[meteorite-list] Venus Catastrophic Resurfacing Hypothesis Challenged

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Mon, 26 Apr 2010 20:30:08 -0500
Message-ID: <1767FCA298054C948B452AB1F9BF07C4_at_ATARIENGINE2>

Kelly, Paul, List,

> ... large-scale cratering only 600 million years ago
> would leave an unmistakable gravity signature...

    For the benefit of the audience (if there is any),
the gravity signature (of terrestrial carters, the only
ones we can compare closely to surface geology) is
caused by post-impact loss of density inside the crater,
creating the distinctive central low.

    Besides the loss of excavated material, in small
simple craters this is due to an interior breccia lens
made from the local rock. In complex craters, models
suggest the gravity anomaly is from fractured target
rocks all across the floor of the crater. All the models
and realities elsewhere are for a surface of cool solid
rock which can form a breccia of solid rocks or
fractured strata.

    There are only 900-odd craters on Venus (and no
small ones due to the protection of the atmosphere),
as contrasted with over 200,000 on Mars. The surface
of the Moon is saturated with millions of craters at every
scale. The Earth has less than 200 verified craters.

    The largest crater on Venus (270-280 km) is the same
size as the largest crater on the Earth (300 km). The size
distribution is similar on the two planets. The larger
number of Venus craters is roughly proportional to
the Earth's IF you accept the view that Venus has had
no tectonic activity great enough to remove craters
since the time of the re-surfacing of Venus. The lack
of tectonic activity is widely accepted because a variety
of other evidences.

> ...unmistakable gravity signature - and there ain't none.

    Here's one of that none:

    Mead is the biggest crater on Venus. It has a signature.
It is the only crater whose gravity signature has a paper(s)
and I suspect it is the only one analyzed. Why the biggest
crater on Venus? Because it's only one where you would
have the ghost of a chance to detect any signature at all.

    "In summary, as a rule of thumb, we suggest that useful information
about the subsurface structure of impact basins cannot be gained at
spacecraft altitudes greater than about 1/10 the diameter of the basin.
At altitudes approaching the basin's diameter even the signature
associated with the topography of a basin may be difficult to
separate from surrounding structures."

    Gee, maybe that's why there's no crater signatures.

    It doesn't explain why the standard NASA handout on
Magellan states unequivocally that the gravity signatures
follow the local topography closely (implying no crater
signatures when we have one signature and others are
likely undetectable). Press Release Science -- my favorite.

      Some sources conclude that the depth of Venus craters
is greater than expected by the standard crater modeling.

    Other sources say the depth of Venus's craters is exactly
the same as predicted by the models, except for very small
and very large craters which are shallower. It's Venus -- why
should anyone agree about anything? If crater depths are
greater, there is no agreement as to what would cause it,
and certainly the great depth of Cleopatra and some others
needs explaining.

    One more wrinkle. Existing craters would not be from
the episode of crust-melting impactors. The existing craters
could not form in plastic crust. They would be the record
of impacts from the time the crust because rigid enough to
form craters up the present day. The "age of the surface" is
the age the post-molten solid crust. Since there is no
agreement on the depth and plasticity of the crust, crucial
factors in crater formation analysis, there's nothing very
certain about the other formation issues.

    Lastly, the tessera occupy 8% to 12% of the Venusian
surface (depending on who you read) and the tessera (RTT)
contain 8.7% of Venus's craters. Since the RTT is obviously
subject to deformation forces (20% of the tessera craters are
distorted), a few craters may have been lost, but only a few.

    However, the number of craters on tessera relative to
area is essentially the same as the rest of Venus. IF the
RTT were 4 to 5 times older than the rest of Venus, dating
to ancient planetary formation times, wouldn't you think
the crater density would be a LITTLE more than on the
young terrain of the rest of the planet? I would. Even if
it was only twice as old, there would be some numerically
distinct marker.

   Thanks, but I think I'll stand pat with my cards from
the last go-round-the-table.

Sterling K. Webb
----- Original Message -----
From: "Kelly Beatty" <jkellybeatty at comcast.net>
To: <meteorite-list at meteoritecentral.com>
Cc: "'Paul H'" <bristolia at yahoo.com>
Sent: Monday, April 26, 2010 3:35 PM
Subject: Re: [meteorite-list] Venus

> Sterling and all...
>> A decade or so was wasted on "geological" mechanisms
>> that could re-surface without outside interference. They
>> were largely hooey that convinced no one, posing improbable
>> mechanisms to accomplish world-wide simultaneous subduction.
> *way* too sweeping a generalization. it's true that there's no
> consensus on
> what triggered the resurfacing of Venus, but the discussion mostly
> surrounds
> whether it's periodic (i.e. it'll happen again) or was a one-time
> event that
> fundamentally altered the planet's internal heat flow.
> regardless, large-scale cratering only 600 million years ago would
> leave an
> unmistakable gravity signature - and there ain't none.
> clear skies,
> Kelly
> ****************
> J. Kelly Beatty
> Senior Contributing Editor
> 617-416-9991
> SkyandTelescope.com list
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Received on Mon 26 Apr 2010 09:30:08 PM PDT

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