VOL.
76, NO. 2
JOURNAL
OF GEOPHYSICAL
RESEARCH
JANUARY
10, 1971
The Surface of Mars
i.
Cratered
Terrains •
BRUCEC. MURRAY,LAURENCEA. SODERBLOM,
ROBERTP. SHARP,AND JAMESA. CUTTS
Division of GeologicalSciences
California Institute of Technology,Pasadena 91109
Mariner 6 and 7 pictures show that craters are the dominant landform on Mars and that their
occurrenceis not correlated uniquely with latitude, elevation, or albedo markings. Two distinct
morphological classesare recognized: small bowl-shaped and large flat-bottomed. The former
showlittle evidenceof modifications,whereasthe latter appear generally more modified than hmar
upland cratersof comparablesize.A regionalmaria/uplands dichotomylike the moon has not yet
been recognized on Mars. Crater modification on Mars has involved much greater horizontal
redistribution of material than in the lunar uplands. It is possiblethat there are erosionalprocesses
only infrequently active. Analysisof the natures and fluxesof bodiesthat have probably impacted
the moon and Mars leads to the likelihood that most of the large flat-bottomed craters on Mars
have survived from the final phasesof planetary accretion.Significant crater modification, however, has taken place more recently on Mars. Inasmuch as the present small bowl-shaped craters
evidence little modification, the postaccretion crater-modification processon Mars may have
been primarily episodic rather than continuous.The size-frequencydistribution of impacting
bodiesthat producedthe present small Martian bowl-shapedcraters differs from that responsible
for post-mare primary impacts on the moon by a marked deficiency of large bodies. Survival of
crater topography from the end of planetary accretion would make any hypothetical earthlike
phasewith primitive oceansthere unlikely. The traditional view of Mars as an earthlike planetary
neighbor in terms of its surface history is not supported by the picture data.
Perhapsthe most excitingresult of the Mariner
4 televisionexperiment in 1965 was the discovery
of cratered terrains on Mars, a generally unexpected addition to the annual developmentof
the frost caps, seasonaldarkenings, and other
presumedearthlike phenomena.The television
pictures returned by Mariners 6 and 7 in 1969
greatly extendedknowledgeof Martian cratered
Mariner 6 and 7 television pictures as they bear
on the nature of the Martian surface.The present
paper deals with the nature and significanceof
cratered
terrains.
The
second is concerned
with
uncratered terrains [Sharp et al., 1971a]. The
third paper discussesphotographic observations
relevant to the nature of light and dark markings
[Cutts et al., 1971], and the fourth describes
terrains and showed two new uncratered terrains
observationsand implications of surfacefeatures
as well. Close-up pictures were obtained of the of the south polar cap [Sharp et al., 1971b]. We
south polar cap and also of certain prominent shall refer to them as papers 1, 2, 3, and 4,
light-dark boundaries. These and other results respectively.
of the television experimentswere first discussed
The picture data used are principally the
in three preliminary reports published shortly 'maximum-discriminability' versions displayed
after receipt of the picture data [Leightonet al., in the accompanying papers by Dunne et al.
1969a,b, c]. The objectiveof this paper and of the [1971] in this issue, supplemented by other
three companionpapersis to describethe results versions of the near- and far-encounter photogof subsequentanalysis and interpretation of the raphy. Image-processing techniques are discussedin a separate paper in this issue [Rindfieis& et al., 1971]. Picture locationand notation
i Contribution 1891, Division of Geological
are summarized in the preceding article in this
Sciences, California Institute of Technology,
issueby Leighton and Murray.
Pasadena.
Craters
Copyright •
are visible
in 52 of the
55 near-en-
counter frames (best resolution 0.3 km) and
1971 by the American Geophysical lJnion.
313
314
M•URRAY ET AL.
constitute the principal landform observed. At
this early stageof Martian exploration we define
cratered terrains to be regions of the surface
in which craters are the dominant, often the
only, topographic forms recognizable at the
resolution of the Mariner 6 and 7 pictures.
surface processesand history of Mars inferred
from the evidenceof the terrains already recognized will provide a useful framework for more
detailed information and knowledge to be acquired by future spacemissions.
In the following, the geographic distribution
Intercrater areas are included in this definition.
of cratered terrain is reviewed and possible
As better pictures become available, more correlations,with elevation, latitude, and albedo
sophisticated
criteriamay haveto be developed are considered.Morphology of local features and
to categorizewhat may well be a variety of crater abundances is then treated. Martian
}Martian terrains.We feel, nevertheless,
that the observations are compared with those of the
4N14
Fig. la. Positionsof Mariner 4 photographs4N7 through 4N14 plotted on Mariner 7 farencounterphotograph7F76 with a sub-spacecraft
longitudeof 199øE.The ringedstructure,Nix
Olympica, is visible in the northern hemisphere.
SURFACEOF MARs--CRATERED TERRAINS
315
4N9
Fig. lb. Crateredterraindisplayedin Mariner 4 frames4N7 through4N14, taken across
Zephyria,Mare Sirenurn,
Mare Cimmerium,
and Phaethontis.
(Seepaper3 for identification
of namedfeatures.)Frames4N8 and 4N13 containprominentlight/dark boundaries.
Individual
framesare about 250 km on a side. Longitude,latitude, and solar elevationangle rangefrom
186øE,13øS,61ø for 4N7 through209øE,42øS,29ø for 4N14. Detailsare availablein Leighton
ct al. [1967].
lunar-crateredterrains,leadingto the conclusion
that many largeMartian craters,like thoseof the
lunaruplands,have survivedsincethe last phases
of planetary accretion. The later history of
or dark areas or with any particular elevation
range, and it contains two distinct types of
craters: large fiat-bottomed and small bowlshaped. In this section we summarize the
Mars, however,appearsto differ significantly
characteristics of Martian
from that of the moon. Finally, the traditional
view of Mars as once having experiencedearthlike conditions is recvaluated in light of the
compareit with the crateredterrain of the moon.
Geographicdistribution. Craters ranging in
similarities of the Martian
cratered terrain to the
lunar uplands.
