Characterization of Venera 15/16 Geologic Units from ... · Analysis of models for the formation of...

ICARUS 77, 3-20 (1989)

Characterization of Venera 15/16 Geologic Units from Pioneer Venus Reflectivity and Roughness Data

D. L. BINDSCHADLER AND J. W. HEAD

Department of Geological Sciences, Box 1846, Brown University, Providence, Rhode Island 02912

Received March 4, 1987; revised May 25, 1988

Geologic units have been defined for the surface of Venus from Venera 15/16 image data. A characterization of these geologic units is carried out using information on surface properties derived from Pioneer Venus (PN0 reflectivity and rms slope data. The geologic context provided by Venera 15/16 units allows additional, more specific interpretations of surface radar properties to be made. Characterization of Venera units results in the definition of four groups of Venera units: (I) smooth rocky units, (!I) rough rocky units, (llI) rough high dielectric units, and (IV) diffusely scattering units. On the basis of correlations of surface morphology to spatial and statistical distributions in ruts slope and reflectivity data, we test models for the origin of the surface properties of some units. We conclude that plains and tectonic units can be contrasted in terms of the average roughness of the surface and that tectonic deformation appears to roughen the surface at 0.5- to 10-m and 5- to 50-cm scales. This tectonic weathering process appears to dominate the erosional regime of Venus. Unlike Earth or Mars, production and transport of soils dominates only a small portion (~:5%) of the surface. Some of the Venera units display distinctive spatial and statistical distributions of PV radar data. In particular, apparent low reflectivity in the tesserae appears to be caused by small (5-50 cm) rock fragments on the surface which cause diffuse scattering at Pioneer Venus wavelengths. Analysis of models for the formation of these fragments suggests that they are due to the pervasive deformation undergone by the tesserae. Finally, aspects of this study have been used to extend results of Venera image data analysis southward of 30°N lat, resulting in a prediction of the distribution of tessera. Such results can aid in Magellan investiga- tions. ~ 1989 Academic Press, Inc.

INTRODUCTION

The collection of side-looking radar data by the Venera 15 and 16 spacecraft (Rzhiga 1984) presents us with an opportunity to link 1- to 3-km morphologic information to the regional and global scale surface properties derived from Pioneer Venus (PV) data (Head et al. 1985). In a preceding pa- per (Bindschadler and Head 1988a) we dis- cussed geologic interpretation of the PV data based on Head et al. (1985), and using additional information from the PV SAR (synthetic aperture radar) experiment. The resulting Pioneer Venus units and their interpretations are listed in Table I and form a

basis for analysis of the radar properties of Venera units defined on the basis of 1- to 3- km scale morphology.

Venera Resu l t s

In November of 1983, the Soviet spacecraft Venera 15 and Venera 16 began acqui- sition of synthetic aperture radar images of the surface of Venus north of about 30°N lat (Kotelnikov et al. 1985, Rzhiga 1984). The two orbiters were placed in highly elliptical orbits (periapsis -1000 km above 62°N, apoapsis 62,000 km), inclined 95 ° relative to the equator. Images were obtained at a res- olution of 1 to 3 km (Barsukov et al. 1984).

The Venera SAR instrument operated at

3 0019-1035/89 $3.00

Copyright © 1989 by Academic Press, Inc. All rights of reproduction in any form reserved.

4 BINDSCHADLER AND HEAD

T A B L E I

R E V I S E D P I O N E E R V E N U S U N I ' i S

S u r f a c e t y p e Def in i t ion A r e a " ( ~ )

E l e v a t i o n h (h)

L o w l a n d s h < 0 .0 25

Rol l ing p l a in s 0 .0 < h < 2.0 63

H i g h l a n d s 2 . 0 < h < 4.5 I 0

M o u n t a i n o u s r e g i o n s h .> 4.5 2

R M S s lope (0)

S m o o t h 1.0 ° < # < 2.5 ° 52

T r a n s i t i o n a l 2.5 ° < 0 < 5.0" 43 R o u g h 5 .0 ° < 0 < 10.0 ° 5

Re f l ec t i v i t y (p)

D i f fuse s c a t t e r e r s ' p < 0 .10 26

R o c k d o m i n a t e d 0 .10 < p < 0 .20 68

H i g h d i e l ec t r i c p > 0 . 2 0 6

" T o t a l a r e a o f V e n e r a 15/16 m a p is 92 .2 × 10 ~ km: .

h E l e v a t i o n s a re in k i l o m e t e r s r e l a t ive to the r a d i u s

o f 6051 kin . ' As m u c h as 5 % o f the s u r f a c e s i n t e r p r e t e d as dif-

f u s e l y s c a t t e r i n g m a y be soil d o m i n a t e d I scc

B i n d s c h a d l e r a n d H e a d 1988a).

a wavelength of 8 cm and mean incidence angle of 10 ° (Koteinikov et al. 1985). For this angle and for rms slope (0) values typi- cal of Venus (1.5 ° < 0 < 4.0°; Pettengill et

al. 1980, Head et al. 1985), the backscatter cross section of the surface is most strongly dominated by the topographic slope at the several- to 10-km scale, while variations in reflectivity and rms slope exert less influ- ence. This effect can be deduced from Hagfors law (Hagfors 1970), shown in Fig. I. The series of curves plotted here de- scribe radar cross section as a function of incidence angle for a variety of rms slope values. An incidence angle of 10 ° lies within the steeper portions of these curves and near the region where they cross. This means that small variations in surface slopes will alter the amount of backscatter more than will the same variation in rms slope. Since the radar cross section is linear in reflectivity, variations in this parameter are also less likely to be seen in the images, except where the surface is relatively pla- nar over several tens of kilometers. Thus, while roughness and reflectivity variations

may be visible in plains regions, they are often obscured in regions of more rugged topography.

Using photomosaics as a base, Barsukov et al. (1986) produced a geologic-morphologic map of the surveyed region of Venus (Fig. 6). The map covers Venus north of approximately 30°N lat, from 265°E east- ward to 210°E long and consists of 13 distinct units (Barsukov et al. 1986, Basi- ievsky et al. 1986, Bindschadler 1986). Table II summarizes the defining characteristics, type locations, and interpretations of these units. Six units are types of plains, denoted by " 'P" in the key to Fig. 6. They are distinguished from one another on the basis of the presence of features such as small ridges, domes, or radar-bright linear and arcuate features. Figure 2 shows the percent area covered by each of the Venera units, demonstrat ing that the plains cover nearly three-fourths of the mapped surface. The remaining quarter of the surface consists of two types of terrain: (1) Five units (total area - 2 5 % ) that are distinguished on the basis of various morphologies and ori- entations of the ridges and grooves that

. . . . . . . . . . . 7 . . . . . ~ . . . . : . . . . . I () ~

. ., 0 - 1 75 ~,

• ! . . ..

1 0 l / "~' i ~ 0 = 3 5 : ' j . : v,

' I

.c t o o . . . . , " 0 7 5 ~ - '

10 1 _ ~ \ • _~

i " i , o - 2 ~ . . . . . . - - . . . . - i

10 0 10 2 0 3 0

I n c i d e n c e A n g l e ( d e g

FiG. 1. H a g l b r s l aw c u r v e s s h o w i n g the e f fec t o f

v a r y i n g r m s s lope on the spec i f ic r a d a r c r o s s s ec t i on .

V e n e r a 15/16 i n c i d e n c e a n g l e is -- 10°; i n c i d e n c e a n g l e s

for a PV a l t i m e t e r m e a s u r e m e n t r a n g e f r o m 0 ° to 5 ° at p e r i a p s i s a n d u p to 8 ° at the a l t i t ude limit o f the ins t ru -

m e n t .

