Coronae on Venus are large, generally circular surface features that have distinctive tectonic, volcanic, and topographic expressions. They range in diameter from less than 200 km to at least 1000 km. Data from the Magellan spacecraft have now allowed complete global mapping of the spatial distribution of coronae on the planet. Unlike impact craters, which show a random (i.e., Poisson) spatial distribution, the distribution of coronae appears to be nonrandom. We investigate the distribution here in detail, and explore its implications in terms of mantle convection and surface modification processes

Bindschadler, D. L.

Janes, D. M.

Moersch, J. E.

Moore, W.

Olson, P.

Ratcliff, J. T.

Schubert, G.

Squyres, S. W.

Stofan, E. R.

Turcotte, D. L.

English

NASA Technical Reports Server

LP! Contribution No. 789 119
more nearly symmetric topographic profiles, such as DcvanaChasrna,
and on the basis of small-scale morphology are clearly extensional
rifts.
Much of the tectonic behavior on Venus appears to be more
reminiscent of aedvely deforming continental regions than of
oceanic regions on Earth. In particular, as in tectonically active
continental areas, deformation is typically distributed across broad
zones one to a few hundred Idlometers wide separated by compara-
tively stronger and less deformed blocks having dimensions of
htmdreds of kilometers. On Earth, the continental lithosphere in
tectonically active areas is weaker than typical oceanic lithosphere
because of the greater thickness of more easily deformable cnzst. As
noted above, because of the higher surface temperature on Venus,
the likely comparable lithospheric thermal gradients on Venus and
Earth [20,21], and the strong temperature dependence of ductile
behavior, the lithosphere on Venus should behave in a weak manner
for crustal thicknesses less than are typical of continental regions on
Earth.
Status of Models: A major challenge in unraveling the tec-
tonic evolution of Venus is to tmderstand the interaction between
mande convection and the lithosphere. The hotspot [22] and coldspot
[23] models for the formation and evolution of major highlands on
Venus are distinguishable on the basis of the predicted sequence of
events and the time-dependent relationship between topography and
gravity. Both models fa.cc difficulties in their present forms, how-
ever. at least partly because both the theology of the upper crust of
Venus and the observed patterns ofmagmatism and deformation arc
more complex than current models for the deformation and mag-
marie response of the crust to mantle flow• Any global tectonic
model, of course, must also consider the formadon and character-
istics of the lowlands, including the large apparent depths of
isostade compensation [22] and relatively recent lowland volcan-
ism [24]. All dynamical models to date require special pleading to
explain Ishtar Terra. Both the rifted, volcanically active highlands
and at least the larger coronae are generally regarded as sites of
upwelling mantle flow and magma generation by pressure-release
partial melting of mantle material. These two classes of features,
however, have very different tectonic and morphological manifes-
tations at the Venus surface• If both are products of mantle up-
welling, then multiple scales of mantle convection are indicated and
the different morphologies of the two classes of features must be
related to differences in the geometry, buoyancy flux, or duration of
flow in the two types of upwelling regions. The assessment of
existing dynamical models for the tectonic evolution of Venus and
the development of the next generation of models will require an
understanding of geological relationships at all scales, from the
highest resolution available to global patterns. High-resolution
measurements of the global gravity field later in the Magellan
mission will provide key data for testing and reining models.
References: {1] Masursky H. ct at. (1980) JGR, 85, 8232.
[2] Campbell D. B. et al. (1983) Science, 22I, 644. [3] Barsukov
V. L. et at. (1986)Proc.LPSCI6th, inJGR, 91, D378. [4] Pettcngi]l
G. H. et at. (1991) Science, 252,260. [5] Solomon S. C. et al. (1991)
Science, 252,297. [6] Solomon S. C. et al. (1992) JGR. in press.
[7] Phillips R. J. et at. (1991) Science, 252,288. [8] Schaber G. G.
et al. (1992) JGR, in press. [9] Weertman J• (1979) PEPI, 19, 197.
[10] Solomon S. C. and Head J. W. (1984) JGR, 89, 6885•
[I1] Smrekar S. E. and Solomon S. C. (1992) JGR, in press.
[12] Baker V. R. et al. (1992)JGR, in press• [13] McGill G. E. et al.
(1981)GRL, 8, 737. [14] Grimm R. E. and Solomon S. C. (1988)
JGR, 93, 11911• [151 Zuber M. T. and Parmentier E. M. (1990)
Icarus, 85, 290• [ 16] Head J. W. and Crumpler L. S. (1987)Science,
238, 1380• [17] McKenzie D. et al. (1992)JGR, in press. [18] Ford
P. G. and Pettengill G. H. (1992) JGR, in press. [19] Sandwell D. T.
and Schubert G. (1992) JGR, in press. [20] Solomon S• C. and Head
L W. (1982) JGR, 87, 9236. [21] Phillips R. L and Matin M. C.
(1984) Amut. Rev. Earth Planet. Sci., 12, 411. [22] Phillips R• L et
al. (1991) Science, 252, 651. [23] Bindschadler D. L. eta]. (1992)
JGR, in press. [24] Head J. W. et tl. (1991) Science, 252, 27_.
'N93-14383 : ! 7 =', "
