viscous type or her lunar/mercurian type. Mouginis-Mark also found strong correlations of crater type occurrence with geologic unit. Secondary craters seem ...
NOTICE
THIS DOCUMENT HAS BEEN REPRODUCED FROM MICROFICHE. ALTHOUGH IT IS RECOGNIZED THAT CERTAIN PORTIONS ARE ILLEGIBLE, IT IS BEING RELEASED IN THE INTEREST OF MAKING AVAILABLE AS MUCH INFORMATION AS POSSIBLE
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Planetary Geological Studies Final Report Contract NASW-3208
Karl R. Blasius 28 February 1981 s -^i J#
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1
1 we have assembled a global data base for study of Mars crater ejecta morphology. Here we review some of the background of this work, describe some of our preliminary efforts to check the data base with ftiindings of other workers, and layout additional investigations. PREVIOUS STUDIES Craters with unusual ejecta morphology were first described in detail by Carr et al. (1977) in an early publication arising from the Viking Orbiter mission, although suggestion of such features had been discussed earlier by Head ( 1976). The first global inventory of crater ejecta morphology on Mars was compiled by Johansen (1978 and 1979). From an analysis of this inventory she concluded that the apparent viscosity of ejecta flows is correlated with latitude so as to suggest that variable content of liquid water and ice is responsible for morphology variations:
' s
a.
Ejecta deposits similar to those of the Moon or Mercury (presumed, by Johansen, to be emplaced ballistically and drys' are most common near the equator ( + 25 0 latitude and very rare beyond 400 lati t^ude.f
b.
Ejecta deposits with the lowest apparent viscosity, lobate structures with liaised distal ridges, are co$centrated within +30 latitude and absent beyond 60 latitude.
c. Ejecta deposits with intermediate to high apparent viscosity, massive or multilobate structures with marginal scarps of variable slo pe, and sometimes having an aureole of a type ( b) de p8sit, are most common at higher latitudes than +30 . d. The largest diameter ejecta flows, compared to crater rim diameter, are found in the two polar regions.
)
Observations 3. to c. are explained qualitatively by Johansen (1978) with a model of Mars having near surface liquid water and dry materials in warm equatorial regions and subsurface ice progressively thicker and nearer the surface at higher latitudes The extremely broad ejecta deposits in polar regions are not explained. As interesting as Johansen's results are, they cannot be taken as definitive. The study was carried out as a preliminary survey. No record was kept of the number or individual sizes and types of craters examined in each area, only the presence of one or more of six types was noted. The relative proportions of the crater types are un'-_ •:c •..n except where only one type was present. In the statements a ,through d. above, the latitudinal abundance of a crater type is based upon the proportion of regions examined which exhibited that crater type at any level of areal density greater than 0. The results of Johansen's study have stimulated us and others to undertake more thorough studies, documenting our data sets more completely and recordinq additional data on environmental parameters which might affect ejecta morphology. Mouginis-Mark (1979) performed such a study with a global sample of 1558 martian craters. He examined the relationship of ejecta morp hology to crater diameter, latitude, altitude, and target material, assigning each crater to one of six morphology types. These classes are different from those of Johansen but share many common features. Mouginis-Mark (1979) found no strong latitudinal variation in the occurrence of any class of craters, except that small craters with very extensive ejecta blankets seemed concentrated in polar regions. a
The sizes of craters in the six classes of Mouginis-Mark are very different. The diameters of the polar craters with very extensive ejecta are, on average, much smaller than type: of craters corresponding to Johansen's (1978) intermediate-tohigh viscosity ejecta craters and those in turn seemed significantly smaller than classes corresponding to Johansen's least viscous type or her lunar/mercurian type. Mouginis-Mark also found strong correlations of crater type occurrence with geologic unit. Secondary craters seem more abundant on younger lava flows, ejecta de p osits with strong radial lineations were most common on Tharsis and El y sium lavas, and the small craters with extensive ejecta blankets seemed most abundant on "fractured terrain, old lavas, and channel materials".