PV CHARACTERISTICS OF VENERA UNITS

TABLE II

DESCRIPTION OF VENERA 15/16 UNXTS _ m

Name (abbrev.) Description Type location ~ Interpretation

Smooth plains (Ps) Featureless, except for Sedna Planitia (Figs. 31, Flood basalts lobate bright and dark 32) patches

Same as Ps, also contain two large calderae

Abundant lobate dark and bright patches, gently rolling topography

Numerous 5- to 20-km domes, small ridge and groove "buttes"

Highland smooth Lakshmi Planum (Figs. 8, plains (Phs) 9, I 1)

Patchy, rolling Plains west of Nightingale plains (Ppr) (Fig. 19)

Dome and butte Northern Niobe Planitia plains (Pdb) (Fig. 40)

Band and ring Linear/arcuate ridges and Plains west of Bell Regio plains (Pbr) bright bands forming (Fig. 33)

arachnoid features

Ridge and band Linear ridges and bright Central Atalanta Planitia plains (Prb) bands, widely spaced (Fig. 23)

(~50 km) Volcanoes (vol) None given Feature west of

Nightingale (Fig. 19) Beta Regio, Bell Regio

(Figs. 29, 35) Mnemosyne Regio (Figs.

24, 25)

Domelike uplifts (du)

Coronae (or ovoids) (ov)

Banded terrain (bt)

Ridge belts (rb)

Furrow belts (fb)

Tesserae (or parquet terrain) (pq)

1000-km-scale topographic domes

Concentric + / - radial ridges and valleys, complex interiors

Subparallel linear ridges, spaced -10-20 km apart

Subparallel ridges forming sinuous belts -50-100 km wide

Subparallel linear grooves in a linear belt

Orthogonal to obliquely intersecting ridges and valleys

Q Figure numbers in Barsukov et al. (1986).

d o m i n a t e their surfaces (Fig. 3). These ridges and g rooves are thought to be tectonic in origin (Bas i levsky et al. 1986). (2) The vo l canoes uni t and the dome- l ike uplift uni t (total a rea - 4 % ) .

ANALYSIS

Compar i son o f P V and Venera Data

The P ionee r Venus a l t imete r opera ted at

Maxwell Monies (Figs. 10, 12)

E. Atalanta Planitia (Fig. 20)

1000 km SE of Maxwell (Fig. 34)

Tellus Regio (Fig. 39). Laima Tessera (Fig. 15)

Same as Ps

Low, overlapping shield volcanoes

Plains are flood basalts, domes are volcanoes; Buttes are inliers of tessera

Relaxed volcanoes (-Sukhanov 1987), Calde- rae (?) (Stofan and Head 1988)

Compression of plains (?)

Effusive volcanism

Rifting, volcanism, mantle "hotspot?"

Several postulated (see Stofan and Head 1987)

Compressional foldbelts (Campbell et al. 1983)

Compression? Extension? (numerous authors)

Extensional fractures? (reclassified in recent maps)

Several postulated (Sukhanov 1986. Bindschadler and Head 1988b)

a 17-cm wave l eng th and m a x i m u m angles of i nc idence f rom 5 ° to 8 ° (Pettengil l et al. 1980). Despi te the s imilar i ty in wave leng ths and inc idence angles to Venera , d i f ferences in reso lu t ion and the form in which data is col lec ted m e a n that the PV and Vene ra 15/ 16 radar i n s t r u m e n t s view the surface in very d i f ferent ways (Figs. 4 and 5). F igure 4 shows a topographic profile of a hypothe t i -


~b bl 021 du

5X I ~ l 2 ( OV lO~V~ 1 o |

6~Z

[•. Plains U.qltS

] Tectonic Units

Volcanic U,"HtS

FUG. 2. Pie chart showing the area covered by each of the Venera units. Total area of Venera unit map is 92.2 x 106 kmL See caption to Fig. 6 for key to unit abbreviations.

cal su r face , wi th re f lec t iv i ty (/9) and rms s lope (0) for the su r f ace profi le s h o w n be- low. The v o l c a n o and the r idges in the eas t - e rn par t o f the prof i le w o u l d not be d is t in- gu i shab le in PV a l t i m e t r y , but do have d i s t inc t su r f ace p r o p e r t i e s . The w e s t e r n po r t i on o f the prof i le is c h a r a c t e r i z e d by s m o o t h to t r ans i t i ona l r o c k y su r faces (Ta- ble I), qu i te t yp i ca l o v e r mos t o f the su r face o f Venus . The v o l c a n o in the eas t is cha r ac - t e r i zed by a t r ans i t i ona l high d i e l ec t r i c sur-

Ovo+O

Furrow belt

Banded lertain

Ridge belt

ParQuel terrain

. " . ' - . e ¢~" ~ : I x l~' /

<.:, - "~:..::~ i)~.~ e "~ ..

::'_5:..""

g r o o v e l

I ' idgol

" ' " " linoament

broad va l ley

FIG. 3. Schematic sketch of ridge and groove terrain units. Scale is somewhat variable, ridges are on the order of 1(I-20 km apart. Note the highly contrasting morphologies seen in the parquet terrain: (a) corresponds to the parquet ofSE lshtar Terra, (b) is located in Tellus Regio.

~r ~1 500 krn ~' I~

P V Da ta , S ing le Footpr in t s

FIG. 4. Cross-sectional illustration of a PV view of the surface. Size of PV footprint in the figure (100 kin) is an upper limit. The minimum dimensions are 23 x 7 km (Pettengill et al. 1980). Although PV altimeter cannot resolve details of topography of landforms in the right hand part of the cross section, the reflectivity (p) and rms slope (0) data display distinct correlations with these features. The boxed region is shown in greater detail in Fig. 5.

face , whi le the r idges to the eas t a re rough and d i f fuse ly sca t t e r ing , ind ica t ing a high deg ree o f su r face r o u g h n e s s at bo th the 5- to 50-cm and 0.5- to 10-m sca les .

The inset box in Fig. 4 is s h o w n in deta i l in Fig. 5 f rom the p e r s p e c t i v e o f a s ide- look ing r a d a r s y s t e m such as V e n e r a 15 or 16. It r e p r e s e n t s the V e n e r a o r b i t e r ' s c apa - b i l i ty to r evea l m a n y l a n d f o r m s cha rac t e r i s - tic o f v o l c a n i s m or t e c ton i sm . A l though va r i a t ions in re f l ec t iv i ty and sur face roughness m a y affect the b a c k s c a t t e r c ros s sec- t ion o f the su r face and thus the r a d a r " ' b r i g h t n e s s , " su r face s lope ef fec ts t end to con t ro l the i n t ens i ty o f the r e t u r n e d signal. The e f fec t s o f r e f l ec t iv i ty and rms s lope on the re f lec ted signal c a n n o t be quan t i t a t i ve ly s e p a r a t e d f rom one a n o t h e r o r f rom sur face t o p o g r a p h i c e f fec t s , i t m a y be qui te difficult to s e p a r a t e re f lec t iv i ty e f fec ts f rom rms s lope e f fec t s even in a qua l i t a t ive sense . Thus the PV and V e n e r a d a t a a re c o m p l e - m e n t a r y d e s p i t e nea r ly an o r d e r o f magni - tude d i f f e r ence in su r face r e so lu t ion . I f it we re found , for e x a m p l e , that all the vo lca- noes m a p p e d f rom V e n e r a images were c h a r a c t e r i z e d by a d i s t inc t r ise in va lues o f re f lec t iv i ty , this i n fo rma t ion wou ld be im-

PV CHARACTERISTICS OF VENERA UNITS 7

I ~oo k~ i look ~io~ I

look dir act.ion (

FIG. 5. Schemat ic Venera radar image, in cross-section and map view, demons t ra t ing effects of topography on side-looking radar images. In areas dominated by rapidly varying surface slopes, any radar br ightness effects due to rms slopes or reflectivity will be secondary to the slope effects shown here.

portant for understanding the origin of high dielectric surfaces on Venus.

