- Email: [email protected]
Valles Marineris, Mars: Wet debris flows and ground ice
ICARUS72, 411-429 (1987)
Valles Marineris, Mars: Wet Debris Flows and Ground Ice B A E R B E L K. L U C C H I T T A u . s . Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 86001
Received January 12, 1987; revised May 19, 1987 Detailed study of the Valles Marineris equatorial troughs suggests that the landslides in that area contained water and probably were gigantic wet debris flows: one landslide complex generated a channel that has several bends and extends for 250 km. Further support for water or ice in debris masses includes rounded flow lobes and transport of some slide masses in the direction of the local topographic slope. Differences in speed and emplacement efficiency between Martian and terrestrial landslides can be attributed to the entrainment of volatiles on Mars, but they can also be explained by other mechanisms. Support that the wall rock contained water comes from the following observations: (1) the water within the landslide debris must have been derived from wall rock; (2) debris appears to have been transported through tributary canyons; (3) locally, channels emerged from the canyons; (4) the wall rock apparently disintegrated and flowed easily; and (5) fault zones within the troughs are unusually resistant to erosion. The study further suggests that, in the equatorial region of Mars, material below depths of 4 0 0 - 8 0 0 m was not desiccated during the time of landslide activity (within the last billion years of Martian history). Therefore the Martian ground-water or groundice reservoir, if not a relic from ancient times, must have been replenished. © 1987 Academic Press, Inc. 1. INTRODUCTION
One conclusion is that the landslides in the Valles Marineris contained water when Are the Valles Marineris landslides wet emplaced and p r o b a b l y formed gigantic wet or dry? Is the equatorial belt of Mars desic- debris flows. Several previous studies of cated or not? A n s w e r s to these questions these landslides (Sharp 1973, Blasius e t al. have profound implications regarding the 1977, Lucchitta 1979) did not conclude extent and timing of ground-ice reservoirs whether their origin was wet or dry, mainly on Mars and ultimately for an understandbecause c o m p a r i s o n s with large terrestrial ing of the evolution of Martian volatiles and landslides were undiagnostic. F u r t h e r m o r e , climate. the availability of w a t e r to lubricate the This p a p e r attempts to address these Martian slides was in doubt. questions. The observations presented are The second conclusion of this report is based largely on recent detailed mapping in that the equatorial region of Mars was not the central Valles Marineris, where excel- desiccated relatively recently. Arguments lent, high-resolution stereoimages permit a for the wet nature of landslides and obserthree-dimensional view into the u p p e r 7-10 vations of the m o r p h o l o g y of the walls of k m o f the crust. The evidence is mostly Valles Marineris suggest that the wall rock observational except for a few quantitative contained ice or water. U n d e r present atconsiderations, which are based on topo- mospheric conditions the equatorial region graphic m a p s (U.S. Geological Survey of Mars is desiccated near the surface, 1980, in press, Wu and H o w i n g t o n 1986). owing to disequilibrium b e t w e e n volatiles T w o main conclusions were reached. in the ground and in the a t m o s p h e r e (Fan411 0019-1035/87 $3.00 Copyright © 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.
412
BAERBEL K. LUCCHITTA
ale 1976, Fanale and Jakosky 1982, Clifford therefore, the Maritan landslides may also and Hillel 1983, Fanale et al. 1986). How- have been dry. That study was based only ever, this desiccation apparently does not on early Viking images; subsequently, extend to great depth, and therefore many additional images have become ground-ice-collapse depressions and liqui- available and offer further insights. On the fled crater ejecta should be expected in this basis of the more recently available inforregion. mation, it is suggested here that the landIn this report I first discuss landslides and slides in the Valles Marineris contained present observations suggesting that they water, in fact that they may have been were wet when emplaced; second, I discuss gigantic wet debris flows. the walls of Valles Marineris and explain why they may have been underlain by 2.1. Observations Supporting Wet Emplacement mixtures of rock and ice. Each of these sections is further separated into two parts, 2.1.1. Ophir Chasma. The best evidence one listing observations that I consider to that landslides contained water and ice be best explained by the stated hypotheses, comes from three landslides that fell from the other listing considerations that are the north wall of Ophir Chasma and gave consistent with the hypotheses but can rise to a plain and a channel that extend for equally well be otherwise explained. A final a combined distance of 250 km at a gradient section addresses the temporal aspects of of approximately 4 m/km (Fig. 1). The both landslide emplacement and the pres- slides fell from about a 7-km height, and, ence of ground ice in the equatorial area of near their heads, they have rugged slump Mars. blocks that extend 20 to 30 km from the detachment scar. The slump blocks grade 2. LANDSLIDES into smoother, longitudinally grooved deLandslides in the Valles Marineris bris aprons that extend an additional 40 kin. troughs were first seen on Mariner 9 images The outer two landslides have well-defined (Sharp 1973), but they were not studied in aprons, and the central one merges gradudetail until Viking images revealed their ally with a plain that covers the floor of a large number and extraordinary morphol- north-south-trending trough previously cut ogy. The slides are similar to terrestrial into eroded mesas of layered deposits. The landslides in many respects (Christiansen plain extends southward for 70 km from the and Head 1978, Lucchitta 1978a, 1979) but trough entrance, and it terminates against a differ in having longitudinally grooved, thin barrier ridge of chasma-wall material that debris aprons with lobate fronts that extend separates Ophir and Candor Chasmata. The far across the valley floors; terrestrial slides ridge is breached on the west side by a 2- to generally display transverse ridges. Excep- 5-km-wide slot extending to the floor level tions are the longitudinally grooved land- of the trough. South of the barrier ridge, the slides in Alaska that moved over glacier trough in Candor Chasma contains a chanice, such as the Sherman landslide (Shreve nel that has high, level terraces and a hilly 1966, Marangunic and Bull 1968). floor deposit. Several of the hills are Many of the Martian landslides are large, doughnut shaped (Fig. 2), and their density covering hundreds of square kilometers; increases southward toward a fault scarp large landslides on Earth are commonly that cuts across the trough floor. South of emplaced as dry-rock avalanches. A com- the fault scarp, the raised trough floor is parative study of large Martian and terres- longitudinally fluted. Where the trough trial landslides (Lucchitta 1979) showed merges with a low, level region in central that the efficiency of runout of the Martian Candor Chasma (Fig. 1), the channel makes and terrestrial slides is similar and that, a sharp bend to the east and hugs the
FIG. 1. Landslides in Ophir C h a s m a . T h r e e large landslides fell from the north wall of Ophir C h a s m a and created an o u t w a s h plain and a channel extending 250 k m from the source. Also note fractured, level plain e m b a y i n g m e s a s near b o t t o m o f s c e n e and wall-rock ridges with faulted crestlines in left and center of scene. Part of p h o t o m o s a i c m a p (U.S. Geological Survey 1984).
414
B A E R B E L K. L U C C H I T T A
FI6. 2. Hummocky deposits with circular depressions ("doughnut-shaped hills"). (A) Moraine in southern Saskatchewan. Note circular depressions (kettle holes, arrow), formed by melted-out ice blocks. Aerial photograph A 17844-108, National Air Photo Library, Ottawa, Canada. (B) Deposit on channel floor in Candor Chasma. Similarity of circular depressions (arrow) to those in (A) suggests that deposit contained ice blocks. Part of Viking Orbiter image 815A50.
eroded base of a wind-fluted interior-mesa slope. T h e c h a n n e l a n d its d e p o s i t s c a n be t r a c e d to t h e e a s t e r n m o s t c o r n e r o f this level region. The landslides and their debris aprons
a p p e a r to b e d i r e c t l y r e l a t e d to t h e plain, t h e c h a n n e l , a n d its d e p o s i t s : the l a n d s l i d e s merge with the plain, from which the channel e m e r g e s ; a n d , e x c e p t i n g w i n d f e a t u r e s , all c o n t a i n t h e y o u n g e s t d e p o s i t s in the
DEBRIS FLOWS AND GROUND ICE
415
FIG. 2--Continued.
area. The observations are best explained by the following setting: The landslides generated wet debris which became dammed against the barrier ridge, and, beyond the ridge, became a debris-charged flood. The flood waters lost some of their load when they impinged on the fault scarp; beyond the scarp, they apparently eroded
the raised trough floor. The water then made a sharp eastward bend and eroded the base of the north channel bank. Eventually, the remaining flood debris came to rest as hummocky material on the level plain. I conclude that the landslide deposits, debris aprons, and plain and channel material contained water for several reasons. (1)
416
BAERBEL K. LUCCHITTA
The materials traveled 250 km from their source on a low gradient and negotiated several bends. This distance is much farther then that of most landslides in the Valles Marineris, which traveled a maximum of 100 km, a reach commensurate with their potential energy (Lucchitta 1979). (2) The landslide and channel materials were capable of erosion beyond 100 km from their source; they breached a bedrock barrier, carved flutes in the lower trough floor, and eroded the channel banks. Finally, (3) the doughnut-shaped hills resemble terrestrial kettle holes (Fig. 2A), formed by the melting of ice blocks; the resemblance strongly suggests that ice blocks were contained in the channel materials. Overall, it appears that ice and water contained in the chasma-wall rock became incorporated in the lanslide material and created a debris-laden flood. 2.1.2. Flow lobes. Other evidence that landslides in the Valles Marineris contained water is based on observations that suggest that some deposits flowed downvalley, apparently influenced by gravitational forces added to the potential energy of the initial drop. Figure 3 shows a small landslide that blocked a tributary canyon to the Valles Marineris. A flow lobe emerged from the landslide deposit at an acute angle to the direction of transport of the slide and flowed down the tributary for 8 km. No uphill lobe is seen. If the lobe resulted only from the energy of the initial fall, lobes extending up and down the valley would be expected, even though the uphill lobe might be shorter than the one heading downhill. On Earth, long-distance downvalley debris flows commonly occur where they contain water, as in the Vaiont landslide in Italy (Mueller 1968) and the Huascaran slide in Peru (PlaNer and Erickson 1978). Even the Mount St. Helens avalanche, which raced downvalley for more than 20 km, was thought to have contained an average of about 9% water by weight (Voight et al. 1981). By analogy, the Martian landslide lobe (Fig. 3) probably also contained water.
