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Structural analysis of deformation in the interior of Artemis, Venus 34°S 132°E
By
Roger A. Bannister
Masters thesis proposal
Advisor: Dr. Vicki L. Hansen
University of Minnesota Duluth
April 26, 2005
Abstract
Artemis is the largest terrestrial circular feature in the known universe, measuring
~2600 km diameter. Artemis could easily be seen from space if it were not for the thick
atmosphere that obscures our view of the Venusian surface. The NASA Magellan
mission penetrated the thick cloud cover using synthetic aperture radar (SAR) to provide
high resolution (~100 m) images of Artemis. The origin of Artemis is disputed despite
being actively researched for over a decade. Four hypotheses have been proposed
suggesting that Artemis is: 1) a zone of NW-SE directed convergence and subduction, 2)
the surface expression of a mantle plume, 3) the surface expression of a meteorite impact
prior to 3.9 Ga, and 4) the interior is a metamorphic core complex. These hypotheses
make testable predictions for deformation in the interior region of Artemis. This study
will attempt to constrain the evolution and geologic history of the interior region through
geologic mapping to test the various hypotheses. This work provides further insight into
the origin of Artemis.
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Introduction
Artemis is by far the largest circular feature, natural or otherwise, in the known
universe with a diameter of ~2600 km. An observer could see Artemis from space with
the naked eye if not for the thick atmosphere that obscures our view of the Venusian
surface. Fortunately, radio waves penetrate the dense clouds and are able to provide high
resolution radar images of Artemis suitable for scientific investigation. Artemis has been
actively researched over the past decade however no explanation for its formation is
widely agreed upon. In fact, four separate and distinctly different hypotheses have been
proposed for the origin of Artemis. Some researchers propose the existence of a
subduction zone at Artemis (McKenzie et al., 1992, Schubert and Sandwell, 1995, Brown
and Grimm, 1995, 1996), others propose a mantle plume origin (Griffiths and Campbell,
1991, Smrekar and Stofan, 1997, Hansen 2002), while still others argue for a meteorite
impact origin (Hamilton, 2004) or a possible metamorphic core complex at Artemis
(Spencer, 2001). The disagreement and variety of hypotheses reflects the complex
history and unusual character of Artemis.
Artemis’ most prominent characteristic is Artemis Chasma, a 1-2 km deep, ~2100
km diameter arcuate trough that surrounds an interior topographic high. The enigmatic
nature of Artemis Chasma has perplexed researchers since it was first identified in
Pioneer Venus data. Artemis is topographically similar to some coronae, circular to
quasi-circular features unique to Venus characterized by an annulus of fractures with
~200 km average diameter generally interpreted to form by the interaction of a rising
diapir with the lithosphere (Squyres et al., 1992, Stofan et al., 1997). However, Artemis
is approximately an order of magnitude larger than the average corona and is more than
twice the size of the next smallest corona, Heng-O. In map view, Artemis’ size and
shape resembles crustal plateaus and volcanic rises. Volcanic rises, thought to represent
the surface expression of a mantle plume impinging on a contemporary thick lithosphere
(Phillips and Hansen, 1998), describe broad domical regions. Crustal plateaus, thought to
represent the surface expression of either a mantle plume impinging on an ancient thin
lithosphere (Phillips and Hansen, 1998), mantle downwelling (Ivanov and Head, 1996),
or a large meteor impact into thin lithosphere (Hansen, 2005), describe steep sided
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regions with flat tops. However, Artemis differs in topographic profile from both
geomorphic groups with its distinctive trough.
The interior of Artemis contains complex structures that record a rich history of
deformation and volcanism. Hansen (2002) identified five quasi-circular features (~300
km diameter) within Artemis that resemble coronae and record a complex history of
temporally and spatially overlapping volcanism and tectonism. Brown and Grimm
(1995) inferred ~50-250 km of displacement along strike-slip deformation zones in the
northeast and southwest of the interior. Trough-parallel contractional features occur
along the outer slope of the trough and extensional features occur along the inner slope.