THE SPATIAL DISTRIBUTION OF C()RONAE OI_VENUS.
S. W. Squyres !, G. Schubert 2, D. L. Bindschadlc#, D. M. Janes t.
L E. Moersch t , W. Moore 2, P. Olson 3, J. T. Ratclift n. E. R. S to fan 4,
D• L. Turcotte t, tComell University, 2University of California, Los
Angeles, 3Johns Hopkins University, 4Jet Propulsion Laboratory,
USA.
Coronae on Venus arelarge,generallycircularsurfacefeatures
thathave distinctivetectonic,volcanic,and topographic expres-
sions.They range in diameter from lessthan 200 km to atleast
1000 kin.Data from the Magellan spacecrafthave now allowed
complete globalmapping of the spatialdistributionof coronae on
the planet.Unlike impact craters,which show a random (i.e.,
Poisson)spatialdistribution[I],thedisWibutionofcoronac appears
tobe nonrandom. We investigatethedistributionhere indetail,and
explore its implications in terms of mantle convection and surface
modification processes.
Figure 1 shows the distribution of coronae and _a-related
features on Venus, in a simple cylindrical (Plate Carr6e) projection.
The map gives the locations of 311 features identified in all of the
Magellan data taken to date• These features include coronae; they
also include radially fractured domes, which are believed to be
eoronae in an early or arrested stage of development [2]. The map
gives the qualitative sense that the distribution is nonrandom, with
distinct areas of clustering and sparseness. However, this assertion
requires testing, especially because there are some significant gaps
in the Magellan data that artificially introduce areas of sparseness
in the corona distribution.
In order to test for nonrandomness, we apply a simple nearest-
neighbor test. In this test, we measure the distance, r, of each feature
from its nearest neighbor on the planet. The percentage of points
with a nearest-neighbor distance greater than r is then plotted as a
function of r in Fig. 2. The next step is to compare this result to the
result expected for a spatially random distribution. There exist
theoretical tleatments for a nearest-neighbor curve of the sort in
P
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/
i ••• ." • , •
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nl :::• " "
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. • •"
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l.
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o 45 9o
, , _ , , T_ • _ . T _ ,--7
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.• •.%,• •
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135 180 2;'5 _70 31S 360
Fig. 1. Map showing the distribution of coronae and related features on
Venus.
https://ntrs.nasa.gov/search.jsp?R=19930005195 2020-03-17T08:43:27+00:00Z
118 International Colloquium on Venus
dependent viscositywiththehelpofa recentlydeveloped approach
[8]and thethermalevolutionof Venus with meltingand differen-
tiationis calculated.
The model predicts thatonlyasmall amount ofmelt isextracted
inthefirstperiodofevohdon; however, thiswould resultinasn-ong
depictioninradioactiveelernentsand devolatilizadonof a partof
theupper mantle.Indcpendendy of the thicknessof thisdifferenti-
'ated layer,only about a 300-kin layer contributesto the crust
formation(adepth below which the melt issupposed tobe denser
thanthesurroundingrocks).A smallbuoyancy ofthisdepletedlayer
issufficienttostabilizethislayerwithrespecttotheundifferentiated
mantle.Thispreventssupply of theundifferentiatedmaterialtothe
meltingregion.Convection occurs eventuallyin both layers.The
lower, undifferentiatedlayer is heated from within.The upper°
differentiatedlayerismainly heated from below. The temperature
increasesand reaches the anhydrous solidus only afterseveral
billionyears depending on the rhcologicalmodel. The young
basalticcrustobserved on Venus isproduced by mehing of the
anhydrous layer.Thismeltingand thecrustgrowth areweak mainly
because of the lower heat fluxconsistingof the radiogenichcat
production in the undifferentiated lower layer and almost not
contributed from the secular-cooling heat flux from the core (it is
even negative) and the remaining radioactivity in the differentiated
layer. An additional consequence of the model is that the magnetic
field was never generated in venusian history.
References: [1] Zuber M. T. and Parmemier E. M. (1987)
Proc. LPSC 17th, in JGR, 92, E541. [2] Zuber M. T. and Parmentier
E. M. (1990) Icarus, 85, 290. [31 Crrimm R. E. and Solomon S. C.
(1988) JGR, 93, 11911. [4] Zharkov V. N. and Solomatov V. S.
(1992) In Venus Geology, Geochemistry, and Geophysics, 281,
Univ. of Arizona, Tucson. [5] Solomon S. C. and Head J. W. (1991)
Science, 252, 252. [6] Williams D. R. and Pan V. (1990) GRL, 17,
1397. [71 Kaula W. M. (1990)Science, 247, 1191. [8] Solomatov V.
S. (1992) In Flow and Dynamic M odefing of the Earth and Planets,
Univ. ofAlaska, in == ''N93-14382 :
THE TECTONICS OF V_NU-S: AIN:OVERVII_W. Scan C.
Solomon, Department of Earth,Atmospheric, and PlanetarySci-
ences, Massachusetts Instituteof Technology. Cambridge MA
02139, USA.
lntrod uctlon: While the Pioneer Venus altimeter, Earth-based
radar observatories, and the Venera 15-16 orbital imaging radars
provided views of large-scale tectonic features on Venus at ever-
