One intriguing aspect of martian impact crater morphology is the change of crater cavity and ejecta characteristics from the mid-latitudes to the polar regions. This is thought to reflect differences in target properties such as an increasing presence of ice in the polar regions. Previous image-based efforts concerning martian crater morphology has documented some aspects of this, but has been hampered by the lack of adequate topography data. Recent Mars Orbiter Laser Altimeter (MOLA) topographic profiles provide a quantitative perspective for interpreting the detailed morphologies of martian crater cavities and ejecta morphology. This study is a preliminary effort to quantify the latitude-dependent differences in morphology with the goal of identifying target-dependent and crater modification effects from the combined of images and MOLA topography. We combine the available MOLA profiles and the corresponding Viking Mars Digital Image Mosaics (MDIMS), and high resolution Viking Orbiter images to focus on two transitional craters; one on the mid-latitudes, and one in the North Polar region. One MOLA pass (MGS Orbit 34) traverses the center of a 15.9 km diameter fresh complex crater located at 12.8degN 83.8degE on the Hesperian ridge plains unit (Hvr). Viking images, as well as MOLA data, show that this crater has well developed wall terraces and a central peak with 429 m of relative relief. Three MOLA passes have been acquired for a second impact crater, which is located at 69.5degN 41degE on the Vastitas Borealis Formation. This fresh rampart crater lacks terraces and central peak structures and it has a depth af 579 m. Correlation between images and MOLA topographic profiles allows us to construct basic facies maps of the craters. Eight main units were identified, four of which are common on both craters.
Two geodetic airborne laser altimeter (ALA) systems coupled to Global Positioning System receivers acquired submeter-resolution topographic profiles of the lower parts of Breidamerkurjökull and Skeidarárjökull, Iceland, in May 1989 and September 1991 (Skeidarárjökull) and of Jakobshavns Isbræ, Greenland, in April 1992. Maximum measured crevasse depths on Breidamerkurjökull, Skeidarárjökull and Jakobshavns Isbræ were 20.7, 36.1 and 50.2 m, respectively. Crevasse spacings were 43 m (45 crevasses km −1 ) for Breidamerkur jökull, 46–51 m (25 crevasses km −1 ) for Skeidarárjökull and 20–40 m (lower part) or 50–80 m (upper part) of Jakobshavns Isbræ (27 crevasses km −1 ). Surface slopes were ~2.4° for the lower 11 km of Breidamerkurjökull, ~0.8° for the lower 10 km of Skeidarárjökull and 1.55° for the lower 28 km of Jakobshavns Isbræ (with a range of 0.55° for the final 17 km to ~6.3° for a steep central part several km in length). Average longitudinal strain-rate values, estimated by assuming a bulk ice temperature of 0°C and a density of 880 kg m −3 , ranged from 0.12 a −1 for Breidamerkurjökull, to 0.63 a −1 for Skeidarárjökull; values for Jakobshavns Isbræ fell between 1.3 and 1.7 a −1 . Remote sensing of glacier microtopography by ALA offers a potential new tool for determining crevasse morphology, spatial density and spacing, meter-scale local slopes, long-wavelength gradients and derived strain rates.