M e t h o d

In order to examine the PV radar properties of the various Venera units, the geo- morphic unit map of northern Venus (Bar-

sukov et al. 1986) was digitized and placed into an equal-area map projection as were the PV altimetry, reflectivity, and rms slope data sets. Various images combining PV data and the Venera unit map were created to facilitate examination of spatial correlations between the data sets. Two images that best illustrate the comparison of the two datasets are shown in Figs. 6 and 7. Figure 6 consists of a shaded relief map of Venus topography overlain with colors corresponding to the various Venera map units. Figure 7 also uses the shaded relief map as a base, but colors correspond to the subdivisions in rms slope and reflectivity of Head et al. (1985) recently revised (Bindschadler and Head 1988a). Venera units are shown only in outline form. Figure 8 is a sketch map showing several promi- nent topographic regions and many of the locations referred to throughout the text.

To examine the average PV radar properties of each Venera unit, histograms and cumulative distribution curves of elevation, reflectivity, and rms slope were plotted for each unit (Bindschadler 1986). Table III summarizes these data by showing the per-

T A B L E I11

PV RADAR PROPERTIES OF VENERA UNITS

Venera Percent area for each PV unit b units ~

A B C D E F G H 1

Ps 12.0 9.4 0.8 43.8 30.8 1.0 0.7 1.3 0.2 Phs 11. I 4.7 0.0 55.3 22.8 1.3 3.4 1.0 0.5 Ppr 9.2 7.7 0.6 47.9 24.6 1.3 5. l 3.4 0.4 Pdb 10.3 12.4 I.O 41.5 29.5 0.6 1.5 2.8 0.5 Pbr 8.2 5.5 0.3 54. I 26.8 ! .0 2.3 1.8 0.0 Prb 2.8 0.9 0.0 56.0 15.7 0.3 20.5 4.6 0.2 vol 5.9 5.4 0.8 47.6 27.9 2.3 6.2 2.8 0.0 du 11.8 27.8 2.9 23.0 26.7 4.8 0.8 1.7 0.6 ov 1 i. I 10.5 1.8 28.6 35.4 3.3 2.8 5.0 1.5 bt 8.5 15.2 2.3 5.5 24.2 12.5 1.5 8.8 21.6 rb 9.5 16.0 2.2 30.2 36.9 3.0 0.6 1.2 0.4 fb 14.6 21.8 7.9 23.4 20.6 i 1.8 0.0 0.0 0.0 pq 13.2 32.5 10.2 15.0 18.2 7.0 0.7 1.6 1.5

° Key to abbrevia t ions for h Let ters des ignate var ious

al. (1985).

unit names is given in Table II. PV radar property units and are given in Head et


320* A~ 68 t I

.e J I I r

230* S0* b t pq fb ov r b

Phs vol du Ppr Pb r P r b

r e x " ( ~ l l ~ ~ ~ / FIG. 9. Mean altitudes of Venera units. Error bars are for one standard deviation. Units are ranked ac-

200* ~ ~ ~ , l ~ . , . ~ V / a 0 . cording to mean altitude. Note the general separation between the plains and other units.

170° ~ 1 1 0 " 140"

[ ] Study Area • Highlands [ ] Lowlands • Mountainous

FIG. 8. Sketch map of study area. Topographic features and names of regions commonly referred to throughout the text are shown here.

centage of each geologic unit characterized by the various subdivisions in PV reflectivity and rms slope.

The mean and standard deviation of elevation, rms slope, and reflectivity of each Venera unit are shown in Figs. 9-11. In general, the plains units are characterized by lower mean elevations and lower mean values of rms slope, while the volcano and ridge and groove units are characterized by higher mean values of these parameters. Such a grouping is not consistent with information from reflectivity (Fig. I1). In addition, there is a great deal of overlap in the range of rms slopes, reflectivity, and eleva-

tions seen within the various units; for most units, the mean value of each parameter falls within one standard deviation of the corresponding value for nearly every other unit.

Several methods might be used to characterize the Venera 15/16 units in terms of their PV radar properties. For example, statistical cluster analysis (e.g., Davis et al.

1986) might be used to separate out charac- teristic radar properties for the Venera units. However, the high degree of similar- ity of many of the Venera units leads us to pursue a more qualitative approach to characterization.

In order to establish a context for comparison of the Venera units to one another, the reflectivity and roughness properties of these units were compared to the distribution of these parameters over the entire portion of Venus mapped in Fig. 6 and covered by PV data. Figures 12 and 13 are histograms showing the distribution of rms

FIG. 6. Venera 15/16 geologic units overlain onto topography. Shaded relief of PV topography was created with light source at 0 ° azimuth and 35 ° above the horizon. Units are identical to those in Barsukov et al. (1986, Fig. 41). Map is in a Lambert azimuthal (equal area) projection. Map Key: Ps, smooth plains; Phs, highland smooth plains; Ppr, patchy rolling plains; Pdb, dome and butte plains; Pbr, band and ring plains; Prb, ridge and band plains; vol, volcanoes; du, dome-like uplifts; ov, coronae (also ovoids); bt, banded terrain; rb, ridge belts; fb, furrow belts; pq, tessera (also parquet terrain).

FIG. 7. Map of PV units in reflectivity and rms slope for study area. Units are shown as colors overlain onto topographic shaded relief map, similar to Fig. 6. Venera units are displayed in outline form (white lines).

10 B I N D S C H A D L E R A N D H E A D

7 "

o 4

3

© 2

1

0 ' I bt

Pq

' i fb

rb

I ' I ' ov Pdb vol

du Ps

r i I Ppr I P r b

Phs Pbr

FIG. I0. Mean rms slope values of Venera units. Error bars are for one standard deviation. Units are ranked according to mean rms slope. Note the differences between the plains and the tectonic units.

slope and reflectivity for this portion of Ve- nus. The data were binned according to di- visions in reflectivity and roughness noted previously (Table I). Similar distributions were calculated for each of the 13 Venera units. In order to compare these distributions to one another, the appropriate average distribution (Fig. 12 or Fig. 13) was sub- tracted from each unit's histogram. The result is a measure of the deviation from average surface properties for each of the Venera units in terms of residual percent area and is plotted in Figs. 14 and 15. With this technique as well as information on the spatial correlation of Venera and PV units (Figs. 6 and 7), we may establish groups of

0 3 -

>.

0 2 - III I O0 ' I ' I ' I

bt vol P b r Prb Ppr o v

' [ ' I ' I ' - - - - Phs Ps du pq

Pdb rb lb

FIG. I I. Mean reflectivity values of Venera units. Error bars are for one standard deviation. Units are ranked according to mean reflectivity. The ordering present in Figs. 9 and 10 is disrupted here. Prb and bt are adjacent, but disparate in terms of rms slope and altitude. Pq and bt are very different but were similar in Figs. 9 and 10.

6 0

5O

~, 4 0 <

~. 2o

10

0 smooth t rans i t i ona l rough

FIG. 12. Histogram showing distributions of rms slope units over the Venera 15/16 unit map.

Venera units based on their surface radar properties.

R E S U L T S

Groups Derived from RMS Slope Data

The portion of Venus mapped by the Venera orbiters is predominantly smooth to transitional in roughness and is comparable in roughness to the whole of Venus. Only about 5% of the surface is characterized by rms slopes >5 ° (Fig. 12). The residual percent area of smooth, transitional, and rough surfaces for all the Venera units is shown in Fig. 14. The ridge and band plains unit (Prb) is the "most smooth" of the Venera units and the banded terrain the "least smooth." Rough surfaces are nearly equally distributed among all units except for the furrow belts, tessera, and banded terrain, which are all characterized by relatively abundant rough surfaces.

