Recent works argue that the venusian crust is thin: less than 10-30 km. However, any convective model of Venus unavoidably predicts melting and a fast growth of the basaltic crust, up to its maximum thickness of about 70 km limited, by the gabbro-eclogite phase transition. The crust is highly buoyant due to both its composition and temperature and it is problematic to find a mechanism providing its effective recycling and thinning in the absence of plate tectonics. There are different ways to solve this contradiction. This study suggests that a thin crust can be produced during the entire evolution of Venus if Venus avoided giant impacts

Solomatov, Viatcheslav S.

Stevenson, David J.

English

NASA Technical Reports Server

LP1 Contribution No. 789 117
Fig. 1. Original Magellan SAR data (top), bandpass filtered version of same
(center), and detected regions of interest (at 1/4 resolution) (bottom).
Ongoing Work: We anticipate that a much higher classifica-
tion accuracy cart be achieved by incorporating prior knowledge
about the imaging and geologic processes, i.e., noise properties,
surface radar reflectivity, expected volcano diametcrs, and so forth.
By _eating the output activations of the nctwork as estmlates of
posterior class probabilities, both data-driven evidence and prior
knowledge cart be integrated directly in terms of a coherent prob-
ability model such as a Bayesian network, which incorporates
appropriate conditional independence assumptions. Note that if the
posterior probabilities at a given level are not confident enough (not
close to 0 or 1), the Laplacian hierarchy can be d_ded for a
higher-resolution analysis. Another significant issue is the ineorpo-
radon of global context models (spatial correlation of geologic
features) with local evidence. In the context of cun-ently available
image analysis algorithms and tools, these issues somewhat push
the state-of-the-art.
Conclusions: In terms of pattern recognition, even though
100% accuracy will not be achievable due to the inherent ambiguity
in the image data, the general method has significant practical
benefit as a basic tool for aiding rapid scientific exploration of the
large Magellan database. A short-term scientific benefit will be to
answer the basic question regarding the approximate number and
distribution of these volcanos on the surface of Venus. Long-term
scientific benefits would include subsequent spatial cluster analysis
of the volcano locations and the association of the volcanos with
local smmtural patterns. It is reasonable to suggest that the applica-
tion of pattern recogrtidon techniques will enable basic scientific
research that otherwise would not be possible by manual methods.
Acknowledgments: The research described in this paper was
performed at the Jet Propulsion Laboratory, California Institute of
Technology, under a contract with the National Aeronautics and
Space Administration.
References: [1] Science, special issue on Magellan data, April
12, 1991. [2] Guest J. E. et al. (1992) JGR, in press. [3] Aubele J. C.
and Slyuta E. N. (1990) Earth Moon Planets, 50/51, 493--532.
[4] Head J. W. et al. (1992)JGR, in press. [5] Skingley J. and Rye
A. J. (1987) Pattern Recognition Letters, 6, 61-67. [6] Cross A. M.
(1988) Int. J. Remote Sensing, 9, 1519-1528. [7] Quegan S. et al.
(1988) Phil. Trans. R. Soc. Lond., A324, 409-421. [8] Wiles C. R.
and Forshaw M. R. B. (1992) LPSC XXIil, 1527-1528. [9] Van
Essen D. C. and Anderson C. H. (1992) Science. [10] Butt P. J. and
Adelson E. H. (1983) IEEE Trans. Comm., 31,532-540. [11] Burr
P. J. (1988) Proceedings of the DARPA Image Understanding
Workshop, 139-152. [12] Miller J. W. et al. (1992) IEEE Trans.
lnfo. Theory, submigg¢l.
N93-14381
MELTING AND DIFFERENTIATION IN VENUS WITH A
"COLD" START: A MECHANISM OF THE THIN CRUST
FORMATION. Viatchcslav S. Solomau)v and David J.Stevenson,
Division of Geological and Planetary Sciences, 170-25, California
Institute of Technology, Pasadena CA 91125, USA.
Recent works [ 1-3] argue that the venusian crust is thin: less than
10-30 km. However, any convective model of Venus unavoidably
predicts melting and a fast growth of the basaltic crust up to its
maximum thickness about 70 km limited by the gabbro-eclogite
phase transition [4]. The crust is highly buoyant due to both its
composition and temperature and it is problematic to Fred a mecha-
nism providing its effective recycling and thinning in the absence of
plate tectonics. There are different ways to solve this contradiction
[5,6]. This study suggests that a thin crust can be produced during
the entire evolution of Venus if Venus avoided giant impacts [7].
The absence of giant impacts means that Venus' interiors were