3
Having recorded both crater diameter and maximum ejecta radius for each crater, Mouginis-Mark (1979) was able to compare the mobility of ejecta with parameters such as latitude, altitude, and geologic unit. He found ejecta travels farthest from crater rims at low altitudes and high latitudes. Mouginis-Mark concludes that the relationship of ejecta mobility to altitude and latitude suggests varying proportions of volatiles in the martial crust may control viscosity of ejecta. No explanation is offered for the varied morphological classes of craters except that geologic unit characteristics of some kind seemed -:o control some aspects of morphology. Mouginis-Mark (1979) acknowledges that his too is a preliminary analysis of an undersized data base. He plans to expand the data set to reduce strong geographical clustering due to clustering of high resolution Viking imaging. There it a limit to such expansion, however, since enormous regions of Mars have not been photographed at better than 150m per pixel resolution, characteristic of data used for most of his analysis. A more serious issue is the selection of six crater classes used for the study. No particular arguments are offered as to why these are a more appropriate set of classes than those of Johansen (1978) for example. More consideration of crater morphology classes might turn up a system which would yield stronger correlations with latitude within the chosen set of 1558 craters. Allen (1979) also reports a global survey of crater ejects morphology similar to Johansen's (1978). He reports that rampart craters (apparently meaning with ejects de posits having well-defined ridge-like terminations) occur at all latitudes, on all types of terrain, and over a wide range of altitudes. His results appear to be more consistent with those of MouginisMark than those of Johansen, but the lack of information on the relative proportions of different types of craters in individual areas makes direct comparison difficult.
4
Other crater ejecta morphometry studies (Mouginis-Mark, 1978; Mutch and Wornow, 1980) have uncovered evidence that ejecta flows of Martian craters may have a characteristic thickness independent of crater diameter. This is a very exciting result tending to confirm the viscous flow model of ejecta emplacement first suggested by Carr, et al. (1977). These results were derived from samples of craters from a small fraction of the surface of Mars so they are tentative, pending confirmation from a more representative sample from varied latitudes, altitudes, and geologic units.
F.
APPROACH
The craters studied by us were classified as to morphology using individual photographic prints of Viking Orbiter frames. Positional and scale information were derived by fitting digitized mosaic coordinates to latitude-longitude coordinates of surface features (geographic control) from the Mars geodetic control net (Davies, et al, 1978) and feature coordinates from the U.S.G.S. series oT-1:9,000,000 scale shaded relief maps. The series of steps required to assemble the data base is shown in Fig. Al. The final products are a crater data base file holding a specific set of data for each crater (Table Al) and a file record of all the areas studied. The geo graphic coordinates, lighting and viewing data, and scale information for each crater are derived from Viking engineering (SEDR) data describing the spacecraft location and time the images were acquired, mosaic coordinates of classified craters, and geographic control point data. A X 2 minimization procedure was used to find an optimum perspective transformation of mosaic coordinates to Mars fixed coordinates for the geograph..c control points. This transformation was then used to estimate the geographic coordinates of the classified craters and the mosaic boundary points. The accuracy of this procedure is remarkably good considering that the mosaics are essentially uncontrolled. The average error in position of projected geodetic control points is just 8km or about 0.13 0 latitude. Crater morphology characteristics recorded (Table Al) are of two classes - attributes of each ejects deposit and other crater characteristics. rjecta deposit topography at the outer margin is characterized as a distinct ridge, a simple scarp, or indistinct. The overall form of the deposit is characterized as simple (massive) or multilobate (made up of tongues of material). The surface appearance of the ejecta deposit was also characterized as to small scale topography, either rough or smooth and the presence of straight radial ridges noted. A second set of descriptors characterize the crater floorsmooth, rough, central peak, or central pit, rim (circularity), and the presence of secondary craters. The validity of compiling all these qualitative judgements into a data base for intercomparison is critically dependent on the choice of an image data set with fairly uniform characteristics of resolution, lighting, and viewing conditions. We also recognize that the apparent absence of some crater characteristics may be a result of actual absence or insufficient image resolution. For each crater we have stored information sufficient to derive feature dimensions in image resolution elements, so we shall be able to distinguish these two cases.