Two groups of units are suggested by Fig. 14. The first consists of all plains units

8 0

6 0

< 4 0 -

m 2 0 "

d i f f u s e r o c k h z g h d t e l o c t r l c

sca t te ro rs

FIG. 13. Histogram showing distributions of reflectivity units over the Venera 15/16 unit map.


40

~ i ooth f x, .

= .....

~ I I I [ I I I I [ I I I T I Prb Pbr vol Pdb rb du bt

Phs Ppr Ps ov fb pq

FIG. 14. Comparison of rms slope properties of the Venera units. Plotted quantity is the differential percent area covered by smooth, transitional, or rough surfaces between a given Venera 15/16 unit and the entire Venera unit map. For example, the points along the unbroken line (smooth surfaces) are calculated by subtracting the percent area within the entire unit map covered by smooth surfaces from the percent area covered by these surfaces within a given unit. Dotted line gives this quantity for transitional surfaces, dashed line corresponds to rough surfaces (see Table I for PV unit definitions). For example, Prb is characterized by -5% less than average abundance of rough surfaces, 20% less than average abundance of transitional surfaces, and nearly 30% greater than average abundance of smooth surfaces. Note the break between plains (and vol) and tectonic units in differential percent smooth surfaces.

and the volcanoes unit and is characterized by more smooth surfaces, fewer rough surfaces, and predominantly fewer transitional surfaces than average. The second group consists of the tectonic ridge and groove terrains with the addition of the dome-like uplift unit. This group is characterized by a depletion in smooth surfaces, enrichment in transitional surfaces, and a slight to strong enrichment in rough surfaces.

Groups Derived from Reflectivity Data

In contrast to the relatively simple grouping of the Venera units by rms slope, the same analysis for reflectivity reveals more complexity. The mapped region is dominated by rocky surfaces, with diffuse scatterers found over approximately 30% of the surface and high dielectric surfaces over only 6% of the map (Fig. 13). This is quite similar to the global distribution of reflectivity (Garvin et al. 1985). Residual percent

area of diffusely scattering, rocky, and high dielectric surfaces for the Venera units are shown in Fig. 15.

On the basis of Fig. 15, we recognize three groups of units. The first consists of the dome-like uplift, furrow belt, and tessera units, all characterized by surfaces that are enriched in diffuse scatterers and slightly depleted in high dielectric phases. Rocky surfaces are depleted in this group in a fashion that is complementary with their enrichment in diffusely scattering surfaces. This is consistent with the interpretation (Bindschadler and Head 1988a) that the in- trinsic reflectivity of the surface of these units is in the rock-dominated range (0 . I0 - 0.20) and that diffuse scattering is mostly responsible for the low reflectivity values in the uncorrected data set.

The second group consists of the plains units and the volcanoes, ridge belt, and corona. These units possess average to less than average abundances of diffusely scattering surfaces, are average in terms of area covered by high dielectric surfaces, and are enriched in rocky surfaces.

The banded terrain and ridge and band plains might be combined to form a third reflectivity group by virtue of their abundant high dielectric surfaces. However ,

o_

i- i

I I I I l l l l l l l l l

~', hlgh dieletnc /

.o

k / d i f f u s e s c a t t e r e ~

~ l _ . l l l l l l l l [ I t bt vol Phs Ps Pdb du pq

Prb Pbr Ppr ov rb

FIG. 15. Comparison of reflectivity properties of the Venera units. Plotted quantity is same as in Fig. 14, but for diffusely scattering, rocky, and high dielectric surfaces. Dotted line is for rocky surfaces, dashed line is high dielectric surfaces, and solid line corresponds to diffusely scattering surfaces (see Table I for PV unit definitions). Behavior of units is complex; see text for analysis.


close examination of these two units shows more differences than similarities in their PV radar properties. The residual percent area covered by rocky and diffusely scattering surfaces are very different, and detailed histograms of the reflectivity of the banded terrain and ridge and band plains (Bindschadler 1986) show that the two arc not very similar for p > 0.20. In particular, 25% of the banded terrain is characterized by t') > 0.25, but this is the case for only 6% of the Prb. We also note that high reflcctivity areas within Atalanta Planitia (Figs. 7 and 8) are largely responsible for the abundance of high dielectric surfaces in the Prb (Fig. 6) and that there is little spatial correlation between the high reflectivity values and the limits of the Prb. The higher rcflec- tivity of the Prb appears to reflect the regional high reflectivity in Atalanta rather than effects which might be linked to sur- lace morphology. Thus. the banded terrain is distinct and the ridge and band plains unit more properly belong to the second group of units (plains, volcanoes, ridge belts, and coronae).

Group,~ in Re,[lectiuity and R M S Slope

Combining reflcctivity and roughness information on unit groups, wc define four groups of Venera units. Each group is characterized relative to a surface with average surface properties for the Vencra unit map (Figs. 12 and 13).

Smooth rocky units. The first group consists of plains and the volcanoes units and is the largest in both areal cxtent and number of units. It covers almost 75% of the mapped region (Fig. 2) and is characterized by more smooth and rocky surfaces and fewer transitional and diffusely scattering surfaces than the average. Thc dome and butte plains and smooth plains, notable for their respective associations with tesserac and ridge belts (Bindschadler 1986) are the least smooth and most diffusely scattering of the smooth rocky units and arc the most similar to the rough rocky units Isce be- low). The volcano unit is very similar to the

plains in terms of surface properties. This is consistent with the idea that the plains are chiefly volcanic in origin, as is suggested by the presence of lobate radar-bright and -dark patches and/or domes within most plains units (Barsukov et al. 1986) and by surface morphology and geochemistry at the Venera 10, 13, and 14 landing sites (Florensky e t a / . 1977, Garvin et al. [984, Surkov 1983, Moroz 1983).

Rough rocky units. The second group of units consists of the coronae and ridge belts. Although similar to the smooth rocky units in terms of reflectivity, both coronac and ridge belts are characterized by fewer smooth surfaces and by more transitional and rough surfaces than are seen among the plains units group. Although the two groups of units may appear to be more similar than they are different on the basis of Figs. 14 and 15, the contrasts in surface properties between them as shown in the two figures are likely to be lower limits. The width in plan of both the ridge belts and of the ridges and grooves that define each corona is on the order or" the size of a tbotprint of the PV radar system. This suggests that individual PV measurements arc likely to have been influenced by the surface properties of both plains and coronae/r idgc belts, thereby di- minishing the measured contrasts in surface properties. Other indications of contrasts between the two groups comes from examination of Figs. 6 and 7. The coronae Night- ingale and Earhart (Fig. 7) arc relatively large and Nightingale is morphologically fresh, in contrast to the subdued, partially buried appearance of" many coronae (Barsu- kov et al. 1986, Stofan and Hcad 1986). These two features possess distinct reflectivity and roughness signatures (Fig. 7) that may represent the surface properties of relatively fresh, unaltered coronac. In addition, a large region west and south of Ata- lanta Planitia and dominated by ridge belts tFig. 6) is also a broad area of enhanced roughness. In some parts of this ridge belt province, diffuse scattering is observcd and linear zones of enhanced roughness corre-


late with the mapped locations of ridge belts.

