Table 2 - Terrain groups described in this report
(in order of decreasing abundance)

Symbol Group Terrain Units Included
(See Table 1 for explanation
of symbols)

LP Lowland Plains ps,pi,pm,pc,dh

LFT Lineated & Fractured Terrain 11,12,f,re

T Tessera tl,twf,tef,th,tg,tf,tu

RT Ridged Terrain r,rbl,rb2,rs

UP Upland Plains psl,pt

SPT Shield/Patera/Tholus s,cp,dm

DU Domed Uplands btd,bld

co Corona co1,co2

LMB Low Marginal Belt mt

HMB High Marginal Belt ml

HT Hilly Terrain ht

CRT>45km Suspected Impact Crts>45 km c,ce

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46


Institute of Technology Center for Space Research,
Cambridge, Mass. 12 p.

Ivanov, B.A., 1989, Morphometry of impact craters on Venus [in
Russian}: Astronomicheskii Vestnik, 23, 39-49, English
translation in Solar System Research, 23, 23-31.

Ivanov, B.A. and Basilevsky, A.T., 1987, A comparison of crater
retention ages on the Earth and Venus [in Russian]:
Astronomicheskii Vestnik, 21, 136-143, English Translation
in Solar System Research, 21, 84-88.

Ivanov,B.A., BaSilevsky, A.T., Krychkov, V.P., and Chernaya,
I.M., 1986, Impact craters of Venus: Analysis of Venera 15
and 16 data: Journal of Geophysical Research, v. 91, p.
D413-D430.

Jurgens, R.F., Slade, M.A., and Saunders, R.S., 1988, Evidence
for highly reflecting materials on the surface and
subsurface of Venus: Science, v. 240, p. 1021-1023.

Kozak, R.C., and Schaber, G.G., 1989, New evidence for global
tectonic zones on Venus: Geophysical Research Letters, v.
16, p. 175-178.

Masursky, Harold, Eliason, Eric, Ford, P.G., McGill, G.E.,
Pettengill, G.H., Schaber, G.G., and Schubert, Gerald, 1980,
Pioneer Venus radar results: Geology from images and
altimetry: Journal of Geophysical Research, v. 85, p. 8232-
8260.

Pettengill, G.H., Eliason, E., Ford, P.G., Loriot, G.B.,
Masursky, Harold, and McGill, G.E., 1980, Pioneer Venus
radar results: Altimetry and surface properties: Journal of
Geophysical Research, v. 85, p. 8261-8270.

Pettengill, G.H., Ford, P.G., and Chapman, B.D., 1988, Venus:
Surface electromagnetic properties: Journal of Geophysical
Research, v. 93, p. 14,881-14,892.

Schaber, G.G., and Kozak, R.C., 1990, Geologic/geomorphic and
structure maps of the northern quarter of Venus: U.S.
Geological Survey Open-File Report 90-24, Denver, Colo.,
scale 1:15,000,000.

Schaber, G.G., Kozak, R.C., and Masursky, Harold, 1987a,
Cleopatra Patera on Venus: Evidence for a volcanic origin:
Geophysical Research Letters, v. 14, p. 41-44.

Schaber, G.G., Shoemaker, E.M., and Kozak, R.C., 1987b, The
surface age of Venus: Use of the terrestrial cratering
record [in Russian]: Astronomicheskii Vestnik, v. 21, p.144-
151, English translation in Solar System Research, v. 21, p.
89-94,

Shoemaker, E.M. and Shoemaker, C.S., in press, Solar System
roulette: The collision of solid bodies, in The new solar
system: Cambridge, Mass., Sky Publishing.

Sukhanov, A.L., and Pronin, A.A., 1987, Evidence of tectonic
spreading on Venus [in Russian]: Doklady Akademii Nauk SSSR,
v.- 294, p. 661-665; English translation in Transactions of
the USSR Academy of Sciences, 1987, v. 294, p. 66-70.

Sukhanov, A.L., Pronin, A.A., Burba, G.A., Nikishin, A.M.,
Kryuchkov, V.P., Basilevsky, A.T., Markov, M.S., Kuzmin,

14


NUMBER OF CRATERS/1000000KM2

FOR TERRAIN GROUPS

| cro45Km |
co

HT

SPT

ue

RT

|
i

T
LP
TERRAIN GROUP

ZW» OOCCOOL/SHALVYO YAEWNN

56


PER GEOLOGIC MAP UNIT

NUMBER CRATERS/1000000KM2

RESSSSBBOL
RRR
igegecececececececece md

Kexesiexocieseosicy” &

sechgechctctchctctcices Mite ct
estatetetatettatetets
POS

earerarararareravereravere
 egatetetetasetetasetetete

P5656

SESSILIS
 Segecececececececececenececey
 oesececeetgtctetetctchetete
BIN

on

pi
GEOLOGIC UNIT

| tu

SKK KKH IN
RK RK

eararavaverererererererorerererere
IXRRRRRR RRR

50%
ERE OIPO?

secesectshctchchetehchctchctetets
SKK KKK RNY

pt

echshchctatchctstchctstchce
SKK KK KKK
Perens

PSST III III
seetetatetetetetethetetetetsteteteleletets
ERRNO NIHR IOND

 eracecesececachchctcecectctctcecerectvirectctcte
ecececerecerecececececececesecerecerecereecenes

ech segecictcechctchecectchvecacirivinictraceree
PRIS IRN D

RRR o Neovo roo rree
PERRIER RII III

eetetstehelctelehctctehchetctvichctcichctctcecte’s
pesteeresecesrneeenseersemneeoere

G
57676 7476%4747570%670747470 0747475747475 475704%
sostletatatatetelctctotetetetetetstcicictclstchctets
POSSI ILO MII

Sac asesecnen areca enerenacecerenenacacecerecenece®
RRR HI NY

ZWAOOOOCOOI\SYALV UO YAsWNN

54

Fig. 28


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Table 5 - Continued

tu
tu
mt
12
ps
pm
pi
12
ps
pi
pi
pi
ps
ps
pe
12
rb1
11
ps
pm
ps
pi
12
12
ps
11
ps
tl
ps
ps
rs
11
ps
11
ps
dh
dh
ps
ps
ps
11
ps
11
12
pt
11
bld
rb1
ps
ps
12
tl
12
bld
ps
ps
ps
tf

