A large number of volcanic features exist on Venus, ranging from tens of thousands of small domes to large shields and coronae. It is difficult to reconcile all these with an explanation involving deep mantle plumes, since a number of separate arguments lead to the conclusion that deep mantle plumes reaching the base of the lithosphere must exceed a certain size. In addition, the fraction of basal heating in Venus' mantle may be significantly lower than in Earth's mantle reducing the number of strong plumes from the core-mantle boundary. In three-dimensional convection simulations with mainly internal heating, weak, distributed upwellings are usually observed. We present an alternative mechanism for such volcanism, originally proposed for the Earth and for Venus, involving Rayleigh-Taylor instabilities driven by melt buoyancy, occurring spontaneously in partially or incipiently molten regions

Scott, D. R.

Stevenson, D. J.

Tackley, P. J.

English

NASA Technical Reports Server

LPI Contribution No. 789 123
al.[12] also considered the high-frequencybursts as possible
lightningevents.They found thatwhilethehigh-frequencyevents
did show some longitudinal dependence, the data were better
ordered by local time. with the peak rates occurring in the dusk local
time sector. Consequently, it is now thought that the fightning is not
associated with volcanic activity. Rather, it is due to weather
processes in an analogous manner to lightning at the Earth, which
tends to peak in afternoon local time sector [13].
The evidence for lighming at Venus from the VLF data now falls
intotwo classes.The higher frequencyburstsshow thelocaltime
dependence, and the ratealsodecreasesmost quicklyas a function
of altitude[14].These burstsare thoughtbe a localresponseto the
lightningdischarge,and thereforearcbestsuitedfor determining
planetwiderates.The ratesare found tobe comparable toterrestrial
rates.However, itisstillnot clearhow thehigh-frequencysignals
entertheionosphere.On the otherhand, about50% of the 100-Hz
burstsareclearlywhistlermode signals,as evidenced by the wave
polarization[15].These signalscan propagatesome distanceinthe
surfaceionospherewaveguide beforeenteringtheionosphere.The
100-Hz burstsare thereforeIcssreliableindetermining the light-
ningrate,or the main source location.
Perhaps the least ambiguous evidence for lightning on Venus has
come from the plasma wave data acquired by the Galileo spacecraft
during the Venus flyby [16]. Unlike Pioneer Venus, Galileo was
able to measure plasma waves at frequencies up to 500 kHz. The
plasma wave experiment detected nine impulsive signals that were
several standard deviations above the instrument background while
the spacecraft was at a distance of about five planetary radii.
Although some of the lower-frequency bursts could possibly have
been l.,angmuir wave harmonics, the higher-frequency bursts were
probably due to lighming. The bursts were at sufficiently high
frequency to pass through the lower-density nightside ionosphere as
freely propagating electromagnetic radiation.
While there is a strong body of evidence for the existence of
lightning at Venus, there are still many questions that remain. From
an ionospheric physics point of view, it is not clear how high-
frequency signals can propagate through the ionosphere. The low-
frequency signals do appear to be whistler mode waves, although
there is still some doubt [ 17]. Also, although whistler mode propa-
gation may be allowed locally, it is not necessarily certain that the
waves can gain access to the ionosphere from below. For example,
whistler mode propagation requires that the ambient magnetic field
passes through the ionosphere into the atmosphere below. It is
possible that the ionosphere completely shields out the magnetic
field. With regard to atmospheric science, there are several ques-
tions that require further study. First, can charge separation occur in
clouds at Venus? Is there sufficient atmospheric circulation to cause
a local time dependence as observed in the VLF data? Do Venus
clouds discharge to the ionosphere, and so cause strong local
electromagnetic or electrostatic signals that could explain the high-
frequency VLF bursts? While some of these questions may be
answered as low-altitude data are acquired during the Finalentry
phase of the Pioneer Venus Orbiter, many questions will still
remain.
Acknowledgments: I wish to thank Christopher T. Russell for
invaluable discussions on the evidence for lightning at Venus. This
work was supported by NASA grant NAG2--485.
References: [I] Inan U. S. and Carpenter D. C. (1987)JGR.
92, 3293-3303. [2] Russell C. T. et al. (1989)Adv. Space Res., 10(5),
37--40. [3] Franz R. C. et al. (1990) Science, 249, 48-51.
[4] Ksanfomaliti L. V. et al. (1983) In Venu._, 565--603, Univ. of
Arizona, Tucson. [5] Krasnopol'sky V. A. (1983) In Venus, 459--483,
Univ. of Arizona, Tucson. [6] Taylor W. W. L. et al. (1979) Nature,
279, 614--616. [7] Scarf F. L. et al. (1980) JGR, 85, 8158--8166.
[8] Scarf F. L and Russell C. T. (1983) GRL, 10, 1192-1195.
[9] Taylor H. A. Jr. et al. (1985) JGR, 90, 7415-7426. [10] Taylor