Rough high dielectric units. The third group consists solely of the banded terrain unit. This unusual terrain can be characterized by rougher than average surfaces that also contain relatively large amounts of high dielectric materials. With the excep- tion of Vesta Rupes, this unit stands out distinctly in all three Pioneer Venus data sets (Fig. 7) and displays a highly nonuni- form statistical distribution of rms slope and reflectivity values. This distribution is in part a function of the differences between the surface properties of Maxwell Montes as compared to Vesta, Akna, and Freyja, but is also due to the large range of values of rms slope and reflectivity measured within Maxwell. A comparison of Fig. 7 with the statistical distributions in Figs. 14 and 15 suggests that Maxwell con- trols the mean of the surface properties in the banded terrain. Although somewhat similar in morphology to the ridge belts (Fig. 3), the mountains of Akna and Freyja are far more topographically distinct and are among the few regions on Venus where rough rocky surfaces are seen (brown in Fig. 7). Maxwell Montes, although very rough compared to Akna and Freyja, is most distinctively characterized by its extremely high radar reflectivity.

Rough diffusely scattering units. The fourth and final group of Venera units consists of the tesserae and furrow belts. It is characterized by relatively few smooth surfaces, abundant transitional and rough surfaces and most distinctly by abundant diffusely scattering surfaces. Figure 7 highlights this correlation most distinctly within the tesserae, which appears as predominantly white regions (transitional diffusely scattering surfaces) with associated magenta regions (rough diffusely scattering surfaces). The furrow belts share this distinct appearance, although they are extremely limited in extent (Fig. 6).

Within the dome-like uplift unit, the spatial distribution of diffusely scattering sur-

faces is different than the tesserae and furrow belts, despite their similar appearances in Figs. 14 and 15. Bell Regio contains some areas where diffuse scattering is important, but is dominated by smooth and transitional rocky surfaces. Most surfaces characterized by low values of uncorrected reflectivity within Beta Regio lie on the northern flanks. The interior of Beta, which was not mapped by Veneras 15 and 16, is quite distinct from the flanks in terms of surface properties. Reflectivity and roughness in- crease inward toward the central rift/volcanic complex (Fig. 7), consistent with Earth-based observations of high radar backscatter in the central rift complex (Campbell et al. 1984). A more complete characterization of Beta and Bell Regiones might group them with the plains, although the distribution of surface properties within these regions are complex and may be distinctive.

In summary, four groups of Venera units can be defined from analysis of the reflectivity and roughness properties of these geologic units: (I) the smooth rocky units, consisting of the plains and volcanoes units; (2) the rough rocky units, which con- sist of the coronae and ridge belts; (3) rough high dielectric units, consisting of the banded terrain; and (4) diffusely scattering units, including the tesserae, furrow belts, and domelike uplifts.

DISCUSSION

Implications for Interpretation of RMS Slope Data

Reflectivity and rms slope data have been interpreted in terms of the material properties of the surface (rocky or high dielectric) and relative roughness at the 0.5- to 10-m scale (smooth, transitional, and rough surfaces) and the 5- to 50-cm scale (diffusely scattering surfaces) (Bindschadler 1986, Bindschadler and Head 1988a). While quite useful for characterizing the surface in a relative sense, the abstract and generalized character of terms such as "diffuse scatter-


e r s" makes it difficult to establish the small scale morphology of the surface in a geological context . In isolation from other information, roughness data cannot distinguish between the continuously undulating surface of a dune field and the more disorderly surface of a boulder field. However , data from the Venera orbiters and landers provides a geologic context which facilitates interpretation of the PV data. We examine the relatively high rms slope values found within the tectonic units, attempting to establish more specific interpretations of the character of the surface.

The PV measurements of small scale surface roughness show that the tectonic units mapped by Veneras 15 and 16 all possess enhanced 0.5- to 10-m roughness as compared to the plains units (Fig. 14). More- over, areas of enhanced roughness often match closely with the boundaries of such units as banded terrain, ridge belts, tesserae, and coronae (Fig. 7). This correspon- dence of enhanced roughness to tectonic units suggests a genetic link and an explanation of the observed positive correlation of rms slope to elevation (Pettengill et al. 1980).

The relative paucity of Venusian soils (Head et al. 1985, Garvin et al. 1985, Bindschadler 1986, Bindschadler and Head 1988a) argues against dunes or wind ripples as the predominant cause of such surface roughness. An interpretation more consistent with all the available data is that fracture and deformation of the surface during tectonism leads to the creation of topographic slopes. Blocky surfaces then de- velop by downslope mass wasting, resulting in increased surface roughness. We will refer to this process as " tec tonic weathering." Tectonic weathering will result in the formation of block-littered slopes and talus piles at the base of ridges. It is expected to be most intense in areas of steepest ridge topography and/or most pervasive deformation. Of the units mapped by the Venera orbiters, the steepest slopes at the 10- to 20- km scale are to be found within the banded

terrain. This unit also possesses the highest mean value and the highest absolute values of rms slope of the Venera units (Figs. 10 and 14). Although not as topographically extreme as the banded terrain, the tessera is also characterized by a high mean value of rms slope (Fig. 10). Coronae and ridge belts are less distinctive in terms of their rms slope properties, in keeping with gen- tler topography and possibly less intense deformational histories.

Thus, the correlation of high values of rms slope with tectonic units when coupled with the observation that soils are relatively sparse on the surface of Venus suggests that tectonic deformation and weathering of the surface arc closely linked. The correlation of elevation and rms slope can then be viewed as a coincidence. It is simply due to the fact that tectonically disrupted regions also tend to lie at high elevations and does not imply the presence of elevation-depen- dent chemical weathering processes. The combination of PV and Venera data suggests that tectonics are a primary cause of mechanical weathering and subsequent roughening of the surface of Venus at the 0.5- to 10-m scale.

Impl icat ions f o r Venus Geology

The erosional environment on Venus has been the object of several studies which have at tempted to assess its importance to the evolution of the surface of the planet (e.g., Brass and Harrison 1982, Greeley et al. 1984). The extremely high atmospheric pressure leads to an environment in which the transport of even relatively large particles at low wind speeds is quite efficient (threshold velocities for l-mm particles of ~-I0 cm/sec) (Greeley et al. 1984, Greeley and Marshall 1985, Bougan et al. 1986). However , for erosion to occur, some combination of chemical and mechanical weathering must also produce material (i.e., soils) suitable for transport.

Most evidence suggests, however, that there is little soil to transport, despite the


fact that crater counts indicate that the average surface age of the northern 25% of Venus is -0 .5 to 1.0 byr (Ivanov et al.

1986, Schaber et al. 1987). Venera 15/16 data reveal only sparse evidence for aeolian landforms at the 1- to 10-km scale (Barsu- kov et al. 1986, Basilevsky et al. 1986). Im- ages from the Venera landing sites do not reveal any typically aeolian landforms (e.g., dunes, sand ripples), and only at the Venera 9 landing site is there a significant possibility of soil depths greater than several centimeters. Erosion rates calculated from the frequency of occurrence of radar- bright (rough) haloes around morphologically fresh impact craters are less than I m/ myr (Ivanov et al. 1986). Finally, corrected PV reflectivity data (Ford and Pettengiil 1984) suggest that only - 5 % of the surface is dominated by soil thicknesses great enough to affect PV radar (-10 's of cm).

Although weathered materials in the form of soils do not dominate the majority of the surface of Venus, they could still be important within restricted regions. Some of the largest erosional deposits on Earth are submarine deltas related to nearby or- ganic belts (e.g., the Bay of Bengal fan). Given the relatively steep topographic slopes and fractured surfaces of most tectonic units, weathering is likely to be more intense within these units, and soils could be deposited in and around banded terrain, tessera, and other tectonic units. However, tectonic units and their surrounding regions are typically characterized by high values of rms slope rather than by low values of reflectivity (Table III, Fig. 7). Even the low reflectivity of the tessera is due to small scale (5-50 cm) roughness, rather than broadly distributed soils (Bindschadler 1986, Bindschadler and Head 1988a).

