TO:  Venus Mappers            FROM:  Geological Mapping Subcommittee
DATE:  July 22, 1996          SUBJECT:  Guidelines from Reno meeting
   Following are guidelines generated during our recent meeting in Reno.
The
letter symbols for map units will follow a standard sequence.
VERY IMPORTANT STUFF BETWEEN THE STARS!!!
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STANDARD SEQUENCE IS AS FOLLOWS:
major unit type --> geographic modifier --> descriptive modifier -->
numerical or letter modifier (essentially members).
All of these are optional except the major unit type; thus it is OK, for
example, to use t for undifferentiated tessera materials, fm for mottled
flow
materials, etc.
The geographic modifier may be upper case.  It turns out that this is OK
with
the USGS as long as the geographic modifier does not come first in the
sequence; leading upper case letters are reserved for accepted formal time-
stratigraphic names (such has H for Hesperian on Mars), and no such system
exists for Venus at present.  All other letters should be lower case.
If used, the letter or number modifier at the end of the sequence will
likely
be subscripted on the published map, as has been done on a number of recent
Mars maps for members of formations.  It is possible that the upper case
geographical letters also will be subscripted, although it is not clear at
present if this will be esthetically pleasing.
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SPECIFIC EXAMPLES
1. Regional plains materials  (pr)  -- these are the areally extensive
plains
present in most quadrangles, and dominant in many.  The governing criterion
is regional persistence, not relative abundance in a given quadrangle.  If
a
mapper is in doubt about the continuity of a plains unit over a wide area,
the symbol "p" should be used rather than "pr" (see below).  Acceptable
descriptive modifiers include:  undifferentiated, variegated, lineated,
ridged, homogeneous, mottled.  Modifiers to avoid include:  smooth, rough,
bright, dark, deformed, fractured.  In an earlier set of guidelines, we
recommended that letters be used to indicate fine divisions (at the member
level) rather than numbers.  The use of numbers for an age sequence of
regionally extensive units is risky because the oldest member (#1) in one
quadrangle might correlate with the #2 member in an adjacent quadrangle.
Some examples:
   prGh = Guinevere homogeneous regional plains material.
   prm = mottled regional plains material.
   prra = ridged regional plains material, member a.
2. Plains Materials  (p) -- plains with more localized occurrences,
possibly
(but not necessarily) associated with fields of cones or shields, coronae,
or
small edifices.  The use of the term "shield plains" is acceptable, but
must
be explained in the DOMU because many of the volcanic constructs in the
plains may be better described as cones, domes, etc.  Or one could use the
term "edifice plains" to avoid this extra explanation.  Numbers implying an
age sequence may be used to designate members for these more local units.
Some examples:
   pl = lineated plains material.
   pRvb = Russalka variegated plains material, member b.
   pHh3 = Heng-O homogeneous plains material, member 3 (clearly implying
that
this member is younger than members 1 and 2).
3. Tessera materials  (t) -- materials in terrain characterized by two or
more non-parallel ridge or linear sets.  This terminology is broad, given
all
the variations in structural style, but a suite of modifiers may be used to
delineate individual material units within this group.  Possible modifiers
include:  undifferentiated, ridged, grooved, lineated.  The use of the term
"complex ridged terrain" (CRT) is strongly discouraged because large areas
of
this terrain are more characterized by linears than ridges, and because the
word "complex" implies more information than we really know about the
material's deformation history.  For blocks of tessera extending across two
or more quadrangles the use of numbers for members is discouraged.  For
smaller, isolated blocks of tessera numbers are acceptable because it is
not
really possible to correlate tessera from one isolated block to another
anyway.  Examples:
   tLr2 = Laima ridged tessera material, member 2.
   t = tessera material, undifferentiated for quadrangle-wide undivided
tessera.
   tu = tessera material, undifferentiated to distinguish undivided tessera
materials from other named tessera materials in the quadrangle.
4. Flow materials  (f) -- Typically digitate or lobate deposits from a
variety of sources.  Possible modifiers:  digitate, lobate, variegated.
Examples:
   fTl1 = Tepev Mons lobate flow material, member 1.
   f = flow material, undifferentiated.
5. Crater materials  (c)  -- Floor, wall, rim, central peak, and ejecta
deposits of impact craters.  For most craters, there is little reason to
separate all of these parts, and thus c alone can designate all of them.
Flow (not "outflow") materials can be designated by cf.  If
stratigraphically
useful, separate names can be used for individual important craters; thus
cfP
would be Potanina crater flow material.  Likewise, a mapper may designate
floor, wall, rim, central peak or ejecta materials for a given crater with
different modifiers if deemed important.  Thus, for example, cflA, cwA,
crA,
cpA, ceA for materials of floor, walls, rim, central peak, and ejecta of
crater Annia Faustina.
6. Surficial materials or surficial modification -- use stipple or other
pattern as an overlay to indicate the presence of surficial materials on
top
of mappable rock units, or to indicate the presence of surface modification
by some process that has not obliterated mappable rock units.  An important
example would be dark or bright parabolas or halos associated with some
impact craters.  If these are very extensive, such that stippling will
result
in cluttering the map, it may prove more practical to include a separate
text
figure showing their extent.
Partially Unresolved Issues:
1. Prominent belts of ridges, linears, or both -- These are the "ridge
belts", "fracture belts", and "deformation belts" of earlier reports.
