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°

120° 125° 130° 135° 140° 145°

-45°

-40°

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C

120° 125° 130° 135° 140° 145°

120° 125° 130° 135° 140° 145°

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6049 6050 6051 6052 6053 6054 6055 6056 6057

120° 125° 130° 135° 140° 145°

120° 125° 130° 135° 140° 145°

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0 500 1000

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-45°

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B

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

illumination

wf

θi

illumination wf

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illumination

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a=a' a=a'

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c=c'

c=c'

c=c' c=c'

d'

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illumination wf

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illumination wf

θi

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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References

Brown, C.D., and Grimm, R.E., 1995, Tectonics of Artemis Chasma: a Venusian "plate"

boundary: Icarus, v. 117, p. 219-249.

Brown, C.D., and Grimm, R.E., 1996, Lithospheric rheology and flexure at Artemis

Chasma, Venus: Journal of Geophysical Research, v. 101, no. 5, p. 12697-12708.

Ford, J.P., Plaut, J.J., Weitz, C.M., Farr, T.G., Senske, D.A., Stofan, E.R., Michaels, G.,

and Parker, T.J., 1993, Guide to Magellan image interpretation: National

Aeronautics and Space Administration Jet Propulsion Laboratory Publication:

Pasadena, California, JPL Publication 93-24, 148 p.

Griffiths, R.W., and Campbell, I.H., 1991, Interaction of mantle plume heads with the

Earth’s surface and onset of small-scale convection: Journal of Geophysical

Research, vol. 96, no. B11, p. 18295-18310.

Grimm, R.E. and Hess, P.C., 1997, The crust of Venus in Bouger, S. W., Hunten, D. M.,

and Phillips, R. J., eds., Venus II, University of Arizona Press, p. 1205-1244.

Hamilton, W.B., 2004, Hundreds of rimmed circular structures on Venus formed from

impacts before 3.9 Ga, not from young plumes [abs.]: Eos, (Transactions,

American Geophysical Union), v. 85, no. 47, Fall Meeting Supplement., V53C-

06.

Hansen, V.L., 2000, Geologic mapping of tectonic planets: Earth and Planetary Science

Letters, v. 176, no. 3-4, p. 527-542.

Hansen, V.L., 2002, Artemis: Surface expression of a deep mantle plume on Venus:

Geological Society of America Bulletin, v. 114, no. 7, p. 839-848.

Hansen, V.L., 2003, Unpublished teaching materials.

Hansen, V.L., 2005, Crustal plateaus as ancient large impact features: a hypothesis [abs.]:

XXXVI Lunar and Planetary Science Conference, 2251.pdf.

Ivanov, M.A., and Head, J.W., 1996, Tessera terrain on Venus; a survey of the global

distribution, characteristics, and relation to surrounding units from Magellan data:

Journal of Geophysical Research, vol. 101, no. 6, p. 14861-14908.

Kaula, W.M., 1990, Venus: a contrast in evolution to Earth: Science, vol. 247, no. 4947,

p. 1191-1196.

21

Kirk, R., Soderblom, L., and Lee, E., 1992, Enhanced visualization for interpretation of

Magellan radar data: Journal of Geophysical Research, v. 97, p. 16371-16380.

McKenzie, D., Ford, P.G., Johnson, C., Parsons, B., Sandwell, D., Saunders, S., and

Solomon, S.C., 1992, Features on Venus generated by plate boundary processes:

Journal of Geophysical Research, vol. 97, p. 13533-13544.

Moores, E.M. and Twiss, R.J., 1995, Tectonics, W. H. Freeman and Company, 415 p.

Phillips, R.J., and Hansen, V.L., 1998, Geological evolution of Venus: rises, plains,

plumes, and plateaus: Science, v. 279, p. 1492-1497.

Schubert, G., and Sandwell, D.T., 1995, A global survey of possible subduction sites on

Venus: Icarus, vol. 117, no. 1, p. 173-196.

Smrekar, S.E., and Stofan, E.R., 1997, Corona formation and heat loss on Venus by

coupled upwelling and delamination: Science, vol. 277, p. 1289-1294.

Spencer, J., 2001, Possible giant metamorphic core complex at the center of Artemis

Corona, Venus: Geological Society of America Bulletin, v. 113, p. 333-345.

Squyres, S.W., Janes, D.M., Baer, G., Bindschadler, D.L., Schubert, G., Sharpton, V.L.,

and Stofan, E.R., 1992, The morphology and evolution of coronae on Venus:

Journal of Geophysical Research, v. 97, p. 13611-13634.

Stofan, E. R., Sharpton, V. L., Schubert, G., Baer, G., Bindschadler, D. L., Janes, D. M.,

and Squyres, S. W., 1992, Global distribution and characterisitics of coronae and

related features on Venus: Implications for origin and relation to mantle

processes: Journal of Geophyscial Research, v. 97, p. 13347-13378.

Stofan, E.R., Hamilton, V.E., Janes, D.M., and Smrekar, S.E., 1997, Coronae on Venus:

morphology and origin in Bouger, S. W., Hunten, D. M., and Phillips, R. J., eds.,

Venus II, University of Arizona Press, p. 931-968.

Tanaka, K.L., Moore, H.J., Schaber, G.G., Chapman, M.G., Stofan, E.R., Campbell,

D.B., Davis, P.A., Guest, J.E., McGill, G.E., Rogers, P.G., Saunders, R.S., and

Zimbelman, J.R., 1994, The Venus geologic mappers’ handbook (second edition):

U.S. Geological Survey Open-File-Report 94-438, 68 p.

Yamaoka, K., Fukao, Y., and Kumazawa, M., 1986, Spherical shell tectonics: effects of

sphericity and inextensibility on the geometry of the descending lithosphere:

Reviews of Geophysics, vol. 24, no. 1, p. 27-53.

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