Geology topics
Randolph L. Kirk
Publications and source records attributed to Randolph L. Kirk.
RADAR: The Cassini Titan Radar Mapper
The Cassini RADAR instrument is a multimode 13.8 GHz multiple-beam sensor that can operate as a synthetic-aperture radar (SAR) imager, altimeter, scatterometer, and radiometer. The principal objective of the RADAR is to map the surface of Titan. This will be done in the imaging, scatterometer, and radiometer modes. The RADAR altimeter data will provide information on relative elevations in selected areas. Surfaces of the Saturn’s icy satellites will be explored utilizing the RADAR radiometer and scatterometer modes. Saturn’s atmosphere and rings will be probed in the radiometer mode only. The instrument is a joint development by JPL/NASA and ASI. The RADAR design features significant autonomy and data compression capabilities. It is expected that the instrument will detect surfaces with backscatter coefficient as low as −40 dB.
The nucleus of Comet Borrelly: A study of morphology and surface brightness
Stereo images obtained during the DS1 flyby were analyzed to derive a topographic model for the nucleus of Comet 19P/Borrelly for morphologic and photometric studies. The elongated nucleus has an overall concave shape, resembling a peanut, with the lower end tilted towards the camera. The bimodal character of surface-slopes and curvatures support the idea that the nucleus is a gravitational aggregate, consisting of two fragments in contact. Our photometric modeling suggests that topographic shading effects on Borrelly's surface are very minor (<10%) at the given resolution of the terrain model. Instead, albedo effects are thought to dominate Borrelly's large variations in surface brightness. With 90% of the visible surface having single scattering albedos between 0.008 and 0.024, Borrelly is confirmed to be among the darkest of the known Solar System objects. Photometrically corrected images emphasize that the nucleus has distinct, contiguous terrains covered with either bright or dark, smooth or mottled materials. Also, mapping of the changes in surface brightness with phase angle suggests that terrain roughness at subpixel scale is not uniform over the nucleus. High surface roughness is noted in particular near the transition between the upper and lower end of the nucleus, as well as near the presumed source region of Borrelly's main jets. Borrelly's surface is complex and characterized by distinct types of materials that have different compositional and/or physical properties.
Imaging Borrelly
The nucleus, coma, and dust jets of short-period Comet 19P/Borrelly were imaged from the Deep Space 1 spacecraft during its close flyby in September 2001. A prominent jet dominated the near-nucleus coma and emanated roughly normal to the long axis of nucleus from a broad central cavity. We show it to have remained fixed in position for more than 34 hr, much longer than the 26-hr rotation period. This confirms earlier suggestions that it is co-aligned with the rotation axis. From a combination of fitting the nucleus light curve from approach images and the nucleus' orientation from stereo images at encounter, we conclude that the sense of rotation is right-handed around the main jet vector. The inferred rotation pole is approximately perpendicular to the long axis of the nucleus, consistent with a simple rotational state. Lacking an existing IAU comet-specific convention but applying a convention provisionally adopted for asteroids, we label this the north pole. This places the sub-solar latitude at ∼60° N at the time of the perihelion with the north pole in constant sunlight and thus receiving maximum average insolation.
Comparison of USGS and DLR topographic models of Comet Borrelly and photometric applications
Stereo analysis of images obtained during the 2001 flyby of Comet Borrelly by NASA's Deep Space 1 (DS1) probe allows us to quantify the shape and photometric behavior of the nucleus. The shape is complex, with planar facets corresponding to the dark, mottled regions of the surface whereas the bright, smooth regions are convexly curved. The photometric as well as textural differences between these regions can be explained in terms of topography (roughness) at and below the image resolution, without invoking significant variations in single-particle properties; the material on Borrelly's surface could be quite uniform. A statistical comparison of the digital elevation models (DEMs) produced from the three highest-resolution images independently at the USGS and DLR shows that their difference standard deviation is 120 m, consistent with a matching error of 0.20 pixel (similar to reported matching accuracies for many other stereo datasets). The DEMs also show some systematic differences attributable to manual versus automatic matching. Disk-resolved photometric modeling of the nucleus using the DEM shows that bright, smooth terrains on Borrelly are similar in roughness (Hapke roughness θ =20°) to C-type asteroid Mathilde but slightly brighter and more backscattering (single-scattering albedo w =0.056, Henyey–Greenstein phase parameter g =−0.32). The dark, mottled terrain is photometrically consistent with the same particles but with roughnesses as large as 60°. Intrinsically darker material is inconsistent with the phase behavior of these regions. Many local radiance variations are clearly related to topography, and others are consistent with a topographic explanation; one need not invoke albedo variations greater than a few tens of percent to explain the appearance of Borrelly.
