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The surfaces of Phobos and Deimos
Vistas in Astronomy, Voi.22, pp.163-192. © Pergamon Press Ltd. 1978. Printed in Great Britain
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THE SURFACES OF PHOBOS AND DEIMOS J. Veverka Laboratory for Planetary Studies, Cornell University, Ithaca, New York 14853
i.
INTRODUCTION
Although Phobos and Delmos were discovered i00 years a g o , almost nothing was known until recently about their surfaces. Accurate photometry and colorimetrv of the satellites usin~ earth-based telescopes is difficult due to the intense scattering light from Mars which tends to contaminate measurements. Estimates of the apparent ma~nltudes of the satellites culminated in the work of G. P. Kuiper during the favorable opposition of Mars in lO56, who found mean opposition magnitudes of V o = +11.6 for Phobos and +12.8 for Deimos (Harris, 1961). These values are in reasonable agreement with the most recent photoelectric determinations of +11.4 and +12.5, respectively (Zellner and CaDen, I974). Since the absolute sizes of the satellites cannot be determined directly from Earth, Kuiver estimated the diameters assuming that the albedos were similar to that of the lunar surface, and obtained diameters of 12 km for Phobos and 6 km for nelmos. Kuiper also published a B-V color estimate of +0.6 for the satellites (Harris, IQ61), which in the case of Deimos has been substantiated by more recent photometry (Zellner and Caoen, 1974). He concluded that the satellites did not share the red color of Mars, but rather were grey llke many asteroids, confirming the impression of early visual observers.
2.
FIRST SPACECRAFT DATA:
MARINER 7
The first direct information about the physical nature of the satellites came during the Mariner 7 Mission in 1969. As Mariner 7 flew by Mars, it took several dozen close-up images of the planet; in at least one of them the silhouette of Phobos was caught azainst the disk of Mars. From this picture, Smith (1970) was able to infer that Phobos is elliptical in cross-sectlon and about twice as big as Kuiper had estimated. Thus the geometric albedo of the Surface is only 5 or 6% rather than the 11-12% typical of the Moon.
3.
FIRST SYSTEMATIC EXPLORATION:
MARINER 9
The first systematic reconnaissance of the satellites of Mars was planned for the Mariner 9 Mission in 1971-2. This mission produced several dozen hlgh-resolutlon images of Phobos and Delmos which provided our first comprehensive data about the physical nature of the satellite surfaces. These results have been reviewed by Veverka et al. (1974), Pollack (1977), and Duxbury (1977). Mariner 9 produced 200 meter resolution coverage of most of the surface of Phobos (vi~. I) and of about one-half of the surface of Delmos (Fig. 2). Both satellites were found to be approximately elllpsoidal in shape. SurDrlsinglv, the shapes of the two satellites appeared to be similar, with the ratio of the longest to the shortest axes eaual to about 1.4 to i (Fig. 3). Phobos was found to be about 27 x 21 x 19 km across: Deimos, about 15 x 12 x ii kin, or about half as big as Phobos (Duxburv, 1974). The surface of Phobos was found to be heavily cratered (Pollack et el, 1972; Thomas and Veverka, 1976), with the landscape dominated by three very large craters: Roche (5 km in diameter), Hall (6 km) and Stickney (10 kin) (Fig. 4). As expected on the basis of tldal theory, (Burns, 1972), both satellites were found to have spin periods synchronous with their orbital periods (7.6 and 30.3 hours, respectively), and to be moving in such a way that their longest axis always points toward Mars. Since the orbit of Mariner @ was entirely within that of Delmos, the tidal locking of the satellltes meant that only the Mars facing side of Delmos could be imaged. 163
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At the best available resolution (200 meters) the surface of Deimos appeared to be smoother than that of Phobos, and differed from that of the inner satellite in having conspicuous patches of higher albedo material (Fig. 5). (At Mariner 9 resolution the surface of Phobos appeared to be homogeneous in albedo (cf. Fig. i)). Initial crater counts on Phobos (Fig. 6) showed that the density of impact craters was very close to that found in the lunar uplands (Pollack et al, 1972; Thomas and Veverka, 1976). For Deimos, the Mariner 9 crater density data were of marginal statistical si~niflcance, but suggested that the surface of the outer satellite was also "saturated" with imoact craters. This led to the dilemma that Deimos was apparently as heavily cratered as Phobos, yet appeared to be much smoother. Fortunatelv this problem has now been resolved by the high resolution imagery of Deimos obtained bv Viking Orbiter II (Section 7).
4.
SURFACE TEXTURE AND COMPOSITION:
MARINER 9 DATA
Mariner 9 yielded little data on the actual composition of the satellite surfaces. Various investigators derived mean albedos of 5-6% for Phobos and 6-7% for Deimos (Veverka, 1977) while Noland and Veverka (1976) derived a value of 1.15 ± 0.I0 for the relative brightness ratio (Deimos/Phobos) of the two surfaces from Mariner q photometry. Thus both satellites have almost equally dark, black surfaces, but the average surface of Deimos is slightly brighter. While the low albedos and grey colors suggested a composition similar to that of C-asteroids and of carbonaceous meteorites (Veverka, 1977) a unique identification of composition could not be made on the basis of the scant information available. Noland and Veverka (1977a) found that the bright albedo material on Delmos was about SO% brighter than the average background surface, and suggested that the presence of these patches could account for the slightly higher albedo of Deimos relative to Phobos. The bright patches could be explained as deposits of fine-grained eJecta from imoacts~ and suggested the possibility that some flne-grained ejecta was retained even on bodies as gravitationally weak as Phobos and Deimos. (On Phobos, g=10-3g~; on Deimos g is about half as large as on Phobos). This possibility was confirmed by a series of studies which showed that the satellites surfaces have the intricate texture of a fine-~ralned regollth, and not that of solid, uncommlnuted rocks. Noland and Veverka (1976; 1977a,b) found that the light scattering properties of the surfaces were very similar to those of the lunar surfaces and concluded that the surfaces must have the intricate texture of a flne-grained regolith, similar to that of the lunar soil. Unfortunately, it is impossible to determine the depth of a regollth bv photometric techniques. Specifically, Noland and Veverka (1976; 1977a,b) found that the surfaces of Phobos, Deimos and of the Moon have similar photometric functions. The phase coefficient for the surface material on Phobos was found to be 0.019 mag/deg, compared with 0.01~ mag/deg for the lunar surface. The phase coefficient for the surface material of Deimos was less, 0.017 mag/deg, consistent with the smoother visual appearance of the outer satellite. These photometric results supported the conclusion drawn from some earlier Earth-based polarization measurements of Delmos by Zellner (1972). Zellner found that the polarization curve of Deimos has a deep negative branch at small phase angles, which is a characteristic signature of regoliths. Additional strong evidence for the presence of a re~olith on Phobos was derived from temperature measurements made by the Mariner 9 Infrared Radiometer (Gatley e t a l , 1974). By measuring the rate at which the surface of Phobos heats up following an eclipse by the shadow of Mars, one can calculate the thermal inertia of ~he surface layer. Values for the thermal inertia obtained by Gatley et al (10 -3 cal cm-sec -~ K -l) are characteristic of lunar-llke regoliths. Thus the photometric, polarimetric and radiometric data all indicate that the surfaces of both satellites are covered with a fine-gralned regolith derived, most probably, from the comminution of the surfaces by impacts. Noland and Veverka (1976;1977 a,b) presented photometric evidence that on both satellites these regoliths are homogeneous in texture on scales of 200 meters. They also concluded that the regolith is homogeneous in albedo on such scales on Phobos, but not on Deimos (Figure 5). With the exception of some UV reflectance measurements between 0.2 and 0.4 ~m, which have only been reduced recently (Pang et al 1977a; 1977b), Mariner 9 obtained little direct data on the composition of the satellite surfaces. However, the Mariner 9 data did establish the texture of the surface layers, and proved, for the first time, that regollths are present on objects even as small as Phobos and Delmos. It has been suggested that in this respect Phobos and Deimos may not be representative of asteroids of comparable size since, unlike the asteroids, the two satellites exist deep in the potential well of a big planet. Soter (1971) calculated that while impacts easily eject fragments from Phobos (or Deimos), many of these do not escape from Mars but end un in Phobos-like or Deimos-like orbits. Thus the satellites have an opportunity to recapture their own debris and Soter's calculation suggests that this re-capture process may be
The Surfaces of Phobos and Deimos
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Mariner 9 view of phobos, taken from a range of 7170 fan at a phase angle of (Picture DAS 04470630; Rev. 80).
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Figure 5: Mariner 9 view of Deimos showing conspicuous bright patches which are about 30% brighter than their surroundings (Noland and Veverka, 1976). (Picture DAS 05553383; Rev. 111; Range=7220 km; Phase=31°).
Figure 7: Apparent crater chain faintly visible in Mariner 9 frame DAS 04790460 (Rev. 89' Range=5760 km; Phase=80°). The individual small craters are about 200-300 meters across.
The Surfaces of Phobos and Deimos
167
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Fisure 3: Approximate shapes and dimensions of Phobos and Deimos. The shapes of both satellites can be approximated by triaxial ellipsoids (Duxburv, 1974).
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Fisure 6: Crater counts on Phobos from Mariner 9 data (from Thomas and Veverka, 1977). The solid llne represents the crater density in the lunar uplands. The fall-off at crater diameters less than i km is now known to be a resolution effect (compare with recent Viking data Fig. 15).