56.5
56.4
55.7
53.9
52.45
51.9
51.9
51.8
51.75
51.7
51.3
51.15
51.05
50.25
49.9
48.45
48.4
48.3
47.8
47.8
47.2
46.8
46.6
45.5
45.5
45.45
45.35
45.35
45
44.9
44.45
44.2
44.1
43.4
42.55
41.95
41.8
40.55
40.1
39.5
39.4
38.7
37.95
37.65
36.6
36.25
35.7
35
34.95
34.6
33.75
32.7
32.3
31.75
30.2
30
29.65
28.6

6051.71
6051.74
6051.57
6051.38
6050.76
6051.11
6051.58
6051.64
6050.54
6051.66
6051.33
6051.34
6051.55
6051.75
6051.54
6051.55
6050.51
6051.33
6050.61
6052.08
6051.54
6051.76
6051.41
6051.04
6050.57
6050.32
6050.58
6051.32
6051.01
6051.92
6050.95
6051.49
6051.23
6051.67
6051.22
6050.72
6051.25
6051.11
6050.92
6051.15
6051.17
6051.27
6051.21
6051.46
6052.43
6051.21
6051.29
6051.88
6051.46
6051.22
6051.61
6052.72
6051.70
6051.69
6051.76
6051.28
6051.73
6051.82

24

ORLOVA
UNNAMED
WHARTON
HAYASI
UNNAMED
COCHRAN
UNNAMED
DOLORES
OLYA
UNSET
STEPHANIA
LOTTA
JULIE
LIND

NANA
LAURA
BROOKE
YBLOCHKNA
GALINA
KEMBLE
CATHER
ALMEIDA
VALENTINA
BARTO
UNNAMED
FEDOSOVA
ZVEREVA
UNNAMED
UNNAMED
UNNAMED
NADIRA
PRICHARD
ARIADNE
RUTH
POLINA
ZINA
TAGLIONI
UNNAMED
JEANNE
ANYA

LENA
UNNAMED
UNNAMED
STINA
UNNAMED
UNNAMED
VOYNICH
UNNAMED
IRINA
MOSES
UNNAMED
BERNHARDT
EDGEWORTH
POTANINA
LILLYA
REGINA
MUKHINA
TSERSKAYA


MEAN AND SD IN ELEVATION

OF TERRAIN GROUPS

PLZZ B

ERROR KK RS
DSSS
CLMLMIUMMMMMM MMs

REESE
PRK

B

HMB

oso9go+
(W>) NOILVAZ1A

42

TERRAIN GROUP

Ky) HIGH ELEVATION

LZ LOW ELEVATION

[S888] MEAN ELEVATION

16

Fig.


INTRODUCTION

The Venera 15 and 16 spacecraft provided radar-image and
altimetric data of sufficient radar cell resolution (1.5- 2.5 km)
to permit compilation of pre-Magellan maps of regional
geologic/geomorphic terrains and structural elements within 111.4
X 10° km’ of the northern quarter of Venus (Sukhanov et al.,

1989; Schaber and Kozak, 1990). Pioneer-Venus and Earth-based
radar data provided a significant amount of additional
information for part of the mapped area. The present report
describes digital correlations and analyses of various physical
aspects of the mapped terrain units described in Schaber and
Kozak (1990) and was prepared to be used in conjunction with that
report.

The interrelations of the geologic/geomorphic terrain units
and terrain groups presented here are based primarily on analyses
of pre-Magellan radar images and altimetric data. Therefore,
these data and the correlations made among them, including
densities of suspected impact craters, should be taken as
tentative prior to verification or rejection based on a thorough
analysis of higher resolution Magellan data. The current results
are provided here primarily as a guide to future investigations
by Magellan teams and by the planetary geoscience community at
large.

METHODS

The contacts between the 34 different geologic/geomorphic
terrain units mapped by Schaber and Kozak (1990 Plate 1) were
digitized and converted to a Sinusoidal Equal-Area projection.
The projection was then registered with a merged Pioneer
Venus/Venera 15/16 altimetric database (E. Eliason and P. Ford,
oral communications, 1990), root mean square (rms) slope values
(C-power’/*), and radar reflectivity (rho) values derived from
Pioneer Venus (Pettengill et al., 1980, 1988). The resulting
information, shown here principally in tables and graphs,
includes comparisons of individual terrain units (Table 1) and
the terrain groups to which they are assigned (Table 2; Figs. 1-
12) in regard to percentage of the map area covered, elevation,
rms slopes, reflectivity, and density of suspected impact craters
>10 km in diameter and their latitudinal and longitudinal
variations (Tables 3-5; Figs. 13-34). [The concept of terrain
groups did not figure in Schaber and Kozak (1990), but it is
useful here because of the difficulty in portraying the areal
distribution of the thirty-four terrain units in page-size black
and white illustrations]. The areal distribution of individual
terrain units, as well as the location of features named in this
report, is given on Plate 1 and Table 1 of Schaber and Kozak
(1990).

AREAL DISTRIBUTION OF TERRAIN MATERIALS
The percentages of map area covered by individual terrain
units and terrain groups, respectively, are shown in Figures 13
and 14. The high and low marginal belt groups and the hilly

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27


Table 5 - Suspected Impact Craters >10 km in Diameter
Between lat 25° N. and 90° N. (in order of decreasing latitude)

Terrain Latitude Longitude Elevation Crater Diameter
Unit (°) (*) (km) Name (km)

