- Email: [email protected]
Mars: Evidence for dynamic processes from mariners 6 and 7
ICARUS
19,
Mars:
180-194
(1973)
Evidence
for
Dynamic
WOLFGANG Department
of Geology,
Processes
E. ELSTON University
Received
26 April
AND
of New
Mexico,
1972;
revised
from
EUGENE Albuquerque,
10 January
Mariners
6 and 7’
I. SMITH2 New
Mexico
87131
1973
Mariners 6 and 7 photographs of the equatorial region of Mars document a threestage evolution of that part of the Martian surface: (1) Highand intermediatealbedo cratered terrains in Meridiani Sinus, Margaritifer Sinus-Thymiamata, Deucalionis Regio-Sabaeus Sinus, and Hellespontus; (2) low-albedo moderately cratered terrain and dark crater fill in Meridiani Sinus, Thymiamata, and Deucalionis Regio-Sabaeus Sinus and possible volcanism in the Hellas-Hellespontus border; and (3) high-albedo surficial deposits, banked-up crater fill, a possible bright-ray crater in Meridiani Sinus, chaotic terrain on the edge of the Margaritifer Sinus mesa, featureless terrain in Hellas and Edom, sinuous channel-like reentrants on scarps at the Hellas-Hellespontus boundary. Regional faulting seems to have occurred following formation of the old cratered plains and prior to formation of low-albedo plains in Meridiani Sinus and also prior to formation of canyonlike reentrants and featureless terrain along the Hellas-Hellespontus boundary. Mars has had a complex history of dynamic evolution, possibly analogous to the more stable regions of Earth. Its geochemical differentiation and thermal regime should account for long-term postaccretional tectonic and volcano-tectonic processes as well as for fluid media on its surface sufficient to cause erosion, including the cutting of large canyons.
manuscript was shortened for publication in October 1972.’ When Mariner 4 returned the first space-probe photographs from Mars in 1965, members of the Investigators Team (Leighton et al., 1965) concluded that the cratered surfaces of the planet were 2 to 5 x lo9 yr old, and that “the principal topographic features of Mars.. . have not
INTRODUCTION
The paper deals with preliminary analysis of some surface features of the equatorial region of Mars photographed by Mariners 6 and 7. Observations were made on maximum-discriminability pictures (Dunne et al., 1971) supplemented with selected photometric images from which effects of the cuber and automatic gain control had been removed (Rindfleisch et al., 1971). The work was done in 1970 and 1971, before Mariner 9 data became available. Summaries were published by Smith and Elston (1971) andElstonandSmith (1971). This manuscript was completed in late February 1972. Footnotes pertaining to Mariner 9 findings were added when the
3 Sections on crater classification, crater distribution, and tectonic patterns were omitted on editing, because this material has been covered by other authors. Otherwise the body of the manuscript is unchanged as written before release of Mariner 9 photographs. A shorter version of this paper was submitted to another journal in July 1971 and rejected in October 1971 after a reviewer dismissed our tentative identification of tectonic features, canyon-like features and other evidence of dynamic activity on Mars as “fantasy.” Our intention to publish this material before Mariner 9 data became available was unfortunately frustrated.
i Contribution No. 34 of the Planetary Geology Group, University of New Mexico. z Present address : University of Wisconsin, Parkside, Kenosha, Wisconsin, 53140. Copyright 0 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.
180
PYRRHAE
MERIDIANI
INSUFFICIENT
?-
400
ESCARPMENT
HELLAS
J-
/-
HELLAS
I-
SINUS
SINUS
RESOLUTION
ZONE
REGIO
SINUS
REGIO
BORDER
H - HELLESPONTUS
G - DEUCALIONIS
F - SABAEUS
E - EDOM
D-
C - THYMIAMATA
B - MARGARITIFER
A-
EXPLANATION
~ HIGH
ALBEDO
. . . . . . . . . .+:*:*:z*:* STAGE
STAGE
I]
3200
I
INTERMEDIATE
2
ALBEDO
0 400 L I I %I, SCALE
KM
182
ELSTOXAND
been produced by stress and deformation within the planet.. .” This conclusion was reiterated by Leighton et al. (1967) in spite of criticisms of the inferred ages of craters by Anders and Arnold (1965), Baldwin (1965), and Witting et al. (1965). In the light of greatly superior imagery from Mariners 6 and 7 the investigators modified their conclusions only marginally (Leighton et al., 1969; Murray et al., 1971; Sharp et al., 1971; Cutts et al., 1971). Large craters were said to have survived” . . . for billions of years, and indeed most probably from the end of planetary accretion” (Murray et al., 1971). Locally, a distinct sequence of events was recognized, as at the boundary between Hellas and Hellespontus (Fig. l), but most of the features were interpreted as having originated prior to the end of the planetary accretion. There was speculation by Sharp et al. (1971) that chaotic terrain may be “an early expression of geothermal maturity” and that volcanism and tectonism will play a greater role in the future than in the past. In general, we find evidence for a sequence of events that does not follow the order described by the investigators. Our interpretations suggest a longer period of development for bedrock terrain and a greater role for internal processes. Murray et al. (1971) stressed the apparent absence of Mars of large fresh craters, resembling Copernicus and Tycho on the Moon. We have noted a feature in Meridiani Sinus that could be interpreted as a brightray crater although the evidence is far from certain. The investigators as well as their critics have consistently ascribed all Martian craters to impact processes. This is a prerequisite for, and a consequence of, all planetary models which interpret Mars as dynamically inactive except, possibly, in relatively recent time. Elsewhere, we have interpreted the craters on the Moon as a mixed population impact and volcanotectonic craters (Smith, 1971; Elston, 1971). Murray et al. (1971) concluded from the resemblance of Mars to the Moon that crustal deformation played little or no part in the evolution of Mars. We conclude from the same resemblance that
SMITH
crustal deformation and volcanism may well have played a major role. Like other writers, we ascribe many features to surficial processes of erosion and blanketing, but would go farther to include the cutting of large canyon-like reentrants, as on the border between Hellas and Hellespontus. Murray et al., (1971) have commented on the lack of simple correlation between albedo, terrain, and crater density, quite unlike the terra-mare dichotomy on the Moon. In at least some areas of Mars, such as Meridiani Sinus, the correlation between albedo and terrain can be made more consistent by assuming two kinds of high-albedo terrains, one of bedrock and the other a surficial deposit. Our view of Mars is of a planet with decipherable dynamic history, involving both internal and surficial processes. As this paper was being completed, a new series of articles by the investigators (Steinbacher et al., 1972 ; Masursky comment et al., 1972) and an editorial (Hammond, 1972) indicate that the investigators now accept the view of Mars as a dynamic planet in the light television imagery and other experimental results from Mariner 9.
CLASSIFICATIONS OF MARTIAN TERRAINS
Leighton et al. (1969) and Sharp et al. recognized three basic types of Martian terrains : cratered, chaotic, featureless. We have accepted this classification, refined it, and examined terrain boundaries. There seem to be at least four types of cratered terrains: (1) High- and intermediate-albedo cratered terrains with relatively smooth intercrater areas, as in Deucalionis Regio, (6N21) (Fig. 2), (2) relatively smooth low-albedo plains, as in eastern Meridiani Sinus (6N13) (Fig. 3), ( 3) high-albedo hummocky and textured cratered plains seen on the border between Deucalionis Regio and Sabaeus Sinus (6N19, 6N21, 6N23) (Fig. 2), and (4) cratered terrain in the south polar region characterized by lineaments and irregular depressions, possibly a landscape modified (1971)
MARS:EVIDEIWE
Pk. 2. Mariner 6 photograph orthographic projection.
6N21
FOR DYNAMIC
of Deucalionis
by glacial and periglacial processes (7N15, 7N17, 7N19). The south polar region is not considered further in this article. Featureless terrain was described by Sharp et al. (1971) from the floor of Hellas basin (7N25, 7N27). It is characterized by high albedo and smooth texture. On A-camera images faint outlines of a few large craters can be resolved but on Bcamera close-ups, where small bowl-shaped craters ordinarily appear, no craters can be seen. Sharp et al. (1971) regarded featureless terrain as unique to Hellas within the coverage of Mariner photography and speculated that “Hellas may be mantled by a type of material markedly different from that covering other parts of the Martian surface.. .” We consider it likely that the terrain in Hellas is basically similar to that on the floors of many other craters, which seem to be partly blanketed by featureless material. As Cutts et al. (1971) have pointed out, many craters in Meridiani Sinus (6N13) show bright markings on the north sides of the crater floor (Fig. 3a) possibly the result of transportation and deposition of materials by wind. We agree that this is probable. Carrying it one step farther, it seems likely that the floors of the largest depressions,
Regio,
PROCESSES
maximum
183
discriminability
version,
such as the parts of Hellas and Edom photographed by Mariners 6 and 7 are largely or entirely mantled by windtransported material to form featureless terrain. The dust storm during the Mariner 9 mission is additional evidence for the efficacy of eolian processes on Mars. Chaotic terrain was described by Sharp et al. (1971) from B-camera photographs 6N6, 6N8, and 6Nl0, and extrapolated to 1.5 x lPkm*. Only future photography can show whether the extrapolation is justified.4 We find chaotic terrain in the predicted position around the base of the Margaritifer Sinus scarp in 6N9 and agree with Sharp et al. (1971) who pointed out that it is relatively young and formed at the expense of cratered terrain. CRATERS
A Possible Bright-Ray
Crater
The absence on Mariner Mars photographs of fresh large Copernican craters 4 Mariner 9 has shown chaotic terrain much more limited and to be related apparent source areas of stream channels equatorial region.
