Clear Filter Images Research Articles

High-resolution Ceres Low Altitude Mapping Orbit Atlas derived from Dawn Framing Camera images

The Dawn spacecraft Framing Camera (FC) acquired over 31,300 clear filter images of Ceres with a resolution of about 35m/pxl during the eleven cycles in the Low Altitude Mapping Orbit (LAMO) phase between December 16 2015 and August 8 2016. We ortho-rectified the images from the first four cycles and produced a global, high-resolution, uncontrolled photomosaic of Ceres. This global mosaic is the basis for a high-resolution Ceres atlas that consists of 62 tiles mapped at a scale of 1:250,000. The nomenclature used in this atlas was proposed by the Dawn team and was approved by the International Astronomical Union (IAU). The full atlas is available to the public through the Dawn Geographical Information System (GIS) web page [http://dawngis.dlr.de/atlas] and will become available through the NASA Planetary Data System (PDS) (http://pdssbn.astro.umd.edu/).

The Camera of the MASCOT Asteroid Lander on Board Hayabusa 2

The MASCOT Camera (MasCam) is part of the Mobile Asteroid Surface Scout (MASCOT) lander’s science payload. MASCOT has been launched to asteroid (162173) Ryugu onboard JAXA’s Hayabusa 2 asteroid sample return mission on Dec 3rd, 2014. It is scheduled to arrive at Ryugu in 2018, and return samples to Earth by 2020. MasCam was designed and built by DLR’s Institute of Planetary Research, together with Airbus-DS Germany. The scientific goals of the MasCam investigation are to provide ground truth for the orbiter’s remote sensing observations, provide context for measurements by the other lander instruments (radiometer, spectrometer and magnetometer), the orbiter sampling experiment, and characterize the geological context, compositional variations and physical properties of the surface (e.g. rock and regolith particle size distributions). During daytime, clear filter images will be acquired. During night, illumination of the dark surface is performed by an LED array, equipped with $4\times36$ monochromatic light-emitting diodes (LEDs) working in four spectral bands. Color imaging will allow the identification of spectrally distinct surface units. Continued imaging during the surface mission phase and the acquisition of image series at different sun angles over the course of an asteroid day will contribute to the physical characterization of the surface and also allow the investigation of time-dependent processes and to determine the photometric properties of the regolith. The MasCam observations, combined with the MASCOT hyperspectral microscope (MMEGA) and radiometer (MARA) thermal observations, will cover a wide range of observational scales and serve as a strong tie point between Hayabusa 2’s remote-sensing scales ( $10^{3}$ – $10^{-3}\mbox{ m}$ ) and sample scales ( $10^{-3}$ – $10^{-6}\mbox{ m}$ ). The descent sequence and the close-up images will reveal the surface features over a broad range of scales, allowing an assessment of the surface’s diversity and close the gap between the orbital observations and those made by the in-situ measurements. The MasCam is mounted inside the lander slightly tilted, such that the center of its 54.8° square field-of-view is directed towards the surface at an angle of 22° with respect to the surface plane. This is to ensure that both the surface close to the lander and the horizon are observable. The camera optics is designed according to the Scheimpflug principle, thus that the entire scene along the camera’s depth of field (150 mm to infinity) is in focus. The camera utilizes a $1024\times1024$ pixel CMOS sensor sensitive in the 400–1000 nm wavelength range, peaking at 600–700 nm. Together with the f-16 optics, this yields a nominal ground resolution of 150 micron/px at 150 mm distance (diffraction limited). The camera flight model has undergone standard radiometric and geometric calibration both at the component and system (lander) level. MasCam relies on the use of wavelet compression to maximize data return within stringent mission downlink limits. All calibration and flight data products will be generated and archived in the Planetary Data System in PDS image format.

High-resolution Ceres High Altitude Mapping Orbit atlas derived from Dawn Framing Camera images

The Dawn spacecraft Framing Camera (FC) acquired over 2400 clear filter images of Ceres with a resolution of about 140m/pixel during the six cycles in the High Altitude Mapping Orbit (HAMO) phase between August 18 and October 21, 2015. We ortho-rectified the images from the first cycle and produced a global, high-resolution, controlled photomosaic of Ceres. This global mosaic is the basis for a high-resolution Ceres atlas that consists of 15 tiles mapped at a scale of 1:750,000. The nomenclature used in this atlas was proposed by the Dawn team and was approved by the International Astronomical Union (IAU). The full atlas is available to the public through the Dawn Geographical Information System (GIS) web page [http://dawngis.dlr.de/atlas] and will become available through the NASA Planetary Data System (PDS) (http://pdssbn.astro.umd.edu/).