ps 87.2 290.5 6050.67 UNNAMED

pe 85.35 214.5 6051.30 TATYANA 22
pi 84.6 79 6050.20 LANDOWSKA 38
pe 84.05 16.5 6050.49 RUSLANOVA 44
pe 82.55 273.5 6051.40 UNNAMED 26
rb1 81.35 200 6051.85 UNNAMED -
pe 81.15 285.5 6051.51 LAGERLOF 58
pi 81.05 223.5 6051.62 EFIMOVA 30
dh 78.45 173.9 6051.78 RUDNEVA 34
tef 78.25 306.5 6052.82 DSAHKOVA 50
rb1 78.1 76.5 6050.70 GINA 22
dh 76.9 192.5 6052.11 TUNDE 16
twt 76.25 17 6054.44 FERNANDEZ 27
11 76.2 104.5 6051.29 KLENOVA 144
pi 75.95 55.5 6051.47 ULRIQUE 22
11 75.8 127 6051.40 DELEDDA 32
11 75.55 96 6051.54 RADKA 15
pi 75.5 272 6051.99 VALBORG 20
rb2 74.7 176.6 6051.85 DICKINSON 72
12 72.25 121.5 6051.85 MONIKA 30
ps 72.1 174.8 6051.65 ERIKA 14
ml 71.2 322.75 6055.16 WANDA 22
psl 70.9 335.25 6055.20 RITA 11
re 70.7 300 6052.65 COTTON 52
ml 70.65 320.5 6055.80 OSIPENKO 32
pt 70.5 289.75 6051.99 OBUKHOVA 44
pm 70.1 107.25 6051.61 LAFAYETTE 46
rb1 69.25 236.25 6051.52 ZOYA 22
pm 68.45 94.2 6052.33 GLORIA 23
pm 68.4 90.5 6052.07 JADWIGA 15
pt 68.2 303.75 6052.31 IVKA 18
12 68 292 6051.32 DUNCAN 46
s 67.1 273 6051.96 NATALIA 12
tu 67.05 329.5 6055.32 MAGDA 16
11 65.45 168.75 6050.15 UNNAMED 34
pt 65.1 299 6051.64 ZDRAVKA 14
vs 64.6 334.5 6054.78 ZLATA 10
pm 64.4 139.5 6051.90 KOIDULA 72
11 64.2 159.5 6050.02 UNNAMED -_
11 64.1 289.75 6050.83 INDIRA 16
pt 63.45 314.5 6053.63 SIGRID 21
pi 62.85 88 6051.69 MASHA 21
vs 62.15 329.75 6054.42 LYUDMILA 20
psl 62.1 321.75 6054.23 BERTA 28
ml 61.55 317.5 6054.36 TAMARA 10
pt 61.35 308 6051.58 AKHMATOVA 44
ps 61.3 223 6051.39 BARSOVA 80
pe 60.4 154.5 6050.40 EMOLOVA 66
pi 60.3 286.5 6051.00 GOLUBKINA 28
12 60 272.9 6051.27 MARGIT 16
rs 59.8 65.6 6051.37 FEDORETS 52
12 59.3 279.7 6051.02 MONTESORI 44
tu 58.3 336.9 6053.79 MAGNANI 28
pt 56.9 6.5 6054.86 ROSSETTI 24

23


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Fig. 10

36


corrected elevation. Elevations were normalized by using the
hypsometric data shown in figure 15. Elevation data plotted in
0.5-km bins.

Figure 34 - Relation between diameter and elevation of 116
suspected impact craters within map area. Crater diameters taken
from Basilevsky et al. (1987). Crater names for some of the
highest and largest craters are given for convenience. Cleopatra
is queried because of the controversy concerning its impact or
volcanic origin (Ivanov et al., 1986; Basilevsky et al., 1987;
Schaber et al., 1987).

18


average (0.127) of the other 32 mapped units. This average value
for the northern quarter of Venus is about the same as that
(0.1340.03) reported by Pettengill et al. (1980) for the the
global mean reflection coefficient. Anomalously high reflection
coefficients of 0.40+0.1 (Pettengill et al., 1988) to 0.60
(Alexandrov et al., 1990) for Maxwell Montes, and 0.40 and 0.45
for Akna and Freja Montes, respectively (Alexandrov et al., 1990)
have strongly influenced the mean in reflectivity and standard
deviation given here for the Lakshmi-type marginal belt unit.
Such anomalies have been suggested to result from concentrations
of some type of electrically conducting minerals (possibly
sulfides or Fe/Ti oxides) embedded in the host rock (Pettengill
et al., 1988). Jurgens et al. (1988) and Alexandrov et al. (1990)
have suggested that high-reflectivity materials could be present
on the surface and subsurface in the Venusian lowlands, as well
as in the highland regions of anomalously high reflectivity
reported by Pettengill et al. (1980, 1988).

Two large volcanic shields, Tepev and Sekmet Montes, have been
described by Alexandrov et al. (1990) as possessing enhanced
reflectivities of 0.35 and 0.40, respectively. In the present
report, correlation of the Pioneer Venus reflectivity data with
the shield unit indicates a mean value of 0.136+0.078, only 9%
higher than the mean for the most pervasive plains unit, the
smooth plains.

The craters of suspected impact origin >45 km in diameter
(unit c) combined with their ejecta (unit ce) were found to have
11% higher mean reflectivity (0.13840.080) than the smooth plains
unit (0.12740.077) but only 5% higher reflectivity than the
lowland plains group (0.13310.083).

The mean reflectivity and standard deviations for the 12
terrain groups illustrate the anomalous radar behavior of the
high marginal belt group relative to the other terrain groups.
The other groups have more consistent mean reflectivities, but
their standard deviations differ significantly.

DISTRIBUTION OF SUSPECTED IMPACT CRATERS

Approximately 150 craters, whose morphologies are similar to
those of impact craters elsewhere in the Solar System, were
recognized by Ivanov et al. (1986) and Basilevsky et al. (1987)
from Venera 15/16 data obtained between about lat 20° and 90° N.
Schaber and Kozak (1990; plate 2) included 116 such structures,
but they considered the remaining 34 to be of uncertain origin at
the resolution of Venera 15/16 data. [Cleopatra Patera, a 100-
km diameter crater located between 7.5 and 8.0 km above the MPR
on the east flank of Maxwell Montes has been the subject of some
controversy. This enigmatic feature has been interpreted from
analysis of Venera 15/16, Pioneer Venus, and Earthbased radar
data to have originated either through volcanic or volcano-
tectonic processes (Masursky et al., 1980; Schaber et al., 1987a;
Schaber and Kozak, 1990), or through impact-cratering processes
(Ivanov et al., 1986; Basilevsky et al., 1987; Ivanov and
Basilevsly, 1987; Ivanov, 1989; Basilevsky and Ivanov, 1990).

10


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30


in the northern quarter of Venus. The unit has a mean elevation
of 6056.5742.28 km, which is 4.7 km above the MPR, and reaches
its maximum elevation of about 11.5 km at the summit of Maxwell
Montes. Tessera-type marginal belt terrain (unit mt), the sole
unit of the low marginal belt group, is associated with the
periphery of large tessera blocks of lower mean elevation and
most commonly occurs along the contacts between the tessera and
plains units. The unit has a mean elevation only about 0.7 km
above the MPR.

