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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Mars gravity - High-resolution results from Viking Orbiter 2

Doppler radio-tracking data have provided detailed measurements for a Martian gravity map extending from 30 deg S to 65 deg N in latitude and through 360 deg of longitude. The feature resolution is approximately 500 km, revealing a huge anomaly associated with Olympus Mons, a mascon in Isidis Planitia, and other anomalies correlated with volcanic structure. Olympus Mons has been modeled with a 600 km surface disk having a mass of 8.7 times 10 to the 21st grams.

Sjogren, W. L.↗
NASA NTRS1979

The JPL orbit determination software system

The Jet Propulsion Laboratory (JPL) orbit determination software system is described. Consisting of a set of computer programs, this system has been developed at JPL for the primary purpose of determining the flight path of deep-space mission spacecraft in NASA's planetary program. Secondary uses include science studies in celestial mechanics and extraterrestrial atmospherics. Attention is given to such features as the capability for processing 30 data types from up to 15 tracking stations, and for precise navigation to natural satellites of outer planets. In addition, the system contains models for such conditions as mascons, atmospheric drag, attitude control, and finite motor burns, to provide for extensive solve-on capabilities.

Ekelund, J. E.↗
NASA NTRS1980

Future lunar gravity measurements

Future missions to the moon should include a detailed high-resolution global gravity survey from a low (15-30 km) polar orbiting spacecraft. The use of gravity gradiometer instruments on board the spacecraft will give higher-resolution data at lower total mission cost than the present Doppler tracking technique. Simulations show that although a three axis gradiometer system is preferred, and can even be used to estimate spacecraft attitude and altitude variation, a properly oriented single rotating gravity gradiometer can be used to resolve closely spaced mascons in both the along-track and cross-track directions.

Forward, R. L.↗
NASA NTRS1980

Ridge systems of Caloris - Comparison with lunar basins

It has been found that the wrinkle ridge systems of the Caloris basin on Mercury display many of the traits which are characteristic of ridges in the mare-filled lunar multiring basins. The considered investigation is concerned with the ridge systems within the Caloris basin on Mercury, and implications for the origin of ridges within basins on both Mercury and the moon. The observed features are found to indicate that the early evolution of Caloris was similar to that of lunar mascon basins. The morphology of ridges within Caloris compares favorably with lunar ridges when viewed on similar resolution earth-based lunar photographs. Ridges in Caloris occur from 1000 to 1320 km diameter range, and are situated within the boundary delineated by topographic benches in the northeastern and southeastern parts of the basin. The orientation of Caloris ridges is more dominantly concentric than ridge orientations in lunar basins.

Maxwell, T. A.↗
NASA NTRS1982

Support of topographic and other loads on the moon and on the terrestrial planets

The existence of mascons on the moon indicates that the lunar elastic lithosphere can support substantial loads for about three billion years. Lunar topography also appears to be uncompensated. Observations of gravity on Mars show that the Tharsis uplift is only partially compensated. A number of authors have attributed this support to lithospheric flexure. In this paper it is shown that membrane stresses play an important role in the support of loads on the moon, Mars, and Mercury. For loads that have been expressed in terms of spherical harmonics analytical expressions for the degree of compensation are obtained as a function of the degree of the load. The results are compared with the observed dependence of the ratio of gravitational potential to topography on degree. It is also concluded that membrane stresses can support a significant ellipticity for a tidally despun planetary body.

Willemann, R. J.↗
NASA NTRS1985

The non-hydrostatic figures of the terrestrial planets

Solid state creep being exponentially dependent on temperature must dominate the mechanical behavior of the mantles of terrestrial planets beneath their lithospheres. General arguments suggest that the lithospheres of the Moon and Mars are about 200 km thick; the Earth, Venus and Mercury much less. Short wavelength gravity anomalies are explained by the finite strength of the lithosphere: the lunar mascons being an example. The good correlation of the Venus and Mars gravity anomalies with topography up to spherical harmonics of degrees 10-15 is in striking contrast to the lack of correlation between the long wavelength components of the geoid and the continent-ocean distribution or even the plates. Attempts have been made to explain the former correlations by isostatic models but the depths of compensation seem implausible. Low degree harmonics of the gravity fields of the terrestrial planets as is certainly the case in the Earth must arise from the density variations driving solid state convection. In the case of Venus the less dense differentiated materials of the highlands seems to be positioned over the singular points of the convection pattern. Thus the correlated gravity field does not arise from the highlands but from the density difference in the convecting interior. In the Earth lack of correlation seems to arise from the fact that the plates have moved relative to the convection pattern the last 100 M yr.