increasing resolution [I-3], the radar im ages from Magellan consti-
tute an improvement in resolution of at least an order of magnitude
over the best previously available [4]. A summary of early Magellan
observations of tectonic features on Venus has been published [5],
but data available at that time were restricted to the first month of
mapping and represented only about 15% of the surface of the
planet. Magellan images and altimetry are now available for more
than 95 % of the Venus surface. Thus a more global perspective may
be taken on the styles and distribution of lithospheric deformation
on Venus and their implications for the tectonic history of the planet
[61.
Generalizations: Tectonic features on Venus are widespread
and diverse, and comparatively few regions are undefonned. Areas
lacking tectonic features at Magellan resolution are restricted to
relatively young volcanic deposits and the younger impact craters
and associated ejecta. Most of the surface, during at least some
period over approximately the last 500 m.y. [7,8] has experienced
horizontal strain sufficient to fault or fold near-surface material.
Tectonic activity on Venus has continued until geologically
recent time, and most likely the planet is tectordcnlly active at
present. Several arguments support this inference. The great relief
and steep slopes of the mountains and plateau scarps of Ishtar Terra
and of the equatorial chasm systems are difficult to reconcile with
long-term passive SUplXn't by crustal strength. Because of the high
surface temperature on Venus, temperatures at which crustal rocks
fail by ductile flow should be reached at much shallowet" depths than
on Earth [9,10]. Numerical models suggest that areas of high relief
and steep slope in the Ishtar region should spread under self-gravity
by ductile flow of the weak lower crust on time scales less than about
10 m.y. [ 11 ]. Thus the prtw.esses that build relief and steepen slopes
must have been active within the last 10 m.y. Further, a number of
features produced by geological processes that have operated more
or less steadily during the past 500 m.y. show evidence of subse-
quent tectonic activity. About one-third of all preserved impact
craters on Venus have thoroughgoing faults and fractures, and 1 in
12 are extensively deformed [81. The longest lava channel on the
plains of Venus does not progress monotonically to lower elevations
downstream, indicating that differential vertical motions have
occurred since the channel was formed [12].
Compared with the Earth, horizontal displacements on Venus
over the last 500 m.y. have been limited. Most of the tectonic
features require modest strains and horizontal displacements of no
more than a few tens to perhaps a few hundreds of kilometers. Plains
thousands of kilometers across record horizontal strains of order
10 -_ or less. The great rift systems of Beta and Aria Regiones need
have extended no more than a few tens of kilometers, on the basis
of topographic profiles, extended feamares such as Somerville
Crater in Devana C"hasma, and analogy with continental rifts on
Earth [13]. For compressional features, the amount of crustal
thickening can be estimated from topographic relief and isostatic
considerations, but this approach provides only a lower bound on
horizontal displacements if any crustal material is recycled into the
mantle at zones of underthrusting. For ridge belts 100 km in width
with up to 1 km of relief, horizontal displacements of no more than
100 km are required for crustal thicknesses of 10--20 km beneath the
adjacent plains [14,15]. Mountain belts are exceptional in that
greater horizontal displacements are required. For a two- to four-
fold thickening of the crust beneath the 500-kin width of Maxwell
Momes, the implied minimum horizontal displacement is 1000-
2000 km.
Unlike the Earth, Venus does not show evidence for a global
system of nearly rigid plates with horizontal dimensions of 103-
l0 t km separated by narrow plate boundary zones a few kilometers
to tens of kilometers across. Predictions prior to Magellan that
Aphrodite Terra would show features analogous to terrestrial spread-
ing centers and oceanic fracture zones [16] now seem to be incor-
rect. Evidence for shear is present in the ridge and fracture belts and
in the mountain belts, but the shearing tends to be broadly distrib-
uted and to accompany horizontal stretching or shortening. Few
clear examples have yet been documented of long, large-offset
strike-slip faults such as those typical of oceanic and many conti-
nental areas on Earth; two such features have been identified in the
interior of Artemis Corona [17]. A number of the chasm systems of
Venus have arcuate planforms, asymmetric topographic profiles,
and high relief [ 18] and have been likened to deep-sea trenches on
Earth [17]. These include Dali and Diana Chasmata [17] and the
moat structure of Artemis Corona; such trenches may be the
products of limited underthrusting or subduction of lithosphere
surrounding large coronae [19]. Elsewhere, however, chasm sys-
tems of somewhat lesser relief display more linear segments and


The spatial distribution of coronae on Venus

The spatial distribution of coronae on Venus

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