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COMPARISON WITH PREVIOUS WORK We have analyzed some aspects of our test data set for comparison with the results of previous investigations. We have translP.ted the classes of Johansen ( 1978 and 1979) and Mouginis-Mark (1979) into our data base code, Tables A2 and A3, respecti-ely. Figure A2 shows the latitude distribution of tie occurrence of four classes of craters by ;,ohansen. In Figure .A3 we show the most nearly comparable plot we could easily construct from our test data set. Johansen gives the percent of areas examined in a latitude band in which a class of craters occurs at some (nonzero) areal density. She examined craters from 2 to 25 km diameter. We show the number of craters in a class as a percent cf all craters examined in a particular latitude band. Only the occurrence of Class 2 ("Flower") craters seem to be a similar function of latitude in our two plots. Ejecta flows with terminal ridges, those with the least apparent viscosity according to Johansen, are significantly more common within 30 1 cf the equator than at higher latitudes. Class 2 ("Lunar") craters are present in our test data set in too small numbers to exhibit significant variations with latitude. Class 3 craters with two ejects flow deposits each show no strong variations in either data set. Class 4 craters, having a single ejecta flow deposit with a terminal scarp, have a distribution which is a strong function of latitude in Johansen's plots, but a similar trend is only suggested by our test data set. Finally, we find 15 to 50% of our craters in each latitude bin cannot be identified with one of Johansen's four classes. The differences between the results found with the two data sets may be due to one or more of several factors: 1.
We plot types as fractions of total crater populations while Johansen only plots occurrence in sampling areas on Mars. This almost certainly accounts for our inconclusive results for Class 1.
2.
Our data set does not include many 2 to 6km diameter craters; Johansen's may.
3.
Our crater classification scheme allowed for classification of ejecta deposit margins as indistinct in topographic character but clearly non-lunar (not Class 1). For example, a deposit may clearly be composed of multiple flow lobes but the distal margin indistinct. We allowed also for the combination of two ejects deposits with margin types different from Johansen's "Flower" or "Composite" classes. All but a few
8
TABLE A2
INTERPRETATION OF CRATER CLASSES OF JOHANSEN (1979)
TYPE
OUR INTERPRETATION
CLASS
SECC: EJDN: (MTYPE, DF, DS, W i t i - 1 to EJDN
1; 1; (1,2,1 or 2, 1)]
1
Lunar!Mercurian
10 or
Flower (lowest apparent viscosity)
[0;1 or 2; (2, 1 or 2, 1 or 2, 1 or 2)]
2
Composite
[0; 2; (3, 1 o. 2, 1 or 2, 1 .)r 2); (2, 1 or 2, 1 or 2, 1 or 2)]
3
Mound or Lump
[0;1; (3, 1 or 2, 1 or 2, 1 or 2)]*
Intermediate Apparent viscosity in order of increasing viscosity:
Polar
4
[0;1; (3, 1 or 2, 1 or 2, 1 or 2)]*
Johansen distinguished these two types by steepness of marginal can scarps. We do not believe such qualitative lie drawn with our data set of images.
consistently
distinctions
9
percent of our "Other" craters have outer ejecta deposits with "indistinct" margin topography (Figure AC. These are not all so classified because of marginal image resolution, many of the craters a: •e among the largest in our data set (Figure A5, A6). We have also examined the latitudinal distributions in our sample data set of the six types of craters defi...A by Mouginis-Mark (1979). Figure A7 is taken from Mouginis-Mark (1979) while Figure A8 is the same type of plot of our data for six ejecta morphology classes defined to be as close as possible to those of Mouginis-Mask (Table 3). Classes 1, 2, and 6 of Mouginis-Mark's data seem to rise sufficiently above a background level of a few percent to draw some conclusions. He judged that only Class 6 exhibited significant dependence upon latitude. in our data only two of his six classes (as we interpret them) rise to similar levels. These are Classes 1 and 2. The results from our test data strongly suggest Type 1 craters are more abundant at low latitudes. We also find large fraction of craters at all latitudes which do not fit into one of the six classes. The latitudinal distribution of Class 2 craters in our test data set shows no simple relation to latitude.
