Albedo Markings Research Articles

Petrophysical Insights Into Lunar Mafic Extrusive Basalts Over Reiner Gamma Formation

The enigmatic swirls of Reiner Gamma Formation (RGF) have been found to be associated with localized magnetic surges and high albedo markings. The intriguing diversity within deep-seated lithologies unravels the compositional evolution of the RGF soil, thereby providing new insights into the geological formation of lunar swirls. The present work contributes to the petrophysical significance of possible mafic units by utilizing the multisensor data from recent lunar missions. The compositional characterization of mafic-rich regolith facilitates an improved high-resolution eigenvector-based pyroxene spectroscopy. This provides evidence of endogenic magmatic water in the spectra of high Mg orthopyroxene with pronounced olivine content. Some regions in the western part contribute to the presence of chromium with the possible emplacement of mafic-rich basalts in the oxygen-deficient environment. The sensitivity of the radar echo to the petrographic units has been utilized to derive the physical characteristics of the regolith grains. The regions with enhanced hydration attribute to an increased dielectric permittivity with an associated surge in bulk density and circular polarization ratio. This is in concordance with the degree of maturity in the regolith due to space weathering. Moreover, a relatively compact confinement of the soil pattern has been observed in the immature mafic-rich patches, emphasizing the mechanical stability of the regolith. The regional geomorphology indicates the emanation of wrinkled ridges, wherein higher mafic abundance zone attributes to the primitive source of extruded magma. In essence, this study proposes the inclusion of local charged dust variant in the global formation perspective of the RGF.

Topographic Correlations Within Lunar Swirls in Mare Ingenii

AbstractThe Moon's bright albedo markings, known as swirls, are defined by broad, bright, on‐swirl areas separated by darker off‐swirl lanes. Their formation mechanism has long been debated and is key for understanding the processing of the lunar surface, the mobility of the lunar soil particles, and the effects of the space environment on planetary surfaces. Here we present, for the first time, evidence that these features do not necessarily cross the surface without regard to topography or local terrain. Within portions of Mare Ingenii on the lunar far‐side, brighter on‐swirl areas have statistically lower mean elevations than adjacent, darker, off‐swirl lanes. These topographic characteristics provide constraints on the plausible formation mechanisms for the swirls in Mare Ingenii, which in turn provide insight into lunar soil migration and evolution. We believe this correlation with topography argues for highly mobile dust transport across the lunar surface.

Reiner Gamma: A Magnetized Elliptical Disk on the Moon

AbstractThe geologic origin of the Moon's crustal magnetic anomalies is unknown. Reiner Gamma is one of the most studied anomalies, and it is correlated with symmetric bright albedo markings known as swirls. Here we propose that its source magnetization arises from a uniformly magnetized elliptical disk, resulting from the melt sheet or floor deposits of an oblique impact crater. The magnetization was likely acquired in a dynamo field and may be as high as ~70 A/m, perhaps due to incorporation of impactor materials. The disk produces vertical fields at its edges that may channel solar wind flux to the surface, producing an elliptical dark region, while neighboring regions remain shielded and bright. Interestingly, the disk appears to be magnetized along its semiminor axis. Measurements of the low altitude magnetic field at Reiner Gamma would test these predictions and help answer additional questions about its interaction with the solar wind.

A small lunar swirl and its implications for the formation of the Reiner Gamma magnetic anomaly

The Moon does not currently possess a dynamo, but its crust contains numerous magnetic anomalies detected from orbit. The geologic origins of these anomalies are still unknown, including the archetypal Reiner Gamma magnetic anomaly. To gain insight, we study a small magnetic anomaly, herein called the octopus, which is possibly associated with Reiner Gamma. The octopus has curving bright albedo patterns characteristic of features known as swirls. We use high cadence 9 Hz Lunar Prospector magnetometer data, along with constraints provided by this swirl's albedo pattern, to perform inversions for the swirl's magnetic source body characteristics. We use three different inversion methods, and they all return similar results. We also estimate the depth of magnetization from characteristics of the horizontal component of the magnetic field and the albedo pattern. We find that performing inversion for source body properties at small swirls has advantages compared to larger anomalies, or anomalies without albedo markings. We find the octopus is magnetized in the same direction as the main Reiner Gamma anomaly (within 1σ uncertainties), suggesting they formed contemporaneously. The large spatial distance between these coeval anomalies and their inferred shallow source body depths are compatible with formation in a hot ejecta deposit that cooled in the presence of a dynamo field, as suggested by Hood et al. (2001). However, a key remaining enigma is why the northeastern Reiner Gamma “tail” formation has a magnetization direction ∼60° different from the main body and octopus.

