Prepared for the National Aeronautics and Space Administration

Geologic Map of the Niobe Planitia Quadrangle (V–23), Venus

By Vicki L. Hansen

Pamphlet to accompany

Scientific Investigations Map 3025

2009

U.S. Department of the Interior

U.S. Geological Survey

75°

50°

25°

75°

25°

–25°

0°

50°

0°

0° 30° 60° 90° 120° 150° 180°

–25°

–50° –50°

–75° –75°

V–1

V–23 V–24 V–22

V–34

V–62

V–35

V–21

V–20

V–9

V–8

V–10

V–33 V–36

V–12

V–45 V–48

V–11

V–46 V–47

V–57

V–3

V–2

V–13

V–44 V–49

V–25

V–32 V–37

V–4

V–56 V–58

QUADRANGLE LOCATION

Photomosaic showing location of map area. An outline of 

1:5,000,000-scale quadrangles is provided for reference.

i

Contents

The Magellan Mission...................................................................................................................................1

Magellan radar data.............................................................................................................................1

Niobe Planitia quadrangle............................................................................................................................1

Introduction............................................................................................................................................1

Image data ....................................................................................................................................2

Image interpretation....................................................................................................................2

Terminology...................................................................................................................................3

Geologic setting ....................................................................................................................................3

Geologic relations.................................................................................................................................4

Basal terrain units........................................................................................................................4

Tessera and intratessera terrain......................................................................................4

Fracture terrain ...................................................................................................................6

Shield terrain material ................................................................................................................6

23N/103E...............................................................................................................................6

21N/113E...............................................................................................................................7

13N/111E...............................................................................................................................7

23N/117E...............................................................................................................................7

13N/119E...............................................................................................................................7

Flow units ......................................................................................................................................8

Flows originating within V–23...........................................................................................8

Rosmerta flows ...................................................................................................................8

Impact features.....................................................................................................................................8

Tectonic structures...............................................................................................................................9

Regional structural suites ..........................................................................................................9

Regional extensional fractures ........................................................................................9

Wrinkle ridges .....................................................................................................................9

Inversion structures ...........................................................................................................9

Fabric anisotropy and implications for stress-strain relations ...................................9

Coronae ................................................................................................................................................10

Maya Corona ..............................................................................................................................10

Dhisana Corona..........................................................................................................................10

Allatu Corona..............................................................................................................................11

Bhumiya Corona.........................................................................................................................11

Omeciuatl Corona ......................................................................................................................11

Rosmerta Corona .......................................................................................................................11

Formation of circular lows .......................................................................................................12

Dorsa ..................................................................................................................................................12

Geologic history ..................................................................................................................................13

Implications for Venus lowland resurfacing processes........................................................................14

Summary........................................................................................................................................................16

References cited..........................................................................................................................................18

Figure

12. Illustrations showing proposed evolution of Venusian surface.......................................................17

Tables

1. Numbers and densities of definite and combined shields................................................................21

2. V–23 impact craters.................................................................................................................................22

1

The Magellan Mission

The Magellan spacecraft orbited Venus from August 10, 

1990, until it plunged into the Venusian atmosphere on October 

12, 1994. Magellan Mission objectives included (1) improving 

the knowledge of the geological processes, surface properties, 

and geologic history of Venus by analysis of surface radar characteristics, topography, and morphology and (2) improving the 

knowledge of the geophysics of Venus by analysis of Venusian 

gravity.

The Magellan spacecraft carried a 12.6-cm radar system to 

map the surface of Venus. The transmitter and receiver systems 

were used to collect three data sets: (1) synthetic aperture radar 

(SAR) images of the surface, (2) passive microwave thermal 

emission observations, and (3) measurements of the backscattered power at small angles of incidence, which were processed 

to yield altimetric data. Radar imaging and altimetric and radiometric mapping of the Venusian surface were accomplished in 

mission cycles 1, 2, and 3 from September 1990 until September 1992. Ninety-eight percent of the surface was mapped with 

radar resolution on the order of 120 m. The SAR observations 

were projected to a 75-m nominal horizontal resolution, and 

these full-resolution data compose the image base used in 

geologic mapping. The primary polarization mode was horizontal-transmit, horizontal-receive (HH), but additional data for 

selected areas were collected for the vertical polarization sense. 

Incidence angles varied between about 20° and 45°.

High-resolution Doppler tracking of the spacecraft took 

place from September 1992 through October 1994 (mission 

cycles 4, 5, 6). Approximately 950 orbits of high-resolution 

gravity observations were obtained between September 1992 

and May 1993 while Magellan was in an elliptical orbit with a 

periapsis near 175 km and an apoapsis near 8,000 km. An additional 1,500 orbits were obtained following orbit-circularization 

in mid-1993. These data exist as a 75° by 75° harmonic field.

Magellan Radar Data

Radar backscatter power is determined by (1) the morphology of the surface at a broad range of scales and (2) the intrinsic 

reflectivity, or dielectric constant, of the material. Topography at 

scales of several meters and larger can produce quasi-specular 

echoes, and the strength of the return is greatest when the local 

surface is perpendicular to the incident beam. This type of 

scattering is most important at very small angles of incidence, 

because natural surfaces generally have few large tilted facets 

at high angles. The exception is in areas of steep slopes, such as 

ridges or rift zones, where favorably tilted terrain can produce 

very bright signatures in the radar image. For most other areas, 

diffuse echoes from roughness at scales comparable to the radar 

wavelength are responsible for variations in the SAR return. In 

either case, the echo strength is also modulated by the reflectivity of the surface material. The density of the upper few wavelengths of the surface can have a significant effect. Low-density 

layers, such as crater ejecta or volcanic ash, can absorb the incident energy and produce a lower observed echo. On Venus, a 

rapid increase in reflectivity exists at a certain critical elevation 

above which high-dielectric minerals or coatings are thought to 

be present. This leads to very bright SAR echoes from virtually 

all areas above that critical elevation.

The measurements of passive thermal emission from 

Venus, though of much lower spatial resolution than the SAR 

data, are more sensitive to changes in the dielectric constant 

of the surface than to roughness. They can be used to augment 

studies of the surface and to discriminate between roughness 

and reflectivity effects. Observations of the near-nadir backscatter power, collected using a separate smaller antenna on 

the spacecraft, were modeled using the Hagfors expression 

for echoes from gently undulating surfaces to yield estimates 

of planetary radius, Fresnel reflectivity, and root-mean-square 

(rms) slope. The topographic data produced by this technique 

have horizontal footprint sizes of about 10 km near periapsis 

and a vertical resolution on the order of 100 m. The Fresnel 

reflectivity data provide a comparison to the emissivity maps, 

and the rms slope parameter is an indicator of the surface tilts, 

which contribute to the quasi-specular scattering component.

Niobe Planitia Quadrangle

Introduction

Niobe Planitia, namesake of the Niobe Planitia quadrangle 

(V–23), is named for Niobe, Queen of Thebes, married to King 

Amphion. Niobe was perhaps the most tragic figure of Greek 

mythology. In a bout of arrogance, Niobe bragged about her 

14 children, mocking Leto who had only two children, Apollo 

(god of prophecy and music) and Artemis (virgin goddess of the 

wild). Upset by Niobe’s mocking, Leto sent Apollo and Artemis 

to Earth to destroy Niobe’s children; Apollo killed Niobe’s 

seven sons and Artemis killed Niobe’s seven daughters. In the 

heat of battle, King Amphion either committed suicide or was 

killed by Apollo. As a result of Niobe’s haughtiness, her entire 

family died. Niobe fled to Mount Sipylus in Asia Minor where 

she turned to stone and her ceaseless tears formed a stream (the 

Achelous). Niobe, the symbol of eternal mourning, weeps to 

this day on Mount Sipylus, where a fading female image carved 

into a porous limestone cliff weeps after it rains.

The Niobe Planitia quadrangle (V–23) encompasses 

approximately 8,000,000 km2 of the Venusian equatorial region 

extending from lat 0° to 25° N. and from long 90° to 120° E. 

(approximately 9,500 15-minute quadrangles on Earth). The 

map area lies along the north margin of the equatorial highland, 

Aphrodite Terra (V–35), and extends into the lowland region 

to the north, preserving a transition from southern highlands 

to northern lowlands (figs. 1, 2, map sheet). The northern 

parts of the crustal plateau, Ovda Regio and Haasttse-baad 

Tessera, mark the south margin of the map area; Niobe and 

Sogolon Planitiae make up the lowland region. The division 

between Niobe and Sogolon Planitiae is generally topographic, 

and Sogolon Planitia forms a relatively small elongate basin. 

Mesolands, the intermediate topographic level of Venus, are 

essentially absent or represented only by Gegute Tessera, which 

forms a slightly elevated region that separates Niobe Planitia 
2

from Llorona Planitia to the east (V–24). Lowlands within 

the map area host five features currently classified as coronae: 

Maya Corona (lat 23° N., long 97° E.) resides to the northwest 

and Dhisana, Allatu, Omeciuatl, and Bhumiya Coronae cluster 

loosely in the east-central area. Lowlands extend north, east, 

and west of the map area.

Mapping the Niobe Planitia quadrangle (V–23) provides an 

excellent opportunity to examine a large tract of lowlands and 

the adjacent highlands with the express goal of clarifying the 

processes responsible for resurfacing this part of Venus and the 

resulting implications for Venus evolution. Although Venus lowlands are widely considered to have a volcanic origin (Banerdt 

and others, 1997), lowlands in the map area lack adjacent coronae or other obvious volcanic sources.

Image Data

The Magellan Mission provided east-directed-illumination 

(left-looking, cycle 1), S-band (12.6 cm wavelength), and SAR 

images that cover over 80 percent of Niobe Planitia quadrangle 

(V–23) and nearly complete (~95%) coverage of right-looking 

(cycle 2) SAR; cycle 3 left-look stereo SAR coverage is not 

complete and is somewhat degraded in many locations across 

the map area (Ford and others, 1993). Digital Compressed Once 

Mosaicked Image Data Records (C1-MIDR; 275 m/pixel) SAR 

data from the regional data base and map base and the hardcopy 

full-resolution radar map (FMAP; 75 m/pixel) dataset were used 

in constructing the map, as well as digital, full-resolution (~75-

125 m/pixel) SAR images downloaded from the Map-a-Planet 

web site (http://pdsmaps.wr.usgs.gov/). Ancillary data included 

the Global Topographic Data Record 3 (GTDR 3) that has an 

effective horizontal resolution of 10 km and similar products 

representing Fresnel reflectivity at 12.6-cm wavelength, average 1- to 10-m-scale slope, and derived 12.6-cm emissivity 

data (GRDR, GSDR, and GEDR, respectively). GTDR data 

were combined with SAR images to produce synthetic stereo 

anaglyphs (Kirk and others, 1992) using NIH-Image macros 

developed by D.A. Young. These images played a critical role 

in elucidating the relations between geology and topography 

and, in particular, the interaction of flows, primary and secondary structures, and topography.

Image Interpretation

The interpretation of features in Magellan SAR images 

is key to developing a geologic history for the Niobe Planitia 

quadrangle (V–23). Ford and others (1993) explore the subject 

of radar image interpretation in depth.

