Patrick J. McGovern

Mars Orbiter Laser Altimeter: Experiment summary after the first year of global mapping of Mars

The Mars Orbiter Laser Altimeter (MOLA), an instrument on the Mars Global Surveyor spacecraft, has measured the topography, surface roughness, and 1.064-μm reflectivity of Mars and the heights of volatile and dust clouds. This paper discusses the function of the MOLA instrument and the acquisition, processing, and correction of observations to produce global data sets. The altimeter measurements have been converted to both gridded and spherical harmonic models for the topography and shape of Mars that have vertical and radial accuracies of ~1 m with respect to the planets center of mass. The current global topographic grid has a resolution of 1/64° in latitude × 1/32° in longitude (1 × 2 km^2 at the equator). Reconstruction of the locations of incident laser pulses on the Martian surface appears to be at the 100-m spatial accuracy level and results in 2 orders of magnitude improvement in the global geodetic grid of Mars. Global maps of optical pulse width indicative of 100-m-scale surface roughness and 1.064-μm reflectivity with an accuracy of 5% have also been obtained.

Localized gravity/topography admittance and correlation spectra on Mars: Implications for regional and global evolution

From gravity and topography data collected by the Mars Global Surveyor spacecraft we calculate gravity/topography admittances and correlations in the spectral domain and compare them to those predicted from models of lithospheric flexure. On the basis of these comparisons we estimate the thickness of the Martian elastic lithosphere (T_e) required to support the observed topographic load since the time of loading. We convert T_e to estimates of heat flux and thermal gradient in the lithosphere through a consideration of the response of an elastic/plastic shell. In regions of high topography on Mars (e.g., the Tharsis rise and associated shield volcanoes), the mass-sheet (small-amplitude) approximation for the calculation of gravity from topography is inadequate. A correction that accounts for finite-amplitude topography tends to increase the amplitude of the predicted gravity signal at spacecraft altitudes. Proper implementation of this correction requires the use of radii from the center of mass (collectively known as the planetary “shape”) in lieu of “topography” referenced to a gravitational equipotential. Anomalously dense surface layers or buried excess masses are not required to explain the observed admittances for the Tharsis Montes or Olympus Mons volcanoes when this correction is applied. Derived T_e values generally decrease with increasing age of the lithospheric load, in a manner consistent with a rapid decline of mantle heat flux during the Noachian and more modest rates of decline during subsequent epochs.

Correction to “Localized gravity/topography admittance and correlation spectra on Mars: Implications for regional and global evolution”

[1] In the paper ‘‘Localized gravity/topography admittance and correlation spectra on Mars: Implications for regional and global evolution’’ by Patrick J. McGovern, Sean C. Solomon, David E. Smith, Maria T. Zuber, Mark Simons, Mark A. Wieczorek, Roger J. Phillips, Gregory A. Neumann, Oded Aharonson, and James W. Head (Journal of Geophysical Research, 107(E12), 5136, doi:10.1029/ 2002JE001854, 2002), the thickness of the lithosphere and lithospheric heat flow for a number of regions of Mars and as functions of time were inferred on the basis of gravity/topography admittance spectra. Observed admittances, derived from spherical harmonic expansions localized with the scheme of Simons et al. [1997], were compared with those predicted from models for the flexural response to lithospheric loading [e.g., Turcotte et al., 1981]. Gravity was calculated according to the finite-amplitude scheme of Wieczorek and Phillips [1998]. Estimates for the thickness of the elastic lithosphere Te at the time of loading for each region were converted to equivalent thermal gradient dT/dz and heat flux q by means of an elastic-plastic stressenvelope formalism [McNutt, 1984]. Here we describe a correction required in the calculation of the modeled gravity anomalies; we report new estimates of Te, load density rl, dT/dz, and q from corrected model admittances; and we discuss the implications of the new results. [2] The source of the required correction is a difference in reference radius values. As defined by McGovern et al. [2002], the planetary shape was taken to equal the radius from the center of mass of Mars to the Martian surface expressed as a spherical harmonic expansion and referenced to the mean equatorial radius Req = 3396 km:

State of stress, faulting, and eruption characteristics of large volcanoes on Mars

The formation of a large volcano loads the underlying lithospheric plate and can lead to lithospheric flexure and faulting. In turn, lithospheric deformation affects the stress field beneath and within the volcanic edifice and can influence magma transport. Modeling the interaction of these processes is crucial to an understanding of the history of eruption characteristics and tectonic deformation of large volcanoes. We develop models of time-dependent stress and deformation for the Tharsis volcanoes on Mars. By means of a finite element code, we calculate stresses and displacements due to a volcano-shaped load emplaced on an elastic plate overlying a viscoelastic mantle. Models variously incorporate growth of the volcanic load with time and a detachment between volcano and lithosphere. The models illustrate the manner in which time-dependent stresses induced by lithospheric plate flexure beneath the volcanic load may affect eruption histories, and the derived stress fields can be related to tectonic features on and surrounding Martian volcanoes. As a result of flexure there are three regions where stresses become sufficiently large to cause failure by faulting, according to the Mohr-Coulomb criterion: at the surface of the plate just outward of the volcano, near the base of the elastic lithosphere beneath the center of the volcano, and on the upper flanks of the volcano early in its growth history. Normal faulting is the dominant mode of failure predicted for the first region, consistent with circumferential graben observed around the Tharsis Montes and with the scarp at the base of Olympus Mons, interpreted as a large-offset, listric normal fault. Normal faulting, mostly radially oriented, is predicted for the second region. Failure in the third region is predicted to consist of thrust faulting, circumferentially oriented on the upper and middle flanks and radially oriented on the lower flanks. In models simulating a growing volcano, this portion of the edifice is subsequently covered by later units which exhibit lower stresses and are not predicted to fail; this volume of early failure remains the most highly stressed area in the edifice. Concentric terraces, interpreted by some workers as thrust faults, on the upper flanks of Olympus Mons may correspond to the predicted circumferential thrust features, if the most recent increments of volcano growth were relatively large, or in the presence of local material property or stress field variations. For volcanoes detached from the plate, predicted failure in the edifice takes the form of radial normal faulting near the volcano base. The addition of a local extensional stress arising from the regional topographic slope yields a pattern of predicted faulting which closely matches that observed on the Tharsis Montes, including the development of radial rifts on the lower volcano flanks to the northeast and southwest and the asymmetric formation of circumferential flank graben. This stress state is also consistent with an interpretation of the aureole deposits of Olympus Mons as the result of gravity sliding along a basal detachment. Our models also suggest an explanation for the lack of strike-slip features, predicted by previously published flexural models, around the Tharsis volcanoes. For a given load increment, the first mode of near-surface failure for most of the area immediately outward of the load is circumferential normal faulting and graben formation. As the volcano grows and the flexural response to the increasing load proceeds, the predicted failure mode in a portion of this annular region surrounding the volcano changes to strike-slip faulting. Because normal faulting has been predicted to have taken place earlier, however, it is likely that release of later stresses will occur by reactivation and growth of these normal faults and graben rather than by the formation of new strike-slip faults.

Ancient Igneous Intrusions and Early Expansion of the Moon Revealed by GRAIL Gravity Gradiometry