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The Surfaces of Phobos and Deimos
169
Fisure 8: View of Phobos from Viking Orbiter 2 (039B84). The large crater at top (Roche) lies close to the north pole of Phobos and is about 5 km across.
170
J. Veverka
Figure 9: Close-up of Phobos grooves obtained by Viking Orbiter i during February 1977. Range=ll0 kin. The picture is about 3 km across and shows detail as small as about 5 meters.
The Surfaces of Phobos and Deimos
171
significant. But it is unlikely that the presence of rezoliths on Phobos and Deimos can be attributed entirely to Soter's hypothetical mechanism since observations of many small asteroids, such as 433 Eros (Zellner, 1976), suggest that flne-gralned regollths occur on all small bodies.
4.
SURFACE MORPHOLOGY:
MARINER 9 RESULTS
Mariner 9 imaged the satellite at resolutions sufficient to show the basic heavily cratered morphology of both surfaces. Craters of varying sizes and varying degrees of degradation were seen, but no unexpected surface features were discovered. True, in the case of Phobos, a few chains of small craters were noticed (Figure 7), but their significance was not appreciated fully. However, Pollack et al (1973) did suggest that such chains may represent the results of outgassing along fractures following a large impact such as that which produced Stickney, the largest crater on Phobos. Although the resolution of the Mariner 9 images was inadequate for a detailed study of crater morphology, it was sufficient to show that eJecta blankets were not prominent. None of the craters showed any evidence of a central peak, consistent with the idea that the formation of central peaks is a gravity dependent phenomenon (Hartmann, 1972). For a surface gravity of only about 10-3g@, one would not expect central peaks to form in any crater small enough to occur on the surface of the satellite.
5.
PLANNED VIKING STUDIES:
1976-1977
The general purpose of the Viking Orbiter investigations of Phobos and Deimos in iq76-1977, was to study the surfaces in more detail and at higher resolution. Although no new major discoveries were anticipated, the intent was to concentrate on those Darts of the surfaces which had not been imaged adequately by Mariner 9. One purpose of this coverage was to provide data for crater counts down to very small diameters to make it possible to look for crater density variations on the satellite surfaces. Such variations could exist if significant chunks of a satellite had been spalled off by "recent" impacts (Pollack et al, 1973). Another major objective of the Viking studies was to obtain information about the comoositions of Phobos and Deimos, both by making color measurements and by determinin~ the masses of the satellites during very close encounters of the spacecraft with the satellites. The masses could be used to determine the mean densities of the satellites. From accurately determined mean densities one could distinguish a dark grey object made of basaltic rock from one made of primitive carbonaceous chondrite rock, since the density of the first would be about 3.5 g/cm 3 while the second would have a density of 2.5 g/cm 3 or less.
6.
VIKING RESULTS
The Viking Orbiter Missions not only achieved the initial satellite objectives outlined above but have obtained a number of totally unexpected and spectacular results. (Duxburv and Veverka, 1977; Veverka and Duxbury, 1977). The Viking Orbiters began to study the satellites during the summer of 1976. Durin~ the Primary Mission (summer and fall of 1976) numerous encounters with the two satellites at minimum ranges of 3000 to 5000 km occured. These minimum ranges were smaller than any achieved by Mariner 9 (Veverka et al, 1974). This fact combined with the better performance of the Viking Orbiter cameras led to images with much higher resolution than those obtained by Mariner 9. One unusually close encounter with Phobos (range about 880 km) occured during the Primary Mission and yielded images with an effective resolution of 50 meters! As part of the Viking Extended Mission, it was planned to make two very close passages to Phobos with Viking Orbiter l, in order to determine the mass and obtain very high resolution images. The first of these encounters took place in February iq77, the second in May 1977, at minimum distances of about 100 and 300 km, respectively. A close encounter between Deimos and Viking Orbiter II took place in October 1977, at a minimum distance of less than 30 km. The image obtained during the 880 km flyby of Phoboe by Viking Orbiter I in late iq76, revealed that the surface is not only heavily cratered, but is cries-crossed bv numerous grooves (Fig. 8). At the moderate resolution then available (about 50 meters), it seemed that there were both trough-like grooves and chains of elongated depressions on the surface. Some of these chains resembled chains of secondary craters on the Moon (Veverka and Duxbury, 1977).
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Figure i0: Close-up of Phobos grooves obtained by Viking Orbiter I during May 1977 from a range of about 300 km. The crater Stickney is over the limb at left. The largest crater near the center of the mosaic is about 1 km in diameter. Two d~fferent enhancements are shown.
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The Surfaces of Phobos and Deimos
175
Various theories of the origin of the grooves were presented, and it became Imoerative to obtain more data on the nature and distribution of the grooves during the February and May 1977 close encounters in order to constrain the range of speculation. These data (Figure 9 and I0) proved convincingly that the grooves are old, are associated with the crater Stlckney, and are probably surface expressions of deep fractures within the body of Phobos (Thomas et al, 1977a). A map of the distribution of the grooves on the surface of Phobos (Thomas et al, 1977b) shows that the grooves are best developed near the crater Stlckney and die away near the antipodal point (Figure ii). Individual grooves can be followed for uD to 12 km. Typically they are 100-200 meters wide and only 10-20 meters deep. By counting impact craters superimposed on the grooves (Figure 12), Thomas et al (lq77a,b) find that the grooves are not significantly younger than most of the surface features on Phobos (Fig 13), and most probably are older than about 3 billion years. The distribution of grooves on the surface of Phobos and the fact that the grooves appear to be best developed in the vicinity of Stlckney and that they die out near the point antipodal to this crater, suggest that the formation of the crater and of the grooves were intimately connected events. It has been suggested that some of these grooves represent fractures produced by Martian tides (Soter and Harris, 1977). At the present time Phobos is deep within the gravitational field of Mars and Soter and Harris have calculated that Martian tides may be pulling Phobos apart. If tidal forces are important in producing the grooves, then some of th~ grooves should be very young, since Phobos is spiralling into Mars on a time scale of I0 years (Pollack, 1977). However, it now apoears that the grooves are at least 3 billion years old (Thomas et al, 1977b). The Sorer-Harris mechanism also fails to explain the global groove pattern and its relationship to Stlckney. Even if one imagines that a large impact (Stickney) pre-fractures the satellite and that tides later exploit this preimposed fracture pattern, some of ~he grooves should be young, since it is unllkelv that Phobos has spent as much as 3 x i0 = years close to the Roche limit of Mars. Close examination of the grooves indicates that many segments are pitted (Figure 14). Such pitting can be explained either by sub-surface drainage of a regolith into fractures or by outgassing from the interior (Veverka et al, 1977). Since gravity is very weak on Phobos, the subduction of a regolith is probably an inefficient process. The fact that some seRments of the grooves may have subtle raised rims (Thomas et al, 1978a) further suoDorts the possibility that outgassing was involved in modifying the appearance of some grooves. One possible scenario is that a very large imoact, specifically that which produced Stlckney, fractured the satellite throughout most of its interior and at the same time heated some parts of it enough to release volatiles, a situation already anticioated in part by Pollack et al (1973), who suggested that some of the "crater chains" barely visible in Mariner 9 images (Figure 7) were due to outgassing of volatiles from the interior of Phobos following a large impact. The inferred composition of Phobos is consistent with such a scenario. By combinin~ spectral data obtained by the Viking 1 Lander (Pollack et al, 1977) with those from the Mariner 9 Ultra-violet Spectrometer (Pang et al, 1977a), a spectral reflectance curve of Phobos between 0.2 and 0.9 m has been constructed. In terms of its spectral reflectance curve, the surface of Phobos appears to be very similar to the type of carbonaceous material found in Type I or Type II carbonaceous chondrites and on the surface of the largest asteroid Ceres. In fact, Pang (1977) has argued that the UV soectrum of Phobos contains a feature characteristic of clay minerals found in Type I and Type II carbonaceous chondrites. Such materials (Table I) contain 10-20% bound water bv weight (Mason, 1971) axtd are unstable at temperatures above 400°K (Anders and Owen, 1977). Since the mean subsurface temperature of Phobos probably exceeds 250°K, a large impact need only raise the temperature locally by 150 K before outgasslng the water will occur. Such outgassing will take place preferentially alone fractures, and can explain both the pitted appearance of some of the grooves as well as the possible occurrence of raised rims (Veverka et al, 1977). The carbonaceous chondrite composition of Phobos seems to be confirmed by the low mean density of the satellite derived from the Viking mass determination: about 2 g/cm 3 (Tolson et al, 1977). A further confirmation could be obtained by detecting the 1.6 m absorption feature of water of hydration in the spectrum of Phobos. This feature, prominent in the reflectance spectra of Type I and Type II Carbonanceous Chondrltes. has recently been detected in the snectrum of Ceres by Lebofskv (1977). The suggested carbonaceous chondrite composition of Phobos not only explains the color and low albedo of the satellite, but explains why Phobos might tend to fracture during a severe impact, since low density carbonaceous material is known to be mechanically very weak. Such a composition also explains the possible release of volatiles along some of the fractures leading to the formation of pits and possible raised rims.
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J. Veverka
CRATER DENSITIES ON GROOVES
td
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10
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Diameter, meters
Figure 13: Surface density of impact craters within grooves (points) compared with that on the rest of Phobos (dashed line) and on the lunar uplands (solid llne). The lunar curve is extrapolated from Hartmann (1974). Within the accuracy of the data we consider the dashed and solid curves to be identical. The data points for the grooves fall into two groups since two sets of images of differing resolution were used, (After Thomas et al, 1977b).