Western (unit twf) and eastern (unit tef) Fortuna-type
tesserae, Hina-type tessera (unit th), and intensely fractured
tessera (unit tf) terrains are the highest tessera units of
significant areal extent in the northern quarter of Venus, where
they are associated primarily with Fortuna Tessera in Ishtar
Terra. The mean elevations of these tessera units above the MPR
range from a high of 3.53 km for the Hina-type tessera unit to a
low of 1.53 km for the intensely fractured tessera terrain. The
Laima-type tessera terrain (unit tl) and the tessera, undivided,
terrain (unit tu) average only 0.64 km and 0.38 km above the MPR,
respectively, and appear to have been regionally disrupted by
tectonic and volcanic processes. These two units are commonly
overlapped by units of the lowland plains group and mantled by
the tessera-mantling plains unit (Schaber and Kozak, 1990). Such
overlap and mantling is apparent in the regions south of Laima
Tessera and north and east of Tellus Regio in the regions of
Dekla, Meni, Ananke, and Kutue Tesserae (see U.S. Geological
Survey, 1989).

Lineated terrain, type 1 (unit 1,) and lineated terrain, type
2 (unit 1,) have mean elevations 0.87 km and 0.54 kn,
respectively, below the MPR and -0.17 km and +0.16 km relative to
the mean elevation of the smooth plains unit. Fractured terrain
(unit f) has a mean elevation 0.75 km above the MPR and 1.45 km
above the mean level of the smooth plains unit. The mean
elevation of the ridge belt, type 1 terrain (unit rb,) is close
to that of lineated terrain, type 1 (unit 1,): 0.88 km below the
MPR and 0.18 km below the mean elevation of the smooth plains
unit. According to the merged Pioneer Venus and Venera 15/16
altimetric database used in this study, lineated terrain, type 1
and ridge belt, type 1 have the lowest mean elevations of the
major terrain units mapped by Schaber and Kozak (1990). Three
less extensive ridged terrains--the ridge complex (unit rc),
ridge swarm (unit rs), and ridge belt, type 2 (unit rb,)--have
mean elevations at, 0.05 km below, and 0.52 km below the MPR,
respectively.

Craters >45 km in diameter of suspected impact origin (unit c)
and their probable ejecta deposits (unit ce) have mean elevations
of 0.78 km and 0.51 km, respectively, below the MPR. The
standard deviations in elevation of these crater units are only
+0.65 km and +0.45 km, respectively.

The mean elevations and their standard deviations of the major
terrain groups in the northern quarter of Venus illustrate well
the overall hierarchy of terrains. The highland terrains are

7


terrain group each consist of a single terrain unit. Smooth
plains (unit ps), intermediate plains (unit pi), mottled plains
(unit pm), complex plains (unit pc), and domical hill or hill
field (unit dh) units form the lowland plains terrain group,
which covers 52.6% of the mapped area. (Domical hill or hill
field terrain (unit dh) is here included in the lowland plains
group because of the unit's obvious genetic association with
plains situated at or near the mean planetary radius; see below.)

The smooth plains unit alone covers 30.0% of the mapped area
but is present in only minor amounts between lat 20° and 90° N,
long 0° and 150° (Fig. 1b). This region is dominated instead by
more complex plains units, lineated terrains, and tesserae. The
three most areally extensive groups following the lowland plains
group are the lineated and fractured terrain group (14.9% of map
area), the tessera group (9.6%) and the ridged terrain group
(9.0%). The upland plains group, consisting of tessera-mantling
plains (unit pt) and smooth plains of Lakshmi Planum (unit psl),
covers only 4.8% of the map area. Combined, the groups noted
above cover about 91% of the area mapped within the northern
quarter of the planet.

HYPSOMETRY

Elevation values included in this report are radial distances
to the center of mass of Venus computed during the digital
processing and merging of Pioneer Venus and Venera 15/16
spacecraft altimetry databases (E. Eliason and P. Ford, oral
communications, 1990). The mean planetary radius (MPR), the
datum, used here is 6051.9 km as derived by Ford (1986) from
Pioneer Venus data. Elevations of terrain units described below
are given in Figure 15 and Table 3; elevations of terrain groups,
in Figure 16 and Table 4.

The relation of the percentages of area to specific elevations
were determined through a hypsometric analysis of the merged
Pioneer Venus and Venera 15/16 altimetry database coinciding with
the mapped part of the northern quarter of Venus (Fig. 17). The
hypsometric curve of the mapped region is highly unimodal. A
similar situation was shown to be true for Venus on a global
basis following analysis of the entire Pioneer venus altimetric
database (Masursky et al., 1980; Pettengill et al., 1980).
Because ocean basins are absent on Venus, the large areal extent
of the lowland plains terrain group strongly influences the
location and peakedness of the hypsometric curve. (All five
terrain units of the lowland plains group have mean elevations
that lie below the MPR.)

The only highly elevated smooth plains unit, smooth plains of
Lakshmi Planum (unit psl), forms the surface of the Lakshmi
plateau and is about 3.0 km above the MPR. Tessera-mantling
plains (unit pt) has a mean elevation about 1 km above the MPR.

Lakshmi-type marginal belt terrain (unit ml) is the only unit
in the high marginal belt group. It forms major linear belts of
mountains (e.g., Maxwell, Freja, and Akna Montes) that nearly
surround Lakshmi Planum. These mountains dominate the landscape

6


Table 1 - Terrain units mapped by Schaber and Kozak (1990)
(in order of decreasing abundance)

Symbol Unit Name

ps Smooth Plains

pi Intermediate Plains

11 Lineated Terrain, Type 1

12 Lineated Terrain, Type 2
rbl Ridge Belt, Type 1

pm Mottled Plains

pt Tessera-Mantling Plains

tl Laima-Type Tessera

dh Domical Hill or Hill Field
pe Complex Plains

tu Tessera, Undivided

rs Ridge Swarm

s Shield

twit Western Fortuna-Type Tessera
re Ridge Complex

£ Fractured Terrain

mt Tessera-Type Marginal Belt
col Corona, Type 1

bld Domed Terrain of Bell Regio
tef Eastern Fortuna-Type Tessera
cp Crater (Patera)

psl Smooth Plains of Lakshmi Planum
btd Domed Terrain of Beta Regio
ml Lakshmi-Type Marginal Belt
rb2 Ridge Belt, Type 2

tt Fractured Tessera

col Corona, Type 2

r Ridge

h Hilly Terrain

th Hina-Type Tessera

ce Ejecta of Suspected Impact Crater
c Suspected Impact Crater

dm Domical Mesa (Tholus)

tg Grooved Tessera

19


mapped part of the domed terrain of Bell Regio (unit bld) has a
mean rms slope of 1.72°, which is even smoother than that of the
smooth plains terrain. The tessera-mantling plains unit has a
mean rms slope of 3.10°+2.38°; thus it is considerably rougher
than the four plains units found at the lower elevations.