to be to the in the
184
ELSTON
FIG. 3a. Mariner
6 photograph
6N13
FIG. 3b. Enlargement
AND
SMITH
of Meridiani Sinus, maximum of part of the eastern side
is fundamental to the conclusion of Murray et al. (1971) that the small bowl-shaped craters of Mars and the bright-ray craters of the Moon belong to different populations. Yet, there is a feature in western Meridiani Sinus that could be interpreted as a brightray crater, even though resolution is so
discriminability of Fig. 3a.
version.
poor that entirely different interpretations are possible (Fig. 3a, A). It is a crater, about 25km in diameter, with a wide high-albedo lip surrounded by a dark collar, widest on the north side of the crater. This is in turn surrounded by a high-albedo collar which grades into radial
MARS
: EVIDENCE
FOR
bright streaks on the south side and into a bright irregular patch that extends for about 6 crater diameters on the north side. A few bright streaks radial or tangential to the cr: hter (Fig. 3a, B) extend from a bright
FIG.
crater (7N19).
4. Possible i n Deucalionis
craters of endogenic origin Regio (6N20). (b) Craters
DYNAMIC
PROCESSES
185
patch up to 8 diameters from the era ,ter. The bright patch is not an instrumel ntal artifact. It is visible from Earth and on far-encounter Mariner photographs, smuch as 6F31 and 7F67 as part of the high-alb
or modified with large
by endogenic central peaks
processes. (a) Doublering in the south polar rel ;ion
186
ELSTONAND SMITH
area between the two northeast-trending low-albedo “horns” of Meridiani Sinus. The entire area gives an impression of a lunar bright-ray crater with a ray system even more asymmetrical than that of Kepler. The crater is superimposed on what appears to be a northeast-trending lineament of regional extent (Fig. 3a, C); the bright patch could be interpreted as a ray system that terminates abruptly against the lineament. This can best be seen on the enhanced photometric version of 6N13. The bright patch can also be seen as the brightest spot on 7N5 and 7N7. Suggestions of a ray pattern can be made out in 7N5. Much of Meridiani Sinus has low albedo, and 6N13 and 7N5 were taken with a relatively low phase angle of 52 and 44”, respectively. Conditions would be favorable for detecting a bright-ray crater. While the phase angle favors discrimination between variations in albedo it is unfavorable for determining relief. On 6N13 the nature of the crater lip and rim and of the lineament on which the crater is superimposed remain in doubt. In summary, while it cannot be claimed that a bright-ray crater definitely exists in Meridiani Sinus, neither can the possibility be dismissed. Like most observers, we attribute lunar bright-ray craters to impact processes.5
Other Unusual Craters A number of Martian craters have unusual features that makes them difficult to classify. A good example is a crater that is nearly square in plan, with partly scalloped edges, partly filled with hummocky material and 150km in diameter, in 6N21 (Fig. 2, A). Other examples include a double-ring crater on 6N20 that merges with a feature resembling a lunar mare ridge (Fig. 4A) and a crater with a disproportionately large central peak in ’ Preliminary
examinations of Mariner 9 shed no new light on this possible bright-ray crater. Phase angles are high and one would not expect to see rays. Also, many superficial markings have shifted as a result of dust storms.
photography
has
7N19 (Fig. 4B). Volcanic processes may have played a part in their development.
POSSIBLEFAULTSCARPS Scarps are common on Mars. Some are irregular and resemble terrestrial erosional features, such as the scarps on the edge of mesas. The scarp separating chaotic terrain and a cratered upland surface in Margaritifer Sinus is an example (6N9). In Meridiani Sinus (6N13) a number of discontinuous and branching linear scarps resemble terrestrial fault scarps. The largest can be traced with interruptions for about 700km (Fig. 3a, DD’). In several places, especially adjacent to the southwest rim of Edom, scarps are prominent at the border of high- and low-albedo terrains. Most of them trend northwesterly ; one prominent set of linear features, in an echelon pattern, trends to the northeast (Fig. 3a, CC’). Patches of low-albedo terrain, also elongated to the northeast run parallel to segments of this scarp. The entire system of northeasterly scarp segments and low-albedo patches is in the position of one of the dark linear markings (“canals”) reported by many telescopic observers and shown on the ACIC Map of Mars. It runs northeast from the eastern “horn” of Meridiani Sinus. Some northtrending scarps on the boundary between Hellas and Hellespontus (7N25, 7N27) also resemble fault scarps.
SEQUENCEOFGEOLOGICEVENTS Principles On Earth, the relative sequence of geologic events in a particular region can be worked out on the basis of superposition and transection of geologic features, i.e., a relatively young geologic feature either covers or cuts across older features. These principles were applied to lunar interpretations by Shoemaker and Hackman (1962). Their scheme has been modified in detail but the basic principles have stood the test of time. The Moon has old high-albedo cratered
MARS
: EVIDENCE
FOR
highlands on which the smooth low-albedo maria are superimposed. It therefore lends itself to a relativelv simnle threefold classification (Preinto (1) p&mare imbrian), (2) mare (Imbrian), and (3)
0
100 Approximate
FIG.
orthographic
5. Reconnaissance projection.
200
DYNAMIC
postmare (Eratosthenian and Copernican) features. More detailed sequences have been worked out locallv. Observers of Mars have noted an apparent lack of correlation between albedo, elevation and
300 scale
photogeologic map Geologic interpretation
187
PROCESSES
400
MO
In Kilometers
of
the Meridiani by W. E. Elston,
Sinus April
area of 1971.
Mars.
Photo
6N13
188
ELSTON
crater density. Nevertheless, we found that sequences of events can be worked out in individual areas. An attempt has been made to establish a time scale of three stages, but correlations between different areas are so uncertain that we have assigned numbers to the stages rather than formal names. The validity and extent of the three-stage classification will have to be determined from Mariner 9 photography. Stage 1 is the oldest, Stage 3 the youngest. Interpretations are greatly simplified by the observation that there seems to be two types of mappable terrains on Marsconsolidated bedrock and transported material. Contacts between map units interpreted as bedrock are sharp and are usually associated with differences in relief. Where contacts are associated with provincial boundaries, they are parallel to scarps, ridges, and sinuous channels that seem to be of tectonic, volcanic, or erosional origin. Bedrock surfaces may have considerable relief. Contacts between bedrock and transported material tend
AND
SNITH
to be gradational and veneers of transported material tend to subdue relief.6 In the discussion that follows, individual areas are discussed in terms of three-stage time classifications. Photography of Margaritifer Sinus, Meridiani Sinus, Sabaeus Sinus, and Deucalionis Region is fairly continuous, allowing particular terrains to be traced for long distances. Correlation between disconnected areas is much more uncertain. The three stages must be regarded as exceedingly broad. In terms of terrestrial geology, they may be equivalent to early Precambrian, all geologic time from early Precambrian through Tertiary, and Quaternary. In terrestrial geology, features as diverse as volcanoes of the Cascades, coastal plain deposits of the Gulf Coast, and glacial features of New England are all classified 6 In addition to stratigraphic evidence presented here, the classification of craters has served as a check on the ages assigned to surfaces on Mars. The degree of maximum degradation of craters, and the total number of craters, become progressively greater on older surfaces.
