 meter) deposits. The rate of darkening may depend more on the thickness of the ray material than on the rates of various darkening processes. Darkening rate may also be a function of crater size. It is observed that rays of craters more than 1 b.y. old remain bright, whereas those older than Class 1 generally fade to imperceptibility.

Allen, C. C.

Mercurian global tectonics - A consequence of tidal despinning

The surface of Mercury exhibits a global tectonic system consisting of an ancient set of NE and NW tending lineaments and a younger set of planimetrically arcuate escarpments interpreted as thrust of high-angle reverse faults. The trends, distribution, and age relations of these tectonic features can be explained by a combination of tidal despinning and global contraction of the planet. In our model, early tidal despinning resulted in conjugate shear fractures trending roughly N60 deg E and N60 deg W which were subsequently modified by a variety of surface processes to produce the presently visible set of lineaments. Continued despinning plus global contraction produced thrust faults with roughly north-south trends. Final contraction may have postdated despinning and produced randomly oriented thrust faults. All of these events predated the formation of Caloris basin, because basin-associated deposits blanket both lineaments and arcuate thrust faults.

Melosh, H. J.

The tectonic and volcanic history of Mercury as inferred from studies of scarps, ridges, troughs, and other lineaments

Tectonic and volcanic modification of the Mercurian surface is discussed, and nine landform classes are defined. An evolutionary chronology, based on reported interpretations of scarp, ridge, trough, and other lineament features, is presented, and the roles (in chronological order) of accretion and differentiation, tidal spindown, plains volcanism, heavy bombardment, cooling/contraction, Caloris impact, intense surface modification, basin subsidence, local plains volcanism, isostatic basin uplift, and light cratering are considered. The observational data were obtained by Mariner 10.

Dzurisin, D.

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.

The intercrater plains of Mercury and the Moon: Their nature, origin and role in terrestrial planet evolution. Constuction of the paleogeologic maps

The Post Caoris surface was derived from the geologic map by plotting all Class 1 and 2 features. To construct the Caloris surface, Class 3 craters were plotted onto the map, as well as all Class 3 plains. However, if P3 plains were adjacent to P2 units, and appeared continuous with other exposures of P3 material, the P2 unit was assumed to overlie the C3 and P3 material. The younger superposed craters were ignored with respect to the Class 3 surface. The boundaries of P3 materials were then continued under the superposed units, using a minimum of reasonable assumptions. For instance, if P2 and P4 plains were adjacent units, no P3 plains were presumed to lie under the P2 material. Similarly, all C3 craters were considered to have some deposits of impact melt after formation, even if they are mapped containing younger units. C3 craters which were superposed with younger units, C1 or C2 craters, and perhaps P2 plains, were redrawn as if later materials had not been emplaced, i.e., in their post impact, pre-degradation states.

Leake, M. A.

The intercrater plains of Mercury and the Moon: Their nature, origin and role in terrestrial planet evolution. Chronology of surface history of Mercury

Phases in the history of the planet Mercury include: (1) condensation and accretion; (2) heating; (3) planetary expansion during heavy bombardment; (4) tidal spin-down and lineament formation; (5) P5 plains emplacement; (6) P4 plains emplacement; (7) peak planetary volume in P3 period; (8) scarp formation; (9) Caloris Basin formation, late class 3; (10) scarp formation and P2 plains formation; (11) smooth plains formation in and around large basins; (12) late or local tectonic stress; and (13) quiescent class 1 period. Although the cooling and contraction of the lithosphere are complete, the core remains molten as an active dynamo, producing the magnetic fields detected by Mariner 10. Plains produced since core formation (P3 to P-1) should record its magnetic activity. Cratering during the Class 2 and Class 1 periods is probably not enough to distribute ballistic materials and homogenize any color differences.

Leake, M. A.