Venus appears to be characterized by an erosional regime that is quite different from that of the other terrestrial planets. On the Earth, physical and chemical weathering and transport of material are most strongly associated with the presence of liquid water, with the aeolian processes comprising a

secondary component. Rates of subaerial erosion are significant enough to expose deep structural layers at the surface and to ensure that most of the land surface is covered by soils. On the Moon and Mercury, physical weathering and transport due to the impact process dominates the erosional regime. On these two bodies, there has been little erosional activity since -3 byr ago but activity prior to that time was intense enough to produce a global regolith (soil layer). Mars occupies an intermediate position between the smaller bodies and Earth. The surface of Mars partially re- cords a time of high impactor flux, but this record is modified by aeolian processes. Like Earth, Moon, and Mercury, the surface of Mars is predominantly soil covered. The presence of large amounts of soil (i.e., sufficient to form near-global soil layers) on these planets is largely due to three processes: (I) chemical weathering in the presence of liquid water, (2) physical weathering due to impact, and (3) physical weathering due to aeolian processes.

On Venus there is no liquid water, and the scarcity of impact craters relative to the smaller terrestrial planets suggests that impact-derived soils are likely to cover only minor portions of the surface. While both aeolian weathering and nonhydrous chemical weathering may be active, the low abundance of soils, and particularly their lack in association with tectonic units, strongly re- stricts their relative importance. In terms of the area covered, the most significant weathering process on Venus is the process responsible for the enhanced small scale roughness of the tectonic units, which we suggest is a process of tectonic weathering. This process is not strictly erosional, since little lateral transport of material is in- volved. We interpret the available observations on the erosional regime of the surface of Venus to indicate that the morphology of the surface at scales of 1 km or greater is largely unaltered by erosional processes. Interestingly, this provides an opportunity to examine surface manifestations of tec-


tonism in a near-pristine state, in striking contrast to the situation on the Earth.

Surface Manifestations o f Volcanism and Tectonism

As seen by the PV orbiter, the predominant surface manifestation of tectonism on Venus appears to be the increased surface roughness within the ridge and groove terrains at the 0.5- to 10-m scales. There are some hints that surface manifestations of tcctonism (and volcanism as well) can be detected in the PV data within some of the plains units. The roughest of the plains units (at rms slope scales) are the smooth plains (Ps) and dome and butte plains (Pdb) (Figs. 10 and 14), respectively noted for their associations with ridge belts and with volcanic domes and ridge and groove " 'but tes" (Barsukov et al. 1986, Bind- schadler 1986). Ridge belts, as noted previously, are enhanced in 0.5- to 10-m scale roughness. All mapped occurrences of the Pdb are partially bounded by tesserac. The " b u t t e s " that help to characterize the Pdb ('[,able 1I) may be outliers of tcssera and thus share the enhanced roughness of the tesserae.

The presence of volcanic domes may also contribute to the roughness of the Pdb through rough lava flows and rough deposits due to explosive activity. The high atmospheric pressure inhibits but does not completely rule out explosive activity (Head and Wilson 1986), especially if strombolian in nature. However , examination of the size distribution of domes suggests that they may be analogous to sea- mounts, and thus predominantly effusive in nature (Aubelc et al. 1988, Slyuta et ul. 1988). Thus the roughness of the Ps may be related to the occurrence of tectonic ridge belts (and thus originates due to tectonic weathering), while the roughness of the Pdb may be related to outliers of tessera as well as to volcanic constructs primarily duc to effusive activity.

Tessera is among the most unusual of the units mapped by the Venera orbiter. This

unit is notable not only for its unusual morphology at !- to 3-km scales (Barsukov et al. 1986, Basilevsky et al. 1986), but also for its unusual radar characteristics (Table 11I, Figs. 7, 14, and 15), partly due to the presence of abundant small (5-50 cm) rock fragments (Bindschadler 1986, Bindschad- let and Head 1988a). The distinctive correlation between these small fragments and the boundaries of the tesserae suggests that surface fragmentation is an important as- pect of the origin and/or evolution of this terrain. We investigate three models to explain the abundance of diffuse scatterers within the tessera.

Model I. The tessera is relatively old compared to other surface units. This age relationship was suggested for eastern lshtar Terra and Tellus Regio on the basis of low values of uncorrected reflectivity in these regions, which were interpreted to indicate abundant soils (Head et al. 1985). "['he corrected reflectivity data of Ford and Pettengill (1984) reveal that these low values in the uncorrected data are due to abundant diffuse scatterers, interpreted to be 5- to 50-cm rock fragments. Thus, in Model I the abundance of 5- to 50-cm rock fragments is taken to be a function only of the age of surface rocks; the degree or type of tectonic activity in the tesserae does not exert any effect.

According to Model I, diffuse scattering is not generally observed in other terrains because they are younger. Some evidence exists in support of the hypothesis of a relatively old tesserae in the Venera image data. Small regions of tessera are almost always obviously embayed by plains, and embayment of some of the larger regions (e.g., Fortuna Tessera) has been suggested, although stratigraphic relations are often ambiguous (Barsukov et al. 1986). In addition, the appearance of the tesserac in map view (Fig. 6) is often suggestive of embayment by plains (e.g., the SE lshtar - -Tel lus region). Crater counts reveal no large age differences between the tesserae and other units, but may reflect a deformational age


rather than the time since surface rocks were formed. Thus Model 1 is consistent with presently available data, but other pos- sibilities remain.

Model 2. The intensive deformation that created the tessera resulted in the pervasive fracture o f the surface, a process o f tectonic weathering. Several models have been proposed to explain the unusual morphology of the tessera (Sukhanov 1986, Ko- zak and Schaber 1986, Bindschadler and Head 1988b). Deformation causing ridge and valley formation and mechanical fracture of the surface would lead to tectonic weathering as proposed to explain the high rms slope values observed for the tectonic units. The enhanced 5- to 50-cm scale roughness of the tessera relative to other tectonic units may be due to a greater amount of horizontal strain, or to a different deformational regime.

Problems with Model 2 may be presented by the radar characteristics of the banded terrain. This unit is thought to be due to intense compressional tectonics (Campbell et al. 1983, Solomon and Head 1984) and thus tectonic weathering would be expected to occur within this unit also. The banded terrain is characterized by high values of rms slope, but the reflectivity signa- ture of the unit lies mostly in the rock-dominated to high dielectric range of values (Table III).

An explanation for the differing radar characteristics of the banded terrain and tessera may be found in the PV data. Por- tions of Aphrodite are characterized by values of rms slope and reflectivity similar to those found in the banded terrain (Head et al. 1985). A map of diffuse scattering derived from PV SAR data (and thus available only for the equatorial region of Venus) (Bindschadler and Head 1988a, Ford and Pettengill 1984) shows that these regions in Aphrodite are also diffusely scattering. This suggests the possibility that, as in Aphrodite, diffuse scattering occurs in the banded terrain but is masked by the high reflectivity of the surface materials. Are-

cibo data could potentially be used for Maxwell in the same fashion as PV SAR data (Ford and Pettengill 1984) was used in Aphrodite to test for the presence of diffuse scattering.

Model 3. The tessera is different in composition and thus more susceptible to either chemical or mechanical weathering than other surfaces. The scarcity of soils on the surface of the tesserae tends to argue against chemical weathering process, while mechanical weathering still requires either greater age (Model 1) or tectonic weathering (Model 2) for the tessera. However, the fact that regions of tesserae tend to rise at least a few hundred meters above the surrounding plains suggests the possibility that the terrain is composed of less dense materials than the plains (Nikolaeva et al. 1988).

