Is Phobos Magnetized?

The available experimental data and theoretical reasons allow us to point out only the upper limit of Phobos' magnetic moment, which does not exceed 10¹⁸ G cm³.     

Mass-Wasting Processes on the Surface of Phobos

This paper considers morphologic signatures of mass-wasting processes on the surface of Phobos. Two types of downslope movement of material are distinguished: (i) intracrater volume landslides inside impact craters and (ii) downslope near-surface movement of material. Crater statistics for the Stickney area (based on new images of Phobos) showed that the landslide in the crater Stickney could have been formed after resurfacing of the outer rim of the crater in the process of meteorite bombardment. An estimate of the volume of the landslide in Stickney (1–2 km³) and simulation of its movement allowed us to classify the landslide as a long-runout one. The possibility of forming a hummocky topographic relief to the east of Stickney due to the crater ejecta and the emplacement of the frontal part of the long-runout landslide is discussed.     

Slope variations on the surface of Phobos

It has been suggested that slope fluctuations on the scale of pixel dimensions could be determined by statistical photoclinometry. A closer study of the surface of Phobos reveals variations in the scattering properties of single particles and micro-structures formed by the particles. In the present context, the photoclinometric method of brightness moments is extended to account for these variations by allowing statistical fluctuations in the phase function of the assumed Lommel-Seeliger scattering law. The mean slope on the investigated regions of Phobos has been found to vary from approximately 12 degrees on a 61m scale to approximately 7 degrees on a 216-272m scale. On the same scales, a value of the order of 2% has been obtained for the standard deviation of the scattering phase function. Hints of a fractal-like scale-invariance have been noticed in the covariance function of brightness.     

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.     

Phobos: Control network analysis

Initial analysis of the Mariner 9 high resolution pictures of Phobos surface features has been completed. A control network of 38 landmarks has been established and used to determine the physical size, shape, orientation, libration, and topography properties of Phobos. The results verified the synchronous rotation of Phobos and revealed a libration of approximately 5° in the orbit plane of Phobos. A preliminary map of Phobos, based on the control network analysis, is given.     

The photometric functions of Phobos and Deimos. III. Surface photometry of Phobos

At least three large areas on the surface of Phobos are covered by a dark material of complex texture which scatters light according to the Hapke-Irvine Law. The average 20° to 80° intrinsic and disc-integrated phase coefficients of this material are β_i = 0.020 ± 0.001 mag/deg and β = 0.033 mag/deg, respectively. These values are slightly greater than the values found for Deimos in Paper II (preceding article). On the largest scale the surface of Phobos is rougher than the surface of Deimos, perhaps accounting for the slightly greater phase coefficients. Contrary to the situation on Deimos, no definite regions of intrinsically brighter material are apparent on Phobos. This difference could account for the slightly lower average reflectance of Phobos relative to Deimos. No evidence for large exposures of solid rock has been found in the three areas studied.     

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.     

Project of the mission to Phobos

This article provides the main scientific objectives and characteristics of the Phobos-Soil project, intended to fly to the Martian satellite Phobos, deliver its soil samples to the Earth, as well as explore Phobos, Mars, and the Martian environment with onboard scientific instruments. We give the basic parameters of the ballistic scenario of the mission, spacecraft, and some scientific problems to be solved with the help of the scientific instruments installed on the spacecraft.     

A note on the gravitational field of Phobos

Phobos' proximity to the Roche limit, and some of its consequences deserving attention by the Phobos' mission, are the subject of this note.     

Near-Infrared Spectrophotometry of Phobos and Deimos

We have observed the leading and trailing hemispheres of Phobos from 1.65 to 3.5 μm and Deimos from 1.65 to 3.12 μm near opposition. We find the trailing hemisphere of Phobos to be brighter than its leading hemisphere by 0.24±0.06 magnitude at 1.65 μm and brighter than Deimos by 0.98±0.07 magnitude at 1.65 μm. We see no difference larger than observational uncertainties in spectral slope between the leading and trailing hemispheres when the spectra are normalized to 1.65 μm. We find no 3-μm absorption feature due to hydrated minerals on either hemisphere to a level of ∼5-10% on Phobos and ∼20% on Deimos. When the infrared data are joined to visible and near-IR data obtained by previous workers, our data suggest the leading (Stickney-dominated) side of Phobos is best matched by T-class asteroids. The spectral slope of the trailing side of Phobos and leading side of Deimos are bracketed by the D-class asteroids. The best laboratory spectral matches to these parts of Phobos are mature lunar soils and heated carbonaceous chondrites. The lack of 3-μm absorption features on either side of Phobos argues against the presence of a large interior reservoir of water ice according to current models of Phobos' interior (F. P. Fanale and J. R. Salvail 1989, Geophys. Res. Lett.16, 287-290; Icarus88, 380-395).     