Reiner Gamma albedo features reproduced by modeling solar wind standoff

All lunar swirls are known to be co-located with crustal magnetic anomalies (LMAs). Not all LMAs can be associated with albedo markings, making swirls, and their possible connection with the former, an intriguing puzzle yet to be solved. By coupling fully kinetic simulations with a Surface Vector Mapping model, we show that solar wind standoff, an ion–electron kinetic interaction mechanism that locally prevents weathering by solar wind ions, reproduces the shape of the Reiner Gamma albedo pattern. Our method reveals why not every magnetic anomaly forms a distinct albedo marking. A qualitative match between optical remote observations and in situ particle measurements of the back-scattered ions is simultaneously achieved, demonstrating the importance of a kinetic approach to describe the solar wind interaction with LMAs. The anti-correlation between the predicted amount of surface weathering and the surface reflectance is strongest when evaluating the proton energy flux.

Inferring planetary obliquity using rotational and orbital photometry

The obliquity of a terrestrial planet is an important clue about its formation and critical to its climate. Previous studies using simulated photometry of Earth show that continuous observations over most of a planet's orbit can be inverted to infer obliquity. However, few studies of more general planets with arbitrary albedo markings have been made and, in particular, a simple theoretical understanding of why it is possible to extract obliquity from light curves is missing. Reflected light seen by a distant observer is the product of a planet's albedo map, its host star's illumination, and the visibility of different regions. It is useful to treat the product of illumination and visibility as the kernel of a convolution. Time-resolved photometry constrains both the albedo map and the kernel, the latter of which sweeps over the planet due to rotational and orbital motion. The kernel's movement distinguishes prograde from retrograde rotation for planets with non-zero obliquity on inclined orbits. We demonstrate that the kernel's longitudinal width and mean latitude are distinct functions of obliquity and axial orientation. Notably, we find that a planet's spin axis affects the kernel -- and hence time-resolved photometry -- even if this planet is East-West uniform or spinning rapidly, or if it is North-South uniform. We find that perfect knowledge of the kernel at 2-4 orbital phases is usually sufficient to uniquely determine a planet's spin axis. Surprisingly, we predict that East-West albedo contrast is more useful for constraining obliquity than North-South contrast.

Vesta’s north pole quadrangle Av-1 (Albana): Geologic map and the nature of the south polar basin antipodes

As part of systematic global mapping of Vesta using data returned by the Dawn spacecraft, we have produced a geologic map of the north pole quadrangle, Av-1 Albana. Extensive seasonal shadows were present in the north polar region at the time of the Dawn observations, limiting the ability to map morphological features and employ color or spectral data for determination of composition. The major recognizable units present include ancient cratered highlands and younger crater-related units (undivided ejecta, and mass-wasting material on crater floors). The antipode of Vesta’s large southern impact basins, Rheasilvia and Veneneia, lie within or near the Av-1 quadrangle. Therefore it is of particular interest to search for evidence of features of the kind that are found at basin antipodes on other planetary bodies. Albedo markings known as lunar swirls are correlated with basin antipodes and the presence of crustal magnetic anomalies on the Moon, but lighting conditions preclude recognition of such albedo features in images of the antipode of Vesta’s Rheasilvia basin. “Hilly and lineated terrain,” found at the antipodes of large basins on the Moon and Mercury, is not present at the Rheasilvia or Veneneia antipodes. We have identified small-scale linear depressions that may be related to increased fracturing in the Rheasilvia and Veneneia antipodal areas, consistent with impact-induced stresses (Buczkowski, D. et al. [2012b]. Analysis of the large scale troughs on Vesta and correlation to a model of giant impact into a differentiated asteroid. Geol. Soc. of America Annual Meeting. Abstract 152-4; Bowling, T.J. et al. [2013]. J. Geophys. Res. – Planets, 118. http://dx.doi.org/10.1002/jgre.20123). The general high elevation of much of the north polar region could, in part, be a result of uplift caused by the Rheasilvia basin-forming impact, as predicted by numerical modeling (Bowling, T.J. et al. [2013]. J. Geophys. Res. – Planets, 118. http://dx.doi.org/10.1002/jgre.20123). However, stratigraphic and crater size–frequency distribution analysis indicate that the elevated terrain predates the two southern basins and hence is likely a remnant of the ancient vestan crust. The lack of large-scale morphological features at the basin antipodes can be attributed to weakened antipodal constructive interference of seismic waves caused by an oblique impact or by Vesta’s non-spherical shape, or by attenuation of seismic waves because of the physical properties of Vesta’s interior. A first-order analysis of the Dawn global digital elevation model for Vesta indicates that areas of permanent shadow are unlikely to be present in the vicinity of the north pole.