MARS
: EVIDENCE
FOR
as Quaternary. This implies merely that they are young; it does not imply that they are contemporaneous or similar in origin. Similarly the assigment of chaotic terrain Sinus and featureless in Margaritifer terrain in Hellas to Stage 3 merely implies that both are relatively young. The age assignment does not depend on any assumed origin for the features involved. ,~feridiani Sinus-Edom In the area of Meridiani Sinus and Edom, Stage 1 is represented by high- and intermediate-albedo cratered plains, Stage 2 by dark cratered plains and fillings of craters, and Stage 3 by high-albedo surficial material (Fig. 5). Cutts et al. (1971) stated that on the northeastern boundary of Meridiani Sinus, adjacent to Edom “the light area is low with respect to the dark area.” We regard the light area as higher. This applies to local conditions at the contracts between dark and light materials, not to the elevation of Meridiani Sinus as a whole, which Herr et al. (1970) have shown to be high relative to Sabaeus Sinus. The dark material appears to invade light material and form puddles in low spots, including craters, somewhat in the manner in which lunar maria invade margins of the older highlands. The evidence can best be seen in Fig. 3b, A, B, C. We assign the low dark material to Stage 2. It is separated by scarps on the northeast and southwest sides from Stage 1 material that is high in albedo and elevation. In Fig. 3, the Sun is to the west. The scarp on the northeast side of the dark patch is brightly illuminated and therefore faces southwest. The scarp on the southwest side is shadowed and therefore faces northeast. This would only be possible if the dark patch were lower than the surrounding bright terrain. Cutts et al. (197 1) stated that “numerous small dark outliers are visible in the light area; similar detached light areas are not observed on the dark side of the boundary.” We regard the dark patches as inliers rather than outliers and we can detect
DYNAMIC
PROCESSES
189
numerous detached light areas in dark areas, as in Fig. 3b, D and E. The dark material on crater floors is interpreted as the same Stage 2 material as that which forms dark plains. At least one crater (Fig. 3a, E) is horseshoeshaped and the dark material on its floor is continuous with surrounding dark plainsforming material. The darkstage 2 material seen in eastern Meridiani Sinus becomes progressively brighter toward the west. The increase in albedo is gradational and we suggest that it results from a surficial veneer of transported (wind-blown! ) Stage 3 material. Some of the bright veneer surrounds the problematical bright-ray crater discussed previously, but most of it forms irregular patches elongated from NNW to SSE. As Cutts et al. (1971) have pointed out, it is significant that many craters in this area have light markings on the north side of the crater floor. In a few craters, including the large crater Edom, the photographed part of the floor is covered with bright material. A bright outer rim is visible on the south side of a few craters (Fig. 3a, F). It appears that dust or sand has been banked up against south-facing crater walls, both on the north side of the crater floors and on the outer rim. By this interpretation, the prevailing winds in Meridiani Sinus would be from NNW to SSE (Bagnold, 1942). To summarize, we suggest the following history for most of the large craters in Fig. 3: (1) Crater formation on a highalbedo plain of Stage 1, (2) inundation of low-lying terrain, including crater floors, by dark material of Stage 2, and (3) partial burial of crater floors, especially along south-facing crater walls, by sand or dust.’ One prominent lineament interpreted as a fault scarp appears to cut off the southwestern rim of Edom, interpreted as a Stage 1 feature. In Fig. 3b, B, it can be seen that the scarp has been embayed by a small reentrant, which was covered by 7 In general, this interpretation has been confirmed by Mariner 9 photography. There are significant changes in material interpreted as veneer, but not in material interpreted as bedrock.
ELSTOX
190
low-albedo material of Stage 2. The fault line has no surface expression in the patch of low-albedo material. Faulting then seems to have occurred after at least some cratering during Stage 1 and before formation of low-albedo plains of Stage 2. Thymiamata, Margcwitifer Sinus, Chryse, and Xanthe The terrain of Meridiani Sinus continues eastward (6N11,6N9) across Thymiamata, but the resolution becomes poorer. As far as can be determined, the part of Thymiamata shown in the eastern part of 6N9 consists of cratered Stage 1 material and irregular patches of low-albedo Stage 2 material. The terrain seems to be more or less mantled by a light Stage 3 veneer to give it a mottled appearance and blur contacts. Craters have the same dark floors, and bright patches against southfacing slopes, as in Meridiani Sinus (Fig. 3a).
Mariner 6 photograph garitifer Sinus and Thymiamata, criminability version, orthographic FIG.
6.
6N9 of Marmaximum disprojection.
AND
SMITH
The cratered surface of the Margaritifer Sinus “mesa” (Fig. 6, A) resembles Stage 1 terrain in Meridiani Sinus. It seems to be an erosional remnant ; its former extent reduced by formation of chaotic terrain. Chaotic terrain appears to have eaten the edge of the mesa, where it encountered craters and partly destroyed them (Fig. 6B). As Sharp et al. (1971) recognized, chaotic terrain is relatively young and fresh, here assigned to Stage 3. West of Margaritifer Sinus in the region of Chryse and Xanthe, large elongated depressions with irregular courses can be seen on 6N3, 6N5, and 6N7 toward the southwest. They are over 1OOOkm long, 1OOkm wide, and widen toward the southwest. Their floors have high albedo but resolution is too poor to permit conclusions on their nature or origin.8 Sabaeus Sinus- Deucalionis Regio Area In Deucalionis Regio (6N19, 6N21, 6N23) high-albedo cratered terrain of Deucalionis Regio is, again, the oldest and is assigned to Stage 1. In Sabaeus Sinus, near the boundary with Deucalionis Regio, terrain is textured and cratered (Fig. 2B). Sabaeus Sinus has lower albedo than Deucalionis Regio but because of the high phase angle, albedo contacts are difficult to see on near-encounter images. Basically, it seems to be a cratered Stage 1 terrain, similar to that of Deucalionis Regio further south, textured by surficial or endogenic processes. Cutts et al. (1971) have shown that there is no difference between the size-frequency distributions of craters in Deucalionis Regio and the Sabaeus Sinus-Meridiani Sinus area. Craters in these areas belong to every class and some of them have dark smooth patches on their floors, similar to Stage 2 material in Meridiani Sinus (Fig. 2C). The texturing partially obscures a few craters (Fig. 2D) and it therefore superimposed on Stage 1 terrain. It is a hummocky terrain dissected by sinuous 8 These features have been shown to be the eastern end of the equatorial canyonlands. They were also recognized as valley-like features by Cross (1971).
MARS
FIG. version,
7. Mariner orthographic
7 photograph projection.
: EVIDENCE
7N27
of
FOR
DYNAMIC
Hellas
and
channel-like depressions up to 200 km long (Fig. 2E).g Most of the channels are routed around craters. Unlike sinuous rilles on the Moon, the channels do not begin in a crater. Some of them begin at the boundary between Deucalionis Regio and Sabaeus Sinus and join to form trunk channels, meandering toward the north. Herr et al. (1970) have shown that this terrain slopes north and we suggest that the channels were formed by surface agents in response to the slope. Sinuous courses suggest erosion by a running liquid or fluidized medium rather than by wind. Sharp appearance is evidence of relative youth of the channels, here designated as having formed in Stage 3. One highly irregularly shaped large crater (Fig. 2A) is partly filled on its north side with hummocky material (Fig. 2F) assigned to Stage 3. The material has fanshaped striations that appear to converge toward the north. The patch of hummocky material superficially resembles an enorg Mariner of this type 8
9 photography in many places.
has
shown
channels
191
PROCESSES
Hellespontus,
maximum
mous mudflow, lava flow, fan almost 1OOkm wide.
discriminabl
or
ility
alluvial
Hellas-Hellespontus Boundary The cratered terrain of Hellespontus is basically similar to Stage 1 cratered terrain in other areas described in this article. The featureless terrain of Hellas, about 5.5km lower (Herr et al., 1970), is young because it largely obscures craters, which are just faintly visible in a few places (Fig. 7A). The featureless terrain is not entirely featureless ; in addition to relict craters it has at least one dome (Fig. 7B) and a sinuous channel (Fig. 7C). There does not seem to be a direct way of determining the age of the Hellas basin. For instance, it cannot be determined whether the faint craters that can be seen on the floor of Hellas formed before or after the basin subsided. The scarp-andridge terrain at the boundary between Hellas and Hellespontus shows evidence of faulting and, probably, structurally controlled volcanism that occurred subsequent to formation of large craters in
192
ELSTON
AND
Stage 1 but prior to filling of the Hellas basin. It is entirely possible that the boundary structures formed at the same time as the Hellas basin as a whole, which would then be a relatively young feature (Stage 22) ; alternatively Hellas is an old feature with a rejuvenated boundary. Scarps are characteristic of the boundary between Hellas and Hellespontus. Some are nearly straight (Fig. 7D), like terrestrial fault scarps. They are parallel to a set of echelon ridges which Sharp et al. (1971) interpreted as older than craters because they “do not crosscut the craters; rather, they are distorted by them.” It is not clear whether they are referring to the large craters characteristic of Hellespontus or to small irregular summit pits on the ridges. We interpret the ridges as younger than the large craters. We can see several places (Fig. 7E) where ridges seem to cut large craters, although, admittedly, the craters traversed are faint. More typically, ridges that intersect crater walls change direction, run around the edge of the crater for about half its circumference and then continue in their former direction (Fig. 7F). If this is the distortion mentioned by Sharp et al. (1971), we know of no way in which a crater can distort a preexisting ridge to the extent shown in Hellespontus. Neither the height nor the width of the ridge is changed. The ridges are more prominent than the crater walls. Impact and volcanic processes would both totally destroy a ridge that formerly ran directly across the site of a crater. If the ridges were later than the craters, protrusion of a large dike-like igneous intrusion would be possible mechanism. Magma could ascend along fractures on the margin of Hellas and locally be diverted along fractures peripheral to craters. The small irregular flat-floored craters on the summit of domelike bulges on the ridges (7N26) are interpreted as analogs of summit craters formed by explosion and/or subsidence on terrestrial volcanic domes (Smith, 1970). Again we seen no evidence that these small craters are distorted by the ridges. Summit pits on domes can be seen in other parts of Mars, for example, on a dome-like bulge on the rim of Edom (Fig. 3bF).