The methodology for defining geological units and structural fabrics builds on standard geological analysis detailed in 

Wilhelms (1990) and Tanaka and others (1994), considering 

cautions of Hansen (2000), Zimbelman (2001), Skinner and 

Tanaka (2003), and McGill and Campbell (2004). Map units 

represent material emplaced within an increment of geological history, to which standard stratigraphic methods have some 

limited application; however, some units may be composite, 

because they might not be stratigraphically coherent over their 

entire represented area and (or) they may have been emplaced 

over an extended period of time, particularly in relation to other 

units and (or) formation of secondary structures. Attempts are 

made to clearly separate secondary structures from material 

units; locations, orientations, and relative densities of primary 

and secondary structures are shown independent of material 

units. Further complicating the process of unraveling both 

temporal constraints and history, evidence for reactivation of 

secondary structures is common across the map area.

Criteria for distinguishing discrete geological units in the 

map area include (1) the presence of sharp, continuous contacts; 

(2) truncation of, or interaction with, underlying secondary 

structures and topography; and (3) primary structures, for example flow channels or edifice topography, that allow a reasonable 

geological interpretation and hint at three-dimensional geometry. Some mapped units fail to fit these constraints, limiting 

their use in constructing stratigraphic interpretations. Composite 

units, in particular, cannot provide robust temporal constraints, 

even of a relative nature.

Unlike surface crater statistics for planetary bodies that 

have old surfaces and high crater densities, such as the Moon 

and Mars, Venus impact crater statistics cannot place constraints 

on the age of surface units that cover the small areas visible in 

the map area (Hauck and others, 1998; Campbell, 1999). Age 

constraints may be established only where units are in mutual 

contact and (or) interact with the same suite of secondary structures. Such temporal constraints are only locally applicable and 

cannot be robustly extended across the map area.

Ranges of structures, both primary (depositional or 

emplacement-related) and secondary (tectonic), are identified 

in the Magellan SAR data. Primary structures include channels, 

shields, lobate flow fronts, and impact crater haloes and rims. 

Channels represent sinuous, low-backscatter troughs hundreds 

of kilometers long and a few kilometers wide; locally, they may 

lack apparent topographic relief (Baker and others, 1992, 1997). 

Shields are small (generally 1–15 km diameter; rarely 20 km 

diameter), quasi-circular to circular, radar-dark or radar-bright 

features with or without topographic expression and with or 

without a central pit (Guest and others, 1992). The size of individual shields is difficult to constrain because bases of individual 

shields are typically poorly defined, and deposits commonly 

blend smoothly into a composite layer that cannot be robustly 

treated as a time line or marker unit. Pits, sharply defined depressions, or pit chains, which are linear arrays of pits, likely represent regions marked by subsurface excavation and, as such, they 

may mark the surface expression of dilatational faults or dikes 

(Okobu and Martel, 1998; Bleamaster and Hansen, 2005; Ferrill 

and others, 2004; Schultz and others, 2004). Pits or pit chains 

can be considered primary structures or secondary structures, 

depending on the question at hand; pits are primary structures 

relative to pit-related materials, yet they may be secondary structures relative to the units they cut or are emplaced within.

Most radar lineaments represent secondary structures. 

Stofan and others (1993) provide an excellent introduction to 

the interpretation of secondary structures in Magellan SAR 

imagery. Extremely fine, sharply defined, continuous radarbright lineaments, typically occurring in the lowlands, have 

commonly been interpreted as fractures (Banerdt and Sammis, 
3

1992; Banerdt and others, 1997). If a fracture is associated with 

pits, it may represent the surface expression of a subsurface dike 

or a dilatational fault. Paired parallel dark and light lineaments 

that are separated by more than a few kilometers and describe 

linear troughs are generally interpreted as graben.

In SAR imagery, the opposite of a groove (linear trough) is 

a ridge (positive linear topography marked by parallel light and 

dark lineaments with the light lineament closest to illumination 

direction). On Venus, ridges typically have either parallel edges 

with moderate sinuosity at the 10-km scale and an across-strike 

gradation in backscatter (a form specifically called ridges in 

this text), or they have a more erratic plan view with common 

high-angle interruptions at the 10-km scale and variations in 

across-strike width (wrinkle ridges). Warps consist of positive 

linear topographic features, on the order of a hundred kilometers across and as much as thousands of kilometers long; warps, 

which are too subtle to appear prominently in SAR data, are 

discernible in topographic data.

Parallel bright and dark lineaments that form a fabric 

marked by alternating parallel ridges and troughs and have 

typical wavelengths of 2–5 km are called ribbons, ribbon fabric, 

or ribbon terrain (fig. 3, map sheet; Hansen and Willis, 1996, 

1998). Bindschadler and others (1992a) called ribbons “narrow 

troughs.” Ribbon fabrics are commonly spatially associated with 

folds; fold crests typically trend at a high angle (generally 90°) 

to the ribbon lineaments. Graben, also commonly spatially associated with ribbons, typically parallel ribbon trends, but they can 

be differentiated from ribbons based on smaller length:width 

ratios. Graben that occur within ribbon terrain typically cut 

across fold crests, resulting in a lens-shape plan view. Collectively referred to as ribbon-tessera terrain, or ribbon terrain 

for short, this distinctive composite tectonic fabric commonly 

marks tessera terrain. The composite fabric may reflect a progressive increase in the depth to the rheological brittle-ductile 

transition with time and fabric development (Hansen and Willis, 

1998; Ghent and Hansen, 1999; Brown and Grimm, 1999). For 

a discussion of ribbon-terrain controversies, see Gilmore and 

others (1998) and Hansen and others (2000).

Terminology

Clarification of the terminology used to describe and 

discuss Niobe Planitia quadrangle (V–23) is necessary, because 

terminology often carries stated and unstated assumptions 

regarding genetic implications or resulting from individual 

experience.

The term “lowland” is used in a topographic sense to 

describe broad regional long-wavelength (>100s of kilometers 

across) basins. The term “planitia/planitiae” refers to individual geomorphic basins or lowland regions. The general term 

“plains” describes geologic units only when the referenced 

published work uses that term.

The term “terrain” describes a texturally defined region, 

for example, a region where tectonism imparted a surface with 

a penetrative deformation that disallows interpretation of the 

original unit or units (Wilhelms, 1990). Characteristic texture 

could imply a shared history, such as a terrestrial tectonothermal 

history or an event that melds possibly previously unrelated 

rock units (any combination of igneous, metamorphic, and 

sedimentary rocks) into gneissic terrain; no unique history is 

inferred or required prior to the event(s) that melded potentially 

separate units into the textural terrain. Events prior to terrain 

formation are unconstrained in time or process unless specifically noted. Examples of terrain in the map area include ribbontessera terrain (also called ribbon terrain), fracture terrain, and 

shield terrain.

Eruptive centers are called “shields” (terminology from 

Guest and others, 1992). An area where shields and associated eruptive materials appear to coalesce into a thin, regionally extensive layer is called “shield terrain”. Shield terrain is 

a composite unit terrain that consists of tens of thousands of 

individual shields and coalesced flow material, referred to as 

“shield-paint” for its apparent low viscosity (like liquid paint) 

during emplacement (Hansen, 2005). Shield paint, which 

acquires post-emplacement mechanical strength (like dry 

paint), could be formed from any combination of lava flows, 

air-fall deposits, or pyroclastic flows (Guest and others, 1992). 

Note that flooding by this radar-smooth flow material highlights gently sloping topographic features that would not normally be visible in Magellan SAR data. Shield terrain contains 

rocks with an interpreted shared emplacement mechanism (represented by primary structures), which differs from ribbon and 

fracture terrains that contain rocks with an interpreted shared 

deformation history (represented by secondary structures). 

Absolute time involved in terrain formation is completely 

unconstrained. Shield terrain in the map area includes edifices 

(shields) and associated deposits (shield paint).

The term “inversion structure” is used to record a sequence 

of geologic events, which could be genetically related or unrelated; the nature and relative order of the events, rather than 

the genetic relation between events, are critical to inversion 

structure formation. Initial formation of extensional fractures 

results in linear negative topography called “fracture troughs,” 

which are subsequently filled by a thin layer of surface material 

(DeShon and others, 2000). Later contraction causes inversion 

of the trough fill, resulting in the formation of linear structural 

ridges, or inversion structures, parallel to host fractures.

Geologic Setting

The vast lowlands display an extensive suite of generally north-striking fractures and a suite of east-northeast to 

east-trending wrinkle ridges. These lowlands also host tens of 

thousands of individual shields. The shields and associated eruptive materials coalesce into a thin, regionally extensive shield 

terrain layer that forms a regionally thin, locally absent veneer 

across much of the lowlands. Shield terrain is locally deformed 

by (predates) and locally masks (postdates) secondary structures 

including regional fractures and wrinkle ridges. In general, wrinkle ridges concentrate where shield terrain is presumed slightly 

thicker than average, and fractures dominate at the surface where 

shield terrain is presumed slightly thinner or absent.

Isolated kipukas of ribbon-tessera terrain (Hansen and 

Willis, 1998) locally protrude through the shield-terrain veneer, 

preserving evidence of local surface processes that predated 
4

shield-terrain emplacement, as well as providing evidence for 

the extensive regional development of tessera-terrain fabrics. 

Ribbon-tessera fabrics are similar to the fabrics that characterize 

high-standing crustal plateaus, such as nearby Ovda and Thetis 

(V–35 and V–36) Regiones. Ribbon-bearing kipukas may 

preserve evidence of ancient collapsed crustal plateaus (Phillips 

and Hansen, 1994; Ivanov and Head, 1996; Hansen and Willis, 

1998; Ghent and Tibuleac, 2002), or they may record different, 

but rheologically similar, processes. Ribbon terrain and associated shield terrain extend eastward into V–24 and northward 

into V–11 and V–12, where shield terrain was first recognized 

as an areally extensive geologic unit (Aubele, 1996).

Geologic Relations

The Niobe Planitia quadrangle lies along the north edge of 

Aphrodite Terra, where it preserves a transition from southern 

highlands to northern lowlands. Mesolands (the intermediate 

topographic level of Venus), as a topographic province, are 

essentially absent or represented only by Gegute Tessera.

Ovda Regio and Haasttse-baad-Gegute Tesserae also display intratessera basin material (itbO and itbHG, respectively) 

and marginal intratessera flow material (fitm). The Niobe Planitia fracture terrain (frN) occurs as isolated kipukas, in locally 

elevated regions such as rims of coronae, and within the cores 

or interior highs of dorsa, as well as scattered outcrops across 

the map area. Neither the host unit nor the fractures likely 

record temporally distinct events. The host is likely a composite 

unit; the fractures commonly parallel the outcrop pattern of unit 

frN and may have formed synchronously with local basal terrain 

modification. Although the two styles of basal terrains occur 

across the map area, unit rtu dominates the eastern lowland, 

whereas unit frN dominates the western lowland. These basal 

terrains are variably covered with an apparently thin veneer 

of material called shield terrain. Shield-terrain material (s) is 

marked by extensively distributed small (~1–10 km diameter) 

shield edifices and associated local deposits (Hansen, 2005). 

Unelanuhi Dorsa (~100 km wide and over 2,000 km long), 

trends northwest parallel to the northeast boundary of Ovda 

Regio and extends into V–22 to the west; basal terrain resides 

within its core. The vast lowland within the map area displays 

an extensive suite of generally north-striking fractures and a 

suite of east-northeast- to east-trending wrinkle ridges, which 

variably cut units frN and s. The lowland also includes localized deposits (smooth flows, undivided, unit fsu) marked by 

relatively smooth radar character. This unit is interpreted as 

relatively radar smooth flows that may represent a mixture of 

both geologic and radar units.

