Of all the volcanic features identified in Magellan images, by far the most extensive and really important are lava flow fields. Neglecting the widespread lava plains themselves, practically every C1-MIDR produced so far contains several or many discrete lava flow fields. These range in size from a few hundred square kilometers in area (like those fields associated with small volcanic edifices for example), through all sizes up to several hundred thousand square kilometers in extent (such as many rift related fields). Most of these are related to small, intermediate, or large-scale volcanic edifices, coronae, arachnoids, calderas, fields of small shields, and rift zones. An initial survey of 40 well-defined flow fields with areas greater than 50,000 sq km (an arbitrary bound) has been undertaken. Following Columbia River Basalt terminology, these have been termed great flow fields. This represents a working set of flow fields, chosen to cover a variety of morphologies, sources, locations, and characteristics. The initial survey is intended to highlight representative flow fields, and does not represent a statistical set. For each flow field, the location, total area, flow length, flow widths, estimated flow thicknesses, estimated volumes, topographic slope, altitude, backscatter, emissivity, morphology, and source has been noted. The flow fields range from about 50,000 sq km to over 2,500,000 sq km in area, with most being several hundred square kilometers in extent. Flow lengths measure between 140 and 2840 km, with the majority of flows being several hundred kilometers long. A few basic morphological types have been identified

Guest, J. E.

Head, James W., III

Lancaster, M. G.

Roberts, K. M.

