Approximately 22,000 small dome-like hills were recognized on the northern 20 percent of the surface of Venus imaged by Verera 15/16. These features were described as generally circular in planimetric outline, with a range in basal diameter from the effective resolution of the Venera images (1 to 2 km) up to 20 km. The General Characteristics, Dome Distribution and Terrain Unit and Geologic Feature Associations are discussed

Aubele, J. C.

Slyuta, E. N.

English

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N90" 12425 l
CHARACTERISTICS, DISTRIBUTION AND GEOLOGIC/TERRAIN ASSOCIATIONS
OF SMALL DOME-LIKE HILLS ON VENUS
J.C. Aubele, Dept. of Geol. Sciences, Brown University, Providence, R.I. 02912
E.N. Slyuta, Vernadsky Institute, USSR Academy of Sciences, Moscow, U.S.S.R.
Introduction. Approximately 22,000 small dome-like hills have been recognized on the northern 20% of the surface
of Venus imaged by Venera 15/16. These features have been described [1] as generally circular in planimetric
outline, with a range in basal diameter from the effective resolution of the Venera images (1-2kin) up to 20 kin.
General Characteristics. The small hills have been called domes [1,2] following the lunar nomenclature, be-
cause of their broad apparent form. The nomenclature used here, "small dome-like hills", is preferred because of
the strict volcanological definition of a dome and terrestrial lithologic connotations. The actual number of these
features, and their true range of diameter and modal diameter Is unknown; since there is undoubtedly a population,
of unknown size, of dome-like hills __1 km that cannot be resolved in the Venera images. The maximum diameter
of 20 km is, in a sense, an artificially imposed size range since similar features occur on Venus In the 20 to 100 km
diameter range. However the 20 km cutoff reflects a significant change in the overall number of edifices, and In
number versus basal diameter frequency distribution. Using diameter measurements In the along-range direction,
best estimate modal diameter was determined for a typical cluster of small dome-like hills in Tethus Regio [3]. The
444 total ranged in basal diameter from 2-8 kin, with a mode of 3-4 kin. Based on constraints on the appearance
of features imaged by this radar system, individual dome-like hltl slopes are all less than 10 °. Assuming these
slopes, simple geometric models imply maximum height of approximately 1 km, and average height less than t km.
Recent radarclinometrlc data confirms these estimates [4] ; profiles of seven small dome-like hills, ranging In basal
diameter from 7.2 to 19.2 kin, show a slope that is slightly convex near the summit, relatively uniform (maximum
slope 5 °) on the flank, and slightly concave (minimum slope 1_) near the base. The maximum height ranges from
150 to 650 m; and increased height is generally related to increased basal diameter, with some variations. Most of
the small dome-like hills show no individually associated features; however, a small number exhibit summit pits, low
basal topographic rises or platforms, and radial or lobe-like bright features. Summit pits are infrequently observed
in dome-like hills larger in basal diameter than 8 kin, and more frequently in ones larger than 15 km. The total
number with visible summit pits Is less than 5%. Bright circular aureoles, without apparent topographic relief,
appear to be associated with small dome-like hills northeast of Atalanta Planitia.
Dome Distribution. Slyuta, et al [2,5] have produced dome density contour maps and find that most dome-like
hills occur in groups of several tens within areas of I0 s km2. Adjacent groups form clusters consisting of 10-20
groups within areas of 105 km 2. The greatest concentration of clusters of dome-like hills occurs in the general area
of 60°N, 1200E. Major concentrations of clusters are located in Tethus Regio (65°N, I t0"E), Atalanta Planitta
(60ON, 155°E), Ananke Tessera (55'_N, 138_E), and Akkruva Colles (from Niobe Planitia, 35_N, 130_-E, to Allat
Dorsa, 65'_N,70°E). In general, these major areas of cluster concentration are approximately equidimenslonal in
areal extent, with the exception of Akkruva Colles which is elongated in the NW-SE directions. Akkruva is also
associated with a linear positive gravity anomaly of 25 mgal in the Pioneer Venus data set. The cluster concentra-
tions in Tethus, Ananke and Akkruva are associated with broad regional topographic highs. However, many small
groups or clusters of groups occur on low plains or inside circular depressions and the cluster concentration In
Atalanta occurs in the general area of the topographically lowest region of Venus. Within a specific cluster, the
dome-like hills appear to occur In relatively evenly scattered groups. Counts in a representative cluster In the Tethus
Regio region show that overall density of the dome-like hills, within a specific cluster, is _=_ 0.4/100 km2 and maxi-
mum density is 1.0/100 km 2 [3]. A few dome-like hills appear to exhibit alignment due to minor local structural
control, but there Is no evidence of large-scale structural control or dominant trend directions.
Terrain Unit and Geologic Feature Associations. Almost all of the areas of cluster concentration occur on
mottled plains units designated as "rolling plains interpreted to be of volcanic origin" [6]. The outline of the major
region of cluster concentration also generally corresponds to the Plains-Corona-Tessera Assemblage described by
Head [7]. Major clusters frequently occur on plains units at the margins of areas of tessera, while very small groups
occasionally occur in intra-tessera plains near the margins of large tessera units. Where it is possible to evaluate