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CARBONACEOUS CHONDRITES
Table I: Major characteristics of dffferent types of Carbonaceous Chondrite meteorftes (after Mason, 1971).
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J. Veverka
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10 PHOBOS COMPOSITE] CRATER DENSITY " #
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The Surfaces of Phobos and Deimos
179
102
DEIMOS CRATER DENSITY
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Figure 15: Crater density on Phobos and Delmos based on Viking Orbiter images compared with the crater density in the lunar upland (solid llne). a)
Phobos
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Deimos; d a s h e d l i n e r e p r e s e n t s a f i t
t o t h e Phobos p o i n t s i n F i g . 15a.
180
7.
J. Veverka
OTHER VIKING RESULTS
The Viking imagery has made it possible to extend crater counts down to very small crater diameters (~i0 meters). The density of imnact craters on the surfaces of both satellites is identical and within the error bars cannot be distinguished from that obtaining in the lunar uplands (Figure 15). Thus, llke the lunar uplands the surfaces of both satellites appear to be in an equilibrium state as far as cratering is concerned and one can estimate a minimum exposure age of at least 3 billion years (Thomas et al, 1978b). There is no evidence on Phobos that different areas of the surfaces differ in age: thus no large-scale spallation has occurred in recent times. Specifically the surface density of craters in the "ungrooved" region antipodal to Stickney is identical to the average value for Phobos. Our coverage of Deimos at the highest resolution is too limited to permit a comoarable search for variations in the density of small impact craters over the surface of the outer satellite. However, these images (Figure 16) have explained why Deimos appears to be smoother than Phobos at low resolution, even though the surface density of craters is the same: many of the craters on Deimos, unlike those on Phobos, have apparently been partially filled by eJecta. In fact, the difference in the amount of apparent blanketing of the two surfaces is remarkable (Figure 17). On Phobos there is essentially no evidence that craters have been filled in significantly following their formation. Typically one finds that the walls and floors of many larger craters are "saturated" with small craters indicating that little blanketing (and incidentally, little gravity slumping) has occurred. On Deimos, however, it is quite evident that many craters are partially filled to depths of about 5 meters. This fill appears to be eJecta, a view supported by the ubiquitous presence on Delmos of numerous blocks and patches of higher albedo material (Figure 16). Isolated blocks appear to be rarer on Phobos (although we do not have 3 meter resolution images of the inner satellite), and isolated albedo patches are almost non-existent. The reason why the surface of Deimos appears to be blanketed much more than that of Phobos by both fine (fill and patches) and coarse (blocks) material remains unclear. Incidentally, we now understand why the phase coefficient of Phobos is larger than that of Deimos (Noland and Veverka, 1976): the surface of Phobos is macroscoplcally rougher than that of Deimos due to deeper craters and to the presence of grooves. Thomas and Phobos and appears to roughness,
Veverka (1977b) applied DN variance techniques to study the relative roughness of Deimos and found similar variances for the two satellites. This result now have been fortuitous since on Phobos the variances were determined by surface whereas on Delmos they were controlled by albedo variations.
The Viking images are making it possible to study the details of crater morphology on the satellites (Figure 18). On Phobos, the freshest craters have depths which are about 20% of the crater diameter, a relationship very similar to that found for small fresh lunar craters by Pike (1974). As expected, due to the low gravity of Phobos, rayed eJecta blankets are not evident, but bright albedo rings (5-10% brighter than their surroundings) are conspicuous around many craters at low phase angles (Figure 19), suggesting the presence of coarse textured "ejecta" near the crater rims. Some craters on Phobos contain dark albedo patterns on their floors which are most conspicuous at large phase angles (Figure 20). Goguen et al (1977) interpret these as areas of very rough texture whose albedo near zero phase is similar to that of the mean Phobos surface (~6%), but whose phase coefficient is much larger. Typically, the contrast of such areas is less than 10% near opposition but reaches 100% near phase angles of 90 ° . Goguen et al have suggested that these dark areas represent patches of solidified impact melt which remain consplcious on Phobos because there is so little fall-back of eJecta into the crater due to the satellite's negligible gravity. How deep are the regoliths on Phobos and Delmos? Although the craters on Phobos show no evidence of being filled in, there are signs of possible layering in some of the crater walls to depths of several hundred meters (Figure 21), suggesting that the regolith may be at least that deep. As mentioned above, on Delmos we see craters filled in to depths of about 5 meters; thus the regolith is at least this deep. One of the most exciting results from the October flyby of Deimos, is the apparent absence of grooves on the outer satellite (Figure 22). While it is true that this absence is consistent with the tidal hypothesis of Soter and Harris, it does remain the case that tides do not provide an adequate explanation of grooves on Phobos. It is more likely that Delmos does not have grooves because (a) no sufficiently large crater exists on its surface, and/or (b) that Delmos is made of mechanically stronger material than Phobos. The largest crater definitely imaged on the surface of Delmos is only 3 km in diameter; but we have
The Surfaces of Phobos and Deimos
Fi6ure 14: Portion of Viking Orbiter 1 frame 246A05 taken from a range of about 250 km. Note the conspicuous pitting of the grooves. The grooves are tvpically 100-200 meters wide.
181
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The Surfaces of Phobos and Deimos
Figure 17: One of the highest resolution images of Phobos obtained by Viking Orbiter i from a range of about ii0 kin. The frame is about 3 km across. Detail as small as 5-6 meters is visible.
183
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J. Veverka
Figure 18: Viking Orbiter i image of Phobos taken from a range of 530 km. visible on some of the craters at top.
Raised rims are
The Surfaces of Phobos and Deimos
185
Figure 19: Viking Orbiter 1 view of Phobos at a phase angle of 14 °. The conspicuous bright rings around many of the craters probably represent areas of unusually intricate texture. Range = 370 km.
186
J. Veverka
Figure 20: Viking Orbiter 1 view of Phobos from a range of about 310 km. spicuous dark marking in the 2 km crater at upper left.
Note the con-
The Surfaces of Phobos and Deimos
Fi6ure 21: View of Phobos from about ii0 km obtained by Viking Orbiter layer in the crater wall at top. The frame is about 3 km across.
187
i.
Note the dark
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Figure 22: Viking Orbiter 2 view of Deimos from a range of i000 km. The resolution is about 50 meters. Note the absence of grooves and the conspicuous bright markings.
The Surfaces of Phobos and Deimos
189
only seen about one half of the surface of the outer satellite. The possibility that Deimos may consist of a mechanically stronger material than Phobos is consistent with the available data about its composition (French and Veverka, 1977; Pang et al, 1977b). One cannot argue that Phobos-like grooves once existed on Deimos, but have since been obliterated by ejecta deposits: the depth of fill appears to be too small to allow such a hypothesis. The grooves on Phobos are typically 10-20 meters deep (Thomas et al, 1977a), the depth of fill in the Deimos craters appears to less than 5 meters in most cases. If the suggestion that the grooves on Phobos are associated with the formation of the large crater Stickney is correct, grooves should be a common phenomenon on small bodies in the asteroid belt which are made of mechanically weak carbonaceous material, and which have very large craters on their surfaces.
8.
CONCLUSIONS
The likely primitive carbonaceous chondrite composition of Phobos constrains possible theories of the origin of the satellites (see Burns, this meeting). Current theories of the chemical condensation of solids from the primeval nebula predict that primitive carbonaceous material forms only in the outer half of the asteroid belt (Lewis, 1974). Thus Phobos, and by analogy Deimos, probably originated in the asteroid belt and were captured by Mars. The traditional problem of capturing small bodies and bringing them into nearly circular, equatorial orbits has been solved by Pollack and Burns (1977), and independently bv Hunten (1977). The scheme involves capturing the satellites early, in fact, during the closing stages of the accretion of Mars. It is supposed that at the time the planet is still surrounded by an extensive primitive atmosphere which provides the necessary drag to effect the capture. Thus, Phobos and Deimos can be considered to be two pieces that survived the terminal stages of the accretion of Mars. It is remarkable that within one hundred years of the discovery of Phobos and Deimos by Asaph Hall, we have learned so much about their physical characteristics and gained significant insights into their evolutionary history, and probable origin. It is even more remerkable that the bulk of this knowledge was gained within seven years from only two spacecraft missions: Mariner 9 in 1971 and Viking in 1976-77.
ACKNOWLEDGEMENT I am grateful to J. Burns, T. Duxbury, D. Pascu and P. Thomas for numerous helpful discussions and comments. This work was supported by the Viking Project and by the NASA Planetary GeoloK-y Program under Grant NSG-7156.
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REFERENCES
Anders, E. and Owen, T. (1977) Science, 198, 453-465.
Mars and Earth:
Origin and Abundance of Volatiles.
Burns, J. A. (1972) Dynamical Characteristics of Phobos and Deimos. Phys., IO, 463-483. Duxbury, T. C. (1974)
Phobos:
Rev. Geonhys. Space
Control Network Analysis. Icarus, 23, 290-99.
Duxbury, T. C. and Veverka, J. (1977) Viking Imaging of Phobos and Deimos: the Primary Mission. J. Geoph. Res., 82, 4203-4211. French, L. M. and Veverka, J. (1977) Bull. Am. Astro. Soc., 2, 518.
An Overview of
Are Phobos and Deimos Made of the Same Material?
