Herbert V. Frey

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.

An altitude-normalized magnetic map of Mars and its interpretation

Techniques developed for the reduction and analysisofterrestrialsatellitemagneticelddataareusedto better understand the magnetic eld observations made by Mars Global Surveyor. A global distribution of radial (Br) magnetic eld observations and associated uncertainties is invertedfor an equivalentsource magnetization distribution and then used to generate an altitude- normalized map of Br at 200 km. The observations are well-represented by a potential function of crustal origin, consistent with a rema- nent origin for the Martian magnetic features. The correla- tion between the 40546 Br observations andBr calculated from the magnetization solution at observation locations is 0.978. For a magnetization distribution connedto a 50 km layer,calculatedmagnetizationsrangefrom-22to+17A/m. We see correlations with tectonics that were only hinted at in earlier maps. Magnetic features appear to be truncated against Valles Marineris and Ganges Chasma, suggestive of a major change in crustal properties associated with fault- ing.

Space-Age Geodesy: The NASA Crustal Dynamics Project

The NASA Crustal Dynamics Project has deployed satellite laser ranging (SLR) systems and very-long-baseline-interferometer (VLBI) systems for measurements of global and regional crustal motions and Earth rotation parameters. In 1984, the several year buildup of the network approached full capability. During 1984, the SLR systems measured 142 unique baselines between stations for the purpose of determining plate motion between the North American, Pacific, South American, Nazca, Australian, and Eurasian plates; 31 baselines for internal deformation of the same plates; and 9 baselines for regional deformation in the area of the western North American plate boundary. During 1984, the VLBI systems measured 24 baselines for plate motion between the North American, Pacific, and Eurasian plates; 37 baselines for internal deformation of the same plates; and 101 baselines for regional deformation in the area of the western North American plate boundary. Both the SLR and VLBI stations operated throughout the September 1983 to October 1984 international MERIT campaign of measurements of the Earths polar motion and changes in rotation rate. NASAs future plans are to continue these measurements through at least 1990 and to initiate a joint NASA/ European SLR measurement campaign for determining regional crustal deformation in the Mediterranean area.

The MOLA Topographic signature at the crustal dichotomy boundary zone on Mars

MOLA data collected during the aerobraking hiatus provide good longitudinal sampling of the crustal dichotomy boundary zone in eastern Mars and reveal a significant topographic signature associated with the transition from cratered highlands to lowland plains. Total relief between the two major units is always >2.5 km and sometimes >6 km; median elevation differences are about 4 km. Regional slopes are very low (< 0.02°) in both cratered terrain south and in lowland plains north of the boundary. Within the transition zone where cratered terrain changes to detached plateaus, mesas and knobby terrain, slopes are 50-100 times steeper and over a few hundred kilometers average about 1°. The topographic signature of the boundary zone is best described as a 2-4 km step function between two nearly flat surfaces, similar in slope and amplitude to some terrestrial passive margins. This supports previous conclusions of a significant difference in crustal properties (thickness, composition, or both) between the lowland plains and cratered terrain, and suggests this difference occurs over a relatively short distance of a few to several hundred kilometers.

Constraints on the crustal nature and tectonic history of the Kerguelen Plateau from comparative magnetic modeling using MAGSAT data

Abstract The Kerguelen Plateau in the southern Indian Ocean is associated with the Broken Ridge Plateau because of their location with respect to the Southeast Indian Ridge. The Kerguelen Plateau lies south of the mid-ocean ridge and the Broken Ridge Plateau is located at a nearly equal distance to the north. Both plateaus have positive MAGSAT crustal magnetic anomalies associated with them. Comparative modeling of these anomalies places constraints on the crustal nature, the thermal structure, and the tectonic history of the plateaus. Our results support the theory that both plateaus are underlain by oceanic crust, are structurally and lithologically similar, and were formed as a single structure which has since been split by seafloor spreading. Active volcanoes on the northern part of the Kerguelen Plateau suggest that it lies on a hotspot which has raised the Curie isotherm to within the crust of the plateau, decreasing the crustal volume contributing to the magnetic anomaly and thus decreasing the amplitude of the anomaly.