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SMALL DOME-LIKE HILLS ON VENUS: Aubele, J.C. and Slyuta, E.N.
the evidence, it appears that the plains, on which the dome-like hills occur, post-date the formation of the tessera
terrain. Smaller cluster concentrations occur in regions of predominant arachnoids, in the area between Sedna
Planitia and Bell Regio and in the area south of the ridge belt province at 40°N, 215_E; and also occur at Beta
Regio. Although domes occur on the plains to the north and south of Ishtar Terra, Lakshmi Planum and the horizon-
tal compressional fold belts [8] of Akna and Freyja Montes exhibit one of the lowest densities of small dome-like hills
on Venus. No major concentrations of dome-like hills occur In the major ridge belt areas, although small groups and
Isolated dome-like hills occur near the ends of a few ridge belts. Small groups of dome-like hills always appear to
be associated [g] with the following specific geologic features: (1) coronae - dome-like hills occur predominantly
inside the annular concentric ridges of coronae [10]; (2) arachnoids - dome-like hills occur predominantly in the
areas of llneations on the plains surrounding and between adjacent arachnoids; (3) intermediate (20-100 km) sized
hills interpreted to be volcanic constructs - these generally occur as isolated features on plains units near groups or
clusters of small dome-like hills; (4) large volcanic centers (___100 kin) - dome-like hills occur predominantly on the
lower flanks, or beyond the distal edges, of the bright radial markings associated with these centers; (5) calderas -
isolated dome-like hills generally occur on the rims and periphery [11]; and (6) large circular features of uncertain
origin - dome-like hills occur predominantly within these features. The spatial association between these specific
geologic features and groups of dome-like hills is ubiquitous, even when one of these geologic features occurs in an
area with low overall density of small dome-like hills. Geologic mapplng provides evidence that formation of the
dome-like hills post-dated the development of tessera terrain; may have coincided with the emplacement of plains
units interpreted to be volcanic; but may have pre-dated the final development of many large volcanic centers found
spatially associated with the dome-like hills.
Discussion, Because they are so numerous, the origin and mechanism of formation of the small dome-like hills of
Venus is a significant question that has Implications for the geologlc processes and evolution of the planet. Since
many clusters of dome-like hills occur on plains units that are adjacent to, and appear to be younger than, tessera
terrain, it Is reasonable to question whether they merely represent partially buried and isolated topography. The
large number of these features, their randomly scattered occurrence, their consistent circularity, the apparent lack
of major structural alignments, and the frequent occurrence of summit pits suggest that the dome-like hills do not
merely represent partially flooded pre-existing ridges. Large volcanic features have been previously recognized on
Venus, such as the calderas in Laksml Planum and the large volcanic edifices associated with Bell Regio. Smooth
plains, frequently appearing to embay other terrain units, have been interpreted to be volcanic in origin. Circular
features, such as corona or arachnoids, have also been interpreted to be associated in some way with volcanism
[8]. In addition, detailed study of the Venera images has resulted in the recognition of approximately 1000 edifices,
with diameter >___20 kin, that are interpreted to be volcanic. As the geologic associations described above have
demonstrated, the small dome-like hills of Venus occur spatially associated with other features which are recog-
nized as volcanic or interpreted to be volcanic by other researchers. The dome-like hills can, therefore, be
reasonably interpreted to be small volcanic edifices themselves. Examples of small edifices, interpreted to represent
predominantly effusive volcanism, occur on Earth in the form of low shield volcanoes and seamounts, and also
occur on Mars and Earth's moon. The morphology of volcanic edifices is a complex function of fundamental vol-
canologtcal properties and variables; however, the general appearance and characteristics of these dominantly
effusive volcanic edifices appear to be similar to those of the Venus dome-like hills [3]. The existence of dome-like
hills (_20 km), intermediate sized volcanic edifices (20-100 kin), and large volcanic centers (_100 km) Implies a
continuum of volcanic edifices on Venus. There is a distinct distribution of number versus size range such that the
number of edifices increases as the size decreases; this is similar to that observed for volcanic edifices on Earth.
References, [1] Barsukov, V.L. et al, 1986, Proc. LPSC XVI, JGR, 91, B4, D378; [2] Slyuta, E.N., et al, 1988,
LPSC XIX (Abst), 1097; [3] Aubele, J.C., et, al, 1988, LPSC XIX (Abst), 21; [4] Sinilo and Slyuta, 1989, LPSC XX
(Abst), 1016; [5] Slyuta, E.N., et al, 1988, Aston. Vestnik, 22, #4, 287 (in Russian); [6] Barsukov, V.L and
Basilevsky, A., 1986, Piroda, 24; [7] Head, J.W., 1989, LPSC XX (Abst), 392; [8] Crumpler, L.S., et al, 1986,
Geology, 14, 1031; [9] Aubele, J.C., 1989, LPSC XX (Abst), 28; [10] Stofan, E.R. and Head, J.W.,1989, Sub-
mitted to Icarus; Stofan, E.R. and Head, J.W., 1988, LPSC XIX (Abst), 1127; [11] Magee, K.P. and Head, J.W.,
1988, LPSC XIX (abst), 711 and 713; Roberts, K.P.M. and Head J.W., 1989, LPSC XX (abst), 910.


Characteristics, distribution and geologic/terrain associations of small dome-like hills on Venus

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