The lowlands also host five coronae, each marked by 

subdued or basin-like topography: Dhisana, Allatu, Omeciuatl, 

and Bhumiya Coronae cluster loosely in the east-central map 

area and Maya Corona occurs in the northwest (lat 23° N., long 

97° E.). Rosmerta Corona (outside of V–23) spawns flows (unit 

fcR) that spilled across topographic lows within Haasttse-baad 

Tessera. The five coronae within the map area display no obvious flow units that can be uniquely attributed to these features at 

map scale; each feature is marked by topographic and structural 

elements as discussed in the sections on tectonic structures and 

coronae. A volcanic structure, marked by a flat-topped edifice 

and radial flows (units fEa and fEb), occurs in the northwest 

corner of the map area.

Basal Terrain Units

Basal units in the Niobe Planitia quadrangle (V–23) are 

commonly associated with local highs (100s of meters high, 

generally >10 km across); but not all basal units correlate with 

local highs and not all local highs correlate with the exposure of 

basal units. The basal units form two packages differentiated by 

structural character: ribbon-tessera/intratessera and fracture terrains. Ribbon-tessera terrain characterizes the highland, occurs 

in large tracts across Ovda Regio (rtO) and Haasttse-baad and 

Gegute Tesserae (rtHG), and occurs in lowland kipukas across 

much of map area (rtu).

Tessera and Intratessera terrain

Tessera terrain within the map area includes ribbon-tessera 

terrain of Ovda Regio (unit rtO), ribbon-tessera terrain of 

Haasttse-baad and Gegute Tesserae (unit rtHG), and undivided 

ribbon-tessera terrain (unit rtu). Each tessera-terrain unit hosts 

ribbon structures (Hansen and Willis, 1998) and commonly 

hosts fold ridges. In general, ribbons and folds trend perpendicular to one another, although tectonic fabric character in unit 

rtu is more difficult to delineate with confidence.

A composite tectonic fabric comprised of folds, ribbons, 

and complex graben characterizes ribbon-tessera terrain (fig. 3, 

map sheet). This fabric, which varies in detail from location to 

location, is characterized by (1) relative orientation of ribbons, 

folds, and complex graben, (2) relative spacing of structural 

suites, and (3) the penetrative development of the composite 

fabric relative to the scale of image resolution. Ribbon structures generally trend perpendicular to fold crests, as do late 

graben (Bindschadler and Head, 1991; Hansen and Willis, 

1998, 1996; Pritchard and others, 1997; Brown and Grimm, 

1999; Ghent and Hansen, 1999). At many locations, three or 

more distinct fold wavelengths are visible in altimetry data 

(Hansen, 2006): approximately 0.5 to 2–5 km, 10 to 15 km, and 

very long wavelengths of 30 to 100 km. Short- and mediumwavelength folds occur within the troughs, limbs, and crests 

of longer-wavelength folds. Short-wavelength folds locally 

curve across longer-wavelength fold crests with angles generally greater than 30°. Low viscosity material (lava?) locally fills 

troughs of short-, medium-, and long-wavelength folds. Flooded 

short-wavelength fold troughs occur along long-wavelength 

fold troughs, limbs, and crests. Locally filled basins occur at a 

range of scales from the troughs of short-wavelength folds and 

extensional structures to intratessera basins (Banks and Hansen, 

2000) visible at map scale (fig. 4, map sheet).

Large intratessera basins (Bindschadler and Head, 1991; 

Banks and Hansen, 2000) that contain radar-smooth fill generally trend parallel to the fold troughs. Intratessera basin material, generally radar smooth in character, is defined as two 

separate map units, intratessera basin material of Ovda Regio 

tessera terrain (unit itbO) and intratessera basin material of 
5

Haasttse-baad and Gegute Tesserae terrain (unit itbHG), based 

on location and host tessera terrain. Relatively large regions 

of these units are visible at map scale, although they are also 

visible at smaller scales (figs. 3, 4, map sheet). Locally, short, 

closely spaced extensional structures cut orthogonally across 

short-wavelength fold crests, typically with little to no indication that these same structures cut long-wavelength fold crests, 

indicating that both early layer contraction and early layer 

extension contributed to ribbon-terrain-fabric formation. Ribbon 

structures are commonly deformed by medium- to long-wavelength folds, indicating relative early ribbon formation (Hansen 

and Willis, 1998), but short wavelength fold fabrics are clearly 

truncated by short to medium extensional structures, indicating that layer contraction and extension occurred throughout 

ribbon-terrain evolution. Widely spaced extension structures 

form complex graben locally; complex graben cut all folds 

that form late in the evolution of ribbon-terrain fabric (Bindschadler and others, 1992a; Gilmore and others, 1997; Hansen 

and Willis, 1996; 1998; Brown and Grimm, 1999; Ghent and 

Hansen, 1999).

Ribbon-tessera-terrain units (rtO, rtHG, rtu) range topographically from highest to lowest instead of stratigraphically, 

because relative temporal relations are unconstrained. Unit rtO, 

confined to the southwestern part of the map area, represents the 

northeastern part of Ovda Regio and extends into adjacent V–22, 

V–34, and V–35 quadrangles. Unit rtHG, which crops out in the 

southeastern and eastern parts of the map area, extends eastward 

into V–24 and southward into V–35. Unit rtu occurs as isolated 

kipukas across the eastern and central map area.

Ribbon-fold fabrics in unit rtO describe a coherent pattern. 

Folds trend northwest parallel to the local northwest margin of 

Ovda Regio, curving to a more northward trend toward the east 

and into V–35 to the south (Bleamaster and Hansen, 2005). 

Ribbon structures generally trend perpendicular to fold crests, as 

do late graben. Intratessera basins with radar-smooth fill generally trend parallel to the fold troughs. In the southwestern map 

area, the structural fabrics preserved in unit rtO and the intratessera basin fill (intratessera terrain volcanic material of Ovda 

Regio, unit itbO) describe more complex patterns: folds trend 

north with local sinuous traces; ribbons trend north-northeast and 

northwest; and aerially extensive intratessera basins comprise 

two general trends, parallel to northwest-trending fold troughs in 

the northeast and parallel to north-trending folds in the south.

Elevation and structural fabric patterns delineate unit rtHG

from unit rtO. Whereas unit rtO generally lies above 6,054 km 

elevation, unit rtHG lies within the range 6,051.5 to 6,053.5 km 

with lower elevations to the north. Near the boundary between 

Haasttse-baad and Gegute Tesserae to the east and Ovda Regio 

to the west, folds and intratessera basin fill (unit itbHG) in unit 

rtHG trend northeast and ribbons trend northwest perpendicular 

to fold crests. To the north within Haasttse-baad and Gegute 

Tesserae, fold crests and intratessera basins also trend northeast. 

Ribbon fabrics trend perpendicular to the fold crests, but shortwavelength folds parallel the fold crests. At least some of the 

northwest-trending lineaments appear to be ribbon ridges and 

troughs, although, locally, some may be short-wavelength folds. 

Data resolution does not allow confident structure identification in each case. Within the region between the northeast- and 

northwest-trending intratessera basins, some basins display 

possible hybrid orientations and patterns. Detailed geologic and 

structural mapping of the intratessera basins is outside the scope 

of this study, although construction of such maps would likely 

provide clues to basin formation in relation to the structural 

evolution of the hosting tessera terrain. Although Haasttse-baad 

and Gegute Tesserae define separate geographic locations, they 

provide no clear geological character that distinguishes the two 

regions; hence, they are considered a single geological unit. As 

noted, terrain elevation and the occurrence of intratessera basins 

decrease to the north, which could relate to overall elevation and 

the related ability to delineate intratessera basins.

Ribbon-tessera terrain, undivided (unit rtu), occurs in 

numerous kipukas across the central, northern, and eastern 

map area. The highly irregular boundaries of unit rtu reflect the 

detailed structural topography that results from the composite 

ribbon-fold fabric and the interaction of this topography with 

younger overlying deposits. Isolated kipukas of unit rtu show 

gradational contacts with overlying material; many exposures of 

unit rtu exist well below the 1:5,000,000 map scale. Each kipuka 

of unit rtu preserves a penetrative fabric, consisting of one or two 

lineament trends; the fabric defines the kipuka material as tessera 

terrain. In some kipukas, the lineaments can be distinguished as 

ribbons or as fold crests; in other kipukas, lineament trend, but 

not character, can be determined. Lineaments among all of the 

rtu kipuka—no matter how small or how widely separated—

show parallel trends. These trends also parallel the fabric trends 

that are preserved in and that characterize unit rtHG. Although 

spatial relations and relative ages among the rtu kipukas cannot 

be uniquely defined, the parallelism of penetrative fabrics is 

consistent with the interpretation that unit rtu forms a continuous subsurface unit and that the kipukas preserve synchronously 

formed terrain, recording a common history. The similar character and parallelism of fabrics in units rtu and rtHG might reflect 

a shared history of these two spatially separate units, as well; this 

interpretation is consistent with, but not required by, the current data. However, tectonic fabric patterns preserved in tessera 

terrain associated with Ovda Regio and with Haasttse-baad and 

Gegute Tesserae and in tessera terrain inliers describe different 

regional strain patterns consistent with, but not required by, the 

interpretation that these two tracts of tessera-terrain (rtO, rtHG) 

formed as separate morphological features (Bindschadler and 

Parmentier, 1990; Bindschadler and others, 1992b; Bindschadler, 

1995; Phillips and Hansen, 1994, 1998; Hansen and Willis, 

1998; Hansen and others, 1999, 2000).

Marginal intratessera terrain flow material (unit fitm), 

characterized by radar-smooth material, fills local lows within 

the associated ribbon terrain along the margins of Ovda Regio 

crustal plateau. Unit fitm does not have any clear, internal, 

crustal-plateau sources and occurs marginally along Ovda Regio. 

These flows locally host structures that resemble ribbons and 

folds. However, unit fitm blankets the structures; thus, tessera 

structures may have been reactivated to deform unit fitm, which 

suggests that unit fitm dominantly postdated the formation of 

tessera fabrics. Timing between the formation of units itbO and 

fitm is difficult to determine, and the relative order of emplacement may vary from location to location. Unit fitm flows may 

be attributed to similar emplacement processes as unit itbO and 
6

simply show a different surface morphology due to flow thickness and depositional environment, or they may be unrelated.

Fracture Terrain

Kipukas preserved mostly in the northwestern part of 

the map area host another basal unit, Niobe Planitia fracture 

terrain (unit frN). Kipukas of unit frN show sharp to diffuse, 

but extremely detailed, contacts and appear locally buried or 

partially obscured by surface material, shield-terrain deposits 

(unit s). Unit frN hosts generally north-striking fractures where 

exposed, but individual exposures also show fracture patterns 

that correspond to local topography, likely indicating that the 

terrain records both a shared history of modest easterly directed 

extensional strain, as well as localized low strain deformation related to topographic warping. The relative age among 

individual kipukas is not constrained, nor is the relative timing 

of surface material that crops out at some distance from the 

terrain-bearing kipukas. At the location of the contact, shield 

terrain clearly postdates development of the tectonic fabric and, 

therefore, postdates the host material as well. Strain fabrics 

recorded in the kipukas could have formed in a spatially localized fashion, which explain many exposures of unit frN where 

the fracture pattern mimics the local topography.

Shield Terrain Material

Shield terrain (unit s) is the most aerially extensive terrain across the map area. Individual shields are radar-dark or 

radar-bright, quasi-circular to circular features (flat-topped or 

flat shield, dome, cone) with or without a central pit (fig. 5, map 

sheet). Embayment relations indicate that flows spread across the 

surface into local topographic lows and around local topographic 

highs, coalescing with material from adjacent edifices to form a 

thin layer of shield paint (fig. 6, map sheet). Shield paint forms 

a thin, regionally extensive but locally discontinuous layer that 

appears lacelike and locally hides and thinly veils stratigraphically lower unit frN and (or) ribbon-tessera-terrain units (fig. 