[1] After an intensive 3-month review of several high science value mission candidates during the summer of 2000, the Mars Exploration Rover (MER) mission was officially selected by NASA in August 2000 as an approved flight mission for launch in the 2003 launch window. The decision to select twin MER rovers for launch in 2003 was based in part on arguments related to the reduction of the risks associated with launch failure as well as the unknowns of Martian environmental conditions at the time of landing. The MER mission will significantly enhance our scientific understanding of the local Martian surface by combining a powerful science payload with mobility measured at the scale of several orbital remote-sensing pixels (i.e., hundreds of meters). There is a vast storehouse of orbital remote-sensing data now available to the science community, and these new data sets show that Mars is a more dynamic, perhaps more water-rich, profoundly layered planet in comparison to Viking-based interpretations developed during the period from the mid-1970s to the middle 1990s. MER will provide invaluable ground truth observations with which to place the wealth of orbital remote-sensing data from the ongoing Mars Global Surveyor and Mars Odyssey missions in scientific context and to improve community-based assessments of the validity of orbital-based observations. By sending each rover to a specific landing site that shows evidence of past liquid water activity [Golombek et al., 2003], the MER mission will broaden scientific understanding of the role of water on Mars and directly explore the possibility of biologically hospitable environments having existed at certain locations on Mars. The MER mission employs an integrated suite of synergistic scientific instruments [Squyres et al., 2003] to analyze and understand whether surficial rocks and soils formed or were modified in the presence of liquid water, thereby addressing one of the major thematic objectives of the current Mars Exploration Program. [2] The scientific results of the MER mission will exert an important impact on both the present and upcoming decade of Mars exploration (Figure 1) [Garvin et al., 2001]. Immediately following MER, the 2005 Mars Reconnaissance Orbiter (MRO) will be deployed to identify the most scientifically compelling sites where the effects of liquid water or hydrothermal systems are suggested and thereby a suite of high-priority future landing sites relevant to key issues associated with biologic “habitability.” Results from MER will feed directly into the prioritized targeting of the two high-resolution MRO science instruments, Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), a hyperspectral imaging spectrometer, and High Resolution Imaging Science Instrument (HiRISE), a submeter-resolution, multicolor camera, and will serve to provide local validation for the entire suite of MRO observations. The MER experience will influence the design and development of the 2009 Mars Science Laboratory (MSL), which will be sent to the most accessible, high-science-priority site identified on Mars via MRO for the purpose of understanding one specific paleoenvironment (i.e., potentially one associated with preserved layered aqueous sediments), searching for potential biosignatures, and characterizing the building blocks of life, if any are preserved. The MSL mission is currently baselined to make use of a radioisotope power system, with a landing precision about an order of magnitude better than that available to the 2003 MER mission (i.e., <10 km semimajor access of the landing error ellipse, 3 sigma). Because a major objective of MSL is to perform definitive analytical measurements of rocks and soils in order to search for evidence of potential biosignatures, MER represents a stepping-stone toward the challenging goal of understanding how to look for ancient life on Mars that can be directly applied to targeting MSL analytical measurements. By the completion of the MER mission, the new understanding of the surface of Mars provided by these roving laboratories will be used as guidance to select future landing sites for MSL, as well as the types of investigations that must be accomplished as part of the MSL mission. [3] This special section of JGR-Planets assembles a collection of papers that describe the Mars Exploration Rover mission, its science payload and specific investigations, current candidate landing sites, and the science activities that could be carried out at those candidate sites. It also includes updated calibration and results from the Pathfinder Alpha Proton X-Ray Spectrometer, which are directly relevant to results to be attained from the MER Alpha Particle X-Ray Spectrometer. Several of the papers in this section are a result of the high level of interest generated at four Mars Exploration Rover landing site workshops that were open to the entire science community [Golombek et al., 2003], which were held 24–25 January 2001, 17–18 October 2001, 26–28 March 2002, and 8–10 January 2003. These workshops provided essential inputs to the process of characterizing and selecting landing sites for the Mars Exploration Rover mission and helped develop community consensus concerning those science hypotheses that could be tested with the rovers and their associated payloads at the final candidate landing sites selected by the NASA Associate Administrator for Space Science.