SMITH
There is at least a possibility that some of the craters on the western border between Hellas and Hellespontus are themselves volcanic in origin, that the ridges are dikes that locally turn into ring-dike segments, and that the entire boundary zone is strongly modified by volcanotectonic processes. This interpretation would be consistent with the views of Elston (1971) on the role of volcanotectonic processes on Earth and Moon. The scarp system that faces the western border of Hellas has a number ofreentrants, resembling terrestrial canyons. The one most easily seen is bordered on its western side by part of a fault scarp (Fig. 7G). It resembles, in this respect, many structurally, controlled terrestrial valleys. At the head of the reentrants a crater was partially destroyed, as it would have been be headward erosion on Earth. At the limit of resolution, a fan-shaped deposit can barely be discerned at the mouth of the reentrant (Fig. 7H). The meandering channel mentioned earlier runs from this canyon-like reentrant for at least 150 km across the floor of Hellas. The length of the canyon is about 130km, and its width at the mouth is about 30km. Because of uncertainties in resolution, the depth cannot be measured accurately by the shadow technique but is of the order of kilometers. lo Another canyon-like reentrant is shown in Figure 71. It is more difficult to see because its direction is parallel to the direction of illumination of the photographs and because of some of the artifacts of image processing. It does not seem to be controlled by a fault, otherwise, its features are about the same as in the other canyon-like reentrant, as far as can be resolved. Sharp et al. (1970) accepted the view of Murray et al. (1970) that large Martian craters formed in the late stages of planetary accretion. Since they concluded that Hellas was even older than the craters, this would make Hellas very old indeed. They considered three possibilities for the
the
lo This light
interpretation of Mariner
remains 9 photography.
unchanged
in
MARS:EVIDENCE
FOR
formation ofat$e Hellas basin: impact, isostatic subsidence, volcanism, inhomopossibly above “an undigested geneity inherited from the planet’s early accretion.” They dismissed the first two possibilities on grounds of scale and lack of evidence and favored the third possiAccretionary inhomogeneities bility. have no known terrestrial analogs and would be preserved on Mars only if the planet lacked postaccretionary differentiation and dynamic activity except in most recent times, as favored by Sharp et al. (1971). We regard Hellas as a postaccretionary feature. Its diameter and depth are no greater than those of many intracratonic basins on terrestrial continents. For example, the part of the Karroo basin of southern Africa preserved on the African plate is at least 1OOOkm in diameter and has a mean depth of at least Bkm (Furon, 1963). Faults and ring-dike complexes concentric with the Karroo basin are preserved in its northern margin. The interpretation of Hellas as a postaccretionary tectonic feature requires that the thermal history permits dynamic activity in the distant past but well after planetary accretion. DISCUSSION
The view of Mars that emerges from our study is that of a moderately dynamic planet. As yet no evidence has been found of characteristically Earth-like tectonic features, such as moving plates and their complexly deformed borders. There are signs however, of internal dynamic activity on the scale of the interior of the African plate. ’ * In evidence, there are scarps resembling fault scarps in Meridiani Sinus and on the Hellas-Hellespontus border. Ridges associated with craters on the boundary of Hellas and Hellespontus are most easily explained by volcanic activity and at least some of the associated craters may themselves be of volcanic origin. l1 Rifting, a tectonic event characteristic of eastern Africa, also seems to have played a part in initiating canyon formation in the Aurorae Sinus region photography.
of
Mars,
as shown
by
Mariner
9
DYNAMIC
193
PROCESSES
A few large craters that looks as if they might be volcanic have been found, as well as some smaller ones. Ridges on the HellasHellespontus border and part of the wall of Edom seem to bulge out into cratered swellings or domes. A sequence of events can be worked out for different parts of Mars, and an attempt has been made at a crude threefold geologic time scale (Fig. 1). The sequences suggests that dynamic activity was confined neither to the earliest nor the latest periods of Martian history. It is incompatible with the suggestion of Murray et al. (1971) and Sharp et al. (1971) that Mars has been dynamically inactive except during planetary accretion and, possibly, a recent episode of “geothermal maturing.“12 Evidence for surface erosion and deposition is seen in chaotic terrain, possible wind-blown veneers, progressive degradation of craters, sinuous channels with tributaries, and canyon-like reentrants. If interpretation of Mars as a moderately dynamic planet is correct, internal geochemical differentiation and defluidization are likely to have occurred and to have provided constituents for an atmosphere and, possibly, a hydrosphere. Murray et al. (1971) considered the lack of postaccretional dynamic activity a reason for lowering the probability for biological activity on Mars. To that extent, our view of Mars a’s a moderately dynamic planet increases the probability of finding life on Mars. ACKNOWLEDGMENTS This research is funded by the Planetology Program Office, Office of Space Science NASA Headquarters under Grant NGR-32-004-062. We are grateful to Dr. R. C. Rhodes, Geological Survey of South Africa, for information on African geology. REFERENCES ANDERS, E. A., AND Age of the craters 1494-1496.
ARNOLD, on Mars.
J.
R. Science
(1965), 149.
l2 Mariner 9 has shown that Martian features most clearly related to large-scale tectonism and volcanism (e.g., the equatorial canyons, the Nix Olympical volcano) are relatively young. This would support the hypothesis of “geothermal maturing” of Sharp et al. (1971).
194
ELSTON
BAGNOLD, R. A. (1942).
“The Physics of WindSand and Desert Dunes” 265 pp. New William Marrow and Co. BALDWIN, R. B. (1965). Mars: An estimate of the age of its surface. Science 149, 1498-1499. CROSS, C. A. (1971). Mars Chart, R-757-NASA, Rand Corp., Santa Monica, California. CUTTS, J. A., SODERBLOM, L.A., SHARP, R.P., SMITH, B. A., AND MURRAY, B. C. (1971). The Surface of Mars : Light and dark markings. J. Geophys. Res. 76, 343-356. DUNNE, J. A., STROMBERG, W. D., RUIZ, R. M., COLLINS, S., AND THORPE, T. E. (1971). Maximum discriminability versions of the near encounter Mariner pictures. J. Geophys. Res. 78, 438-472. ELSTON, W. E. (1971). Evidence for lunar volcano-tectonic features. J. Geophys. Res. 76, 5690-5702. ELSTON, W. E., AND SMITH, E. I. (1971). Stratigraphy and classification of Martian terrains photographed by Mariners 6 and 7 (abstr.): Int. Union Geol. and Geophys., Program, 15th Gen. Assembly, Moscow, p. 154. FURON, RAYMOND (1963). “Geology of Africa,” 337 pp. New York, Hafner. HAMMOND, A. L. (1972). Mars as an active planet : The view from Mariner 9. Science 175, 286-287. HERR, K. C., HORN, DIETER, MCAFEE, J. M., AND PIMENTEL, G. C., (1970). Martian topography from Mariner 6 and 7 infrared spectra. Astronom. J. 75, 833-894. LEIGHTON, R. B., HOROWITZ, N. H., MURRAY, B.C., SHARP, R.P., HERRII\IAN, A.G., YOUNG, blown York,
A.T.,SMITH,B.A.,DAVIES,M.E.,ANDLEROY, C. B. (1969). Television observations from Mariners 6 and 7: Mariner-Mars 1969, a preliminary report, NASA, SP-225, pp. 3782. LEIGHTON,R. B., MURRAY, B.C., SHARP, R.P., ALLEN, J. D., AND SLOAN, R. K. (1965). Mariner IV photography of Mars: Initial results. 8cience 149, 627-630. LEIGHTON, R. B., MURRAY, B. C., SHARP, R. P., ALLEN, J. D., AND SLOAN, R. K., (1967).
AND
SMITH Mariner IV pictures of Mars: Mariner Mars 1964 Project Report : Television Experiment, Pt. 1. Investigators’ Report. Jet Propulsion Lab. Tech. Rept. 32-884, 178 pp. MASURSKY, HAROLD and 30 others (1972). Mariner 9 television reconnaissance of Mars and its satellites : Preliminary results. Science 175, 294-305. MURRAY, B.C., SODERBLOM,L. A., SHARP,R. P., AND CUTTS, J. A. (1971). The Surface of Mars 1. Cratered terrains, J. Geophys. Res. 76, 313-330. RINDFLEISCH, T. C., DUNNE, J. A., FRIEDEN, H. J., STROMBERG, W. D., AND RUIZ, R. M. (197 1). Digital processing of Mariner 6 and 7 pictures. J. Geophys. Res. 76, 394-417. SHARP, R. P., SODERBLOM, L. A., MURRAY, B. C., AND CUTTS, J. A. (1971). The surface of Mars 2. Uncratered terrains. J. Geophys. Res. 76, 331-342. SHOEMAKER,E. M. AND HACKMAN, R.J.(1962). Stratigraphic basis for a lunar time scale. In. “The Moon” (Zdenek Kopal and Z. K. Mikhailov, eds.), London, pp. 289-306. Academic Press. SMITH, E. I. (1970). Comparison of selected lunar and terrestrial volcanic domes : Ph.D. dissert. (unpub.), Albuquerque, Univ. New Mexico, 200 pp. SMITH, E. I. (1971). Determination of origin of small lunar and terrestrial craters by depthdiameter ratio. J. Geophys. 76, No. 23, 56835689. SMITH, E. I., AND ELSTON, W. E. (1971). Martian stratigraphy and terrain classification : A basis for the geological mapping of Mars (abstr.). EOS, Amer. Geophys. Union Trans. 52, 263. STEINBACHER, R. H., KLIORE, A., LORELL, J., HIPSHER, H., BARTH, C. A., MASURSKY, H., MUNCH, G.,PEARL, J. AND SMITH, B.(1972). Mariner 9 science experiments : Preliminary results. Science 175, 293-294. WITTING, J., NARIN, F., AND STONE, F.A.(1965). Mars: Age of its craters. Science 149, 14961498.