OBSERVATIONS OF MARTIAN CRATERED TERRAINS
Cratered terrain on Mars was first revealed
in televisionpicturesreturned by Mariner 4 in
1965. Mariners 6 and 7 extended the observations
cratered terrain
and
diameter from a few hundred meters to a few
hundred kilometers are visible in the Mariner 6
and 7 photographs.If this sampleis representative, cratered terrain (including intercrater
areas) constitutesat least 90% of the Martian
surface.
Somevery largecratersare recognizable
in
the far-encounter frames of Mariner 6 and 7,
of cratered terrain sufiqcientlyto demonstrate especiallyin the dark area Syrtis Major (frame
that it is the dominant Martian landscape;it is 7F87) and Mare Cimmerium(frame 7F82). The
not uniquelycorrelatedin occurrence
with light
featureNix Olympica,longrecognizedas a bright
316
MURRAY ET AL.
patch from earth-basedobservations,is seenat
Figures3 and 4 of paper2 and Figures1 and 2 of
higherresolution
(frame7F77) to be a multiple- paper 4).
ring structureof maximumdimension
exceeding Figureslb, 2, and3 in thispaperandFigure3
500 kms. Presumablythis is at least the remnant in paper4 are mosaics
composed
of the bestnearof a very large crater. Circular features almost encounterphotographyfromMariners4, 6, and7.
as large are faintly visiblein bright areaswithin They all show predominatelycratered terrain.
frames 7F83 and 7F84. The polar cap edge Each of thesemosaics
is discussed
brieflyin the
observed in the late far-encounter frames of both
context of the regional associationsof Martian
Mariner6 andMariner7 is partlydelineated
by
cratered
largec•atersup to 110 km in diameter(see
terrain.
Craters from 4 to 350 km in diameter are
Fig. 2. Area of mosaicof frames6N9 through6N23, shownin outlineon the far-encounter
frame7F67,i•mluding
the dark areasMeridianiSinusand Sabaeus
Sinusand the light area
l)eucalionis
Regio.(Geographic
namesarelocatedin paper3.) Solarelevationangles,
which
rangefrom 52ø downto 3ø are shownat the lowerright-handcornerof the B frames.The outline
of thesenarrow-angle
framesisshownin whiteonthemosaic
of A frames,alongwithlatitudeand
lo•gitudegridli•tes.IndividualA framesareof the orderof 1000km in the longdirection;
B
framesare l/10 scaleof A frames.One degreeof latitude on Mars is 59 km. Maximum discrim-
inability versionshave beenusedthat greatlyaccentuate
topographic
detail but distortand
suppresssome albedo marking, especially the outlines of Meridiani Sinus. The crater counts
usedi•t Figre'es
4, 5, and6 arederivedfromframes6N16to 6N23andreferprincipally
to the
I)eucalionis Regio area.
SURFACEOF •/•ARS--CRATEREDTERRAINS
317
318
M•RR^¾
visible in Mariner 4 frames 4N5 through 4N14,
as is shown in Figures l a and lb. A prominent
ET
AL.
By way of comparison, on the moon two distinct
kinds of cratered
terrain
are evident
that
bear a-
light/dark boundary,the northernboundaryof simple relationship to height and light and dark
Mare Sirenurn, is included in the pair of frames
markings. On the moon, large, highly-modified
4N?/4NS. Crateredtopographydoesnot appear craters are restricted to the uplands. Mare
to vary
significantly across this boundary.
The
frames from
formation presumably obscuredmost of the older
preexisting craters in the maria basins. So,
nearly all encompasscratered terrain except for whereasthe lunar uplands are rough, elevated,
small areas of chaotic terrain (see Figure 2 of bright terrains containing numerous old fiatpaper 2•. The mosaic of Figure 2 covering the floored craters, the lunar maria are dark, low in
equatorial areas Meridiani Sinus and Deucalionis elevation, smooth, and essentially devoid of
Regio exhibits cratered terrain especially well old highly modified craters. Mariner 6 and 7
and illustrates again that the general character of photographs do not reveal a similar regional
cratered terrain can remain unchanged across associationof Martian craters. First, both light
near-encounter
Mariner
6
light/dark boundaries.
and
dark
areas
of
Martian
cratered
terrain
The near-encounter frames of Mariner ?,
included in the mosaic of Figure 3, also display
principally cratered terrains except for the
contain large fiat-bottomed craters generally
in comparable abundance. The lunar correlation
of large degraded craters with high albedo and
featureless floor of Hellas. These frames show the
uplands is not evident on Mars. Hence, the
presence of craters at southern midlatitudes characteristicassociationof crater form, eleva(20ø to 45øS).
tion, and albedo denoted by the term 'mare' on
The mosaicin paper 4 (Figure 3) demonstrates the moon, is apparently absent on Mars, at
the occurrence of numerous craters under the
least in those areas photographed in the nearsouth polar cap. Thus, cratersare abundant in the encountersof Mariners 4, 6, and 7. Hellas, the
equatorial, midlatitude, and south polar regions large circular area of featurelessterrain discussed
of Mars, evidencing no marked latitudinal in paper 2 of this series, might conceivably
dependence of cratered terrains. The continua- representa mare-like surfaceeven though it is of
tion of cratersacrossthe Noachis/Hellespontushigh albedo. However, the marked absence of
boundary (Figure 3) again demonstrates the any visible impact featureswould require it to be
absenceof any unique correlation between light extremelyyoung.A continuousprocessmodifying
and dark areas and cratered terrain.
the floor of Hellas seemsmore likely.
The distribution
of cratered terrain has been
Topographic relief.