To some extent, the three models may be evaluated on the basis of observations. Model 3 appears to be the least likely given the scarcity of soils within the tessera. In addition, measurements of of ridge spac- ings (lvanov 1988) suggest that the elevation of regions of tessera could be explained by crustal thickening. Both Models I and 2 appear to be reasonable and largely consistent with the PV and Venera data. It is possible that the relative age of the tcssera (Model I) is the cause of its unusual radar properties. However, the lack of a distinctly older surface within the tesserae and the correlation of small scale roughness to tectonic units is more consistent with a model of tectonic weathering (Model 2) for the formation of diffuse scatterers in the tessera.

CONCI.USIONS

The study of the surface of Venus using radar data has begun to reveal details of the geologic processes that shape the surface at several scales.

(1) Surface morphology-surface property correlations. Distributions of PV surface units within the Venera geologic units demonstrates correlations between surface morphology and surface properties such as


small scale roughness, diffuse scattering, and composition.

(2) Complementary nature of PV and Venera data. The Venera 15/16 images pro- vide a geologic context that allows for more detailed interpretation of PV-measured surface properties. In turn, the reflectivity and roughness data act to constrain models for the formation of Venera units.

(3) Venera units groups based on PV data. Characterization of the Venera units on the basis of PV radar properties shows that four groups of units can be distinguished.

(l) Plains and volcanoes units (smooth rocky units) are characterized by smooth to transitional roughness and rocky surfaces (predominantly volcanic in origin).

(ll) Coronae and ridge belts (rough rocky units), which are similar to the plains in porosity and bulk density but rougher at 0.5- to 10-m scales, are likely to be composed of deformed plains material.

(111) Banded terrain (high dielectric units) is characterized by extremely high values of reflectivity, rms slope, and topography. This unit is compositionally unusual; it also appears to have undergone extreme deformation (Crumpler et al. 1986. Vorder Bruegge et al. 1986).

(IV) Tesserae and furrow belts (diJ: fusely scattering units) arc characterized by enhanced surface roughness at both the I- to 10-cm and 0.5- to 10-m scales.

(4) Erosional regime of Venus. The low abundance of soil, lack of water, lack of abundant impact craters, and age of the sur- lace imply that production and transport of soils is inefficient in comparison to Earth or Mars. Thus, the geomorphology and radar properties of the surface should bc pre- served in a relatively pristine state and be directly related to volcanic and tectonic processes.

(5) 72,ctonic weathering. In general, the plains and tectonic units can be contrasted in terms of percent area covered by smooth surfaces (Fig. 14). with plains characterized

by more smooth surfaces. One of the effects of tectonic deformation of the surface appears to be a mass-wasting process (tectonic weathering) that causes progressive roughening of the surface at both the 5- to 50-cm and 0.5- to 10-m scales.

(6) Distinct radar properties of the tessera. The tessera terrain unit is the most distinctive of the Venera units in terms of PV reflectivity and roughness values. It is characterized by surfaces that are relatively rough at both the 0.5- to 10-m and 5- to 50- cm scales. The smaller scale of roughness produces diffuse scattering of the 17-cm wavelength PV radar and appears to be ex- pressed as 5- to 50-cm-size rock fragments. Examination of models to explain the presence of these fragments in the tesserae (as well as their absence in most other regions) suggests that they are primarily due to weathering induced by tectonic delbrma- tion, with older relative age of the tessera as a possible contributing factor.

(7) Extension o f results to the south of Venera coverage. Some results of this study can be extended to the south. Linking surface properties to groups of geologic units has led us to characterize the regions surrounding the Vega landing sites as most similar to the smooth rocky units (Bindschadler and Head 1986). Kreslavsky et al. (1987) have produced a prediction of the distribution of tessera for the entire PV data set using PV reflectivity and roughness information. Evaluation of this prediction (Bindschadler et al. 1988) using PV SAR data, Arecibo data, and a map of diffuse scattering (Bindschadler and Head 1988a) suggests that it is quite accurate. As an example, Alpha Regio (-20°S, 3°E) is pre- dicted to be tesserae on the basis of PV reflectivity and roughness properties. Are- cibo radar image data of Alpha Regio show alternating bright and dark bands spaced approximately 10-20 km apart that form a very complex structure. Following the interpretation of Campbell et al. (1983), these bright and dark bands are roughness variations due to talus shed from topographic


slopes. Thus Alpha Regio is very likely to be tessera. Such information will aid inves- tigators in targeting analysis of Magellan data.

ACKNOWLEDGMENTS

We thank Ellen Stofan and Scott Murchie for con- structive criticism, Paul Fisher for computer assis- tance, and Susan Slavney for special image photogra- phy. Special thanks to V. L. Barsukov and A. T. Basilevsky of the Vernadsky Institute for a copy of the Venera unit map. Thanks to G. H. Pettengill and an anonymous reviewer for their comments and sugges- tions. We gratefully acknowledge the support of NASA Grant NAGW-713, NASA Contract JPL- 957088, and the William Marlar Foundation.

REFERENCES

AtJBEI.E, J. C., J. W. HEAD, E. N. SLYUTA, AND A. T. BASILEVSKY 1988. Characteristics of domes on Ve- nus and a comparison with terrestrial cinder cones and oceanic volcanic edifices. Lunar Planet. Sci. XIX, 21-22.

BARSUKOV, V. L., A. T. BASlLEVSKY, G. A. BURBA, N. N. BOBINA, V. P. KRYUCHKOV, R. O. KUZMIN, O. V. NIKOLAEVEA, A. A. PRONIN, L. B. RONCA, I. M. CHERNAYA, V. P. SASHKINA, A. V. GARANIN, E. R. KUSHKY, M. S. MARKOV, A. L. SUKHANOV, V. A. KOTELNIKOV, O. N. RZHIGA, G. M. PETROV, Yu. N. ALEXANDROV, A. 1. SIDORENKO, A. F. BO- GOMOLOV, G. I. SKRYPNIK, M. Yu. BERGMAN, L. V. KUDRIN, I. M. BOKSHTEIN, M. A. KRONROD, P. A. CHOCHIA, YU. S. TYUFLIN, S. A. KADNI- CHANSKY, AND E. L. AKIM 1986. The geology and geomorphology of the Venus surface as revealed by the radar images obtained by Veneras 15 and 16. Proc. Lunar Planet. Sci. Conf. 16th 91, D378-D398.

BARSUKOV, V. L., A. T. BASILEVSKY, R. O. KUZMIN, A. A. PRONIN, V. P. KRYUCHKOV, O. K. NIKOLAY- EVA, I. M. CHERNAYA, G. A. BURBA, N. N. BO- BINA, V. P. SASHKINA, M. S. MARKOV, AND A. L. SUKHANOV 1984. Geology of Venus from the results of analysis of radar images taken by Venera 15 and 16 probes: Preliminary data. Geochimia 12, 1811- 1820.

BASII.EVSKY, A. T., A. A. PRONIN, L. B. RONCA, V. P. KRYUCHKOV, A. L. SUKHANOV, AND M. S. MAR- KOV 1986. Styles of tectonic deformation on Venus: Analysis of Veneras 15 and 16 data. Proc. Lunar Planet. Sci. Conf. 16th 91, D399-D411.

BINDSCHADLER, D. L. 1986. Characterization of Venera 15/16 Geologic" Units from Pioneer Venus Reflectivity and Roughness Data, M.Sc. thesis, Brown University.

BINDSCHADLER, D. L., AND J. W. HEAD 1986. Vega landing sites: Venera 15/16 unit analogs from Pio-

neer Venus reflectivity and RMS slope data. Geophys. Res. Len. 13, 1415-1418.