The corona, type 1 terrain (unit co,) is characterized by
circular or oblong sets of concentric ridges, while the corona,
type 2 terrain (unit co,) has smooth, annular ridges at Venera
15/16 image resolution (Schaber and Kozak, 1990); these
differences in associated ridge roughness are reflected in the
mean rms slopes computed for these units (3.48°+2.52° and
2.72°+1.04°, respectively).

The shield terrain (unit s) has a mean rms slope of
2.12°+0.89°, which is about the same as that of the smooth plains
unit. Thus, any roughness associated with lava flow fields on
the flanks of the shields is probably at a scale or slope-length
too small to be determined from analyses of radar echoes returned
from the nadir-viewing Pioneer Venus altimeter (Pettengill et
al., 1980). The moderate incidence angles at which Magellan
radar image data will be acquired will be better suited to sort
out subtle differences in the small-scale roughness of lava flow
fields.

The distribution of rms slopes for the 34 terrain units shows
a general increase with elevation (Fig. 20; Table 3). A major
exception however, is the smooth plains of Lakshmi Planum unit
which is both relatively smooth (rms = 2.00°+0.77°) and about 3.6
km above the level of the lowland plains group.

ow! 1 ion Coefficient (Reflectivit
Extremely weak radar echo returns and computed very low

reflectivity values within parts of some tessera units (e.g.,
western and eastern Fortuna-type, intensely fractured, Laima-
type, and undivided), the fractured terrain, and the tessera-
type marginal belt units are considered to be misleading on the
Pioneer Venus database used in this report. These values may
result from excessive diffuse scattering of the radar echos from
extremely rough terrains. Pettengill et al. (1988) used the
imaging-mode data from Pioneer Venus to estimate the fraction of
unmodeled, diffusely scattering surface between lat 20° S. and
50° N., and thus to correct the Pioneer Venus reflectivity values
obtained from at least part of the map area. However, these data
have not yet been merged and correlated with the terrain units
mapped by Schaber and Kozak (1990) and the terrain groups
described in this report.

Figure 21 and Table 3 give Pioneer Venus-derived reflectivity
data for the included terrain units; Figure 22 and Table 4, for
the included terrain groups.

The variations in the mean reflectivity computed for the 34
terrain units are relatively small, with a few exceptions. The
Lakshmi-type marginal belt and Hina-type tessera units have
reflectivities 2.2 and 1.6 times, respectively, higher than the

9


Table 5 - Continued

ps
ps
pi
pi

28.4 4.7
28.2 106.8
28.1 72.65
25.9 72.7

6051.30
6050.52
6050.92
6050.62

25

BROWNING
ANTONINA
CHRISTIE
UNNAMED

26
17
28
57


DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY

Venus: Quantitative Analyses of Terrain Units Identified from
Venera 15/16 Data and Described in Open-File Report 90-24

by
Gerald G. Schaber’

Open-File Report 90-468
1990

Prepared for the National Aeronautics and Space Administration
Under NASA Contracts W15,415 and WO8,777

This report is preliminary and has not been reviewed for
conformity with U.S. Geological Survey editorial standards. Any
use of trade, product, or firm names is for descriptive purposes
only and does not imply endorsement by the U.S. Government.

> 2255 N. Gemini Drive

Flagstaff, Arizona 86001


MEAN AND SD IN REFLECTIVITY

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[S888] MEAN REFLECTIVITY

Fig. 22


The latitudinal distribution of suspected impact craters
between lat 25.9° and 87.2° N. has been computed for both 1° and
5° latitudinal bands (Figs. 24, 25). Eleven of the 1°
latitudinal bands between lat 25° and 87° were not crossed by a
crater; thus, the considerable deviations of the curve in figure
24 show no global trend. However, when the craters are placed in
5° latitudinal bins (Fig. 25), a strong trend of generally
decreasing numbers of impact craters with decreasing latitude
between 80° and 25° emerges. Regional deviations from that
general trend can also be seen.

Crater Population and Density

Figure 26 shows the population of suspected impact craters
recognized from Venera 15/16 data on the 22 cratered terrain
units. Although the areally extensive smooth plains terrain has
twice as many suspected impact craters as any other unit, a
comparison of its areal coverage with its crater population shows
its crater density to be only about half that of the other
cratered terrains (Fig. 27). This discrepancy is also seen in
the interpolated crater densities of the 22 cratered units (Fig.
28) and the deviations in crater density of 21 cratered units
relative to the smooth plains unit (Fig. 29). On the basis of
available data, we see that Lakshmi-type marginal belt terrain
has the highest crater density, followed by the smooth plains of
Lakshmi Planum terrain. The smooth plains unit has the fifth
least cratered surface of the 22 cratered terrain units
recognized. On the graph of crater densities of the 12 terrain
groups (Fig. 30), the high marginal belts have the highest
density-- more than 4 times that of the low plains group that
contains the smooth plains. If Cleopatra Patera is shown by
Magellan to be originally of impact origin, then the crater
density of the high marginal belt unit (and group) is even
greater relative to the other mapped units (and groups).

The reality and significance of these preliminary results--as
yet statistically unsupportable--must await detailed scrutiny of
the higher resolution Magellan data. A high-priority objective
of the Magellan mission is to determine statistically valid
crater size-frequency distributions from which at least relative
surface ages can be determined for the different units.
Knowledge of absolute surface ages will require
reasonable estimates of the cratering rate at Venus over the past
500 million years or more.