A striking difference
examined for possiblecorrelation with elevation. between the Martian and lunar surfaces is the
A limited quantity of reliable data on regional lower relief of the walls of large craters (diameter
elevation has been obtained from ground-based > 15 km) and of the intercrater areas on Mars.
radar observations[Goldsteinet al., 1970; Lincoln This was first recognizedfrom the Mariner 4 TV
Laboratory, 1970] and from equivalent width experiment [Leightonet al., 1965], although in
variations in C02 absorptionbands measuredby that instance large spurious background light
the onboardinfrared spectrometersof Mariners 6 levels originating from some unknown optical
and 7 [Herr et al., 1970]. Cratered terrain occurs degradation weakened the credibility of this
over a wide range of elevations;no unique cor- conclusion[Young, 1969]. The Mariner 6 and 7
relation with height is evident.
camera systems were free of such anomalous
In summary, the cratered terrain of Mars is effects;the gentlenessof Martian topographyis
the most extensive terrain on the planet. It again indicated.
occurs over a wide range of latitudes and elevaSeveral specificlines of evidenceare available.
tions and is not confinedto either light or dark As is evident in Figure 7a, the horizontal width
areas or to a particular elevation or latitude of walls of large fiat-bottomed craters (diameter
range.
> 15 km) on Mars is generally several times
Although variations in the character of the smaller than the width of walls of similar size
cratered terrain do occur [Cutts et al., 1971], craters on the moon. Thus the relief across such
they are not clearly defined,and the relationships walls must be much less than on the moon,
with elevation, albedo,and latitude are complex. provided that the wall slopeson Mars are not
319
correspondinglygreater. It is clear that such is
not the case because the conspicuousshadows
that would be cast by such steep walls are not
found. In addition, slopesof walls of large fiatbottomed craters can be estimated from oblique
views independently of the lighting conditionsor
photometric properties of the surface. This is
done by assumingthat these large craters are
approximately radially symmetric and by comparing the projected widths of the near and far
walls measuredon a photograph. If the slopesare
low, these apparent widths are nearly the same
over a wide range of viewing angles. If the
slopesare great (i.e., the order of the emission
angle), then the relative apparent widths vary
rapidly with emission angle. Such tests were
made on two large fiat-bottomed craters in the
northeast corner of 6N16 (emission angle
•43ø). Such estimates give slopesof the order of
10ø; certainly they are not more than 20 ø.
Finally, low average slopes on Mars are suggestedby the rather narrow width of the marginal
zone of the south polar cap. As is discussedmore
fully in paper 4, the latitudinal width of that
zone is probably controlled by the magnitude of
crater slopes;average slopesof the order of 5 ø or
less on scale of several kilometers
are indicated
along the polar-cap edge. In summary, the
relief of large fiat-bottomed craters (diameter
> 15 km) on Mars is less, perhaps several times
less, than that found on the moon, and the
average slopes of large crater walls and of
surroundingterrain also may be less than those
found in the lunar uplands. Small bowl-shaped
craters, however, may or may not differ significantly in form from fresh craters of the same
diameter on the moon; that analysis is not yet
complete.
Morphology of craters and associatedfeatures.
Two distinct types of craters are distinguished
in the areasviewed by Mariners 6 and 7 and are
referred to here as small bowl-shapedand large
flat-bottomed craters. No gradation between
highly modified from their presumed initial
appearance as impact craters. Rims are not
detectable or greatly subdued, central peaks are
rare, and other impact-associatedfeatures, such
as secondarycrater swarmsand cjccta blankets,
have been greatly modified, usually beyond
recognition.Some of these craters, here termed
vestigialor ghost,have been so greatly modified
that the relief of their walls is only faintly
visible. Other large flat-bottomed craters are
much more easily recognized. These two states
of preservation are well shown in frame 6N16.
We do not see a clear gradation between these
two types of flat-bottomed crater; they may
reflect a complex episodicphase of early Martian
history.
Small bowl-shaped craters represent the
majority of cratersobservedsofar with diameters
below 10 to 15 kin. These smaller craters exhibit
almost all the associated impact phenomena
found with lunar primary craters of similar
size. The associatedslump blocksand secondary
crater swarms are not visible, as expected, as
these would
be below
the
limit
of resolution.
The small bowl-shaped craters appear to be of
uniform morphology, indicative either of very
little or very uniform modification.
A parallel is found in comparing the crater
populations on Mars with those on the lunar
uplands. Most of the large craters in the lunar
uplands also have suffered significant modification (Figure 7a), as a result of which their
ejccta blankets are no longer recognizableand
secondary crater swarms have been obliterated.
Many exhibit smooth, flat floors. Some have
lost rims and central peaks as well. Also present
in the lunar uplands are abundant fresh small
craters (Figure 7b), which have sufferedlittle, if
any, modification. Thus, both the lunar upland
and the Martian surfacesgenerallydisplaytwo
families of craters:abundant, fiat-flooredcraters
that have been significantly modified, and
smallercratersthat appearrelatively unmodified.
these two types has been observedso far; they However, on Mars both classesshow less variaare discussed separately here. A few ringed tion in the degreeof modification.
structures (6N20) and a considerablenumber of
Important differences do exist between the
polygonal craters (6N13) have been recognized. lunar uplands and the Martian terrains. The
This latter similarity to the moon may have Martian large flat-bottomed craters are less
structural implications.
numerous and more highly modified than those
Large flat-bottomed craters seen in Mariner observed on the moon. The intercrater areas on
6 and 7 photographs range in diameter from Mars are much smoother. Although the large
about 15 km to severalhundred km. They are flat-floored craters both on Mars and on the moon
320
MuRR^¾
must have sufferedperiodsof intensemodification
before the formation of the small fresh craters,
the intensity or period,or both, of suchmodification on Mars, not only must have been greater,
but also must have occurred principally after
the formation of most of the large craters. Otherwise, some large fresh craters would be easily
recognizable in the Mariner pictures, just as
Copernicus,Tycho, and Kepler are conspicuous
,,iO
4
'
I
'
I
6N,48,20
AND
22
4000
on the moon.
Martian cratered terrains also display a
variety of positive and negative local features
other than craters. Included
400
z
<•
.1-
are sinuous channels
?
6 N ,'17,'19 AND 21 AVERAGED
and ridges visible in many B-camera frames
(see, for example, 6N16, 6N18, and 6N20). A
series of subparallel short linear markings, in
some instances composing polygonal patterns
similar to those of the moon, is visible in the
dark area, SabaeusSinus, in the northern parts
4O
<
of 6N19 and 6N21.