Short-wavelength infrared (1.3-2.6 μm) observations of the nucleus of Comet 19P/Borrelly
During the last two minutes before closest approach of Deep Space 1 to Comet 19P/Borrelly, a long exposure was made with the short-wavelength infrared (SWIR) imaging spectrometer. The observation yielded 46 spectra covering 1.3–2.6 μm; the footprint of each spectrum was ∼160 m × width of the nucleus. Borrelly's highly variegated and extremely dark 8-km-long nucleus exhibits a strong red slope in its short-wavelength infrared reflection spectrum. This slope is equivalent to J–K and H–K colors of ∼0.82 and ∼0.43, respectively. Between 2.3–2.6 μm thermal emission is clearly detectable in most of the spectra. These data show the nucleus surface to be hot and dry; no trace of H 2 O ice was detected. The surface temperature ranged continuously across the nucleus from ⩽300 K near the terminator to a maximum of ∼340 K, the expected sub-solar equilibrium temperature for a slowly rotating body. A single absorption band at ∼2.39 μm is quite evident in all of the spectra and resembles features seen in nitrogen-bearing organic molecules that are reasonable candidates for compositional components of cometary nuclei. However as of yet the source of this band is unknown.
A new Mars Digital Image Model (MDIM 2.1) control network
The U.S. Geological Survey has recently completed a final revised version of its 231 m/pixel global Viking image mosaic of Mars that has substantially improved geodetic accuracy compared to versions released in 1991 and 2001. This mosaic, known as MDIM 2.1, is currently available in the USGS ISIS file format (see http://astrogeology.usgs.gov/Projects/MDIM21/) and will be formatted and submitted to the NASA Planetary Data System (PDS) in the near future for archiving as a single ~5-MB DVD volume. Positional control for MDIM 2.1 comes from a new geodetic/photogrammetric solution of the global Mars Mariner 9 and Viking image control network. The details of this network solution are described here. This network incorporates 1,054 Mariner 9 and 5,317 Viking Orbiter images. Accuracy of the new solution is improved primarily as the result of constraining all 37,652 control points to radii from Mars Orbiter Laser Altimeter (MOLA) data and adding 1,232 "ground control points" whose horizontal coordinates are also constrained by MOLA. The MOLA data are believed to have an absolute accuracy on the order of 100 m horizontally. Additional improvements result from use of updated timing and orientation data for the Viking Orbiter images, improved reseau measurements and hence distortion correction of the images, and careful checking and remeasurement of control points with large residuals. The RMS error of the solution is 15.8 µm (~1.3 Viking pixels, ~280 m on the ground). The IAU/IAG 2000 coordinate system is used for the network and the mosaic.
Topographic mapping of Mars: From hectometer to micrometer scales
We describe USGS topomapping of Mars at resolutions from 100 m to 30 µm with data from the latest spacecraft missions. Analysis of NASA 2001 Mars Odyssey Thermal Emission Imaging System (THEMIS) data combining daytime visible reflected, daytime IR emitted, and nighttime IR emitted images allows us to isolate the physical effects of topography, albedo, and thermal inertia. To a good approximation these physical influences interact linearly so that maps showing topographic shading, albedo, and relative thermal inertia can be produced by simple algebraic manipulation of the coregistered images. The shading map resembles an airbrush shaded relief portrayal of the surface, and can be used as the input for quantitative reconstruction of topography by photoclinometry (PC) at 100-m resolution over most of the planet. The High Resolution Stereo Camera (HRSC) of the ESA Mars Express orbiter includes a 9-line scanner for color and stereo imaging and a Super-Resolution Channel (SRC). We analyze these images with a combination of USGS ISIS cartographic software and commercial photogrammetric software, providing an independent check on the stereo processing pipeline developed by the HRSC team. In particular, we are producing very high resolution digital elevation models (DEMs) from the SRC images by photoclinometry and by stereoanalysis, using Mars Orbiter Camera images to complete the stereopair. The NASA Mars Exploration Rovers (MER) carry a diverse set of cameras: two wide-angle hazard camera pairs, panoramic stereo imagers (Pancam and Navcam), and a Microscopic Imager (MI) that images a 3-cm-square area at 30 µm/pixel resolution. Our work emphasizes MI data and includes geometric calibration, bundle-adjustment, mosaicking, generation of DEMs by stereo analysis and focal sectioning, and combination of MI images with color data from Pancam. The software being developed to support these analyses can also be used to produce high-precision controlled mosaics, DEMs, and other products from the Pancam and Navcam images.