Distribution in Elevation

The density of suspected impact craters on all cratered
terrain units is plotted against elevation in Figure 31. No
overall trends are apparent; however, a direct relation is
Clearly indicated for Lakshmi-type marginal belt terrain (unit
ml) and the smooth plains of Lakshmi Planum (unit psl). [The
western Fortuna-type tessera terrain (unit twf) is elevated but
contains very few craters recognizable from the Venera 15/16
data]. A simple elevation plot of all 116 suspected impact

12


Magellan data should provide sufficient surface detail to confirm
whether Cleopatra is indeed a volcanic patera, an impact basin,
or possibly some combination of both. Cleopatra Patera has thus
been intentionally left off the list of craters (Table 5)
strongly suspected of being of impact origin based on pre-
Magellan data].

The recognition of only 116 craters with impact-like
morphology within a mapped area of 111.4 X 10° km’ strongly
suggests that the "average" surface of the northern quarter of
Venus is much younger than the ancient surfaces of the Moon,
Mars, and Mercury. Estimates as high as 1.0+0.5 b.y. have been
suggested for the average age of that part of the northern
quarter of Venus mapped by Venera 15/16; in making estimates
workers have applied the lunar cratering rate averaged over the
past 3.3 Ga (Ivanov et al., 1986; Basilevsky et al., 1987; Ivanov
and Basilevsky, 1987). Assuming that the terrestrial cratering
rate over the past 0.5 b.y. is more applicable to Venus, Schaber
et al. (1987b) and Shoemaker and Shoemaker (in press) have
suggested that the average surface in the northern quarter of
Venus may be much younger than 1.0 Ga, perhaps a maximum of 450
m.y.---equivalent to the retention age estimated for the North
American and European cratons on Earth.

Unfortunately, the small number of suspected impact craters >10
km in diameter within the northern quarter of Venus cannot
support statistically confident determinations of crater size-
frequency distributions and relative ages for individual terrain
units or terrain groups. Arvidson et al. (1990), in addressing
the nature and rate of resurfacing of Venus, favored a model in
which crater-retention ages >>0.4 Ga possibly exist only for
plains not subjected to recent volcanism or tectonism (governed
by crater production and viscous relaxation); in their model are
removed rapidy (>several km/Ga) by volcanism and tectonisn.
Significantly, their resurfacing model hat both volcanism and
tectonism vary in magnitude over space and time.

What follows is a simple description of the correlations
between actual numbers and densities of craters with elevation
and mapped terrain units and terrain groups based on the
available pre-Magellan crater data (Table 5). Statistical
confirmation of the crater distributions and interpretations
presented in this preliminary report must await the analyses of
the complete Magellan dataset.

Longitudinal and Latitudinal Distribution

The longitudinal distribution of the craters (Fig. 23) appears
to be rather uniform, except within two sectors. The sector
between long 270° and 310° has slightly more craters than the
longitudinal average, while the sector between long 177° and 270°
appears to have slightly fewer. The first sector, immediately
west of Lakshmi Planum, contains the maximum concentration of
large coronae, while the second sector includes a large fan of
ridge belts whose axis is centered along long 210° (Sukhanov and
Pronin, 1987; Kozak and Schaber, 1989).

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dominated by the high marginal belt (6056.57+2.28 km) which
averages 4.7 km above the MPR. The tessera group (6053.46t1.55
km) and the the upland plains group (6053.35+1.42 km) form a
second elevation level at a mean of 1.5 km above the MPR. A
third level with a mean elevation of about 6052 km is formed by
the hilly terrain group, lower marginal belt group,
shield/patera/tholus group, and domed upland group. The corona
group (6051.88+0.62 km) forms a transitional fourth hypsometric
level with a mean elevation exactly at the MPR, while the fifth
and lowest elevation level at about 6051.3 km is made up of the
lineated and fractured terrain group, the lowland plains group,
the group of suspected impact craters >45 km in diameter, and the
ridged terrain group. The mean elevation of this group actually
lies 0.6 km below the MPR.

SURFACE PHYSICAL PROPERTIES

Scattering properties of part of surface of Venus were
obtained by the Pioneer Venus and the Venera 15/16 spacecraft
(Pettengill et al., 1980 1988; Abramov et al., 1989; Alexandrov
et al., 1990). The parameters derived from the radar echos from
these spacecraft included the C-parameter and rho, the power
reflection coefficient (reflectivity) for normal incidence. The
C-parameter corresponds to the inverse of the mean square, meter-
scale surface slopes (in radians), or rms slope.

Data in a satisfactory digital and cartographic format were
available to the author only from Pioneer Venus at the time of
the preparation of this report. Values of rms slopes (C-power
2) and the power reflection coefficient were acquired by Pioneer
Venus only in the area between lat 60° S. and 75° N. (Pettengill
et al., 1980). Thus, the terrain units mapped by Schaber and
Kozak (1990) between 75° and 90° N. are not included in the
following discussion of rms slope and reflectivity.

Rms _ Surface Slopes
Figure 18 and Table 3 give rms data for the included units;

Figure 19 and Table 4, for the included groups.

The high marginal belt, low marginal belt, and tessera groups
dominate the more rugged terrains, while the ridged terrain,
lowland plains terrain, and lineated and fractured terrain groups
form the most extensive surfaces with mean rms slopes between 2°
and 3°.

Hina-type tessera and Lakshmi-type marginal belt terrains have
the largest mean rms slopes and standard deviations of any unit
mapped in the northern quarter of Venus (8.09°+4.00° and
7.45°+4.19°, respectively). (However, a single surface exposure
of the Hina-type tessera unit in southern Fortuna Tessera covers
only 0.23% of the map area.) Of the eight terrain units with the
highest rms slopes, seven are tesserae with mean rms slopes
between 4° and 8°.

The five ridged terrains have mean rms slopes between 2.3° and
3.0°, and thus they are intermediate between the rougher tesserae
and the smoother lowland plains and lineated terrains. The

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impact craters >10 km in diameter in 5° latitudinal
bins.

Figure 26. Histogram showing the number of suspected impact

craters >10 km in diameter recognized on 22 terrain
units.

Figure 27. Graph showing the relation between population of

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Table
Table
Table
Table

Table

WNr

impact craters >10 km in diameter recognized on 22
terrain units and percentage of map area covered by
those units.

28. Histogram showing the density of suspected impact
craters >10 km in diameter per 1,000,000 km? for 22
terrain units.

29. Histogram showing deviation in density of suspected
impact craters >10 km in diameter for 21 terrain units
relative to that of smooth plains unit.

