Motion of blocks on the surface of Phobos: New constraints for the formation of grooves

Although grooves are common on asteroids, on Phobos the grooves have been the subject of various formation hypotheses for two reasons: the fact that Phobos is orbiting around Mars close to the Roche limit, implies an unusual gravity environment as well as the presence of the huge crater impact, Stickney, which seems to be related at least to a family of grooves. Among many hypotheses, it has been suggested that the grooves could have been dug by rolling Stickney ejecta, but this hypothesis was questioned using two main arguments: no block was observed at the end of the grooves, nor do they run downslope. Thus, the study of surface and near surface dynamics on Phobos can clarify the soundness of these controversial arguments.The present study explores this idea by computing the trajectory of a test mass gliding on the surface of Phobos for any initial position and velocity. An ellipsoidal model of Phobos is used for both the gravity and surface geometry, and several orbiting distances from Mars are considered. It is shown that, due to the Phobos rotation, the trajectories generally do not run downslope even for velocities as low as ∼1 m/s. In consequence the corresponding argument against the rolling blocks hypothesis is not applicable. This hypothesis, however, is clearly ruled out for the sets of parallel grooves in the polar regions because of the curvature of the computed trajectories. On the contrary the trajectories issued from the rim of Stickney close to the equator, with velocities of a few m/s, show similitude with the corresponding groove patterns, in particular for the east–west asymmetry. In some conditions the trajectories leave the surface, which may account for regions free of grooves and for trails without blocks at the end. Consequently, the rolling block hypothesis cannot be rejected outright until further analysis is carried out. Damping of the motion on the regolith is simulated introducing a solid damping coefficient in the equations. A relatively small but not unrealistic coefficient of ∼0.1 accounts for the length of the long main western hypothetic block trails. The simulations with damping confirm that the trajectories turn downslope only at the very end for velocities ?1 m/s, but valuable comparison with the observations requires a better model.     

Spectral heterogeneity on Phobos and Deimos: HiRISE observations and comparisons to Mars Pathfinder results

The High-Resolution Imaging Science Experiment (HiRISE) onboard Mars Reconnaissance Orbiter (MRO) has been used to observe Phobos and Deimos at spatial scales of around 6 and 20 m/px, respectively. HiRISE (McEwen et al., JGR, 112, CiteID E05S02, DOI: 10.1029/2005JE002605, 2007) has provided, for the first time, high-resolution colour images of the surfaces of the Martian moons. When processed, by the production of colour ratio images for example, the data show considerable small-scale heterogeneity, which might be attributable to fresh impacts exposing different materials otherwise largely hidden by a homogenous regolith. The bluer material that is draped over the south-eastern rim of the largest crater on Phobos, Stickney, has been perforated by an impact to reveal redder material and must therefore be relatively thin. A fresh impact with dark crater rays has been identified. Previously identified mass-wasting features in Stickney and Limtoc craters stand out strongly in colour. The interior deposits in Stickney appear more inhomogeneous than previously suspected. Several other local colour variations are also evident.Deimos is more uniform in colour but does show some small-scale inhomogeneity. The bright “streamers” (Thomas et al., Icarus, 123, 536–556,1996) are relatively blue. One crater to the south-west of Voltaire and its surroundings appear quite strongly reddened with respect to the rest of the surface. The reddening of the surroundings may be the result of ejecta from this impact.The spectral gradients at optical wavelengths observed for both Phobos and Deimos are quantitatively in good agreement with those found by unresolved photometric observations made by the Imager for Mars Pathfinder (IMP; Thomas et al., JGR, 104, 9055–9068, 1999). The spectral gradients of the blue and red units on Phobos bracket the results from IMP.     