The detailed relationship between the formation of Artemis Chasma and the interior
deformation of Artemis is crucial to understanding how Artemis formed. My thesis
research focuses on the following questions: Can the age relationship between trough and
interior be determined? What implications arise if the interior deformation predates
trough formation? What if the interior deformation is contemporaneous with or younger
than the trough? Which models can we refute/which models warrant further
consideration based on this evidence? To answer these questions I will analyze
deformation in the interior region with emphasis on short-wavelength structures in the
interior region of Artemis and cross-cutting relationships between interior structures and
trough structures to provide critical constraints for the evolution of Artemis as a whole. I
will also test proposed models for the formation of Artemis by detailed geologic mapping
of areas critical to the predictions of these models. Previous studies have focused on the
trough morphology with little emphasis on the relationship between the trough and the
interior. I aim to place additional constraints on the history of Artemis’ interior and
improve our understanding of how the largest terrestrial circular structure in the solar
system formed.
Background
Venus
Venus, commonly called Earth’s sister planet, is comparable to the Earth in size,
bulk density, and distance from the sun (table 1) suggesting that the bulk composition and
heat budget of Venus should be broadly similar to the Earth (Grimm and Hess, 1997). In
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spite of these similarities, the Earth and Venus evolved differently resulting in a diversity
of surface deformation styles. Conditions at the Venusian surface are inhospitable with a
temperature of ~730 K (~450° C), atmospheric pressure of ~92 atm, and a near absence
of water. The Earth’s crust is differentiated into granitic continental crust and basaltic
oceanic crust as reflected in the bimodal distribution of the hypsometric curve for the
Earth. Geochemical evidence for Venusian crustal composition is limited but indicates
Venus’ surface is mainly basaltic. The hypsometric curve for Venus displays a unimodal
distribution implying the crust is not differentiated in the same manner as the Earth. The
plate tectonic process on Earth, manifested in curvilinear features such as orogenic belts,
subduction zones, oceanic spreading centers, and transform fault zones, provides a
mechanism for heat to escape the planet. Venus lacks such globally pervasive curvilinear
features and instead circular features dominate the surface, suggesting that Venus losses
its heat in a different manner.
Magellan
The NASA Magellan mission to Venus launched in 1989 to obtain near global
radar images of Venus at a resolution better than 300 m and a near global topographic
map with horizontal resolution of ~10km and vertical accuracy of 80 m or better (Ford et
al., 1993). Magellan collected correlated radar images, emissivity, rms slope, reflectivity,
and altimetry data across three mapping cycles and gravity data across two cycles. This
wealth of data provides the opportunity to better understand the geologic characteristics
of Venus. The Magellan datasets as they apply to this research are discussed further in
the data and methodology section of this text.
Artemis
Artemis Chasma, named for the Greek goddess of the hunt, was first identified in
low resolution radar data from the NASA Pioneer Venus Orbiter mission. Stofan et al.
(1992) categorized Artemis (figure 1) as a corona after Magellan data became available,
though it is not clear if Artemis is still included in Stofan’s corona database. The term
corona (from the Latin word meaning “crown,” plural coronae) originated as a
descriptive term covering any quasi-circular structure defined primarily by an annulus of
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concentric fractures and/or ridges. The term developed genetic connotations as the body
of corona research grew even though the diversity of morphologies and range of sizes
indicates that coronae may form by a variety of processes (Stofan et al., 1997). Although
Stofan et al. (1992, 1997) suggest that the various morphologies represent corona in
different stages of development, corona display no obvious age progression where they
occur in chains or clusters (Stofan et al., 1997). Artemis dwarfs the mean coronae (~200
km diameter) by an order of magnitude and is over twice the size of the next smallest
corona, Heng-O (~1000 km diameter). This vast size difference makes classification of
Artemis as a corona questionable.