7, map sheet). The scale of lacey holes in the shield-paint veil 

ranges from tens to hundreds of square kilometers. The lacey 

holes suggest that shield paint probably flowed across the surface, rather than being emplaced as an air-fall layer, which would 

form a more continuous blanket. Shield paint hosts spaced 

fractures, wrinkle ridges, and inversion structures that indicate 

the acquisition of some inherent mechanical strength following 

unit emplacement.

The smallest size of individual shield edifices is difficult 

to determine because many pixels are needed to image a single 

shield; thus, although full-resolution Magellan SAR data has 

approximately 100 m/pixel resolution, the effective resolution 

for shield size is likely approximately 0.5 to 1 km diameter 

(Zimbelman, 2001; Guest and others, 1992). Minimum discernable shield diameter depends on size, morphology, radar 

incidence angle, radar contrast with surroundings, texture of 

surroundings, and character and number of adjacent shields. 

Because approximately 0.5- to 1-km-diameter shields can be 

identified locally in the map area, it is likely that even smaller 

shields also exist. Guest and others (1992) reached a similar 

conclusion. In addition, shield size is difficult to constrain 

because shield bases are commonly poorly defined; individual 

edifices typically appear to blend smoothly into a base layer 

of coalesced shield paint. In some places, a slight difference 

in radar backscatter or truncation of pre-existing underlying 

structural fabric can define the apparent limit of individual 

shields, or it might mark a change in thickness of shield paint. 

Figure 5 illustrates a range of shield morphologies: (1) aprons 

that blend outward into surrounding terrain, (2) edifices that are 

distinct from surroundings, (3) edifice morphologies that range 

from steep to broad and from cone shaped to flat topped, and (4) 

extremely flat regions that contain low-viscosity material that 

flowed into subtle topographic lows.

The density of shields is also difficult to determine because 

shield identification depends on image resolution, as well as 

the experience of the geologist. In the map area, shields were 

ranked by confidence levels (table 1): definite shields (obvious 

shields) and potential shields (possibly controversial features). 

The major distinction between these divisions relates to data 

resolution and geologists’ experience. Shields likely exist 

below SAR resolution, making maximum estimates difficult to 

determine. Minimum estimates of definite-shield densities range 

from 3,550–10,500 shields/106 km2. Combined (both definite and potential) shield densities range from 15,650–33,675 

shields/106 km2; these values are neither minimum nor maximum values, simply estimates.

No direct correlation exists between the number of definite 

and combined shields in the map area: the area centered at lat 

23° N., long 103° E. (fig. 6A) hosts the fewest combined shields, 

yet the most definite shields and the area centered at lat 13° N., 

long 111° E. (fig. 8A, map sheet) has the fewest definite shields 

and the most combined shields. The average number of definite 

shields (247) and combined shields (938), representing average 

total densities of approximately 6,255 and approximately 23,445 

shields/106 km2, respectively, are slightly greater than, but similar to, the shield plains of Aubele (1996; 4,500 shields/106 km2).

Figures 6–10 (map sheet) show details of five 2°x2° 

images from the map area that illustrate shield patterns, densities, relations with basal-terrain units, and secondary structures. 

Each area (1) has both right- and left-illumination SAR images 

that lack large data gaps; (2) lacks large impact craters, large 

tracts of ribbon terrain, or extensive basal fracture terrain, all of 

which could hamper shield identification and interrupt shield 

patterns; (3) hosts regional easterly trending wrinkle ridges and 

northerly striking fractures; and (4) yields clues for shield-terrain formation. Areas shown in figures 6–10 are named for their 

location, for example figure 6, centered at lat 23° N., long 103° 

E. is referred to as 23N/103E.

23N/103E

Area 23N/103E (fig. 6A) contains a similar distribution of 

definite shields (402) and combined shields (754). An approximately 20 km wide data gap obscures the north-central part of 

the map area. Temporal relations between shields and wrinkle 

ridges are difficult to determine robustly because these primary 

and secondary structures have relatively small size and spacing and the shields or variations in shield-paint thickness could 
7

contribute to strain partitioning during wrinkle-ridge formation. Locally, a shield appears to truncate a wrinkle ridge along 

trend and, therefore, would postdate wrinkle-ridge formation. It 

is also possible that the shield was emplaced prior to wrinkleridge formation, and ridge formation ended as it approached the 

shield representing an area of greater layer thickness and, hence, 

strength. In addition, the large number of individual shields 

and individual wrinkle ridges may infer that wrinkle-ridge and 

shield formation were partly penecontemporaneous.

In the region between wrinkle ridges, polygonal wrinkleridge structure is best developed away from shield centers, 

indicating that the polygonal structures most likely formed after 

individual shields. The pattern likely reflects strain partitioning resulting from a thicker material layer near shield centers. 

But early polygonal fabric formation is also possible, which 

suggests that the subsequent thin shield flows cover the preexisting ridges. The polygonal fabric shows either a preferred 

longitudinal shape parallel to the trend of the wrinkle ridges or 

no preferred shape.

In the southeastern part of the image, shields are superposed on a locally preserved basal layer cut by closely spaced, 

east-trending, anastomosing lineaments (fig. 6D). The patch-like 

outcrop pattern of these kipukas of basal terrain, which are not 

visible at map scale, indicates the extremely thin nature of the 

overlying shield terrain unit. The penetrative fabric is discussed 

further in the section on regional structures.

Locally, north-northwest-striking fractures cut the shield 

paint, but the discontinuous nature of many fracture traces suggests that these fractures were locally reactivated and original 

fractures likely predated most shield paint. These relations are 

also consistent with a thin shield-terrain layer.

21N/113E

Area 21N/113E (fig. 7A), which hosts almost an order of 

magnitude more combined shields (1,080) than definite shields 

(167), shows similar relations between wrinkle ridges and fractures. This region also preserves patches of extremely fine scale 

polygonal fabric that has diffuse to relatively sharp boundaries 

(fig. 7C). Both definite and potential shields occur within the 

region defined by fine polygonal fabric without obvious flow 

boundaries. Potential shields are more prominent in the region 

deformed by polygonal fabric, which may simply reflect a 

contrast with a subtly textured substrate. Mottled areas free of 

small-scale polygonal fabric could be aggregates of subresolution shields. Wrinkle ridges cut the polygonal fabric boundary 

with no obvious spatial pattern relative to the fine polygonal 

fabric, indicating that there is likely a relative mechanical coherence in the layer across this boundary, although layer thickness 

may differ across the boundary. The region between wrinkle 

ridges preserves a fine-scale polygonal fabric that appears to 

grade in scale across the enlarged image (fig. 7A). Northerly 

striking fractures parallel to the regional trend are covered by 

shield paint, yet fractures locally cut individual shield edifices. The fractures show a spatial correlation with the areas of 

smaller-scale polygonal fabric. Both observations are consistent 

with the interpretation that the shield terrain layer is thinner in 

the area of small-scale polygonal fabrics. An approximately 15 

km diameter circular depression marks the northeast corner of 

21N/113E; extremely fine, commonly covered fractures concentric to this structure extend 60–70 km from its center.

13N/111E

Area 13N/111E (fig. 8), located in Sogolon Planitia, also 

shows an order of magnitude more combined shields (1,340) 

than definite shields (140), east-trending wrinkle ridges and 

north-striking fractures. Locally, shields postdate formation of 

north-trending lineaments that both cut and invert shield-paint 

material (fig. 8C). The lineaments are apparently extension 

fractures, in places filled with shield-paint material, which was 

subsequently shortened, resulting in formation of inversion 

structures (DeShon and others, 2000). Fracture fill (shield paint) 

was apparently extremely localized and deposited as a thin, 

lace-like layer, indicated by (1) open fractures transitioning into 

inversion structures along strike and (2) the close spatial location of open fractures and inversion structures.

23N/117E

Area 23N/117E (fig. 9A) hosts four times as many combined shields (1,000) as definite shields (250), as well as 

inversion structures. In the northeast corner of this area, kipukas 

of ribbon-tessera terrain peek through a veil of shield terrain. 

Shield paint embays the detailed ribbon-terrain topography, 

comprised of alternating, parallel, north-trending ridges and 

troughs. Shield paint blends into a coherent layer; shields are 

locally visible due to topographic expression. Isolated patches 

of fine-scale polygonal fabric occur locally (fig. 9D) and 

gradually increase in spacing away from the shields, which is 

consistent with an interpretation that shield paint is thicker near 

the edifices. Locally a secondary structural fabric, marked by 

delicate, closely spaced (~500 m or less), short (~5–20 km), 

northeast-trending lineaments (fractures?), transects the surface 

discontinuously (fig. 9E). The fabric is visible in patches that 

have a parallel lineament trend and similar lineament spacing. 

Fabric continuity across spatially separate regions supports the 

interpretation that this fabric is secondary. The tight lineament 

spacing likely reflects deformation of a thin layer, interpreted as 

shield paint in this image. Similar subtle lineament fabrics (with 

various orientations) across much of the Niobe Planitia map 

area are not visible at map scale.

13N/119E

Area 13N/119E (fig. 10A) includes ribbon-terrain kipukas, 

wrinkle ridges, fractures, and definite (270) and combined (630) 

shields. Ribbon terrain shows relatively high relief, rising from 

several hundred meters along the northern edge of the image. 

Ribbon trends parallel those preserved in isolated kipukas 

across the map area. Shields and shield paint occur across the 

topographic range of ribbon terrain, forming high, isolated 

deposits (fig. 10C). Even at high elevation, shield paint that 

flowed into localized lows gently masks ribbon fabrics, indicating that (1) shield terrain postdates ribbon-terrain formation, (2) 

shield terrain lies directly above ribbon terrain, (3) shield terrain 

forms discontinuously across local high relief (that is, it cannot 
8

be connected by a continuous datum indicative of low-elevation 

embayment), and (4) shield paint is locally very thin. Regional 

continuity of ribbon trends between kipukas is consistent with 

the interpretation that ribbon terrain extends beneath shield 

paint across much of the map area and that shield terrain is thin 

at both a local and a regional scale. Delicate wrinkle ridges 

that cut shield paint east of the ribbon-terrain kipukas parallel 

regional trends. North-striking fractures both cut and are covered by shield paint, indicating that at least a part of the activity 

on these structures (likely reactivation) was penecontemporaneous with shield-terrain formation.

Several independent observations indicate that shield 

terrain forms a thin veneer, both locally and regionally: (1) 

extremely complex contacts between shield terrain (unit s) and 

underlying basal terrains across the central and northern map 

area, marked by low topographic relief; (2) detailed contacts 

between shield terrain and underlying units; (3) lace-like 

character of shield terrain; (4) local fracture filling of shield 

terrain and subsequent structural inversion with changes along 

strike from fracture trough (unfilled) to inversion structures; 

(5) occurrence of fine-scale polygonal fabric on shield terrain; 

(6) widespread reactivation of subterrain fractures; and (7) the 

occurrence of numerous small kipuka of ribbon terrain across 

huge expanses of very low topographic relief. Guest and others 

(1992) estimated that shield deposits are likely tens of meters or 

less in thickness. Robust quantification of layer thickness is difficult using available SAR data, but layers tens of meters thick 

or less are consistent with observations 1–7.