,
Certain diagnosticlunar and terrestrial surface
features were looked for but not recognizedin
the Mariner pictures. Especially conspicuousis
the absence of fresh large craters and their
associated rays and secondary swarms. Such
craters, like Tycho and Copernicus,are among
the more striking featuresobservedon the moon.
Sinuousrilles, flow fronts, and partially flooded
craters
that
characterize
the lunar
maria
have
not been observed. Finally, regional terrestrial
featuresresemblingfoldedmountains, islandarcs,
continental/oceanicplates, and rift valleys are
not recognizedon Mars.
Size-frequencydistribution of Martian craters.
Cratered terrain on the moon traditionally has
been described according to the number of
craters per unit area as a function of diameter.
In Figure 4 the cumulativenumber of cratersper
unit area larger than a given diameter is plotted
against that diameter for the Martian area
Deucalionis Regio. The relevant data are listed
in Table
1.
.,f
SBC
I
,
4
CRATER DIAMETER
-
LF
I
,1o
,1oo
D (KM)
Fig. 4. Crater abundancesof Deucalionis Regio.
The cumulative crater-diameter relationship is
plotted by using the numerical data of Table 1.
The vertical axis is a logarithmic scale of crater
diameter. The range of crater diameters for small
bowl-shaped and large fiat-bottomed craters,
respectively, is indicated approximately by arrows
overlapping in the range of 10-15 km. Error bars
have been derived on the basis of the expected
statistical uncertainty in the actual number of
craters counted. Thus, the errors are largest for
the largest (and therefore fewest) craters counted
in either
A or B frames.
any genuine geographic variation or small
solar-elevation angle effect.
It would be desirable to have size-frequency
data separately for the large flat-bottomed and
small bowl-shaped populations. Unfortunately,
the resolution of the A frames is inadequate to
confidently separate the two classesin the important 5-15 kilometer range, and the areal
coverageof the B frames, which do possessadequate resolution, is insufficient to provide adequate crater counts in the same interval, as
shown by the error bars in Figure 4. Thus we
have included both populations in Figure 4 and
merely note on it the approximate size ranges of
Figure 5 shows the crater counts for the
individual A and B frames that are averaged in
Figure 4. Figure 5 demonstratestwo aspectsof
the data: (1) major geographicvariations in the
density of large fiat-bottomed craters are not
present within Deucalionis Regio, nor are the
apparent densities correlated with either solar- the two classes.
elevation angle or A-camera filter, and (2) the
statistical
noise in the measurements
of small
bowl-shaped craters is large enough to mask
MARTIAN
The
SURFACE PROCESSES AND HISTORY
observed
characteristics
of the Martian
SURFACE OF MARS--CRATERED
TERRAINS
321
crater terrains have been compared with those
of the uplands and maria of the moon. This
TABLE 1. Martian Crater Abundances (I)eucasection examines the implications of this comlionis Regio Area)
parisonfor Martian history and surfaceprocesses.
Review of lunar impact history. A valuable
Diameter
Cumulative
Interval,
Number Cumulative Number/10 6 approach to understanding the evolution of the
km
Counted
Number
km 2
lunar surfacehas been the analysis of crater sizefrequencydistributions.These crater populations
Average of Wide-Angle Frames 6N17, 6N19,
record the energy-frequency distributions of
and 6N21
impacting bodies (i.e., the mass-frequencydis< 4 1
579 + 24
3
25o 4- 10.3
tribution for given impact velocities) as well as
4 245
7
572 + 23
247 4- 10.2
4 65 0
34
538 + 23
233 4- 10.0
the net effectivenessof crater-removal processes.
5 15 5
46
492 + 22
213 4- 96
Analysis of surfacesof differingage can elucidate
5 61-
6 0
35
1-
7 0
8 0
9 0
10 0
58
25
28
34
6
7
8
9
10
111-
11 0
20
11
1-
12
1-
13
15
18
1-
12
13
15
18
20
24
30
35
40
45
50
60
70
70
16
13
31
29
22
33
37
29
12
13
16
17
19
5
1121-
25313641465161-
0
0
0
0
457
399
374
346
312
+
+
+
+
4-
21
20
19
18
17
292 4- 17 1
276 4- 166
263 4- 16 2
232 4- 15 2
203 4- 14 2
181 4- 13 5
148 4- 12 2
111 4- 106
82 4- 9 1
70 4- 84
57 4- 75
41 4- 64
24 4- 49
22
54-
198
172
162
150
136
126
119
114
100
88
78
44444444444-
93
86
404
84
8 1
77
74
72
70
4000
66
62
58
64 4- 52
48 4- 45
35 4- 38
30 4- 36
25 4- 33
18 4- 28
2 1
1041
2.24-
400
--
4000
'
i
0
A vetageof Narrow-A ngle Frames 6N18, 6N20,
and 6N22
<
0 560 66-
0
0
0
1
1
i
768696061626-
1 41-
1 712
16-
23142 713.64.65.87.91021-
0 55
11
0 65
0 75
0 85
0 95
1 05
1 15
1 25
1 4o
1 70
2 15
2 30
2 7o
35
45
57
78
9.9
20
27
27
3
16
10
12
10
4
8
8
12
8
6
2
2
3
3
1
2
1
i
113
110
94
84
72
62
58
5O
42
444444444-
10.6
10.5
97
92
85
79
76
71
65
30 4- 55
22 4- 47
40
37
12 4- 35
9430
6425
22
544420
1 4
24-
164144-
140
1
5600
55OO
4700
4200
4444-
530
52O
480
450
3600
3100
2900
2500
2100
1500
1100
800
44444444-
420
390
380
356
320
270
230
200
400
'•.
40
700 4- 190
6OO 4- 170
450
300
250
4- 150
4- 120
4- 11o
200 4- lOO
lOO 4- 70
50 4- 50
6N
25
_
"•xx6N49
•
'10
CRATER DIAMETER
4OO
D (KM)
Fig. 5. Plots of crater abundances similar to
those in Figure 4 are presented for individual
Mariner frames. The narrow-angle B frames (top)
show considerablevariation in the large craters,
mainly becauseof the limited number of craters;
it is also possible that some minor geographic
variations
in
crater
abundances
are
included.
