James G. Acker
May-21-93 05:54AM
Venus Resurfacing II

Organization: NASA Goddard Space FLight Ceneter -- InterNetNews site
From: jgacker@news.gsfc.nasa.gov (James G. Acker)
Message-ID: <1timu9$iie@skates.gsfc.nasa.gov>
Newsgroups: talk.origins


      "An Assessment of Resurfacing Models for Venus"

      Walter S. Kiefer (Lunar and Planetary Institute, Houston, TX
77058;  phone number and email address on request -- JGA)

      Interpretations of the Venus cratering record have focused on
two endmember models:  (1) catastrophic resurfacing of the entire planet
about 500 My ago, with little subsequent resurfacing, or (2) continuous
resurfacing at a constant rate of small, randomly located patches over
the last billion years.  Both the Earth's resurfacing history and
numerical modeling provide a context for assessing these alternatives.
Although there is considerable evidence for variability in the Earth's
resurfacing rate, there is apparently no evidence for the near-absence
of resurfacing for 500 m.y. periods.  Several approaches for producing
catastrophic resurfacing have been proposed, including (1) a recent
transition from time-dependent to steady-state mantle convection;
(2) the effects of chemical buoyancy from a depleted mantle layer; and
(3) a recent transition from a layered to whole mantle convection.
However, steady-state convection does not preclude continual resurfacing.
In this case, the resurfacing will have a non-random spatial
distribution, in violation of observations.  Chemical buoyancy can
cause episodic variations in volcanism with a 500 m.y. period, but the
resurfacing rate remains high for most of this period, contrary to
hypothesis 1.  A transition from layered to whole mantle convection
could produce an intense resurfacing episode, but provides no mechanism
for preventing later resurfacing, again contrary to hypothesis 1.
These considerations favor a continuous resurfacing model as more likely
for Venus.  Existing Monte Carlo models of continuous resurfacing
presume randomly distributed resurfacing patches of a fixed size and
constraint the allowed patch size.  More realistic models would
allow for a spectrum of resurfacing sizes and for some temporal and
spatial coherence in the resurfacing pattern.  In such a model, the
observed randomness of the crater distribution probably reflects
chaotic reorganization of the mantle's convective platform.  The
observed crater distribution may place an upper-bound on the
time-scale for this reorganization.

      Abstract for the AGU Spring Meeting, May 24-28, 1993, Baltimore,
MD.

      Jim Acker