19,
Mars:
180-194
(1973)
Evidence
for
Dynamic
WOLFGANG Department
of Geology,
Processes
E. ELSTON University
Received
26 April
AND
of New
Mexico,
1972;
revised
from
EUGENE Albuquerque,
10 January
Mariners
6 and 7’
I. SMITH2 New
Mexico
87131
1973
Mariners 6 and 7 photographs of the equatorial region of Mars document a threestage evolution of that part of the Martian surface: (1) Highand intermediatealbedo cratered terrains in Meridiani Sinus, Margaritifer Sinus-Thymiamata, Deucalionis Regio-Sabaeus Sinus, and Hellespontus; (2) low-albedo moderately cratered terrain and dark crater fill in Meridiani Sinus, Thymiamata, and Deucalionis Regio-Sabaeus Sinus and possible volcanism in the Hellas-Hellespontus border; and (3) high-albedo surficial deposits, banked-up crater fill, a possible bright-ray crater in Meridiani Sinus, chaotic terrain on the edge of the Margaritifer Sinus mesa, featureless terrain in Hellas and Edom, sinuous channel-like reentrants on scarps at the Hellas-Hellespontus boundary. Regional faulting seems to have occurred following formation of the old cratered plains and prior to formation of low-albedo plains in Meridiani Sinus and also prior to formation of canyonlike reentrants and featureless terrain along the Hellas-Hellespontus boundary. Mars has had a complex history of dynamic evolution, possibly analogous to the more stable regions of Earth. Its geochemical differentiation and thermal regime should account for long-term postaccretional tectonic and volcano-tectonic processes as well as for fluid media on its surface sufficient to cause erosion, including the cutting of large canyons.
manuscript was shortened for publication in October 1972.’ When Mariner 4 returned the first space-probe photographs from Mars in 1965, members of the Investigators Team (Leighton et al., 1965) concluded that the cratered surfaces of the planet were 2 to 5 x lo9 yr old, and that “the principal topographic features of Mars.. . have not
INTRODUCTION
The paper deals with preliminary analysis of some surface features of the equatorial region of Mars photographed by Mariners 6 and 7. Observations were made on maximum-discriminability pictures (Dunne et al., 1971) supplemented with selected photometric images from which effects of the cuber and automatic gain control had been removed (Rindfleisch et al., 1971). The work was done in 1970 and 1971, before Mariner 9 data became available. Summaries were published by Smith and Elston (1971) andElstonandSmith (1971). This manuscript was completed in late February 1972. Footnotes pertaining to Mariner 9 findings were added when the
3 Sections on crater classification, crater distribution, and tectonic patterns were omitted on editing, because this material has been covered by other authors. Otherwise the body of the manuscript is unchanged as written before release of Mariner 9 photographs. A shorter version of this paper was submitted to another journal in July 1971 and rejected in October 1971 after a reviewer dismissed our tentative identification of tectonic features, canyon-like features and other evidence of dynamic activity on Mars as “fantasy.” Our intention to publish this material before Mariner 9 data became available was unfortunately frustrated.
i Contribution No. 34 of the Planetary Geology Group, University of New Mexico. z Present address : University of Wisconsin, Parkside, Kenosha, Wisconsin, 53140. Copyright 0 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.
180
PYRRHAE
MERIDIANI
INSUFFICIENT
?-
400
ESCARPMENT
HELLAS
J-
/-
HELLAS
I-
SINUS
SINUS
RESOLUTION
ZONE
REGIO
SINUS
REGIO
BORDER
H - HELLESPONTUS
G - DEUCALIONIS
F - SABAEUS
E - EDOM
D-
C - THYMIAMATA
B - MARGARITIFER
A-
EXPLANATION
~ HIGH
ALBEDO
. . . . . . . . . .+:*:*:z*:* STAGE
STAGE
I]
3200
I
INTERMEDIATE
2
ALBEDO
0 400 L I I %I, SCALE
KM
182
ELSTOXAND
been produced by stress and deformation within the planet.. .” This conclusion was reiterated by Leighton et al. (1967) in spite of criticisms of the inferred ages of craters by Anders and Arnold (1965), Baldwin (1965), and Witting et al. (1965). In the light of greatly superior imagery from Mariners 6 and 7 the investigators modified their conclusions only marginally (Leighton et al., 1969; Murray et al., 1971; Sharp et al., 1971; Cutts et al., 1971). Large craters were said to have survived” . . . for billions of years, and indeed most probably from the end of planetary accretion” (Murray et al., 1971). Locally, a distinct sequence of events was recognized, as at the boundary between Hellas and Hellespontus (Fig. l), but most of the features were interpreted as having originated prior to the end of the planetary accretion. There was speculation by Sharp et al. (1971) that chaotic terrain may be “an early expression of geothermal maturity” and that volcanism and tectonism will play a greater role in the future than in the past. In general, we find evidence for a sequence of events that does not follow the order described by the investigators. Our interpretations suggest a longer period of development for bedrock terrain and a greater role for internal processes. Murray et al. (1971) stressed the apparent absence of Mars of large fresh craters, resembling Copernicus and Tycho on the Moon. We have noted a feature in Meridiani Sinus that could be interpreted as a brightray crater although the evidence is far from certain. The investigators as well as their critics have consistently ascribed all Martian craters to impact processes. This is a prerequisite for, and a consequence of, all planetary models which interpret Mars as dynamically inactive except, possibly, in relatively recent time. Elsewhere, we have interpreted the craters on the Moon as a mixed population impact and volcanotectonic craters (Smith, 1971; Elston, 1971). Murray et al. (1971) concluded from the resemblance of Mars to the Moon that crustal deformation played little or no part in the evolution of Mars. We conclude from the same resemblance that
SMITH
crustal deformation and volcanism may well have played a major role. Like other writers, we ascribe many features to surficial processes of erosion and blanketing, but would go farther to include the cutting of large canyon-like reentrants, as on the border between Hellas and Hellespontus. Murray et al., (1971) have commented on the lack of simple correlation between albedo, terrain, and crater density, quite unlike the terra-mare dichotomy on the Moon. In at least some areas of Mars, such as Meridiani Sinus, the correlation between albedo and terrain can be made more consistent by assuming two kinds of high-albedo terrains, one of bedrock and the other a surficial deposit. Our view of Mars is of a planet with decipherable dynamic history, involving both internal and surficial processes. As this paper was being completed, a new series of articles by the investigators (Steinbacher et al., 1972 ; Masursky comment et al., 1972) and an editorial (Hammond, 1972) indicate that the investigators now accept the view of Mars as a dynamic planet in the light television imagery and other experimental results from Mariner 9.
CLASSIFICATIONS OF MARTIAN TERRAINS
Leighton et al. (1969) and Sharp et al. recognized three basic types of Martian terrains : cratered, chaotic, featureless. We have accepted this classification, refined it, and examined terrain boundaries. There seem to be at least four types of cratered terrains: (1) High- and intermediate-albedo cratered terrains with relatively smooth intercrater areas, as in Deucalionis Regio, (6N21) (Fig. 2), (2) relatively smooth low-albedo plains, as in eastern Meridiani Sinus (6N13) (Fig. 3), ( 3) high-albedo hummocky and textured cratered plains seen on the border between Deucalionis Regio and Sabaeus Sinus (6N19, 6N21, 6N23) (Fig. 2), and (4) cratered terrain in the south polar region characterized by lineaments and irregular depressions, possibly a landscape modified (1971)
MARS:EVIDEIWE
Pk. 2. Mariner 6 photograph orthographic projection.
6N21
FOR DYNAMIC
of Deucalionis
by glacial and periglacial processes (7N15, 7N17, 7N19). The south polar region is not considered further in this article. Featureless terrain was described by Sharp et al. (1971) from the floor of Hellas basin (7N25, 7N27). It is characterized by high albedo and smooth texture. On A-camera images faint outlines of a few large craters can be resolved but on Bcamera close-ups, where small bowl-shaped craters ordinarily appear, no craters can be seen. Sharp et al. (1971) regarded featureless terrain as unique to Hellas within the coverage of Mariner photography and speculated that “Hellas may be mantled by a type of material markedly different from that covering other parts of the Martian surface.. .” We consider it likely that the terrain in Hellas is basically similar to that on the floors of many other craters, which seem to be partly blanketed by featureless material. As Cutts et al. (1971) have pointed out, many craters in Meridiani Sinus (6N13) show bright markings on the north sides of the crater floor (Fig. 3a) possibly the result of transportation and deposition of materials by wind. We agree that this is probable. Carrying it one step farther, it seems likely that the floors of the largest depressions,
Regio,
PROCESSES
maximum
183
discriminability
version,
such as the parts of Hellas and Edom photographed by Mariners 6 and 7 are largely or entirely mantled by windtransported material to form featureless terrain. The dust storm during the Mariner 9 mission is additional evidence for the efficacy of eolian processes on Mars. Chaotic terrain was described by Sharp et al. (1971) from B-camera photographs 6N6, 6N8, and 6Nl0, and extrapolated to 1.5 x lPkm*. Only future photography can show whether the extrapolation is justified.4 We find chaotic terrain in the predicted position around the base of the Margaritifer Sinus scarp in 6N9 and agree with Sharp et al. (1971) who pointed out that it is relatively young and formed at the expense of cratered terrain. CRATERS
A Possible Bright-Ray
Crater
The absence on Mariner Mars photographs of fresh large Copernican craters 4 Mariner 9 has shown chaotic terrain much more limited and to be related apparent source areas of stream channels equatorial region.
to be to the in the
184
ELSTON
FIG. 3a. Mariner
6 photograph
6N13
FIG. 3b. Enlargement
AND
SMITH
of Meridiani Sinus, maximum of part of the eastern side
is fundamental to the conclusion of Murray et al. (1971) that the small bowl-shaped craters of Mars and the bright-ray craters of the Moon belong to different populations. Yet, there is a feature in western Meridiani Sinus that could be interpreted as a brightray crater, even though resolution is so
discriminability of Fig. 3a.
version.
poor that entirely different interpretations are possible (Fig. 3a, A). It is a crater, about 25km in diameter, with a wide high-albedo lip surrounded by a dark collar, widest on the north side of the crater. This is in turn surrounded by a high-albedo collar which grades into radial
MARS
: EVIDENCE
FOR
bright streaks on the south side and into a bright irregular patch that extends for about 6 crater diameters on the north side. A few bright streaks radial or tangential to the cr: hter (Fig. 3a, B) extend from a bright
FIG.
crater (7N19).