Surface features of Phobos and Deimos

Viking Orbiter images have provided nearly complete coverage of the two satellites of Mars and have been used to construct maps of the surface features of Phobos and Deimos. The satellites have radically different appearances although nearly all features on both objects were formed directly or indirectly by impact cratering. Phobos has an extensive network of linear depressions (grooves) that probably were formed indirectly by the largest impact recorded on Phobos. Deimos lacks grooves as well as the large number of ridges that occur on Phobos. Craters on Deimos have substantial sediment fill; those on Phobos have none. Evidence of downslope movement of debris is prominent on Deimos but is rare on Phobos. Many of the differences between Phobos and Deimos may be caused by modest differences in mechanical properties. However, the lack of a very large crater on Deimos may be responsible for its lack of grooves.     

The origin and specific features of the martian satellites in the context of the euruption concept

The numerous problems related to the origin and evolution of Phobos and Deimos, as well as to specific features of their topography, are readily accounted for in the context of a concept presupposing the possibility of detonation (or burning) of electrolyzed ices.The explosion of an icy asteroid of mass m _A \> 10²³ g within the gravitational sphere of Mars resulted in the capture of secondary fragments into satellite orbits and the formation of a ring of icy and rocky particles. The motion of satellites in the ring reduced the eccentricities of their orbits and rendered their mutual collisions impossible. The thick regolith of Deimos is the material captured from the ring.The impact responsible for the Stickney crater on Phobos initiated a detonation in the material in the crater, and the detonation, in its turn, ignited the products of electrolysis in the bulk of the satellite. Phobos lost its regolith because of detonation-induced acceleration. As a result of the burning out of Phobos's ices, its density somewhat exeeds that of Deimos.Different grooves on Phobos have differing origins. The grooves of groups B, E, D (as specified by P. Thomas et al., J. Geophys. Res., 84, 8457–8477, 1979) are due to fracturing caused by the Sickney event; group C (which is normal to the minor axis of the Phobos ellipsoid) reflects the layered nature of the parent asteroid; while the closely arranged group A grooves, which are normal to Phobos's major axis, were formed during the contraction of the satellite as a result of the loss of burnt-out ices. A number of conclusions are drawn which can be tested by future missions to the satellites.     

Results of TV imaging of Phobos (Experiment VSK-Fregat)

From February to March 1989 the Phobos 2 spacecraft took 37 TV images of Phobos at a distance of 190-1100 km. These images complement Mariner-9 and Viking data by providing higher-resolution coverage of a large region West of the crater Stickney (40-160 degrees W) and by providing disk-resolved measurements of surface brightness at a greater range of wavelengths and additional phase angles. These images have supported updated mapping and characterization of large craters and grooves, and have provided additional observations of craters' and grooves' bright rims. Variations in surface visible/near-infrared color ratio of almost a factor of 2 have been recognized; these variations appear to be associated with the ejecta of specific large impact craters. Updated determinations of satellite mass and volume allow calculation of a more accurate value of bulk density, 1.90 +/- 0.1 g cm-3. This is significantly lower than the density of meteoritic analogs to Phobos' surface, suggesting a porous interior perhaps containing interstitial ice.     

A model of Phobos

A model of the Martian satellite Phobos was constructed at a scale of 1:60 000 using 25 Mariner 9 photorecords and a solar-simulation technique. Measurements of the crater diameters D, depths d, ratios $\text{d}\text{D}$, longitude and latitude locations of the centers, IAU designations, crater shapes, and rim class are given in a catalog of 260 depressions. An open-ended indexing of the craters is based on their locations by octant and diameter magnitude. Six craters were found with sharply defined rims. At least 28 craters have raised rims. The range of the $\text{d}\text{D}$ ratios is from 0.002 to 0.26, with a mean $\text{d}\text{D}$ of 0.10. The mean diameter of Stickney is interpreted to be 11.1 km, its minimum 9.6 km, and the diameter of Hall 5.9 km. A 100-m contour-interval topographic map has been drawn from measurements of the model. This is rendered on an elliptical form of a Lambert equal-area polar projection. The topographic map made it possible to estimate vector lengths from the center of Phobos to vertices on a 6-frequency octahedron that fits the sattelite. A mean radius of 11.0 km results from averaging the vector lengths to the 146 well-distributed vertices of the polyhedron. A volume of 5620 km³ is deduced.     

