Tesserae are regions of elevated terrain characterized by two or more sets of ridges and grooves that intersect orthogonally. Tesserae comprise 15-20 percent of the surface of Venus, but the nature of their formation and evolution is not well understood; processes proposed to account for their characteristics are many and varied. Two types of tessera boundaries have been described: type 1 are generally embayed by plains; and type 2 boundaries are characterized by being linear at the 100-km scale and often associated with steep scarps or tectonic features. Margins such as the western edge of Alpha have been described as type 2. Some of the tessera have boundaries that display deformation of both the edge of the tessera and the adjoining plains. This study focuses on the western edge of Alpha Regio in an effort to characterize on occurrence of this type of boundary and assess the implications of the style in general. Using Magellan SAR imagery, lineament lengths, orientations, and spacing were measured for ten 50 x 60 km areas spanning 500 km of the western boundary. Structural characteristics and orientations were compared to stratigraphic units in order to assess the sequence and style of deformation

Gilmore, M. S.

Head, James W., III

English

NASA Technical Reports Server

34 InternationalColloquium on Venus
temperature is quite significant compared with the effects of gas-
eous SO 2 and liquid H2SO 4. Thus, we can state that the variations
observed by de Pater et al. [ 1 ] are most likely due to the variations
in the abundance of gaseous H2SO 4 and not to liquid H2SO 4 or
gaseous sulfuric dioxide as previously suggested.
A plot of the calculated millimeter-wave spectrum of Venus
based on the presence of one or more constituents is shown in
Fig. 5. The results reported in this figure show the effect that HxSO,t
(g) has on the MMW spectrum of Venus. In addition, the results
show that there are specific millimeter-wave frequencies that are
especially sensitive to the abundance of H2SO 4 vapor in the lower
atmosphere of Venus.
References: [1] de Pater 1. et al. (1991) Icarus, 90, 282-298.
[2] Fahd A. K. and Steffes P. G. (1992) Icarus, in press. [3] Fahd
A. K. and Steffes P. G. 0991)JGR, 96, 17471-17476. [4] Steffes
P. G. (1985) Icarus, 64, 576:-585.
- _ __ • _j
RADAR SCATTERING PROPERTIES OF PANC-AKELIKE I
DOMES ON VENUS. P.G. Ford and G. H. Pettengill, Center for
Space Research. Massachusetts Institute of Technology, Cam-
bridge MA 02139, USA.
Magellan radar images have disclosed the presence of a large
number of almost peffecdy circular domes, presumably of volcanic
origin, in many regions of Venus [1]. several with diameters of
30 km or more. Their high degree of symmetry has permitted
measurements of their shape, as determined by the Magellan
altimeter [2], to be compared with models of dome production from
the eruption of high-viscosity magmas [3].
In this work, we examine in detail the radar images of domes in
Rusalka Planifia (2.8°S, 150.9°E) and Tinatin Planitia (12.2°N.
7.5°E), selected for their circular symmetry and apparent absence of
modification due to large-scale slumping or tectonic rifting. Assum-
ing that these domes are shaped according to the model of reference
[3], we can orthorectify the available Magellan SAR image swaths
(F-BIDRs: Full-Resolution Basic Image Data Records) to generate
three-dimensional plots of the radar scattering cross-section o 0 (r,
0, O) as a function of distance from center of dome (r), scattering
angle (0), and azimuthal coordinate (O)-
The behavior of a 0 with reslx_t to changes in 0 has been
determined from Pioneer Venus radar data for many broad classes
of Venus surface type [4], and parameterized as a combination of a
quasispecular scattering component (tqs and a diffuse component
(td:
(t0(0) = (tq,(0) + od(0) =
0t CP(cos4 0 + Csin 2 0) -312 + (1 - a)pK0 v
2
where a represents the fraction of the surface that contributes to
quasispecular scattering, C is the Hagfors parameter [5], p is the