English

NASA Technical Reports Server

62 International Colloquium on Venus
To understand the reflection process it is convenient to divide it
into two steps. In the F_rst step the photon is stopped (absorbed) by
the reflecting surface and the photon's momentum is transferred to
the vane. In the second step a photon is immediately emitted from
the reflecting surface of the vane, By the principle of equal but
opposite reaction a momentum equal to that of the emitted photon
must be transferred to the vane during this second step. Therefore,
it can be seen that the rotor must rotate so that the absorbing surface
turns toward the radiation when opposite vanes are equally illumi-
nated and equal in surface area.
It has been long known that Venus has a high albedo due to the
scattering (similar to the reflection process) of solarradiation by the
cloud droplets in its atmosphere. The radiation not scattered, bm
intercepted by the planet and its atmosphere, is mainly absorbed
within the cloud layers. Therefore, momentum (equal, more or less,
to that of the solar radiation intercepted) is continually transferred
to the venusian atmosphere. An atmospheric system is different
from the radiometer in that it presents a symmetrical surface (same
radiation-matter interaction) toward the solar radiadon at its morn-
ing and evening limbs (Fig. 3). If the cross-sectional areas at both
limbs were equal as illustrated, the momentum transfer at the
morning limb would decelerate the atmosphere's rotation while at
the evening limb the same transfer would accelerate the rotation an
equal amount. The net result of this is that the overall rate of rotation
would be unchanged.
Such a symmetrical configuration is not likely since the atmo-
sphere must be warmed as it rotates across the planet's day hemi-
sphere and cooled as it rotates across the planet's night hemisphere.
This warming and cooling must result in a formation of an asym-
metrical configuration (Fig. 4). It is apparent that the momentum
transfer at the evening limb must be greater than that at the morning
limb because the atmosphere's greater cross section at the evening
limb intercepts a greater amount of solar radiation. It should be
noted that very little of the solar radiation is u-ansmitted through the
cloud layers, especially at or near the limbs where the atmospheric
path length of the radiation is long. This net momentum transfer
must be continually added to the angular momentum of the atmo-
spheric system at the same time angular momentum is continually
removed from the atmosphere by the frictional drag imposed on the
atmosphere by the slowly rotating planet's surface. This completes
the description of this mechanism.
There is great pressure to quantify the mechanism just imxo-
duced in an effort to evaluate its potential effectiveness. This
pressure is resisted for the following reason. What physics professor
would demonstrate the existence of radiation pressure and make the
success or failure of the demonstration dependent upon the ability
to predict the expected rate of rotation of an unknown apparatus? It
is enough that the rotor turns in the direction predicted when the
radiation source is set before the apparatus,
Fig. 3.
) Evenm_
>
)
>
Atmosphere not to scale.
It
A
D
!
A i
I"
I
0
N
EveniJagLi
)
>
MorningLimb
Fig, 4, Atmosphere n_ to scale.
It should be enough that the qualitative details of the known
superrotation of the venusian atmosphere are entirely consistent
with the simple radiation pressure mechanism presented for this
mechanism to receive serious consideration. An analysis of the
frictional drag expected for the nearly laminar flow found beneath
the cloud deck is much beyond my talent, to say nothing about the
frictional coupling that exists in the turbulent cloud layer. It is
possible that the mechanism might betested if such friction al effects
were reasonably well known.
The mechanism does suggest a possible phenomenon other than
superrotation. The acceleration and deceleration that occur at the
evening and morning limbs must compress the rotating atmosphere
at some morning location and rarefy it at some afternoon location.
A more detailed analysis of the expected atmospheric tides due to
this mechanism is the subject of a nearly completed separate work.
A simple mechanism involving the phenomenon known as
radiation pressure has been proposed to explain the superrotadon of
the venusian atmosphere. According to basic principles of physics
it cannot be denied that the process must be active. It has been shown
that support of the proposed mechanism by predictive, quantitative
calculations is not presently possible because critical properties of
the real system are unknown, or at best, poorly known, The
possibility of atmospheric tides that, if observed, would be consis-
tent with the mechanism has been noted. It should also be noted that
the solar wind might replace the solar radiation in a similar
mechanism. This has not been considered because the physics of the
solar wind as it encounters a planet in its path is beyond my abilities.
Reference: [1] Behon M. L S. et al. (1991) Science, 253,
1531-1536. fI'his article not only reports results from the Galileo
vennsian encounter, but also is a review of the superrotation
phenomenon and is extensively referenced
N93-14339 _4 _ _j "0_!
UARGE-V()LUME LAVA FLOW FIELDS ON VENUS: DI-
MENSIONS AND MORPHOLOGY. M. G. Lancaster t, J. E.
Gues0, K. M. Roberts 2. and J. W. Head 2, 1University of London
Observatory, University College London, London NW7 2QS, UK,
ZDepartment of Geological Sciences, Brown University, Provi-
dence RI 02912, USA.
Of all the volcanic features identified irt Magellan images, by far
the most extensive and areally important are lava flow fields.
Neglecting the widespread lava plains themselves, practically every
C1-MIDR produced so far contains several or many discrete lava