Some landslide lobes, at great distance from their sources, apparently flowed in directions at angles to the original direction of transport. A large complex of landslides in Ius Chasma, which fell from walls about 6 km high, has several superposed lobate debris aprons showing longitudinal ridges and grooves parallel to the direction of transport (Fig. 4). Locally, transverse pressure ridges developed near the margins of the aprons. The lobe that traveled farthest, superposed on the others, extends as much as I00 km from the chasma-wall landslide scar and has pressure ridges at its terminus that indicate travel at a right angle (east) to the initial direction of transport of the slide masses (south). This lobe apparently broke off from a subsidiary scarp within the upper landslide mass and flowed across the westward extension of a spur of wall rock. The change in direction of flow (Fig. 4) at such great distance from the source of the slide must have been influenced by local topographic effects; apparently the lobe was diverted in the direction of the presumed regional slope along the axis of Ius Chasma. The lobe was probably lubricated to facilitate the flow down the local gradient similar to the downvalley flows mentioned above. Also, the even, rounded margin of the lobe has the appearance of a wet debris flow. Rounded lobes have been suggested as evidence for wet emplacement (Rib and Liang 1978), whereas tonguelike margins indicate dry-rock avalanches. Smaller landslides emerging from large gullies on the Valles Marineris walls resemble mudflows on Earth; they form level, thin, spreading debris lobes (Fig. 5). The absence of obvious break-away scars in many gullies indicates that the debris probably was derived from surficial material on the walls. Many mudflows on Earth also derive their debris from unconsolidated material concentrated in valleys or on slopes and mobilized by floods (Johnson and Rahn 1970); the similarity suggests that water was involved in the emplacement of small Martian slides.
DEBRIS F L O W S A N D G R O U N D ICE
417
FIG. 3. Landslide deposits (Is) in tributary to western Ophir Chasma. Deposit fell from one-third the height of south wall (dashed line), traveled in direction of white arrow, and came to rest on older hummocky deposits (h) on floor. Flow lobe (1) emerged from deposit and flowed downhill in direction of black arrow, angling back from direction of transport of slide mass. No equivalent lobe is seen on other side of landslide deposit. Also note small debris flow (d) coming off south side of trough wall farther east. Part of Viking Orbiter image 915AI0.
2.1.3; Valleys and "ponds." T h e p r e s e n c e o f fluids in l a n d s l i d e d e p o s i t s is a l s o s u g g e s t e d b y s m a l l v a l l e y s in slide d e p o s i t s . F i g u r e 6 s h o w s a v a l l e y t h a t b e g i n s at a t h e a t e r - s h a p e d i n d e n t a t i o n w i t h i n a slide deposit and that has a V-shaped cross pro-
file, m u c h like t h e c a n y o n s ' t r i b u t a r y to t h e V a l l e s M a r i n e r i s , w h o s e origin is g e n e r a l l y a t t r i b u t e d to s a p p i n g ( S h a r p 1973, Pieri 1980). T h e v a l l e y ' s s h a p e is m o r e r e g u l a r a n d c r i s p e r t h a n a r e the s h a p e s o f o t h e r linear depressions that may be caused by
418
B A E R B E L K. L U C C H I T T A
FIG. 4. Landslide complex in Ius Chasma. Landslides contain hummocky deposits (h) in upper part and longitudinally grooved debris lobes (1) in lower part. Flow directions shown by white arrows. Southernmost lobe originated at subsidiary scarp (s) and has pressure ridges (dark arrows) indicating flow at angles to flow in upper part of slide. Mosaic of Viking Orbiter images 919-A15 through A18.
d i f f e r e n t i a l m o v e m e n t o f t h e slide m a s s . I f this v a l l e y w a s i n d e e d f o r m e d like t r i b u t a r y c a n y o n s , t h e n it s u g g e s t s t h a t a fluid drained from the landslide deposit. The last piece of morphologic evidence that the landslide contained water comes
f r o m o b s e r v a t i o n s o f a l e v e l , f r a c t u r e d plain occupying the lowermost area of central C a n d o r C h a s m a (Fig. 1). T h e p l a i n e m b a y s s u r r o u n d i n g e r o d e d m e s a s o f l a y e r e d terrain, w h i c h i m p l i e s e m p l a c e m e n t o f fluids ( W i l h e l m s 1972). T h i s p l a i n h a s b e e n inter-
DEBRIS FLOWS AND GROUND ICE
419
FIG. 5. Small landslides (Is) emerging from large gullies in walls of Valles Marineris. Debris aprons are thin and spread out and have lobate fronts like terrestrial mudflows. Part of Viking Orbiter image 910A15.
preted to represent a former pond, playa, or alluvial flat where wasted materials were concentrated (Lucchitta and Ferguson 1983, Lucchitta et al. 1986). Debris from two landslide complexes (including the landslides shown in Fig. 1) apparently flowed into this flat, level plain and probably contributed to the fill. 2.2. Observations Consistent with Wet E m p l a c e m e n t 2.2.1. Landslide efficiency. Some aspects of the landslides in the Valles Marineris are consistent with wet emplacement, but other mechanisms may equally well have been
responsible. For instance, Scheidegger (1973) noted that terrestrial landslides have increased efficiency with increased volume (Fig. 7 and Table I). The efficiency o f a slide is obtained by calculating the tangent of the slope from the top of the landslide scar to the tip o f the slide deposit; the lower the tangent (the lower the slope), the higher the efficiency. This slope is also an approximate measure of the coefficient of friction (Scheidegger 1973). A few landslides in the Valles Marineris, whose volumes were obtained by computing the volume missing from landslide scars, were plotted on Scheidegger's graph of efficiency versus
420
B A E R B E L K. L U C C H I T T A
FI6.6. Valley in landslide. The valley (arrow) originates in a landslide deposit (ls) and has a blunt, cirquelike head and a V-shaped cross profile, similar to valleys attributed to sapping (Sharp 1973, Pieri 1980). Part of Viking Orbiter image 64A16.
volume (Fig. 7 and Table I). I transposed all volumes into weights in order to introduce the proper terms for terrestrial and Martian gravities. The graph shows that, in general, the Martian landslides fit Scheidegger's graph remarkably well. However, the trend
of increased efficiency with increased weight appears to level out slightly on Mars (but this observation is unreliable because of the highly scattered data and the uncertainties in the Martian slope and volume measurements). If a difference in trends
10 .................
0.5
t
0.2
2°
i'
0.1
~ O.OS
1" 3 .
6. 45"
7t 9"10"11"
x. Earth * . Mors
002 00~
1014
iOIs
|O16
]017
i0 ls
I019
i0 =o
i0:1
WEIGHT IN DYNES
FIG. 7. Graph of landslides, modified after Scheidegger (1973). Tangent of slope from top of landslide scar to tip of the slide deposit, a measure of efficiency, is plotted against landslide weight. Weight calculated by using g = 980 cm/sec z for Earth and g = 372 cm/sec 2 for Mars, and assuming density of 2.5 g/cm 3 for all slides. Terrestrial slides marked x (after Scheidegger), Martian slides represented by numbered dots (for location see Table I). Position of Martian slides on graph approximate because of uncertainties in slope and volume. Dashed line is Scheidegger's correlation curve. Graph shows that Martian and terrestrial slides are remarkably similar. However, whereas terrestrial slides generally increase in efficiency (decreasing tangent) with increasing weight, Martian slides show a slight tendency to retain their efficiency regardless of weigtit.
1022
DEBRIS FLOWS AND GROUND ICE TABLE I MARTIAN LANDSLIDES PLOTTED IN FIGURE 7 Landslide ID
1 2 3 4 5 6 7 8 9 10 11
Location" Lat.
Long.
- 10030 ' -4°30 ' -9o00 ' -8045 ' -8°15 ' - 3o00 ' - 12045 ' -3030 ' -9000 ' -7000 ' -8o00 '
68o20 , 70o00 , 44o30 ' 69°20 ' 71o00 ' 73°45 ' 69020 ' 71o00 ' 41015 ' 45°15 ' 41°15 '
C e n t e r o f l a n d s l i d e scar.
really exists, it could indicate that (I) Martian and terrestrial landslides have a threshold weight above which efficiency no longer increases with increased weight or, more likely, (2) some aspects of the environments or the emplacement mechanisms are different. Such differences may be due to the composition of the slide material or the substrate or to the entrainment of volatiles. When the morphologic evidence for water in the landslides is considered, the difference between the Martian and terrestrial slides, if real, is consistent with the hypothesis that the Martian landslides were wet debris flows, whereas large catastrophic landslides on Earth are mostly dryrock avalanches. 2.2.2. Landslide speeds. Another difference between Martian and terrestrial landslides is their speed: the Martian landslides apparently m o v e d much faster than terrestrial ones. Minimum speeds for the Martian landslides can be calculated wherever the slide masses overtopped obstacles or m o v e d through topographic lows and uphill on opposite slopes. Minimum speeds correspond to heights regained during travel and can be calculated by compar-
421
ing the kinetic energy of the slide to the potential energy needed to raise the slide mass to the regained height [½my2 = mgh, and v = (2gh) ½ where v is the minimum speed, m the mass of the slide, g the gravitational acceleration, and h the regained height]. Speeds calculated for a few Martian landslides are compared to speeds of terrestrial landslides or debris avalanches in Table II. The Martian landslides generally moved faster than terrestrial slides, even though Martian gravity is only about one-third that of Earth. The most likely explanation for the higher Martian speeds is that the slides on Mars fell from greater heights and had a larger mass than most terrestrial slides; therefore they also had a higher potential energy. The Huascaran debris avalanche in Peru is the only terrestrial one that, in its upper part, matched the Martian ones in speed (Plafker and Erickson 1978); it fell from a height (4 km) similar to heights on Mars. The Huascaran avalanche is also the only large terrestrial one among those cited
T A B L E II SPEED OF LANDSIDES Earth
km/hr
V a i o n t , I t a l y ( M u e l l e r 1968) E l m , S w i t z e r l a n d ( H e i m 1932) M a d i s o n , M o n t a n a ( H a d l e y 1966) G r o s V e n t r e , W y o m i n g ( A l d e n 1928) F r a n k , A l b e r t a ( D a l y et al. 1912) B l a c k h a w k , C a l i f o r n i a ( S h r e v e 1968) Huascaran, upper section, Peru (Plafker and E r i c k s o n 1978)
90 150 160 165 175 235 360
Mars Ius, e a s t ( - 7 ° 3 0 ' ; 78°00') ° T i t h o n e u m , c e n t e r ( - 4 ° 2 0 ' ; 85°30 ') I u s , w e s t ( - 6 ° 2 0 ' ; 87°00 ') I u s , c e n t e r ( - 6 ° 2 0 ' ; 85015 ') T i t h o n i u m , e a s t ( - 4 ° 3 0 ' ; 80o00 ') T i t h o n i u m , e a s t ( - 4 ° 3 0 ; 79030 ') C a n d o r ( - 4 5 ° 4 5 ' ; 75o20 ' ) M e l a s ( - 8 ° 1 5 ' ; 71o00 ')
250 250 300 300 300 300 400 450
a L a t i t u d e a n d l o n g i t u d e at c e n t e r o f l a n d s l i d e scar.