Ceres Survey Atlas derived from Dawn Framing Camera images

The Dawn Framing Camera (FC) acquired almost 900 clear filter images of Ceres with a resolution of about 400m/pixels during the seven cycles in the Survey orbit in June 2015. We ortho-rectified 42 images from the third cycle and produced a global, high-resolution, controlled mosaic of Ceres. This global mosaic is the basis for a high-resolution Ceres atlas that consists of 3 tiles mapped at a scale of 1:2,000,000. The nomenclature used in this atlas was proposed by the Dawn team and was approved by the International Astronomical Union (IAU). The whole atlas is available to the public through the Dawn GIS web page [http://dawn_gis.dlr.de/atlas].

Spectral edge: gradient-preserving spectral mapping for image fusion.

This paper describes a novel approach to image fusion for color display. Our goal is to generate an output image whose gradient matches that of the input as closely as possible. We achieve this using a constrained contrast mapping paradigm in the gradient domain, where the structure tensor of a high-dimensional gradient representation is mapped exactly to that of a low-dimensional gradient field which is then reintegrated to form an output. Constraints on output colors are provided by an initial RGB rendering. Initially, we motivate our solution with a simple "ansatz" (educated guess) for projecting higher-D contrast onto color gradients, which we expand to a more rigorous theorem to incorporate color constraints. The solution to these constrained optimizations is closed-form, allowing for simple and hence fast and efficient algorithms. The approach can map any N-D image data to any M-D output and can be used in a variety of applications using the same basic algorithm. In this paper, we focus on the problem of mapping N-D inputs to 3D color outputs. We present results in five applications: hyperspectral remote sensing, fusion of color and near-infrared or clear-filter images, multilighting imaging, dark flash, and color visualization of magnetic resonance imaging diffusion-tensor imaging.

Geologic mapping of ejecta deposits in Oppia Quadrangle, Asteroid (4) Vesta

Oppia Quadrangle Av-10 (288–360°E, ±22°) is a junction of key geologic features that preserve a rough history of Asteroid (4) Vesta and serves as a case study of using geologic mapping to define a relative geologic timescale. Clear filter images, stereo-derived topography, slope maps, and multispectral color-ratio images from the Framing Camera on NASA’s Dawn spacecraft served as basemaps to create a geologic map and investigate the spatial and temporal relationships of the local stratigraphy. Geologic mapping reveals the oldest map unit within Av-10 is the cratered highlands terrain which possibly represents original crustal material on Vesta that was then excavated by one or more impacts to form the basin Feralia Planitia. Saturnalia Fossae and Divalia Fossae ridge and trough terrains intersect the wall of Feralia Planitia indicating that this impact basin is older than both the Veneneia and Rheasilvia impact structures, representing Pre-Veneneian crustal material. Two of the youngest geologic features in Av-10 are Lepida (∼45km diameter) and Oppia (∼40km diameter) impact craters that formed on the northern and southern wall of Feralia Planitia and each cross-cuts a trough terrain. The ejecta blanket of Oppia is mapped as ‘dark mantle’ material because it appears dark orange in the Framing Camera ‘Clementine-type’ color-ratio image and has a diffuse, gradational contact distributed to the south across the rim of Rheasilvia. Mapping of surface material that appears light orange in color in the Framing Camera ‘Clementine-type’ color-ratio image as ‘light mantle material’ supports previous interpretations of an impact ejecta origin. Some light mantle deposits are easily traced to nearby source craters, but other deposits may represent distal ejecta deposits (emplaced >5 crater radii away) in a microgravity environment.