Several hypotheses have been proposed for the formation of Artemis. These
hypotheses make numerous predictions for surface deformation that can be tested by
detailed geologic mapping of Artemis’ interior. Temporal relationships between interior
deformation and the trough formation will provide a critical constraint for these
hypotheses. Orientations of contractional and extensional features and the order in which
they formed provide additional constraints. Critical questions include: 1) Is the
deformation in Artemis’ interior older, younger, or contemporaneous with trough
structures? 2) Is there evidence for strike-slip displacement in the northeast and
southwest of Artemis’ interior? 3) What is the temporal relationship between the corona-
like features in Artemis’ interior? Figure 2 summarizes some of the testable predictions
made by each model.
Subduction hypothesis
The subduction hypothesis (Brown and Grimm, 1995, 1996, Schubert and
Sandwell, 1995, McKenzie et al., 1992) makes numerous testable predictions about
deformation in Artemis Chasma and the interior region of Artemis. Brown and Grimm
(1995, 1996) interpret Artemis Chasma as a zone of northwest oriented convergence
based on the topographic similarities between the southeast portion of the trough and
terrestrial subduction zones. If northwest directed subduction is occurring at Artemis
then the trough should transition to a zone of left-lateral strike-slip motion to the
northeast and right-lateral strike-slip motion to the southwest. Normal faulting should be
present at the rise in the down-going slab as it bends down into the subduction zone. The
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trench should be characterized by folding normal to convergence, analogous to an
accretionary complex on Earth. Additionally, a subduction zone with the curvature
present at Artemis (~300° of arc) requires a very shallow subduction angle (Moores and
Twiss, 1995, Yamaoka et al., 1986). The subduction hypothesis predicts a shallow
apparent depth of compensation (ADC) and a gravity anomaly above the “cold”
subducting slab. On Earth, dewatering reactions in the subducting slab causes melting of
the overlying mantle, however this would not be the case on an ultra-dry Venus. The
presence of a shallowly subducting slab beneath Artemis would lower the geothermal
gradient in the area and likely preclude volcanism in the interior after the initiation of
subduction.
Mantle plume hypothesis
Plume hypotheses are favored by many authors to form circular structures on
Venus (Stofan et al., 1992, 1997, Squyres et al., 1992). Individual models and methods
differ but agree in many key aspects. Each predict broad domical uplift resulting in radial
fracturing and volcanism followed by lateral spreading of the plume head forming
concentric folds and fractures. Smrekar and Stofan’s (1997) numerical model produced a
trough that migrates outward with time. Scaling the model to the size of Artemis (~2600
km diameter) produces a trough with ~1.2 km of relief, consistent with the observed
relief of ~1-2 km. Giffiths and Campbell (1991) observed the formation of a trough that
migrated inwards with time in their physical model of plume heads interacting with a free
surface. Their model also produced small-scale convection cells as the plume head
spread laterally that could account for the observed corona-like structures in the interior
of Artemis. Hansen (2002) suggests that a rising plume head might spawn compositional
diapirs as it flattens that could also account for the interior corona, however this process
was not investigated in detail.
Metamorphic core complex hypothesis
Spencer (2001) observed a possible metamorphic core complex, an area where
extensive extension of the crust exposes deep crustal rocks, in the center of Artemis. A
rising plume tail arriving after a flattening plume head purportedly caused approximately
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170 km of inferred northwest-southeast directed extension in the center of Artemis. The
author admits that it is unclear how a rising plume tail would produce a linear
deformation belt rather than radial/concentric deformation because no regional stresses
are evaluated. Spencer offers no mechanism for the denudation and exhumation of the
lower plate, an unlikely feat in the absence of high erosion rates. The metamorphic core
complex hypothesis is difficult to evaluate because it lacks a regional context.
Meteorite impact hypothesis
Hamilton (2004) flatly stated that all rimmed circular structures on Venus are the
product of meteor impacts prior to 3.9 Ga. An impact origin calls for the obliteration of
any preexisting structures, therefore the interior of Artemis cannot predate the formation
of the trough. Artemis does not topographically resemble any form of currently
recognized impact structure, however due to the large diameter of Artemis it is
conceivable that the morphology would be unlike smaller impact craters. Hamilton does
not provide a method to distinguish between impact craters and structures created by
mantle plumes or rising diapirs.