Flow Units

Flows originating within V–23

Smooth flows, undivided (unit fsu), occur in the southwest 

lowland region just north of Ovda Regio. This unit is interpreted 

as radar-smooth flows that may represent a mixture of both 

geologic and radar units. Unit fsu typically lies in contact with 

unit s (shield terrain), although locally it abuts unit rtu (ribbontessera terrain). The contact between units fsu and s is typically 

gradational. Shield edifices lie within the mapped limits of unit 

fsu, and unit fsu likely locally embays unit s. Units s and fsu

are interpreted as mutually time transgressive. In some places, 

unit fsu postdates the emplacement of locally defined unit s, as 

evidenced by the burial of wrinkle ridge structures that deform 

and, therefore, postdate local occurrences of unit s (lat 11° N., 

long 96° E.).

In the northwest corner of the map area, Ezili Tholis (~100 

km diameter, as high as 6,053.5 km) perches above the surrounding lowland. Two suites of associated flows, units fEa

and fEb, are distinguished on the basis of SAR and altimetry 

data. Each unit comprises numerous intermediate- to highradar-backscatter lobate flows that radiate outward from the 

topographic edifice. Unit fEb flows are proximal to the summit; 

are marked by higher-radar backscatter, likely indicating a 

rougher surface; and occur along a steep slope. Unit fEb may be 

younger than unit fEa, but relative timing between the two units 

is unconstrained. Unit fEa distal flows extend outward from 

the volcano as far as 200 km. The flows were affected by local 

topography, where they are buttressed to the east by a local high 

extending above 6,052 km and are decorated with basal terrain 

(Niobe Planitia fracture terrain, unit frN). Local high areas, as 

high as 6,052.5 km, also deflect unit fEa flows to the south; a 

local high along the quadrangle boundary (lat 22.5° N., long 90° 

E.) splits local flows. Kipukas of unit frN are surrounded by unit 

fEa. Wrinkle ridges cut units fEa and fEb, which also contain 

small shield edifices.

The five coronae within the map area display no obvious 

flow units that can be identified at map scale.

Rosmerta flows

Rosmerta Corona is a 300-km-diameter, radial, domical 

corona that lies outside of the map area along the boundary 

between V–24 and V–36 (centered at lat 0° N., long 124.5° E.; 

Thetis Regio quadrangle). In the southeastern map area, flows 

that emanate from Rosmerta Corona flow northwest and fill 

local topographic basins within Haasttse-baad Tessera (unit 

rtHG). Rosmerta Corona flow material (unit fcR) shows mottled 

to radar-smooth character and can be traced to the central region 

of Rosmerta Corona. Some of the material mapped as Haasttsebaad and Gegute Tessera intratessera basin fill (unit itbHG) may 

have originated from Rosmerta Corona, having been connected 

with unit fcR either through narrow surface passages in the 

tessera-terrain fabric that are not visible at map scale or through 

the subsurface. Both unit fcR and unit itbHG show local shield 

edifices.

Impact Features

At least 24 impact craters, ranging from 3 to 63 km in 

diameter, dot the map area (table 2). Essentially all of the 

impact craters show ejecta blankets, whereas only three show 

clear halo deposits; 17 craters have radar-smooth interiors, 

interpreted as interior fill. Crater densities for individual craters 

range from 1.91 to 3.82 craters/106 km2, with an average crater 

density of 2.97 (Herrick and others, 1997). Half of the craters 

have densities >3.0 craters/106 km2 and seven craters have 

densities >3.5 craters/106 km2, which is higher than the Venus 

global average (~2 craters/106 km2; Phillips and Izenberg, 1995; 

McKinnon and others, 1997). Although there are too few craters 

to constrain the ages of individual surfaces on Venus (Hauck 

and others, 1998; Campbell, 1999), the relatively high crater 

density and the paucity of crater haloes may indicate that the 

map area represents a relatively old composite surface that is 

mostly older than the Venusian average model surface age (Phillips and Izenberg, 1995; Hansen and Young, 2007).

None of the impact craters formed during a unique event; 

therefore, the crater units represent clear diachronous unit 

emplacement. Unit cu (crater material, undivided) represents 

ejecta deposits associated with local, time-transgressive bolide 

impact. Some impact-crater interiors include a radar-smooth 

material that is also mapped as unit cu. Two additional impactcrater-related units appear spatially associated with Ferrier 

crater. Units coa and cob are interpreted as radar-rough and 

radar-smooth outflow facies, respectively. Unit coa, which is 

proximal to Ferrier crater, displays a heterogeneous texture; 
9

whereas, unit cob, the distal facies, is relatively radar smooth. 

Both units appear to locally bury wrinkle ridge structures that 

deform locally preserved shield terrain (unit s). Thus, units 

coa and cob, and Ferrier Crater by association, appear to have 

formed after the formation of local wrinkle ridges. In general, 

units coa and cob cover north-striking fractures, as evidenced 

by the general truncation of fractures in spatial correlation with 

the limits of units coa and cob. However, locally, units coa and 

cob are cut by north-striking fractures, which is likely the result 

of local structural reactivation.

Tectonic Structures

Both local and regional tectonic structures occur across 

Niobe Planitia quadrangle (V–23). Local structural patterns are 

generally areally confined or associated with individual features, 

such as coronae; whereas, regional structures describe coherent 

patterns across a much larger area. The timing of local structures 

likely corresponds to the formation, or stages of formation, of 

the individual features with which they are associated. Temporal 

evolution of regional tectonic structures is more difficult to constrain, may be time transgressive, and (or) involve reactivation.

Regional structural suites

Regional structures in the map area include four types of 

tectonic structural suites: (1) ribbon-tessera fabrics, (2) extensional fractures, (3) wrinkle ridges, and (4) inversion structures. 

Ribbon-tessera fabrics are described above in the section called 

Tessera and Tessera Terrain.

Regional Extensional Fractures

Regional extensional fractures, which generally strike 

north-northwest, display coherent tectonic patterns across 

the northern two thirds of the map area. These lineaments are 

interpreted as extensional fractures on the basis of their straight, 

sharp character and lack of notable topographic expression, 

or along strike offset. Fractures shown on the geologic map 

indicate the general trend and character of the fracture suite, 

however fracture spacing, 1–2 km or less (locally down to the 

resolution of the current SAR data set), is too detailed to show 

on the 1:5,000,000-scale map. Extension fractures, best preserved in local topographic highs across the lowland, are both 

locally covered by, and cut, shield terrain material, indicating 

that the fractures were formed and (or) reactivated through time. 

Although the fractures generally describe a northerly trend, 

locally, fractures mimic local topography, indicating that some 

fractures formed (or were reactivated) during local uplift along 

topographic warps.

Wrinkle Ridges

Wrinkle ridges (figs. 6–10) define low sinuous ridges a 

few kilometers wide and as much as a few hundred kilometers 

long that are found on most terrestrial worlds, especially on 

large flat expanses of volcanic flows (Watters, 1988). Wrinkle 

ridges, which trend east-northeast, occur across much of the 

northern two thirds of the map area as previously noted (Bilotti 

and Suppe, 1999). In Sogolon Planitia, wrinkle ridges trend 

northeast to locally northward, parallel to (1) northward striking 

fractures, (2) the Sogolon Planitia-Haasttse-baad Tessera boundary, and (3) northeast to north-trending warps and ridges within 

Sogolon Planitia. North-trending Sogolon wrinkle ridges likely 

represent inversion structures, described in more detail below. 

Like the fractures, wrinkle ridges occur at a range of spacing, 

down to small-scale wrinkle ridges too closely spaced to show 

on the 1:5,000,000-scale geologic map. Locally, wrinkle ridges 

define fine-scale polygonal fabrics (fig. 7) much too small to 

show on the 1:5,000,000-scale geologic map. However, closely 

spaced wrinkle ridges parallel the trends shown on the geologic 

map. Small-scale wrinkle ridges wrap around individual shields, 

indicating that wrinkle ridges deformed a thin mechanical layer, 

very low-angle fault elements participated in the formation of 

these wrinkle ridges, or both. Wrinkle ridges locally deform 

shields but are also covered by shields, indicating diachronous 

formation of both shields and wrinkle ridges (fig. 6). Wrinkle 

ridges are notably absent within exposures of ribbon-tessera 

terrain in high-resolution imagery (figs. 5, 6), although wrinkle 

ridges extend to the contact between tessera terrain and shield 

terrain. These relations suggest that ribbon-tessera terrain is not 

rheologically amenable to wrinkle ridge formation (it lacks a 

thin deformable layer); whereas, the thin shield terrain veneer 

readily forms wrinkle ridges. Thus, the presence or absence of 

wrinkle ridges in this case is likely related to rheological criteria 

rather than temporal considerations.

Inversion Structures

Inversion structures (Buchanan and Buchanan, 1995) 

within the map area generally occur as north-trending ridges. 

Although the inversion structures are too small to appear on 

the 1:5,000,000-scale geologic map, their occurrence provides 

critical clues to the nature of shield terrain. Inversion structures 

record a sequence of geologic events, which could be genetically related or unrelated; the nature and relative order of the 

events, rather than the genetic relation between events, are 

critical to inversion structure formation. Initial formation of 

extension fractures results in linear negative topography. Fracture troughs are subsequently filled by a thin layer of surface 

material; later contraction causes inversion of the trough fill, 

resulting in the formation of linear structural ridges parallel to 

host fractures. Inversion structures noted elsewhere on Venus 

(DeShon and others, 2000) follow a similar sequence of events. 

In the map area, northerly trending fracture troughs were locally 

filled with shield terrain material and later shortened, inverting 

the fill to form northerly trending ridges (figs. 7, 8).

Fabric anisotropy and implications for stressstrain relations

Extension fractures and wrinkle ridges are examples of 

regionally distributed strain fabrics, interpreted as parallel to 
10

maximum principle shortening and extension, respectively. The 

orientations of strain fabrics on Venus are commonly interpreted 

as a reflection of principle stress directions during wrinkle ridge 

(or fracture) formation (Sandwell and others, 1997; Solomon 

and others, 1999; Anderson and Smrekar, 1999; Banerdt and 

others, 1997; McGill, 2004). Such an interpretation requires 

that the host material was originally homogeneous and isotropic. Across the map area this assumption might not be robust. 

Wrinkle ridges and inversion structures provide illustrative 

examples.

In figure 6, basal terrain, which displays a penetrative 

fabric (closely spaced at the scale of observation) marked by 

local east-trending lineaments, is exposed through the lace-like 

veil of shield terrain. The east-trending linear fabric parallels 

the trend of adjacent wrinkle ridges, and it is likely that the 

older anisotropic fabric strongly influenced the orientation of 

younger wrinkle ridges. The penetrative fabric clearly predated 

shield terrain formation, which in turn must have predated 

the formation of wrinkle ridges; therefore, the host crust was 

anisotropic prior to wrinkle ridge formation. To illustrate the 

importance of an existing anisotropy in stress-strain relations, 

consider a section of corrugated cardboard (fig. 11, map sheet). 

The corrugations, small wavelength folds, define a penetrative 

structural/mechanical fabric anisotropy. Application of a wide 

range of orientations of principle stress axes would each result 

in the same orientation of longer wavelength folds (strain); in 

each case the resulting fold axes would parallel the original 

corrugations. Thus, the resulting strain can be almost independent of principle stress orientation. The strength anisotropy is so 

marked that application of principle compressive stress parallel to the corrugations will likely not result in formation of any 

shortening fabric. Corrugations or other mechanical anisotropy 

can either increase the overall strength of the material in the 

direction of the anisotropy, greatly inhibiting the formation of 

strain structures, or weaken the strength parallel to the anisotropy, and thus, greatly influencing the orientation of younger 

strain features, almost independent of differential stress orientation. A similar relation exists for fractures and later formation 

of inversion structures. In this case, a wide range of possible 

orientations of principle stress axes results in structural inversion of fracture fill and the formation of ridges parallel to the 

original fractures. Original fracture orientation, rather than the 

paleostress field that accompanied shortening, is responsible 

for inversion structure orientation (Withjack and others, 1995, 

1998).