Reprint of: Resolved photometry of Vesta reveals physical properties of crater regolith

During its year-long orbital mission, the Dawn spacecraft has mapped the surface of main-belt asteroid Vesta multiple times at different spatial resolutions and illumination and viewing angles. The onboard Framing Camera has acquired thousands of clear filter and narrow band images, which, with the availability of high-resolution global shape models, allows for a photometric characterization of the surface in unprecedented detail. We analyze clear filter images to retrieve the photometric properties of the regolith. In the first part of the paper we evaluate different photometric models for the global average. In the second part we use these results to study variations in albedo and steepness of the phase curve over the surface. Maps of these two photometric parameters show large scale albedo variations, which appear to be associated with compositional differences. They also reveal the location of photometrically extreme terrains, where the phase curve is unusually shallow or steep. We find that shallow phase curves are associated with steep slopes on crater walls and faults, as calculated from a shape model. On the other hand, the phase curve of ejecta associated with young impact craters is steep. We interpret these variations in phase curve slope in terms of physical roughness of the regolith. The lack of rough ejecta around older craters suggests that initially rough ejecta associated with impact craters on Vesta are smoothed over a relatively short time of several tens of Myr. We propose that this process is the result of impact gardening, and as such represents a previously unrecognized aspect of Vesta space weathering (Pieters et al., 2012). If this type of space weathering is common, we may expect to encounter this photometric phenomenon on other main belt asteroids.

PRELIMINARY ANALYSIS OF SOHO/STEREO OBSERVATIONS OF SUNGRAZING COMET ISON (C/2012 S1) AROUND PERIHELION

We present photometric and morphological analysis of the behavior of sungrazing comet C/2012 S1 ISON in SOHO and STEREO images around its perihelion on 2013 November 28.779 UT. ISON brightened gradually November 20-26 with a superimposed outburst on November 21.3-23.5. The slope of brightening changed about November 26.7 and was significantly steeper in SOHO's orange and clear filter images until November 27.9 when it began to flatten out, reaching a peak about November 28.1 ($r_H{\sim}17 R_\odot$), then fading before brightening again from November 28.6 ($r_H{\sim}5 R_\odot$) until disappearing behind the occulting disc. ISON brightened continuously as it approached perihelion while visible in all other telescopes/filters. The central condensation disappeared about November 28.5 and the leading edge became progressively more elongated until perihelion. These photometric and morphological behaviors are reminiscent of the tens of meter sized Kreutz comets regularly observed by SOHO and STEREO and strongly suggest that the nucleus of ISON was destroyed prior to perihelion. This is much too small to support published gas production rates and implies significant mass loss and/or disruption in the days and weeks leading up to perihelion. No central condensation was seen post-perihelion. The post-perihelion lightcurve was nearly identical in all telescopes/filters and fell slightly steeper than $r_H^{-2}$. This implies that the brightness was dominated by reflected solar continuum off of remnant dust in the coma/tail and that any remaining active nucleus was <10 m in radius.

High-resolution Vesta Low Altitude Mapping Orbit Atlas derived from Dawn Framing Camera images

The Dawn Framing Camera (FC) acquired close to 10,000 clear filter images of Vesta with a resolution of about 20m/pixel during the Low Altitude Mapping Orbit (LAMO) between December 2011 and April 2012. We ortho-rectified these images and produced a global high-resolution uncontrolled mosaic of Vesta. This global mosaic is the baseline for a high-resolution Vesta atlas that consists of 30 tiles mapped at a scale between 1:200,000 and 1:225,180. The nomenclature used in this atlas was proposed by the Dawn team and was approved by the International Astronomical Union (IAU). The whole atlas is available to the public through the Dawn GIS web page [http://dawn_gis.dlr.de/atlas].

Resolved photometry of Vesta reveals physical properties of crater regolith

During its year-long orbital mission, the Dawn spacecraft has mapped the surface of main-belt asteroid Vesta multiple times at different spatial resolutions and illumination and viewing angles. The onboard Framing Camera has acquired thousands of clear filter and narrow band images, which, with the availability of high-resolution global shape models, allows for a photometric characterization of the surface in unprecedented detail. We analyze clear filter images to retrieve the photometric properties of the regolith. In the first part of the paper we evaluate different photometric models for the global average. In the second part we use these results to study variations in albedo and steepness of the phase curve over the surface. Maps of these two photometric parameters show large scale albedo variations, which appear to be associated with compositional differences. They also reveal the location of photometrically extreme terrains, where the phase curve is unusually shallow or steep. We find that shallow phase curves are associated with steep slopes on crater walls and faults, as calculated from a shape model. On the other hand, the phase curve of ejecta associated with young impact craters is steep. We interpret these variations in phase curve slope in terms of physical roughness of the regolith. The lack of rough ejecta around older craters suggests that initially rough ejecta associated with impact craters on Vesta are smoothed over a relatively short time of several tens of Myr. We propose that this process is the result of impact gardening, and as such represents a previously unrecognized aspect of Vesta space weathering (Pieters et al., 2012). If this type of space weathering is common, we may expect to encounter this photometric phenomenon on other main belt asteroids.