Predicted Results
The subduction hypothesis predicts normal faulting at the topographic swell in the
subducting plate due to the flexure of the slab. However, the outer rise is free of such
deformation. Brown and Grimm (1996) performed flexural modelling of the outer-swell
and proposed that faulting might not occur in the presence of a high in-plane force,
though the in-plane forces required to prevent brittle failure may be implausibly high.
Northwest-southeast convergence requires roughly parallel portions of the trough to
accommodate motion through strike-slip displacement. The topographic trough observed
in these areas is unlikely to form in a strike-slip zone. The shallow subduction angle
required by the curvature of the arc would cause the subducting slab to emerge on the
northwest side of Artemis, like slicing a knife through an orange. These first-order
geometric problems call the subduction hypothesis into question.
The metamorphic core complex hypothesis lacks a regional context and makes
few testable predictions for deformation in Artemis’ interior. The absence of
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geologically quick erosion rates makes denudation of the lower plate to expose deep
crustal rocks unlikely. The hypothesis fails to relate the metamorphic core complex to
other structures in Artemis’ interior and to the formation of the trough. The metamorphic
core complex hypothesis does not address the big picture and lacks sufficient detail to
evaluate it properly.
The meteorite impact hypothesis contains few details regarding crater
morphologies, the thickness of impacted lithosphere, or the size of the impactor required
to form Artemis. The blanket statement that all rimmed circular structures resulted from
meteor impacts is questionable considering the variety of morphologies and deformation
styles of these structures.
The mantle plume hypothesis predicts overlapping temporal and spatial evolution
of fractures and lava flows in Artemis’ interior and of a trough with concentric folds and
fractures. The differences between mantle plume models become important if Artemis
meets these first order conditions. Smrekar and Stofan’s (1997) model predicts relatively
simple deformation and volcanism in Artemis’ interior in contrast to Hansen’s (2002)
hypothesis and Griffiths and Campbell’s (1991) experiment which predict complex
deformation and volcanism in the interior. Smrekar and Stofan’s (1997) model predicts a
trough that migrates outward with time whereas Griffiths and Campbell’s (1991)
experiment produced a trough that migrates inward with time Another important aspect
to consider is the time scale over which deformation occurs. The crust may behave
ductily at low strain rates, precluding the existence of brittle deformation observed at
Artemis. Smrekar and Stofan’s model forms coronae over ~400 Ma. A shorter time
scale seems more geologically reasonable.
Data and Methodology
The NASA Magellan Mission (1991-1994) produced an amazing digital
correlated geophysical data set for Venus with near global (~98%) coverage (Ford et al.,
1993). The Magellan radar sensor acquired data in three modes; synthetic aperture radar
(SAR), radiometer, and altimeter mode. The system used a 3.7 m-diameter parabolic
high-gain antenna (HGA) fixed 25° off nadir perpendicular to the trajectory of the
spacecraft in SAR and radiometer modes. The SAR operated with a 12.6 cm wavelength
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at 2.385 GHz (S-band) with horizontal parallel transmit/receive polarization (HH) in
order to penetrate the thick, CO2-dominated cloud cover. SAR images were produced in
3 cycles with varying look geometries (left-looking, right looking, and stereo left-
looking) to image ~98% of the planet surface. The altimeter mode used a smaller
altimeter horn antenna (ALTA) fixed in the nadir direction. Figure 3 illustrates the
geometry of Magellan observations. Table 2 summarizes the radar and orbital
characteristics. Due to the elliptical orbit the SAR incidence angle was varied with
latitude to provide an optimum signal to noise ratio. In the Artemis region incidence
angles range from ~38-23°, ~25°, and ~20-14° from north to south for cycles 1, 2, and 3
respectively. SAR image resolution is approximately 100 m. The size of the altimeter
footprint varies with spacecraft altitude and therefore varies with latitude. In the Artemis
region footprints measure 8-11km in along-track dimension and 19-24km in cross-track
dimension (Ford et al., 1993). Combining the surface to spacecraft distance with the
known position of the spacecraft relative to the planetary center produces a global
topographic data record (GTDR) with horizontal resolution ~10km and vertical resolution
~80m. Reflectivity and rms slope data sets were also derived from the altimetry data.