The geology of the terrestrial planets

Knowledge regarding the geology of the terrestrial planets has increased considerably during the last four years. The present investigation provides a brief summary of work during these years on the geology of Mercury, Venus, moon, and Mars. Following the Mariner 10 encounter with Mercury in 1974, the geologic history of the planet was broadly outlined by Strom (1979). McCauley et al. (1981) recognized several facies of ejecta around Caloris. Hostetler and Drake (1980) showed that unless Mercury received more than 60-70 percent of its thermal energy from tidal interactions, it must have undergone early, almost global melting. Knowledge of the Venusian surface has increased substantially over the last few years both through improvement of earth-based observations and in connection with the Pioneer Venus mission. A topographic map of Venus is presented. Attention is also given to the composition and character of lunar highlands, the evolution of different maria, photographs obtained of almost the entire planet Mars, and Martian craters and volcanism.

Carr, M. H.

The dependence of reflectance spectra of Mercury on surface terrain

Reflectance spectra of Mercury covering the spectral range of 0.3-1.1 micron, obtained during 1963-1976, were examined for any correlations with surface terrain. Mercury's 6.1385-deg/day rotational rate, the phases of the planet around maximum elongations, and bidirectional-reflectance-spectroscopy theory were used to identify the surface area associated with each spectrum. Data from 1974-1975, rereduced with improved standard-star-flux ratios, show a weak absorption band in the near infrared not seen in earlier analyses. Older spectra suggest that the western longitudes of the unimaged side of Mercury are similar to the rest of the planet. Spectra of the intercrater plains in the 0-90-deg quadrant suggest a possible absorption band. Spectra of areas dominated by Caloris Basin with the encompassing smooth plains may show Fe(2+) abundances in the soil comparable to lunar-highlands soil. No striking differences between spectra of intercrater plains and smooth plains are found. The absorption features seen in spectra of Mercury are generally weaker than features seen in lunar spectra.

Vilas, F.

The determination of nutritional requirements for Safe Haven Food Supply System (emergency/survival foods)

The Space Station Safe Haven Food System must sustain 8 crew members under emergency conditions for 45 days. Emergency Survival Foods are defined as a nutritionally balanced collection of high density food and beverages selected to provide for the survival of Space Station flight crews in contingency situations. Since storage volume is limited, the foods should be highly concentrated. A careful study of different research findings regarding starvation and calorie restricted diets indicates that a minimum nutritional need close to RDA is an important factor for sustaining an individual's life in a stressful environment. Fat, protein, and carbohydrates are 3 energy producing nutrients which play a vital role in the growth and maintenance process of human life. A lower intake of protein can minimize the water intake, but it causes a negative nitrogen balance and a lower performance level. Other macro and micro nutrients are also required for nutritional interrelationships to metabolize the other 3 nutrients to their optimum level. The various options for longer duration than 45 days are under investigation.

Ahmed, Selina

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.

Detecting potassium on Mercury

A critical comment on the work of A.L. Sprague et al. (1990) is presented. It is argued that, in attributing an enhanced emission in the potassium D lines on Oct. 14, 1987 in the equatorial region of Mercury to a diffusion source centered on Caloris Basin, Sprague et al. misinterpreted the data. Sprague et al. present a reply, taking issue with the commenters.

Killen, R. M.

Source processes for the alkali metals in the atmosphere of Mercury

A large (fivefold) increase in Mercury's potassium (K) column abundance on October 14, 1987, has been reported by Sprague et al. (1990), who attributed the enhancement to diffusion through the surface in the Caloris Basin, from depths of order 10 km. The postulated source rate is much larger than any previously estimated diffusion rate, and, if true, certainly affects consideration of the origin of other atmospheric species. However, Killen et al. (1991) have pointed out that the claim is not supported by the published observations of K or sodium as a whole. Sprague et al. (1991) have responded by further hypothesizing the existence of several other sources of gas diffusing out of the regolith, all of which are time variable. In any case, the Sprague et al. data indicate large variations in abundance, and it is important to understand the cause. With this issue in mind, the available abundance estimates for correlation with possible controlling physical parameters has ben examined. A significant correlation between the average zenith K column abundance and indices of solar activity has been found.

Shemansky, D. E.