The unusual dynamical environment of Phobos and Deimos

The nonintuitive dynamical environment of Phobos and Deimos is explored using a three-dimensional numerical model. Surface gravity, escape speeds, and ejecta impact contours are calculated, both for the satellites at their present orbit distances and for orbit distances they may have had in the past. Impact loci for Stickney ejecta are computed and compared with the observed groove locations in order to evaluate a possible secondary impact origin for the grooves on Phobos. Possible effects of the dynamical environment on shaping the satellites' surfaces are discussed.     

Impact fracture patterns on phobos ellipsoids

Phobos-ellipsoid models made of clay were fragmented by the impact of high-velocity projectiles to examine the idea proposed by P. Thomas, J. Veverka, and T. Duxbury ((1978) Nature273, 282–284) that the grooves on Phobos are the manifestation of fractures produced by the Stickney-forming impact. The fracture lines on the models consist of two sets. One is concentric around the impact site and along E lines, which are defined as the intersecting lines of the ellipsoid surface and a set of spherical surfaces with the center of the spheres at the impact site. The other runs radially from the impact site and along P lines, which are defined as the lines crossing E lines perpendicularly on the ellipsoid surface. Some patterns of the grooves originating radially from the crater Stickney on Phobos are very similar to the P lines. The gridded topography, hummocky groove sections, and smooth topography on Phobos could have been formed by the fracture or associated surface disturbances due to the wave induced by the Stickney-forming impact, because they are distributed along the E lines surrounding the converging point of the P lines. All the models except one showed that the density of the fractures east of the impact site is greater than that of those to the west. Fracture patterns similar to one of the most prominent groove sets, which converge and diminish into the region of about (270°, 0°) were not produced by the impact on the ellipsoid of uniform constituent. These grooves would have been produced by the opening of preexisting cracks by the Stickney-forming impact. Other grooves also seem to be affected by such latent cracks.     

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 km³. 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.     

Morphology and Size–Frequency Distribution of Kilometer-Scale Impact Craters on Callisto and Ganymede Derived from Galileo Data

Using high-resolution Galileo images, we counted the number of craters (larger than 1 km) on two of Jupiter's satellites—Callisto (outside and inside the Asgard impact basin) and Ganymede (in the dark cratered Galileo region)—and classified these craters morphologically. Based on the degree of preservation of crater rims, three morphological classes, A, B, and C (from the most preserved to the most degraded), have been identified. The A : B : C ratios, equal, respectively, to 1 : 3 : 5, 1 : 3 : 7, and 1 : 2.5 : 6.5 for fragments of the territory outside and inside the Asgard basin and within Galileo Regio, indicate that these crater populations reached a considerably high degree of maturity. The degradation of kilometer-scale craters on Callisto proceeds by the narrowing of their rims and their disintegration into chains of knobs, probably due to the sublimation of ice that composes the rim material. Comparing the density of craters of different classes in the regions inside and outside Asgard shows that class A craters on the territories examined were formed after the event that formed this impact basin. Kilometer-scale craters on Ganymede degrade through the expansion and smoothing of their rims and the dissection of them by radial furrows. This implies the involvement in the crater destruction of a downslope movement triggered by the seismic activity that accompanied the formation of tectonic grooves. It is possible that ice sublimation also took part in the destruction of craters on Ganymede, but its effect was less prominent than the effect of downslope movements.     

Phobos,Deimos, and the Moon: Size and distribution of crater ejecta blocks

The size and radial distributions of ejecta blocks around craters (D = 0.8 to 10 km) on Phobos and Deimos have been compared to those around lunar craters (D = 0.2 to 3.5 km). The radial distribution of blocks was found to be similar on Phobos and the Moon, but more dispersed on Deimos. For the best imaged crater on Deimos (D = 800 m), the size distributions of blocks and the fraction of excavated volume present as blocks are similar to those on the Moon. The wider dispersal of blocks on Deimos is consistent with other findings on the spread of finer ejecta over the satellite.     