Emissivity data was derived from radiometry measurements interleaved with the SAR
observations. Gravity data was collected during cycles 4 and 5 before the spacecraft was
intentionally crashed in an effort to circularize the orbit to increase data resolution in the
polar regions.
This study primarily uses SAR image mosaics from cycles 1, 2, and 3, including
both left-looking and right-looking images (figure 4). Compressed once (C1-MIDR) and
compressed twice (C2-MIDR) SAR images are used for regional analysis while full
resolution images (F-MIDR) are used for detailed analysis of areas of interest (resolution
of SAR images are ~225, ~675, and ~100 m respectively). GTDR data is used for
analysis of regional topography and to assist visualization in three dimensions. Geologic
mapping will be conducted according to guidelines set forth by Tanaka et al. (1994) and
Hansen (2000).
Radar image brightness is a function of the roughness, topography, and electrical
properties of the imaged surface. Basically, surfaces that are inclined towards the
incident radar and/or are rough on or above the scale of the radar wavelength (~12cm)
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will appear radar-bright while surfaces that are inclined away from the incident radar
and/or are smooth (below the scale of the radar wavelength) will appear radar-dark
(figure 5). The opposite is true when the SAR images are inverted. Inverted SAR images
are useful for structural analysis because structural elements (typically lineaments) tend
to show up better in negative SAR images (figure 6). Due to the geometry of radar
imagery, echoes off areas of high elevation will return to the antenna before areas of low
elevation. This will cause mountain peaks to be imaged forward of their actual position,
known as foreshortening, while the opposite occurs on the back slope, known as
elongation. If the echo from the peak returns before the echo from the forward toe, then
the peak will be imaged on top of the base, an effect known as lay-over. A surface
inclined away from the radar look direction at an angle greater than the incidence angle
will fall in radar shadow and will not be imaged. These effects complicate SAR image
interpretation but once understood they provide a tool for determining short-wavelength
topography.
Stereo imagery greatly enhances interpretation of landforms by displaying data in
three dimensions. Parallax differences between two images with different viewing
geometries produce a sense of depth when viewing the images with a stereoscope (or
with red-blue 3-D glasses if the images are combined as a red-blue anaglyph). Radar
stereo pairs with opposite look directions are difficult to visually merge because the
illumination direction changes drastically between images; however stereo pairs with the
same look direction but different incidence angles (i.e. cycle 1 and cycle 3 left-look SAR
images) are more easily combined. Unfortunately, cycle 3 coverage of Artemis is patchy
and limited to a small portion of the north east, therefore true stereo coverage of Artemis
is poor.
However, cycle 1 left-looking SAR and cycle 2 right-looking SAR coverage of
Artemis is nearly complete. Synthetic stereo pairs (Kirk et al., 1992) for each data set are
created by introducing distortion to the image based on the GTDR for the same area
using the displace filter in Adobe Photoshop®. Combining the images by placing the
original image on the red channel and the distorted image on the blue and green channels
produces a red-blue anaglyph that can be viewed with standard red-blue 3D glasses.
Regional topographic trends are easily gleaned from synthetic stereo images. Synthetic
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stereo images do not resolve subtle topographic features seen in true stereo images and
there is a sacrifice of image resolution in the synthetic stereo pair because of the lower
resolution of the altimetry data (figure 7). The method for creating synthetic stereo pairs
may also be applied to the other geophysical data sets (emissivity, rms slope, reflectivity,
and gravity) for enhanced three-dimensional visualization of those data sets.