Gatley, I., Keiffer, H., Miner, E. and Neugebauer, G. (1974) Infrared Observations of Phobos from Mariner 9. Astrophys. ~., 190, 497-503. Goguen, J., Veverka, J. and Duxbury, T. (1977) Photometry and Distribution of Dark Material on Phobos. Bull. Am. Astro. Soc., 2, 517. Harris, D. L. (1961) Photometry and Colorimetry of Planets and Satellites. In Planets and Satellites (G. P. Kuiper and B. M. Middlehurst, eds). University of Chicago Press. Hartmann, W. K. (1972) Interplanet Variations in Scale of Crater Morphology - Earth, Mars, Moon. Icarus, 17, 707-713. Hartmann, W. K. (1973) Martian Cratering. 4. Mariner 9 Initial Analysis of Cratering Chronology. J. Geophys. Res., 78, 4096-4116. Hunten, D. M. (1977) Lewis, J. L. (1974)
Private communication. The Temperature Gradient in the Solar Nebula.
Lebofsky, L. A. (1977) Asteroid I Cores: Ast. Soc., in press. Mason, B. (1971) York.
Science, 186, 440-443.
Evidence for Water of Hydration.
Handbook of Elemental Abundances in Meteorites.
Mort. Not. Roy.
Gordon and Breach, New
Noland, M. and Veverka, J. (1976) Photometric Function of Phobos and Delmos. I. Integrated Photometry. Icarus, 28, 405-414.
Disc-
Noland, M. and Veverka, J. (1977a) Photometric Function of Phobos and Deimos. II. Photometry of Phobos. Icarus, 30, 200-211.
Surface
Noland, J. and Veverka, J. (1977b) Photometric Function of Phobos and Deimos. III. Face Photometry of Deimos. Icarus, 30, 212-223.
Sur-
Pang, K. D. (1977)
Private cormnunication.
Pang, K. D., Pollack, J. B., Veverka, J., Lane, A. L. and AJello, J. M. (lq77a) The Composition of Phobos: Evidence of a carbonaceous Chrondrite Surface from Spectrum Analysis. Science, in press. Pang, K. D., Rhoads, J. W., Lane, A. L., and Ajello, J. M. (1977) Spectral Albedo and Composition of Deimos: Inferences on the Origin of the Martian Satellites. Bull. Am. Astro. Soc., 2, 518. Pike, R. J. (1974) Depth/Diameter Relations of Fresh Lunar Craters: craft Data. Geophy. Res. Lett., ~, 291-294. Pollack, J. B. (1977) Fhobos and Deimos. University of Arizona Press. Pollack, J. B. and Burns. J. (1977) Bull. Am. Astro. Soc., 2, 518.
Revision from Space-
In Planetary Satellites (J. Burns, ed.)
An Origin by Capture of the Martian Satellites?
Pollack, J. B., Veverka, J., Noland, J., Sagan, C., Hartmann, W. K., Duxbury, T. C., Born, G. H., Milton, D. J. and Smith, B. A. (1972) Mariner 9 Television Observations of Phobos and Deimos. Icarus, 17, 394-407.
The Surfaces of Phobos and Deimos
191
Pollack, J. B., Veverka, J., Noland, M., Sagan, C., Duxbury, T. C., Acton, C. H., Jr., Born, G. H., Hartmann, W. K. and Smith, B. A. (1973) Mariner 9 Television Observations of Phobos and Deimos. 2. J. Geophys. Res., 78, 4313-4326. Pollack, J. B., Veverka, J., Pang, K., Colburn, D., Lane, L. and AJello, J. M. (1977) Multi-color observations of Phobos with the Viking lander Cameras: Evidence for a carbonaceous Chondrite composition. Science, in press. Smith, B. A. (1970)
Phobos:
Preliminary Results form Mariner 7.
Sorer, S. (1971) The Dust Belts of Mars. Cornell University, Ithaca, N. Y. Sorer, S. and Harris, A. (1977) Nature, 268, 421-422.
Science, 16__88,828-830.
Cntr. RadioDhysics Space Res. Report No. 462.
Are Striations on Phobos Evidence for Tidal Stress?
Thomas, P. and Veverka, J. (1977a) 2__O0,595-597.
Phobos:
Surface Density of Impact Craters.
Icarus,
Thomas, P. and Veverka, J. (1977b) A Quantitative Comparison of the Surface Roughness of Phobos and Deimos. In Reports of Planetary Geology Program 1976-77. NASA Technical Memorandum X-3511. Thomas, P., Veverka, J. and Duxbury, T. (1977a) Grooves on Phobos: Morphology, Distribution, Age and Possible Origin. Bull. Am. Astro. Soc., 9, 517. Thomas, P., Veverka, J. and Duxbury, T. (1977b) Nature, in press.
The Origin of the Grooves on Phobos.
Thomas, P., Veverka, J., Bloom, A., and Duxbury, T. (1978a) The Morphology and Possible Origin of the Grooves on Phobos. In preparation. Thomas, P., Veverka, J., Duxbury, T. and Hartmann, W. K. (1978b) The density of impact craters on the surfaces of Phobos and Deimos. In preparation. Tolson, R. H., Duxbury, T. C., Born, G. H., Christensen, E. J., Hildebrand, C. E., Mitchell, R. J., Molko, P. M., Morabito, L. A., Relchert, R. J., TaraJi, H., Veverka, J. and Neugebauer, G. (1977) Preliminary results of the first Viking close encounter with Phobos. Science, in press. Veverka, J. (1977) Photometry of satellite surfaces. Ed.) University of Arizona Press.
In Planetary Satellites (J. Burns,
Veverka, J. and Duxbury, T. C. (1977) Viking observations of Phobos and Deimos: nary results. J. Geophys. Res., 82, 4213-4223.
Prelimi-
Veverka, J., Noland, M., Sagan, C., Pollack, J. B., Quam, L., Tucker, R. B., Fross. B., Duxbury, T. C. and Green, W. (1974) A Mariner 9 atlas of the moons of Mars. Icarus. 23, 206-209. Veverka, J., Thomas, P. and Duxbury, T. (1977) The surface of Phobos: latest Viking results. Bull. Am. Astro. Soc., 9, 517. Zellner, B. H. (1972) 103-185. Zellner, B. (1976)
Minor planets and related objects. VIII. Delmos. Astron. J., 77,
Physical properties of asteroid 433 Eros. Icarus, 28, 149-153.
Zellner, B. H. and Capen, R. D. (1974) Icarus, 23, 437-444.
J,P v ^ - - 2 2 / 2 - - 0
Summary of the
Photome£rlc properties of the Martian satellites.
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DISCUSSION:
Comment: I think next year when we take a closer picture of Phobos, we will have another test of the tidal forces, because at that time, I think we will find it all torn apart. Question:
Do you have any idea of the age of these bodies?
Answer: The problem is that to determine an age, you will have to calculate how long it takes for a surface to become saturated with craters at the orbit of Mars. For that you have to know what the flux of impacting bodies has been at the orbit Mars for the last 4½ billion years, and of course we don't know that. So, if you assume that the flux has been similar to that at the surface of our Moon, you get an age of about 4 billion years. But other reasonable estimates would lower this age down to something like 1.5 billion years. The problem is that we don't really know what the flux of impacting objects at Mars has been. Comment: I think the tidal stress of Phobos is comparable to the rotational stress of Phobo~ for seven and one half hour period. I think there would be a lot more tidal stress induced if the objects were out of synchronous rotation and rotating faster in the past. If you knock them out of synch you're going to create a very large stress in the process of knocking them out of synch. Question: Are ages of the grooves that you quoted based on the ages for the striations coming out of Stickney? Answer: Yes, we believe that all of the grooves are associated with Stickney and that all are about the same age. There definately is no evidence that any grooves have opened up in the last 108 years. Comment: It would appear that possibly the striations are due to stresses building up on Phobos, and then when it's hit, that's when it cracks.
RESPONSE: Yes that's a possibility. But Stickney is a very old crater, and 4 billion years ago Phobos certainly wasn't as close to Mars as it is today. Thus the tidal stresses must have been much smaller when Stickney, and presumably the grooves, formed. Question: You discuss the chemistry of the satellite surfaces? information?
What is the source of such
RESPONSE: It is based on measurements of the spectral reflectance of Phobos between 0.2 and 0.9 ~m. The spectrum and the low albedo are characteristic of carbonaceous condrite material. There is no other cosmically abundant material which has a similar spectrum and an equally low albedo. Question: You mention that the value of gravity on the surface of Phobos is 10 -3 , of terrestlal gravity. A person on Earth can Jump about two feet; wearing a space suit maybe he could Jump about one foot, but on Phobos that would be a thousand feet. In view of the large value of the solar perturbations, would it be safe to jump? Answer: I guess what I would do is that I would pick a big crater. Stickney seems to be about 1500 meters deep, so if you stood in Stlckney you might Just be able to Jump to the rim. So I would try it in Stickney, but not in the smaller craters, it might be dangerous. Question: I noticed on one of the pictures, that the rim of an ancient crater? Answer: Question:
there is a big circular ark type shape.
Is
I think you're probably referring to what is sometimes called Kepler ridge. Do you think this ridge is an old crater rim?
RESPONSE: It's a feature which is conspicuous only under certain viewing and lighting conditions. We're not quite sure whether it is part of an old crater or not.
0083-6656/78/0701-0163
$05.00/0
THE SURFACES OF PHOBOS AND DEIMOS J. Veverka Laboratory for Planetary Studies, Cornell University, Ithaca, New York 14853
i.