Horizontal transport of the regolith,modification of features,and erosion rates on the lunar surface

New lunar soils, freshly deposited as impact ejecta, evolve into more mature soils by a complex set of processes involving both near-surface effects and mixing. Poor vertical mixing statistics and interregional exchange by impact ejection complicate the interpretation of soil maturization. Impact ejecta systematics are developed for the smaller cratering events which, with cumulative crater populations observed in young mare regions and on Copernicus ejecta fields, yield rates and a range distribution for the horizontal transport of material by impact processes. The deposition rate for material originating more than 1 m away is found to be about 8 mm m.y.^?1 Material from 10 km away accumulates at a rate of about 0.08 mm m.y.^?1, providing a steady influx of foreign material. From the degradation of boulder tracks, a rate of 5±3 cm m.y.^?1 is computed for the filling of shallow lunar depressions on slopes. Mass wastage and downslope movement of bedrock outcroppings on Hadley Rille seems to be proceeding at a rate of about 8 mm m.y.^?1 The Camelot profile is suggestive of a secondary impact feature.     

Regolith transport in craters on Eros

Images of Eros from the NEAR Shoemaker spacecraft reveal bright and dark albedo features on steep crater walls unlike markings previously observed on asteroids. These features have been attributed to the downslope movement of space-weathered regolith, exposing less weathered material (Science 292 (2001) 484; Meteor. Planet. Sci. 36 (2001) 1617; Icarus 155 (2002) 145). Here we present observations of the interiors of large craters (>1 km in diameter) to test this hypothesis and constrain the origin of the features. We find that bright regions in these craters correspond to steep slopes, consistent with previous work. The geographic distribution of craters with albedo variations shows no pattern and does not resemble the distribution of ponds, another phenomenon on Eros attributed to regolith movement. Shadows and other indications of topography are not observed at feature boundaries, implying that the transported layer is ?1 m thick. The presence of multiple bright and dark units on long slopes with sharp boundaries between them suggests that mobilized regolith may be halted by frictional or other effects before reaching the foot of the slope. Features on crater walls should darken at the same rate as bright ejecta deposits from crater formation; the lack of observed, morphologically fresh craters with bright interiors or ejecta suggests that the albedo patterns are younger than the most recently formed craters greater than about 100 m in diameter. Smaller or micrometeorite impacts, which would not necessarily leave evident deposits of bright ejecta, remain possible causes of albedo patterns. Although their effectiveness is difficult to assess, electrostatic processes and thermal creep are also candidates.     

The geology of 433 Eros

Abstract— The global high‐resolution imaging of asteroid 433 Eros by the Near‐Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft has made it possible to develop the first comprehensive picture of the geology of a small S‐type asteroid. Eros displays a variety of surface features, and evidence of a substantial regolith. Large scale facets, grooves, and ridges indicate the presence of at least one global planar structure. Directional and superposition relations of smaller structural features suggest that fracturing has occurred throughout the object. As with other small objects, impact craters dominate the overall shape as well as the small‐scale topography of Eros. Depth/diameter ratios of craters on Eros average ～0.13, but the freshest craters approach lunar values of ～0.2. Ejecta block production from craters is highly variable; the majority of large blocks appear to have originated from one 7.6 km crater (Shoemaker). The interior morphology of craters does not reveal the influence of discrete mechanical boundaries at depth in the manner of craters formed on lunar mare regolith and on some parts of Phobos. This lack of mechanical boundaries, and the abundant evidence of regolith in nearly every high‐resolution image, suggests a gradation in the porosity and fracturing with depth. The density of small craters is deficient at sizes below ～200 m relative to predicted slopes of empirical saturation. This characteristic, which is also found on parts of Phobos and lunar highland areas, probably results from the efficient obliteration of small craters on a body with significant topographic slopes and a thick regolith. Eros displays a variety of regolith features, such as debris aprons, fine‐grained “ponded” deposits, talus cones, and bright and dark streamers on steep slopes indicative of efficient downslope movement of regolith. These processes serve to mix materials in the upper loose fragmental portion of the asteroid (regolith). In the instance of “ponded” materials and crater wall deposits, there is evidence of processes that segregate finer materials into discrete deposits. The NEAR observations have shown us that surface processes on small asteroids can be very complex and result in a wide variety of morphologic features and landforms that today seem exotic. Future missions to comets and asteroids will surely reveal still as yet unseen processes as well as give context to those discovered by the NEAR Shoemaker spacecraft.     