Coronae

The map area hosts five coronae—all of which are probably old coronae (Chapman and Zimbelman, 1998), or circular 

lows (McDaniel and Hansen, 2005). Each of these coronae is 

marked by an amphitheater-like depression and by concentric 

fractures or faults that are spatially correlative with a circular 

ridge. In addition, each of these features lacks well-developed 

radial fractures and obvious large flows, features typical of 

many coronae (Stofan and others, 1992, 1997) and particularly associated with postulated young coronae (Chapman and 

Zimbelman, 1998). Although the lack of large distinctive flows 

could be a function of relatively old age and resulting radar 

homogenization (Arvidson and others, 1992), it is also possible 

that these features simply never had large obvious flows. We 

refer to these features as circular lows to emphasize descriptive 

character, rather than to hypothesize genetic evolution. Each 

circular low is described briefly below.

Maya Corona

Maya Corona (200 km diameter; lat 23° N., long 98° E.), 

in northwestern V–23, represents a circular basin as much as 

900 m deep (relief from ridge to basin floor), surrounded by a 

low incomplete ridge (best developed along the western margin) 

and a suite of concentric fractures. The concentric fracture zone 

is best preserved on the basin interior and basin walls, although 

locally concentric fractures occur in the surrounding lowlands. 

Shield terrain (unit s) variably covers the fracture zone; locally 

the fractures show evidence of structural reactivation following 

local shield emplacement. Regional north-northwest-striking 

fractures and northeast-trending wrinkle ridges variably cut and 

are cut by shield terrain material. Wrinkle ridges show a slight 

change in trend inward toward the Maya basin. Horner impact 

crater, which lies along the northwest interior wall of Maya 

basin, clearly postdates formation of Maya and likely postdates 

shield terrain and regional fracture and wrinkle ridge formation, 

although reactivation of these suites of structures cannot be dismissed. Maya lacks radial fractures, evidence of obvious flows, 

and any evidence of early domical development that might have 

accompanied corona formation. Maya basin and the concentric 

fracture suite formed synchronously and predated the formation of shield terrain and the formation of the fracture suite and 

wrinkle ridges.

Dhisana Corona

Dhisana Corona (~100 km diameter; lat 14.5° N., long 

111.5° E.) displays a 300–400-m-deep pear-shaped basin 

surrounded by concentric fractures and (or) faults developed 

along the basin wall and within the surrounding lowlands. 

The concentric fracture zone is mostly developed on the basin 

interior and basin walls, although locally concentric fractures 

occur in the surrounding lowlands and are visible through a 

thin cover of shield terrain. Locally, the fractures show evidence of structural reactivation following emplacement of 

individual shields. The basin hosts a central bulge decorated by 

shields. Two partially collapsed pancake domes, adorned with 

shields, lie along the northwest boundary of the basin, clearly 

overprinting the concentric fracture zone. Shield material, unit 

s, locally covers parts of the basin wall and floor, as well as 

the surrounding lowland. Regional north-striking fractures cut, 

and are covered by, shield material. Wrinkle ridges, parallel to 

regional trends, extend across Dhisana and deform unit s. The 

local geologic history that emerges includes (1) formation of 

unit rtu; (2) basin formation accompanied by development of 

the concentric fracture/fault zone; (3) interior mound formation 

and, presumably, pancake dome development within the same 
11

time slice; (4) shield terrain evolution, including formation of 

north-striking fractures and generally east-trending wrinkle 

ridges, all developing in a time-transgressive fashion, broadly 

overlapping in time and space. Dhisana notably lacks radial 

fractures, obvious flows, and evidence of early dome development.

Allatu Corona

Allatu Corona (150 km diameter; lat 15.5° N., long 114° 

E.) forms a circular basin 300–400 m deep, marked by an 

approximately 75-km-wide zone of concentric fractures/faults. 

The fracture zone, which is locally blanketed by discontinuous shield terrain material (unit s), is best preserved along the 

northwest and southeast basin walls, although locally concentric 

fractures occur in the surrounding lowland region. The fracture zone is completely covered along the southwest margin. 

A pancake dome covers the fracture zone along the west basin 

margin, yet concentric fractures of the same trend locally cut the 

dome, recording structural reactivation. Shield terrain material (unit s) variably covers the surrounding lowland, parts of 

the fracture zone, the basin interior, and an interior peak that 

lies off center in the northeast part of the basin. North-striking 

fractures cut and are covered by unit s. Wrinkle ridges, parallel to the regional trend, deform shield terrain material (unit 

s). The concentric fracture zone has a slight asymmetric form 

with a narrower footprint along the west basin margin, which 

is also marked by outcrops of unit rtu. The local geohistory 

that emerges includes (1) formation of rtu; (2) basin formation 

accompanied by the development of a concentric fracture/fault 

zone; (3) pancake dome formation, with local reactivation of 

concentric fractures; and (4) shield terrain evolution, including 

formation of north-striking fractures and generally east-trending 

wrinkle ridges, all developing in a time-transgressive fashion, 

broadly overlapping in time and space. Unit rtu is notably free 

of concentric fractures or any evidence that it was affected 

by the development of Allatu. Like Dhisana, Allatu shows a 

notable lack of radial fractures, obvious flows, and any evidence 

of early dome formation.

Bhumiya Corona

Bhumiya Corona (150 km diameter; lat 15° N., long 118° 

E), an approximately 600 m deep circular basin nestled among 

exposures of unit rtu, displays a circular 75–100-km-wide 

fracture/fault zone. The fracture zone, best preserved along the 

north-northeastern, southwestern, and western margins is variably masked by unit s. Unit s occurs in a lace-like continuous 

fashion from the surrounding lowlands, across the basin walls 

and into the basin interior. Bhumiya lacks an interior peak or 

high, but two pancake domes occur to the northwest of Bhumiya, barely overlapping the location of the concentric fracture 

zone. Wrinkle ridges trend eastward across the structure, parallel to regional trends, and deform shield terrain material. The 

local geohistory that emerges includes (1) formation of rtu and 

rtHG; (2) basin formation accompanied by the development of 

a concentric fracture/fault zone; (3) pancake dome formation; 

and (4) shield terrain evolution, including formation of northstriking fractures and generally east trending wrinkle ridges, 

all developing in a time-transgressive fashion, broadly overlapping in time and space. Bhumiya lacks radial fractures, obvious 

flows, and any evidence that an earlier domical structure existed 

during its evolution.

Omeciuatl Corona

Omeciuatl Corona (125 x 175 km diameter; lat 16.5° N., 

long 118.5° E.), located just north of Bhumiya, includes an 

elongate 600–700-m-deep basin. The deeper depth of these two 

circular lows is likely a function of their location nestled among 

ribbon-tessera terrain and, hence, higher elevation surroundings. Omeciuatl’s 30–100-km-wide concentric fracture/fault 

zone conforms to the shape of the elongated basin with fractures 

developed on the basin walls and cutting the adjacent region, 

including locally cutting nearby tessera terrain. Wrinkle ridges, 

parallel to regional trends, deform unit s across the lowland, 

within and across Omeciuatl basin walls and floor, and between 

individual exposures of unit rtu. The local geohistory that 

emerges includes (1) formation of rtu and rtHG; (2) basin formation accompanied by the development of concentric fracture/

fault zone; and (3) shield terrain evolution, including formation 

of north-striking fractures and generally east-trending wrinkle 

ridges, all developing in a time-transgressive fashion, broadly 

overlapping in time and space. Omeciuatl lacks radial fractures, 

obvious large flows, and any evidence of early uplift or doming 

history.

The concentric fracture zones of Omeciuatl and Bhumiya 

just barely touch one another, but relative temporal relations 

between the two are indistinguishable—both circular low 

structures postdate formation of tessera terrain tectonic fabrics 

and predate the formation of northerly striking fractures, shield 

terrain development, and wrinkle ridge formation. Allatu 

shows similar relative temporal relations within its local environment. Dhisana lies too far from rtu inliers (~150 km) to be 

able to place robust temporal constraints on Dhisana tesseraterrain formation, but the preserved relations are certainly 

consistent with temporal relations delineated for the other 

circular lows.

Rosmerta Corona

Rosmerta Corona (~300 km diameter; centered at lat 0° N., 

long 124.5° E. in V–24 and V–36) is more typical of a radial 

concentric corona (Stofan and others, 1992) marked by welldeveloped radial fractures (which do not extend into the map 

area) and flows. Although the center of Rosmerta lies outside 

the map area, Rosmerta flows flood local lows across southeastern Haasttse-baad Tessera within the map area. Radial fractures 

clearly crosscut ribbon-tessera fabric in V–24, providing clear 

evidence that Rosmerta Corona formed after the development 

of the distinctive tessera-terrain fabric. Rosmerta flows also fill 

local structural basins within unit rtHG, indicating that intratessera basin formation predated Rosmerta formation. Rosmerta 

flows are generally radar smooth with local shields.
12

Formation of circular lows

The five circular lows share many characteristics, yet they 

differ from typical radial concentric coronae. The circular lows 

are marked by (1) circular basins that lie 300–900 m below 

local base level, with (2) apparent spatial association with tessera terrain, and (3) well-developed concentric fracture/fault 

zones that correlate spatially with basin wall topography but 

also extend into the adjacent lowlands. The structures show a 

notable lack of radial fractures, long flows, and evidence for 

early doming or uplift. In contrast, radial concentric coronae, 

as represented by Rosmerta Corona, are marked by positive 

topography, radial fracture suites, and variably developed 

volcanic flows (Stofan and others, 1992, 1997, 2001). Radial 

concentric coronae are also commonly clustered in spatial 

association with volcanic rises, occur as chains associated with 

chasmata, and occur between volcanic rises (Stofan and others, 

1992, 2001; Hamilton and Stofan, 1996; DeLaughter and 

Jurdy, 1999), whereas circular lows may reside more typically 

within the lowlands (Shankar and Hansen, 2008). Although 

many workers accept that (all) coronae represent the surface 

expression of endogenic diapirs that impinged on the crust or 

lithosphere (Janes and others, 1992; Squyres and others, 1992; 

Stofan and others, 1992, 1997; Janes and Squyres, 1995; Koch 

and Manga, 1996; Smrekar and Stofan, 1997; DeLaughter and 

Jurdy, 1999), geologic relations preserved at the circular lows 

within the map area may be difficult to justify within a diapiric 

hypothesis. Efforts aimed at modeling the surface expression of 

diapir emplacement consistently predict early surface doming 

and development of radial fractures that extend beyond the 

limits of the subsurface diapir (Withjack and Scheiner, 1982; 

Cyr and Melosh, 1993; Koch and Manga, 1996; Smrekar and 

Stofan, 1997). Could circular lows represent diapiric structures, 

yet lack radial fractures? Perhaps radial fractures formed, but 

were buried by later flows (for example, the volcanic loading 

model of Cyr and Melosh (1993)). Such an explanation would 

not address the lack of radial fractures in adjacent exposed 

tessera terrain, which should preserve a record of radial 

fractures (if they had formed), nor does it address the fact that 

circular lows form topographic basins, not domes. The lack 

of radial fractures might be attributed to regional stress fields 

that accompanied diapir emplacement (Withjack and Scheiner, 

1982), but this would result in formation of an elliptical central 

domain, which should display positive rather than negative 

topography. Although Dhisana and Omeciuatl describe somewhat elongate basins, there is no evidence that regional deviatoric stresses of the required orientation (east-west-oriented 

maximum compressive stress; north-south-oriented minimum 

compressive stress) existed during basin evolution. The long 

axis of both Dhisana and Omeciuatl are incompatible with 

possible fabric weakness imposed by tessera-terrain fabrics, 

and north-striking extension fractures might record principle 

stress orientations 90° from the principle stress orientations 

predicted by basin shape. Perhaps the rheological character of 

the local crust was such that radial fractures never formed, or 

the host ribbon terrain somehow limited radial fracture formation. Rosmerta Corona seems to challenge these explanations, 

because radial fractures associated with Rosmerta extensively 

dissect and crosscut ribbon-terrain fabric of Haasttse-baad 

Tessera (Bleamaster and Hansen, 2005). Therefore, it would 

seem that (1) there is something unique about the rheological 

conditions associated with the formation of circular lows as 

diapirs, (2) circular lows formed by a different mechanism than 

radial-domical coronae, or (3) both.