Neither A nor B frames show any correlation with
solar elevation angle, thus ruling out a potential
source of systematic error.
322
MURRAY ET AL.
temporal variations in impact fluxes and in
surface processes.The size-frequency distributiens of lunar craters and the impact-flux histories implied for the earth-moon environment
are reviewed here as background for the discussionof Mars.
Figure 6 shows typical crater size-frequency
distributionsfor lunar surfacesof differing age:
two maria and two upland regions. The crater
populations on the younger surfaces,the maria,
are less complicated by historical variation in
impact flux, and it is appropriateto begin the discussionthere. Craters of diameter larger than
about 3 km are generally attributed to primary
impacts [Shoemaker,1965]. Such craters appear
sharp and fresh, with raised rims, rays, ejecta
blankets, and surrounding secondaries;in the
larger size range, they exhibit central peaks.
Although the magnitude of this primary crater
distribution may vary significantly between
maria of different age, the form of the distribution remains nearly unchanged[Kuiper et al.,
1966; Trask, 1.966].For.example, Trask found
that the exponentsof the powerfunctionsrepresentingthe size-frequencydistributionsof craters
at the Ranger 7 and 8 sites are very nearly the
same,whereasthe total number of cratersin this
size range (diameter >3 km) differ by about a
factor of 3. Thus through the periodof the evolution of the lunar maria the form of the mass-frequencydistribution of impacting bodieshas been
constant.
It is possible to predict the population of
secondaryimpact craters to be expected with
this primary distribution. By studying the
secondary populations of nuclear craters and
large lunar primaries, Shoemaker [1965] and
Brinkmann [1966] predicted that the secondary
distribution will have a greater ratio of small to
largecratersand shouldexceedthe primary popu-
•,0 5 ,
MARS COMPARED WITH
LUNAR
o
,104 _
MARS COMPARED WITH
MARIA
LUNAR
'
UPLANDS
•-RANGER
'•nT
•
MARE TRANOUIL-
n.'
Lu
o
LU '1000
LITA TIS
-
--
SOUTHPOLAR
REGION
-
'
'z.
r•
.
Z
••
•00 --
LU
_
•
-
•-
.4
RANGER•IT
MARE
-
CO GNI TUM
4
'
40
TSIOLKOVSKY
'100
CRATER
DIAMETER
'I
REGION
dO
'•00
D (KM)
Fig. 6. The Deucalionis Regio data of Figure 4 are compared with crater abundances of
the lunar maria (left) and the uplands (right). The Ranger 7 and 8 data are from Trask [1966].The
uplands data were compiledfor this paper from Orbiter 4 frame 88, taken in the vicinity of the
south pole, and Orbiter 3 frame 121 from the lunar backsidenear Tsiolkovsky, chosenbecause
they appear to be free of effects of Mare formation.
8UIIFACE OF MAIls--CIIATERED
lation
for craters
of diameter
smaller
TEIIItAINS
323
than about
the moon. (We shall ignorethe possiblecomplexities ill the actual events that may have constituted the terminal phasesof accretionand the
immediate period thereafter. We presume a
smooth and ral•id decline of cratering rates at
the end of accretion of both the moon and Mars.)
Below diameters of about 100 meters the rate
To return to the significanceof the Martian
of increaseof lunar crater density with decreasing crater abundances, a comparison of size-frediameter falls to a lower value than expected for quency distributions of craters in Deucalionis
a secondarypopulation. Craters in this size class Regio and in the lunar maria and uplandsis also
rangefrom fresh and pristine to nearly obliterated presentedin Figure 6. Martian cratered terrain
shallow irregular depressions.This distribution and the lunar uplandsboth displaylarge subdued
of morphologies has led observers to conclude craters and small bowl-shapedcraters. The overthat these smaller craters form a steady-state all forms of the distributions are similar. Howpopulation in which craters are formed and ever, the size-frequencydistribution of the Mardestroyed at the same rate [Moore, 1964; Shoe- tian small bowl-shaped craters is significantly
maker, 1965]. Soderblom[1970] has shown that different from that of comparably sized craters
the relationship between the steady-state popu- on the moon. The implications of these similarlation and the secondary production curve can ities and differences are considered in the next
be derived analytically from a model of down- two sections.
slope transport of material by impacts that
Age of largefiat-bottomed
Martian craters. The
produce negligible individual topographic age of large fiat-bottomed Martian craters can
changes.
be estimatedby extrapolationof lunar cratering
In summary, the various segmentsof the crater history. Lunar inpact fluxescan be extrapolated
frequency curves for craters in the lunar maria to Mars on some basis, and the Martian surface
are related in recognizableways. Primaries larger ages obtained by scaling with known surface
than a few kilometers produce abundant secon- ageson the moon. This was first attempted by
daries smaller than 1 kin. The erosive action of
Anders and Arnold [1965]. A more complete
small secondaries (diameter •10 meters)on
analysis was performed by Hartmann [1966]
other large secondariesproducesa steady popu- using Mariner 4 crater counts. Hartmann's
lation in which craters are generated and ob- analysis indicated that these Martian craters are
1 kin. The steep part of the maria curves, for
craters smaller than 3 kin, is compatible with
these predictions (Figure 6). The distribution
of large telescopically observable secondaries
supports this hypothesis [Shoemaker, 1965].
literated
at the same rate.
Interpretation of the size-frequency relationship of craters in the lunar uplands is complicated
by superpositionof the later impact history recorded in the lunar maria on earlier cratering,
which produced the very numerouslarge highlymodified craters. Under the assumption of constant flux rates throughout the history of the
moon, the relative densities of large craters
(% 20 km in diameter) on the ,•4-b.y.-old mare
surfacein Mare Tranquilitatis [Albeeet al., 1970;
Silver, 1970] and in the lunar uplands would
imply an age for the uplands greater than 50 b.y.