4. Possible i n Deucalionis
craters of endogenic origin Regio (6N20). (b) Craters
DYNAMIC
PROCESSES
185
patch up to 8 diameters from the era ,ter. The bright patch is not an instrumel ntal artifact. It is visible from Earth and on far-encounter Mariner photographs, smuch as 6F31 and 7F67 as part of the high-alb
or modified with large
by endogenic central peaks
processes. (a) Doublering in the south polar rel ;ion
186
ELSTONAND SMITH
area between the two northeast-trending low-albedo “horns” of Meridiani Sinus. The entire area gives an impression of a lunar bright-ray crater with a ray system even more asymmetrical than that of Kepler. The crater is superimposed on what appears to be a northeast-trending lineament of regional extent (Fig. 3a, C); the bright patch could be interpreted as a ray system that terminates abruptly against the lineament. This can best be seen on the enhanced photometric version of 6N13. The bright patch can also be seen as the brightest spot on 7N5 and 7N7. Suggestions of a ray pattern can be made out in 7N5. Much of Meridiani Sinus has low albedo, and 6N13 and 7N5 were taken with a relatively low phase angle of 52 and 44”, respectively. Conditions would be favorable for detecting a bright-ray crater. While the phase angle favors discrimination between variations in albedo it is unfavorable for determining relief. On 6N13 the nature of the crater lip and rim and of the lineament on which the crater is superimposed remain in doubt. In summary, while it cannot be claimed that a bright-ray crater definitely exists in Meridiani Sinus, neither can the possibility be dismissed. Like most observers, we attribute lunar bright-ray craters to impact processes.5
Other Unusual Craters A number of Martian craters have unusual features that makes them difficult to classify. A good example is a crater that is nearly square in plan, with partly scalloped edges, partly filled with hummocky material and 150km in diameter, in 6N21 (Fig. 2, A). Other examples include a double-ring crater on 6N20 that merges with a feature resembling a lunar mare ridge (Fig. 4A) and a crater with a disproportionately large central peak in ’ Preliminary
examinations of Mariner 9 shed no new light on this possible bright-ray crater. Phase angles are high and one would not expect to see rays. Also, many superficial markings have shifted as a result of dust storms.
photography
has
7N19 (Fig. 4B). Volcanic processes may have played a part in their development.
POSSIBLEFAULTSCARPS Scarps are common on Mars. Some are irregular and resemble terrestrial erosional features, such as the scarps on the edge of mesas. The scarp separating chaotic terrain and a cratered upland surface in Margaritifer Sinus is an example (6N9). In Meridiani Sinus (6N13) a number of discontinuous and branching linear scarps resemble terrestrial fault scarps. The largest can be traced with interruptions for about 700km (Fig. 3a, DD’). In several places, especially adjacent to the southwest rim of Edom, scarps are prominent at the border of high- and low-albedo terrains. Most of them trend northwesterly ; one prominent set of linear features, in an echelon pattern, trends to the northeast (Fig. 3a, CC’). Patches of low-albedo terrain, also elongated to the northeast run parallel to segments of this scarp. The entire system of northeasterly scarp segments and low-albedo patches is in the position of one of the dark linear markings (“canals”) reported by many telescopic observers and shown on the ACIC Map of Mars. It runs northeast from the eastern “horn” of Meridiani Sinus. Some northtrending scarps on the boundary between Hellas and Hellespontus (7N25, 7N27) also resemble fault scarps.
SEQUENCEOFGEOLOGICEVENTS Principles On Earth, the relative sequence of geologic events in a particular region can be worked out on the basis of superposition and transection of geologic features, i.e., a relatively young geologic feature either covers or cuts across older features. These principles were applied to lunar interpretations by Shoemaker and Hackman (1962). Their scheme has been modified in detail but the basic principles have stood the test of time. The Moon has old high-albedo cratered
MARS
: EVIDENCE
FOR
highlands on which the smooth low-albedo maria are superimposed. It therefore lends itself to a relativelv simnle threefold classification (Preinto (1) p&mare imbrian), (2) mare (Imbrian), and (3)
0
100 Approximate
FIG.
orthographic
5. Reconnaissance projection.
200
DYNAMIC
postmare (Eratosthenian and Copernican) features. More detailed sequences have been worked out locallv. Observers of Mars have noted an apparent lack of correlation between albedo, elevation and
300 scale
photogeologic map Geologic interpretation
187
PROCESSES
400
MO
In Kilometers
of
the Meridiani by W. E. Elston,
Sinus April
area of 1971.
Mars.
Photo
6N13
188
ELSTON
crater density. Nevertheless, we found that sequences of events can be worked out in individual areas. An attempt has been made to establish a time scale of three stages, but correlations between different areas are so uncertain that we have assigned numbers to the stages rather than formal names. The validity and extent of the three-stage classification will have to be determined from Mariner 9 photography. Stage 1 is the oldest, Stage 3 the youngest. Interpretations are greatly simplified by the observation that there seems to be two types of mappable terrains on Marsconsolidated bedrock and transported material. Contacts between map units interpreted as bedrock are sharp and are usually associated with differences in relief. Where contacts are associated with provincial boundaries, they are parallel to scarps, ridges, and sinuous channels that seem to be of tectonic, volcanic, or erosional origin. Bedrock surfaces may have considerable relief. Contacts between bedrock and transported material tend
AND
SNITH
to be gradational and veneers of transported material tend to subdue relief.6 In the discussion that follows, individual areas are discussed in terms of three-stage time classifications. Photography of Margaritifer Sinus, Meridiani Sinus, Sabaeus Sinus, and Deucalionis Region is fairly continuous, allowing particular terrains to be traced for long distances. Correlation between disconnected areas is much more uncertain. The three stages must be regarded as exceedingly broad. In terms of terrestrial geology, they may be equivalent to early Precambrian, all geologic time from early Precambrian through Tertiary, and Quaternary. In terrestrial geology, features as diverse as volcanoes of the Cascades, coastal plain deposits of the Gulf Coast, and glacial features of New England are all classified 6 In addition to stratigraphic evidence presented here, the classification of craters has served as a check on the ages assigned to surfaces on Mars. The degree of maximum degradation of craters, and the total number of craters, become progressively greater on older surfaces.
