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[95] TWO MOONS orbit Mars: Phobos (mean diameter, 22.0 km) and Deimos (mean diameter, 14.0 km). As part of the centennial celebration commemorating Asaph Hall's discovery of Phobos and Deimos in 1877, an extensive exploration of the two Martian moons was conducted with the Viking orbiters. The spectacular high-resolution imaging data obtained have rivaled in resolution any previous flyby or orbiter imaging data on any body in our solar system.


These data provided much more knowledge of the moons' surface morphology and their physical and dynamical properties. Phobos was observed to be somewhat smaller than determined by Mariner 9 (~5200 km3 rather than 5700 km3 ), and Deimos was somewhat larger (~1200 km3 rather than 1000 km3). Both satellites are locked into a stable, synchronous rotation about Mars, with their longest axes pointing toward Mars and their shortest axes normal to their orbit planes (which are within a few degrees of Mars' equator). Both satellites have topographic variations as large as 20 percent of local mean radii, and Deimos has a few large flat areas.


Viking found Phobos and Deimos to be within 10 to 15 km of their predicted positions based on Mariner 9 images. Precessing ellipses accurately model the orbits of the two moons, with short-period Mars gravity perturbations having displacement amplitudes of less than a few kilometers on Phobos' orbit, and solar perturbations having displacement amplitudes of less than 5 km for Deimos (except for one 110-km, 54-year periodic longitude perturbation).


Phobos, one of the three satellites in our solar system whose period (7h 39m ) is less than the rotational period of the primary planet (24h 37m for Mars), is losing orbital energy to surface tides it raises on Mars. As the orbit of Phobos decays and gets closer to Mars, Phobos may eventually be torn apart when the tidal forces of Mars overcome the cohesive bond between its particles. Phobos, already inside the "Roche Limit" where internal gravity alone is too weak to hold it together, could conceivably become a ring plane about Mars within the next 50 million years.


Phobos and Deimos are both uniformly gray. Albedos of ~0.06 put both in a class with the darkest objects in our solar system. These dark surfaces appear to be layers of regolith with depths of a few hundred meters for Phobos and at least 5 to 10 meters for Deimos. Cratering of the surface of Phobos continued during and after the formation of the regolith, and the regolith is saturated with craters. However, on Deimos it appears that the regolith continued to develop after the cratering subsided, and the smaller craters (<100 meters) are partially filled or covered. This obscuration of the smaller craters gives Deimos a much smoother appearance than Phobos when [96] viewed at ranges of more than a few hundred kilometers, because the filled craters are near or below the resolving power of the cameras and therefore are not visible.


In contrast to the smooth appearance of Deimos, the surface of Phobos is dominated by sharp, fresh-looking craters of all sizes and a vast network of linear features resembling crater chains. These linear grooves, up to tens of kilometers long and hundreds of meters across, appear to be surface fractures associated with the formation of Stickney, the largest crater on Phobos. Crater densities on both satellites are comparable to densities on the lunar uplands, a fact that suggests ages of up to a few billion years. However, impact fluxes may have been significantly higher for Phobos and Deimos because of ejecta being thrown into orbit about Mars and then recollected as the satellites swept it up in their orbits.


Similar networks of striations have not been identified on Deimos; however, they may have been covered by regolith, and picture resolution may not have been sufficient to identify such features. For example, a large depression 10 km across at the south pole of Deimos may have been caused by a single impact or may have been the result of fragmentation if Deimos was once part of a larger body. Linear features radiating from the center of this depression are suggested by the data, but low picture resolution has limited any interpretation of these features or determination of the origin of the large depression.


The close encounters with Phobos and Deimos have yielded preliminary mass determinations of approximately 1 X 1016 and 2 X 1015 kg, respectively. Using the volumes mentioned earlier, mean densities of about 1900kg/m3 for Phobos and 1400 kg/m3 for Deimos are obtained. These low densities, as well as their colors and albedos, make Phobos and Deimos compositionally similar to Type-l carbonaceous chondrites found in the asteroid belt. These data strongly suggest capture as the origin of the two asteroid-like moons of Mars.