Runcorn, S. K.↗
NASA NTRS1985

Fault type predictions from stress distributions on planetary surfaces - Importance of fault initiation depth

The prediction of fault type on planetary surfaces from model stresses calculated at depth is discussed. These fault-type predictions yield different faults than those predicted using the surface criteria commonly employed in geophysical models. For elastic-plate flexure models of mascon loading on the moon, stresses calculated at the surface predict the occurrence of strike-slip faulting at the radial distance where grabens are found. Normal faults bounding lunar grabens and thrust faults responsible for wrinkle ridges are analyzed. It is found that the former initiate at the mechanical discontinuity that separates the breccia of the megaregolith from in situ fractured rock and that the latter initiate at the mechanical discontinuity between basalt layers and the underlying basin floor. The difference between elastic constants for the outer few kilometers of brecciated megaregolith and the underlying lunar lithosphere are evaluated. Superposing nonisotropic stresses resulting from the weight of overburden to the depth of the relevant mechanical discontinuity yield stresses that predict wrinkle ridges in the basin centers and grabens outside the basin margin, and eliminate the predicted zone of strike-slip faults.

Golombek, M. P.↗
NASA NTRS1987

Investigation of lunar crustal structure and isostasy

The lunar mascon basins have strongly free air gravity anomalies, generally exceeding 100 milligals at an elevation of 100 km. The source of the anomalies is a combination of mantle uplift beneath the impact basins and subsequent infilling by high-density mare basalts. The relative contribution of these two components is still somewhat uncertain, although it is generally accepted that the amount of mantle uplift greatly exceeds the thickness of the basalts. Extensive studies have been carried out of the crustal structure of mare basins, based on gravity data, and their tectonic evolution, based on compressive and extensional tectonic features. The present study endeavored to develop a unified, self-consistent model of the lunar crust and lithosphere incorporating both gravity and tectonic constraints.

Thurber, Clifford H.↗
NASA NTRS1986

On the early thermal state of the moon

New theories for the formation of the moon from an accretion disk thrown into circumterrestrial orbit after the collision of a planet-sized object with the earth have led to a reexamination of the tectonic consequences of an initially molten moon. Even the smallest estimates of radial contraction that would accompany cooling of the moon from an initially molten state predict accumulated near-surface horizontal compressive stresses considerably in excess of the compressive strength of the upper lunar crust, estimated to be 0.5 to 1 kbar on the basis of topographic relief, the stress levels necessary to form mare ridges in mascon mare basins, and measurements of rock friction. Various mechanisms for relieving or modifying such large near-surface stresses are considered, including viscoelastic effects, widespread development of major fault systems, impact gardening, and opposing stresses arising from other global-scale processes. All of these mechanisms face substantial difficulties when tested against geological and mechanical information from the moon and other terrestrial planets. These considerations pose a serious problem for theories of lunar origin that call for an initially molten state.

Solomon, Sean C.↗
NASA NTRS1987

Implications of lithospheric bending and faulting in lunar mare-terra tectonics

A qualitative model of lithospheric flexure and faulting is proposed which accounts for certain rille fault features around mare regions of the moon. It is suggested that the subsidence of lunar mare basins may be due to the existence of their mascons and their volcanic fills as loads that flexed the lithosphere, in addition to the effects of drying up and cooling of internal hot volumes. Differences in the internal attack against the lunar crust from above and below the mean surface are considered. The rille formation is shown to be characterized by peripheral bulging and bending, and it is suggested that asthenosphere-related effects in lunar tectonics may have been very important.

Raitala, Jouko↗
NASA NTRS1989

Buried topography of Utopia, Mars - Persistence of a giant impact depression

Knobs, partially buried craters, ring fractures, and some mesas permit a qualitative determination of the topography buried beneath younger northern plains materials. These features are widely distributed in the Utopia area but are absent in a large, roughly circular region centered at about 48 deg N, 240 deg W. This implies the existence of a circular depression about 3300 km in diameter buried beneath Utopia Planitia that is interpreted to represent the central part of a very large impact basin. The presence of buried curved massifs around part of this depression, and a roughly coincident mascon, lend further support. Present topography, areal geology, and paleotopography of buried surfaces all point to the persistence of this major depression for almost the entire history of Mars.