Simulated three-dimensional perspective views, 3D models, and fly-by
animations provide supplemental visualization of Artemis. These visualization
techniques are well suited for illustrative purposes, however caution should be exercised
when making scientific interpretations due to the vertical exaggeration (usually ~20
times) used to create these images. Color overlay methods to combine elevation and
SAR images are also used to a limited extent in this study. Images are produced by
breaking-down a color coded topography image into hue, saturation, and intensity
components and substituting a SAR image for the intensity component (Ford et al.,
1993). This technique allows visualization of topography without the loss of resolution
in the SAR image that occurs in the synthetic stereo process and is compatible with our
eyes natural ability to discern intensity differences with more acuity than color
differences. Additionally these images convey topography to those who cannot
successfully view stereo imagery.
Summary
Artemis records a complex history of volcanism and deformation. The
relationship between deformation in the interior and the formation of Artemis Chasma is
crucial to the understanding of the evolution of Artemis as a whole. I will analyze
deformation in the interior through geologic mapping using high resolution SAR
imagery. My goal is to determine if the interior of Artemis formed before, during, or
after the formation of Artemis Chasma. I will test existing hypotheses in light of this new
mapping in order to rule out or refine them. My work will help constrain models for the
evolution of Artemis and provide further insight into the formation of the largest
terrestrial circular structure in the known universe.
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Table 1 – Summary of physical characteristics of the Earth and Venus
Earth Venus Mass (kg) 5.976e+24 4.869e+24 Equatorial radius (km) 6,378.14 6,051.8 Mean density (gm/cm3) 5.515 5.25 Mean distance from the Sun (km) 149,600,000 108,200,000 Rotational period (days) 0.99727 243.0187 (retrograde) Orbital period (days) 365.256 224.701 Equatorial surface gravity (m/sec2) 9.78 8.87 Satellites 1 0 Magnetic field Yes No Mean surface temperature 15°C 482°C Atmospheric pressure (bars) 1.013 92 Atmospheric composition N2 - 77%
O2 - 21% Other - 2%
CO2 - 96% N2 - 3+%
Trace amounts of: Sulfur dioxide, water vapor, carbon monoxide, argon,
helium, neon, hydrogen chloride, and hydrogen fluoride
Table 2 – Magellan radar system and orbital characteristics (from Ford et al., 1993)
Radar system characteristics Wavelength 12.6 cm Operating Frequency 2.385 GHz Modulation Bandwidth 2.26 MHz Transmitted pulse length 26.5 μs SAR Antenna
Gain 36.0 dB Angular beamwidth 2.1° x 2.5°
Altimeter Antenna Gain 19.0 dB Angular beamwidth 10° x 30°
Polarization HH Effective slant-range resolution 88 m Along-track resolution 120 m
Orbit characteristics Periapsis altitude 289 km Periapsis latitude 9.5° N Altitude at pole 2000 km Inclination 85.5° Period 3.259 hrs Repeat cycle 243 days
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Figure 1 – SAR image of Artemis. Image is left-looking SAR with data gaps filled first with stereo left-looking then with right-looking SAR images. Projection of this and the following SAR images is mercator.
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Subduction Mantle Plume Metamorphic Core Complex Meteorite Impact
multi-ringed?
central peak?
ejecta blanket?
Predictions:1) interior volcanism and northwest-southeast shortening predates subduction2) Trough transitions from convergence to strike-slip in the northeast and southwest3) Southwest portion of the annulus forms prior to the northeast portion4) gravity anomaly over the “cold” subducting slab5) shallow apparent depth of compensation (i.e. in the lithosphere)6) normal faulting in the outer rise in response to bending of the subducting plate.
Interior predictions:1) Volcanism predates the trough2) NW-SE shortening occured first
References: Brown and Grimm (1995, 1996), Schubert and Sandwell (1995), McKenzie et al. (1992)
Predictions:1) interior volcanism and deformation contemporaneous with annulus formation2) entire trough forms coherently and migrates either inward or outward with time3) gravity anomaly centered over the interior4) deep apparent depth of compensation (i.e. in the mantle)
Interior predictions:1) lineaments cut and are cut by other interior lineaments and trough structures2) lava flows overlap in time and space
References: Griffiths and Campbell (1991), Hansen (2002), Smrekar and Stofan (1997)
Predictions:1) ~170 km northwest-southeast directed extension in the center exposes deep crustal and possibly mantle rocks2) extension in center due to rising plume tail
Interior predictions:1) Minimum of ~170 km NW-SE extension2) Extensive shear zones trending NW-SE3) Upper plate boundary and presence of klippe
References: Spencer (2001)
Predictions:1) any prexisting deformation in the interior was obliterated by impact, i.e. interior deformation cannot predate trough formation.2) Artemis formed prior to 3.9 Ga3) Trough and paired rises may represent multi-ringed impact structure
Interior predictions:1) All structures formed nearly instantaneously2) Central peak or peak ring usually associated with large impact craters3) Ringed margin
References: Hamilton (2004)
?