Viking also obtained pictures of Phobos and Deimos, or their shadows, against Mars. The transit pictures were used in refining knowledge of the shapes of the satellites, and the shadow pictures helped locate Viking Lander 1. The satellite and shadow images were used to improve map coordinates of features on Mars surrounding the images.



Phobos from 480 Kilometers. Viking Orbiter 1 flew within 480 km of Mars' inner satellite, Phobos, to obtain the pictures in the mosaic of the asteroid-size moon. As seen here, Phobos is nearly 75% illuminated and is about 21 km across and 19 km from top to bottom. Some features as small as 20 meters across can be seen. Surface features include grooves resembling linear chains of craters and small hummocks which appear to be resting on the surface. The regolith-covered surface is saturated with craters. Hummocks, mostly seen near the terminator (right), are about 50 meters in size and may be surface debris from impacts. [211-5353]

Phobos Closeup. The photomosaic on top was taken at a range of 300 km as Viking Orbiter 1 was approaching Phobos. The areas covered by three pictures taken at a range of 110 to 130 km are outlined on the photomosaic. The upper right area of the photomosaic shows a region dominated by grooves. The grooves are probably fractures in the surface of Phobos from a large impact. Two large craters with dark material on their floors are seen near the bottom of the photomosaic. These flat-bottomed craters give evidence that Phobos is covered by a regolith of up to a few hundred meters thick. The three pictures show the heavily cratered surface; craters as small as 10 to 15 meters are visible. [Top 211-5356. Left 244A03, Center 244A04, Right 244A06]

[98] Fractures Radiating from Stickney Crater. Viking Orbiter 1 flew within 300 km of Phobos in May 1977 to obtain this photomosaic. Raw pictures are at the top and computer-enhanced pictures, to show small surface detail, are at the bottom. The northern hemisphere of Phobos is visible from about 30° above the equator (Phobos' orbit plane), with the side of Phobos facing Mars at the lower right. Phobos presents an illuminated area of about 17 km from top to bottom and 23 km across. The rim of Stickney, the largest crater on Phobos, is seen at the lower left, with a large network of grooves radiating from it. A large, 2-km diameter crater with a slumped wall is seen just below the middle of the picture. [343A27, 29, 31 (P-19133)]


High Resolution View of Grooves on Phobos. This picture shows a northern area on Phobos which is dominated by grooves. An area near the terminator (7.5 X 9.0 km) is seen with visible features as small as 20 meters. Craters of all sizes abound, with a significant portion formed later than the grooves. The grooves radiate from the antipodal point of Stickney, and are probably surface fractures caused by the impact that formed this large crater. Possible outgassing of volatiles during formation could have caused the raised d rims along the fractures by ejecting regolith. [246A06]

Stereoscopic Views of Phobos. The upper pair shows the side facing away from Mars at a range of 500 km from the orbiter. The large craters near the limb are about 4 km across and a few w hundred meters deep. The lower pair shows the side facing Mars at a range of 300 km. The grooves are radiating from Stickney and are tens of kilometers long, hundreds of meters wide, and can be tens of meters deep. [Upper left 246A76, ripper right 246A66, Lower left 343 A08, Lower right 343A25]

[100] Phobos Overflying Ascraeus Mons. This spectacular picture, taken by Viking Orbiter 2, is the first picture ever taken showing such detail on both a satellite and primary planet. Viking (Orbiter 2 was about 13 000 km above the surface of Mars and about 8000 km above Phobos, which increases the apparent size of Phobos relative to features on Mars. Phobos is about 22 km across, and Ascraeus Mons is over 300 km across at its base. The complete outline of Phobos is seen from direct and reflected sunlight. Transit pictures such as this are used to determine the size and shape of the satellite as well as improve the map coordinates of features on Mars registered near the satellite's image. A unique tie between Mars surface (map) coordinates and inertial space can be made when the inertial positions of the satellite and spacecraft are known accurately. [304B88]

Phobos Overflying the Mouth of Ares Vallis. These mosaics of pictures from Viking Orbiter 1 show Phobos passing beneath the spacecraft with the' surface of Mars in the background. These mosaics, taken about a minute apart, show an apparent motion of Phobos across the surface of Mars of about 50 km. Orbiter 1 was 13 700 km above the Margaritifier Sinus region of' Mars, and Phobos was 6700 km from the spacecraft. Phobos, four times darker than Mars, appears black against Mars in these unenhanced pictures. This region of Mars contains chaotic terrain along the equator; it is near the head of Arcs Vallis, a major channel leading to Chryse basin where Viking Lander 1 is located. [451A03-10: 1° N, 19° W]