THERMAL SHADOWS AND COMPOSITIONAL STRUCTURE IN COMET NUCLEI

We use a fully 3-dimensional thermal evolution model to examine the effects of a non-uniform surface albedo on the subsurface thermal structure of comets. Surface albedo markings cast "thermal shadows", with strong lateral thermal gradients. Corresponding compositional gradients can be strong, especially if the crystallization of amorphous water ice is triggered in the hottest regions. We show that the spatial extent of the structure depends mainly on the obliquity, ther- mal conductivity and heliocentric distance. In some circumstances, subsurface structure caused by the thermal shadows of surface features can be maintained for more than 10 Myr, the median transport time from the Kuiper Belt to the inner solar system. Non-uniform compositional structure can be an evolutionary product and does not necessarily imply that comets consist of building blocks accumulated in different regions of the protoplanetary disk.

The remarkable surface homogeneity of the Dawn mission target (1) Ceres

Dwarf-planet (1) Ceres is one of the two targets, along with (4) Vesta, that will be studied by the NASA Dawn spacecraft via imaging, visible and near-infrared spectroscopy, and gamma-ray and neutron spectroscopy. While Ceres’ visible and near-infrared disk-integrated spectra have been well characterized, little has been done about quantifying spectral variations over the surface. Any spectral variation would give us insights on the geographical variation of the composition and/or the surface age. The only work so far was that of Rivkin and Volquardsen ([2010], Icarus 206, 327) who reported rotationally-resolved spectroscopic (disk-integrated) observations in the 2.2–4.0 μm range; their observations showed evidence for a relatively uniform surface. Here, we report disk-resolved observations of Ceres with SINFONI (ESO VLT) in the 1.17–1.32 μm and 1.45–2.35 μm wavelength ranges. The observations were made under excellent seeing conditions (0.6″), allowing us to reach a spatial resolution of ∼75 km on Ceres’ surface. We do not find any spectral variation above a 3% level, suggesting a homogeneous surface at our spatial resolution. Slight variations (about 2%) of the spectral slope are detected, geographically correlated with the albedo markings reported from the analysis of the HST and Keck disk-resolved images of Ceres (Li et al. [2006], Icarus 182, 143; Carry et al. [2008], Astron. Astrophys. 478, 235). Given the lack of constraints on the surface composition of Ceres, however, we cannot assert the causes of these variations.

The apparent lack of lunar-like swirls on Mercury: Implications for the formation of lunar swirls and for the agent of space weathering