30. Histogram showing the density of suspected impact
craters >10 km in diameter for 9 terrain groups.

31. Graph showing relation between the density of
suspected impact craters >10 km in diameter on 22
terrain units and mean elevation of those units.

32. Graph showing the elevation of 116 suspected impact
craters >10 km in diameter within map area.

33. Graph showing the density of suspected impact craters
>10 km in diameter relative to area-corrected
elevation.

34. Graph showing the relation between diameter and
elevation of 116 suspected impact craters within map
area.

LIST OF TABLES

- Terrain units mapped by Schaber and Kozak (1990)
- Terrain groups described in this report
- Physical parameters and crater densities for the 34
terrain units mapped by Schaber and Kozak (1990)
Physical parameters and crater densities of the 12
terrain groups described in this report
- Suspected impact craters >10 km in diameter

between lat 25° N. and 90° N.


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R.O., Bobina, N.N., Shashkina, V.P., Slyuta, E.N., and
Chernaya, I.M., 1989, Geomorphic/geologic map of part of the
northern hemisphere of Venus: U.S. Geological Survey
Miscellaneous Investigations Series Map I-2059, scale
1:15,000,000.

U.S. Geological Survey, 1989, Maps of part of the northern
hemisphere of Venus: U.S. Geological Survey Miscellaneous
Investigations Series Map I-2041, 3 sheets, scale
1:15,000,000.

FIGURE CAPTIONS
(In Figures 1-12, latitude bounding data at long 0° is 28.3° N.;
at long 90° it is 27.7° N.; at long 180° it is 46.5° N.; and at
long 270° it is 31.5° N.]

Figure la - Distribution of lowland plains terrain group. Group
consists of the following geologic/geomorphic terrain units:
smooth plains, intermediate plains, mottled plains, complex
plains, and domical hill or hill field.

Figure 1b - Distribution of smooth plains terrain unit

Figure 2 - Distribution of lineated and fractured terrain group,
consisting of lineated terrain, type 1;, lineated terrain, type
2; fractured terrain; and ridge complex terrain.

Figure 3 - Distribution of tessera terrain group, consisting of
Laima-type tessera; western Fortuna-type tessera; eastern
Fortuna-type tessera; Hina-type tessera; intensely fractured
tessera; and tessera, undivided, units.

Figure 4 - Distribution of ridged plains group, consisting of
ridge; ridge belt, type 1; ridge belt, type 2; and ridge swarm
units.

Figure 5 - Distribution of upland plains terrain group,
consisting of smooth plains of Lakshmi Planum and tessera-
mantling plains units.

Figure 6 - Distribution of shield/patera/tholus terrain group,
consisting of shield, crater (patera), and domical mesa (tholus)
units.

Figure 7 - Distribution of domed upland terrain group,
consisting of domed terrain of Beta Regio and domed terrain of
Bell Regio.

Figure - Distribution of corona terrain group, consisting of
corona, type 1, and corona, type 2, units.

Figure - Distribution of low marginal belt terrain group.
Contains only tessera-type marginal belt unit.

15


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Figure 23 - Longitudinal distribution of 116 suspected impact
craters >10 km in diameter within the area mapped by Schaber and
Kozak (1990).

Figure 24 - Relative abundance of suspected impact craters >10 km
in diameter in 1° latitudinal bins. Decrease in circumference of
latitude bands with increasing latitude was taken into
consideration by arbitrarily assigning a value of 1 to the
highest latitude band containing a crater (87.2° N.), to which
all other latitudinal bands were normalized.

Figure 25 - Relative abundance of suspected impact craters >10 km
in diameter in 5° latitudinal bins. Bands normalized as
described in figure 24.

Figure 26 - Number of suspected impact craters >10 km in diameter
recognized on 22 terrain units. Remaining 12 units mapped had no
recognizable impact craters. See Table 1 for explanation of

symbols.

Figure 27 - Relation between population of impact craters >10 km
in diameter recognized on 22 terrain units and percentage of map
area covered by those units. Combined populations of
intermediate (unit pi), mottled (unit pm), and complex (unit pc)
fall at top center. See Table 1 for explanation of other

symbols.

Figure 28 - Density of suspected impact craters >10 km in
diameter per 1,000,000 km’ for each of the 22 cratered terrain
units. See Table i for explanation of symbols.

Figure 29 - Deviations in density of suspected impact craters >10
km in diameter for 21 cratered terrain units relative to that of
smooth plains unit, which has a value of 0 on the y axis. See
Table 1 for explanation of symbols.

Figure 30 - Density of suspected impact craters >10 km in
diameter for nine terrain groups. No impact craters were
recognized on the remaining terrain groups. See Table 2 for
explanation of symbols.

Figure 31 - Relation between density of suspected impact craters
>10 km in diameter on the 22 cratered terrain units and mean
elevation of those units. See Table 1 for explanation of

symbols.

Figure 32 - Elevation of 116 craters >10 km in diameter of
suspected impact origin within map area. Note clustering of
craters into two elevation groups.

Figure 33 - Density of suspected impact craters relative to area-

17


LIST OF FIGURES
Maps showing areal distribution of 12 terrain groups and 1
terrain unit in northern quarter of Venus (from Schaber and

Kozak,

1990). Latitude bounding data at long 0° is 28.3°N.; at

long 90° it is 27.7° N.; at long 270° it is 31.5° N.

Figure la. Lowland plains terrain group
"

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1b. Smooth plains terrain unit

" 2. Lineated and fractured terrain group

" 3. Tessera terrain group

4. Ridged plains terrain group

" 5. Upland plains terrain group

" 6. Shield/patera/tholus terrain group

" 7. Domed upland terrain group

" 8. Corona terrain group

" 9. Low marginal belt terrain group

" 10. High marginal terrain group

" 11. Hilly terrain group

" 12. Suspected craters >45 km in diameter terrain

group.

13. Histogram showing percentage of map area covered by
each of the 34 terrain units.

14. Histogram showing percentage of map area covered by
each of the the 12 terrain groups.

15. Histogram showing the mean elevations and standard
deviations of the 34 terrain units.

16. Histogram showing mean elevations and standard
deviations of the 12 terrain groups.

17. Hypsometric diagram showing percentages of map area at
various elevations.

18. Histogram showing mean rms slopes and standard
deviations for the 34 terrain units between lat 25° N
and 75° N.

19. Histogram showing mean rms slopes and standard
deviations for the 12 terrain groups between lat 25°
N.and 75° N.

20. Graph showing relation between rms slopes and
elevation for the 34 terrain units between lat 25°
N.and 75° N.

21. Histogram showing mean reflectivities and standard
deviations for the 34 terrain units between about lat
25° N. and 75° N.