The Holy GRAIL? The gravity field of a planet provides a view of its interior and thermal history by revealing areas of different density. GRAIL, a pair of satellites that act as a highly sensitive gravimeter, began mapping the Moons gravity in early 2012. Three papers highlight some of the results from the primary mission. Zuber et al. (p. 668, published online 6 December) discuss the overall gravity field, which reveals several new tectonic and geologic features of the Moon. Impacts have worked to homogenize the density structure of the Moons upper crust while fracturing it extensively. Wieczorek et al. (p. 671, published online 6 December) show that the upper crust is 35 to 40 kilometers thick and less dense—and thus more porous—than previously thought. Finally, Andrews-Hanna et al. (p. 675, published online 6 December) show that the crust is cut by widespread magmatic dikes that may reflect a period of expansion early in the Moons history. The Moons gravity map shows that the crust is cut by extensive magmatic dikes, perhaps implying a period of early expansion. The earliest history of the Moon is poorly preserved in the surface geologic record due to the high flux of impactors, but aspects of that history may be preserved in subsurface structures. Application of gravity gradiometry to observations by the Gravity Recovery and Interior Laboratory (GRAIL) mission results in the identification of a population of linear gravity anomalies with lengths of hundreds of kilometers. Inversion of the gravity anomalies indicates elongated positive-density anomalies that are interpreted to be ancient vertical tabular intrusions or dikes formed by magmatism in combination with extension of the lithosphere. Crosscutting relationships support a pre-Nectarian to Nectarian age, preceding the end of the heavy bombardment of the Moon. The distribution, orientation, and dimensions of the intrusions indicate a globally isotropic extensional stress state arising from an increase in the Moons radius by 0.6 to 4.9 kilometers early in lunar history, consistent with predictions of thermal models.

Lunar interior properties from the GRAIL mission

The Gravity Recovery and Interior Laboratory (GRAIL) mission has sampled lunar gravity with unprecedented accuracy and resolution. The lunar GM, the product of the gravitational constant G and the mass M, is very well determined. However, uncertainties in the mass and mean density, 3345.56 ± 0.40 kg/m3, are limited by the accuracy of G. Values of the spherical harmonic degree-2 gravity coefficients J2 and C22, as well as the Love number k2 describing lunar degree-2 elastic response to tidal forces, come from two independent analyses of the 3 month GRAIL Primary Mission data at the Jet Propulsion Laboratory and the Goddard Space Flight Center. The two k2 determinations, with uncertainties of ~1%, differ by 1%; the average value is 0.02416 ± 0.00022 at a 1 month period with reference radius R = 1738 km. Lunar laser ranging (LLR) data analysis determines (C − A)/B and (B − A)/C, where A < B < C are the principal moments of inertia; the flattening of the fluid outer core; the dissipation at its solid boundaries; and the monthly tidal dissipation Q = 37.5 ± 4. The moment of inertia computation combines the GRAIL-determined J2 and C22 with LLR-derived (C − A)/B and (B − A)/C. The normalized mean moment of inertia of the solid Moon is Is/MR2 = 0.392728 ± 0.000012. Matching the density, moment, and Love number, calculated models have a fluid outer core with radius of 200–380 km, a solid inner core with radius of 0–280 km and mass fraction of 0–1%, and a deep mantle zone of low seismic shear velocity. The mass fraction of the combined inner and outer core is ≤1.5%.

GRAIL gravity constraints on the vertical and lateral density structure of the lunar crust

We analyzed data from the Gravity Recovery and Interior Laboratory (GRAIL) mission using a localized admittance approach to map out spatial variations in the vertical density structure of the lunar crust. Mare regions are characterized by a distinct decrease in density with depth, while the farside is characterized by an increase in density with depth at an average gradient of ∼35 kg m −3 km −1 and typical surface porosities of at least 20%. The Apollo 12 and 14 landing site region has a similar density structure to the farside, permitting a comparison with seismic velocity profiles. The interior of the South Pole-Aitken (SP-A) impact basin appears distinct with a near-surface low-density (porous) layer 2-3 times thinner than the rest of the farside. This result suggests that redistribution of material during the large SP-A impact likely played a major role in sculpting the lunar crust.