Processes of lunar crater degradation: Changes in style with geologic time

Lunar crater degradation can be divided into two time periods based on differing styles and rates of crater degradation processes. Comparison of lunar radiometric age scales and the relative degradation of crater morphologic features for craters larger than about 5 km diam shows that Period I, prior to about 3.85–3.95 b.y. ago, is characterized by a high influx rate and by formation of large, multi-ringed basins. Period II, from about 3.85–3.95 b.y. to present, is characterized by a much lower influx rate and lack of large multi-ringed basins. Craters formed throughout Period II show generally constant morphologic characteristics. Craters formed in Period I show markedly different characteristics although their residence time could not have increased more than 15% over the total time of Period II. The vast majority of crater degradation of Period I craters took place nearly coincident with their time of formation. Elements of crater degradation and modification during Period I include destruction of crater exterior, rim, and wall facies and structures, decrease in crater depth, and increase in crater floor width. Examination of fresh crater geometry reveals that major changes in crater depth and floor width parameters can occur with the addition of only minor volumes of material as crater fill. Volumes sufficient to produce these characteristic changes are readily available in the surrounding crater wall and rim deposits and can be derived by erosion associated with the observed morphologic changes. Depositional mechanisms associated with lunar landslides are capable of moving material across the crater floor-wall boundary while maintaining and propagating the characteristic break in slope. A prime source of crater degradation during Period I is related to the formation of multiringed basins. The widespread ballistic sedimentation associated with the formation of these basins produces a near-saturation bombardment which excavates and mobilizes large volumes of local material and preferentially moves it into nearby low regions. Seismic effects contribute to degradation by enhancing slope instability and by mobilizing material for downslope movement. The net effect for a crater influenced by multi-ringed basin formation is a tendency toward destruction of crater facies and structure by near-saturation bombardment and seismic effects, the erosion and mobilization of crater material, and the redeposition of this material in nearby low regions, primarily on the crater floor. This process appears to be of major importance in the degradation and modification of craters, in generation of interior crater fill, and in the formation and propagation of Cayley-type plains surfaces.     

Crater densities on the satellites of Mars

The density of craters larger than 1 km in diameter has been determined for the entire surface of Phobos, and half that of Deimos. Densities of craters as small as 10 m on Phobos and 5 m on Deimos have been measured for small areas of the satellites. On both objects, crater densities are similar and yield plots which have slopes close to -1.9 on both incremental and cumulative log-log graphs. These densities are close to those expected to obtain under equilibrium conditions. They are also near the maximum predicted, based on the fragmentation lifetimes of the two objects: that is, the densities are near to the maximum possible before such objects are likely to suffer an impact severe enough to disrupt them. While the observed crater densities cannot be converted to absolute ages in any rigorous fashion, they can be understood if the flux at Mars has been similar to that at the Moon and if the surfaces that we see today generally date back to the end of the period of heavy bombardment some 4 billion years ago. It is extremely unlikely that the surfaces are younger than 1 billion years. There are no large areas on Phobos for which crater densities differ by more than a factor of 3 from the average.     