[101] Phobos Shadow Transit over the Viking Lander 1 Site. The passage of the Phobos shadow over Viking Lander 1 was imaged simultaneously from Viking Orbiter 1 and Viking Lander 1. The time of shadow passage, as observed by the Lander,, was used to locate the position of the shadow (and therefore the position of the lander) in the orbiter pictures. This picture shows the shadow of Phobos (~60 X 120 km across) in the Chryse Planitia region a few kilometers directly north of Viking Lander 1. To the left of bottom center is Maumee Vallis, approximately 420 km southwest of the lander's location. [463A21]

Deimos from Near and Far. A two-picture photomosaic showing the complete side of Deimos visible from Viking Orbiter 2 is on the left, and a high resolution three-picture mosaic of a small area near the terminator is on tile right. The two-picture photomosaic, taken at 500 km, shows a smooth surface with limited cratering and a few large flat areas. No linear grooves are seen: however, bright patches of material near the intersection of the large flat areas are visible. The three-picture photomosaic taken at about 50 km gives a completely different view of Deimos than does the two-picture (lower resolution) photomosaic. A surface saturated with craters and strewn with boulders is revealed by the factor-of-10 increase in resolution.. (Craters have been partially filled or covered by regolith, which gives a smooth appearance to the surface at lower resolution (a range of 500 km or more). A "wind streaking"" effect from upper right to lower left probably resulted from a base surge phenomenon when ejecta material was transported and deposited downtrack by the impact of an incoming meteoroid. A few dark-rimmed craters are seen. [Left 428B10-1 1. Right 423B61-63]


Deimos from 30 Kilometers. Deimos was observed on October 15, 1977 when Viking Orbiter 2 passed within 30 km of the surface. This is one of the highest resolution pictures ever taken of any body in our solar system by an orbiting or flyby spacecraft. The picture covers an area of 1.2 X 1.5 km, and shows features as small as 3 meters. Viking Orbiter 2 would have been visible from the surface of Deimos during this exceptionally close flyby. 'The surface of Deimos is saturated with craters. A layer of crust appears to cover craters smaller than 50 meters, making Deimos look smoother than Phobos. Boulders as large as houses (10 to 30 meters across) are strewn over the surface - probably blocks ejected from nearby craters. Long shadows are seen east by these boulders (sunlight is coming from the' left). The image was taken when the Sun was only 10 above the horizon. [423B03]

[103] Phobos and Deimos-Similar but Not Identical. In the upper images, the surfaces of Phobos and Deimos arc compared at a range of 1400 km. Features as small as 100 meters are detectable. Phobos is viewed at a 90° phase angle and Deimos at a 60° phase angle. Grooves and craters dominate the uniformly dark surface of Phobos at this resolution. Deimos, however, appears to be very smooth, with few craters and with areas of bright albedo. The grooves on Phobos radiate from the large crater Stickney (10 km across) at the left. The bright patches on Deimos are near the intersection of large flat areas. Higher resolution imaging in the bottom images dispels the initial impression of a smooth surface for Deimos, by showing a surface saturated with craters that have been obscured by regolith. [Upper left 357A64, Upper right 413B83, Lower left 244A05, Lower right 423A61]



Phobos and Deimos in Color. Color pictures of the two Martian moons have confirmed Earth-based spectra by also showing both satellites to be gray. The Viking imaging data showed the surfaces to be uniformly gray over the complete surface to a resolution of a few hundred meters. No significant color differences were seen 011 either surface, including areas around craters and those within the bright albedo features on Deimos. The color indicates composition is of a carbonaceous chondritic material. Phobos (a) is at a range of 4200 km, and Deimos (b) is at a range of 2100 km. In these pictures, color differences have be en exaggerated. Most of the color differences are due to noise or are artifacts of the processing, especially around craters and the limb. [Left 357A03-07, Right 355B01-09]