INTRODUCTION
Although Phobos and Delmos were discovered i00 years a g o , almost nothing was known until recently about their surfaces. Accurate photometry and colorimetrv of the satellites usin~ earth-based telescopes is difficult due to the intense scattering light from Mars which tends to contaminate measurements. Estimates of the apparent ma~nltudes of the satellites culminated in the work of G. P. Kuiper during the favorable opposition of Mars in lO56, who found mean opposition magnitudes of V o = +11.6 for Phobos and +12.8 for Deimos (Harris, 1961). These values are in reasonable agreement with the most recent photoelectric determinations of +11.4 and +12.5, respectively (Zellner and CaDen, I974). Since the absolute sizes of the satellites cannot be determined directly from Earth, Kuiver estimated the diameters assuming that the albedos were similar to that of the lunar surface, and obtained diameters of 12 km for Phobos and 6 km for nelmos. Kuiper also published a B-V color estimate of +0.6 for the satellites (Harris, IQ61), which in the case of Deimos has been substantiated by more recent photometry (Zellner and Caoen, 1974). He concluded that the satellites did not share the red color of Mars, but rather were grey llke many asteroids, confirming the impression of early visual observers.
2.
FIRST SPACECRAFT DATA:
MARINER 7
The first direct information about the physical nature of the satellites came during the Mariner 7 Mission in 1969. As Mariner 7 flew by Mars, it took several dozen close-up images of the planet; in at least one of them the silhouette of Phobos was caught azainst the disk of Mars. From this picture, Smith (1970) was able to infer that Phobos is elliptical in cross-sectlon and about twice as big as Kuiper had estimated. Thus the geometric albedo of the Surface is only 5 or 6% rather than the 11-12% typical of the Moon.
3.
FIRST SYSTEMATIC EXPLORATION:
MARINER 9
The first systematic reconnaissance of the satellites of Mars was planned for the Mariner 9 Mission in 1971-2. This mission produced several dozen hlgh-resolutlon images of Phobos and Delmos which provided our first comprehensive data about the physical nature of the satellite surfaces. These results have been reviewed by Veverka et al. (1974), Pollack (1977), and Duxbury (1977). Mariner 9 produced 200 meter resolution coverage of most of the surface of Phobos (vi~. I) and of about one-half of the surface of Delmos (Fig. 2). Both satellites were found to be approximately elllpsoidal in shape. SurDrlsinglv, the shapes of the two satellites appeared to be similar, with the ratio of the longest to the shortest axes eaual to about 1.4 to i (Fig. 3). Phobos was found to be about 27 x 21 x 19 km across: Deimos, about 15 x 12 x ii kin, or about half as big as Phobos (Duxburv, 1974). The surface of Phobos was found to be heavily cratered (Pollack et el, 1972; Thomas and Veverka, 1976), with the landscape dominated by three very large craters: Roche (5 km in diameter), Hall (6 km) and Stickney (10 kin) (Fig. 4). As expected on the basis of tldal theory, (Burns, 1972), both satellites were found to have spin periods synchronous with their orbital periods (7.6 and 30.3 hours, respectively), and to be moving in such a way that their longest axis always points toward Mars. Since the orbit of Mariner @ was entirely within that of Delmos, the tidal locking of the satellltes meant that only the Mars facing side of Delmos could be imaged. 163
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J. Veverka
At the best available resolution (200 meters) the surface of Deimos appeared to be smoother than that of Phobos, and differed from that of the inner satellite in having conspicuous patches of higher albedo material (Fig. 5). (At Mariner 9 resolution the surface of Phobos appeared to be homogeneous in albedo (cf. Fig. i)). Initial crater counts on Phobos (Fig. 6) showed that the density of impact craters was very close to that found in the lunar uplands (Pollack et al, 1972; Thomas and Veverka, 1976). For Deimos, the Mariner 9 crater density data were of marginal statistical si~niflcance, but suggested that the surface of the outer satellite was also "saturated" with imoact craters. This led to the dilemma that Deimos was apparently as heavily cratered as Phobos, yet appeared to be much smoother. Fortunatelv this problem has now been resolved by the high resolution imagery of Deimos obtained bv Viking Orbiter II (Section 7).
4.
SURFACE TEXTURE AND COMPOSITION:
MARINER 9 DATA
Mariner 9 yielded little data on the actual composition of the satellite surfaces. Various investigators derived mean albedos of 5-6% for Phobos and 6-7% for Deimos (Veverka, 1977) while Noland and Veverka (1976) derived a value of 1.15 ± 0.I0 for the relative brightness ratio (Deimos/Phobos) of the two surfaces from Mariner q photometry. Thus both satellites have almost equally dark, black surfaces, but the average surface of Deimos is slightly brighter. While the low albedos and grey colors suggested a composition similar to that of C-asteroids and of carbonaceous meteorites (Veverka, 1977) a unique identification of composition could not be made on the basis of the scant information available. Noland and Veverka (1977a) found that the bright albedo material on Delmos was about SO% brighter than the average background surface, and suggested that the presence of these patches could account for the slightly higher albedo of Deimos relative to Phobos. The bright patches could be explained as deposits of fine-grained eJecta from imoacts~ and suggested the possibility that some flne-grained ejecta was retained even on bodies as gravitationally weak as Phobos and Deimos. (On Phobos, g=10-3g~; on Deimos g is about half as large as on Phobos). This possibility was confirmed by a series of studies which showed that the satellites surfaces have the intricate texture of a fine-~ralned regollth, and not that of solid, uncommlnuted rocks. Noland and Veverka (1976; 1977a,b) found that the light scattering properties of the surfaces were very similar to those of the lunar surfaces and concluded that the surfaces must have the intricate texture of a flne-grained regolith, similar to that of the lunar soil. Unfortunately, it is impossible to determine the depth of a regollth bv photometric techniques. Specifically, Noland and Veverka (1976; 1977a,b) found that the surfaces of Phobos, Deimos and of the Moon have similar photometric functions. The phase coefficient for the surface material on Phobos was found to be 0.019 mag/deg, compared with 0.01~ mag/deg for the lunar surface. The phase coefficient for the surface material of Deimos was less, 0.017 mag/deg, consistent with the smoother visual appearance of the outer satellite. These photometric results supported the conclusion drawn from some earlier Earth-based polarization measurements of Delmos by Zellner (1972). Zellner found that the polarization curve of Deimos has a deep negative branch at small phase angles, which is a characteristic signature of regoliths. Additional strong evidence for the presence of a re~olith on Phobos was derived from temperature measurements made by the Mariner 9 Infrared Radiometer (Gatley e t a l , 1974). By measuring the rate at which the surface of Phobos heats up following an eclipse by the shadow of Mars, one can calculate the thermal inertia of ~he surface layer. Values for the thermal inertia obtained by Gatley et al (10 -3 cal cm-sec -~ K -l) are characteristic of lunar-llke regoliths. Thus the photometric, polarimetric and radiometric data all indicate that the surfaces of both satellites are covered with a fine-gralned regolith derived, most probably, from the comminution of the surfaces by impacts. Noland and Veverka (1976;1977 a,b) presented photometric evidence that on both satellites these regoliths are homogeneous in texture on scales of 200 meters. They also concluded that the regolith is homogeneous in albedo on such scales on Phobos, but not on Deimos (Figure 5). With the exception of some UV reflectance measurements between 0.2 and 0.4 ~m, which have only been reduced recently (Pang et al 1977a; 1977b), Mariner 9 obtained little direct data on the composition of the satellite surfaces. However, the Mariner 9 data did establish the texture of the surface layers, and proved, for the first time, that regollths are present on objects even as small as Phobos and Delmos. It has been suggested that in this respect Phobos and Deimos may not be representative of asteroids of comparable size since, unlike the asteroids, the two satellites exist deep in the potential well of a big planet. Soter (1971) calculated that while impacts easily eject fragments from Phobos (or Deimos), many of these do not escape from Mars but end un in Phobos-like or Deimos-like orbits. Thus the satellites have an opportunity to recapture their own debris and Soter's calculation suggests that this re-capture process may be
The Surfaces of Phobos and Deimos
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Figure 5: Mariner 9 view of Deimos showing conspicuous bright patches which are about 30% brighter than their surroundings (Noland and Veverka, 1976). (Picture DAS 05553383; Rev. 111; Range=7220 km; Phase=31°).
Figure 7: Apparent crater chain faintly visible in Mariner 9 frame DAS 04790460 (Rev. 89' Range=5760 km; Phase=80°). The individual small craters are about 200-300 meters across.
The Surfaces of Phobos and Deimos
167
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The Surfaces of Phobos and Deimos
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Figure 9: Close-up of Phobos grooves obtained by Viking Orbiter i during February 1977. Range=ll0 kin. The picture is about 3 km across and shows detail as small as about 5 meters.
The Surfaces of Phobos and Deimos
171
significant. But it is unlikely that the presence of rezoliths on Phobos and Deimos can be attributed entirely to Soter's hypothetical mechanism since observations of many small asteroids, such as 433 Eros (Zellner, 1976), suggest that flne-gralned regollths occur on all small bodies.
4.
