The record of impact cratering on Venus, recently revealed from high-resolution radar imaging by the Magellan spacecraft, has been interpreted as indicating that the planet underwent catastrophic global resurfacing about 500 million years ago. Although other interpretations of the crater characteristics have been suggested, the possibility of geologically rapid global resurfacing on the planet that most resembles our own in terms of mass, density, and bulk composition has generated widespread interest. The mechanisms that could cause such a catastrophe are discussed

Solomon, Sean C.

English

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F= ii_
NEWS AND VIEWS
VENUS
Keeping that youthful look
Sean C. Solomon
THE record of impact cratering on
Venus, recently revealed from high-
resolution radar imaging by the Magel-
lan spacecraft, has been interpreted as
indicating that the planet underwent
eatastrophic global resurfadng about 500
million years ago'. Although other inter-
pretatiom of the crater characteristics
have been suggested2, the possibility of
geologically rapid global resurfacing on
the planet that most resemblesour own in
terms of mass, density and bulk compo-
sition has generated widespread interest:
what mechanisms could cause such a
catastrophe? Several ideas were discus-
sed at the Fall Meeting of the American
Geophysical Union last month*.
The average age of Venus's surface
has been estimated from the number of
larger impact craters (the planet's thick
atmosphere has tended to protect it from
the smaller bodies that would give cra-
ters smaller than 30 km in diameter) and
from the cratering rate on Earth and the
Moon, or from estimates of the number
of asteroids whose orbits cross that of
Venus _. The answer, 500 million years z-2
(a tenth of the age of the Solar System),
is not in itself remarkable for a planet
which, like the Earth, has a hot, dyna-
mic inter/or -- despite the absence of
significant erosion. What is remarkable
is that the craters at all sizes are indis-
tinguishable from a random population
(that is, spatial variations in crater
density are not larger than those due to
chance) and almost none has been signi-
ficantly modified" by tectonic strain or. by12
volcanic flows beyond the crater nm
(Fig. 1), even though Magellan images
show volcanic and tectonic features to be
nearly ubiquitous on Venus.
The interpretation of Sehaber and
FIG. 1 Magellanradar image of an impact crater on Venus,
approximately70 km in diameter. Radar-bright, rough-
texturedejecta extending up to 60 km from the crater rim
and the remarkable bright flow features extending more
than 300 kmfrom the craterwalls are thoughtto date from
the impactz and have not been modifiedto any significant
degreeby laterdeformationor volcanismoutside the crater
(although the generally radar-dark, smooth floor of the
crater maycontainyoungervolcanicdeposits).
114
others I is that most of the surface dates
from the end of a global resurfacing
event that ceased about SO0 million years
ago, and that the small fraction of cra-
ters volcanically embayed or modified by
deformation indicate that volcanic and
tectonic activity has since been at much
lower levels. Phillips and colleagues'
have argued that a correlation of loca-
tions of modified craters with areas of
low crater density and an inverse cor-
relation between crater density and
Magellan radar backscatter (a quantity
elevated in regions of high topography
and high roughness, both thought to be
signatures of comparative geological
youth) indicate that the Venus surface
exhibits a spectrum of ages. (The paucity
of small craters, however, hampers the
use of crater density in determining the
relative ages of geological units, a
method used with the other solid planets
and satellites.) Phillips et aL have de-
scribed an alternative model in which
resurfacing occurs episodically in small
patches a few hundred kilometres in
extent, with a characteristic time be-
tween events of the order of I0 s years
(Fig. 2).
Nonetheless, the catastrophic hypoth-
esis and its possible mechanisms have
captured the lion's share of the atten-
tion. At the meeting, D. L. Turcotte
(Cornell University) proposed that
global lithospheric overturn operates
episodically on Venus, and that for the
past 500 million years the lithosphere has
been cooling and mechanically stable.
Although the mechanism for stabiliza-
tion of the lithosphere is not an explicit
component of Turcotte's scheme, he
argues that a cool and thick lithosphere
is in better agreement (than one in
steady-state conductive
equilibrium) with the large
values obtained from
topographic profiles for the
rigidity of the lithosphere
at sites of presumed litho-
spheric bending 4 and with
the large ratio of
long-wavelength gravity
anomaly to long-wave-
length topographic relief at
highland areas'.
Parmentier and Hess 6,
following Stevenson and
Bittker 7, have recently
suggested that lithospheric
stabilization may occur on
Venus because of a de-
crease in the density of
lithospheric mantle follow-
ing melt extraction. But
they add that the cooling
pMMgl_NG PAGE BLANK NOT FILMED
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: 0.0
