James G. Acker
May-21-93 05:41AM
Venus Resurfacing I

Organization: NASA Goddard Space FLight Ceneter -- InterNetNews site
From: jgacker@news.gsfc.nasa.gov (James G. Acker)
Message-ID: <1tim6m$iie@skates.gsfc.nasa.gov>
Newsgroups: talk.origins


      "The Resurfacing Controversy for Venus:  An Overview and
a Mechanistic Perspective"
      Sean C. Solomon (DTM, Carnegie Inst. of Wash., 5241 Broad
Branch Rd., NW, Washington, D.C., 20015)

      Two remarkable aspects of the population of impact craters on
Venus are that craters of all sizes are indistinguishable from a random
population and that most craters have not been significantly modified
by tectonic strain or by volcanic flows external to the crater rim,
despite evidence from Magellan images that volcanic and tectonic
features are widespread on Venus.  One interpretation of these observations
is that most of the surface dates from the end of a catastrophic global
resurfacing event that ceased about 500 My ago, and that the small
fraction of craters volcanically embayed or modified by deformation
indicate that volcanic or tectonic activity subsequent to that time has
been at much lower levels.  A competing scenario, in which resurfacing
occurs episodically in patches a few hundred kilometers in extent and
there is a wider spectrum of surface ages, also appears to be consistent
with the characteristics of impact craters on Venus.  While geological
and statisitical studies of the crater population on Venus offer some
promise for distinguishing between these two hypotheses, consideration of
the possible mechanisms for catastrophic and episodic resurfacing
provides an independent perspective.  Potential mechanisms for catastrophic
resurfacing of Venus range from geologically sudden convective destabilization
of the global lithosphere to strongly time-dependent heat flux and melt
generation in the underlying mantle.  For most of these mechanisms,
resurfacing occurs implicitly or explicitly by volcanism.  An alternative
hypothesis is that, at least in the geologically recent history of
Venus, the primary resurfacing mechanism has been tectonic deformation
rather than volcanism.  Because the rate of surface strain should be
controlled by the temperature-dependent strength of the lower crust,
a geologically rapid transition in surface strain rates should be the
natural result of planetary cooling.  This transition would occur at
comparable times for areas of similar crustal thickness and heat flow
(e.g. plains), but would be delayed for regions of thicker and hotter
crust (highlands).  The end of the era of high rates of tectonic
resurfacing could thus appear as a "catastrophe" over 80% of the
planet with an elevation within 1 km of the mean, while continued
deformation would give rise to "episodic" resurfacing to much
younger times in the highlands, a result consistent with lower
crater densities seen in highland regions.


      Abstract for the AGU Spring Meeting, May 24-28, Baltimore,
MD.