Mcgill, George E.↗
NASA NTRS1992

Simulations of lunar gravity field determination for Lunar Observer

The current plan for the Lunar Observer (LO) mission is to launch in the late 1990s and insert LO into a 100 km polar circular mapping orbit. However, prior to the mapping orbit, LO will be placed in a gravity calibration orbit (GCO) at a higher altitude to determine the gravity field of the moon. This paper examines the abilities of two GCO orbits (at 200 and 500 km altitudes) to recover a high degree and order gravity truth model that includes spherical harmonics and mascons by estimating different degree and order gravity fields with spherical harmonics only. This is achieved by comparing radial accelerations from the true and estimated models at the mapping altitude and by comparing trajectory propagations. For the gravity fields estimated (up to 30th degree and order), the 500 km GCO was just as successful as the 200 km GCO in determining the gravity field.

Konopliv, Alex↗
NASA NTRS1993

Importance of expansion and contraction in the formation of tectonic features on the Moon

The lack of globally distributed tectonic features on the lunar surface has been used to argue against significant changes in the radius of the Moon since the formation of the presently observed surface, which dates to the end of heavy bombardment about 3.9 Ga. This observation has been used previously to limit the maximum stresses to approximately 100 MPa that could be supported by the lunar lithosphere without the formation of globally distributed tectonic features, which in turn limits the maximum radius changes to plus or minus 1 km for a purely elastic lithosphere. In a previous abstract, limits on the elastic expansion or contraction of the Moon were reexamined with respect to realistic failure stresses necessary to produce actual lunar tectonic features. In addition, limits on the permanent (plastic) strain that could be accommodated by non-mascon grabens and wrinkle ridges were considered with more severe constraints placed on the total reasonable expansion and contraction of the Moon since 3.9 Ga. In this abstract, considerations of the distribution and mechanisms of formation due to a planetary radius change or their accommodating much permanent plastic planetary expansion or contraction.

Golombek, M. P.↗
NASA NTRS1993

Possible flexural signatures around Olympus and Ascraeus Montes, Mars

The effective elastic thickness (h) of the lithosphere provides a measure of the thermal and mechanical state of a planet's shallow interior. An estimate of h in the vicinity of a feature that constitutes a load on a planetary surface can be determined from the flexural response of the lithosphere to the load. This approach has been applied to Mars by calculating radial stresses associated with lithospheric flexure associated with surface loads, and comparing the results to the positions of circumferential graben surrounding the major Martian shield volcanoes and mascon basins. However, many prominent surface loads on Mars, most notably the Olympus Mons volcano, do not exhibit flexural graben. In these instances application of the above method can provide only a lower limit of effective elastic thickness. An alternative method of determining h is to calculate the vertical displacements associated with the flexural loading and to compare the amplitude and shape of the flexural profile to observed topography. This method has not been applied to flexural problems on Mars because of the poor resolution of Martian topographic data. However, previous analyses have shown that the lithosphere around major volcanic shields should exhibit vertical deflections of order 1 km over horizontal baselines of order 100 km. We were thus motivated to search for the presence of flexural troughs in the existing Mars topography data.