MARS
: EVIDENCE
FOR
as Quaternary. This implies merely that they are young; it does not imply that they are contemporaneous or similar in origin. Similarly the assigment of chaotic terrain Sinus and featureless in Margaritifer terrain in Hellas to Stage 3 merely implies that both are relatively young. The age assignment does not depend on any assumed origin for the features involved. ,~feridiani Sinus-Edom In the area of Meridiani Sinus and Edom, Stage 1 is represented by high- and intermediate-albedo cratered plains, Stage 2 by dark cratered plains and fillings of craters, and Stage 3 by high-albedo surficial material (Fig. 5). Cutts et al. (1971) stated that on the northeastern boundary of Meridiani Sinus, adjacent to Edom “the light area is low with respect to the dark area.” We regard the light area as higher. This applies to local conditions at the contracts between dark and light materials, not to the elevation of Meridiani Sinus as a whole, which Herr et al. (1970) have shown to be high relative to Sabaeus Sinus. The dark material appears to invade light material and form puddles in low spots, including craters, somewhat in the manner in which lunar maria invade margins of the older highlands. The evidence can best be seen in Fig. 3b, A, B, C. We assign the low dark material to Stage 2. It is separated by scarps on the northeast and southwest sides from Stage 1 material that is high in albedo and elevation. In Fig. 3, the Sun is to the west. The scarp on the northeast side of the dark patch is brightly illuminated and therefore faces southwest. The scarp on the southwest side is shadowed and therefore faces northeast. This would only be possible if the dark patch were lower than the surrounding bright terrain. Cutts et al. (197 1) stated that “numerous small dark outliers are visible in the light area; similar detached light areas are not observed on the dark side of the boundary.” We regard the dark patches as inliers rather than outliers and we can detect
DYNAMIC
PROCESSES
189
numerous detached light areas in dark areas, as in Fig. 3b, D and E. The dark material on crater floors is interpreted as the same Stage 2 material as that which forms dark plains. At least one crater (Fig. 3a, E) is horseshoeshaped and the dark material on its floor is continuous with surrounding dark plainsforming material. The darkstage 2 material seen in eastern Meridiani Sinus becomes progressively brighter toward the west. The increase in albedo is gradational and we suggest that it results from a surficial veneer of transported (wind-blown! ) Stage 3 material. Some of the bright veneer surrounds the problematical bright-ray crater discussed previously, but most of it forms irregular patches elongated from NNW to SSE. As Cutts et al. (1971) have pointed out, it is significant that many craters in this area have light markings on the north side of the crater floor. In a few craters, including the large crater Edom, the photographed part of the floor is covered with bright material. A bright outer rim is visible on the south side of a few craters (Fig. 3a, F). It appears that dust or sand has been banked up against south-facing crater walls, both on the north side of the crater floors and on the outer rim. By this interpretation, the prevailing winds in Meridiani Sinus would be from NNW to SSE (Bagnold, 1942). To summarize, we suggest the following history for most of the large craters in Fig. 3: (1) Crater formation on a highalbedo plain of Stage 1, (2) inundation of low-lying terrain, including crater floors, by dark material of Stage 2, and (3) partial burial of crater floors, especially along south-facing crater walls, by sand or dust.’ One prominent lineament interpreted as a fault scarp appears to cut off the southwestern rim of Edom, interpreted as a Stage 1 feature. In Fig. 3b, B, it can be seen that the scarp has been embayed by a small reentrant, which was covered by 7 In general, this interpretation has been confirmed by Mariner 9 photography. There are significant changes in material interpreted as veneer, but not in material interpreted as bedrock.
ELSTOX
190
low-albedo material of Stage 2. The fault line has no surface expression in the patch of low-albedo material. Faulting then seems to have occurred after at least some cratering during Stage 1 and before formation of low-albedo plains of Stage 2. Thymiamata, Margcwitifer Sinus, Chryse, and Xanthe The terrain of Meridiani Sinus continues eastward (6N11,6N9) across Thymiamata, but the resolution becomes poorer. As far as can be determined, the part of Thymiamata shown in the eastern part of 6N9 consists of cratered Stage 1 material and irregular patches of low-albedo Stage 2 material. The terrain seems to be more or less mantled by a light Stage 3 veneer to give it a mottled appearance and blur contacts. Craters have the same dark floors, and bright patches against southfacing slopes, as in Meridiani Sinus (Fig. 3a).
Mariner 6 photograph garitifer Sinus and Thymiamata, criminability version, orthographic FIG.
6.
6N9 of Marmaximum disprojection.
AND
SMITH
The cratered surface of the Margaritifer Sinus “mesa” (Fig. 6, A) resembles Stage 1 terrain in Meridiani Sinus. It seems to be an erosional remnant ; its former extent reduced by formation of chaotic terrain. Chaotic terrain appears to have eaten the edge of the mesa, where it encountered craters and partly destroyed them (Fig. 6B). As Sharp et al. (1971) recognized, chaotic terrain is relatively young and fresh, here assigned to Stage 3. West of Margaritifer Sinus in the region of Chryse and Xanthe, large elongated depressions with irregular courses can be seen on 6N3, 6N5, and 6N7 toward the southwest. They are over 1OOOkm long, 1OOkm wide, and widen toward the southwest. Their floors have high albedo but resolution is too poor to permit conclusions on their nature or origin.8 Sabaeus Sinus- Deucalionis Regio Area In Deucalionis Regio (6N19, 6N21, 6N23) high-albedo cratered terrain of Deucalionis Regio is, again, the oldest and is assigned to Stage 1. In Sabaeus Sinus, near the boundary with Deucalionis Regio, terrain is textured and cratered (Fig. 2B). Sabaeus Sinus has lower albedo than Deucalionis Regio but because of the high phase angle, albedo contacts are difficult to see on near-encounter images. Basically, it seems to be a cratered Stage 1 terrain, similar to that of Deucalionis Regio further south, textured by surficial or endogenic processes. Cutts et al. (1971) have shown that there is no difference between the size-frequency distributions of craters in Deucalionis Regio and the Sabaeus Sinus-Meridiani Sinus area. Craters in these areas belong to every class and some of them have dark smooth patches on their floors, similar to Stage 2 material in Meridiani Sinus (Fig. 2C). The texturing partially obscures a few craters (Fig. 2D) and it therefore superimposed on Stage 1 terrain. It is a hummocky terrain dissected by sinuous 8 These features have been shown to be the eastern end of the equatorial canyonlands. They were also recognized as valley-like features by Cross (1971).
MARS
FIG. version,
7. Mariner orthographic
7 photograph projection.
: EVIDENCE
7N27
of
FOR
DYNAMIC
Hellas
and
channel-like depressions up to 200 km long (Fig. 2E).g Most of the channels are routed around craters. Unlike sinuous rilles on the Moon, the channels do not begin in a crater. Some of them begin at the boundary between Deucalionis Regio and Sabaeus Sinus and join to form trunk channels, meandering toward the north. Herr et al. (1970) have shown that this terrain slopes north and we suggest that the channels were formed by surface agents in response to the slope. Sinuous courses suggest erosion by a running liquid or fluidized medium rather than by wind. Sharp appearance is evidence of relative youth of the channels, here designated as having formed in Stage 3. One highly irregularly shaped large crater (Fig. 2A) is partly filled on its north side with hummocky material (Fig. 2F) assigned to Stage 3. The material has fanshaped striations that appear to converge toward the north. The patch of hummocky material superficially resembles an enorg Mariner of this type 8
9 photography in many places.
has
shown
channels
191
PROCESSES
Hellespontus,
maximum
mous mudflow, lava flow, fan almost 1OOkm wide.
discriminabl
or
ility
alluvial
Hellas-Hellespontus Boundary The cratered terrain of Hellespontus is basically similar to Stage 1 cratered terrain in other areas described in this article. The featureless terrain of Hellas, about 5.5km lower (Herr et al., 1970), is young because it largely obscures craters, which are just faintly visible in a few places (Fig. 7A). The featureless terrain is not entirely featureless ; in addition to relict craters it has at least one dome (Fig. 7B) and a sinuous channel (Fig. 7C). There does not seem to be a direct way of determining the age of the Hellas basin. For instance, it cannot be determined whether the faint craters that can be seen on the floor of Hellas formed before or after the basin subsided. The scarp-andridge terrain at the boundary between Hellas and Hellespontus shows evidence of faulting and, probably, structurally controlled volcanism that occurred subsequent to formation of large craters in
192
ELSTON
AND
Stage 1 but prior to filling of the Hellas basin. It is entirely possible that the boundary structures formed at the same time as the Hellas basin as a whole, which would then be a relatively young feature (Stage 22) ; alternatively Hellas is an old feature with a rejuvenated boundary. Scarps are characteristic of the boundary between Hellas and Hellespontus. Some are nearly straight (Fig. 7D), like terrestrial fault scarps. They are parallel to a set of echelon ridges which Sharp et al. (1971) interpreted as older than craters because they “do not crosscut the craters; rather, they are distorted by them.” It is not clear whether they are referring to the large craters characteristic of Hellespontus or to small irregular summit pits on the ridges. We interpret the ridges as younger than the large craters. We can see several places (Fig. 7E) where ridges seem to cut large craters, although, admittedly, the craters traversed are faint. More typically, ridges that intersect crater walls change direction, run around the edge of the crater for about half its circumference and then continue in their former direction (Fig. 7F). If this is the distortion mentioned by Sharp et al. (1971), we know of no way in which a crater can distort a preexisting ridge to the extent shown in Hellespontus. Neither the height nor the width of the ridge is changed. The ridges are more prominent than the crater walls. Impact and volcanic processes would both totally destroy a ridge that formerly ran directly across the site of a crater. If the ridges were later than the craters, protrusion of a large dike-like igneous intrusion would be possible mechanism. Magma could ascend along fractures on the margin of Hellas and locally be diverted along fractures peripheral to craters. The small irregular flat-floored craters on the summit of domelike bulges on the ridges (7N26) are interpreted as analogs of summit craters formed by explosion and/or subsidence on terrestrial volcanic domes (Smith, 1970). Again we seen no evidence that these small craters are distorted by the ridges. Summit pits on domes can be seen in other parts of Mars, for example, on a dome-like bulge on the rim of Edom (Fig. 3bF).