422
BAERBEL K. LUCCHITTA
in Table II that contained much ice and water because it incorporated parts of an ice cap. Indeed, Plafker and Erickson (1978) attributed the high speed not only to the height of the fall but also to the entrainment of perhaps as much as 5 to 10% (by volume) of ice and water. Similarly, ice and water in the Martian landslides may have been a contributing factor in their high speeds. 2.2.3. Longitudinal grooves. A further difference between Martian and terrestrial landslides is the presence of longitudinal ridges and grooves on debris aprons of the Valles Marineris slides (Figs. 1 and 4; Lucchitta 1978a, Figs. 2 and 4), as opposed to the presence of transverse ridges on most terrestrial slides. Exceptions are longitudinally grooved Alaskan landslides that moved across glacier ice, such as the wellstudied Sherman landslide. Shreve (1966) and Marangunic and Bull (1968) suggested that these longitudinal grooves arose from shearing between rapidly moving debris traveling forward at slightly different speeds or times. In a study of longitudinal flow patterns in debris of impact ejecta, volcanic ash flows, and landslides on Earth, Moon, and Mars, Lucchitta (1979) concluded that the most important conditions for the formation and preservation of longitudinal patterns in debris aprons are high initial energies resulting in high speeds, and low frictional resistance within the slide mass or of the subjacent ground. The development and preservation of grooves in the Sherman landslide were thus probably caused by its travel over smooth glacier ice, a condition that resulted in very low frictional resistance of the substrate. In Martian slides, grooves may have formed because of both high initial energies and low frictional resistance. If the landslides in the Valles Marineris indeed contained water, this water probably contributed to the lowering of frictional resistance within the Martian slide masses and thus aided in the development of longitudinal grooves.
3. WALL ROCK 3.1. Observations Supporting Ice or Water in Wall R o c k
The walls bordering the Valles Marineris grabens display spur-and-gully morphology, landslide scars, and tributary canyons (Lucchitta 1978b). A different morphology marks the interior deposits, which have conspicuous layers (McCauley 1978). Evidence that ice or water may have been present in the wall rock comes from the morphology of tributary canyons and the contrast between characteristics of the wall rock and the interior deposits. 3.1.1. Tributary canyons. Tributary canyons, some as much as 130 km long, have blunt, cirquelike valley heads, V-shaped cross profiles, and uniform gradients toward the main chasmata. In places they have h u m m o c k y or lobate floor deposits that resemble rock glaciers in Antarctica (Lucchitta 1978b) and that indicate that material was mass wasted from the walls and transported through the canyons. Dry mass wasting alone is an unlikely transportation mechanism because gradients are only a few degrees, much below the angle of repose (about 30°). Wind erosion alone is also unlikely because the uniform gradients indicate the work of a confined fluid with a distinct upper surface. Clearly, the moving debris must have been lubricated. The most readily available lubricating agents would have been ice or water. In the absence of atmospheric precipitates in this region of Mars (Fanale and Jakosky 1982), ground ice, sapped from the chasma walls (Sharp 1973, Pieri 1980), appears to be the most likely lubricating agent. Further evidence that fluids flowed through tributary canyons comes from a tributary that gave rise to a channel below its mouth (Fig. 8). This canyon has a depth of about 400 m near its head and 800 m near its mouth (obtained by measuring the widths of talus slopes within the canyon and assuming that the talus rests at a 30° angle of repose). This fluid, again, most
DEBRIS FLOWS AND GROUND ICE
423
FIG. 8. Tributary canyon and channel. A small channel (arrow) issues from the mouth of a tributary canyon (t) in the plateau segment separating Candor and Melas Chasmata. Apparently a fluid flowed from the canyon. Part of Viking Orbiter image 912A16.
likely came from canyon-wall rock; the implication is that the rocks were not desiccated at a depth of 400-800 m below the surface during the time of channel formation. 3.1.2. Contrast in morphology of walls and interior mesas. The wall-rock morphology is conspicuously different from that of the interior layered deposits. Wall rocks show layers only near the top, display no evidence of wind erosion, and have bifurcating spurs and gullies. By contrast, interior deposits may be smooth or layered throughout; they show conspicuous cliffs in some places and are strongly eroded by wind in other places, as evidenced by regularly spaced flutes (Fig. 9; also see Fig. 10). Wall-rock morphology occurs not only on the Valles Marineris walls but also in their interior, where wall rock projects as spurs or forms isolated erosional remnants. Some
of these wall-rock remnants are as high and have slopes as steep as those of adjacent eroded mesas of layered deposits, yet debris flows, spreading as thin, extensive aprons far across the chasma floors, occur abundantly only on wall-rock slopes. They are conspicuously lacking on slopes of interior deposits (Fig. 9), where I found only two small landslides, both forming minor slumps restricted to the slopes. Clearly, wall rocks and interior deposits had different c o h e r e n c e and coefficients of internal friction. The wall rocks were highly susceptible to efficient flow; therefore they were probably poorly consolidated and contained volatiles. Cliffs on interior deposits, by contrast, suggest that the deposits were generally well consolidated; however, flutes on some layers indicate that these were, perhaps, composed of sand-sized grains susceptible to wind erosion. The wall
424
BAERBEL K. LUCCHITTA
FIG. 9. Wall rock and layered deposits. Mesa (m) of layered deposits having fluted slopes is as high and steep as adjacent wall-rock remnant (w) having irregular spur-and-gully morphology.The layered deposits lack debris flows, whereas wall-rock remnants display abundant debris flows (d) that spread out across the valley floor. Note landslide (Is) issuing from gully. Viking Orbiter image 912A11.
rocks were probably weakly cemented by ice. They may have been similar to the ice-charged highland rocks of the Martian midlatitude belt, which gave rise to "terrain softening" features (Squyres and Carr 1986), but the wall rocks were probably
backed by water trapped b e y o n d exposed surfaces at a depth of only 1 to 2 km, where the 0 ° isotherm would be intersected (Fanale 1976, R o s s b a c h e r and Judson 1981). Therefore, and because of the high, steep scarps of the Valles Marineris, the highland
DEBRIS FLOWS AND GROUND ICE rocks may have collapsed catastrophically and formed gigantic wet debris flows (Lucchitta 1979) rather than deforming by creep as in the "softened" Martian midlatitudes. 3.2. Observations Consistent with Ice or Water in Wall R o c k
Like landslides, some characteristics associated with wall rock have ambiguous origins; they can be explained by a high content of ice or water or by other wallrock properties. One of these characteristics is the unusual competence of structurally disturbed areas. The Valles Marineris grabens are associated with numerous fault zones both on top of the surrounding plateaus and within the
425
grabens. The faults on the plateaus caused chain craters, commonly attributed to collapse (Sharp 1973, Blasius et al. 1977), and they influenced the alignment of tributary canyons. These observations suggest that the faults are planes of weakness along which degradational processes operated more efficiently than in the surrounding undisturbed rock. Yet, within the troughs, the crest lines of most wall-rock spurs are marked by dense fault zones, whereas surrounding lower areas are much less fractured (Fig. 10, also see Fig. 1). Apparently, the fracture zones are more resistant to erosion than are the unfractured areas, a phenomenon different from that of many terrestrial zones where brecciation along
FIG. 10. Fault zones in chasmata. Crestline of wall-rock spur separating western Ophir Chasma from western Candor Chasma is intensely faulted (black arrows). Faults appear to be less dense on slopes of spur. Apparently, the fault zones are more resistant to erosion than is wall rock and thus support the spur. Dark bands along bases of wall-rock spur (white arrows) are probably mafic volcanic rocks (Lucchitta 1987). Note difference in morphologies of wall rock (spurs and gullies) and interior layered mesa (m) (fluted slopes). Mosaic of Viking Orbiter images 815A41 through A48.
426
BAERBEL K. LUCCHITTA
faults tends to make them less resistant. These Martian fault zones may be more resistant because the faults have been intruded by dikes or the fault zones were more lithified than the surrounding country rock owing to hydrothermal activity or other circulation of water. Dikes along faults are common on Earth, but they would have to be unusually abundant in the Valles Marineris and intruded along nearly all faults to uphold the spurs. The explanation that fault zones were lithified implies that the country rock is weaker, and even though readily explained otherwise, also agrees well with the possibility that the wall rock is composed of relatively unconsolidated materials charged with water or ice. If such is the case, the frictional heat and shear stress near the fault planes alone could have consolidated the rocks and driven out the volatiles, making the fault zones more resistant than the surrounding country rock.
4. TIMING OF EVENTS The timing of landslide emplacement and tributary canyon formation is important because it dates the period when water and ice were present in the canyon walls. A relative time for these events can be established by considering the geologic history of the Valles Marineris and its interior; a relative time and a tentative absolute time can be established by crater counts. Geologic relations (Lucchitta 1981, 1985, Nedell et al. in press) indicate that the landslides occurred late in the history of the Valles Marineris; they were emplaced after the canyon walls developed spur-and-gully morphology and after deposition and erosion of the interior layered deposits formed the high-standing interior mesas. The only features younger than the landslides are probable small volcanic vents and eolian forms. The emplacement of tributary canyons is more difficult to date, but they also appear to have formed mostly after the erosional epoch that created the mesas; the
tributary canyons are probably similar in age to the landslides. Crater counts on the combined landslide deposits of Tithonium and Ius Chasmata yielded an average crater frequency of 570 +- 130 craters -> 1 km diameter/106 km 2 (Lucchitta 1979). Individual landslide deposits are too small to give statistically significant results, but on some deposits superposed craters are so scarce that the deposits may be recent. One of these deposits is the landslide complex in Ophir Chasma (Fig. 1), discussed above. It has only a few superposed craters, all of which are -<500 m in diameter. When the average landslide age is plotted against the relative ages of other Martian events (Fig. ll), it becomes apparent that the Valles Marineris grabens probably existed throughout a major part of Martian history, whereas the landslides were emplaced only during the last third of this time span. The landslide emplacement coincides with late activities on the Tharsis volcanoes. When the relative time scale of Scott and Tanaka (1981) is compared with an absolute time scale (Model 2 of Neukum and Hiller 1981) (Fig. 11), it appears that the landslides were emplaced during the last billion years. The subsurface of the equatorial region of Mars, accordingly, was not desiccated during this time. 5. CONCLUSION Landslides in the Valles Marineris probably contained water. The most compelling evidence comes from a large channel that issued from three landslides in Ophir Chasma. Also supporting entrainment of water or ice is the morphology of flow lobes and the observation that landslide debris appears to have contributed to a " p o n d e d " level deposit in the lowest area in central Valles Marineris. If landslides were wet, they derived their water from the Valles Marineris wall rock, which, accordingly, must have contained water or ice. That the wall rock contained volatiles is further supported in varying degrees by (1) tributary
DEBRIS FLOWS AND GROUND ICE Rel. Crater Age (n z lkm/106 km 2)
427
Abs. Age (b.y.) -0
Olympus Mons ! Construct
100-
I''1
-0.5
I I I ! I I
I
500- -1.o
r'l
r.arsis
Plains ~
-1.5
Landslides
1,000-2,0 ~2,5
I Northern /Plains
-3.0 !