High resolution Vesta High Altitude Mapping Orbit (HAMO) Atlas derived from Dawn framing camera images

The Dawn framing camera (FC) acquired about 2500 clear filter images of Vesta with a resolution of about 70m/pixels during the High Altitude Mapping Orbit (HAMO) in fall 2011. We ortho-rectified these images and produced a global high resolution controlled mosaic of Vesta. This global mosaic is the baseline for a high resolution Vesta atlas that consists of 15 tiles mapped at a scale of 1:500,000. The nomenclature used in this atlas was proposed by the Dawn team and was approved by the International Astronomical Union (IAU). The whole atlas is available to the public through the Dawn GIS web page [http://dawn_gis.dlr.de/atlas].

Leading/Trailing Albedo Asymmetries of Thebe, Amalthea, and Metis

Using Galileo clear-filter images (effective wavelength ≈0.64 μm), we have created the first albedo maps of the small inner jovian satellites Thebe, Amalthea, and Metis. These maps clearly show that the leading sides of all three satellites are significantly brighter than their corresponding trailing sides, confirming and extending a result first reported by P. C. Thomas et al. (1998, Icarus 135, 360–371). In particular, on all three moons the leading side is brighter than the trailing side by 25 to 30%. The fact that the direction and size of this albedo asymmetry is identical from satellite to satellite suggests that one common physical mechanism is governing the global albedo patterns of all three moons. The most plausible such mechanism is the impact of macroscopic meteoroids that originated outside the jovian system. These impacts, which eject the dust that forms Jupiter's ring system (M. E. Ockert-Bell et al., 1999, Icarus 138, 188–213; J. A. Burns et al., 1999, Science 284, 1146–1150), are probably also responsible for brightening the leading sides of these small satellites.

Phoebe: Albedo Map and Photometric Properties

Using clear-filter images from Voyager 2 (effective wavelength 0.48 μm), we have constructed the first-ever digital albedo map of Saturn's moon Phoebe. Most normal reflectances in this new map are between 0.07 and 0.11; the albedo histogram is largely bimodal, suggesting that the satellite is covered predominantly by two different types of surface materials. The highest albedos are confined to isolated, quasi-circular spots 40 to 100 km across, including three spots of varying albedo in a band immediately south of the equator and one especially bright spot at latitude 60°N (normal reflectance as high as 0.13, ≈50% brighter than the average surface). The bright northern spot and the brightest of the southern spots occur at approximately the same longitude, an alignment that gives Phoebe its significant rotational lightcurve. The low resolution of the Voyager images does not permit interpretation of the bright spots' origin.Phoebe's global-average photometric function was determined by combining the satellite's telescopic near-opposition phase curve (S. Kruseet al.1986,Icarus68, 168–175) with absolute disk-resolved reflectances measured from the Voyager images (triaxial-ellipsoid shape assumed with radii 115, 110, and 105 km). Modeling of the telescopic observations supports the presence of a significant opposition surge, although scatter in the data does not allow unambiguous determination of the surge's exact strength. The derived photometric function is consistent with the idea that Phoebe is a C-type object—probably a primitive, captured body related to Chiron, Pholus, and the inhabitants of the Kuiper Belt. The albedo map and photometric information will aid in the planning of high-resolution Cassini images of Phoebe, which will represent our first close look at this class of primitive outer Solar-System object.

The Structure of Jupiter's Ring System as Revealed by the Galileo Imaging Experiment