This unrealistic age estimate, coupled with the
observedsevere modification of the large upland
craters leads to two conclusions:(1) the dense
population of large craters in the lunar uplands
records a period during which the cratering rate
was orders of magnitude greater than has occurred sincethe formation of the maria, and (2)
the uplands represent the accretional surface of
at least 4 d- 2 b.y. in age. We argue here that
Hartmann's result is a significant underestimate
of the age of thesecraterswhen reviewed in light
of current
information.
Hartmann
assumed that both the lunar and the
Martian impact fluxes were entirely asteroidal
and used a previous estimate by Anders [1964]
of asteroidal fluxes at Mars higher by a factor of
25. This factor of 25 is probably grossly overestimated, as indicated by a variety of reasons.
First, Wetherill [1968] has shown by dynamical
arguments that the probability of Mars being
impacted by an asteroidal object whose orbit
was perturbed by Mars is very low. This consideration significantly reduces the figure of a
factor of 25. Second,the assumptionthat the
impacting debris would preferentially impact
Mars rather than the moon by a factor of 25 is
not
consistent
with
Studies of meteorite
meteorite
observations.
orbits from observations
of
meteor trails have shown that most of the debris
324
•URRAY
ET AL.
with or postdating the formation of these large
craters.Both showa collectionof younger,fresher
impacting;the earth at present is in highly eccentric orbits with aphelion distances of 4 to 5 AU
[Wetherill, 1969]. Objects in such eccentric orbits
would have almost equal likelihood of striking
Mars and the moon. Furthermore, recent evidence of decreasing impact fluxes during the
period of lunar formation (L. A. Soderblom and
L. A. Lebofsky, unpublisheddata, 1970) suggests
that a significant fraction of the lunar crater
population was produced by cometary impact,
or at least by a sourceother than asteroidal.An
asteroidal impact flux would tend to increase
major surfacemodification.Thus, not only are the
Martian crateredterrains unquestionablyancient,
but they mimic to some extent the sequenceof
events recordedon the lunar uplands. We feel it is
probable, therefore, that both the Martian cratered terrain and the lunar uplands record the
final stagesof planetary accretion.
Differencesin post-accretionalhistories. Major
modification of large fiat-bottomed craters on
with time [Kuiper, 1950; •pik, 1951].Further,
both the moon and Mars must have occurred be-
as will be shown in the next section, the distributions of objects forming the present small
craters (diameter • 10 km) on Mars and on the
craters that
have accumulated
since the end of
fore formation of the present relatively unmodified small crater populations on those surfaces.
Small bowl-shaped craters on Mars appear commoon have been differefit. Hence this factor of
parablein degreeof modificationto cratersof the
25 must be a significant over-estimate, since samesize (diameters1-10 km) on the lunar maria.
impact fluxes on Mars and on the moon cannot A comparison of these two distributions should
both have been entirely asteroidal. The true expose differences and similarities in the size
ratio may be closer to unity, since objects in spectra of impacting objects that have created
cometary orbits have almost equal probability them.
of striking the earth and Mars. Finally, the
The size-frequencydistribution of primaries
large fiat-bottomed Martian craters have been in the lunar maria can be written
severelymodified, and it is likely that many are
N = AD -1'7
no longer rocognizablein the Mariner pictures.
Hence, crater counts are on the low side com- where N is the number of craters per unit area
pared to counts on the moon. All these arguments with diameters larger than D and A is a constant.
tend to increase the Hartmann-model age esti- Thus, for example, the ratio of the numbers of
mate to something far exceeding the age of craters larger than 3 km to the number larger
the solar system. This, of course, is under an than 30 km for the lunar maria is about 50.
assumption of constant flux. The implication is, From Figure 4, the number of small bowl-shaped
then, that these large fiat-bottomed craters on craters larger than 3 km in each A frame (area
Mars were formed, as were those in the lunar •0.6 X 106 km '•) is about 400. Hence, if the
uplands, during the early history of the planet proportion of 3- to 30-km Martian bowl-shaped
under very great fluxes. As in the case of the craters were the same as the proportion on the
moon, we associate such conditions with the moon, one would expect to see about 8 fresh
final phasesof planetary accretion.
bowl-shaped craters larger than 30 km in each
A second set of arguments also suggeststhat A frame. Or, stated differently, 1 in 5 of the
the large fiat-bottomed craters on the moon and large fiat-bottomed craters in Deucalionis Regio
Mars record analogousstagesof planetary evo- that are larger than 30 km in diameter should
lution. The sequenceof early eventson Mars and be as fresh and sharp as the small bowl-shaped
on the moon were evidently similar. Both sur- craters and should appear similar to Kepler and
faces show an old, now-subdued population of Copernicuson the moon. Of the approximately
numerous large craters that are ancient by any 100 large fiat-floored craters in DeucalionisRegio
scaling, although the Martian population has with diameter greaterthan 30 km, noneresembles
been more severely modified and no longer dis- the large youthful lunar craters.
plays the saturation that is seen in the lunar
In order to explain this evident deficiency,one
uplands. The size-frequencydistributions of the might presumethat large Copernicus-likecraters
lunar and Martian large craters are similar in on Mars rapidly assume the morphology of
both amplitude and form. Both show evidenceof large fiat-floored craters. It is indeed conceivable
major epochs of surface modification concurrent that the floors could quickly becomefiat. It is
S•TRr^c•
or M^Rs--C•^T•D
difficult, however, to imagine how rims, ejecta
blankets, and secondariescould be obliterated
without destroying the presently observedsmall
bowl-shapedcraters. It doesnot seem likely that
the large craters of a contemporaneouspopulation could be severly altered without even more
severly modifying the smaller ones. Thus, the
distribution of impacting bodies that formed the
presently observedsmall bowl-shapedcraters on
Mars is deficient, by comparison to the lunar
distribution, in objectsthat would producecraters
like Copernicusand Kepler. That is to say, the
postulated Copernicus-type craters and the
presentsmall bowl-shapedcratersare not part of
the same population (not that there never were
any Copernicus-typeevents).