Compositional interpretation of PFS/MEx and TES/MGS thermal infrared spectra of Phobos

The origin of the Martian satellites presents a puzzle of long standing. Addressing the composition of Phobos will help constrain theories of its formation. Visible and near-infrared spectra of Phobos lack deep absorption features, making the compositional interpretation a tricky task. PFS/MEx and TES/MGS observations in the thermal infrared show several spectral features that can be used to investigate the composition of the surface. Our results show that the majority of the spectra are consistent with the presence of phyllosilicates, particularly in the area northeast of Stickney. This area corresponds to the “blue” region as defined by Murchie et al. (1999). Analysis of PFS and TES observations in the “red” region defined by Murchie et al. (1999) are consistent with tectosilicates, especially feldspars/feldspathoids. We discuss several physical and chemical mechanisms that can act to eliminate or reduce the strength of bands in the VIS/NIR spectra, with possibly little or no effect in the mid-IR. Comparison of the TES and PFS data to the meteorites shows that no class of chondritic meteorites provide significant agreement with the spectral features observed. The lack of consistency of the PFS and TES spectra to analogs of ultraprimitive materials (organic residues) suggests that an origin via capture of a transneptunian object is not supported by these observations, although it cannot be completely ruled out. Derived surface temperatures from PFS and TES data are in very good agreement with brightness temperatures derived from Viking orbiter measurements, Earth-based observations, and values predicted by numerical models. Our results show that the surface temperature of Phobos varies with solar incidence angle and heliocentric distance, reconciling the different results.We collect and summarize the compositional clues for the origin of Phobos discussed in this paper, including our results. Currently, the most likely scenario is the in-situ formation of Phobos, although a capture of achrondrite-like meteorites is not ruled out.     

Phobos: Surface density of impact craters

The preliminary conclusion of the Mariner 9 Television Team that the surfaces of Phobos and Deimos are saturated with craters larger than 0.2 km in diameter is reconsidered on the basis of more extensive and uniform crater counts. For Phobos, it is verified that the surface appears saturated with craters larger than 1 km in diameter. For craters smaller than 1 km, the data points fall below the saturation curve, and it is not clear that all the departure can be explained in terms of loss of resolution. For Deimos, because of the paucity of craters visible in the Mariner 9 images, a statistically meaningful crater density curve cannot be constructed. Definitive crater density curves for subkilometer craters can only be established once additional imagery at a resolution of better than 100 m is obtained. Such imagery will be provided by the 1976 Viking Orbiters.     

Crater morphology in sandstone targets: The MEMIN impact parameter study

Abstract– Hypervelocity (2.5–7.8 km s^?1) impact experiments into sandstone were carried out to investigate the influence of projectile velocity and mass, target pore space saturation, target‐projectile density contrast, and target layer orientation on crater size and shape. Crater size increases with increasing projectile velocity and mass as well as with increasing target pore space saturation. Craters in water‐saturated porous targets are generally shallower and larger in volume and in diameter than craters from equivalent impacts into dry porous sandstone. Morphometric analyses of the resultant craters, 5–40 cm in diameter, reveal features that are characteristic of all of our experimental craters regardless of impact conditions (I) a large central depression within a fragile, light‐colored central part, and (II) an outer spallation zone with areas of incipient spallation. Two different mechanical processes, grain fragmentation and intergranular tensile fracturing, are recorded within these crater morphologies. Zone (I) approximates the shape of the transient crater formed by material compression, displacement, comminution, and excavation flow, whereas (II) is the result of intergranular tensile fracturing and spallation. The transient crater dimensions are reconstructed by fitting quadric parabolas to crater profiles from digital elevation models. The dimensions of this transient and of the final crater show the same trends: both increase in volume with increasing impact energy, and with increasing water saturation of the target pore space. The relative size of the transient crater (in percent of the final crater volume) decreases with increasing projectile mass and velocity, signifying a greater contribution of spallation on the final crater size when projectile mass and velocity are increased.