Zuber, M. T.↗
NASA NTRS1996

An Inversion of Gravity and Topography for Mantle and Crustal Structure on Mars

Analysis of the gravity and topography of Mars presently provides our primary quantitative constraints on the internal structure of Mars. We present an inversion of the long-wavelength (harmonic degree less than or equal to 10) gravity and topography of Mars for lateral variations of mantle temperature and crustal thickness. Our formulation incorporates both viscous mantle flow (which most prior studies have neglected) and isostatically compensated density anomalies in the crust and lithosphere. Our nominal model has a 150-km-thick high-viscosity surface layer over an isoviscous mantle, with a core radius of 1840 km. It predicts lateral temperature variations of up to a few hundred degrees Kelvin relative to the mean mantle temperature, with high temperature under Tharsis and to a lesser extent under Elysium and cool temperatures elsewhere. Surprisingly, the model predicts crustal thinning beneath Tharsis. If correct, this implies that thinning of the crust by mantle shear stresses dominates over thickening of the crust by volcanism. The major impact basins (Hellas, Argyre, Isidis, Chryse, and Utopia) are regions of crustal thinning, as expected. Utopia is also predicted to be a region of hot mantle, which is hard to reconcile with the surface geology. An alternative model for Utopia treats it as a mascon basin. The Utopia gravity anomaly is consistent with the presence of a 1.2 to 1.6 km thick layer of uncompensated basalt, in good agreement with geologic arguments about the amount of volcanic fill in this area. The mantle thermal structure is the dominant contributor to the observed geoid in our inversion. The mantle also dominates the topography at the longest wavelengths, but shorter wavelengths (harmonic degrees greater than or equal to 4) are dominated by the crustal structure. Because of the uncertainty about the appropriate numerical values for some of the model's input parameters, we have examined the sensitivity of the model results to the planetary structural model (core radius and core and mantle densities), the mantle's viscosity stratification, and the mean crustal thickness. The model results are insensitive to the specific thickness or viscosity contrast of the high-viscosity surface layer and to the mean crustal thickness in the range 25 to 100 km. Models with a large core radius or with an upper mantle low-viscosity zone require implausibly large lateral variations in mantle temperature.

Kiefer, Walter S.↗
NASA NTRS1999

The Lunar Crustal Thickness from Analysis of the Lunar Prospector Gravity and Clementine Topography Datasets

The Lunar Prospector spacecraft has mapped the gravity field of the Moon to a level of resolution never achieved before, and a spherical harmonic representation to degree and order 100 is available. When combined with the topography dataset produced by the Clementine mission, the resulting Bouguer anomaly map is interpreted to model the thickness of the lunar crust. Such models are crucial to understanding the lunar thermal history and the formation of geological features such as mascon basins, several more of which have been newly discovered from this dataset. A two-layer planetary model was used to compute the variations of the depth to the lunar Moho. The thickness values ranged from near 0 to 120 km. There is significant agreement with previous work using the Clementine gravitational field data with differences in specific locations such as South Pole-Aitken Basin, for example.

Asmar, S.↗
NASA NTRS1997

Lunar and Planetary Science XXVIII

The present conference discusses such topics as density crossovers in lunar picrites, the geology of the Cassini impact basin, Mars, nanobacteria in carbonates, the properties of shocked aerogels, a chemical model of Comet Halley, lunar mascons, the impact evolution of icy regoliths, the geology of the Venera 8 landing site, the photogeologic mapping of northern Venus, HST observations of Mars, observational constraints on the rotational dynamics of Mars, and primordial magnetic field measurements from the moon. Also discussed are models of the S2 fluorescence spectra of comets, Martian crater ejecta, the heights of Venusian steep-sided domes, cloud-climate interactions on Venus, the Humorum basin geology from Clementine data, an early Amazonian lake in the Gale crater of Mars, nebular fractionations and Mn-Cr systematics, the Rock Chipper planetary surface sample collection, Mariner 10 stereo images of Mercury, remote and local stresses and Calderas on Mars, the electrostatic charging of saltating particles, SO2 detected on Callisto, the Mars Explorer Planetary Data System, an assessment of explosive venting on Europa, the sequential faulting history of the Mars Valles Marineris, a search for Martian sediments, the composition and internal structure of Europa, long-term and 'diurnal' tidal stresses on Europa, and episodic greenhouse climates on Mars.

Source record↗
NASA NTRS2005

GRACE Gravity Data Target Possible Mega-impact in North Central Wilkes Land, Antarctica

A prominent positive GRACE satellite-measured free-air gravity anomaly over regionally depressed subglacial topography may identify a mascon centered on (70 deg S, 120 deg E) between the Gamburtsev and Transantarctic Mountains of East Antarctica. Being more than twice the size of the Chicxulub crater, the inferred Wilkes Land impact crater is a strong candidate for a Gondwana source of the greatest extinction of life at the end of the Permian. Its ring structure intersects the coastline and thus may have strongly influenced the Cenozoic rifting of East Antarctica from Australia that resulted in the enigmatic lack of crustal thinning on the conjugate Australian block.

vonFrese, Ralph R. B.↗