Substantial differences between our data sets may account for differences in observed trends - images chosen by us did not include a subset cat high resolution images so we have fewer small craters with detailed morphology well resolved. Since the number of craters in each data set is similar we have more large diameter craters. Mouginis-Mark (1979) points out that Type 6 craters are on average his smallest class, most are less than 5 km diameter (Figure 3, Mougin.t;-Mark, 1979). Few such craters occur in our test data set (Figure A5). Other reasons for relative abundance anomalies of crater types between our two data sets are illuminated by examining our class "other". This class represents 27 to 90% of the craters in each latitude interval. This class .represents so many craters because the six classes defined in Table 3A do not include craters with a single ejects flow deposit with a marginal scarp. This is Johansen's (1978) icy type (mound, lump, or polar) or Type 4 in Table la. Apparently, Mouginis-Mark (1979) did not distinguish this possible class of czater in setting up his six types for class>4.fication. Alternatively, we may have misinterpreted his Type 1. Perhaps it should include single ejects deposit craters with either marginal ridges or marginal scarps. If this is the case, Mouginis-Mark did not make a critical m-)rphological
Interpreta6.ion ot. crater classes or Mouginis-Mark (1979)
SALI::NT FEATURES
OUR INTE'2PRETA.TION 'SEC'-; EJDN; (MTYPE, DF, DS, R11 i , i=1, EJDN 1
1
Single ejecta deposit, marginal ridge
[0;1;(2,1 or 2,1 or 2,1)
2
Dual ejecta deposits outer of Type 1 inner with marginal scarp
(0;2;(3,1 or 2, 1 or 2, 1) (2,1 or 2, 1 or 2, 2_)]
3
Multiple ejecta deposits of Type 1
[0;2;(2,1 or 2, 1 or 2, 1) (2,1 or 2, 1 or 2,I))
4
Radial ridges on ejecta
[0;1;(1 or 3, 2, 1 or 2, 2)]
5
Multilobate, complex secondary craters
[1;1 or 2;(1 or 2 or 3,1 ,1 or 2,1) X (1 or 2)]
6
Large ejecta area (pancake craters)
[0;1;(3,2,1 or 2, 1 or 2) + (RF1/R > 3) )
TYPE ID
11
distinction which probably prevented him from finding trends with latitude similar to those of Johansen (1978). From our test data set we have sorted out four morphological classes of craters with single ejecta flow deposits. These classes are: 1.
SRS = distal ridge margin, simple structure,
2.
SSS = distal scarp margin, simple structure,
3.
-IRIN I
4.
SSM = distal scarp margin, multilobate structure.
= distal ridge margin, multilobate structure
The distribution of Classes 1 and 3 (Figure A9) is very similar to Mouginis-Mark Type ]_ (Figure A8). These craters, having ejecta deposits with marginal ridges, are m, Dre abundant at mid- and low-latitudes. Classes 2 and 4 of Figure A9 (marginal scarps) have just the opposite distribution with latitude. These tentative results are very similar in character to the findings of Johansen (1978, 1979). We have also made a preliminary analysis of the relationship of ejecta mobility to latitude in our test data set. Figure A10 is reproduced from Mouginis-Mark (1979). He notes that the max-4 mum distance ejecta travels from crater center, normalized to crater radius, appears to increase toward higher latitudes. In Figure All, we plot crate=s binned by a similar function, mean ejecta radius/crater radius, versus latitude. Since our calculation uses mean rather than maximum ejecta radius, our mobility values are binned somewhat differently from those of Mouginis-Mark. We find the same sharp, upward trend in mobility for northern mid-to-high latitudes, but our results for the southern hemisphere are inconclusive. CRATER STRADDLING TERRAIN BOUNDARIES A feature of this data base which is not duplicated, to our knowledge in other crater data bases is the record of ejecta characteristics for craters straddling terrain boundaries. If the ejecta characteristics on the two terrains are different then the source of that difference can be pinpointed. As the material in the projectile that formed the crater, projectile velocity and projectile direction are common and the material that is shocked to form the ejecta deposit is likely to be similar also, only differences in the topographic character of the two terrain surfaces or the properties of the materials near the surface can account for such contrasts. In our preliminar y analyses of the data sets we have found striking differences in the type of ejecta
12
deposit produced by craters stradelling boundaries. On the rough terrain, mutilobate deposite forms dominate (Table A.4) whereas on the smooth terrain the simple deposits are much more numerous. One speculative interpretation in that roughness elements on the rougher surface are responsible for splitting the flow deposit into lobes. However, much more data are needed to confirm this interpretation. SUMMARY Theser p elimi_n^ar nd fragmentary explorations of our growing crater morp — ho ogy data base have confirmed some results or earlier studies and illuminate problems with previous classification schemes. We are confident that with our flexible classification scheme and a larger more uniform global data sample now being collected, we shall be able to clearly delineate the dependence of Martian crater ejecta morphology and morphometry on latitude, elevation, and terrain type.