A new Mars Digital Image Model (MDIM 2.1) control network
The U.S. Geological Survey has recently completed a final revised version of its 231 m/pixel global Viking image mosaic of Mars that has substantially improved geodetic accuracy compared to versions released in 1991 and 2001. This mosaic, known as MDIM 2.1, is currently available in the USGS ISIS file format and will be formatted and submitted to the NASA Planetary Data System (PDS) in the near future for archiving as a single ~5-MB DVD volume. Positional control for MDIM 2.1 comes from a new geodetic/photogrammetric solution of the global Mars Mariner 9 and Viking image control network. The details of this network solution are described here. This network incorporates 1,054 Mariner 9 and 5,317 Viking Orbiter images. Accuracy of the new solution is improved primarily as the result of constraining all 37,652 control points to radii from Mars Orbiter Laser Altimeter (MOLA) data and adding 1,232 "ground control points" whose horizontal coordinates are also constrained by MOLA. The MOLA data are believed to have an absolute accuracy on the order of 100 m horizontally. Additional improvements result from use of updated timing and orientation data for the Viking Orbiter images, improved reseau measurements and hence distortion correction of the images, and careful checking and remeasurement of control points with large residuals. The RMS error of the solution is 15.8 µm (~1.3 Viking pixels, ~280 m on the ground). The IAU/IAG 2000 coordinate system is used for the network and the mosaic.
Preparing for THEMIS controlled global Mars mosaics
We have begun work to prepare for producing controlled 2001 Mars Odyssey THEMIS infrared (IR) and visible (VIS) global mosaics of Mars. This effort is being coordinated with colleagues from Arizona State University and on the THEMIS team who plan to address radiometric issues in making such mosaics. We are concentrating on geometric issues. Several areas of investigation are now in progress, including: a) characterizing the absolute pointing accuracy of THEMIS images; b) investigating whether automatic tie point matching algorithms could be used to provide connections between overlapping THEMIS images; c) developing algorithms to allow for the photogrammetric (bundle) adjustment of the THEMIS IR (line scanner) camera images. Our primary goal in this pilot study effort will be to make several test control THEMIS mosaics and better determine which methods could be used, which require development, and what level of effort is required, in order to make large regional or global controlled THEMIS mosaics.
High-resolution topomapping of candidate MER landing sites with Mars Orbiter Camera narrow-angle images
We analyzed narrow‐angle Mars Orbiter Camera (MOC‐NA) images to produce high‐resolution digital elevation models (DEMs) in order to provide topographic and slope information needed to assess the safety of candidate landing sites for the Mars Exploration Rovers (MER) and to assess the accuracy of our results by a variety of tests. The mapping techniques developed also support geoscientific studies and can be used with all present and planned Mars‐orbiting scanner cameras. Photogrammetric analysis of MOC stereopairs yields DEMs with 3‐pixel (typically 10 m) horizontal resolution, vertical precision consistent with ∼0.22 pixel matching errors (typically a few meters), and slope errors of 1–3°. These DEMs are controlled to the Mars Orbiter Laser Altimeter (MOLA) global data set and consistent with it at the limits of resolution. Photoclinometry yields DEMs with single‐pixel (typically ∼3 m) horizontal resolution and submeter vertical precision. Where the surface albedo is uniform, the dominant error is 10–20% relative uncertainty in the amplitude of topography and slopes after “calibrating” photoclinometry against a stereo DEM to account for the influence of atmospheric haze. We mapped portions of seven candidate MER sites and the Mars Pathfinder site. Safety of the final four sites (Elysium, Gusev, Isidis, and Meridiani) was assessed by mission engineers by simulating landings on our DEMs of “hazard units” mapped in the sites, with results weighted by the probability of landing on those units; summary slope statistics show that most hazard units are smooth, with only small areas of etched terrain in Gusev crater posing a slope hazard.