Growth of large volcanoes on Venus: Mechanical models and implications for structural evolution

The structure, tectonics, and evolution of large volcanoes on Venus, as revealed by data from the Magellan mission, appear to be distinct from those on Earth and Mars. To determine the conditions and processes that account for these differences, we model the evolution of stress and deformation in growing volcanic edifices on Venus using the finite element method. Large volcanoes on Venus are characterized by topographically prominent conical edifices surrounded by relatively flat flow aprons. The surfaces of both edifice and apron consist dominantly of radially oriented flows. Tectonic features, usually the surface expression of shallowly intruded dikes, also have predominantly radial orientations. Features similar to Hawaiian-style linear rift zones or large-scale flank failure, common on large volcanoes on Earth and Mars, are absent. By comparing predictions of faulting for models with detached and welded basal boundary conditions with the observed tectonic features, we determine that a welded condition is more likely. Horizontal compressive stresses transmitted into edifices across welded basal boundaries are inversely proportional to elastic lithosphere thickness Te. Contravening stress increments from magma chamber expansion, differential thermal contraction, or buoyant loading from beneath the crust or lithosphere are required to reorient principal stresses in the edifice so that magma ascent and radial dike formation are favored. Incremental volcano growth results in stress distributions that decrease with height in the edifice. Dike intrusion is easiest in the uppermost (youngest) layers, consistent with the observed shallow radial dikes on Venus volcanoes. Small values of Te greatly inhibit the formation of large shields, in that conical edifice topography cannot be supported and large contravening stresses are required for further growth. Large volcano formation is much more likely at large Te, where more moderate contravening stresses are sufficient for growth. The near-surface manifestation of mantle upwelling on Venus may thus depend on Te in that the response to upwellings beneath thin or nonexistent lithosphere is likely to be dominantly ductile, leading to the formation of coronae, but a thick elastic lithosphere is required to support the growth of the largest volcanoes. The transitional value of Te between these modes of evolution likely depends on the horizontal scale of the upwelling.

Volcanic spreading and lateral variations in the structure of Olympus Mons, Mars

The Olympus Mons volcano on Mars is notable not only for its immense height and width, but also for substantial asymmetries in its structure. The gently sloped northwest flank extends to a much greater distance from the central caldera complex than the more steeply sloped southeast flank. Furthermore, the northwest flank exhibits lower-flank extensional faults, whereas the southeast shows upper-flank compressional terraces and lower-flank upthrust blocks. However, both the northwest and southeast flanks exhibit characteristic concave-upward profiles and steep bounding scarps, in contrast to other sectors. The NW-SE asymmetries are aligned with the regional slope from the Tharsis rise, but an understanding of the underlying causes has remained elusive. We use particle dynamics models of growing, spreading volcanoes to demonstrate that these flank structures could reflect the properties of the basement materials underlying Olympus Mons. We find that basal slopes alone are insufficient to produce the observed concave-upward slopes and asymmetries in flank extent and deformation style that are observed at Olympus Mons; instead, lateral variations in basal friction are required. These variations are most likely related to the presence of sediments, transported and preferentially accumulated downslope from the Tharsis rise. Such sediments likely correspond to ancient phyllosilicates (clays) recently discovered by the Mars Express mission.

Filling of flexural moats around large volcanoes on Venus: Implications for volcano structure and global magmatic flux

The absence of topographic moats and concentric normal faulting around large volcanoes on Venus is attributed to filling of the annular flexural depression by lava flows from the central edifice. Large volcanoes on Venus are characterized by prominent, approximately conical edifices surrounded by relatively flat flow aprons. The surfaces of both of these constructional components consist dominantly of radially oriented flows. From analytic plate flexure models, we generate a synthetic stratigraphy for Venus volcanoes from which we calculate the volume of material filling the flexural moats to the level of the flow apron. The total volume of volcano-associated extrusive lavas, including moat fill, can be an order of magnitude greater than the volume of the edifice alone. Extended to all large volcanoes on Venus, this procedure yields estimates for magmatic flux on Venus comparable to the present terrestrial intraplate extrusive flux, but this scenario is nonetheless consistent with observations of areally limited resurfacing since the most recent global resurfacing event. The absence of flexure-related tectonic features around most large volcanoes, by the arguments advanced here, is attributable to masking by apron flows and low stresses in the apron moat fill. On Venus, a volcanic edifice and associated moat-filling material constitute a single structurally coherent unit. Large volcanoes on Venus thus appear to be structurally distinct from those on Earth and Mars.