??
trough outline
lineament
lava flow
Single rising plume
Small scale convection orsecondary diapirs
Explanation:
Figure 2.14
Figure 3 – Magellan observing geometry. (From Ford et al. 1993)
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120° 125° 130° 135° 140° 145°
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Figure 4 – Magellan data coverage for Artemis. A) Left-looking SAR, B) Right-looking SAR, C) Stereo left-looking SAR, D) Color coded shaded relief derived from altimetry data. Black stripes indicate gaps in data coverage. SAR images have been stretched to enhance contrast.
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θi
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Figure 5 - Radar ground range images (basal strip) resulting from (i) surface roughness (greater than the wavelength of radar), and differnet topographic forms, incidence angle (θ), and illumination direction (ii-x). (ii) Illustrates change in backscatter return as a result of gradual change in slope and resulting orienation toward receiver. (iii-x) illustrate radar return based on straight slopes and illustrate radar foreshorteniing, layover, and radar shadow. Points on topographic forms (a, b, c) project parallel to wavefront (wf, perpendicular to illumination) to points on the ground range image (a', b', c'). Point d' marks the trailing edge of radar shadow on the ground range image. Projected location and size of near slope (ns), far slope (fs) and radar shadow (rs) shown with shades of gray indicative of relative radar return and hence brightness. Gray lines shown where surface locations would not be imaged
(iii) Left illumination of asymmetric topographic form; foreshortening and radar shadow result in apparent symmetric shape. Point a at the base of the near slope 'projects' to its correct location; point b at peak projects to b'; point c at the base of the far slope 'projects' to its correct location, but its presence is lost in radar shadow. The shallow near slope is imaged from a' to b' and 'foreshortened' in the ground range image; the entire far (steep) slope is lost in radar shadow from b' to d'.
(iv) Left illumination and an asymmetric topographic form with steep slope facing radar results in extreme foreshortening, or "layover". Only the far slope is imaged because b projects to b' and a, projected to a', is lost in extreme foreshortening. Although the near slope is lost to layover, none of the far slope is lost to radar shadow in this case.
(v) Right illumination of asymmetric topographic form with steep slope facing away from radar; foreshortening and radar shadow result in an apparent near symmetric image in contrast with topographic reality.
(vi) Left illumination of an assymetric trough with near slope steeper than incidence angle resulting in radar shadow.
(vii) Left illumination of symmetric topographic form; foreshortening and far slope imaging results in apparent asymmetric image in contrast with topographic reality.
(viii) Left illumination of symmetric topographic form with a higher incidence angle than in (vii) leads to less foreshortening, but the entire far slope is in radar shadow.
(ix) Right illumination of symmetric topographic form; foreshortening and far slope imaging results in apparent asymmetric form in contrast with topographic reality. Compare with (vii) and (viii).
(x) Left illumination of an assymmetric trough with shallow near slope.
(From Hansen 2003)
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Figure 6 – Comparison of normal and inverted SAR images. Structural lineaments such as fractures, folds, and faults are much easier to see in inverted images
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Figure 7 – Comparison of synthetic stereo images and true stereo images for the same area of Artemis Chasma. Long-wavelength topography is easily visualized in synthetic stereo (A), subtle short-wavelength topography along the trench wall is more apparent in true stereo (B). The difference is more pronounced when zoomed in (C and D), though viewing at this scale may cause eye strain. Black areas represent data gaps.
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Figure 7 (continued)
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