Selection of the Mars Exploration Rover landing sites
The selection of Meridiani Planum and Gusev crater as the Mars Exploration Rover landing sites took over 2 years, involved broad participation of the science community via four open workshops, and narrowed an initial ∼155 potential sites (80–300 × 30 km) to four finalists based on science and safety. Engineering constraints important to the selection included (1) latitude (10°N–15°S) for maximum solar power, (2) elevation (less than −1.3 km) for sufficient atmosphere to slow the lander, (3) low horizontal winds, shear, and turbulence in the last few kilometers to minimize horizontal velocity, (4) low 10‐m‐scale slopes to reduce airbag spin‐up and bounce, (5) moderate rock abundance to reduce abrasion or strokeout of the airbags, and (6) a radar‐reflective, load‐bearing, and trafficable surface safe for landing and roving that is not dominated by fine‐grained dust. The evaluation of sites utilized existing as well as targeted orbital information acquired from the Mars Global Surveyor and Mars Odyssey. Three of the final four landing sites show strong evidence for surface processes involving water and appear capable of addressing the science objectives of the missions, which are to determine the aqueous, climatic, and geologic history of sites on Mars where conditions may have been favorable to the preservation of evidence of possible prebiotic or biotic processes. The evaluation of science criteria placed Meridiani and Gusev as the highest‐priority sites. The evaluation of the three most critical safety criteria (10‐m‐scale slopes, rocks, and winds) and landing simulation results indicated that Meridiani and Elysium Planitia are the safest sites, followed by Gusev and Isidis Planitia.
Meter-scale slopes of candidate MER landing sites from point photoclinometry
Photoclinometry was used to analyze the small-scale roughness of areas that fall within the proposed Mars Exploration Rover (MER) 2003 landing ellipses. The landing ellipses presented in this study were those in Athabasca Valles, Elysium Planitia, Eos Chasma, Gusev Crater, Isidis Planitia, Melas Chasma, and Meridiani Planum. We were able to constrain surface slopes on length scales comparable to the image resolution (1.5 to 12 m/pixel). The MER 2003 mission has various engineering constraints that each candidate landing ellipse must satisfy. These constraints indicate that the statistical slope values at 5 m baselines are an important criterion. We used our technique to constrain maximum surface slopes across large swaths of each image, and built up slope statistics for the images in each landing ellipse. We are confident that all MER 2003 landing site ellipses in this study, with the exception of the Melas Chasma ellipse, are within the small-scale roughness constraints. Our results have provided input into the landing hazard assessment process. In addition to evaluating the safety of the landing sites, our mapping of small-scale roughnesses can also be used to better define and map morphologic units. The morphology of a surface is characterized by the slope distribution and magnitude of slopes. In looking at how slopes are distributed, we can better define landforms and determine the boundaries of morphologic units.
Athena Microscopic Imager investigation
The Athena science payload on the Mars Exploration Rovers (MER) includes the Microscopic Imager (MI). The MI is a fixed‐focus camera mounted on the end of an extendable instrument arm, the Instrument Deployment Device (IDD). The MI was designed to acquire images at a spatial resolution of 30 microns/pixel over a broad spectral range (400–700 nm). The MI uses the same electronics design as the other MER cameras but has optics that yield a field of view of 31 × 31 mm across a 1024 × 1024 pixel CCD image. The MI acquires images using only solar or skylight illumination of the target surface. A contact sensor is used to place the MI slightly closer to the target surface than its best focus distance (about 66 mm), allowing concave surfaces to be imaged in good focus. Coarse focusing (∼2 mm precision) is achieved by moving the IDD away from a rock target after the contact sensor has been activated. The MI optics are protected from the Martian environment by a retractable dust cover. The dust cover includes a Kapton window that is tinted orange to restrict the spectral bandpass to 500–700 nm, allowing color information to be obtained by taking images with the dust cover open and closed. MI data will be used to place other MER instrument data in context and to aid in petrologic and geologic interpretations of rocks and soils on Mars.
