Various theories for the long, linear depressions on the surface of Phobos are reviewed. Imagery from Viking Orbiters is used to map the surface distribution of the grooves, study their morphology, and date them by means of the density of superimposed impact craters. Data is presented which tends to support the hypothesis that the deep-seated fracturing was caused by a large, nearly catastrophic cratering event. It is suggested that the grooves were produced during the creation of the Stickney crater, rather than as the result of tidal stresses induced by Mars or by drag forces during the hypothetical capture of the satellite by Mars

Duxbury, T.

Thomas, P. C.

Veverka, J.

English

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CRSR 679
THE ORIGIN OF iliE GROOVES O^1 PHOBOS
P. Thomas, J. Veverka, T. Duxbiir%
December 1977
THE ORIGIN OF THE GROOVES ON PHOBOS
P. Thor,.aS
Laboratory for Planetary Studies
and
Department of Geological Sciences
Cornell University
Ithaca, New York	 14853
and
J. Veverka
Laboratory for Planetary Studies
Cornell Ur versity
TChaca, Now York 14853
and
T. Duxbury
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California 	 91103
Submitted to Nature
b-1-
The characteristic surface features of the inner Martian
satclLLtc Phobos arc i.mpacL c+-aLers and lonr„ linear depressions,
here called grooves. The grooves have been explained as surface
maniidstations of deep-seated fractures, but there has been a
divergence of opinion as to the cause of the fracturing. Soter
and Harris l have attributed the fracturing to tidal stresses
induced by Mars; Pollack and Burns 2 have suggested fracturing by
drag forces during the hypothetical capture of the satellite by
Mars; Veverka et a1 3 have attributed the fracturing to a large,
nearly catastrophic cratering event. In this note we present- new
data on the grooves which tend to support the crat;ering hypothesis
and appear to rule out the tidal mechanism.
f.
	
While it was known  from the Mariner 9 images obtained in
1971-72 that the surface of Phobos is heavily cratered, the dis-
covery of the grooves by the Viking, Orbiters came as a surprise.
1	 An effective resolution of <
.
70 meters is needed to see grooves well,
whereas the best resolution achieved by Mariner 9 was ? 200 meters.
Sufficient- Viking Orbiter imagery has now been obtained to map the
surface distribution of grooves, study their morphology, and to date
them by means of the density of superimposed im pact- craters.
Figure 1 is a map of the distribution of grooves on Phobos.
For reference we have sketched in the outline of the largest topo-
graphic feature on Phobos — the 10 km crater Stickney. The region
`	 west of Stickney, between longitudes 70 0
 and 160*W has been imaged
at resolutions of only -150 meters, whereas imagery with a resolution
of ;50 meters exists for the remainder of the satellite. Thus no
1-2-
significance should be attached to the apparent low density of
grooves west of Stiekney evident in Figure 1.
The striking pattern in Figure 1 is that the grooves emanate
from all sides of Stickney and run continuously towards a region
near 270 O W where they die out. This area, although not exactly
180 0 away from the center of Stickney, is in fact antipodal to the
crater when one considers the actual three-dimensional shape of
Phobos. (Phobos 5 is approximately a triaxial ellipsoid with prin-
cipal axes 27 x 21 x 19 km across; the longest axis coincides
with the 0°- 180° longitude direction).
The remarkable pattern of grooves seen in Figure l strongly
suggests that the formation of Stickney and of the grooves were
intimately related events. This ides. is supported by the typical
morphology of the grooves. Individual grooves are best developed
in the neighborhood of Stickney and taper out near longitude 2700.
Most grooves are between 100 and 200 meters wide and 10 to 20 meters
deep, with concave-up cross-profiles. The largest grooves, 700
meters wide and 90 meters deep are near Stickney. Only near the
antipodal point near longitude 270° are the grooves consistently
less than 100 meters in width. .
The grooves have a more complex morphology than that expected
of simple fractures. They commonly have a beaded or pitted ap pear-
ance due to subtle depressions along their lengths. Some grooves,
especially between longitudes 170° and 220'W have wide irregularly
bounded segments of hummocky topography, some of which ap pear to
rise above the surrounding surface. Some straight-walled groove
-3-
segments appear to have subtle, slightly raised rims. Thus the
morphology of the grooves is not that of simple .fractures or of
fault blocks. The grooves almost certainly represent some response
of the Phobos regolith to deeper fractures, and it is probable
that the formation of the grooves involved a variety of processes
of which deep-seated fracturing was dominant. Attempts to eluci-
date some of these processes are underway. 6 Whatever the modifying
processes, the grooves appear to be closely related to the forma-
tion of Stickney,
i
	