22. Histogram showing mean reflectivities and standard
deviations for the 12 terrain groups between lat 25° N.
and 75° N.

23. Graph showing the longitudinal distribution of 116
craters >10 km in diameter of suspected impact origin
within map area

24. Graph showing relative abundance of suspected impact
craters >10 km in diameter in 1° latitudinal bins

25. Graph showing the relative abundance of suspected

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26


TABLE OF CONTENTS

LIST OF FIGURES---

AREAL DISTRIBUTION OF TERRAIN MATERIALS
HY PSOMETRY -------------------
SURFACE PHYSICAL PROPERTIES-
Rms Surface Slopes----
Power Reflection Coefficient (Reflectivity)
DISTRIBUTION OF SUSPECTED IMPACT CRATERS-----
Longitudinal and Latitudinal Distribution-
Crater Population and Density-------- 12
Distribution in Elevation---
Crater Diameter and Elevation
ACKNOWLEDGMENTS---------- 13
REFERENCES CITED:
FIGURE CAPTIONS-



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craters (Fig. 32) and a corrected plot (Fig. 33) show a clear
separation of the same two groups. A more accurate portrayal of
the densities of crater with elevation is revealed when these
data are corrected for the percentage of the map area occurring
at a given elevation (Fig. 17).

Crater Diameter and Elevation
A plot of crater diameter against elevation (Fig. 34) shows

the same elevation clustering as in Figures 32 and 33. All of
the craters between elevations of 6054 and 6056 km are less than
30 km in diameter, while 24 (23%) of the 106 craters found
between 6050.5 and 6052.5 km are greater than 40 km in diameter.
Six of those craters (6%) have diameters between 70 and 144 kn.

ACKNOWLEDGMENTS

The author thanks Sherman Wu and Sherri Cantrell for
digitization of the geologic/geomorphic map (Schaber and Kozak,
1990, Plate 1). Appreciation is also expressed to Eric Eliason,
Nanci Isbell, Chris Isbell, and Tracie Stoewe for their help in
digital processing of the map data and to John Chadwick for his
support in analysis of impact crater data. Support to the U.S.
Geological Survey was provided under Contract W15,415 from the
NASA Planetary Geology and Geophysics Program, and under Contract
WO 8,777 from the NASA Magellan Project Office.

REFERENCES CITED

Abramov, A.V., Grechischev, A.V., Zherihhin, N.V., Zheltikov,
I.A., Kreslavsky, M.A., Levchenko, G.M., and Norozov, A.A.,
1989, Scattering properties of Venus surface derived from
Venera 15/16 data: (abstract). In Lunar and Planetary
Science XX, Lunar and Planetary Institute, p. 3-4.

Alexandrov, Yu. N., Krivtsov, A.P., and Rzhiga, O.N., 1990,
Venera 15 and 16 spacecraft: Some results of Venus surface
reflectivity measurements: (abstract). In Lunar and
Planetary Science XXI, Lunar and Planetary Institute, p. 13-
14.

Arvidson, R.E., Grimm, R.E., Phillips, R.J., Schaber, G.G., and
Shoemaker, E.M., 1990, On the nature and rate of resurfacing
of Venus: Geophysical Research Letters, 19, 1385-1388.

Basilevsky, A.T., and Ivanov, B.A., 1990, Cleopatra crater on
Venus: Venera 15/16 data and impact/volcanic origin
controversy: Geophysical Research Lettters, 17, 175-178.

Basilevsky, A.T., Ivanov, B.A., Burba, G.A., Chernaya, I.M.,
Kryuchkov, V.P., Nikolaeva, 0.V., Campbell, D.B., and Ronca,
L.B., 1987, Impact craters of Venus: A continuation of the
analysis of data from Venera 15 and 16 spacecraft: Journal
of Geophysical Research, v. 92, p. 12,869-12,901.

Campbell, D.B., Head, J.W., Hine, A.A., Harmon, J.K., Senske,
D.A., and Fisher, P.C., 1989, Styles of volcanism on Venus:
New Arecibo high resolution radar data: Science, v. 246, p.
373-377.

Ford, P.G., 1986, Pioneer Venus hypsometry: Massachusetts

13


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Figure 10 - Distribution of high marginal belt terrain group.
Group contains only Lakshmi-type marginal belt unit.

Figure 11 - Distribution of hilly terrain group. Group contains
only hilly terrain unit.

Figure 12 - Distribution of terrain group containing suspected
impact craters >45 km in diameter and their ejecta.

Figure 13 - Percentages of map area covered by each of the 34
terrain units. See Table 1 for explanation of symbols.

Figure 14 - Percentage of map area covered by each of the 12
terrain groups. See Table 2 for explanation of symbols and
included terrain units.

Figure 15 - Mean elevations and standard deviations of the 34
terrain units. See Table 1 for explanation of symbols. High and
low elevation bars represent the standard error, not absolute
elevations.

Figure 16 - Mean elevations and standard deviations of the 12
terrain groups. See Fig. 15 for explanation of high and low
elevation bars.

Figure 17 - Percentages of the region mapped by Schaber and Kozak
(1990) at various elevations.

Figure 18 - Mean rms slopes and standard deviations for the 34
terrain units between about lat 25° N. and 75° N. See Table 1
for explanation of symbols. See Fig. 15 for explanation of high
and low elevation bars.

Figure 19 - Mean rms slopes and standard deviations for the 12
terrain groups between about lat 25°N. and 75° N. See Table 2
for explanation of symbols. See Fig. 15 for explanation of high
and low elevation bars.

Figure 20 - Relation between rms slope and elevation for the 34
terrain units between about lat 25° N. and 75° N. See Table 1
for explanation of symbols.

Figure 21 - Mean reflectivities and standard deviations for the
34 terrain units between about lat 25° N. and 75° N. See Table 1
for explanation of symbols. See Fig. 15 for explanation of high
and low elevation bars.

Figure 22 - Mean reflectivities and standard deviations for the

12 terrain groups between about lat 25° and 75° N. See Table 2

for explanation of symbols. See Fig. 15 for explanation of high
and low elevation bars.

16