Mars Digital Image Model (MDIM) 2.1 control network
Positional control for MDIM 2.1 comes from a new geodetic/photogrammetric solution of the global Mars Mariner 9 and Viking image control network. The details of this network solution are described here. This network incorporates 1,054 Mariner 9 and 5,317 Viking Orbiter images. Accuracy of the new solution is improved primarily as the result of constraining all 37,652 control points to radii from Mars Orbiter Laser Altimeter (MOLA) data and adding 1,232 "ground control points" whose horizontal coordinates are also constrained by MOLA. The MOLA data are believed to have an absolute accuracy on the order of 100m horizontally. Additional improvements result from use of updated timing and orientation data for the Viking Orbiter images, improved reseau measurements and hence distortion correction of the images, and careful checking and remeasurement of control points with large residuals. The RMS error of the solution is 15.8 µm (~1.3 Viking pixels, ~280 m on the ground). The IAU/IAG 2000 coordinate system is used for the network and the mosaic.
Eros: Shape, topography, and slope processes
Stereogrammetric measurement of the shape of Eros using images obtained by NEAR's Multispectral Imager provides a survey of the major topographic features and slope processes on this asteroid. This curved asteroid has radii ranging from 3.1 to 17.7 km and a volume of 2535±20 km 3 . The center of figure is within 52 m of the center of mass provided by the Navigation team; this minimal difference suggests that there are only modest variations in density or porosity within the asteroid. Three large depressions 10, 8, and 5.3 km across represent different stages of degradation of large impact craters. Slopes on horizontal scales of ∼300 m are nearly all less than 35°, although locally scarps are much steeper. The area distribution of slopes is similar to those on Ida, Phobos, and Deimos. Regions that have slopes greater than 25° have distinct brighter markings and have fewer large ejecta blocks than do flatter areas. The albedo patterns that suggest downslope transport of regolith have sharper boundaries than those on Phobos, Deimos, and Gaspra. The morphology of the albedo patterns, their lack of discrete sources, and their concentration on steeper slopes suggest transport mechanisms different from those on the previously well-observed small bodies, perhaps due to a reduced relative effectiveness of impact gardening on Eros. Regolith is also transported in talus cones and in connected, sinuous paths extending as much as 2 km, with some evident as relatively darker material. Talus material in at least one area is a discrete superposed unit, a feature not resolved on other small bodies. Flat-floored craters that apparently contain ponded material also suggest discrete units that are not well mixed by impacts.
Observations of comet 19P/Borrelly by the miniature integrated camera and spectrometer aboard deep space 1
The nucleus of the Jupiter-family comet 19P/Borrelly was closely observed by the Miniature Integrated Camera and Spectrometer aboard the Deep Space 1 spacecraft on 22 September 2001. The 8-kilometer-long body is highly variegated on a scale of 200 meters, exhibiting large albedo variations (0.01 to 0.03) and complex geologic relationships. Short-wavelength infrared spectra (1.3 to 2.6 micrometers) show a slope toward the red and a hot, dry surface (≤345 kelvin, with no trace of water ice or hydrated minerals), consistent with ∼10% or less of the surface actively sublimating. Borrelly's coma exhibits two types of dust features: fans and highly collimated jets. At encounter, the near-nucleus coma was dominated by a prominent dust jet that resolved into at least three smaller jets emanating from a broad basin in the middle of the nucleus. Because the major dust jet remained fixed in orientation, it is evidently aligned near the rotation axis of the nucleus.
USGS and DLR topographic mapping of Comet Borrelly
In the fall of 2001, NASA's Deep Space 1 (DS1) probe imaged Comet Borrelly during a flyby encounter. Three of the Borrelly images have geometry suitable to photogrammetrically map the nucleus, which form two stereopairs with an expected precision (EP) of ~410 m and ~670 m each. DS1 team members at the USGS and DLR have independently produced digital elevation models (DEMs) of Borrelly. Automatic stereo-matching algorithms were used by both USGS and DLR, but the USGS DEM was additionally manually edited in stereo. We accomplished a quantitative statistical comparison of the DEMs and found they have a standard deviation of 120 m, which is small compared to the EP above. There are systematic differences, we estimate the stereomatching error to only 0.20 pixel RMS, which is similar to the level of subpixel matching accuracy obtained in a wide variety of other mapping situations. The resulting DEMs enable a variety of applications such as perspective views, photometric modeling and studies of the energy balance of the nucleus. We hope to use the USGS DEM as a starting point to extrapolate the shape of the hidden side of the nucleus. This would not only let us determine the volume and moments of inertia of the nucleus, but would lead to a calculation of the insolation onto the nucleus averaged over an entire orbit, and thence to a model of the evolution of nuclear shape.