13
Table A.2
Occurrence of Simple and Multilobate Deposit Forms in Craters Straddling Terrain Boundaries with Rough Terrain on One Side and Smooth Terrain on the Other
Terrain Type
Deposit Form Simple No. %
Rough cr.
10
27.0
Smooth cs. or cw.
30
81.1
Multilobate No. % 27
73.0
7
18.9
14
REFERENCES Allen, C. C., 1979, Icarus, 39, 111-123. Carr, M. H., Crumpler, L. S., Cutts, J. A., Greeley, R., Guest, J. E., and Masursky, H., 1977, Journal of Geophysical Res., 82, 4055,4065. Davies, M. E., Katayama, F. Y., and Roth, J. A., 1978, Rand Corporation Report R-2309-NASA, P. 91 Head, J. W. and Roth, R., 1976, Papers PresenteO to the Symposium on Planetary Cratering Mechanics, Flagstaff, Arizona, 13-17, September, 1976, p. 50-52. Johansen, L. A., 1978, Proceedings of the Second Colloquium on Planetary Water and Polar Processes, Hanover, ,pp. 109-110. New Hampshire, lb-ib October, Johansen, L. A. 1979, Reports of Planetary Geology Program 1978, 1979, NASA TM 80339, p. 123-125. Mouginis-Mark, P., 1979, Journal of Geophysical Res., 84, 8011,8022.
I
4
CLRSSES
15
(lrom
JOHRNSEN,1978) + 'CTUi^''
CL ?0
1
A s0
H H
W CL U1
50
0
40
In w
30
Cr
U 20 tl
o
z W
10
U M W
—60 — 50 — 40
—80 —70
11
— 30 — 20 — 10
N -
10
0
30
20
40
50
70
60
80 NORTH
SOUTh
LATITUDE 3
13
10
155
96
178
234
119
26
215
188
' CLASS 3 ( " Composite") ` CLASS 4 ("ICY")
CLASS 1 ("Ballistic Lunar") q CLASS 2 ("rlow+er")
Fig. A3
47
70
24
23
7
'OTHER
Distribution with latitude in our test data set of four classes of crater ejocta morphology patterned after four types of Johnsen (see Fig. A2). Occurrence of other classes is noted.
80
° J 0
`^
•
4
°
°
/
70
:•
1
6° so
o
a
d
°
.o
\
__ 1
40 30
0 0
^. o
^p
10
/ VV
60
50
40
O
20
10
^., 30
^0 o 0
-1C
ff. ^ -20
-30
-40
0 •\\C -50
-60
Latitude On degrees north) .........
•
Ballistic (Lunar type)
^^•
O
Water type (Flo%er)
Icy rypes (Mound; Lump, Polar)
^•^ C
Fig. A2
Corposite (Fla%er and Mound configurctian in one inspoct)
Distribution of crater ejecta morphology by four types (Johnsen, 1979).
i
?r^^zs ^ =r^cT cu ^ _^.
16
r;^^cTr^s
56
0
45 40
J 35 0 CL
30 x W M 25 U 20
W O
15
z u 10 x LLJ 5
0
-30 -40 -30 -20 -10
10
20
30
40
50
60
70
LATITUDE Fig. A4 Distribution with latitude of ejecta deposits with indistinct marginal topography in our test data.
30C
270; 240
o210 L O
b 180
L120 O z
se 60 30 0
B
i6
24
32
40
40
36
64
72
Be
Be
96
Diameter, km Fig. AS Distribution over diameter of craters in our test data set.
7
17
,-^_r-^^ rte! ^.
-
-, '
r,
45 41-2
c^,
L
3
A JG' L
U
25 22
O Z
15 1e 5
e
Se
3 e
2e
SZ
42
7e
se
Be
Be
102
Liameter, km
Fig. A6 Distribution of indistinct ejects margin ejecta with crater diameter in our test data set. This class of crater includes virtually all large craters and a substantial fraction of intermediate size craters (compare with Fig. A5).
h W F