2,000 10,0OO--
I
Ib..I -3.5
ISyria
I,,n2P' .... I Ptanum
-4,0
5O,O00-(Scott and Tanaka, ( Neukurn and 1981) Hiller, 1981) FIG. 11. Sketch showing age relations in Tharsis and Valles Marineris areas. Landslides were emplaced during last third of Valles Marineris history, and may be as young as recent. Stippled area corresponds to age determined from crater counts (Lucchina 1979). Other relative ages after Scott and Tanaka (1981); absolute ages after Model 2 of Neukum and Hiller (1981).
canyons that served as conduits for lubricated debris and that gave rise to channels in places; (2) the contrast between the morphologies of wall rock and interior deposits, which suggests that the wall rock was composed of poorly consolidated material that flowed easily; and (3) the erosional resistance of structurally disturbed zones, which implies that surrounding wall rock was relatively weak. Many of the above observations can be explained otherwise, but the combined evidence strongly supports that water was involved in landslide emplacement and wall-rock disintegration. The postulated ice in wall rock may have filled pore spaces, but perhaps it was so abundant that it formed segregated lenses in places. The ice was probably cementing the walls, but owing to the internal heatflow, was converted to water at a distance
of 1 to 2 km beyond exposed faces. Thus, the walls were exceedingly unstable, susceptible to collapse, and predisposed toward the generation of wet debris flows (Lucchitta 1979). If the Valles Marineris landslides were indeed wet, they are not analogous to terrestrial dry-rock avalanches, and they cannot be cited as examples to support terrestrial, theoretical dryrock avalanche emplacement mechanisms. If ice and water were present in the Valles Marineris walls at the time of landslide emplacement, the equatorial region of Mars below a certain depth was not desiccated at this time. This time period occurred within the last billion years and may even be recent. Consequently, the groundice reservoir on Mars would have been extensive and, perhaps, planetwide fairly recently and, if it is not a relic from ancient times, it must have been replenished (Clif-
428
BAERBEL K. LUCCHITTA
ford and Hillel 1983). These considerations are important for understanding the volatile inventory and climatic history of Mars. REFERENCES ALDEN, W. C. 1928. Landslide and Flood at Gros Ventre, Wyoming. American Institute of Mining and Metallurgical Engineers, Technical Publication 140. BLASlUS, K. R., J. A. CUTTS, J. E. GUEST, AND H. MASURSKY 1977. Geology of the Valles Marineris: First analysis of imaging from the Viking I orbiter primary mission. J. Geophys. Res. 82, 4067-4091. CHRISTIANSEN, E. H., AND J. W. HEAD 1978. Martian Landslides--Classification and Genesis (Abstract). Reports of Planetary Geology Program, 1977-78. NASA Tech. Memo. 79729, pp. 285-287. CLIFFORD, S. M., AND D. HILLEL 1983. The stability of ground ice in the equatorial region of Mars. J. Geophys. Res. 88, 2456-2474. DALY, R. A., W. G. MILLER, AND G. S. RICE 1912. Report o f the Commission Appointed to Investigate Turtle Mountain, Frank, Alberta. Canada Geological Survey Memoir 27. FANALE, F. P. 1976. Martian volatiles: The degassing history and geochemical fate. Icarus 28, 179-202. FANALE, F. P., AND B. M. JAKOSKY 1982. Regolithatmosphere exchange of water and carbon dioxide on Mars: Effects on atmospheric history and climate change. Planet. Space Sci. 30, 819-831. FANALE, F. P., J. R. SALVAIL, A. P. ZENT, AND S. E. POSTAWKO 1986. Global distribution and migration of subsurface ice on Mars. Icarus 67, 1-18. HADLEY, J. B. 1966. The Madison Landslide (Montana) in Yellowstone Earthquake Area. Billings Geological Society, Annual Field Conference Guidebook llth, pp. 45-48. HEIM, ALBERT 1932. Bergsturz and Menschenleben (Landslides and people's lives). Fretz and Wasmuth, Zurich. JOHNSON, A. M., AND P. H. RAHN 1970. MobiLization of debris flows. In Zeitschriftfuer Geomorphologie, Supp. Band 9, pp. 168-186. LUCCHITTA, B. K. 1978a. A large landslide on Mars. Geol. Soc. Am. Bull. 89, 1601-1609. LUCCHITTA, B. K. 1978b. Morphology of chasma walls, Mars. J. Res. U.S. Geol. Surv. 6, 651-662. LUCCHITTA, B. K. 1979. Landslides in Valles Marineris. J. Geophys. Res. 84, 8097-8113. LUCCHITTA, B. K. 1981. Valles Marineris--Faults, Volcanic Rocks, Channels, Basin Beds (Abstract). Reports of Planetary Geology Program--1981. NASA Tech. Memo. 84211, pp. 419-421. LUCCHITTA, B. K. 1985. Valles Marineris Basin Beds: a Complex Story (Abstract). Reports of Planetary Geology Program--1984. NASA Techn. Memo. 87563, pp. 506-508.
LUCCHITTA, B. K. 1987. Recent mafic volcanism on Mars. Science 235, 565-567. LUCCHITTA, B. K., AND H. M. FERGUSON 1983. Chryse Basin channels: Low gradients and ponded flows. J. Geophy. Res. 88, A553-A568. LUCCHITTA, B, K., H. M. FERGUSON, AND C. SUMMERS 1986. Sedimentary deposits in the northern lowland plains, Mars. J. Geophys. Res. 91, E166E174. MARANGUNIC, C., AND C. BULL 1968. The landslide on the Sherman Glacier. In The Great Alaska Earthquake of 1964, Hydrology. National Academy of Science Publication 1603, pp. 383-394. McCAULEY, J. F. 1978. Geologic Map o f the Coprates Quadrangle o f Mars. U.S. Geol. Survey Misc. Invest. Map 1-897, scale 1:5,000,000. MUELLER, L., 1968. New considerations of the VajonI slide. Rock Mech. Eng. Geol. 6, 1-91. NEDELL, S. S., S. W. SQUYRES, AND D. W. ANDERSON (in press). Origin and evolution of the layered deposits in the Valles Marineris. Icarus 70, 409-441. NEUKUM, G., AND K. HILLER 1981. Martian ages. J. Geophy. Res. 86, 3097-3121. PIERI, DAVID 1980. Martian valleys: Morphology, distribution, age, and origin. Science 210, 895-897. PLAFKER, G., AND G. E. ERICKSON 1978. Mechanism of catastrophic avalanches from Nevados Huascaran, Peru. In Rock Slides and Avalanches (B. Voight, Ed.), pp. 227-314. Elsevier, Amsterdam. RIB, H. T., AND T. A. LIANG 1978. Recognition and identification. In Landslides, Analysis, and Control (R. L. Schuster and R. J. Krizek, Eds.), pp. 34-80. National Academy of Sciences Special Report 176, Washington, DC. ROSSBACHER, L. A., AND S. JUDSON 1981. Ground ice on Mars: Inventory, distribution, and resulting landforms. Icarus 45, 39-59. SCHEIDEGGER, A. E. 1973. On the prediction of the reach and velocity of catastrophic landslides. Rock Mech. 5, 231-236. SCOTT, D. H., AND K. L. TANAKA 1981. Mars: Paleostratigraphic restoration of buried surfaces in Tharsis Montes. Icarus 45, 304-330. SHARP, R. P. 1973. Mars troughed terrain. J. Geophy. Res. 78, 4063-4072. SHREVE, R. L. 1966. Sherman landslide, Alaska. Science 154, 1639-1643. SHREVE, R. L. 1968. Leakage and fluidization in air-layer lubricated avalanches. Geol. Soc. Amer. Bull. 79, 653-658. SQUYRES, S. W., AND M. H. CARR 1986. Geomorphic evidence for the distribution of ground ice on Mars. Science 231, 249-252. U.S. Geological Survey 1980. Tithonium Chasma o f Mars. Topographic Orthophoto Mosaic. U.S. Geological Survey Miscellaneous Investigations Map 1-1294, scale I : 500,000.
DEBRIS FLOWS AND GROUND ICE U.S. Geological Survey 1984. Valles Marineris Region, Viking High-Resolution Controlled Mosaic MTM-5072. U.S. Geol. Survey Misc. Invest. Map 1-1592, scale 1:500,000. U.S. Geological Survey (in press). Corprates Northwest. Topographic Map Series. U.S. Geological Survey Miscellaneous Investigations Map 1-1712, scale 1 : 2,000,000. VOIGT, B., H. GLICKEN, R. J. JANDA, AND P. M. DOUGLASS 1981. Catastrophic rock slide avalanche
429
of May 18. In The Eruptions o f Mount St. Helens, Washington. U.S. Geological Survey Prof. Paper 1250, pp. 347-378. WIL~ELMS, D. E. 1972. Geologic Mapping of a Second Planet. lnteragency Report. Astrogeology 55. Wu, S. S. C., AND A. E. HOWlNGTON 1986. A Mars Digital Terrain Model and Sample Correlation with a Mars Digital Image Model (Abstract). Results of Planetary Geology and Geophysics Program--1985. NASA Tech. Memo. 88383, pp. 608-611.