H. A. lr. et al. (1989) JGR, 94, 12087-12091. [11] Ho C.-M. etal.
(1991) JGR, 96, 21361-21369. [12] Russell C. T, et al. (1989)
Icarus, 80, 390-415. [13] Russell C. T. (1991)SpaceSci. Rev., 55,
317-356. [ 14] Ho C.-M. et al. (1992)JGR, in press. [15] Strangeway
R. I. (1991) JGR, 96, 22741-22752. [ 16] Gurnett D. A. et al. (1991)
Science, 253, 1522-1525. [17] Grebowsky J. M. etal. (1991)JGR,
96, 21347-21359. "
IN93-14487 q i
VOLCANISM BY MELT-DRIVEN RAYLEIGH-TAYLOR
INSTABILITIES AND POSSIBLE CONSEQUENCES OF
MELTING FOR ADMITTANCE RATIOS ON VENUS.
P. J. Tackley t, D. 1. Stevenson 1, and D. R. Scott 2, 1Division of
Geological and Planetary Sciences, Caltech, Pasadena CA 91125,
USA, ZDoparta'nent of Geological Sciences, University of Southern
California, Los Angeles CA 90089, USA.
A large nttmber of volcanic features exist on Venus, ranging
from tens of thousands of small domes to large shields and coronae.
It is difficult to reconcile all these with an explanation involving
deep mantle plumes, since a number of separate arguments lead to
the conclusion that deep mantle plumes reaching the base of the
lithosphere must exceed a certain size. In addition, the fraction of
basal heating in Venus' mantle may be significantly lower than in
Earth's mantle, reducing the number of strong plumes from the
core-mantle boundary. In three-dimensional convection simula-
tions with mainly internal heating, weak, distributed upweUings are
usually observed.
Description of Instability: We present an alternative mecha-
nism for such volcanism, originally proposed for the Earth [ 1] and
for Venus [2], involving Rayleigh-Taylor instabilities driven by
melt buoyancy, occurring spontaneously in partially or incipiently
molten regions. An adiabatically upwelling element of rock expe-
riences pressure-release partial melting, hence increased buoyancy
and upweiling velocity. This positive feedback situation can lead to
an episode of melt buoyancy driven flow and magma production,
with the melt percolating through the solid by Darcy flow. The
percolation and loss of partial melt diminishes the buoyancy,
leading to a maximum upwelling velocity at which melt percolation
flux is equal to the rate of melt production by pressure-release
melting.
Application to Venus: The instability has been thoroughly
investigated and parameterized using f'mite-element numerical
models, and hence its applicability to Venus can be assessed.
Numerical convection simulations and theoretical considerations
indicate that Venus' interior temperature is likely hotter than
Earth's, hence the depth of intersection of the adiabat with the dry
solidus may be appreciably deeper. In the regions of distributed
broad-scale upwelling commonly observed in internally-heated
convection simulations, partial melt may thus be generated by the
adiabatically upwelling material, providing the necessary environ-
ment for these instabilities to develop. Scaling to realistic material
properties and melting depths, the viscosity at shallow depth must
be 1019 Pa.s or less, leading to a period of self-perpetuating
circulation and magma production lasting N30 Ma, magma produc-
tion rates of -1000 km3]l_a, and lengthscales of -250 kin.
Geoid, Topography, and Viscosity Profiles: Partialmelt and
buoyant residuum represent density anomalies that are of the same
https://ntrs.nasa.gov/search.jsp?R=19930005199 2020-03-17T09:04:34+00:00Z
124 InternationalColloquium on Venus
time = 0.0000
time = 0.0162
Fig. I. Finite-element ,imulation showing the growth of melt-drivm insta-
bilities at the top of the mantle (depth of box -160 kin) from initially small
(<0.1%) random partial melt perturbations. Temperature (shaded), velocity
(arrows), partial melt distribution (contours. at 0 and intervals of Z5%), melt
production rate per unit area (on top). Tune nondimensiunalized to D2/_.
order as thermal anomalies driving mantle flow. Density anomalies
equivalent to 100 K are obtained by only 1% partial melt or buoy ant
residuum, which is 3% depleted. These density effects are expected
to have an effect on geoid and topography caused by one of these
instabilities, and possibly the geoid and topography caused by
thermal plumes.
Comparison of mantle geotherms for Venus (derived both from
numerical simulations and from observations combined with theory)
with an experimentally determined dry solidus for peridotite [3]
indicate that the average geotherm passes close to or even exceeds
the dry solidus at a depth of around 90 kin. Thus, for any reasonable
viscosity law, such as in [4], a region of lower viscosity is expected
at shallow depth. This contradicts several geoid/topography studies
based on simple mantle models, which require no low-viscosity
zone. We investigate the possibility that the presence of buoyant
residuum and partial melt at shallow depth may account for this
discrepancy.
Global geoid modeling using the technique described in [5]
indicates that a possible way to satisfy the long wavelength admit-
tance data is with an Earth-like viscosity structure and a concentra-
tion of mass anomalies close to the surface, compatible with a