flow fields. These range in size from a few hundred square kilome-
ters in area (like those fields associated with small volcanic edifices
for example), through all sizes up to several hundred thousand
https://ntrs.nasa.gov/search.jsp?R=19930005151 2020-03-17T08:45:28+00:00Z
LPI Contribution No. 789 63
TABLE I. Type examples of great flow fields. Areas arc the total for each flow field, and the lengths and widths refer to flow units within each ficld.
Name Latitude Longitude CI-MI'DR Area/kin2 Lengtht/km Widths/km Morphology Source
Lauma Dona 52N--67N 176-200 60n 180 2.529,070 225-540 65-270 Sheet Ridge belt
60n208
Aitra Mona 49N-54N 264-272 6011263 201,020 270 (aver.) 5-10 Symm. apron Large volcano
45n265
Neago Fluctus 43N-59N 340-355 60n347 744,470 450-1900 90--450 Transitional Kumres
45n350
Sekmet Mona 42N-50N 239-245 45n244 81,170 150-520 10--40 A_rma. apron Large volcano
Mist Fost_e 37N-43N 242--255 45n244 106,050 140-450 5-20 Subparallel Rift zone
Kawelu Planitia 40N-52N 253-275 45n265 779,930 630 (max) 120-360 Sheet Various
Ulfnm/Ganiki 24N-32N 215-223 30n225 197,550 670 (max) 20--45 Subptrallel Rift zone
Rusalka Planitia 10S--SN 170-185 00nlS0 773,470 280 (av.) 135 Transitional Various
15s180 630 (max) (typical)
Ozza Mona (S) 7S-2S 200-212 00n197 120,690 1460 (max) 35-130 Subparallel Fissures
00n215
NW of Phoebe IN-6N 266-273 00n266 137,730 260 (av.) 5-15 Apron Large volcano
420 (max)
Parga Chasma 21S-13S 242-250 15s249 222,600 360 (av.) <90 Asymm. apron Corona
632 (max)
Ammavam 57.S-43S 015-043 45s011 460,000 460 (fan) K30 (fan) Fan C.aldera and
45s032 880 (max) <80 (ponds) shield cluster
Kaiwan Fluems 53S-43S 353-010 45s011 _.315,000 870 (N) 5-35 Subparallel Fissures on
45s350 1250 (max) rift zone
Mylitta Fluctus 63S-49S 349-359 45s350 300,000 10(30 (max) 5-80 Subparallel Caldera on
43S 6Os347 rift zone
square kilometers in extent (such as many rift related fields) 11].
Most of these are related to small, intermediate, or large-scale
volcanic edifices, coronae, arachnoids, calderas [2,3], fields of
small shields [4], and rift zones [5].
An initial survey of 40 well-def'med flow fields with areas
greater than 50,000 km 2 (an arbiurary bound) has been undertaken.
Following Columbia River Basalt terminology, these have been
termed great flow fields [6]. This represents a working set of flow
fields, chosen to cover a variety of morphologies, sources, loca-
tions, and characteristics. The initial survey is intended to highlight
representative flow fields, and does not represent a statistical set.
For each flow field, the location, total area, flow lengths, flow
widths, estimated flow thicknesses, estimated volumes, topographic
slope, altitude, backscatter, emissivity, morphology, and source has
been noted. The flow fields range from about 50,000 km 2 to over
2,500,000 km z in area, with most being several hundred square
kilometers in extent. Flow lengths measure between 140 and 2840
kin, with the majority of flows being several hundred kilometers
long.
A few basic morphological types have been identified. This is
not intended as a genetic classification, and the types identified are
merely end members of a continuous range in morphology. The
main distinction that has been drawn is between sheet flows, which
are irregular in outline and show little or no intenaal structure, and
digitate flows [7], which are made up of distinct flow lobes or lava
streams. Usually, the digitate flows are related to a centralized
source, whereas the sheet flows were erupted from extensive
fissures. The sheet flows cannot be divided into individual flow
units and may therefore be termed simple flows [8], while the
digitate flows, divisible into many flow units, are all compound
(although individual flow units may be treated as simple).
Sheet flows are distinguished by their relatively uniform back-
scatter, lack of internal flow smlcture such as well-defined lava
streams, channels, etc., absence of flow lobes, and irregular bound-
aries. Of the studied set, they range from 66,210 km 2 to 2,529,070
km 2. Sheet widths lie between 60 to 360 krn, with maximum lengths
between 225 and 680 kin. The sheet flow fields are difficult to map
in the sense that internal flow boundaries are essentially absent.
Source regions for sheet flows are typically the fissures associated
with rift zones, but they may also be traced to the annular structures
surrounding some coronae. Some of the larger sheet flow fields may
conceivably be composed of more than one large flow field.
Digitate flow fields are characterized by many discrete flow
lobes or lava streams. They are more common, and show a wider
range of morphology than sheet flows. Relative to sheet flow fields
they display a wider variation in radar backscatter, often being
composed of both radar-bright and -dark flow units within the same
field. They have been subdivided into divergent and subparallel
morphologies on the basis of the distal widening of the overall flow
field and the downstream divergence between flow units. All the
divergent flow fields have central sources, whereas half the
subparallel fields have been erupted from fissures along sections of
64 International Colloquium on Venus
Sheet-------_heet/Digitate Transiti0nal_Di_ oaate
_,_ssetc_ Co¢or_ o¢
!
Divergent
I
I I !
S}nllllClllCa]A_JfOl_ A_).lllll_elllC_lApI_II Fall
Sub-parallel
au Cal_e_ _.
Fig. 1. Morphological types of great flow fields (not to scale).
rift zone. The more centered the source, the greater the degree of
divergence of the flow field, although the local topography may also
control the direction of flow lobes. The divergent fields contain
symmetrical apron and fan end members. However, a large number
of aprons are distinctly asymmetric in plan, and may be considered
mcnsitional between syrmnetricaI aprons and fans. All the sym-
tactical aprons surround large volcanos, while the asymmetrical