Valles Marineris, Mars: Wet Debris Flows and Ground Ice B A E R B E L K. L U C C H I T T A u . s . Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 86001
Received January 12, 1987; revised May 19, 1987 Detailed study of the Valles Marineris equatorial troughs suggests that the landslides in that area contained water and probably were gigantic wet debris flows: one landslide complex generated a channel that has several bends and extends for 250 km. Further support for water or ice in debris masses includes rounded flow lobes and transport of some slide masses in the direction of the local topographic slope. Differences in speed and emplacement efficiency between Martian and terrestrial landslides can be attributed to the entrainment of volatiles on Mars, but they can also be explained by other mechanisms. Support that the wall rock contained water comes from the following observations: (1) the water within the landslide debris must have been derived from wall rock; (2) debris appears to have been transported through tributary canyons; (3) locally, channels emerged from the canyons; (4) the wall rock apparently disintegrated and flowed easily; and (5) fault zones within the troughs are unusually resistant to erosion. The study further suggests that, in the equatorial region of Mars, material below depths of 4 0 0 - 8 0 0 m was not desiccated during the time of landslide activity (within the last billion years of Martian history). Therefore the Martian ground-water or groundice reservoir, if not a relic from ancient times, must have been replenished. © 1987 Academic Press, Inc. 1. INTRODUCTION
One conclusion is that the landslides in the Valles Marineris contained water when Are the Valles Marineris landslides wet emplaced and p r o b a b l y formed gigantic wet or dry? Is the equatorial belt of Mars desic- debris flows. Several previous studies of cated or not? A n s w e r s to these questions these landslides (Sharp 1973, Blasius e t al. have profound implications regarding the 1977, Lucchitta 1979) did not conclude extent and timing of ground-ice reservoirs whether their origin was wet or dry, mainly on Mars and ultimately for an understandbecause c o m p a r i s o n s with large terrestrial ing of the evolution of Martian volatiles and landslides were undiagnostic. F u r t h e r m o r e , climate. the availability of w a t e r to lubricate the This p a p e r attempts to address these Martian slides was in doubt. questions. The observations presented are The second conclusion of this report is based largely on recent detailed mapping in that the equatorial region of Mars was not the central Valles Marineris, where excel- desiccated relatively recently. Arguments lent, high-resolution stereoimages permit a for the wet nature of landslides and obserthree-dimensional view into the u p p e r 7-10 vations of the m o r p h o l o g y of the walls of k m o f the crust. The evidence is mostly Valles Marineris suggest that the wall rock observational except for a few quantitative contained ice or water. U n d e r present atconsiderations, which are based on topo- mospheric conditions the equatorial region graphic m a p s (U.S. Geological Survey of Mars is desiccated near the surface, 1980, in press, Wu and H o w i n g t o n 1986). owing to disequilibrium b e t w e e n volatiles T w o main conclusions were reached. in the ground and in the a t m o s p h e r e (Fan411 0019-1035/87 $3.00 Copyright © 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.
412
BAERBEL K. LUCCHITTA
ale 1976, Fanale and Jakosky 1982, Clifford therefore, the Maritan landslides may also and Hillel 1983, Fanale et al. 1986). How- have been dry. That study was based only ever, this desiccation apparently does not on early Viking images; subsequently, extend to great depth, and therefore many additional images have become ground-ice-collapse depressions and liqui- available and offer further insights. On the fled crater ejecta should be expected in this basis of the more recently available inforregion. mation, it is suggested here that the landIn this report I first discuss landslides and slides in the Valles Marineris contained present observations suggesting that they water, in fact that they may have been were wet when emplaced; second, I discuss gigantic wet debris flows. the walls of Valles Marineris and explain why they may have been underlain by 2.1. Observations Supporting Wet Emplacement mixtures of rock and ice. Each of these sections is further separated into two parts, 2.1.1. Ophir Chasma. The best evidence one listing observations that I consider to that landslides contained water and ice be best explained by the stated hypotheses, comes from three landslides that fell from the other listing considerations that are the north wall of Ophir Chasma and gave consistent with the hypotheses but can rise to a plain and a channel that extend for equally well be otherwise explained. A final a combined distance of 250 km at a gradient section addresses the temporal aspects of of approximately 4 m/km (Fig. 1). The both landslide emplacement and the pres- slides fell from about a 7-km height, and, ence of ground ice in the equatorial area of near their heads, they have rugged slump Mars. blocks that extend 20 to 30 km from the detachment scar. The slump blocks grade 2. LANDSLIDES into smoother, longitudinally grooved deLandslides in the Valles Marineris bris aprons that extend an additional 40 kin. troughs were first seen on Mariner 9 images The outer two landslides have well-defined (Sharp 1973), but they were not studied in aprons, and the central one merges gradudetail until Viking images revealed their ally with a plain that covers the floor of a large number and extraordinary morphol- north-south-trending trough previously cut ogy. The slides are similar to terrestrial into eroded mesas of layered deposits. The landslides in many respects (Christiansen plain extends southward for 70 km from the and Head 1978, Lucchitta 1978a, 1979) but trough entrance, and it terminates against a differ in having longitudinally grooved, thin barrier ridge of chasma-wall material that debris aprons with lobate fronts that extend separates Ophir and Candor Chasmata. The far across the valley floors; terrestrial slides ridge is breached on the west side by a 2- to generally display transverse ridges. Excep- 5-km-wide slot extending to the floor level tions are the longitudinally grooved land- of the trough. South of the barrier ridge, the slides in Alaska that moved over glacier trough in Candor Chasma contains a chanice, such as the Sherman landslide (Shreve nel that has high, level terraces and a hilly 1966, Marangunic and Bull 1968). floor deposit. Several of the hills are Many of the Martian landslides are large, doughnut shaped (Fig. 2), and their density covering hundreds of square kilometers; increases southward toward a fault scarp large landslides on Earth are commonly that cuts across the trough floor. South of emplaced as dry-rock avalanches. A com- the fault scarp, the raised trough floor is parative study of large Martian and terres- longitudinally fluted. Where the trough trial landslides (Lucchitta 1979) showed merges with a low, level region in central that the efficiency of runout of the Martian Candor Chasma (Fig. 1), the channel makes and terrestrial slides is similar and that, a sharp bend to the east and hugs the
FIG. 1. Landslides in Ophir C h a s m a . T h r e e large landslides fell from the north wall of Ophir C h a s m a and created an o u t w a s h plain and a channel extending 250 k m from the source. Also note fractured, level plain e m b a y i n g m e s a s near b o t t o m o f s c e n e and wall-rock ridges with faulted crestlines in left and center of scene. Part of p h o t o m o s a i c m a p (U.S. Geological Survey 1984).
414
B A E R B E L K. L U C C H I T T A
FI6. 2. Hummocky deposits with circular depressions ("doughnut-shaped hills"). (A) Moraine in southern Saskatchewan. Note circular depressions (kettle holes, arrow), formed by melted-out ice blocks. Aerial photograph A 17844-108, National Air Photo Library, Ottawa, Canada. (B) Deposit on channel floor in Candor Chasma. Similarity of circular depressions (arrow) to those in (A) suggests that deposit contained ice blocks. Part of Viking Orbiter image 815A50.
eroded base of a wind-fluted interior-mesa slope. T h e c h a n n e l a n d its d e p o s i t s c a n be t r a c e d to t h e e a s t e r n m o s t c o r n e r o f this level region. The landslides and their debris aprons
a p p e a r to b e d i r e c t l y r e l a t e d to t h e plain, t h e c h a n n e l , a n d its d e p o s i t s : the l a n d s l i d e s merge with the plain, from which the channel e m e r g e s ; a n d , e x c e p t i n g w i n d f e a t u r e s , all c o n t a i n t h e y o u n g e s t d e p o s i t s in the
DEBRIS FLOWS AND GROUND ICE
415
FIG. 2--Continued.
area. The observations are best explained by the following setting: The landslides generated wet debris which became dammed against the barrier ridge, and, beyond the ridge, became a debris-charged flood. The flood waters lost some of their load when they impinged on the fault scarp; beyond the scarp, they apparently eroded
the raised trough floor. The water then made a sharp eastward bend and eroded the base of the north channel bank. Eventually, the remaining flood debris came to rest as hummocky material on the level plain. I conclude that the landslide deposits, debris aprons, and plain and channel material contained water for several reasons. (1)
416
BAERBEL K. LUCCHITTA
The materials traveled 250 km from their source on a low gradient and negotiated several bends. This distance is much farther then that of most landslides in the Valles Marineris, which traveled a maximum of 100 km, a reach commensurate with their potential energy (Lucchitta 1979). (2) The landslide and channel materials were capable of erosion beyond 100 km from their source; they breached a bedrock barrier, carved flutes in the lower trough floor, and eroded the channel banks. Finally, (3) the doughnut-shaped hills resemble terrestrial kettle holes (Fig. 2A), formed by the melting of ice blocks; the resemblance strongly suggests that ice blocks were contained in the channel materials. Overall, it appears that ice and water contained in the chasma-wall rock became incorporated in the lanslide material and created a debris-laden flood. 2.1.2. Flow lobes. Other evidence that landslides in the Valles Marineris contained water is based on observations that suggest that some deposits flowed downvalley, apparently influenced by gravitational forces added to the potential energy of the initial drop. Figure 3 shows a small landslide that blocked a tributary canyon to the Valles Marineris. A flow lobe emerged from the landslide deposit at an acute angle to the direction of transport of the slide and flowed down the tributary for 8 km. No uphill lobe is seen. If the lobe resulted only from the energy of the initial fall, lobes extending up and down the valley would be expected, even though the uphill lobe might be shorter than the one heading downhill. On Earth, long-distance downvalley debris flows commonly occur where they contain water, as in the Vaiont landslide in Italy (Mueller 1968) and the Huascaran slide in Peru (PlaNer and Erickson 1978). Even the Mount St. Helens avalanche, which raced downvalley for more than 20 km, was thought to have contained an average of about 9% water by weight (Voight et al. 1981). By analogy, the Martian landslide lobe (Fig. 3) probably also contained water.