0.0 0.2 :.- OA : =_ "0_ .' _ 0,9 :_::. t.O
: "_..':'. /._., -: ._t: _ -,
Age_noq, ye_) _ :_;-:-
FIG. 2 Two schematic distributions of surface
age on Venus. With catastrophic
resurfacing z (green histogram), most of the
surface is about 500 million years old, but
small areas have younger ages. With episo-
dic resurfacing 2 (red histogram), there Is a
more widespread distribution of surface
ages from 0 to approximately 1.000 million
years. Both possibilities are broadly
consistent with the distribution and states
of preservationof Impact craters on Venus.
(Courtesyof R. J. Phillips.)
of such a layer may subsequently raise its
density to the point where it becomes
unstable. Global overturn of this un-
stable layer, which would sink back into
the mantle, would be followed by partial
melting of the upper mantle, global re-
surfacing, and the gradual development
of a new buoyant layer and another
extended period of stabifity. In simple
one-dimensional models of this process,
the time between lithospheric instability
events is 300-500 million years.
Others point to time-variable mantle
convection rather than iithospheric insta-
bility as a mechanism for global resur-
facing An early effort to simulate such
convection on Venus 8 led to models in
which the characteristic flow speed and
mantle heat flux showed large variations
(by factors of 2-10) at intervals of 100-
200 million years. J. Arkani-Hamed
(McGill University) reported improved
calculations with better spatial resolution
which are still time-varying but which
show significantly weaker fluctuations of
the order of several tens of a per cent.
V. Steinbach (Cologne University) drew
attention to the potential role of upper-
mantle phase transitions in governing the
radial character of mantle convection in
the large terrestrial planets 9. He and D.
Yuen argue that such phase transitions
promote the formation of distinct con-
vecting layers in the upper and lower
mantle, but that as the Rayleigh number
(governing vigour of flow) in the mantle
decreases in response to core cooling,
whole-mantle convection tends to be-
come favoured over layered convection.
If Venus cooled more rapidly than the
Earth early in its history, it may now
"Fall meeting of the American Geoohysical Unio_. San
Francisco. 7-11 December 1992.
NATURE • VOL 361 • 14 JANUARY1993
https://ntrs.nasa.gov/search.jsp?R=19940026607 2020-06-16T12:36:34+00:00Z
have a smaller mantle Rayleigh number
and may have gone through a transition
to whole-mantle convection, while the
Earth may still be characterized by
layered mantle flow. Such a transition
would have been accompanied by over-
turn of the upper mantle, the upward
transport of heat, and probably by global
resurfacing.
"lanelithosphericinstabilitymodels as
appliedto date for Venus sufferfrom
being one-dimensionalm globalsyn-
chroneityis imposed by assumption.
Even ifsuch instabilitymechanisms do
operate,itislikelythatdifferentparts
of the planetwillbe atdifferentstages
in the stabilizationa d destabilization
sequence, and whole-planettemporal
variationswillbe smoothed out by such
regional differences.For both litho-
sphericand mantle-convectionmechan-
isms,attentioneeds to be giventoall
the terrestrialp anetsas a group.Both
the Earth and Mars have gone through
episodesofgreatmagmatic activity,pre-
sumably an indicationofvariablemantle
convectiveflux,butneitherplanetshows
any evidenceof rapid,complete global
resurfacingduringthe past4,000million
years,and nor do Mercury ortheMoon.
Globalresurfacingby large-scaleover-
turnofthe lithosphereor upper mantle
shouldresultin efficientoutgassingof
the upper mantle;considerationof the
4°At abundance of Venus's atmosphere
suggeststhatany such widespreadout-
gassingwas restrictedto times much
earlierthan 500 millionyearsagoI°.
Finally,remember that there are
alternativesto the catastrophicmodel
thatseem to be equallyconsistentwith
the characteristicsof impact craterson
Venus2.Furtherstudy of the Magellan
imagesofimpactfeaturesand continued
development of dynamic models of the
planet'sinteriorshouldsharpenthecom-
petinghypothesesand may lead to an
improved understandingof mantle con-
vectionand melting on all of the terrest-
rial planets, including the Earth. [3
Sean C. Solomon is in the Department
of Terrestrial Magnetism, Camegle In-
stitution of Washington, Washington DC
20015. USA.
1. Sch#aer. G. G. et mL.l. _ys. Res. lKI. 13257-
133O1 (1992)
2. Phillips. R. J. e_ al. J. ¢eophys. Res. 07. 15923--
15948 (1992).
3. Shoemaker, E. M, Wolfe, R. F. & Shoemake¢, C. S.
Lunu_net. SoL 22. 1253-1254 (1991).
4. Sa'Kh_ll. D. T. & Schuber[, G. J. leon./_s. 97,
16069-16083 (1992).
5. Smm_r, S. E. & Phillips, R. J. Ear/h pMr_t. Sol. Left.
107. 582-597 (1991).
6. PannenUer. E. M & Hecs. P. C. ¢?,eophys. Res. Left.
111. 2015-2018 (1992).
7. Stevenson, D. J. & Bittker, S. S. Lunar planet, Sci. 2.t.,
1200-1201 (1@gO).
8 Ad_ni.Hamed. J. & Toks6z, M. N. Phys. Earth planet
Inter. 34, 232-250 (1984}.
9. Steird>ach, V. & Yuen, 0. A. Geophys, Res, tett. _Lg.
2243-2246 (1992).
10. Matsui. T, & Tajika. E, Lurutrplanet ,SoL:[2. 863
(1991).
NATURE - VOL 361 • 14 JANUARY1993
CELL MOTILITY
NEWS AND VIEWS
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