The global distribution of the reflectivity of the surface of Venus as determined by the Pioneer Venus orbital radar instrument has been analyzed in a geological context and statistically correlated with elevation. In addition, a comparison between the reflectivity and rms slope (roughness) correlations with elevation permits radar‐geologic topographic zones to be identified. The radar reflectivity ρ at normal incidence and at a wavelength of 17 cm (1.76 GHz) is a model‐dependent measure of the bulk dielectric constant κ of surfaces dominated by dry rocks and soils and depends on surface material properties such as porosity and conductivity. Only the quasi‐specular component of the radar echo was used in determining the reflectivity values analyzed in this study, and for very rough surfaces the absolute magnitude of ρ may be underestimated by 10–15%. Empirically derived relationships between ρ, κ, and bulk density γ are used to interpret geologically the ρ distribution. The global mean ρ of 0.13 is significantly greater than the average lunar and typical martian values of ∼0.07, suggesting the absence of a continuous soil mantle on Venus. The ρ distribution is well described by a two‐stage Gaussian distribution with modes at 0.11 and 0.14. The close proximity of these modes suggests that there is no fundamental dichotomy of surfaces on Venus insofar as their ρ properties, in contrast with roughness. Less than 15% of Venus has ρ values low enough to indicate a major soil component on the surface. Approximately 27% of the surface is dominated by low‐porosity materials such as bedrock, and less than 15% is enriched in high dielectrics. The rest of the surface (43%) is most simply envisioned as partially mantled bedrock, perhaps an extension of the types of terrain viewed by the Venera 10 and 13 landers. The most plausible model for the highest ρ (and thus highest κ) materials requires enrichment in Ti and Fe (e.g., minerals such as rutile, ilmenite, and magnetite). High‐titanium basalts such as those found on the moon would produce the required enrichment, as would pyrites. Since surface geochemical measurements demonstrate that there are basalts in the Venusian plains, a model in which high‐titanium basalts are exposed at the highest elevations (Maxwell, Theia, Atla, Ovda) is favored. Possible weathering of ilmenite in such basalts to produce rutile could explain the high‐κ materials in less elevated regions. When correlated with elevation, κ exhibits a complex nonmonotonic trend in which both decreases and increases are observed. A major decrease in ρ of ∼0.02 km −1 in the upper plains contrasts with the almost 1° rms km −1 increase in surface roughness over the same interval and may be an expression of increased soil production, perhaps due to enhanced breakdown of silicates into carbonates in the lower highlands. Alternately, the decrease could be due to increased centimeter scale roughening, perhaps caused by the increase in regional slope in the highlands. A major increase in ρ (0.05 km −1 ) in the middle highlands correlates with a rapid rise in rms slope. Unlike rms slope, however, there is no overall correlation of ρ with either elevation or regional slope, suggesting that it may be a more locally controlled parameter. Hierarchical clustering analysis of the ρ, roughness, and regional slope properties of Venus demonstrates that distinct subregions are best defined on the basis of topographic zones in which the radar parameters follow well‐defined trends. The lowland plains are extremely smooth at scales from centimeters to 100 km. In general, no single radar parameter (e.g., ρ) serves to subdivide the surface into distinct geologic regions, but taken together, the radar parameters can be statistically correlated to define meaningful radar geologic units. Such units appear to correlate with the kinds of surfaces that can be seen on Venus at kilometer resolution.
One intriguing aspect of martian impact crater morphology is the change of crater cavity and ejecta characteristics from the mid-latitudes to the polar regions. This is thought to reflect differences in target properties such as an increasing presence of ice in the polar regions. Previous image-based efforts concerning martian crater morphology has documented some aspects of this, but has been hampered by the lack of adequate topography data. Recent Mars Orbiter Laser Altimeter (MOLA) topographic profiles provide a quantitative perspective for interpreting the detailed morphologies of martian crater cavities and ejecta morphology. This study is a preliminary effort to quantify the latitude-dependent differences in morphology with the goal of identifying target-dependent and crater modification effects from the combined of images and MOLA topography. We combine the available MOLA profiles and the corresponding Viking Mars Digital Image Mosaics (MDIMS), and high resolution Viking Orbiter images to focus on two transitional craters; one on the mid-latitudes, and one in the North Polar region. One MOLA pass (MGS Orbit 34) traverses the center of a 15.9 km diameter fresh complex crater located at 12.8degN 83.8degE on the Hesperian ridge plains unit (Hvr). Viking images, as well as MOLA data, show that this crater has well developed wall terraces and a central peak with 429 m of relative relief. Three MOLA passes have been acquired for a second impact crater, which is located at 69.5degN 41degE on the Vastitas Borealis Formation. This fresh rampart crater lacks terraces and central peak structures and it has a depth af 579 m. Correlation between images and MOLA topographic profiles allows us to construct basic facies maps of the craters. Eight main units were identified, four of which are common on both craters.
The reformulation of the Mars program gives NASA a rare opportunity to deliver a credible vision in which humans, robots, and advancements in information technology combine to open the deep space frontier to Mars. There is a broad challenge in the reformulation of the Mars exploration program that truly sets the stage for: 'a strategic collaboration between the Science Mission Directorate (SMD), the Human Exploration and Operations Mission Directorate (HEOMD) and the Office of the Chief Technologist, for the next several decades of exploring Mars'.Any strategy that links all three challenge areas listed into a true long term strategic program necessitates discussion. NASA's SMD and HEOMD should accept the President's challenge and vision by developing an integrated program that will enable a human expedition to Mars orbit in 2033 with the goal of returning samples suitable for addressing the question of whether life exists or ever existed on Mars