Some landslide lobes, at great distance from their sources, apparently flowed in directions at angles to the original direction of transport. A large complex of landslides in Ius Chasma, which fell from walls about 6 km high, has several superposed lobate debris aprons showing longitudinal ridges and grooves parallel to the direction of transport (Fig. 4). Locally, transverse pressure ridges developed near the margins of the aprons. The lobe that traveled farthest, superposed on the others, extends as much as I00 km from the chasma-wall landslide scar and has pressure ridges at its terminus that indicate travel at a right angle (east) to the initial direction of transport of the slide masses (south). This lobe apparently broke off from a subsidiary scarp within the upper landslide mass and flowed across the westward extension of a spur of wall rock. The change in direction of flow (Fig. 4) at such great distance from the source of the slide must have been influenced by local topographic effects; apparently the lobe was diverted in the direction of the presumed regional slope along the axis of Ius Chasma. The lobe was probably lubricated to facilitate the flow down the local gradient similar to the downvalley flows mentioned above. Also, the even, rounded margin of the lobe has the appearance of a wet debris flow. Rounded lobes have been suggested as evidence for wet emplacement (Rib and Liang 1978), whereas tonguelike margins indicate dry-rock avalanches. Smaller landslides emerging from large gullies on the Valles Marineris walls resemble mudflows on Earth; they form level, thin, spreading debris lobes (Fig. 5). The absence of obvious break-away scars in many gullies indicates that the debris probably was derived from surficial material on the walls. Many mudflows on Earth also derive their debris from unconsolidated material concentrated in valleys or on slopes and mobilized by floods (Johnson and Rahn 1970); the similarity suggests that water was involved in the emplacement of small Martian slides.
DEBRIS F L O W S A N D G R O U N D ICE
417
FIG. 3. Landslide deposits (Is) in tributary to western Ophir Chasma. Deposit fell from one-third the height of south wall (dashed line), traveled in direction of white arrow, and came to rest on older hummocky deposits (h) on floor. Flow lobe (1) emerged from deposit and flowed downhill in direction of black arrow, angling back from direction of transport of slide mass. No equivalent lobe is seen on other side of landslide deposit. Also note small debris flow (d) coming off south side of trough wall farther east. Part of Viking Orbiter image 915AI0.
2.1.3; Valleys and "ponds." T h e p r e s e n c e o f fluids in l a n d s l i d e d e p o s i t s is a l s o s u g g e s t e d b y s m a l l v a l l e y s in slide d e p o s i t s . F i g u r e 6 s h o w s a v a l l e y t h a t b e g i n s at a t h e a t e r - s h a p e d i n d e n t a t i o n w i t h i n a slide deposit and that has a V-shaped cross pro-
file, m u c h like t h e c a n y o n s ' t r i b u t a r y to t h e V a l l e s M a r i n e r i s , w h o s e origin is g e n e r a l l y a t t r i b u t e d to s a p p i n g ( S h a r p 1973, Pieri 1980). T h e v a l l e y ' s s h a p e is m o r e r e g u l a r a n d c r i s p e r t h a n a r e the s h a p e s o f o t h e r linear depressions that may be caused by
418
B A E R B E L K. L U C C H I T T A
FIG. 4. Landslide complex in Ius Chasma. Landslides contain hummocky deposits (h) in upper part and longitudinally grooved debris lobes (1) in lower part. Flow directions shown by white arrows. Southernmost lobe originated at subsidiary scarp (s) and has pressure ridges (dark arrows) indicating flow at angles to flow in upper part of slide. Mosaic of Viking Orbiter images 919-A15 through A18.
d i f f e r e n t i a l m o v e m e n t o f t h e slide m a s s . I f this v a l l e y w a s i n d e e d f o r m e d like t r i b u t a r y c a n y o n s , t h e n it s u g g e s t s t h a t a fluid drained from the landslide deposit. The last piece of morphologic evidence that the landslide contained water comes
f r o m o b s e r v a t i o n s o f a l e v e l , f r a c t u r e d plain occupying the lowermost area of central C a n d o r C h a s m a (Fig. 1). T h e p l a i n e m b a y s s u r r o u n d i n g e r o d e d m e s a s o f l a y e r e d terrain, w h i c h i m p l i e s e m p l a c e m e n t o f fluids ( W i l h e l m s 1972). T h i s p l a i n h a s b e e n inter-
DEBRIS FLOWS AND GROUND ICE
419
FIG. 5. Small landslides (Is) emerging from large gullies in walls of Valles Marineris. Debris aprons are thin and spread out and have lobate fronts like terrestrial mudflows. Part of Viking Orbiter image 910A15.
preted to represent a former pond, playa, or alluvial flat where wasted materials were concentrated (Lucchitta and Ferguson 1983, Lucchitta et al. 1986). Debris from two landslide complexes (including the landslides shown in Fig. 1) apparently flowed into this flat, level plain and probably contributed to the fill. 2.2. Observations Consistent with Wet E m p l a c e m e n t 2.2.1. Landslide efficiency. Some aspects of the landslides in the Valles Marineris are consistent with wet emplacement, but other mechanisms may equally well have been
responsible. For instance, Scheidegger (1973) noted that terrestrial landslides have increased efficiency with increased volume (Fig. 7 and Table I). The efficiency o f a slide is obtained by calculating the tangent of the slope from the top of the landslide scar to the tip o f the slide deposit; the lower the tangent (the lower the slope), the higher the efficiency. This slope is also an approximate measure of the coefficient of friction (Scheidegger 1973). A few landslides in the Valles Marineris, whose volumes were obtained by computing the volume missing from landslide scars, were plotted on Scheidegger's graph of efficiency versus
420
B A E R B E L K. L U C C H I T T A
FI6.6. Valley in landslide. The valley (arrow) originates in a landslide deposit (ls) and has a blunt, cirquelike head and a V-shaped cross profile, similar to valleys attributed to sapping (Sharp 1973, Pieri 1980). Part of Viking Orbiter image 64A16.
volume (Fig. 7 and Table I). I transposed all volumes into weights in order to introduce the proper terms for terrestrial and Martian gravities. The graph shows that, in general, the Martian landslides fit Scheidegger's graph remarkably well. However, the trend
of increased efficiency with increased weight appears to level out slightly on Mars (but this observation is unreliable because of the highly scattered data and the uncertainties in the Martian slope and volume measurements). If a difference in trends
10 .................
0.5
t
0.2
2°
i'
0.1
~ O.OS
1" 3 .
6. 45"
7t 9"10"11"
x. Earth * . Mors
002 00~
1014
iOIs
|O16
]017
i0 ls
I019
i0 =o
i0:1
WEIGHT IN DYNES
FIG. 7. Graph of landslides, modified after Scheidegger (1973). Tangent of slope from top of landslide scar to tip of the slide deposit, a measure of efficiency, is plotted against landslide weight. Weight calculated by using g = 980 cm/sec z for Earth and g = 372 cm/sec 2 for Mars, and assuming density of 2.5 g/cm 3 for all slides. Terrestrial slides marked x (after Scheidegger), Martian slides represented by numbered dots (for location see Table I). Position of Martian slides on graph approximate because of uncertainties in slope and volume. Dashed line is Scheidegger's correlation curve. Graph shows that Martian and terrestrial slides are remarkably similar. However, whereas terrestrial slides generally increase in efficiency (decreasing tangent) with increasing weight, Martian slides show a slight tendency to retain their efficiency regardless of weigtit.
1022
DEBRIS FLOWS AND GROUND ICE TABLE I MARTIAN LANDSLIDES PLOTTED IN FIGURE 7 Landslide ID
1 2 3 4 5 6 7 8 9 10 11
Location" Lat.
Long.
- 10030 ' -4°30 ' -9o00 ' -8045 ' -8°15 ' - 3o00 ' - 12045 ' -3030 ' -9000 ' -7000 ' -8o00 '
68o20 , 70o00 , 44o30 ' 69°20 ' 71o00 ' 73°45 ' 69020 ' 71o00 ' 41015 ' 45°15 ' 41°15 '
C e n t e r o f l a n d s l i d e scar.
really exists, it could indicate that (I) Martian and terrestrial landslides have a threshold weight above which efficiency no longer increases with increased weight or, more likely, (2) some aspects of the environments or the emplacement mechanisms are different. Such differences may be due to the composition of the slide material or the substrate or to the entrainment of volatiles. When the morphologic evidence for water in the landslides is considered, the difference between the Martian and terrestrial slides, if real, is consistent with the hypothesis that the Martian landslides were wet debris flows, whereas large catastrophic landslides on Earth are mostly dryrock avalanches. 2.2.2. Landslide speeds. Another difference between Martian and terrestrial landslides is their speed: the Martian landslides apparently m o v e d much faster than terrestrial ones. Minimum speeds for the Martian landslides can be calculated wherever the slide masses overtopped obstacles or m o v e d through topographic lows and uphill on opposite slopes. Minimum speeds correspond to heights regained during travel and can be calculated by compar-
421
ing the kinetic energy of the slide to the potential energy needed to raise the slide mass to the regained height [½my2 = mgh, and v = (2gh) ½ where v is the minimum speed, m the mass of the slide, g the gravitational acceleration, and h the regained height]. Speeds calculated for a few Martian landslides are compared to speeds of terrestrial landslides or debris avalanches in Table II. The Martian landslides generally moved faster than terrestrial slides, even though Martian gravity is only about one-third that of Earth. The most likely explanation for the higher Martian speeds is that the slides on Mars fell from greater heights and had a larger mass than most terrestrial slides; therefore they also had a higher potential energy. The Huascaran debris avalanche in Peru is the only terrestrial one that, in its upper part, matched the Martian ones in speed (Plafker and Erickson 1978); it fell from a height (4 km) similar to heights on Mars. The Huascaran avalanche is also the only large terrestrial one among those cited
T A B L E II SPEED OF LANDSIDES Earth
km/hr
V a i o n t , I t a l y ( M u e l l e r 1968) E l m , S w i t z e r l a n d ( H e i m 1932) M a d i s o n , M o n t a n a ( H a d l e y 1966) G r o s V e n t r e , W y o m i n g ( A l d e n 1928) F r a n k , A l b e r t a ( D a l y et al. 1912) B l a c k h a w k , C a l i f o r n i a ( S h r e v e 1968) Huascaran, upper section, Peru (Plafker and E r i c k s o n 1978)
90 150 160 165 175 235 360
Mars Ius, e a s t ( - 7 ° 3 0 ' ; 78°00') ° T i t h o n e u m , c e n t e r ( - 4 ° 2 0 ' ; 85°30 ') I u s , w e s t ( - 6 ° 2 0 ' ; 87°00 ') I u s , c e n t e r ( - 6 ° 2 0 ' ; 85015 ') T i t h o n i u m , e a s t ( - 4 ° 3 0 ' ; 80o00 ') T i t h o n i u m , e a s t ( - 4 ° 3 0 ; 79030 ') C a n d o r ( - 4 5 ° 4 5 ' ; 75o20 ' ) M e l a s ( - 8 ° 1 5 ' ; 71o00 ')
250 250 300 300 300 300 400 450
a L a t i t u d e a n d l o n g i t u d e at c e n t e r o f l a n d s l i d e scar.