BINDSCHADLER, D. L., AND J. W. HEAD 1988a. Dif- fuse scattering on the surface of Venus: Origin and implications for the distribution of soils. Earth, Moon Planets 42, 133-149.

BINDSCHADLER, D. L., AND J. W. HEAD 1988b. Models for the origin of tessera terrain on Venus. Lunar Planet. Sci. XIX, 78.

BINDSCHADI.ER, D. L., J. W. HEAD, M. A. KRESLAVSKY, YU. G. SHKURATOV, AND A. T. BASI- LEVSKY 1988. Distribution of tesserae on Venus: Prediction using Pioneer Venus and Venera data. Lunar Planet. Sci. XIX, 80.

BOUGAN, S., R. GREELEY, AND J. MARSHELL 1986. The aeolian environment on Venus. Lunar Planet. Sci. XVII, 74-75.

BRASS, G. W., AND C. G. A. HARRISON 1982. On the possibility of plate tectonics on Venus. Icarus 49, 86-96.

CAMPBELL, D. B., J. W. HEAD, J. H. HARMON, AND A. A. HINE 1983. Venus: Identification of banded terrain in the mountains of Ishtar Terra. Science 221,644-647.

CAMPBELL, D. B., J. W. HEAD, J. H. HARMON, AND A. A. HINE 1984. Venus: Volcanism and rift formation in Beta Regio. Science 226, 167-170.

CRUMPLER, L. S., J. W. HEAD, AND D. B. CAMPBELL 1986. Orogenic belts on Venus. Geology 14, 1031- 1034.

DAVIS, P. A., R. C. KOZAK, AND G. G. SCHABER 1986. Global radar units on Venus derived from statistical analysis of Pioneer Venus Orbiter radar data. J. Geophys. Res. 91, 4979-4992.

FLORENSKY, C. P., L. B. RONCA, A. T. BASILEVSKY, G. A. BURBA, O. V. NIKOLAEVA. A. A. PRONIN, A. M. TRAKHTMAN, V. P. VOLKOV, AND V. V. ZA- ZETSKY 1977. The surface of Venus as revealed by Soviet Venera 9 and 10. Geol. Soc. Amer. Bull. 88, 1537-1545.

FORD, P. G., AND G. H. PETTENGILL 1984. Venus radar reflectivity--A reanalysis. Bull. Amer. As- tron. Soc. 16, 697.

GARVIN, J. B., J. W. HEAD, G. H. PETTENGILL, AND S. H. ZISK 1985. Venus global radar reflectivity and correlations with elevations. J. Geophys. Res. 90, 6859-687 I.

GARVIN, J. B., J. W. HEAD, M. T. ZUBER, AND P. HELFENSTEIN 1984. Venus: The nature of the surface from Venera panoramas. J. Geophys. Res. 89, 3381-3399.

GREELY, R., J. IVERSEN, R. LEACH, J. MARSHALL, B. WHITE, AND S. WILLIAMS 1984. Windblown sand on Venus: Preliminary results of laboratory simula- tions. Icarus 57, 112-124.

GREELEY, R., AND J. MARSHALL 1985. Rolling as a possible mode of wind transport on Venus and resulting bedforms. Lunar Planet. Sci. XVI, 292-293.

20 B I N D S C H A D L E R A N D H E A D

HAGFORS, T. 1970. Remote probing of the Moon by microwave and infrared emiss ions and radar. Radio Sci. 5, 189-227.

HEAD, J. W., A. R. PETERFREUND. J. B. GARVIN, AND S. H. ZISK 1985. Surface character is t ics of Ve- nus derived from Pioneer Venus alt imetry, roughness , and reflectivity measu remen t s . J. Geophys. Res. 90, 6873-6885.

HEAD, J. W., AND L. WH.SON 1986. Volcanic processes and landforms on Venus: Theory, predictions and observat ions . J. Geophys. Res. 91, 9407-9446.

IVANOV, B. A., A. T. BASlI.EVSKY, V. P. KRYUCH- KOV, AND 1. M. CHERNAYA 1986. Impact craters on Venus: Analysis of Venera 15 and 16 data. Proc. Lunar Planet. Sei. Conf. 16th, i)413-1)430.

K()IEt NtKt)V, V. A., A. F. BOGOMOLOV, AND O. N. RZHJGA 1985. Radar s tudy of Venus surface by Venera-15 and -16 spacecraft . Adv. Space Res. 5, 5- 16.

KOZAK, R. C.. AND G. G. SCHARER 1986. Gravity- spreading origin of the Venusian tessera , Lunar Planet. Sci. XVII, 444-445.

KRESI.AVSKV, M. A., A. T. BASILEVSKY, AND YU. G. SRKURATOV 1988. Prognosis of the distribution of tessera terrain on Venus using Pioneer Venus and Venera 15/16 data. Astron. Vest., in press.

M o g o z , V. I. 1983. S u m m a r y of preliminary results of the Venera 13 and Venera 14 miss ions . In Venus (D. M. Hunten , 1.. Colin, and T. M. Donahue , Eds.), pp. 45-68. Univ. Arizona Press, Tucson.

NIK()I.AEVA, O. V., A. A. PRONIN, A. T. BASILEVSKY. M. A. iVANOV, AND M. A. KRESLAVSKV 1988. Are tesserae the outcrops of feldspathic crust on Venus'? Lunar Planet. Sei. XIX, 864-865.

PETTENGII.I., G. H., E. ELAISON, P. G. FORD, G. B. LORtOT, H. MASURSKY, AND G. E. McGHE 1980.

Pioneer Venus radar results: Alt imetry and surface properties. J. Geophys. Res. 85, 8261-8270.

RZHIGA. O. N. 1984. Through Venus clouds. Zemla Vselennaja 1, 2.

SCHABER, G. G. , E. M. SHOEMAKER, AND R. C. KO- ZAK 1987. The surface of Venus: Use of the terrestrial cratering record. Astron. Vest. 21, 144-151 tin Russian) . Solar System Res. 21, 89-94.

SI.YUTA, E. W.. O. V. NIKOLAEVA, AND M. A. KRESI.AVSKY 1988. Distribution of small domes on Venus: Venera 15/16 radar data. Lunar Planet. Sci. XIX, 1097-1098.

g()IOMON, ~. ( ' . , ,~NI) J. W. HEAl) 1984. Venus banded terrain: Tectonic models for band formation and their relationship to lithospheric thermal structure. J. Geophys. Res. 89, 6885-6897.

S'rOFAN, E. R., AND J. W. HEAD 1986. Pioneer Venus character is t ics of ow~ids on Venus: Preliminary results, l.unar Planet. Sci. XVII Suppl., 1033-1{}34.

STOI-AN, E. R.. AND J. W. HEAD 1988. Arachnoid s t ructures on Venus: Morphology and distribution. Lunar Planet. Sei. XIX, 1127-1128.

STOFAN, E. R., J. W. HEAD, AND E. M. PARMENTIER 1987. Corona s t ructures on Venus: Models of origin. Lunar Planet. Sci. XVli l , 954-955.

SUKHANOV, A. L. 1986. Parquet: Regions of areal plastic deformation. Geotektonika 4, 60-76.

SUKHANOV, YU. A. 1983. Studies of Venus rocks by Veneras 8, 9, and 10. In Venus (D. M. Hunten, L. Colin, and T. M. Donahue , Eds.), pp. 154-158. Univ. Arizona Press, Tucson .

VORDER BRUEGGE. R. W.. J. W. HEAD, AND D. B. CAMPREtl 1986. Cross-s tr ike discontinuit ies on Maxwell Montes , Venus: Evidence for large-scale strike-slip faulting. Lunar Planet. Sci. XVII. 917- 918.

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