SURFACE MORPHOLOGY:
MARINER 9 RESULTS
Mariner 9 imaged the satellite at resolutions sufficient to show the basic heavily cratered morphology of both surfaces. Craters of varying sizes and varying degrees of degradation were seen, but no unexpected surface features were discovered. True, in the case of Phobos, a few chains of small craters were noticed (Figure 7), but their significance was not appreciated fully. However, Pollack et al (1973) did suggest that such chains may represent the results of outgassing along fractures following a large impact such as that which produced Stickney, the largest crater on Phobos. Although the resolution of the Mariner 9 images was inadequate for a detailed study of crater morphology, it was sufficient to show that eJecta blankets were not prominent. None of the craters showed any evidence of a central peak, consistent with the idea that the formation of central peaks is a gravity dependent phenomenon (Hartmann, 1972). For a surface gravity of only about 10-3g@, one would not expect central peaks to form in any crater small enough to occur on the surface of the satellite.
5.
PLANNED VIKING STUDIES:
1976-1977
The general purpose of the Viking Orbiter investigations of Phobos and Deimos in iq76-1977, was to study the surfaces in more detail and at higher resolution. Although no new major discoveries were anticipated, the intent was to concentrate on those Darts of the surfaces which had not been imaged adequately by Mariner 9. One purpose of this coverage was to provide data for crater counts down to very small diameters to make it possible to look for crater density variations on the satellite surfaces. Such variations could exist if significant chunks of a satellite had been spalled off by "recent" impacts (Pollack et al, 1973). Another major objective of the Viking studies was to obtain information about the comoositions of Phobos and Deimos, both by making color measurements and by determinin~ the masses of the satellites during very close encounters of the spacecraft with the satellites. The masses could be used to determine the mean densities of the satellites. From accurately determined mean densities one could distinguish a dark grey object made of basaltic rock from one made of primitive carbonaceous chondrite rock, since the density of the first would be about 3.5 g/cm 3 while the second would have a density of 2.5 g/cm 3 or less.
6.
VIKING RESULTS
The Viking Orbiter Missions not only achieved the initial satellite objectives outlined above but have obtained a number of totally unexpected and spectacular results. (Duxburv and Veverka, 1977; Veverka and Duxbury, 1977). The Viking Orbiters began to study the satellites during the summer of 1976. Durin~ the Primary Mission (summer and fall of 1976) numerous encounters with the two satellites at minimum ranges of 3000 to 5000 km occured. These minimum ranges were smaller than any achieved by Mariner 9 (Veverka et al, 1974). This fact combined with the better performance of the Viking Orbiter cameras led to images with much higher resolution than those obtained by Mariner 9. One unusually close encounter with Phobos (range about 880 km) occured during the Primary Mission and yielded images with an effective resolution of 50 meters! As part of the Viking Extended Mission, it was planned to make two very close passages to Phobos with Viking Orbiter l, in order to determine the mass and obtain very high resolution images. The first of these encounters took place in February iq77, the second in May 1977, at minimum distances of about 100 and 300 km, respectively. A close encounter between Deimos and Viking Orbiter II took place in October 1977, at a minimum distance of less than 30 km. The image obtained during the 880 km flyby of Phoboe by Viking Orbiter I in late iq76, revealed that the surface is not only heavily cratered, but is cries-crossed bv numerous grooves (Fig. 8). At the moderate resolution then available (about 50 meters), it seemed that there were both trough-like grooves and chains of elongated depressions on the surface. Some of these chains resembled chains of secondary craters on the Moon (Veverka and Duxbury, 1977).
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Fisure 12: Part of Viking Orbiter Picture 246A06 showing impact craters within grooves. The large crater at left is about 750 meters across. (After Thomas et al, 1977b).
The Surfaces of Phobos and Deimos
175
Various theories of the origin of the grooves were presented, and it became Imoerative to obtain more data on the nature and distribution of the grooves during the February and May 1977 close encounters in order to constrain the range of speculation. These data (Figure 9 and I0) proved convincingly that the grooves are old, are associated with the crater Stlckney, and are probably surface expressions of deep fractures within the body of Phobos (Thomas et al, 1977a). A map of the distribution of the grooves on the surface of Phobos (Thomas et al, 1977b) shows that the grooves are best developed near the crater Stlckney and die away near the antipodal point (Figure ii). Individual grooves can be followed for uD to 12 km. Typically they are 100-200 meters wide and only 10-20 meters deep. By counting impact craters superimposed on the grooves (Figure 12), Thomas et al (lq77a,b) find that the grooves are not significantly younger than most of the surface features on Phobos (Fig 13), and most probably are older than about 3 billion years. The distribution of grooves on the surface of Phobos and the fact that the grooves appear to be best developed in the vicinity of Stlckney and that they die out near the point antipodal to this crater, suggest that the formation of the crater and of the grooves were intimately connected events. It has been suggested that some of these grooves represent fractures produced by Martian tides (Soter and Harris, 1977). At the present time Phobos is deep within the gravitational field of Mars and Soter and Harris have calculated that Martian tides may be pulling Phobos apart. If tidal forces are important in producing the grooves, then some of th~ grooves should be very young, since Phobos is spiralling into Mars on a time scale of I0 years (Pollack, 1977). However, it now apoears that the grooves are at least 3 billion years old (Thomas et al, 1977b). The Sorer-Harris mechanism also fails to explain the global groove pattern and its relationship to Stlckney. Even if one imagines that a large impact (Stickney) pre-fractures the satellite and that tides later exploit this preimposed fracture pattern, some of ~he grooves should be young, since it is unllkelv that Phobos has spent as much as 3 x i0 = years close to the Roche limit of Mars. Close examination of the grooves indicates that many segments are pitted (Figure 14). Such pitting can be explained either by sub-surface drainage of a regolith into fractures or by outgassing from the interior (Veverka et al, 1977). Since gravity is very weak on Phobos, the subduction of a regolith is probably an inefficient process. The fact that some seRments of the grooves may have subtle raised rims (Thomas et al, 1978a) further suoDorts the possibility that outgassing was involved in modifying the appearance of some grooves. One possible scenario is that a very large imoact, specifically that which produced Stlckney, fractured the satellite throughout most of its interior and at the same time heated some parts of it enough to release volatiles, a situation already anticioated in part by Pollack et al (1973), who suggested that some of the "crater chains" barely visible in Mariner 9 images (Figure 7) were due to outgassing of volatiles from the interior of Phobos following a large impact. The inferred composition of Phobos is consistent with such a scenario. By combinin~ spectral data obtained by the Viking 1 Lander (Pollack et al, 1977) with those from the Mariner 9 Ultra-violet Spectrometer (Pang et al, 1977a), a spectral reflectance curve of Phobos between 0.2 and 0.9 m has been constructed. In terms of its spectral reflectance curve, the surface of Phobos appears to be very similar to the type of carbonaceous material found in Type I or Type II carbonaceous chondrites and on the surface of the largest asteroid Ceres. In fact, Pang (1977) has argued that the UV soectrum of Phobos contains a feature characteristic of clay minerals found in Type I and Type II carbonaceous chondrites. Such materials (Table I) contain 10-20% bound water bv weight (Mason, 1971) axtd are unstable at temperatures above 400°K (Anders and Owen, 1977). Since the mean subsurface temperature of Phobos probably exceeds 250°K, a large impact need only raise the temperature locally by 150 K before outgasslng the water will occur. Such outgassing will take place preferentially alone fractures, and can explain both the pitted appearance of some of the grooves as well as the possible occurrence of raised rims (Veverka et al, 1977). The carbonaceous chondrite composition of Phobos seems to be confirmed by the low mean density of the satellite derived from the Viking mass determination: about 2 g/cm 3 (Tolson et al, 1977). A further confirmation could be obtained by detecting the 1.6 m absorption feature of water of hydration in the spectrum of Phobos. This feature, prominent in the reflectance spectra of Type I and Type II Carbonanceous Chondrltes. has recently been detected in the snectrum of Ceres by Lebofskv (1977). The suggested carbonaceous chondrite composition of Phobos not only explains the color and low albedo of the satellite, but explains why Phobos might tend to fracture during a severe impact, since low density carbonaceous material is known to be mechanically very weak. Such a composition also explains the possible release of volatiles along some of the fractures leading to the formation of pits and possible raised rims.
176
J. Veverka
CRATER DENSITIES ON GROOVES
td
N % % % % % %
X~
E Z
E
t62L-:
u e-
10
100
1000
10000
Diameter, meters
Figure 13: Surface density of impact craters within grooves (points) compared with that on the rest of Phobos (dashed line) and on the lunar uplands (solid llne). The lunar curve is extrapolated from Hartmann (1974). Within the accuracy of the data we consider the dashed and solid curves to be identical. The data points for the grooves fall into two groups since two sets of images of differing resolution were used, (After Thomas et al, 1977b).
<5
0.5
5
50-70
2.5
~3 t
60 -80 10- t5
:3.6
20
After" MASON (t97:5)
Handbook of Elemental Abundance in Meteorites = Wood- van Schmus tMostly in clay minerals == "hydrated silicates"
3.4
Allende
C5
2.2 2.7
Orgueil
Weight %
(g/cm3) H20 t Organics Clayse° Magnetite
C2 Murchison
Ct
Typee~ Example
F
CARBONACEOUS CHONDRITES
Table I: Major characteristics of dffferent types of Carbonaceous Chondrite meteorftes (after Mason, 1971).
0
0
0 t-h
cn
P o
(b
178
J. Veverka
I
10 PHOBOS COMPOSITE] CRATER DENSITY " #
*\
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"~ ic~~ o
d
Z
I..
rj E
J
,
,
,,,,,I
,
,
,
,,,,,I
tO0 t000 Diameter,meters Figure 15(a).