Another observation supportingthe conclusion
that recent Martian fluxesrecordedby the small
bowl-shaped craters were deficient in objects
capable of forming a Copernicus-typecrater is
the lack of associatedsecondarycraters. If the
Martian primary distribution were similar to the
lunar one, there would be a secondary population much like that represented by the steep
sections of the crater-frequency relations for
Mare Tranquilitatis and Oceanus Procellarum
(Figure 6). The secondarydistribution produced
by the observed
small bowl-shapedcraters,however, shouldexceedthe density of primary craters
only for crater diameters less than 100 meters
which is below the limit
of resolution
in Mariner
1969 pictures. The absenceof not only Copernicus-sizedfresh Martian craters, but of their
associated secondariesas well, reinforces the
important conclusion that the distributions of
impacting objects that produced the present
Martian small bowl-shaped craters and the postmaria lunar craters have been distinctly different.
Two possibleexplanationsof these differences
in accumulated impacts can be suggested.First,
the populations of impacting bodies might be
composedof several families of objects (e.g., asteroids and comets) present in different relative
abundancesat Mars and at the moon. A second,
alternative possibility is that the ratio of cometary to asteroidal impacts has not been significantly different in the vicinities of Mars and
the moon at any particular time but that this
ratio has been changing over time. Thus the two
surfaces now record accumulations
over different
time scales (this could result, e.g., in moon
T•^•s
325
averagesover abundant comets and later asteroids and Mars averagesonly over the later asteroids). Both kinds of effects may be responsible
for the differences in the observed distributions.
The absence of Copernicus-like craters in
Deucalionis Regio indicates still another important
difference
between
lunar
and Martian
his-
tory. Copernicus,and Kepler, for example, presumably were formed on the moon by either
cometary or asteroidal impact. If cometary, approximately the same number of such craters
shouldhave been formed on Mars as on the moon,
since comets are typically in highly eccentric
orbits. If Copernicusand Kepler are of asteroidal
origin, there may have been a greater number of
suchimpactson Mars than on the moon [Anders
and Arnold, 1965; Witting et al., 1965; Baldwin,
1965]. Either way, such craters must have
formed on Mars in at least as great a number as
are now evident in the lunar maria and at about
the sametime. Thus, 10-20 of the approximately
250 large flat-bottomed craters in frames 6N17,
6N19, and 6N21 must be no older than Mare
Tranquilitatis (•4 X 10• yrs). Yet all appear to
have suffered about the same degree of modification, or we shouldrecognizethe younger fraction easily. Hence major modification of the
large Martian craters must have occurredafter
the period of mare formation on the moon, i.e.,
within the last 3 to 4 b.y.
Martian crater-modificationprocesses. Martian crater morphologiesreflect crater-modification processes;therefore, they can supply
insight into the nature of those processes.
Small
bowl-shapedcraters on Mars all display about
the samedegreeof preservation.Those observed
appear fresh, having sufferedonly minor modification,if any. However,any earliergenerations
of Martian bowl-shaped craters have all been
modified beyond recognition. Two possible
explanationsfor these relationshipsmerit examination: (1) episodicsurgesof crater formation
coupled with continuousmodification, and (2)
episodesof catastrophic crater removal superimposed on an approximately continuous rate
of crater formation. It is difficult from present
information to evaluate confidently the relative
importance of these two possibilities.However,
the first appearsto us lesslikely for two reasons.
Studies of the chemical nature and cosmic-ray
exposure ages of terrestrial meteorite falls
[Anders,1964] suggestthat as many as 5 to 20
326
Mmm^¾ ET AL.
fainilies of meteoritic fragments have been producedby collisionsin the asteroidbelt in the last
few hundred million years. Theoretical analyses
of the lifetime of thesefragments[Wetherill, 1967s
Hartmann, 1968] indicate that the half-life for
such families is of the order of l0 s to 10 9 years.
Thus the lifetimes are much longer than the
intervals between the production events. Itcrice
it seemsunlikely that such surgesof impacts of
asteroidal objects have occurred on Mars (although this restrictionmay not apply to cometary
objects). Overlap of differing families would lead
to a distribution in the state of preservation of
small bowl-shaped craters. Secondly, a continuous erosionprocesscapable of rapidly removing
traces of all preexisting small bowl-shaped craters
would probably eventually obliterate the large
craters as well. Infrequent episodes of crater
removal, however, might serve to remove large
numbers of small craters without wearing down
the la.rge fiat-bottomed craters significantly.
Finally, in view of the unusual and recent surface
modificationprocesses
implied by featurelessand
chaotic terrains (seepaper 2), we prefer to place
the emphasis of explanation on hypothetical
episodesof crater removal rather than on hypothetical episodesof formation.
The nature of Martian surface processescan
be further evaluated in the light of another
significant observation. As illustrated in Figure
7a, intercrater areas in Martian cratered terrains
are much smoother than in the lunar uplands
Fig. 7a. Lunar simulation of Mariner A frame. To illustrate the morphology of the lunar uplands at the same surfacescaleof the Mariner 6 and 7 wide-angleframes, a portion of medium-resolution frame 130 of the Lunar Orbiter Mission 4 has been enlarged so that the scale of the left,hand sideof the frame approximatesthat of the accompanyingMariner frames 6N17, 6N19, and
6N21 (upper right,,lowerleft, andlowerright, respectively).Becauseof the smallerradiusand therefore greater curvature, of the moon, complete simulation of wide-angle frames is not possible.
The solarelevationanglein the lunar frame rangesfrom 50øto 14ø, thereby coveringmost of the
rangeof the three Mariner frames.The contrastof the lunar frame hasbeenartificially modified to
resemblethat of the Mariner frames.The lunar area is in the vicinity of the south pole, an upland
region of the moon that has not,been severelyaffectedby adjacent,mare formation and filling.
SURFACE OF MARS--CRATERED
TERRAINS
327
Fig. 7b. Lunar simulation of Mariner B frame. A similar procedure to that of Figure 7a has
been carried out to match the surfacescaleof Mariner narrow-angleframes 6N18, 6N20, and 6N22.