more cold and more water-rich than the Earth's after the accretion
and core formation. The initial temperature distribution after the
core formation is not necessarily convectively unstable: The viscos-
ity is extremely sensitive to the temperature and uncertainties in the
initial thermal state easily cover the transition from conductive to
convective regimes. Convection and conduction-convection transi-
tion arc parametcrized for the temperature-, pressure- and stress-
https://ntrs.nasa.gov/search.jsp?R=19930005193 2020-03-17T08:43:35+00:00Z
118 International Colloquium on Venus
dependent viscosity with the help of a recently developed approach
[8] and the thermal evolution of Venus with melting and differen-
tiation iscalculated.
The model predictsthatonly a small amount of meltisextracted
in the first period of evolution; however, this would result in a stwng
depletion in radioactive elements and devolatilizadon of a part of
the upper mantle. Independently of the thickness of this differenti-
ated layer, only about a 300-kin layer conmbutes to the crust
formation (a depth below which the melt is supposed to be denser
thanthesun-oundingrocks).A smallbuoy ancyofthisdepletedlayer
is sufficient to stabilize this layer with respect to the undifferentiated
mantle. This prevents supply of the undifferentiated material to the
melting region. Convection occurs eventually in both layers. The
lower, undifferentiated layer is heated from within. The upper,
differentiated layer is mainly heated from below. The temperature
increases and reaches the anhydrous solidus only after several
billion years depending on the theological model. The young
basaltic crust observed on Venus is produced by melting of the
anhydrous layer. This melting and the crust growth are weak mainly
because of the lower heat flux consisting of the radiogenic heat
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 Parmentier E. M. (1987)
Prec. LPSC 17th, in JGR, 92, E541. [2] Zuber M. T. and Parmentier
E. M. (1990)Icarus, 85, 290. [3] Grimm R. E. and Solomon S. C.
(1988) JGR, 93, ll911. [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. [7] Kaula W. M. (1990) Science, 247, I 191. [8] Solomatov V.
S. (1992) In Flow and Dynam, jc M odeJing of the Earth and Planets.
Univ'° Alaska'in"  lN93-14382 ="'
THE TECTONICS OF V_NU_'S: A1N:OVERY]I_W. Scan C.
Solomon, Deparunent of Earth, Atmospheric, and Planetary Sci-
ences, Massachusetts Institute of Technology, Cambridge MA
02139, USA.
Introduction: While the PioneerVenus 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 [1-3], theradar images from Magellanconsti-
tute an improvement in resolution of at lea_st 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
[6].
Generalizations: Tectonic features on Venus are widespread
and diverse, and comparatively few regions are undeformed. Areas
lacking tectonic features at Magellan resolution are reswicted 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 tectonically active at
presenL Severalarguments suplx_ thisinference.The greatrelief
and steepslopesof themountains and plateauscarpsofIshtarTerra
and of the equatorial chasm systems are difficult to xt_,oncile with
long-term passive support by crustal str¢_ngth. Because of the high
surface temperature on Venus, temperatures at which crustal rocks
fail by ductile flow should be reached at much shallower 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 re.ales less than about
10 m.y. [11 ]. Thus the processes that build relief and steepen slopes
must have been active within the last 10 m.y. Further, a number of
features produced by geologic.al 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 [8]. 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 ofldlometers. Plains
thousands of kilometers across record horizontal strains of order
10-2 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 features such as Somerville
Crater in Devana Chasma, 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 t bound on
horizontal displacements if any crustal material is recycled into the
mantle at zones of underthrustlng. 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
Montes, 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-
10 a 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 distafib-
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 underflu'usting or subduction of lithosphere
sin-rounding large coronae [19]. Elsewhere, however, chasm sys-
tems of somewhat lesser relief display more linear segments and


Melting and differentiation in Venus with a cold start: A mechanism of the thin crust formation

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