Freanel reflection coefficient, and K and v arc functions of small-
scale surface roughness. Average values of C, p, and ot over an entire
dome are extracted from altimeter measurements.
Variations of(t 0 with respect to radial distance r arc interpreted
as changes in the small-scale roughness of the dome, which would
be expected from the radial dependence of the cooling rate of the
lava, perhaps enhanced by subsequent weathering. The result of
aeolian processes may also be seen in the dependence of ¢t0 on
azimuth angle _, since Free-grained surface material that contributes
to Od may be emplaced or rearranged by the prevailing surface
winds.
References: [1] Head J. W. et al. (1991) Science, 252,276-288.
[2] Ford P. G. and Pettengill G. H. (1992) JGR, in press.
[3] McKenzie D. et al. (1992) JGR, in press. [4] Ford P. G. and
Senske D. A. (1990) GRL, 17, 1361. [5] Hagfors T. (1970) Raddo
189.
N93-14317
SEQUENTIAL DEFORMATION OF PLAINS ALONG
TESSERA BOUNDARIES ON VENUS: EVIDENCE FROM
ALPHA REGIO. M.S. Gilmore and J. W. Head, Department of
Geological Sciences, Brown University, Providence RI 02912,
USA.
Tcsserae arc regions of elevated terrain characterized by two or
more sets of ridges and grooves that intersect orthogonally [1].
Tesserae comprise 15-20% of the surface of Venus, but the nature
of their formation and evolution is not well understood; processes
proposed to account for their characteristics are many and varied
[2]. Two types oftessera boundaries have been described: Type I are
generally embayed by plains; type 1I boundaries are characterized
by being linear at the 100-kin scale and often associated with steep
scarps m-tectonic features [2,3]. Margins such as the western edge
of Alpha have been described by these authors as type H. Some of
the tessera have boundaries that display deformation of both the
edge of the tessera and the adjoining plains [2,3]. This study focuses
on the western edge of Alpha Regio in an effort to characterize one
occurrence of thistype of boundary and assess the implications for
the style in general. Using Magellan SAR imagery, lineament
lengths, orientations, and spacings were measured for ten 50 x 60-
km areas spanning 500 km of the western boundary. Structural
characteristics and orientations were compared to stratigraphic
units in order to assess the sequence and style of deformation.
Alpha Regio is a 1300 x 1500-kin prominent radar-bright upland
feature in the southern hemisphere of Venus that averages 1 km
above the surrounding plains [4]. Ridges and troughs within Alpha
average 33 km long 20 km apart in the north and 35 km long 17 km
apart in the south; their prominent orientation is N20°E [4]. The
ridges and troughs on the western edge of Alpha have an orientation
of N15°E, but differ from the interior ridges as their average spacing
is 4 km (Fig. 1). These lineaments are joined by a second set of
lineaments and graben trending N55°W and extending into the
plains. The deformation producing these northwest-trending linea-
ments has occurred over a period of time separated by several stages
of plains emplacement. Two plains units have embayed the western
edge of Alpha: a radar-dark plains unit (PI 1) that embays the edge of
the heavily deformed tesserae, and a radar-bright unit to the west
that embays the radar-dark unit (Fig. 2). The plains unit closest to
the tessera (PI l) has fewer lineaments than the tessera, but a greater
number of lineaments (spaced at an average of 3 km apart) than the
younger plains unit (P12), which embays and covers unit P1t. The
https://ntrs.nasa.gov/search.jsp?R=19930005129 2020-03-17T08:45:55+00:00Z
LPI Contribution No. 789 35
6054
6053
6052
6051
6050
6049
.... I .... i .... i .... t ....
.... ' .... I .... I .... I''"I
156 357 358
c)
N
-22.7. a_:_ Gaaben
-2 A R,QgeS anO Scalps
HalchlJces on Uplhlown
FauII Block
_maO Sl_eld Volcanoes
/'/ /!'Y ,c \, j(b)
356.6 357,1
Kdometels