Topography, roughness, layering, and slope properties of the Medusae Fossae Formation from Mars Orbiter Laser Altimeter (MOLA) and Mars Orbiter Camera (MOC) data

The enigmatic Medusae Fossae Formation (MFF) is one of the youngest surficial deposits on Mars. Previously proposed modes of origin include pyroclastic volcanism, eolian mantling, polar sedimentation, carbonate platform, and paleoshorelines. In order to assess mapped member boundaries and constrain likely origin processes we analyze five topographic transects from the Mars Orbiter Laser Altimeter (MOLA) collected during the aerobraking hiatus orbit period of the Mars Global Surveyor (MGS) mission together with several simultaneously acquired Mars Orbiter Camera (MOC) images to examine the regional and local variations in the relief, slopes, vertical roughness, and relative elevations of mapped MFF members. We find that the members are not particularly flat or level but have average thicknesses of many hundreds of meters and substantially more internal relief than previously thought. Locally, the MFF members are sometimes distinguishable by slope, elevation, and surface roughness characteristics, but no unique formation-wide characteristics are apparent in this analysis of the limited initial data. The relative elevations of the mapped members are not consistent with undeformed planar horizontal layers but may be consistent with layers draped over preexisting topography with subsequent partial removal. Analysis of the initial coincident MOC and MOLA hiatus data suggests the possible presence of local layers, as well as the mislocation of at least a few MFF member and formation boundaries. We conclude that carbonate and shoreline deposit origins are not as well supported in the initial MGS data as they were in the Viking data and that the MFF formation and member boundaries should be reexamined in conjunction with the incoming new MGS data.

Magsat magnetic anomaly contrast across Labrador Sea passive margins

Many passive margins not complicated by nearby anomalous crustal structure have satellite elevation crustal magnetic anomaly contrasts across them that are recognizable in reduced-to-pole versions of the Magsat and POGO data. In the Labrador Sea region this contrast is particularly well developed with strong positive anomalies overlying the continental crust of Greenland and eastern Canada and prominent negative anomalies situated over the Labrador Sea and Baffin Bay. We use forward modeling of the large-scale crustal bodies in this region (continental, oceanic, passive margin, several anomalous structures) to show that the Magsat anomaly contrast is due simply to the change in crustal susceptibility and thickness at the continental/oceanic crustal transition. Because the thickness varies more than the average susceptibility from continental to oceanic crust, the strong anomaly contrast is essentially an edge effect due mostly to the change in crustal structure.

Discovery of a 450 km diameter, multi‐ring basin on Mars through analysis of MOLA topographic data

Mars Orbiter Laser Altimeter (MOLA) topographic data have revealed a previously unknown, 450 km wide, 2 km deep basin centered at 30N, 312W near the Phison Rupes. This basin, as large as and deeper than the obvious Cassini impact basin located 1000 km to the SW, is not apparent in the existing but good quality Viking imagery. Gridded MOLA data show the feature as a closed depression. Based on analysis of slope breaks readily visible in two MOLA profiles, we suggest this Phison Rupes Basin has three topographic rings with diameters approximately 350, 455 and 670 km. These rings outline a region of lower impact crater density and smoother inter-crater plains. Similar previously unknown features may exist elsewhere on Mars, and MOLA topographic data may be able to locate them.

Satellite magnetic anomalies and the Middle America Trench

Abstract Quantitative modeling of the Middle America Trench satellite-elevation crustal magnetic anomaly, using seismic epicenter data to constrain the slab geometry and a magnetization (representing induced and viscous contributions) similar to that found for the Aleutian Arc slab, agrees well with reduced-to-pole MAGSAT data. Incorporating minor contributions from crustal sources in Mexico and Central America produces only minor improvement in the agreement between the modeled and observed anomaly, supporting the idea that it is the slab which is the major source body for the Middle America Trench anomaly.

Pogo and Pangaea

Abstract Long wavelength magnetic anomalies of crustal origin derived from the POGO and MAGSAT satellite data often display a strong continuity across the now-rifted continental margins when the continents are reassembled into Pangaea. These anomalies predate the breakup of the supercontinent and represent major blocks whose crustal properties are broadly similar even though those blocks are no longer contiguous.