	 The orientations of most grooves are consistent with the con-
cept of planar fractures cutting through much of Phobos. One pro-
minent set of groove planes is nearly parallel to the equatorial
plane of Phobos. A second is roughly p erpendicular to the sub-
Mars direction, while a third is intermediate between the first
two, with southern hemisphere normals on the Mars side of Phobos.
i-)hile the grooves are not radial to the rim of Stickney, the major
planes defined by the grooves are roughly perpendicular to the plane
'	 of the crater's rim.
The age of the grooves can be estimated from the density of
superimposed impact craters (Pig. 2). In Figure 3 we compare the
density of impact craters within the grooves with the average value
for all of Phobos.. Also shown for comparison is an extrapolation
of the crater density curve for the lunar uplands. ?
 Within the
error bars, we consider the density of im pact craters averaged
over Phobos to be equal to the extrapolated value for the lunar
uplands, and the density within the grooves to be at least half
i
-4-
this value. On this basis the absolute age of the grooves can
be estimated using the techniques of Hartmann. 8 A crater produc-
tion rate for Dears twice that of the Moon, scaled to the g on
-0.12 9
Phobos (g -0.5 cm/ see " ; crater climneters scaled as g 	 ) gives
an age	 in excess of 3.4 billion years for the grooves, Extreme
values of fluxes and of gravity scaling ex ponents must be used to
vield ages less than 10 9 years.
The old age of the grooves, their pattern on the surfaces,
as well as their evident association with the crater Stickney
are not consistent with the tidal hypothesis of Soter and Harris.1
If Martian tides were the main cause of grooves, the grooves should
be younger than 10 9 years since Phobos is presently spiraling into
Mars on a timescale of 10 8 years.10
The fracturing of Phobos by drat; forces during its hypothe-
tical capture by Dears does not exp lain Lhe evident association of
grooves with the crater Stickney.
The old age of the grooves and their pattern on the surface
of the satellite are best explained by third hy pothesis according
to which the grooves are modified surface'expressions of deep
seated fractures produced during the formation of Stickney.
Available evidence suggests that Stickney is of comparable age to
the grooves and Pollack et al 
1F	 have calculated that the energy
of impact that formed Stickney may have been within an order of
magnitude of the binding energy of Phobos. It has recently been
suggested 11-13 that Phobos is made of a mechanically weak material
similar to that found in primitive carbonaceous chondrite meteorites.
-5-
Thus it seems quite plausible that a severe impact such as that
responsible for the formation of Stickney could fracture much of
the satellite.
According to our argument other small bodies in the Solar
System made of similar mechanically weak material (some C-asteroids,
for example 14) may be expected to have similar patterns of grooves
associated with the largest impact craters.
Acknowledgement
This research was supported by the Viking Project- and by the
NASA Planetary Geology Program under Grant NSG 7156.
REFERENCES
1 Sover, S. & 11arris, A. NnPnre 268, 421-422(1977).
1 Pollack, J.B., & Burns, J.A. Bull, Am. Astro, Soc, 9, 518-519
(1977).
3 Veverka, J., Thomas, P., & Duxbury, T.C. Bull. Am. Astro. Sci.
9, 517-518 (1977).
4 Pollack, J.B., Veverka, J,, Noland, M., Sagan, C., Hartmann,W.K.,
Duxbury, T.C., Born, G.H., Milton, D,J. & Smith, B.A, Icarus
17, 394-407 (1972).
5 Duxbury, T.C. Icarus 23, 290-299 (1974).
6 Thomas, P., Veverka, J. & Duxbury, T.C. Bull. Am. Astro. Soc.
9, 517 (1977).
7 Hartmann, W.K. J. Geophys. Res, 78, 4096-4116 (1973).
8 Hartmann, W.K. Icarus 31, 260-276 (1977).
9 Johnson, S.W., Smith, J.A., Franklin, E.G., Moraski, L.K., &
s;
Teal, D.J. J. Geophys. Res, 74, 4838-4349 (1969).
10 Pollack, J.B., in Planetary Satellites (ed. J.A. Burns)
319-345 (Univ. of Arizona Press, 1977).
11 Tolson, R.H., Duxbury, T.C., Born, G.H., Christensen, E.J.,
Diehl, R.E., Hildebrand, C.E., Mitchell, R.T., M.)lko, P.M.,
Morabito, L.A., Palluconi,F.D., Reichert, R.J., Taraji, H.,
Veverka, J., Neugebauer, G., & Findlay, J.T. Science (in the
press).
4
REFERENCES con't
12 Pane, K.D., Pollack, J.B., Veverka, J, Lane, A.L., & Ajello,
J,M. Science "in the press).
13 Pollack, J.B., Veverka, J,, Pang, K., Colburn,n., Lane, A.L „
& Ajello, J.M. Science (in the press).
14Pollack, J.B., Veverka, J., Noland, M., Sagan, C., Hartmann,
W.K., Duxbury, T.G., Born, G.H., Milton, D,J., & Smith,
B.A. Icarus 17, 394-407 (1972).
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FIGURE CAPTIONS
Figure 1; Sketch map of Phobos showing the location of the
grooves and of the largest crater Stickney, Stippled
areas are hummocky topography in grooves.
Figure 2: Part of Viking Orbiter Picture 246A06 showing impact
craters within the grooves. The scale bar represents
1 km,
Figure 3: Surface density of impact craters within the grooves
(points) compared with that on the rest of Phobox
(dashed line) and on the lunar uplands (solid line).
The lunar curve is extrapolated from Hartmann,7
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.
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The origin of the grooves on Phobos

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