SMITH
There is at least a possibility that some of the craters on the western border between Hellas and Hellespontus are themselves volcanic in origin, that the ridges are dikes that locally turn into ring-dike segments, and that the entire boundary zone is strongly modified by volcanotectonic processes. This interpretation would be consistent with the views of Elston (1971) on the role of volcanotectonic processes on Earth and Moon. The scarp system that faces the western border of Hellas has a number ofreentrants, resembling terrestrial canyons. The one most easily seen is bordered on its western side by part of a fault scarp (Fig. 7G). It resembles, in this respect, many structurally, controlled terrestrial valleys. At the head of the reentrants a crater was partially destroyed, as it would have been be headward erosion on Earth. At the limit of resolution, a fan-shaped deposit can barely be discerned at the mouth of the reentrant (Fig. 7H). The meandering channel mentioned earlier runs from this canyon-like reentrant for at least 150 km across the floor of Hellas. The length of the canyon is about 130km, and its width at the mouth is about 30km. Because of uncertainties in resolution, the depth cannot be measured accurately by the shadow technique but is of the order of kilometers. lo Another canyon-like reentrant is shown in Figure 71. It is more difficult to see because its direction is parallel to the direction of illumination of the photographs and because of some of the artifacts of image processing. It does not seem to be controlled by a fault, otherwise, its features are about the same as in the other canyon-like reentrant, as far as can be resolved. Sharp et al. (1970) accepted the view of Murray et al. (1970) that large Martian craters formed in the late stages of planetary accretion. Since they concluded that Hellas was even older than the craters, this would make Hellas very old indeed. They considered three possibilities for the
the
lo This light
interpretation of Mariner
remains 9 photography.
unchanged
in
MARS:EVIDENCE
FOR
formation ofat$e Hellas basin: impact, isostatic subsidence, volcanism, inhomopossibly above “an undigested geneity inherited from the planet’s early accretion.” They dismissed the first two possibilities on grounds of scale and lack of evidence and favored the third possiAccretionary inhomogeneities bility. have no known terrestrial analogs and would be preserved on Mars only if the planet lacked postaccretionary differentiation and dynamic activity except in most recent times, as favored by Sharp et al. (1971). We regard Hellas as a postaccretionary feature. Its diameter and depth are no greater than those of many intracratonic basins on terrestrial continents. For example, the part of the Karroo basin of southern Africa preserved on the African plate is at least 1OOOkm in diameter and has a mean depth of at least Bkm (Furon, 1963). Faults and ring-dike complexes concentric with the Karroo basin are preserved in its northern margin. The interpretation of Hellas as a postaccretionary tectonic feature requires that the thermal history permits dynamic activity in the distant past but well after planetary accretion. DISCUSSION
The view of Mars that emerges from our study is that of a moderately dynamic planet. As yet no evidence has been found of characteristically Earth-like tectonic features, such as moving plates and their complexly deformed borders. There are signs however, of internal dynamic activity on the scale of the interior of the African plate. ’ * In evidence, there are scarps resembling fault scarps in Meridiani Sinus and on the Hellas-Hellespontus border. Ridges associated with craters on the boundary of Hellas and Hellespontus are most easily explained by volcanic activity and at least some of the associated craters may themselves be of volcanic origin. l1 Rifting, a tectonic event characteristic of eastern Africa, also seems to have played a part in initiating canyon formation in the Aurorae Sinus region photography.
of
Mars,
as shown
by
Mariner
9
DYNAMIC
193
PROCESSES
A few large craters that looks as if they might be volcanic have been found, as well as some smaller ones. Ridges on the HellasHellespontus border and part of the wall of Edom seem to bulge out into cratered swellings or domes. A sequence of events can be worked out for different parts of Mars, and an attempt has been made at a crude threefold geologic time scale (Fig. 1). The sequences suggests that dynamic activity was confined neither to the earliest nor the latest periods of Martian history. It is incompatible with the suggestion of Murray et al. (1971) and Sharp et al. (1971) that Mars has been dynamically inactive except during planetary accretion and, possibly, a recent episode of “geothermal maturing.“12 Evidence for surface erosion and deposition is seen in chaotic terrain, possible wind-blown veneers, progressive degradation of craters, sinuous channels with tributaries, and canyon-like reentrants. If interpretation of Mars as a moderately dynamic planet is correct, internal geochemical differentiation and defluidization are likely to have occurred and to have provided constituents for an atmosphere and, possibly, a hydrosphere. Murray et al. (1971) considered the lack of postaccretional dynamic activity a reason for lowering the probability for biological activity on Mars. To that extent, our view of Mars a’s a moderately dynamic planet increases the probability of finding life on Mars. ACKNOWLEDGMENTS This research is funded by the Planetology Program Office, Office of Space Science NASA Headquarters under Grant NGR-32-004-062. We are grateful to Dr. R. C. Rhodes, Geological Survey of South Africa, for information on African geology. REFERENCES ANDERS, E. A., AND Age of the craters 1494-1496.
ARNOLD, on Mars.
J.
R. Science
(1965), 149.
l2 Mariner 9 has shown that Martian features most clearly related to large-scale tectonism and volcanism (e.g., the equatorial canyons, the Nix Olympical volcano) are relatively young. This would support the hypothesis of “geothermal maturing” of Sharp et al. (1971).
194
ELSTON
BAGNOLD, R. A. (1942).
“The Physics of WindSand and Desert Dunes” 265 pp. New William Marrow and Co. BALDWIN, R. B. (1965). Mars: An estimate of the age of its surface. Science 149, 1498-1499. CROSS, C. A. (1971). Mars Chart, R-757-NASA, Rand Corp., Santa Monica, California. CUTTS, J. A., SODERBLOM, L.A., SHARP, R.P., SMITH, B. A., AND MURRAY, B. C. (1971). The Surface of Mars : Light and dark markings. J. Geophys. Res. 76, 343-356. DUNNE, J. A., STROMBERG, W. D., RUIZ, R. M., COLLINS, S., AND THORPE, T. E. (1971). Maximum discriminability versions of the near encounter Mariner pictures. J. Geophys. Res. 78, 438-472. ELSTON, W. E. (1971). Evidence for lunar volcano-tectonic features. J. Geophys. Res. 76, 5690-5702. ELSTON, W. E., AND SMITH, E. I. (1971). Stratigraphy and classification of Martian terrains photographed by Mariners 6 and 7 (abstr.): Int. Union Geol. and Geophys., Program, 15th Gen. Assembly, Moscow, p. 154. FURON, RAYMOND (1963). “Geology of Africa,” 337 pp. New York, Hafner. HAMMOND, A. L. (1972). Mars as an active planet : The view from Mariner 9. Science 175, 286-287. HERR, K. C., HORN, DIETER, MCAFEE, J. M., AND PIMENTEL, G. C., (1970). Martian topography from Mariner 6 and 7 infrared spectra. Astronom. J. 75, 833-894. LEIGHTON, R. B., HOROWITZ, N. H., MURRAY, B.C., SHARP, R.P., HERRII\IAN, A.G., YOUNG, blown York,
A.T.,SMITH,B.A.,DAVIES,M.E.,ANDLEROY, C. B. (1969). Television observations from Mariners 6 and 7: Mariner-Mars 1969, a preliminary report, NASA, SP-225, pp. 3782. LEIGHTON,R. B., MURRAY, B.C., SHARP, R.P., ALLEN, J. D., AND SLOAN, R. K. (1965). Mariner IV photography of Mars: Initial results. 8cience 149, 627-630. LEIGHTON, R. B., MURRAY, B. C., SHARP, R. P., ALLEN, J. D., AND SLOAN, R. K., (1967).
AND
SMITH Mariner IV pictures of Mars: Mariner Mars 1964 Project Report : Television Experiment, Pt. 1. Investigators’ Report. Jet Propulsion Lab. Tech. Rept. 32-884, 178 pp. MASURSKY, HAROLD and 30 others (1972). Mariner 9 television reconnaissance of Mars and its satellites : Preliminary results. Science 175, 294-305. MURRAY, B.C., SODERBLOM,L. A., SHARP,R. P., AND CUTTS, J. A. (1971). The Surface of Mars 1. Cratered terrains, J. Geophys. Res. 76, 313-330. RINDFLEISCH, T. C., DUNNE, J. A., FRIEDEN, H. J., STROMBERG, W. D., AND RUIZ, R. M. (197 1). Digital processing of Mariner 6 and 7 pictures. J. Geophys. Res. 76, 394-417. SHARP, R. P., SODERBLOM, L. A., MURRAY, B. C., AND CUTTS, J. A. (1971). The surface of Mars 2. Uncratered terrains. J. Geophys. Res. 76, 331-342. SHOEMAKER,E. M. AND HACKMAN, R.J.(1962). Stratigraphic basis for a lunar time scale. In. “The Moon” (Zdenek Kopal and Z. K. Mikhailov, eds.), London, pp. 289-306. Academic Press. SMITH, E. I. (1970). Comparison of selected lunar and terrestrial volcanic domes : Ph.D. dissert. (unpub.), Albuquerque, Univ. New Mexico, 200 pp. SMITH, E. I. (1971). Determination of origin of small lunar and terrestrial craters by depthdiameter ratio. J. Geophys. 76, No. 23, 56835689. SMITH, E. I., AND ELSTON, W. E. (1971). Martian stratigraphy and terrain classification : A basis for the geological mapping of Mars (abstr.). EOS, Amer. Geophys. Union Trans. 52, 263. STEINBACHER, R. H., KLIORE, A., LORELL, J., HIPSHER, H., BARTH, C. A., MASURSKY, H., MUNCH, G.,PEARL, J. AND SMITH, B.(1972). Mariner 9 science experiments : Preliminary results. Science 175, 293-294. WITTING, J., NARIN, F., AND STONE, F.A.(1965). Mars: Age of its craters. Science 149, 14961498.