The solar elevation angle of the lunar area is 22 ø and those of the Mariner frames are 38 ø, 26 ø,
and 14ø, respectively. The lunar simulation is taken from Orbiter 4 high-resolution frame 107
(longitude 6ø, latitude -42ø).
Both are thought by us to represent original been fundamentally different from those operaccretionary surfaces, but the Martian surface ative on the moon.
has undergonea much greater degreeof leveling
A natural inclination to call on solely aeolian
and horizontal redistribution
of material. This
processesto effect a regional redistribution of
processhas causedthe disappearanceof elevated materials on Mars is complicatedby the following
crater rims and ejecta sheets,yet has permitted considerations.Very effective physical or chemthe survival of what is apparently primary relief ical weathering of consolidatedrock is required
along crater walls (see 6N18, Figure 7b). Local to reduce original relief of hundredsof meters or
processesmodifying lunar terrains •re impact kilometers completely into dust and sand-sized
fragmentation and downslope transport by particles that could then be effectively transsliding, rolling, and impact ejecta, These proc- ported by the Martian winds. By comparison,
cessestend to smooth out local topographic the dominant fragmenting processon the moon
irregularities.If the Martian modificationprocess results from impact of high-velocity particles.
of redistributingmaterial dependedprincipally As pointed out previously [Leiqhtonet al., 1967],
on localimpact events,as on the moon,it would the effectivenessof this processon Mars would
seem that local relief along crater walls should be greatly reduced by the thin atmosphere.
have been smoothed out before the intercrater
Inasmuch as liquid water cannot exist in a free
areas were leveled. Hence, the processeswhich state on Mars owing to the low surfacepressures
have modified the surfaceof Mars may have [Inqer8oll,1970], and that thermal shattering is
328
MURRAY
ET
AL.
most likely ineffective [Ryan, 1962], the mech- Kuiper, 1952, pp. 3, 343-444; Shirk et al., 1965;
anism of effective weathering on Mars remains National Academy o[ Sciences, 1966a, 1966b;
unknown.
Shklovskii and Sagan, 1966]. The pictures from
Episodic crater obliteration might be related Mariners 4, 6, and 7 now provide a concrete
to occasionalbut extreme changesin the nature framework for Martian surface history. The
of the Martian atmosphere. Thus the Martian
survival of large Martian craters for, at the very
terrain, like that of much of the westernUnited least, severalbillion years,and indeedmost probStates where the odd thunderstorm is the major ably from the end of planetary accretion, places
eroding event, may display the effects of ero- strong constraintson the surfacehistory of Mars
sional processesthat are inactive during average relevant to the possibilityof any earthlike period.
conditions.
Unlike earth, neither crustal deformation nor
Summary. Comparison of regional associ- atmospheric erosion has been sufficient to
ations, morphologies, and size-frequency dis- destroy these ancient Martian topographic
tributions
of craters on Mars and on the moon
features. Thus there is, for Mars as well as
suggests the following conclusions concerning for the lunar uplands, the implication of a
stable crust and the absence of an earthlike
Martian surfacehistory and processes:
aqueous
atmospherewith great erosionalcapabil1. The processof mare formation that has
generated unsaturated and relatively unmod-
ifiedcratered
te)rainonthemoonsubsequent
to
accretion
is not evident
on Mars.
ities.
It is difficult to be precise as to exactly how
'lunarlike'
or 'non-earthlike'
Mars
must
be in
2. Many of the large fiat-bottomed craters order to satisfy present observations. At the
on Mars probably have survived since the final very least, it can be argued that the most probableearly history of the surfaceof Mars is much
stagesof accretion of that planet.
like that of the moon. Two kinds of evidence from
3. The large craters on both Mars and the
moon record intense modification. However, the Mariner 6 and 7 picture analysis support this
lack of large fresh Martian craters, comparable conclusion.The first pertains to the magnitude
to Kepler and Copernicuson the moon, implies of and responseto crustal deformation. Like the
that the substantial large-crater modification on lunar uplands, the Martian cratered terrain has
Mars took place during the last 3-4 b.y. after been exceedinglystable for cons, undisturbed by
major modification had ended on the moon. earthlike tectonic activity. Further, the strucHowever, no significant modification process tural properties of the lunar and Martian crusts
appears to have been operative during the inter- must be similar to account for the corresponding
val of the accumulation of the present small development of polygonalizationof large craters
on both
surfaces.
The
second kind
of evidence
bowl-shapedcraters.
4. The average distributions of impacting supporting a 'lunarlike' Mars is the strikingly
bodies that formed the currently visible small similar sequence of early events recorded on
craters on Mars and of the lunar maria have been
both bodies. The
Martian
cratered
terrain
and
the lunar uplands preserveremnants of old crater
populationsthat are similar in morphology,areal
5. Modification
of the surface on Mars has
been accompaniedby much greater horizontal density, and size-frequency distribution, and
that are highly modified. Both surfacesrecord a
redistribution
of material than has occurred on
stage of initial high-impact flux followed by
the moon.
intense surface modification that grossly deCOMPARISON WITH THE SURFACE EVOLUTION
graded craters and ceased before the accuOF EARTH AND Moon
mulation of most of the presently visible small
There has been a traditional interest in Mars as bowl-shaped craters. These similarities strongly
a possibleabode of simple life forms. For many suggest that the near-surface crustal environdecadesprecedingspacecraftinvestigation,Mars ment of Mars has been lunarlike since its earwas viewed as possibly having experiencedan liest history. There is no present evidenceof any
earthlike phase with aqueous atmosphere and terrestrial phase or processes.Thus, there is
possibly primitive oceans in which simple life little more basisfor postulationof ancientoceans
forms might have evolved [de Vaucouleurs,1950; on Mars than on the moon. Brief earthlike pcdifferent.
SURFACE OF MARS--CRATERED TERRAINS
riods cannot be rigorously ruled out for either
body, but sucheventsare lesslikely for moon and
Mars than for Venus, or perhaps even Mercury
whose surface history is not so constrained by
observations
329
Institute of Technology. Cutts has been partly
supported by NASA-105-69836 and Soderblom
by NGL-05-002-003.
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