The jovian ring system was observed during four orbits of Galileo's nominal mission, when 25 clear-filter images of the rings were taken at spatial resolutions of 23 to 134 km/pixel; the ring appeared fortuitously in an additional 11 images. The tenuous jovian ring system (normal optical depths<10−5) has three components: the halo, main ring, and gossamer ring.The innermost component, atoroidal halo, extends radially from approximately 92,000 to about 122,500 km (near the 3 : 2 Lorentz resonance) and has a full-width, half-maximum thickness of 12,500 km; its brightness decreases with height off the equatorial plane and decreases as the planet is approached. Themain ringreaches from the halo's outer boundary across 6440 to 128,940 km, just interior to Adrastea's orbit (128,980 km); at its outer edge, the main ring takes nearly 1000 km to develop its full brightness. The ring's brightness noticeably decreases around 127,850 km in the vicinity of Metis' semimajor axis (127,980 km). The precise location and nature of the main ring's outer periphery may shift slightly from image to image; the same is true for the “notch” near Metis. The main ring has a faint, vertically extended component that thickens as the halo region is approached. Brightness variations of ±10% are visible in the central main ring and may be due to vertical corrugations, density clumps, or “spokes” in the ring. Unexplained differences between the near- and far-arm brightnesses are visible. We have discovered that thegossamer ring, lying exterior to the main ring, has two primary components, each of which is fairly uniform: one originates just interior to Amalthea's orbit (181,366 km) while the other is situated radially interior to Thebe's orbit (221,888 km). Very faint material continues past Thebe, blending into the background at 250,000 km. The gossamer rings have thicknesses that are comparable to the maximum elevations of these satellites off Jupiter's equatorial plane; from Galileo's nearly equatorial view, the gossamer rings present rectangular end-profiles with greater intensities along their top and bottom surfaces. The rings seem to be derived from the satellites.

Search for glazed surfaces on Triton

Near‐infrared reflectance spectra of Triton obtained from Earth since 1978 show several absorption bands between 0.8 and 4 μm, features attributed to CH4 and N2 and inferred to represent materials in the condensed state (Cruikshank and Brown, 1986, and Spencer et al., 1990). Clark et al. (1983) had previously proposed a frost grain metamorphism model for icy satellite surfaces, according to which freshly condensed CH4 and N2crystals would rapidly reconfigure themselves into a glaze or grow to unusually large sizes under the pressure and temperature conditions prevailing on Triton. In either case, the long path lengths apparently needed to explain the prominence of the absorptions could be provided. More recently, the Voyager 2 flyby of Triton revealed the existence of topographically smooth frozen “lava lakes” and active geyserlike plumes. The presence of the former, and the solid‐state greenhouse mechanism proposed by Smith et al. (1989) and Brown et al. (1990) to power the latter, both suggest the likely existence, at least as localized patches, of surficial clear ice. Thus smooth, even glazed areas of N2 and CH4ice might occur on Triton. We report the results of a search, at the pixel scale, for such glazed areas in Voyager 2 high‐resolution images of Triton. Glazed surfaces, if present, are expected to produce a surge in reflectance when viewed under the geometry of specular reflection (Middleton and Mungall, 1952). Atmosphere‐insensitive green and clear filter images acquired at phase angles ranging from 60° to 100°, and containing mirror points occurring over three different types of terrains (the South Polar Cap Mottled Unit, the South Polar Cap Bright Fringe, and the “Frost Band” region), have been surveyed. No smooth, glazed surfaces producing detectable specular reflection were found. The lack of evidence for glazed areas in the southern mid‐latitude regions, at least at the scale of the spatial resolution (∼3–7 km per line pair), indicates that another cause may be responsible for the prominence of Triton's spectral features. We note that the southern higher‐latitude regions in which active geyserlike plumes are located were not viewed under large enough incidence angles by Voyager 2 to allow conclusive searches for specular behavior.

Small satellites of Uranus: Disk-integrated photometry and estimated radii

Disk-integrated photometry from Voyager 2 clear-filter images is used to compare properties of the ten small satellites that orbit Uranus interior to Miranda. Two satellites, 1985U1 Puck and 1986U7 Cordelia, are resolved. Observations of Puck cover phase angles of 15 to 33° and yield a phase coefficient of 0.031 ± 0.003 mag/deg. The mean radius of Puck is 77 ± 3 km; its geometric albedo (Voyager clear filter, λ ∼ 0.48 μm) is 0.074 ± 0.008. Cordelia's albedo is found to be similar to that of Puck. This result suggests that all the small satellites may be low-albedo objects with reflectances similar to, but slightly higher than, those of the ring material. Satellite radii calculated with the assumption that they have the same reflectance as Puck range from 13 ± 2 km (1986U7 Cordelia) to 55 ± 6 km (1986U1 Portia). The ε-ring shepherds have calculated radii of 13 ± 2 and 16 ± 2 km; these radii correspond to approximate masses of 1.4 and 2.5 × 10 19 g, respectively. No useful constraints on the colors of the small satellites can be obtained given the low signal-to-noise ratio of the color data. We estimate that the opposition magnitudes range from +20.4 for Puck to +23.9 and 24.2 for the two ε-ring shepherds.