shallow layer of buoyant residuum and/or partial melt. Geoid and
topography signatures for local features, specifically Rayleigh-
Taylor instabilities and thermal plumes with partial melting, are
currently being evaluated and compared to data.
References: [ 1] Tackley P. J. and Stevenson D. J. (1990) Eos,
71, 1582. [2] Tackley P. J. and Stevenson D. J. (1991) Eos, 72,287.
[3] Takahashi E. (1986)JGR, 91, 9367-9382. [4] Borch R. S. and
Green H. W. (1987) Nature, 330, 345. [5] Hager B. H. and Clayton
R. W. (1989) In Man@ Convection. 657-764. - :,
.,N93-14388
CAN A TIME-STRATIGRAPHIC CLASSIFICATION SYS-
TEM BE DEVELOPED FOR VENUS? K.L. Tanaka and G. G.
Schaber, U.S. Geological Survey, 2255 N. Gemini Drive. Flagstaff
AZ 86001, USA.
Magellan radar images reveal that Venus' exposed geologic
record covers a relatively short and recent time span, as indicated by
the low density of impact craters across the planet [1]. Therefore,
because impact cratering in itself wilt not be a useful tool to det-me
geologic ages on Venus, it has been questioned whether a useful
stratigraphic scheme can be developed for the planet. We believe
that a venusian stratigraphy is possible and that it can be based on
(1) an exarninadort of the rationale and methods that have been used
to develop such schemes for the other planets, and (2) what can be
gleaned from Magellan and other datasets of Venus.
Tmae-stratigraphic classification systems or schemes have been
derived for all other terrestrial planetary bodies (Earth, Moon, Mars,
Mercury) [2-5] because the schemes are useful in determining
geologic history and in correlating geologic events across the entire
surface of a planet. Such schemes consist of a succession of tlrne-
stratigraphic units (such as systems and series) that include all
geologic units formed during specified intervals of time. Our
terrestrial stratigraphy is largely based on successions of fossil
assemblages contained in rock units and the relations of such units
with others as determined by their lateral continuity and superposition
and intersection relations. To define geologic periods on the other
planets, where fossil assemblages cannot be used, major geologic
events have been employed (such as the formation of large impact
basins on the Moon and Mercury [3,5] or the emplacement of
widespread geologic terrains on Mars [4,6]). Furthermore, terres-
trial schemes have been supplemented by absolute ages determined
by radiometric techniques; on the Moon and Mars, relative ages of
stratigraphic units have been def'med according to impact cratering
statistics [3,4].
Stratigraphies are by nature provisional, and they are commonly
revised and refined according to new findings. Boundaries of
stratigraphic units are difficult to def'me precisely on a global scale;
refinements such as newly discovered marker beds may require
changes in nomenclature or minor adjustments in absolute- or
relative-age assignments. Although the martian stratigraphy has
been revised only once [4], the lunar stratigraphy is in its fifth major
version [3] and continues to be ref'med in detail [7]. Terrestrial
stratigraphy has been revised many times in its development over
the past century. Although the Galilean satellites of Jupiter are
being mapped geologically, there has been insufficient basis or need
for the development of formal stratigraphies for them [8].
Given the current state of stratigraphic methods and their appli-
cation to the planets, what are the prospects for a stratigraphy for
Venus? To address this question, we need to examine two related
questions: (1) Has the planet experienced widespread geologic
events or processes resulting in the broad distribution of coeval
materials (which would form the basis for time-stratigraphic units)?
(2) Can we unravel the complex stratigraphic-tectonic sequences
apparent on Venus? (Or can superposition relations and relative age
be reasonably well established from Magellan radar or other
datasets?)
Already a variety of potential venusian stratigraphic markers
based on widespread geologic events and activities are emerging
(Table 1 ). First, the global distribution of impact craters indicates
that Venus had a global resurfacing event or series of events that
ended about 500 + 300 m.y. ago [ 1]. Presumably, most of the plains
material that covers about 80% of the planet formed during that
resurfacing and can be included in a stratigraphic system. Under-
lying much of the plains material are complex ridged or tesselated
terrain materials; they must be older than the plains material, but
their structures may in part be contemporary with its emplacement
[9]. Postdating the plains material are most of the impact craters,
many of the largest volcanic shields, and regional fracture belts [ 1].
Most of the plains are cut by lineations, narrow grabens, and wrinkle
ridges. Perhaps such widespread structures may provide a basis for
defining regional or even nearly global stratigraphic markers to


Volcanism by melt-driven Rayleigh-Taylor instabilities and possible consequences of melting for admittance ratios on Venus

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