Fig. 1. (a) SAR image of portion of western Alpha Regio including plains, t-me-scale lineaments, and ridges and troughs typical of Alpha tes=era. The fabric
of the fine-scale linemlne_ntseleady differs from the more typical tessera fabric. (b) Sketch map of (It) emphasizing line.amenLs and volcanic features. Two sets
of lin_Ls are seen here that increase in number from the plains into the l-me-scale ridges. The northwest-trending lincameats and shields are confined to the
fine-scale fabric. (c) Topographic profile of A-A' in (b).
northern part of Fig. 2 displays some lava flows from PI 2 that were
too thin to cover the northwest lineaments. The boundary between
the plains units is also marked by a change in slope from 2 ° in the
younger unit to 6°; the slope may then increase to as much as 25 ° at
the plains-tessera border. This is interpreted to mean that the dark
plains were emplaced along the margin of the tessera, tilted up, and
deformed, followed by the emplacement of the bright plains, which
also have been tilted upward and deformed. Deformation features
are grabenlike and interpreted to be extensional in origin. The
deformation of the plains follows one of the major trends in the
tessera interior [4,51.
There is a range of interactions between the northeast- and
northwest-trending lineaments; f'me-scale ridges and troughs are
common within western Alpha while large graben up to 6 km in
width occur less frequently. In addition, the tessera along Western
Alpha contains numerous domes, pits, and small shield volcanos
that both predate (Fig. 1) and postdate the intense tessera deforma-
tion. These small shields are very similar to the array of shield
volcanos that have been mapped on the Venus plains [6], and we
interpret this to mean that these tessera regions are formed from
adjacent volcanic plains. In addition, blocks (< 100 km in length) of
crust that have radar characteristics that make them indistinguish-
able from undeformed plains are observed in the tessera along the
western margin. Each of these characteristics, in addition to the
distinct difference in ridge spacing in western Alpha, suggests that
western Alpha Regio may consist of plains that are sequentially
deformed, tilted, and uplifted, ultimately being incorporated into
the tessera. In addition, it has been proposed that f'me-scale ridges
like those of Western Alpha am produced by layering thought to
exist within the plains [5].
We have considered two models to explain the characteristics of
the edge of Western Alpha. The t-h-st is mantle downwelling and
underthrusting of plains units due to compression at the edge of the
tessera block [5,7]. Shortening and isostatic adjustments due to
crustal thickening would cause uplift in Alpha and consequently
cause deformation and uplift along the margins of Alpha. The
second model is one of gravitational relaxation of the Alpha tessera
block. Gravitational relaxation of a plateau is expected to produce
extensional features within the interior of the crustal block that
change to compressional features at the margins of the block [8,5,3].
This is consistent with the presence of relatively young and
undeformed intertesseraplains in Alpha and the tree-scale compres-
sional features on its western margin. We favor the gravitational
relaxation model for these reasons and because it is an ongoing
process favored under Venus conditions of high temperatures and
low erosion rates [9,10]. This model fails to explain, however, why
the deformation seen on the western edge of Alpha does not extend
over the entire margin of the tessera block.
In summary, the western border of Alpha Regio has two sets of
lineaments that extend into the surrounding plains. This deforma-
tion has occurred over a time period marked by at least two sets of
plains emplacement. Western Alpha also contains small shield
36 International Colloquium on Venus
(a)
6054
6053-
6052-
6051
6050
6049
I I f I I I t
t
lc )
I t I I ,I
355 356 357 358
_:_::!_-.'.-_?_:.."_:_:i:.... . :...i::::_::i!_._.-::::_ _,_:::. "__!i:i:t
