From: jgacker@news.gsfc.nasa.gov (James G. Acker)
Subject: Re: Ginenthal on Venus' Surface Phenomena
Organization: NASA Goddard Space Flight Center - Greenbelt, MD USA
X-Newsreader: TIN [version 1.1 PL6]
Date: Tue, 12 Jan 1993 15:08:50 GMT

news (news@fedfil.UUCP) wrote:


Yeah, Ted, I like chemistry.

First important point:
: atmosphere itself is highly corrosive.  The two should have caused lots
: and lots of weathering.  But there is no evidence of this.
: 
: 
:         "THE MISSING VENUSIAN REGOLITH
: 
:     "Geophysicists, in order to explain the physical nature of
:     the Venusian surface, offer the supposition that between
:     100 million and a billion years ago the entire planet turned
:     itself inside out.  If one were to accept this assumption
:     it would require that over that period of time
:     between the covering of the surface with lava flows and
:     the present, erosional forces would break down the
:     surface rock into detritus to form a regolith.
: 
: 
:     "Venus' atmosphere is known to contain hydrochloric and
:     hydrofluoric acid, both of which
:     are very corrosive.  Paolo Maffei explains further that,
:     "the atmosphere of Venus also contains - although
:     in small amounts-hydrogen chloride and hydrogen
:     fluoride, which reacting with sulfuric acid [known to exist
:     in Venus' atmosphere] could form fluosulfuric acid, a
:     very strong acid capable of attacking and dissolving
:     almost all common materials including most rocks."
: 
: 
: 
:     "According to the scientists, Venus has been subjected to
:     this intense weathering of its surface for at least 100
:     million years.  Over this period of time the planet
:     shouict have developed a covering of weathered material.
:     Nevertheless, George McGill et al., inform us that:
: 
: 
: 
:         'Radar and Venera lander observations imply that most of the
:         surface of Venus cannot be covered by unconsolidated
:         wind blown deposits; bulk densities on near surface
:         materials are not consistent with aeolian sediments ...  Thus
:         present-day wind-blown sediments cannot form a continuous
:         layer over the entire planct.
: 
: And from Bruce Murray (JOURNEY INTO SPACE):
:       
:         'Russian close-ups of Venus were surprising.  I had presumed
:         that its surface was buried under a uniform blanket of
:         soil and dust.  Chemical weathering should be intense in
:         such a hot and acid environment,...Unknown processes
:         of topographic renewal evidently manage to outstrip
:         degradation and burial.
: 
: 
:     "In order to explain the lack of a Venusian regolith the
:     scientists imagine a process that has no scientific basis
:     for its action to reconsolidate the detritus on Venus.
:     Nevertheless, let us assume that Venus' erosion rate is
:     extremely weak and that it is not tumed back into rock at the
:     surface by unknown processes.  What do we find? If we
:     allow a tiny erosion rate of one millimeter per hundred
:     years, then in 100 thousand years we produce one meter
:     of loose material on the surface of Venus, which is equal to
:     about 40 inches.  However, in 100 million years we
:     generate a kilometer of detritus, which is over 3000 feet of
:     this loose material.  Under no known condition can this much
:     matter at the surface be turned to solid rock..."

	There's a serious misconception here.  The argument is that an
erosion rate invariably produces a certain quantity of loose material.
Chemical weathering does not necessarily do this;  in fact, boulders of
granite in tropical climes transform almost entirely to clay from the
surface inward.  They still look like hard granite boulders, but they
are in fact hard *clay* boulders.  Keep in mind that chemical weathering
and physical weathering are two different processes.


:     "What we find at the surface of Venus is  the  detritus  of  an  erosion
:     rate that is only a few  thousand  years  old.  Only  by  ignoring  this
:     clear  evidence  can  the astronomers support the view that Venus'
:     surface reflects events  tracing  to  processes  occurring between
:     100 million and one billion years ago.

:        'MageUan scientists strove to explain the paradox of young
:        looking craters  on  a  relatively  old surface.  They  raised
:        the  possibility  that  several  hundred  million  years  ago,
:        a planet-wide outpouring wiped the slate clean, drowning any
:        existing craters in  a  flood  of  lava.  Then the flood would
:        have had to turn off fairly abruptly  so  the  craters  formed  by
:        subsequent impacts would remain pristine.

	Examination of AGU abstracts indicates that this hypothesis is
still being analyzed, but it is a hypothesis being considered.  There is
an analogy in Earth's geologic history:  the Cretaceous, which was a
period of "superplume" volcanism.  The Deccan Traps were kind of the
last "burp" of the superplume era.


:     "No doubt there will be other, equally imaginative, scenarios
:     advanced in order to explain away this dilemma of so few
:     craters showing signs of decay.  To retum to Kerr:
: 
:         'But surface remodeling is going on after afl, Magellan scientists
:         told a large  crowd  at  the AGU  [American  Geological
:         Union]  meeting.  More  recent  images   show   the   ravages
:         of time, but in a fashion that leavesfew aged craters."
: 
: 
: That's like saying that your 90-year-old grandma shows her age, but in a
: manner which draws wolf-whistles in a bikini.  Not too likely, is it?

	Hmmm...  it was hard to find anything that addressed this
statement directly.  However, this is an interesting abstract:

	"Episodic Plate Tectonics on Venus"  EOS, October 27, 1992
Transactions, 1992 Fall Meeting Supplement.  D.L. Turcotte, Dept. of
Geological Sciences, Cornell University, Ithaca, NY 14853.  Phone # on
request.

	We propose as a working hypothesis that the heat loss from the
interior of Venus is strongly time dependent.  In particular, we hypo-
thesize that a global lithosphere on Venus stabilized about 500 MYBP and
has _thickenened_ (emphasis mine) conductively since then with a present
thickness of about 300 km.  This hypothesis is strongly supported by the
number and distribution of craters on the surface.  A mean surface age of
0.5 +/- 0.3 Gyrs is inferred and only 4.5% of the craters are embayed by
lava flows.  These observations led Schaber et al. (1992) to hypothesize that
a global resurfacing event had occurred about 500 Myrs ago and that
_relatively little volcanism_ has occurred since.  A thick lithosphere
allows us to explain the regions of high topography on Venus using either
Airy or thermal isostasy.  The thick lithosphere also gives depths of
compensation of 150 km or more consistent with the observed correlated
gravity anomalies.  Sandwell and Schubert (1992) have shown that the
morphology of several coronae on Venus are in good agreement with
lithospheric flexure models with elastic thicknesses of 15-35 km.  With a
300 km thick lithosphere these depths imply temperatures of 840-960 deg. K,
reasonable values for the base of an elastic lithosphere.  Without a
strong time dependence, the steady-state lithospheric thickness on
Venus would be about 40 km.  __Such a thin lithosphere is not consistent
with the high topography, large associated gravity anomalies, and the
inferred thicknesses of the elastic lithosphere.__

I've got more.  Just hold on, Holden.


	Jim Acker
	jgacker@neptune.gsfc.nasa.gov