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Knowledge acquired from spacecraft observations of the surfaces of Phobos and Deimos is reviewed. Consideration is given to: the first spacecraft data - Mariner 7; the first systematic exploration - Mariner 9; surface texture and composition from Mariner 9 data; surface morphology from Mariner 9; and Viking results. A series of Viking photographs of Phobos and Deimos is presented.
The global distribution, morphology, age, possible origin and significance of the long, linear depressions termed grooves on the surface of Phobos are discussed, based on Viking Orbiter data. The grooves, which consist of linear strings of coalesced and separate depressions in a loose regolith up to 100-200 m in depth, are observed to define the intersection of several sets of parallel planes with the surface of Phobos, with the widest and deepest grooves occurring just outside the rim of the 10-km crater, Stickney. The superposition of the grooves on older craters, crater density within the grooves and the intersections of groove sets suggest that the grooves are all of the same age, with their formation closely following that of Stickney. The evidence of groove morphology and distribution is used to attribute their formation to the enlargement of preexisting fractures or the formation of new fractures by the Stickney impact, causing the mobilization of the regolith along the fractures. The lack of observable grooves on Deimos is explained by the absence of a crater large enough to have severly fractured its surface.
A three-dimensional numerical model is used to study the dynamical environment of Phobos and Deimos. 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 also calculated and compared with the observed groove locations in order to evaluate a possible secondary-impact origin for the grooves on Phobos. Attention is also given to the possible influence of the dynamical environment on shaping the satellites' surfaces.
The shape and orbital characteristics of Phobos, the larger and innermost of Mars' two irregular moons, are discussed and illustrated. Also presented are the high resolution pictures of Phobos that were obtained during the close flybys of Viking Orbiters 1 and 2. The viewing geometry is also given for each picture.
The physical characteristics of Phobos and Deimos, satellites of Mars, are discussed. Phobos and Deimos are used as an example to discuss the probable internal structure of objects of this type and the structural formations on their surfaces. The history of astronomical observations of Mars is also described.
The Manned Mars Explorer (MME) project responds to the fundamental problems of sending human beings to Mars in a mission scenario and schematic vehicle designs. The mission scenario targets an opposition class Venus inbound swingby for its trajectory with concentration on Phobos and/or Deimos as a staging base for initial and future Mars vicinity operations. Optional vehicles are presented as a comparison using nuclear electric power/propulsion technology. A Manned Planetary Vehicle and Crew Command Vehicle are used to accomplish the targeted mission. The Manned Planetary Vehicle utilizes the mature technology of chemical propulsion combined with an advanced aerobrake, tether and pressurized environment system. The Crew Command Vehicle is the workhorse of the mission performing many different functions including a manned Mars landing, and Phobos rendezvous.
Viking Orbiter images of Phobos and Deimos have been measured to establish global control networks for 98 surface features of the former and 53 of the latter; photogrammetric triangulation has yielded body-fixed coordinates of these control-points, as well as mean triaxial radii of 13.3 x 11.1 x 9.3 km for Phobos and 7.5 x 6.2 x 5.4 for Deimos. Expressions are also obtained for the inertial orientations of these bodies' spin axes and prime meridians. While these expressions should be accurate to a few tenths of a deg for the 1971-1980 period, their accuracy will degrade with time as the orbit accuracy degrades.
The figure of Phobos is presently modeled by a sixth-degree-and-order spherical harmonic expansion that fits the observed surface to an accuracy of a few hundred m, as validated through the use of control points, limbs, terminators, map projections, and predicted rotational properties. Only the region around the large crater, Stickney, exhibits lower accuracy. The forced libration amplitude obtained closely matches the observed value; this supports the view of Phobos as possessing a uniform density distribution.
Scientific exploration opportunities for human missions to the Moon, Phobos, Mars, and an asteroid are addressed. These planetary objects are of prime interest to scientists because they are the accessible, terresterial-like bodies most likely to be the next destinations for human missions beyond Earth orbit. Three categories of science opportunities are defined and discussed: target science, platform science, and cruise science. Target science is the study of the planetary object and its surroundings (including geological, biological, atmospheric, and fields and particle sciences) to determine the object's natural physical characteristics, planetological history, mode of origin, relation to possible extant or extinct like forms, surface environmental properties, resource potential, and suitability for human bases or outposts. Platform science takes advantage of the target body using it as a site for establishing laboratory facilities and observatories; and cruise science consists of studies conducted by the crew during the voyage to and from a target body. Generic and specific science opportunities for each target are summarized along with listings of strawman payloads, desired or required precursor information, priorities for initial scientific objectives, and candidate landing sites. An appendix details the potential use of the Moon for astronomical observatories and specialized observatories, and a bibliography compiles recent work on topics relating to human scientific exploration of the Moon, Phobos, Mars, and asteroids. It is concluded that there are a wide variety of scientific exploration opportunities that can be pursued during human missions to planetary targets but that more detailed studies and precursor unmanned missions should be carried out first.