"Fracture belt" and "deformation belt" are not acceptable because they are
genetic.  Material categories called "materials of ridge belts" or
"materials
of ridge and linear belts" are acceptable; these would be columns in the
correlation chart on the map.  Only material units confined to belts would
be
included within these categories; some materials within belts are so
modified
by the ridges or linears that their original character is lost and thus
they
must be designated as "belt materials" of some sort.  In contrast, a
material
that can be traced from surrounding plains into a belt should be designated
a plains unit and listed within a plains column on the correlation chart.
   It is important to map material units within these belts and not just
draw
lines that enclose the ridges and/or linears.  Thus the boundaries of a
ridge
belt, for example, may or may not coincide with material map unit contacts.
Most mappers need not worry about these belts because almost all of them
occur either in the Atalanta/Vinmara area or the Lavinia area.
2. The use of "bright" and "dark" as descriptive terms could be acceptable
for local features, such as bright crater floors, dark mantle deposits, or
small low-return lava flows.  These terms must be avoided in describing any
areally extensive deposit, since the definitions are vague.  It is OK to
use
descriptions in the DOMU such as:  "radar-dark relative to the planetary
average at an incidence angle of 45 degrees".
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Below is the proposed general text to be placed in the margin of all Venus
geological maps.  This will be edited by the USGS (of course), but we
believe
that its content will be essentially as presented here.  There are a few
small changes from the version distributed by Ken Tanaka immediately after
the Reno meeting.
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Overview paragraphs to go on every map to summarize the Magellan mission
   The Magellan spacecraft orbited Venus from August 10, 1990 until it
plunged into the Venusian atmosphere on October 12, 1994. Magellan had the
objectives of: 1) improving the knowledge of the geological processes,
surface properties and geologic history of Venus by analysis of surface
radar
characteristics, topography and morphology, and 2) improving the knowledge
of
the geophysics of Venus by analysis of Venusian gravity.
   The Magellan spacecraft carried a 12.6-cm radar system to map the
surface
of Venus.  The transmitter and receiver systems were used to collect three
data sets: 1) synthetic aperture radar (SAR) images of the surface, 2)
passive microwave thermal emission observations, and 3) measurements of the
backscattered power at small angles of incidence which were processed to
yield altimetric data.  Radar imaging, altimetric, and radiometric mapping
of
the Venusian surface was done in mission cycles 1, 2 and 3, from September
1990 until September 1992.  Ninety-eight percent of the surface was mapped
with radar resolution on the order of 120 meters. The SAR observations were
projected to a 75-m nominal horizontal resolution, and these
full-resolution
data comprise the image base used in geologic mapping.  The primary
polarization mode was horizontal-transmit, horizontal-receive (HH), but
additional data for selected areas were collected for the vertical
polarization sense. Incidence angles varied between about 20 and 45
degrees.
   High resolution Doppler tracking of the spacecraft was done from
September
1992 through October 1994 (mission cycles 4,5,6).  Some 950 orbits of high-
resolution gravity observations were obtained between September 1992 and
May
1993 while Magellan was in an elliptical orbit with a periapsis near 175
kilometers and an apoapsis near 8,000 kilometers.  An additional 1500
orbits
were obtained following orbit- circularization in mid-1993.  These data
exist
as a 75 degree by 75 degree harmonic field.
   Radar backscatter power is determined by: 1) the morphology of the
surface
at a broad range of scales, and 2) the intrinsic reflectivity, or
dielectric
constant, of the material.  Topography at scales of several meters and
larger
can produce quasi-specular echoes, with the strength of the return greatest
when the local surface is perpendicular to the incident beam.  This type of
scattering is most important at very small angles of incidence, since
natural
surfaces generally have few large tilted facets at high angles.  The
exception is in areas of steep slopes, such as ridges or rift zones, where
favorably tilted terrain can produce very bright signatures in the radar
image.  For most other areas, diffuse echoes from roughness at scales
comparable to the radar wavelength are responsible for variations in the
SAR
return.  In either case, the echo strength is also modulated by the
reflectivity of the surface material.  The density of the upper few
wavelengths of the surface can have a significant effect.  Low-density
layers
such as crater ejecta or volcanic ash can absorb the incident energy and
produce a lower observed echo.  On Venus, there also exists a rapid
increase
in reflectivity at a certain critical elevation, above which
high-dielectric
minerals or coatings are thought to be present.  This leads to very bright
SAR echoes from virtually all areas above that critical elevation.
   The measurements of passive thermal emission from Venus, though of much
lower spatial resolution than the SAR data, are more sensitive to changes
in
the dielectric constant of the surface than to roughness.  As such, they
can
be used to augment studies of the surface and to discriminate between
roughness and reflectivity effects.  Observations of the near-nadir
backscatter power, collected using a separate smaller antenna on the
spacecraft, were modeled using the Hagfors expression for echoes from
gently
undulating surfaces to yield estimates of planetary radius, Fresnel
reflectivity, and root-mean-square slope.  The topography data produced by
this technique have horizontal footprint sizes of about 10 km near
periapsis,
and a vertical resolution on the order of 100 m. The Fresnel reflectivity
data provide a comparison to the emissivity maps, and the rms slope
parameter
is an indicator of the surface tilts which contribute to the quasi-specular
scattering component.