aprons are centered on large volcanos (some of which are on rift
zones), coronse, and a cluster of small shields. Of the studied fans,
two are related to shield clusters, while a third may be traced to a set
of fissures. Fans are the least common of all the surveyed fields. The
subparallel fields may be traced to rift zones and fissures, coronae,
calderas, and s cluster of small shields.
In all types the widths of individual flow lobes or streams ranges
from a few kilometers (usually in the proximal regions) to several
tens of kilometers, with distal lobes of asymmetric aprons and
subparallel flow fields up to 130 km in width. The symmetrical
aprons are typically around 300 km in radius, while the maximum
length of the asymmetric aprons are up to 770 km in maxiumum
length. The measured subparallel flow fields range between 140 and
1460 km in length, with typical lengths of a few hundred kilometers.
Most of the symmetrical and assymetrical aprons have relatively
radar-bright proximal regions, while many of the asymmetrical
aprons have distal regions of particularly low backscatter. All the
divergent types may display channels.
A number of flow fields are transitional between the sheet and
digitate types. In these cases, very broad, but sheetlike flow lobes,
up to a few hundred kilometers across, may be discriminated. These
large lobes tend to have somewhat more variable backscatter than
the sheet flows. In several cases these transitional flows appear to
consist of large expanses of ponded lava. The transitional flows are
all associated with fissures. The plains contain numerous examples
of portions of flow fields that cannot be traced to their source. These
flows are usually indistinct, and may represent relatively old,
degraded flows that have been partly resurfaced by later volcanism.
Such indistinct flows occur beyond the distal reaches of some large
flow fields such as Mylitta Fluctus [7] and Kaiwan Fluctus [1]. A
key question regarding the great flow fields is how they relate to
plains development and what their con_bution is to volcanic
resurfacing in general [9]. Another key question concerns the
effusion rates and emplacement times for there great flows, as has
been estimated for Mylitta Fluctus [7]. The set of flow fields has
been chosen to address these questions, with initial emphasis
(mapping, detailed measurements, etc.) being placed on the type
flow fields.
References: [1] Lancaster M. G. etal. (1992) LPSCXXIII, 753--
754. [2] Crumpler L. S. et al. (1992) LPSCXXIII, 277-278. [3] Head
J. W. et al. (1992) LPSC XXIII, 513-514. [4] Aubele J. C. et al.
(1992) LPSC XXIIi, 47--48. [5] Roberts K. M. et al. (1992) LPSC
XXIII, 1157-1158. [6] Tolan T. L. et al. (1989) In Volcanism and
Tectonism in the Columbia River Flood--Basalt Province, Boulder,
Colorado (S. P. Reidel and P. R. Hooper, eds.), GSA Spec. Paper
239. [7] Roberts K. M. et al. (1992) JGR, special Magellan issue,
submitted. [8] Walker G. P. L. (1971) Bull. Volcanol., XXXV-3,
579-590. [9] Head J. W. et al. (1992)LPSCXXIII. 517-518.
N93-14340,
DERIVATION OF SURFACE PROP'ERTIE;S FROM
MAGELLAN ALTIMETRY DATA. Amy ]. L_velP, F. Peter
$chlocrb l, and George E. McCfillZ, lDepartment of Physics and
Astronomy, University of Massachusetts, Amherst MA 01003,
USA, 2Department of Geology and Geography, University of Mas-
sachusetts, Amherst MA 01003, USA.
The fit of the Hagfors model [1] to the Magellan altimetry data
provides a means to characterize the surface properties of Venus.
However, the derived surface properties are only meaningful if the
model provides a good representation of the data. The Hagfors
model is generally a realistic fit to surface scattering properties of
a nadh--directed antenna [2] such as the Magellan altimeter;, how-
ever, some regions of the surface of Venus are poorly described by
the existing model, according to the "goodness of fit" parameter
provided on the ARCDR CDRoms. Poorly characterized regions
need to be identified and fit to new models in order to derive more
accurate surface properties for use in inferring the geological
processes that affect the surface in those regions.
We have compared the goodness of fit of the Hagfors model to
the distribution of features across the planet, and preliminary results
show a correlation between steep topographic slopes and poor fits
to the standard model, as has been noticed by others [3.4]. In this
paper, we investigate possible relations between many classes of
features and the ability of the Hagfors model to fit the observed echo
profiles. In the regions that are not well characterized by existing
models, we calculate new models that compensate for topographic
relief in order to derive improved estimates of surface properties.
Areas investigated to date span from longitude 315 through 45,
at all latitudes covered by Magellan. A survey of those areas yields
preliminary results that suggest that topographically high regions
are well suited to the current implementation of the Hagfors model.
Striking examples of such large-scale good fits are Alpha Regio, the
northern edges of Lads Terra, and the southern edge of Ishtar Terra.
Other features that axe typically well fit are the rims ofcoronae such
as Heng-O and the peaks of volcanos such as Gula Mons. Surpris-
ingly, topographically low regions, such as the ubiquitous plains
areas, are modeled poorly in comparison. However, this generaliza-
tion has exceptions: Lakshmi Planum is an elevated region that is
not well fit compared to the rest of neighboring Ishtar, while the
southern parts of topographically low Guinevere Planitia are char-
acterized quite well by the Hagfors model.
Features that are candidates for improved models are impact
craters, coronae, ridges of significant scale, complex ridged ter-


Large-volume lava flow fields on Venus: Dimensions and morphology

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