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 microstructures 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 about 12 deg on a 61 m scale to about 7 deg on a 216 - 272 m scale. On the same scales, a value of the order of 2 percent 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.
Analytic expressions are derived to model the surface topography and the normal to the surface of Phobos. The analytic expressions are comprised of a spherical harmonic expansion for the global figure of Phobos, augmented by addition terms for the large crater Stickney and other craters. Over 300 craters were measured in more than 100 Viking Orbiter images to produce the model. In general, the largest craters were measured since they have a significant effect on topography. The topographic model derived has a global spatial and topographic accuracy ranging from about 100 m in areas having the highest resolution and convergent, stereo coverage, up to 500 m in the poorest areas.
The dynamics of submicron-sized dielectric particles lost from the Martian moon Phobos are studied in connection with the possible detection of dust by the Phobos 2 spacecraft. The motion of these small dust grains is influenced not only by gravity but also by solar radiation pressure and electromagnetic forces. The plasma environment of Mars is described by applying a hybrid gasdynamic-cometary model. Some of the submicron-sized grains ejected at speeds on the order of a few tens meters per second can stay in orbit around Mars for several months forming a nonuniform and time-dependent dust halo.
Results of two experiments performed onboard the Phobos spacecraft are presented: spectrophotometry in the 300-600 nm range and thermal radiometry, i.e., intrinsic thermal emission in the 6-50 micron range. The thermophysical characteristics of the Phobos regolith are indicated by a thermal inertia coefficient which is similar to that of the moon. The reflectivity of the regolith is inhomogeneous in the region investigated and depends mostly on the relief features, primarily craters and their age. The expected similarity of the reflection spectra with those of carbonacenous chondrites was not confirmed.
A dual-frequency (1.668 and 5.01 GHz) feed was designed for the Deep Space Station (DSS) 14 70-m antenna to support the Soviet Phobos Mission. This antenna system was capable of supporting telemetry, two-way Doppler, and very long baseline interferometry (VLBI). VLBI and two-way Doppler information on the Phobos spacecraft was acquired with this antenna in 1989.
Both observed and synthetic time series of the magnetic field obtained in circular orbit around Mars by the Phobos spacecraft are analyzed. Of the three reported spectral peaks at 8, 12 and 24 h only the 24-h peak could be due to intrinsic sources. However, 24-h spectral peaks can also be produced in synthetic time series with no intrinsic field effects included. Hence, it is concluded that present spectral analyses of time series obtained with the Phobos magnetometers provide no constraints on the size or the existence of an intrinsic magnetic field at Mars.
A preliminary design has been developed for a manned mission to the Martian moon Phobos. The spacecraft is to carry a crew of five and will be launched from Low Earth Orbit in the year 2010. The outbound trajectory to Mars uses a gravitational assisted swingby of Venus and takes eight months to complete. The stay at Phobos is scheduled for 60 days. During this time, the crew will be busily engaged in setting up a prototype fuel processing facility. The vehicle will then return to Earth orbit after a total mission duration of 656 days. The spacecraft is powered by three nuclear thermal rockets which also provide the primary electrical power via dual mode operation. The overall spacecraft length is 110 m, and the total mass departing from Low Earth Orbit is 900 metric tons.
The impact crater Stickney dominates one hemisphere of the Martian moon Phobos; its diameter (11 km) is about half the size of the body (19 x 22 x 27 km). Besides demarking a threshold between cratering and catastrophic disruption, this impact reveals a great deal about the target's interior. Because Phobos has an unusually low density yet exhibits no direct evidence for volatiles such as water ice, it has been supposed that it sequesters volatiles in the deep interior, or that it is made of some exotic substance, or that it is a loosely-aggregated rubble pile. The network of fracture grooves created by the Stickney impact constrain which, if any, of these models accord with observation.
Previous spectra lead to disparate conclusions about Phobos' composition: primitive (like D- and T-asteroids) and processed materials (like lunar soils). Thermal-IR spectra of Phobos obtained by TES are consistent with materials like lunar soils.