Images returned by the MESSENGER spacecraft from the Mercury flybys have been examined to search for anomalous high-albedo markings similar to lunar swirls. Several features suggested to be swirls on the basis of Mariner 10 imaging (in the craters Handel and Lermontov) are seen in higher-resolution MESSENGER images to lack the characteristic morphology of lunar swirls. Although antipodes of large impact basins on the Moon are correlated with swirls, the antipodes of the large impact basins on Mercury appear to lack unusual albedo markings. The antipodes of Mercury’s Rembrandt, Beethoven, and Tolstoj basins do not have surface textures similar to the “hilly and lineated” terrain found at the Caloris antipode, possibly because these three impacts were too small to produce obvious surface disturbances at their antipodes. Mercury does have a class of unusual high-reflectance features, the bright crater-floor deposits (BCFDs). However, the BCFDs are spectral outliers, not simply optically immature material, which implies the presence of material with an unusual composition or physical state. The BCFDs are thus not analogs to the lunar swirls. We suggest that the lack of lunar-type swirls on Mercury supports models for the formation of lunar swirls that invoke interaction between the solar wind and crustal magnetic anomalies (i.e., the solar-wind standoff model and the electrostatic dust-transport model) rather than those models of swirl formation that relate to cometary impact phenomena. If the solar-wind standoff hypothesis for lunar swirls is correct, it implies that the primary agent responsible for the optical effects of space weathering on the Moon is solar-wind ion bombardment rather than micrometeoroid impact.

Extreme Kuiper Belt Object 2001 QG298and the Fraction of Contact Binaries

Extensive time-resolved observations of Kuiper Belt object 2001 QG298 show a lightcurve with a peak-to-peak variation of 1.14 +-0.04 magnitudes and single-peaked period of 6.8872 +- 0.0002 hr. The mean absolute magnitude is 6.85 magnitudes which corresponds to a mean effective radius of 122 (77) km if an albedo of 0.04 (0.10) is assumed. This is the first known Kuiper Belt object and only the third minor planet with a radius > 25 km to display a lightcurve with a range in excess of 1 magnitude. We find the colors to be typical for a Kuiper Belt object (B-V = 1.00 +- 0.04, V-R = 0.60 +- 0.02) with no variation in color between minimum and maximum light. The large light variation, relatively long double-peaked period and absence of rotational color change argue against explanations due to albedo markings or elongation due to high angular momentum. Instead, we suggest that 2001 QG298 may be a very close or contact binary similar in structure to what has been independently proposed for the Trojan asteroid 624 Hektor. If so, its rotational period would be twice the lightcurve period or 13.7744 +- 0.0004 hr. By correcting for the effects of projection, we estimate that the fraction of similar objects in the Kuiper Belt is at least 10% to 20% with the true fraction probably much higher. A high abundance of close and contact binaries is expected in some scenarios for the evolution of binary Kuiper Belt objects.

Are the circular, dark features on Comet Borrelly's surface albedo variations or pits?

The highest resolution images of Comet 19P/Borrelly show many dark features which, upon casual inspection, appear to be low albedo markings, but which may also be shadows or other photometric variations caused by a depression in the local topography. In order to distinguish between these two possible interpretations we conducted a photometric analysis of three of the most prominent of these features using six of the highest quality images from the September 22, 2001 Deep Space 1 (DS1) flyby. We find that: 1. The radiance in the darkest parts of each feature increases as phase angle decreases, similarly to the radiance behavior of the higher albedo surrounding terrain. The dark features could be either fully illuminated low albedo spots or, alternatively, they could be depressions. No part of any of the three regions was in full shadow. 2. One of the regions has a radiance profile consistent with a rimmed depression, the second, with a simple depression with no rim, and the third with a low albedo spot. 3. The regolith particles are backscattering and carbon black is one of the few candidate regolith materials that might explain this low albedo. We conclude that Borrelly's surface is geologically complex to the limit of resolution of the images with a combination complex topography, pits, troughs, peaks and ridges, and some very dark albedo markings, perhaps a factor of two to three darker than the average 3–4% albedo of the surrounding terrains. Our technique utilizing measured radiance profiles through the dark regions is able to discriminate between rimmed depressions, rimless depressions and simple albedo changes not associated with topography.