422
BAERBEL K. LUCCHITTA
in Table II that contained much ice and water because it incorporated parts of an ice cap. Indeed, Plafker and Erickson (1978) attributed the high speed not only to the height of the fall but also to the entrainment of perhaps as much as 5 to 10% (by volume) of ice and water. Similarly, ice and water in the Martian landslides may have been a contributing factor in their high speeds. 2.2.3. Longitudinal grooves. A further difference between Martian and terrestrial landslides is the presence of longitudinal ridges and grooves on debris aprons of the Valles Marineris slides (Figs. 1 and 4; Lucchitta 1978a, Figs. 2 and 4), as opposed to the presence of transverse ridges on most terrestrial slides. Exceptions are longitudinally grooved Alaskan landslides that moved across glacier ice, such as the wellstudied Sherman landslide. Shreve (1966) and Marangunic and Bull (1968) suggested that these longitudinal grooves arose from shearing between rapidly moving debris traveling forward at slightly different speeds or times. In a study of longitudinal flow patterns in debris of impact ejecta, volcanic ash flows, and landslides on Earth, Moon, and Mars, Lucchitta (1979) concluded that the most important conditions for the formation and preservation of longitudinal patterns in debris aprons are high initial energies resulting in high speeds, and low frictional resistance within the slide mass or of the subjacent ground. The development and preservation of grooves in the Sherman landslide were thus probably caused by its travel over smooth glacier ice, a condition that resulted in very low frictional resistance of the substrate. In Martian slides, grooves may have formed because of both high initial energies and low frictional resistance. If the landslides in the Valles Marineris indeed contained water, this water probably contributed to the lowering of frictional resistance within the Martian slide masses and thus aided in the development of longitudinal grooves.
3. WALL ROCK 3.1. Observations Supporting Ice or Water in Wall R o c k
The walls bordering the Valles Marineris grabens display spur-and-gully morphology, landslide scars, and tributary canyons (Lucchitta 1978b). A different morphology marks the interior deposits, which have conspicuous layers (McCauley 1978). Evidence that ice or water may have been present in the wall rock comes from the morphology of tributary canyons and the contrast between characteristics of the wall rock and the interior deposits. 3.1.1. Tributary canyons. Tributary canyons, some as much as 130 km long, have blunt, cirquelike valley heads, V-shaped cross profiles, and uniform gradients toward the main chasmata. In places they have h u m m o c k y or lobate floor deposits that resemble rock glaciers in Antarctica (Lucchitta 1978b) and that indicate that material was mass wasted from the walls and transported through the canyons. Dry mass wasting alone is an unlikely transportation mechanism because gradients are only a few degrees, much below the angle of repose (about 30°). Wind erosion alone is also unlikely because the uniform gradients indicate the work of a confined fluid with a distinct upper surface. Clearly, the moving debris must have been lubricated. The most readily available lubricating agents would have been ice or water. In the absence of atmospheric precipitates in this region of Mars (Fanale and Jakosky 1982), ground ice, sapped from the chasma walls (Sharp 1973, Pieri 1980), appears to be the most likely lubricating agent. Further evidence that fluids flowed through tributary canyons comes from a tributary that gave rise to a channel below its mouth (Fig. 8). This canyon has a depth of about 400 m near its head and 800 m near its mouth (obtained by measuring the widths of talus slopes within the canyon and assuming that the talus rests at a 30° angle of repose). This fluid, again, most
DEBRIS FLOWS AND GROUND ICE
423
FIG. 8. Tributary canyon and channel. A small channel (arrow) issues from the mouth of a tributary canyon (t) in the plateau segment separating Candor and Melas Chasmata. Apparently a fluid flowed from the canyon. Part of Viking Orbiter image 912A16.
likely came from canyon-wall rock; the implication is that the rocks were not desiccated at a depth of 400-800 m below the surface during the time of channel formation. 3.1.2. Contrast in morphology of walls and interior mesas. The wall-rock morphology is conspicuously different from that of the interior layered deposits. Wall rocks show layers only near the top, display no evidence of wind erosion, and have bifurcating spurs and gullies. By contrast, interior deposits may be smooth or layered throughout; they show conspicuous cliffs in some places and are strongly eroded by wind in other places, as evidenced by regularly spaced flutes (Fig. 9; also see Fig. 10). Wall-rock morphology occurs not only on the Valles Marineris walls but also in their interior, where wall rock projects as spurs or forms isolated erosional remnants. Some
of these wall-rock remnants are as high and have slopes as steep as those of adjacent eroded mesas of layered deposits, yet debris flows, spreading as thin, extensive aprons far across the chasma floors, occur abundantly only on wall-rock slopes. They are conspicuously lacking on slopes of interior deposits (Fig. 9), where I found only two small landslides, both forming minor slumps restricted to the slopes. Clearly, wall rocks and interior deposits had different c o h e r e n c e and coefficients of internal friction. The wall rocks were highly susceptible to efficient flow; therefore they were probably poorly consolidated and contained volatiles. Cliffs on interior deposits, by contrast, suggest that the deposits were generally well consolidated; however, flutes on some layers indicate that these were, perhaps, composed of sand-sized grains susceptible to wind erosion. The wall
424
BAERBEL K. LUCCHITTA
FIG. 9. Wall rock and layered deposits. Mesa (m) of layered deposits having fluted slopes is as high and steep as adjacent wall-rock remnant (w) having irregular spur-and-gully morphology.The layered deposits lack debris flows, whereas wall-rock remnants display abundant debris flows (d) that spread out across the valley floor. Note landslide (Is) issuing from gully. Viking Orbiter image 912A11.
rocks were probably weakly cemented by ice. They may have been similar to the ice-charged highland rocks of the Martian midlatitude belt, which gave rise to "terrain softening" features (Squyres and Carr 1986), but the wall rocks were probably
backed by water trapped b e y o n d exposed surfaces at a depth of only 1 to 2 km, where the 0 ° isotherm would be intersected (Fanale 1976, R o s s b a c h e r and Judson 1981). Therefore, and because of the high, steep scarps of the Valles Marineris, the highland
DEBRIS FLOWS AND GROUND ICE rocks may have collapsed catastrophically and formed gigantic wet debris flows (Lucchitta 1979) rather than deforming by creep as in the "softened" Martian midlatitudes. 3.2. Observations Consistent with Ice or Water in Wall R o c k
Like landslides, some characteristics associated with wall rock have ambiguous origins; they can be explained by a high content of ice or water or by other wallrock properties. One of these characteristics is the unusual competence of structurally disturbed areas. The Valles Marineris grabens are associated with numerous fault zones both on top of the surrounding plateaus and within the
425
grabens. The faults on the plateaus caused chain craters, commonly attributed to collapse (Sharp 1973, Blasius et al. 1977), and they influenced the alignment of tributary canyons. These observations suggest that the faults are planes of weakness along which degradational processes operated more efficiently than in the surrounding undisturbed rock. Yet, within the troughs, the crest lines of most wall-rock spurs are marked by dense fault zones, whereas surrounding lower areas are much less fractured (Fig. 10, also see Fig. 1). Apparently, the fracture zones are more resistant to erosion than are the unfractured areas, a phenomenon different from that of many terrestrial zones where brecciation along
FIG. 10. Fault zones in chasmata. Crestline of wall-rock spur separating western Ophir Chasma from western Candor Chasma is intensely faulted (black arrows). Faults appear to be less dense on slopes of spur. Apparently, the fault zones are more resistant to erosion than is wall rock and thus support the spur. Dark bands along bases of wall-rock spur (white arrows) are probably mafic volcanic rocks (Lucchitta 1987). Note difference in morphologies of wall rock (spurs and gullies) and interior layered mesa (m) (fluted slopes). Mosaic of Viking Orbiter images 815A41 through A48.
426
BAERBEL K. LUCCHITTA
faults tends to make them less resistant. These Martian fault zones may be more resistant because the faults have been intruded by dikes or the fault zones were more lithified than the surrounding country rock owing to hydrothermal activity or other circulation of water. Dikes along faults are common on Earth, but they would have to be unusually abundant in the Valles Marineris and intruded along nearly all faults to uphold the spurs. The explanation that fault zones were lithified implies that the country rock is weaker, and even though readily explained otherwise, also agrees well with the possibility that the wall rock is composed of relatively unconsolidated materials charged with water or ice. If such is the case, the frictional heat and shear stress near the fault planes alone could have consolidated the rocks and driven out the volatiles, making the fault zones more resistant than the surrounding country rock.
4. TIMING OF EVENTS The timing of landslide emplacement and tributary canyon formation is important because it dates the period when water and ice were present in the canyon walls. A relative time for these events can be established by considering the geologic history of the Valles Marineris and its interior; a relative time and a tentative absolute time can be established by crater counts. Geologic relations (Lucchitta 1981, 1985, Nedell et al. in press) indicate that the landslides occurred late in the history of the Valles Marineris; they were emplaced after the canyon walls developed spur-and-gully morphology and after deposition and erosion of the interior layered deposits formed the high-standing interior mesas. The only features younger than the landslides are probable small volcanic vents and eolian forms. The emplacement of tributary canyons is more difficult to date, but they also appear to have formed mostly after the erosional epoch that created the mesas; the
tributary canyons are probably similar in age to the landslides. Crater counts on the combined landslide deposits of Tithonium and Ius Chasmata yielded an average crater frequency of 570 +- 130 craters -> 1 km diameter/106 km 2 (Lucchitta 1979). Individual landslide deposits are too small to give statistically significant results, but on some deposits superposed craters are so scarce that the deposits may be recent. One of these deposits is the landslide complex in Ophir Chasma (Fig. 1), discussed above. It has only a few superposed craters, all of which are -<500 m in diameter. When the average landslide age is plotted against the relative ages of other Martian events (Fig. ll), it becomes apparent that the Valles Marineris grabens probably existed throughout a major part of Martian history, whereas the landslides were emplaced only during the last third of this time span. The landslide emplacement coincides with late activities on the Tharsis volcanoes. When the relative time scale of Scott and Tanaka (1981) is compared with an absolute time scale (Model 2 of Neukum and Hiller 1981) (Fig. 11), it appears that the landslides were emplaced during the last billion years. The subsurface of the equatorial region of Mars, accordingly, was not desiccated during this time. 5. CONCLUSION Landslides in the Valles Marineris probably contained water. The most compelling evidence comes from a large channel that issued from three landslides in Ophir Chasma. Also supporting entrainment of water or ice is the morphology of flow lobes and the observation that landslide debris appears to have contributed to a " p o n d e d " level deposit in the lowest area in central Valles Marineris. If landslides were wet, they derived their water from the Valles Marineris wall rock, which, accordingly, must have contained water or ice. That the wall rock contained volatiles is further supported in varying degrees by (1) tributary
DEBRIS FLOWS AND GROUND ICE Rel. Crater Age (n z lkm/106 km 2)
427
Abs. Age (b.y.) -0
Olympus Mons ! Construct
100-
I''1
-0.5
I I I ! I I
I
500- -1.o
r'l
r.arsis
Plains ~
-1.5
Landslides
1,000-2,0 ~2,5
I Northern /Plains
-3.0 !