I
0 i * lii~
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The Surfaces of Phobos and Deimos
179
102
DEIMOS CRATER DENSITY
h \ \
t,J
"6
,o° t,J C
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I#
t
10
t00 Diameter, rn
t000
Figure 15(b).
Figure 15: Crater density on Phobos and Delmos based on Viking Orbiter images compared with the crater density in the lunar upland (solid llne). a)
Phobos
b)
Deimos; d a s h e d l i n e r e p r e s e n t s a f i t
t o t h e Phobos p o i n t s i n F i g . 15a.
180
7.
J. Veverka
OTHER VIKING RESULTS
The Viking imagery has made it possible to extend crater counts down to very small crater diameters (~i0 meters). The density of imnact craters on the surfaces of both satellites is identical and within the error bars cannot be distinguished from that obtaining in the lunar uplands (Figure 15). Thus, llke the lunar uplands the surfaces of both satellites appear to be in an equilibrium state as far as cratering is concerned and one can estimate a minimum exposure age of at least 3 billion years (Thomas et al, 1978b). There is no evidence on Phobos that different areas of the surfaces differ in age: thus no large-scale spallation has occurred in recent times. Specifically the surface density of craters in the "ungrooved" region antipodal to Stickney is identical to the average value for Phobos. Our coverage of Deimos at the highest resolution is too limited to permit a comoarable search for variations in the density of small impact craters over the surface of the outer satellite. However, these images (Figure 16) have explained why Deimos appears to be smoother than Phobos at low resolution, even though the surface density of craters is the same: many of the craters on Deimos, unlike those on Phobos, have apparently been partially filled by eJecta. In fact, the difference in the amount of apparent blanketing of the two surfaces is remarkable (Figure 17). On Phobos there is essentially no evidence that craters have been filled in significantly following their formation. Typically one finds that the walls and floors of many larger craters are "saturated" with small craters indicating that little blanketing (and incidentally, little gravity slumping) has occurred. On Deimos, however, it is quite evident that many craters are partially filled to depths of about 5 meters. This fill appears to be eJecta, a view supported by the ubiquitous presence on Delmos of numerous blocks and patches of higher albedo material (Figure 16). Isolated blocks appear to be rarer on Phobos (although we do not have 3 meter resolution images of the inner satellite), and isolated albedo patches are almost non-existent. The reason why the surface of Deimos appears to be blanketed much more than that of Phobos by both fine (fill and patches) and coarse (blocks) material remains unclear. Incidentally, we now understand why the phase coefficient of Phobos is larger than that of Deimos (Noland and Veverka, 1976): the surface of Phobos is macroscoplcally rougher than that of Deimos due to deeper craters and to the presence of grooves. Thomas and Phobos and appears to roughness,
Veverka (1977b) applied DN variance techniques to study the relative roughness of Deimos and found similar variances for the two satellites. This result now have been fortuitous since on Phobos the variances were determined by surface whereas on Delmos they were controlled by albedo variations.
The Viking images are making it possible to study the details of crater morphology on the satellites (Figure 18). On Phobos, the freshest craters have depths which are about 20% of the crater diameter, a relationship very similar to that found for small fresh lunar craters by Pike (1974). As expected, due to the low gravity of Phobos, rayed eJecta blankets are not evident, but bright albedo rings (5-10% brighter than their surroundings) are conspicuous around many craters at low phase angles (Figure 19), suggesting the presence of coarse textured "ejecta" near the crater rims. Some craters on Phobos contain dark albedo patterns on their floors which are most conspicuous at large phase angles (Figure 20). Goguen et al (1977) interpret these as areas of very rough texture whose albedo near zero phase is similar to that of the mean Phobos surface (~6%), but whose phase coefficient is much larger. Typically, the contrast of such areas is less than 10% near opposition but reaches 100% near phase angles of 90 ° . Goguen et al have suggested that these dark areas represent patches of solidified impact melt which remain consplcious on Phobos because there is so little fall-back of eJecta into the crater due to the satellite's negligible gravity. How deep are the regoliths on Phobos and Delmos? Although the craters on Phobos show no evidence of being filled in, there are signs of possible layering in some of the crater walls to depths of several hundred meters (Figure 21), suggesting that the regolith may be at least that deep. As mentioned above, on Delmos we see craters filled in to depths of about 5 meters; thus the regolith is at least this deep. One of the most exciting results from the October flyby of Deimos, is the apparent absence of grooves on the outer satellite (Figure 22). While it is true that this absence is consistent with the tidal hypothesis of Soter and Harris, it does remain the case that tides do not provide an adequate explanation of grooves on Phobos. It is more likely that Delmos does not have grooves because (a) no sufficiently large crater exists on its surface, and/or (b) that Delmos is made of mechanically stronger material than Phobos. The largest crater definitely imaged on the surface of Delmos is only 3 km in diameter; but we have
The Surfaces of Phobos and Deimos
Fi6ure 14: Portion of Viking Orbiter 1 frame 246A05 taken from a range of about 250 km. Note the conspicuous pitting of the grooves. The grooves are tvpically 100-200 meters wide.
181
a I n 3 o i d au~
• sza~am E-~ ~noqe sT ITe~ap a I q t s I A ~ s a I I e m s aq,~ "ssoaoe m~ g ' I 3noqe ST "u~ 0N ~noqe ~o a S u ~ = ~ moz~ somT~i 7o d n - a s o i o g ~ a 3 I q z o SuI~TA : 9 I azngT~
The Surfaces of Phobos and Deimos
Figure 17: One of the highest resolution images of Phobos obtained by Viking Orbiter i from a range of about ii0 kin. The frame is about 3 km across. Detail as small as 5-6 meters is visible.
183
184
J. Veverka
Figure 18: Viking Orbiter i image of Phobos taken from a range of 530 km. visible on some of the craters at top.
Raised rims are
The Surfaces of Phobos and Deimos
185
Figure 19: Viking Orbiter 1 view of Phobos at a phase angle of 14 °. The conspicuous bright rings around many of the craters probably represent areas of unusually intricate texture. Range = 370 km.
186
J. Veverka
Figure 20: Viking Orbiter 1 view of Phobos from a range of about 310 km. spicuous dark marking in the 2 km crater at upper left.
Note the con-
The Surfaces of Phobos and Deimos
Fi6ure 21: View of Phobos from about ii0 km obtained by Viking Orbiter layer in the crater wall at top. The frame is about 3 km across.
187
i.
Note the dark
188
J. Veverka
Figure 22: Viking Orbiter 2 view of Deimos from a range of i000 km. The resolution is about 50 meters. Note the absence of grooves and the conspicuous bright markings.
The Surfaces of Phobos and Deimos
189
only seen about one half of the surface of the outer satellite. The possibility that Deimos may consist of a mechanically stronger material than Phobos is consistent with the available data about its composition (French and Veverka, 1977; Pang et al, 1977b). One cannot argue that Phobos-like grooves once existed on Deimos, but have since been obliterated by ejecta deposits: the depth of fill appears to be too small to allow such a hypothesis. The grooves on Phobos are typically 10-20 meters deep (Thomas et al, 1977a), the depth of fill in the Deimos craters appears to less than 5 meters in most cases. If the suggestion that the grooves on Phobos are associated with the formation of the large crater Stickney is correct, grooves should be a common phenomenon on small bodies in the asteroid belt which are made of mechanically weak carbonaceous material, and which have very large craters on their surfaces.
8.
CONCLUSIONS
The likely primitive carbonaceous chondrite composition of Phobos constrains possible theories of the origin of the satellites (see Burns, this meeting). Current theories of the chemical condensation of solids from the primeval nebula predict that primitive carbonaceous material forms only in the outer half of the asteroid belt (Lewis, 1974). Thus Phobos, and by analogy Deimos, probably originated in the asteroid belt and were captured by Mars. The traditional problem of capturing small bodies and bringing them into nearly circular, equatorial orbits has been solved by Pollack and Burns (1977), and independently bv Hunten (1977). The scheme involves capturing the satellites early, in fact, during the closing stages of the accretion of Mars. It is supposed that at the time the planet is still surrounded by an extensive primitive atmosphere which provides the necessary drag to effect the capture. Thus, Phobos and Deimos can be considered to be two pieces that survived the terminal stages of the accretion of Mars. It is remarkable that within one hundred years of the discovery of Phobos and Deimos by Asaph Hall, we have learned so much about their physical characteristics and gained significant insights into their evolutionary history, and probable origin. It is even more remerkable that the bulk of this knowledge was gained within seven years from only two spacecraft missions: Mariner 9 in 1971 and Viking in 1976-77.
ACKNOWLEDGEMENT I am grateful to J. Burns, T. Duxbury, D. Pascu and P. Thomas for numerous helpful discussions and comments. This work was supported by the Viking Project and by the NASA Planetary GeoloK-y Program under Grant NSG-7156.
190
J. Veverka
REFERENCES
Anders, E. and Owen, T. (1977) Science, 198, 453-465.
Mars and Earth:
Origin and Abundance of Volatiles.
Burns, J. A. (1972) Dynamical Characteristics of Phobos and Deimos. Phys., IO, 463-483. Duxbury, T. C. (1974)
Phobos:
Rev. Geonhys. Space
Control Network Analysis. Icarus, 23, 290-99.
Duxbury, T. C. and Veverka, J. (1977) Viking Imaging of Phobos and Deimos: the Primary Mission. J. Geoph. Res., 82, 4203-4211. French, L. M. and Veverka, J. (1977) Bull. Am. Astro. Soc., 2, 518.