Perhaps these circular lows do not represent the surface 

expression of subsurface diapirs. Other hypotheses have been 

proposed for corona evolution, including formation as volcanic 

caldera, as sinkholes due to subsurface flow or negative diapers, 

and by exogenic impactors (Shankar and Hansen, 2008; Hansen 

and others, 2008). If circular lows represent caldera, we might 

expect extensive associated flows, as well as collapse features 

within the interior basins walls (Lipman, 2000); but, such 

features are not observed. In addition, the concentric fracture 

zones associated with each circular low extend well outside the 

circular basins into the surrounding lowland, which would not 

be expected in the case of magma chamber collapse. 

Circular lows could represent impact craters that differ 

from their pristine cousins. Vita-Finzi and others (2005) suggested that almost all coronae represent impact craters, and 

Hamilton (2005) proposed that all coronae, as well as all circular features on Venus, represent impact craters. These geologists 

suggest that coronae may represent impact craters modified by 

erosion. If circular lows were modified by erosion, we might 

expect to see drainage patterns indicative of erosive processes 

showing material transport both toward the basin interior and 

outward from it; such features are not observed. In addition, the 

variable blanketing of the concentric fracture zones by shields 

would not seem to be easily accommodated in such a scenario.

Circular lows within the map area could differ from pristine impact craters due to differences in host rheology during 

initial formation, rather than as a result of postformational 

modifications; or they could represent subsurface processes, 

either negative diapers or subsurface flow. Each of the circular 

lows lies nestled among inliers of ribbon terrain, and these features could have formed during the evolution of ribbon terrain. 

This hypothesis might address (1) the basin morphology; (2) the 

concentric fracture zone relative to basin topography; (3) relative timing and spatial association with ribbon terrain; (4) the 

cookie-cutter form of circular lows relative to adjacent ribbon 

terrain; and (5) the lack of obvious lava flows, radial fractures, 

and evidence of an early uplift history.

Dorsa

Unelanuhi Dorsa, a 150–250-km-wide and over 1,000-kmlong ridge belt within the map area, extends westward for 

~1,000 km into V–22. Unelanuhi Dorsa comprises a northwesttrending linear rise, ~500 m high, that preserves an internal 

fabric marked by closely spaced folds and fractures. Internal 

fold fabrics trend parallel to Unelanuhi Dorsa, as well as at 

a slightly oblique angle; fractures strike more northerly. The 

oblique fractures could have formed prior to dorsa formation 

or synchronous with dorsa uplift. Early-formed fractures would 

have been uplifted and protected from burial within the adjacent 

lowlands. Unelanuhi Dorsa trends northwest, parallel to the 

northeast boundary of Ovda Regio. It is possible that the fold 
13

ridges of this dorsa, and perhaps the dorsa itself, are genetically 

related to Ovda crustal plateau formation; however, given the 

paucity of data such an interpretation is speculative.

Geologic History

A relatively simple geologic history of the map area 

emerges. Basal terrain units formed across the map area in a 

time-transgressive manner, followed by local formation of circular low features. Extensively developed shield terrain evolved 

time-transgressively across the region coupled with formation 

and reactivation of regional fractures, formation of wrinkle 

ridges, and the development of inversion structures. Rosmerta 

Corona flows, smooth flows undivided, and flows from the 

northwest volcano flooded local topographic lows. Impact craters, scattered across the surface, likely formed only after local 

ribbon-tessera-terrain material gained strength enough to act 

in a brittle fashion with regard to bolide impact. Circular lows 

may represent exogenic bolide impact or endogenic subsurface 

processes that formed prior to complete strengthening of local 

ribbon-tessera terrain.

Basal terrain units rtO, rtHG, rtu, and frN occur within the 

same stratigraphically constrained position across the map area, 

and temporal relations among the various basal terrains cannot 

be robustly differentiated. This means that there is no evidence 

to suggest that these terrains form synchronously, nor is there 

evidence that requires that the various basal terrains formed 

in a time-transgressive fashion across the map area. Based on 

the similar orientation in structural fabric of rtHG and rtu, the 

simplest interpretation is that this tessera terrain fabric may 

have formed as a coherent unit across much of the map area, 

particularly in the eastern region. Unit rtO defines a distinctly 

different fabric orientation than units rtHG and rtu, and as such, 

it likely records a spatially separate and likely temporally distinct event. Unit rtO also resides at distinctly higher elevations 

than units rtHG and rtu, which is consistent with the interpretation of a separate evolution of rtO. Basal terrain frN dominates 

kipukas preserved across the northwestern part of the map area. 

Although unit frN might exist under much of the northwestern 

map area, it is likely that unit frN represents a composite unit, 

variably developed in both space and time. In many small exposures, unit frN may represent unit rtu veiled by a thin veneer, 

locally reactivated along rtu structures. Intratessera basin units 

(units itbO and itbHG) formed after the development of their 

associated tessera terrain fabric but before adjacent shield terrain, although it is possible that shield terrain began to form 

concurrently with units itbO and itbHG. 

The mode of formation of ribbon-tessera terrain is outside 

the scope of the current discussion. Ribbon-tessera fabrics have 

been variably interpreted as the result of significant crustal 

shortening associated with the interaction of a mantle downwelling on thin lithosphere (Bindschadler and Parmentier, 1990; 

Bindschadler and others, 1992a,b; Bindschadler, 1995), the 

interactions of a large thermal mantle plume with thin lithosphere (Hansen and others, 1997; Phillips and Hansen, 1998; 

Hansen and Willis, 1998; Ghent and Hansen, 1999; Hansen and 

others, 1999, 2000), and progressive solidification and deformation of the surface of huge lava ponds resulting from massive 

partial melting of the mantle as a result of large bolide impact 

with ancient thin lithosphere (Hansen, 2006). The presence of a 

wide range of extensional and contractional wavelengths from 

Magellan SAR effective resolution to tens of kilometers, and 

synchronous flooding of local topographic lows by low viscosity fluid (lava) favors the lava pond hypothesis (Hansen, 2006). 

The downwelling and plume crustal plateau hypotheses call for 

plateau formation associated with crustal thickening, the former 

by crustal shortening and the later by magmatic accretion. In 

both hypotheses it is postulated that crustal plateaus sit high due 

to thickened crust; lowland ribbon-tessera terrain, such as that 

preserved across the map area, would require collapse of the 

plateau crust with time, which may be difficult to accommodate 

based on modeling (Nunes and others, 2004). In the case of 

plateau formation via crystallization of a bolide-impact-induced 

lava pond, low density mantle that results from massive partial 

melting supports the plateau; and it is possible that the low 

density mantle root could be removed through later mantle circulation processes resulting in plateau subsidence to a lowland 

position (Hansen, 2006). At this point it is premature to dismiss 

any hypotheses of crustal plateau and ribbon terrain formation. Perhaps future studies of ribbon terrain at a global scale 

will lead to hybrid hypotheses that accommodate the relations 

preserved within the map area.

Shield terrain consistently lies stratigraphically above the 

basal terrains at each individual location; thus, shield terrain at 

any one location postdates the formation of basal-terrain units at 

that same location. However, given the lack of robust temporal 

relations (relative or absolute) between the various basal terrain 

units, we cannot robustly state (or deny) that all shield terrain 

postdates the formation of all ribbon-tessera terrain. In addition, 

map relations indicate that unit s developed time-transgressively 

at both the local and regional scale.

The five circular lows formed after the formation of their 

local tessera terrain fabric but before dominant formation of 

shield terrain and the development of north-striking fractures 

at each particular location. It is possible that local shield terrain began to evolve early, but in any case, local shield terrain 

formation continued after the formation of each circular low. 

Northerly striking fracture suites postdated formation of each 

local circular low. Fractures were locally reactivated during the 

evolution of shield terrain. Wrinkle-ridge formation overlaps 

temporally with the development of shield terrain. The temporal location of circular lows forming after local ribbon terrain 

fabric, but before complete development of shield terrain, is 

consistent with circular-low formation via bolide impact on an 

ancient, partially crystalline lava-pond surface.

The eruption of the northwest volcanic material and the 

formation of Rosmerta Corona—marked by domical uplift, 

radial fracture formation, and the emergence of radially distributed lava flows—are unconstrained with regard to one another. 

Both events formed relatively recently in recorded local history, 

possibly overlapping in time with the formation of local shield 

terrain. The pristine impact craters within the map area postdate 

local formation of tessera-terrain tectonic fabric, emplacement 

of units itbO and itbHG, and the formation of circular lows. The 

relative timing of local shield terrain and individual impact cra-
14

ters is difficult to robustly constrain given that individual shields 

could form before and after impact craters. Impact craters that 

have radar-smooth interiors and lack haloes are relatively older 

than impact craters that have haloes and radar-rough interiors 

(Izenberg and others, 1994; Phillips and Izenberg, 1995).

Implications for Venus Lowland Resurfacing Processes

Several hypotheses have emerged to address the near 

random distribution of ~970 apparently pristine impact craters 

across the surface of Venus (Schaber and others, 1992; Phillips and others, 1992; Herrick and others, 1997; Basilevsky 

and others, 1997). Because impact-crater density reflects the 

relative age of planet surfaces, the near random spatial distribution of craters indicates that Venus lacks large regions of very 

young and very old surfaces. The paucity of small craters, due 

to screening by Venus’ thick atmosphere, hampers surface age 

determination. In addition, determination of surface age by 

crater density requires binning across a range of crater diameters—a technique not possible on Venus. A datable surface 

on Venus must exceed ~2 x 107 km2 in order to be statistically robust, based on impact crater density alone (Phillips and 

others, 1992), and areas this large require assumptions with 

regard to surface formation that severely limit the uniqueness 

of any temporal or history interpretation (Campbell, 1999). 

Although some workers combine morphologically similar units 

into large composite regions for crater-density dating (Namiki 

and Solomon, 1994; Price and Suppe, 1994; Price and others, 

1996; Basilevsky and Head, 2002), these studies assume all 

terrains formed at the same time, and lack statistical validity 

(Campbell, 1999).

Resurfacing hypotheses are broadly divisible into two 

groups: (1) catastrophic/episodic and (2) equilibrium/evolutionary (Hansen and Young, 2007). Catastrophic/episodic 

hypotheses propose that a global-scale, temporally punctuated 

event or events dominated Venus’ evolution, which is reflected 

in the generally uniform impact crater distribution (Schaber 

and others, 1992; Strom and others, 1994; Herrick, 1994). 