Predicted lightcurves of Phobos and Deimos

Using Mariner 9 results on the shapes, rotation periods and photometric functions of Phobos and Deimos we calculate approximate orbital lightcurves for the two Martian satellites. The prediction is that both Phobos and Deimos should show orbital brightness fluctuations detectable from Earth. For Phobos the detectable amplitude is predicted to be about 0.1 mag; for Deimos, 0.2 mag.     

Radiosounding in the planned mission to Phobos

The opportunities to study Phobos’ internal structure provided by radio methods are considered in this paper. The necessity of these studies is related to solution of the problem of the origin of the Martian moons. Radiosounding is one of the most efficient methods of analyzing the internal structure of small space objects and, in particular, that of Phobos. The new Boomerang project planned according to the Federal Space Program of Russia for 2016—2025 within the Expedition-M program aimed at the exploration of Phobos and delivery of soil samples from its surface to the Earth, as well as the specifics of a ballistic scenario of this expedition, provide a unique opportunity to carry out radioscopy of this space object to discover the internal structure Phobos and to solve the key problem of its origin. The model of Phobos’ internal structure, radiosounding ballistic conditions, analysis of optimum frequency range of sounding, and key parameters of the device required for the experiment are considered in this paper. The significance of proposed studies and opportunities for their implementation are discussed.     

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.     

Phobos sample return mission: Scientific substantiation

The goals and the main scientific objectives of the Phobos-Grunt space mission are considered. The content of the investigations of the main task of the mission—to return a sample of Phobos material to the Earth in order to answer the question about the origin of this satellite of Mars and to reconstruct its history—is discussed.     

Mariner 9 polarimetry of Phobos and Deimos

We have used the Mariner 9 A-camera system to measure the polarization (P) of Phobos and Deimos at large phase angles (α). For Deimos, P = +22 ± 4% at α = 74°; for Phobos P = +20.5 ± 4% at α = 77°, and P = +24.5 ± 4% at α = 81°. These measurements refer to orange light at about 0.57 μm. A comparison with laboratory measurements of powdered rock samples indicates that the observations are consistent with the presence of regoliths on the satellites.     

Arecibo radar observations of Phobos and Deimos

In November 2005, we observed the moons of Mars using the Arecibo 2380-MHz (13-cm) radar, obtaining a result for the OC radar albedo of Phobos (0.056±0.014) consistent with its previously reported radar albedo and implying an upper bound on its near-surface bulk density of . We detected Deimos by radar for the first time, finding its OC radar albedo to be 0.021±0.006, implying an upper bound on its near-surface density of , consistent with a high-porosity regolith. We briefly discuss reasons for these low radar albedos, Deimos' being possibly the lowest of any Solar System body yet observed by radar.     

CHOMIK: a multi-method approach for studying Phobos

CHOMIK is the name of a penetrator constructed for sampling and retrieval of Phobos surface material. It formed an integral part of the Phobos Sample Return Mission. In this paper we present its construction and intended mode of operation, since the concept is still viable for future missions either to Phobos or to other small bodies of similar dimensions. We take Phobos as an example to describe the science case for such an instrument and how it might be utilized to resolve important open issues regarding the origin of the Martian moons. Concerning the latter, we place emphasis on measurement techniques and analysis tools for mapping trace element concentrations in returned sample.     

Interpretation of the surface brightness of Phobos

Analysis of disk resolved images of Phobos obtained by the Phobos 2 spacecraft allows us to study the surface scattering law and albedo variations. From low phase angle images we find variations in local geometric albedo approximately 10%, with a correlation length approximately 1km. The scattering law is reasonably well matched by the recent proposed LPI (Lumme et al. 1990a) model, which allows us to deduce a small scale (approximately 1 mm) surface roughness (approximately 0.5), defined here as the rms. tangent of the local surface normal relative to the mean surface normal in the Duxbury (1991) model of Phobos. This value is very close to what has been found for Mercury and the Moon.     

GRAS-F seismogravimeter for measuring gravity-inertial fields on the surface of Phobos

The problems of measuring the variations in the gravitational and inertial fields on Phobos related to librational oscillations, tidal effects, and seismic impacts have been considered in the paper. It has been indicated that thermal equilibrium noise in a mechanical oscillator, which forms the basis for a sensor, is responsible for the oscillator’s maximal sensitivity at a level of 8 × 10⁻⁹ m/s². The actual sensitivity of the designed three-dimensional seismogravimeter, which was estimated based on the calibration results, is ∼2 × 10⁻⁸ m/s². This makes it possible to measure the anticipated variations in the gravitational field and to obtain information about the seismic noise level with a surface oscillation amplitude resolution at a level of 2.5 × 10⁻⁷ m at frequencies of 0.1 Hz to ∼10⁻¹⁰ m at frequencies higher than 5 Hz.