iiiiii :::
356-3 356.8
K+lom_l_t _
Fig. 2. (a) SAP, image of a portion of western Alpha Regio including two units of plains and t-me-scale lineaments. CO)Sketch map of (a) emphasizing the
nonhwest-trending lineaments and graben and volcanos. In general, these lineaments are more abundant and have a closer spacing in the older unit Pit than in
the younger P12. "l'ae younger unit has embayed the older, covering most lineaments. Cotatinued deformation has caused fracturing to extend through both units.
(c) Topographic profile of B-B' in Co).
volcanos and blocks of undcformed plainslike crust. Each of these
features suggests that Alpha is deforming, uplifting, and possibly
incorporating plains lavas onto its western edge. Gravitational
relaxation of Alpha tessera may be the mechanism producing this
deformation and may contribute to the features found at other type
1I boundaries on Venus. The total length of type H (deformational)
boundaries on Venus is less than type I (embayed by plains lava.s)
boundaries, but type II boundaries occur at some point along many
tessera blocks of all sizes [3]. We are continuing our investigation
of this and other similar boundaries.
References: [1] Basilevsky A. T. et al. (1986) Proc. LPSC
16th, in JGR, 91, D399. [2] Bindschadler D. L. and Head J. W.
(1991) JGR, 96, 5889. [3] lvanov M. A. et al. (1992)LPSC XXIH,
581. [4] Senske D. et al. (1991) Earth Moon Planets, 55, 97.
[5] Bindschadler D. L. et al. (1992) JGR, submitted. [6] Aubele L
oral. (1992)LPSCXXIII,47. [71 Bird P. (1979)JGR,84,7561-7571.
[8] Bindschadler D. L. and Parmentier E. M. (1990) IGR, 95,
21329. [-9] Wgertman J. (1979)PEPI, 19, 197. [10] Smrekar S. E.
anqk._'.olg_,aoFtS. C. (_tp92),a/G_i_h_lres'l.
N93-14318 ..'
VENUSIAN HYDROLOGY: STEADY STATE RECONSID-
ERED. David H. Ca'inspoon, Laboratory for Atmospheric and
Space Physics, University of Colorado, Boulder CO 80309, USA.
In 1987 Grinspoon proposed that the data on hydrogen abun-
dance, isotopic composition, and escape rate were consistent with
the hypothesis that water on Venus might be in steady state rather
than monotonic decline since the dawn of time [ 1,2,3]. This conclu-
sion was partially based on a derived water lifetime against nonther-
mal escape of approximately 10S yr. De Bergh etal. [4], preferring
the earlier Pioneer Venus value of 200 ppm water to the significantly
lower value detected by Bezard etal. [5], found HzO lifetimes of
>109 yr. Donahue and Hodges [6] derived HzO lifetimes of 0.4-5 x
109 yr. Both these analyses used estimates of H escape flux between
0.4 × 10 _ and 1 × 10 ¢em -z s -t from Rodriguez et al. {7]. Yet in more
recent Monte Carlo modeling Hodges and Tinsley [8] found an
escape flux due to charge exchange with hot H ÷ of 2.8 × I0 _ cm -z
s-L McElroy et al. [9] estimated an escape flux of 8 x 1_ cm-Z s -_
from collisions with hot O produced by dissociative recombination
ofO_ +. Brace et al. [10] estimated an escape flux of 5 × 10 _ cm -_
s-z from ion escape from the ionotail of Venus. The combined
estimated escape flux from all these processes is approximately 4 x
107 em -z s -_. The most sophisticated analysis to date of near-IR
radiation from Venus' nightside reveals a water mixing ratio of
approximately 30 ppm [ 11 ], suggesting a lifetime against escape for
water of less than l0 s yr, Large uncertainties remain in these
quantities, yet the data point toward a steady state. Further evalua-
tion of these uncertainties, and new evolutionary modeling incorpo-
rating estimates of the outgassing rate from post-Magellan estimates
of the volcanic resurfacing rate, will be presented.
If Comet Halley has a mass of 10 _sg and is approximately 50%
water ice, then impact of such an object provides roughly 10% the
current atmospheric water inventory on Venus. The terminal Crcta-


Sequential deformation of plains along Tessera boundaries on Venus: Evidence from Alpha Regio

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