Initial mapping and interpretation of lunar crustal magnetic anomalies using Lunar Prospector magnetometer data

Maps of relatively strong crustal magnetic field anomalies detected at low altitudes with the magnetometer instrument on Lunar Prospector are presented. On the lunar nearside, relatively strong anomalies are mapped over the Reiner Gamma Formation on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Oceanus Procellarum. The main Rima Sirsalis anomaly does not correlate well with the rille itself but is centered over an Imbrian‐aged smooth plains unit interpreted as primary or secondary basin ejecta. The stronger Reiner Gamma anomalies correlate with the locations of both the main Reiner Gamma albedo marking and its northeastward extension. Both the Rima Sirsalis and the Reiner Gamma anomalies are extended in directions approximately radial to the center of the Imbrium basin. This alignment suggests that Imbrium basin ejecta materials (lying in many cases beneath the visible mare surface) are the sources of the nearside anomalies. If so, then the albedo markings associated with the stronger Reiner Gamma anomalies may be consistent with a model involving magnetic shielding of freshly exposed mare materials from the solar wind ion bombardment. Two regions of extensive magnetic anomalies are mapped in regions centered on the Ingenii basin on the south central farside and near the crater Gerasimovic on the southeastern farside. These regions are approximately antipodal to the Imbrium and Crisium basins, respectively. The Imbrium antipode anomaly group is the most areally extensive on the Moon, while the largest anomaly in the Crisium antipode group is the strongest detected by the Lunar Prospector magnetometer. A consideration of the expected antipodal effects of basin‐forming impacts as well as a combination of sample data and orbital measurements on the nearside leads to the conclusion that the most probable sources of magnetic anomalies in these two regions are ejecta materials from the respective impacts. In both regions the strongest individual anomalies correlate with swirl‐like albedo markings of the Reiner Gamma class visible on available orbital photography.

Ground-based High-Resolution Imaging of Mercury

Images obtained during the 1974–1975 Mariner 10 flybys of Mercury revealed nearly 50% of the surface of the planet at very high spatial resolutions. Since then, Mercury's close proximity to the Sun has foiled nearly all ground-based and space-based attempts to achieve high-resolution images of the remainder of the planet. We have used a novel technique to obtain nearly diffraction-limited images of Mercury using the Mount Wilson Observatory 60 inch (1.5 m) reflecting telescope. These images are unique in that they show a portion of Mercury that was not imaged during the Mariner 10 mission, including a bright, possibly impact-related feature in the northern hemisphere, as well as other prominent albedo markings across the disk.

Groundbased Imaging Spectroscopy of Mars in the Near-Infrared: Preliminary Results

Groundbased near-infrared (1.30-4.05 μm) imaging spectroscopic measurements of Mars were obtained during the 1990 opposition to define the spectroscopic variability of the Martian surface and atmosphere. Surface albedo features at wavelengths less than 3.5 μm are almost identical in form and contrast to the classical albedo markings observed in the visible, with typical bright/dark contrasts of 2.0 to 3.0. Poleward of 45°N and 70°S, the reflectance at wavelengths between 2.9 and 3.3 pm was 60-90% lower than at mid- and equatorial latitudes, indicating the presence of atmospheric or surface water ice. Maps of absorption band depths in the 1.9-2.4-μm region show significant spectral variability from region to region on Mars. Comparisons with Viking measurements and radiative transfer model results reveal that most of this variability is correlated with topography, albedo, and emission angle differences but some of it appears to be related to surface mineralogic or airborne dust variations.

Charge-coupled device imaging spectroscopy of Mars. 1. Instrumentation and data reduction/analysis procedures

This paper describes the collection, reduction, and analysis of 0.4–1.0 μm Mars imaging spectroscopy data obtained during the 1988 and 1990 oppositions from Mauna Kea Observatory and provides a general outline for the acquisition and analysis of similar imaging spectroscopy data sets. The U.H. 2.24-m Wide Field Grism CCD Spectrograph was used to collect 13 three-dimensional image cubes covering 90% of the planet south of 50°N in the 0.4–0.8 μm region (λ/Δλ=245 at 0.6 μm) and covering 55% of the planet south of 50°N in the 0.5–1.0 μm region (λ/Δλ=293 at 0.75 μm). Spectra extracted from these image cubes reveal the detailed character of the martian near-UV to visible spectrum. Images at red wavelengths reveal the “classical” albedo markings at 100–500 km spatial resolution while images at blue wavelengths show little surface feature contrast and are dominated by condensate clouds/hazes and polar ice. Many of the data acquisition, reduction, and analysis steps discussed here are new or unique to imaging spectroscopy data sets. These techniques exploit the information contained within the spatial domain of data such as these, thus allowing more traditional point-spectral analysis techniques to be expanded into an imaging format.