An Overview of
Are Phobos and Deimos Made of the Same Material?
Gatley, I., Keiffer, H., Miner, E. and Neugebauer, G. (1974) Infrared Observations of Phobos from Mariner 9. Astrophys. ~., 190, 497-503. Goguen, J., Veverka, J. and Duxbury, T. (1977) Photometry and Distribution of Dark Material on Phobos. Bull. Am. Astro. Soc., 2, 517. Harris, D. L. (1961) Photometry and Colorimetry of Planets and Satellites. In Planets and Satellites (G. P. Kuiper and B. M. Middlehurst, eds). University of Chicago Press. Hartmann, W. K. (1972) Interplanet Variations in Scale of Crater Morphology - Earth, Mars, Moon. Icarus, 17, 707-713. Hartmann, W. K. (1973) Martian Cratering. 4. Mariner 9 Initial Analysis of Cratering Chronology. J. Geophys. Res., 78, 4096-4116. Hunten, D. M. (1977) Lewis, J. L. (1974)
Private communication. The Temperature Gradient in the Solar Nebula.
Lebofsky, L. A. (1977) Asteroid I Cores: Ast. Soc., in press. Mason, B. (1971) York.
Science, 186, 440-443.
Evidence for Water of Hydration.
Handbook of Elemental Abundances in Meteorites.
Mort. Not. Roy.
Gordon and Breach, New
Noland, M. and Veverka, J. (1976) Photometric Function of Phobos and Delmos. I. Integrated Photometry. Icarus, 28, 405-414.
Disc-
Noland, M. and Veverka, J. (1977a) Photometric Function of Phobos and Deimos. II. Photometry of Phobos. Icarus, 30, 200-211.
Surface
Noland, J. and Veverka, J. (1977b) Photometric Function of Phobos and Deimos. III. Face Photometry of Deimos. Icarus, 30, 212-223.
Sur-
Pang, K. D. (1977)
Private cormnunication.
Pang, K. D., Pollack, J. B., Veverka, J., Lane, A. L. and AJello, J. M. (lq77a) The Composition of Phobos: Evidence of a carbonaceous Chrondrite Surface from Spectrum Analysis. Science, in press. Pang, K. D., Rhoads, J. W., Lane, A. L., and Ajello, J. M. (1977) Spectral Albedo and Composition of Deimos: Inferences on the Origin of the Martian Satellites. Bull. Am. Astro. Soc., 2, 518. Pike, R. J. (1974) Depth/Diameter Relations of Fresh Lunar Craters: craft Data. Geophy. Res. Lett., ~, 291-294. Pollack, J. B. (1977) Fhobos and Deimos. University of Arizona Press. Pollack, J. B. and Burns. J. (1977) Bull. Am. Astro. Soc., 2, 518.
Revision from Space-
In Planetary Satellites (J. Burns, ed.)
An Origin by Capture of the Martian Satellites?
Pollack, J. B., Veverka, J., Noland, J., Sagan, C., Hartmann, W. K., Duxbury, T. C., Born, G. H., Milton, D. J. and Smith, B. A. (1972) Mariner 9 Television Observations of Phobos and Deimos. Icarus, 17, 394-407.
The Surfaces of Phobos and Deimos
191
Pollack, J. B., Veverka, J., Noland, M., Sagan, C., Duxbury, T. C., Acton, C. H., Jr., Born, G. H., Hartmann, W. K. and Smith, B. A. (1973) Mariner 9 Television Observations of Phobos and Deimos. 2. J. Geophys. Res., 78, 4313-4326. Pollack, J. B., Veverka, J., Pang, K., Colburn, D., Lane, L. and AJello, J. M. (1977) Multi-color observations of Phobos with the Viking lander Cameras: Evidence for a carbonaceous Chondrite composition. Science, in press. Smith, B. A. (1970)
Phobos:
Preliminary Results form Mariner 7.
Sorer, S. (1971) The Dust Belts of Mars. Cornell University, Ithaca, N. Y. Sorer, S. and Harris, A. (1977) Nature, 268, 421-422.
Science, 16__88,828-830.
Cntr. RadioDhysics Space Res. Report No. 462.
Are Striations on Phobos Evidence for Tidal Stress?
Thomas, P. and Veverka, J. (1977a) 2__O0,595-597.
Phobos:
Surface Density of Impact Craters.
Icarus,
Thomas, P. and Veverka, J. (1977b) A Quantitative Comparison of the Surface Roughness of Phobos and Deimos. In Reports of Planetary Geology Program 1976-77. NASA Technical Memorandum X-3511. Thomas, P., Veverka, J. and Duxbury, T. (1977a) Grooves on Phobos: Morphology, Distribution, Age and Possible Origin. Bull. Am. Astro. Soc., 9, 517. Thomas, P., Veverka, J. and Duxbury, T. (1977b) Nature, in press.
The Origin of the Grooves on Phobos.
Thomas, P., Veverka, J., Bloom, A., and Duxbury, T. (1978a) The Morphology and Possible Origin of the Grooves on Phobos. In preparation. Thomas, P., Veverka, J., Duxbury, T. and Hartmann, W. K. (1978b) The density of impact craters on the surfaces of Phobos and Deimos. In preparation. Tolson, R. H., Duxbury, T. C., Born, G. H., Christensen, E. J., Hildebrand, C. E., Mitchell, R. J., Molko, P. M., Morabito, L. A., Relchert, R. J., TaraJi, H., Veverka, J. and Neugebauer, G. (1977) Preliminary results of the first Viking close encounter with Phobos. Science, in press. Veverka, J. (1977) Photometry of satellite surfaces. Ed.) University of Arizona Press.
In Planetary Satellites (J. Burns,
Veverka, J. and Duxbury, T. C. (1977) Viking observations of Phobos and Deimos: nary results. J. Geophys. Res., 82, 4213-4223.
Prelimi-
Veverka, J., Noland, M., Sagan, C., Pollack, J. B., Quam, L., Tucker, R. B., Fross. B., Duxbury, T. C. and Green, W. (1974) A Mariner 9 atlas of the moons of Mars. Icarus. 23, 206-209. Veverka, J., Thomas, P. and Duxbury, T. (1977) The surface of Phobos: latest Viking results. Bull. Am. Astro. Soc., 9, 517. Zellner, B. H. (1972) 103-185. Zellner, B. (1976)
Minor planets and related objects. VIII. Delmos. Astron. J., 77,
Physical properties of asteroid 433 Eros. Icarus, 28, 149-153.
Zellner, B. H. and Capen, R. D. (1974) Icarus, 23, 437-444.
J,P v ^ - - 2 2 / 2 - - 0
Summary of the
Photome£rlc properties of the Martian satellites.
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DISCUSSION:
Comment: I think next year when we take a closer picture of Phobos, we will have another test of the tidal forces, because at that time, I think we will find it all torn apart. Question:
Do you have any idea of the age of these bodies?
Answer: The problem is that to determine an age, you will have to calculate how long it takes for a surface to become saturated with craters at the orbit of Mars. For that you have to know what the flux of impacting bodies has been at the orbit Mars for the last 4½ billion years, and of course we don't know that. So, if you assume that the flux has been similar to that at the surface of our Moon, you get an age of about 4 billion years. But other reasonable estimates would lower this age down to something like 1.5 billion years. The problem is that we don't really know what the flux of impacting objects at Mars has been. Comment: I think the tidal stress of Phobos is comparable to the rotational stress of Phobo~ for seven and one half hour period. I think there would be a lot more tidal stress induced if the objects were out of synchronous rotation and rotating faster in the past. If you knock them out of synch you're going to create a very large stress in the process of knocking them out of synch. Question: Are ages of the grooves that you quoted based on the ages for the striations coming out of Stickney? Answer: Yes, we believe that all of the grooves are associated with Stickney and that all are about the same age. There definately is no evidence that any grooves have opened up in the last 108 years. Comment: It would appear that possibly the striations are due to stresses building up on Phobos, and then when it's hit, that's when it cracks.
RESPONSE: Yes that's a possibility. But Stickney is a very old crater, and 4 billion years ago Phobos certainly wasn't as close to Mars as it is today. Thus the tidal stresses must have been much smaller when Stickney, and presumably the grooves, formed. Question: You discuss the chemistry of the satellite surfaces? information?
What is the source of such
RESPONSE: It is based on measurements of the spectral reflectance of Phobos between 0.2 and 0.9 ~m. The spectrum and the low albedo are characteristic of carbonaceous condrite material. There is no other cosmically abundant material which has a similar spectrum and an equally low albedo. Question: You mention that the value of gravity on the surface of Phobos is 10 -3 , of terrestlal gravity. A person on Earth can Jump about two feet; wearing a space suit maybe he could Jump about one foot, but on Phobos that would be a thousand feet. In view of the large value of the solar perturbations, would it be safe to jump? Answer: I guess what I would do is that I would pick a big crater. Stickney seems to be about 1500 meters deep, so if you stood in Stlckney you might Just be able to Jump to the rim. So I would try it in Stickney, but not in the smaller craters, it might be dangerous. Question: I noticed on one of the pictures, that the rim of an ancient crater? Answer: Question:
there is a big circular ark type shape.
Is
I think you're probably referring to what is sometimes called Kepler ridge. Do you think this ridge is an old crater rim?
RESPONSE: It's a feature which is conspicuous only under certain viewing and lighting conditions. We're not quite sure whether it is part of an old crater or not.