2,000 10,0OO--
I
Ib..I -3.5
ISyria
I,,n2P' .... I Ptanum
-4,0
5O,O00-(Scott and Tanaka, ( Neukurn and 1981) Hiller, 1981) FIG. 11. Sketch showing age relations in Tharsis and Valles Marineris areas. Landslides were emplaced during last third of Valles Marineris history, and may be as young as recent. Stippled area corresponds to age determined from crater counts (Lucchina 1979). Other relative ages after Scott and Tanaka (1981); absolute ages after Model 2 of Neukum and Hiller (1981).
canyons that served as conduits for lubricated debris and that gave rise to channels in places; (2) the contrast between the morphologies of wall rock and interior deposits, which suggests that the wall rock was composed of poorly consolidated material that flowed easily; and (3) the erosional resistance of structurally disturbed zones, which implies that surrounding wall rock was relatively weak. Many of the above observations can be explained otherwise, but the combined evidence strongly supports that water was involved in landslide emplacement and wall-rock disintegration. The postulated ice in wall rock may have filled pore spaces, but perhaps it was so abundant that it formed segregated lenses in places. The ice was probably cementing the walls, but owing to the internal heatflow, was converted to water at a distance
of 1 to 2 km beyond exposed faces. Thus, the walls were exceedingly unstable, susceptible to collapse, and predisposed toward the generation of wet debris flows (Lucchitta 1979). If the Valles Marineris landslides were indeed wet, they are not analogous to terrestrial dry-rock avalanches, and they cannot be cited as examples to support terrestrial, theoretical dryrock avalanche emplacement mechanisms. If ice and water were present in the Valles Marineris walls at the time of landslide emplacement, the equatorial region of Mars below a certain depth was not desiccated at this time. This time period occurred within the last billion years and may even be recent. Consequently, the groundice reservoir on Mars would have been extensive and, perhaps, planetwide fairly recently and, if it is not a relic from ancient times, it must have been replenished (Clif-
428
BAERBEL K. LUCCHITTA
ford and Hillel 1983). These considerations are important for understanding the volatile inventory and climatic history of Mars. REFERENCES ALDEN, W. C. 1928. Landslide and Flood at Gros Ventre, Wyoming. American Institute of Mining and Metallurgical Engineers, Technical Publication 140. BLASlUS, K. R., J. A. CUTTS, J. E. GUEST, AND H. MASURSKY 1977. Geology of the Valles Marineris: First analysis of imaging from the Viking I orbiter primary mission. J. Geophys. Res. 82, 4067-4091. CHRISTIANSEN, E. H., AND J. W. HEAD 1978. Martian Landslides--Classification and Genesis (Abstract). Reports of Planetary Geology Program, 1977-78. NASA Tech. Memo. 79729, pp. 285-287. CLIFFORD, S. M., AND D. HILLEL 1983. The stability of ground ice in the equatorial region of Mars. J. Geophys. Res. 88, 2456-2474. DALY, R. A., W. G. MILLER, AND G. S. RICE 1912. Report o f the Commission Appointed to Investigate Turtle Mountain, Frank, Alberta. Canada Geological Survey Memoir 27. FANALE, F. P. 1976. Martian volatiles: The degassing history and geochemical fate. Icarus 28, 179-202. FANALE, F. P., AND B. M. JAKOSKY 1982. Regolithatmosphere exchange of water and carbon dioxide on Mars: Effects on atmospheric history and climate change. Planet. Space Sci. 30, 819-831. FANALE, F. P., J. R. SALVAIL, A. P. ZENT, AND S. E. POSTAWKO 1986. Global distribution and migration of subsurface ice on Mars. Icarus 67, 1-18. HADLEY, J. B. 1966. The Madison Landslide (Montana) in Yellowstone Earthquake Area. Billings Geological Society, Annual Field Conference Guidebook llth, pp. 45-48. HEIM, ALBERT 1932. Bergsturz and Menschenleben (Landslides and people's lives). Fretz and Wasmuth, Zurich. JOHNSON, A. M., AND P. H. RAHN 1970. MobiLization of debris flows. In Zeitschriftfuer Geomorphologie, Supp. Band 9, pp. 168-186. LUCCHITTA, B. K. 1978a. A large landslide on Mars. Geol. Soc. Am. Bull. 89, 1601-1609. LUCCHITTA, B. K. 1978b. Morphology of chasma walls, Mars. J. Res. U.S. Geol. Surv. 6, 651-662. LUCCHITTA, B. K. 1979. Landslides in Valles Marineris. J. Geophys. Res. 84, 8097-8113. LUCCHITTA, B. K. 1981. Valles Marineris--Faults, Volcanic Rocks, Channels, Basin Beds (Abstract). Reports of Planetary Geology Program--1981. NASA Tech. Memo. 84211, pp. 419-421. LUCCHITTA, B. K. 1985. Valles Marineris Basin Beds: a Complex Story (Abstract). Reports of Planetary Geology Program--1984. NASA Techn. Memo. 87563, pp. 506-508.
LUCCHITTA, B. K. 1987. Recent mafic volcanism on Mars. Science 235, 565-567. LUCCHITTA, B. K., AND H. M. FERGUSON 1983. Chryse Basin channels: Low gradients and ponded flows. J. Geophy. Res. 88, A553-A568. LUCCHITTA, B, K., H. M. FERGUSON, AND C. SUMMERS 1986. Sedimentary deposits in the northern lowland plains, Mars. J. Geophys. Res. 91, E166E174. MARANGUNIC, C., AND C. BULL 1968. The landslide on the Sherman Glacier. In The Great Alaska Earthquake of 1964, Hydrology. National Academy of Science Publication 1603, pp. 383-394. McCAULEY, J. F. 1978. Geologic Map o f the Coprates Quadrangle o f Mars. U.S. Geol. Survey Misc. Invest. Map 1-897, scale 1:5,000,000. MUELLER, L., 1968. New considerations of the VajonI slide. Rock Mech. Eng. Geol. 6, 1-91. NEDELL, S. S., S. W. SQUYRES, AND D. W. ANDERSON (in press). Origin and evolution of the layered deposits in the Valles Marineris. Icarus 70, 409-441. NEUKUM, G., AND K. HILLER 1981. Martian ages. J. Geophy. Res. 86, 3097-3121. PIERI, DAVID 1980. Martian valleys: Morphology, distribution, age, and origin. Science 210, 895-897. PLAFKER, G., AND G. E. ERICKSON 1978. Mechanism of catastrophic avalanches from Nevados Huascaran, Peru. In Rock Slides and Avalanches (B. Voight, Ed.), pp. 227-314. Elsevier, Amsterdam. RIB, H. T., AND T. A. LIANG 1978. Recognition and identification. In Landslides, Analysis, and Control (R. L. Schuster and R. J. Krizek, Eds.), pp. 34-80. National Academy of Sciences Special Report 176, Washington, DC. ROSSBACHER, L. A., AND S. JUDSON 1981. Ground ice on Mars: Inventory, distribution, and resulting landforms. Icarus 45, 39-59. SCHEIDEGGER, A. E. 1973. On the prediction of the reach and velocity of catastrophic landslides. Rock Mech. 5, 231-236. SCOTT, D. H., AND K. L. TANAKA 1981. Mars: Paleostratigraphic restoration of buried surfaces in Tharsis Montes. Icarus 45, 304-330. SHARP, R. P. 1973. Mars troughed terrain. J. Geophy. Res. 78, 4063-4072. SHREVE, R. L. 1966. Sherman landslide, Alaska. Science 154, 1639-1643. SHREVE, R. L. 1968. Leakage and fluidization in air-layer lubricated avalanches. Geol. Soc. Amer. Bull. 79, 653-658. SQUYRES, S. W., AND M. H. CARR 1986. Geomorphic evidence for the distribution of ground ice on Mars. Science 231, 249-252. U.S. Geological Survey 1980. Tithonium Chasma o f Mars. Topographic Orthophoto Mosaic. U.S. Geological Survey Miscellaneous Investigations Map 1-1294, scale I : 500,000.
DEBRIS FLOWS AND GROUND ICE U.S. Geological Survey 1984. Valles Marineris Region, Viking High-Resolution Controlled Mosaic MTM-5072. U.S. Geol. Survey Misc. Invest. Map 1-1592, scale 1:500,000. U.S. Geological Survey (in press). Corprates Northwest. Topographic Map Series. U.S. Geological Survey Miscellaneous Investigations Map 1-1712, scale 1 : 2,000,000. VOIGT, B., H. GLICKEN, R. J. JANDA, AND P. M. DOUGLASS 1981. Catastrophic rock slide avalanche
429
of May 18. In The Eruptions o f Mount St. Helens, Washington. U.S. Geological Survey Prof. Paper 1250, pp. 347-378. WIL~ELMS, D. E. 1972. Geologic Mapping of a Second Planet. lnteragency Report. Astrogeology 55. Wu, S. S. C., AND A. E. HOWlNGTON 1986. A Mars Digital Terrain Model and Sample Correlation with a Mars Digital Image Model (Abstract). Results of Planetary Geology and Geophysics Program--1985. NASA Tech. Memo. 88383, pp. 608-611.