Large Quasi-Circular Features Beneath Frost on Triton

Specially processed Voyager 2 images of Neptune's largest moon, Triton, reveal three large quasi-circular features ranging in diameter from 280 to 935 kilometers within Triton's equatorial region. The largest of these features contains a central, irregularly shaped area of comparatively low albedo about 380 kilometers in diameter, surrounded by crudely concentric annuli of higher albedo materials. None of the features exhibit significant topographic expression, and all appear to be primarily albedo markings. The features are located within a broad equatorial band of anomalously transparent frost that renders them nearly invisible at the large phase angles (alpha > 90 degrees ) at which Voyager obtained its highest resolution coverage of Triton. The features can be discerned at smaller phase angles (alpha = 66 degrees ) at which the frost only partially masks underlying albedo contrasts. The origin of the features is uncertain but may have involved regional cryovolcanic activity.

On the shape and albedo variegation of asteroids: Results from Fourier analysis of synthetic and observed asteroid lightcurves

Using the 2254 lightcurves of 357 asteroids contained in the “Asteroid Photometric Catalogue” (C.-I. Lagerkvist et al. 1987a), we have performed Fourier analysis of those asteroids with acceptably complete phase coverage and point density, and for which a unique period has been identified. We have obtained Fourier coefficients for 188 lightcurves of 91 asteroids, and for 868 synthetic lightcurves obtained from a set of models different in shape and albedo markings and at various orientations. Most of the asteroid lightcurves are composites from more than one night: the total number of single lightcurves used is 385. On the basis of the comparison between Fourier coefficients of the model and asteroid lightcurves, we give a description of possible shape and albedo variations for 18 asteroids. We also try to define the general distribution of shapes of all the analyzed asteroid samples: about 75% of the asteroids are quasi-ellipoidal or spheroidal in shape, while about 25% seem to be irregularly shaped. A fraction of 30–40% of the asteroids (mostly the regular ones) show in the lightcurves indications of the presence of albedo variegations.

Infrared lightcurves of asteroids 532 Herculina and 45 Eugenia: Proof of the absence of significant albedo markings

Time-resolved near-infrared (J and K bands) and mid-infrared (N and Q bands) photometric measurements of asteroids 532 Herculina and 45 Eugenia show reflected and thermal lightcurves that are in phase: i.e., thermal maxima and minima occur at nearly the same time as the maxima and minima in reflected light. Model lightcurves for Herculina based on the albedo distribution of Taylor et al. (1987, Icarus 69, 354–369) are predicted to be 90° out of phase. A comparison of the model and observed lightcurves leads us to conclude that the light variations of Herculina and Eugenia are due primarily to shape rather than to surface albedo variations.

Flattening, pole, and albedo features of 4 vesta from photometric data

A number of large asteroids show irregular lightcurves of relatively small amplitude and/or ambiguous rotational periods. These observations and the fact that their strong gravitational binding probably results in quasi-equilibrium shapes lead to model these bodies as axisymmetric, biaxial ellipsoids covered by albedo markings. We developed a general numerical algorithm for obtaining simulated lightcurves of “spotted” asteroids and varied the most critical geometrical and physical parameters (albedo contrast, size, and position of the spots; polar coordinates, and shape of the asteroid). We then analyzed the case of 4 Vesta by assuming an axisymmetric ellipsoidal shape with a large brighter region on one hemisphere, in agreement with the results of photometric and polarimetric observations. Fitting the numerical simulations to the available data, we obtained the flattening of the ellipsoid (0.79 ± 0.03), the albedo contrast and geometry of the brighter region, and the orientation of the polar axis. If the derived flattenning corresponds to the equilibrium shape of a nearly homogeneous body, a density of 2.4 ± 0.3 g cm −3 can be inferred. These results show satisfactory agreement with values by different techniques. We plan to apply the same method both to other large asteroids and to smaller, irregularly shaped ones; in the latter case, this will allow us to test the uncertainties in current pole determination methods.