Equilibrium/evolutionary hypotheses suggest, instead, that the 

generally uniform crater distribution results from relatively continuous resurfacing in which volcanism and (or) tectonism occur 

across the planet through time, although the style of resurfacing 

could vary spatially and temporally at any given locale (Phillips and others, 1992; Guest and Stofan, 1999). Equilibrium/

evolutionary hypotheses could also involve a progression from 

global, steady-state equilibrium resurfacing to global impact 

crater accumulation as a result of secular changes (Solomon, 

1993; Phillips and Hansen, 1998; Bjonnes and others, 2008).

Catastrophic hypotheses include (1) a one-dimensional 

lithospheric instability hypothesis which calls for wholesale 

lithospheric recycling (Parmentier and Hess, 1992; Turcotte, 

1993; Turcotte and others, 1999) and (2) volcanic burial of a 

preserved lithosphere (Schaber and others, 1992; Steinbach and 

Yuen, 1992; Strom and others, 1994). Because these hypotheses 

assume global synchroneity, they predict a single average model 

surface age (AMSA), marking the time of the last lithospheric 

catastrophe. However, Phillips and Izenberg (1995) document 

three AMSA provinces based on impact crater density and 

crater morphology that is inconsistent with catastrophic resurfacing.

Near-global catastrophic flooding is incorporated into the 

global stratigraphy hypothesis (Basilevsky and Head, 1995, 

1998; Head and Basilevsky, 1998), which predicts global eras 

represented by different geologic processes: (1) early, globally extensive and synchronous tessera-terrain deformation 

(and resulting impact crater destruction), (2) local warping and 

crustal thickening, forming isolated high-standing crustal plateaus and the extensive lowlands; and (3) catastrophic emplacement of 1–3-km-thick flood lava burying earlier formed impact 

craters across the lowlands. Wrinkle ridges deform lowland 

flood lavas, whereas the highland regions marked by crustal 

plateaus preserve remnant tessera terrain, preserving the only 

record of precatastrophic flooding processes.

Evolutionary resurfacing hypotheses, which call for equilibrium or steady-state resurfacing, may include different modes 

of impact crater accumulation through time. Phillips (1993) 

proposed an equilibrium volcanic resurfacing hypothesis, 

which calls for near steady-state impact-crater formation and 

burial (destruction) through spatially random volcanic activity 

across local areas (~107 km2, or less). The resulting equilibrium 

resurfacing represents a balance between crater formation and 

burial integrated across space and time. Although the equilibrium volcanic resurfacing hypothesis is dominantly statistical, it 

specifies that craters were removed by volcanic burial; therefore, it can be tested by geologic observations. The equilibrium 

volcanic resurfacing hypothesis does not specifically address 

tessera-terrain formation or any relation between impact craters 

and tessera terrain. Guest and Stofan (1999) propose a nondirectional equilibrium surface evolution in which coronae, rifts, 

wrinkle ridges, small and large edifices, and large flow fields 

formed throughout the portion of Venus’ history revealed by 

presently exposed rock units. They suggest that the lowlands 

accumulated lavas erupted in a range of spatially and temporally transgressive styles. Their hypothesis does not address the 

formation of tessera-terrain units. However, the nondirectional 

evolution hypothesis seems to call for volcanic burial of ancient 

impact craters.

Each hypothesis calls for emplacement of thick lava flows 

(~1 km), whether emplaced catastrophically or partitioned over 

time, across large expanses of the lowlands in order to bury preexisting impact craters. We can test the requirement of such a 

thick (1 km) layer across much of the map area. Unit rtu occurs 

in small exposures across much of the map area. In many cases 

unit rtu is exposed in regions of locally higher (although still 

quite gentle) topographic slopes. The outcrop of unit rtu north 

of Barrera Crater (lat 18.5° N., long 109.5° E.) has an average 

slope of ~0.286° and shows a break in slope just outside the 

shield-rich region. Rounding this slope to 0.3°, we determine a 

1-km isopach surface, indicating where ribbon-tessera terrain 

could lie beneath a 1-km-thick cover and, therefore, harbor 

buried impact craters. The majority of the map area cannot 

accommodate a 1-km-thick layer stratigraphically above ribbontessera terrain.
15

A similar isopach constructed for unit frN is also shown 

on the geologic map. The slopes associated with exposures of 

unit frN are generally less than the slopes associated with unit 

rtu, therefore maximizing the region that could hide buried 

impact craters. Despite the conservative approach of this 

exercise, taken together, the two isopach contours indicate that 

essentially no part of the map area could accommodate a layer 

>1 km thick stratigraphically above ribbon-tessera terrain or 

the fracture terrain units. Yet both of these units are predicted 

to predate the emplacement of the 1–3 km thick regional plains 

required within the context of the catastrophic resurfacing and 

global stratigraphy hypotheses. Neither the volcanic equilibrium resurfacing hypothesis nor the nondirectional hypothesis 

directly imply that a thick layer lies above tessera terrain, yet 

both hypotheses call for thick lava packages to bury early 

formed impact craters. Each of these hypotheses require large 

areas of the Venusian surface to have thick regional plains, but 

the overall shape of the regional plains at such depths is important. The occurrence of exposures of basal units, those inferred 

to predate regional plains (units rtu, rtHG, rtO, frN) across 

the map area, indicates that first-order geologic map relations 

within the map area are inconsistent with the predictions or 

requirements of the catastrophic resurfacing, global stratigraphy, volcanic equilibrium resurfacing, and nondirectional 

resurfacing hypotheses.

Similar first-order map relations exist across the map area, 

in V–24 to the east and in V–11 and V–12 to the north (Aubele, 

1996). Across each of these four adjacent quadrangles, a thin 

veneer of shield terrain lies directly above ribbon-tessera terrain 

and (or) fracture terrain, presenting a major challenge to all 

resurfacing hypotheses that call for burial of impact craters. In 

fact, these first-order map relations provide strong evidence that 

impact craters could not have formed on surfaces marked by 

ribbon-tessera terrain and fracture terrain and have been subsequently buried.

Solomon (1993) proposed a hybrid two-stage surface 

history for Venus that did not require burial of impact craters. 

Solomon (1993) called for viscous relaxation of ancient impact 

craters, followed by later crater accumulation. According to 

his hypothesis, early steady-state impact-crater formation and 

destruction resulted from global-scale tectonic deformation 

(viscous relation of impact craters) driven by extremely high 

heat flow; cooling led to a change in crustal rheology and 

subsequent accumulation of impact craters across the surface. 

In this hypothesis, Venus’ surface could not preserve impact 

craters until it cooled significantly; once cooled, the surface 

accumulated craters since the time of the global rheological 

transition, interpreted as the global AMSA. This hypothesis 

does not require the emplacement of thick regional plains, 

because it does not require impact crater burial. It does, 

however, require that the surface of Venus was rheologically 

weak at the global scale. Subsequent to the proposal of this 

hypothesis, new flow laws determined for ultra-dry diabase 

(Mackwell and others, 1998) indicated that Venus’ crust is too 

strong, even at elevated surface temperature, to allow for early 

viscous relaxation of impact craters as hypothesized. Although 

Solomon’s (1993) hypothesis seems to have been abandoned in 

light of the new flow laws, the spirit of the hypothesis might be 

feasible if ancient Venus was wetter than contemporary Venus, 

resulting in possible regional or local viscous flow, perhaps 

enhanced by local thermal anomalies. Given that Solomon’s 

hypothesis does not require a thick lava package to bury impact 

craters, this hypothesis remains viable based on map area relations.

The spirit of Solomon’s (1993) hypothesis might also be 

preserved in a modified hypothesis in which craters could be 

destroyed (rather than buried) at local spatial scale and in a 

time-transgressive fashion. Hansen and Young (2007) proposed 

such a two-stage resurfacing hypothesis. That hypothesis, herein 

called the SPatially Isolated Time-Transgressive Equilibrium 

Resurfacing (SPITTER) hypothesis, calls for early equilibrium 

resurfacing, with a later transition to crater accumulation. In 

contrast to Solomon, who calls for global formation of tessera 

terrain, these workers call on spatially isolated, time-transgressive destruction of ancient impact craters through local formation of ribbon terrain. Coherent tracts of tessera terrain could 

have evolved in a spatially isolated but time-transgressive fashion (Bindschadler, 1995; Hansen and Willis, 1996, 1998) and as 

allowed by the relations across the map area (and V–24). The 

SPITTER hypothesis does not depend on a particular mechanism of ribbon-tessera formation (such as downwelling, plume, 

or impact-induced lava pond), but rather focuses on the similarities of each of these hypotheses: complete crater destruction in a 

spatially isolated yet time-transgressive fashion.

According to the SPITTER hypothesis, crustal plateau 

formation and subsequent collapse to form lowland tessera 

terrain, coupled with a transition from thin to thick lithosphere, 

might accommodate the various observations and correlations 

across Venus. Observations related to impact craters include the 

following:

1. Venus shows few partially flooded and few faulted craters 

(Herrick and others, 1997); most craters appear pristine. 

Although studies indicate some modification (Sharpton, 

1994; Wichman, 1999; Herrick and Sharpton, 2000), examples of near complete flooding of craters are rare. Thus, 

crater destruction was an all or nothing endeavor. Craters 

are not preserved, form but viscously relax, are completely 

destroyed by tectonism, are buried beyond recognition, or 

are preserved in a near-pristine state.

2. Large craters (>100 km diameter) have rim-terrain heights 

of ~650 m (Herrick and Sharpton, 2000); thus, burial 

requires strata >650 m. Presumably ancient craters would 

far exceed Venus’ largest crater and likely require ~1-kmthick strata for burial.

3. Mapping across V–23 and V–24 indicates that large tracts 

of lowland tessera show thin blanketing of strata (this 

study). Additionally, huge lowland regions (>106 km2) are 

covered by thin layers or flows tens of meters to 100 m 

thick (Guest and others, 1992; Aubele, 1996; DeShon and 

others, 2000; Addington, 2001; Hansen and DeShon, 2002; 

Stofan and others, 2005).

4. Craters on ribbon-tessera terrain postdate tessera fabric 

formation (Gilmore and others, 1997).

5. Craters record a temporal sequence of degradation in 

which halos are lost and troughs and interiors fill with 

smooth material over time (Izenberg and others, 1994; 
16

Herrick and Sharpton, 2000); thus, craters are divisible 

into broad relative age groups. Young craters display 

haloes and rough interiors. Old craters lack haloes and 

have smooth interiors. Intermediate craters lack haloes and 

display rough interiors.

6. Crater density combined with crater morphology (5) allows 

delineation of three AMSA provinces (Phillips and Izenberg, 1995). Spatial correlation of geologic time-dependent 

criteria (halos) with crater density suggests that AMSA 

provinces reflect true temporal domains, not stochastic 

fluctuations in a random distribution (Campbell, 1999).

In addition, comparison of AMSA provinces with major geologic (Hansen and Young, 2007) features reveals several 

observations:

7. Volcanic rises show no correlation with AMSA.

8. Crustal plateaus correlate mostly with intermediate AMSA.

9. Many lowlands correlate with old AMSA.

10. Beta-Atla-Themis and Lada, regions of volcanotectonic 

activity (Crumpler and others, 1997), correlate with young 

AMSA.

SPITTER (fig. 12) accommodates observations 1–10 

above. The global AMSA might record, broadly, the time of a 

change from near steady-state crater formation and destruction 

(thin lithosphere) to crater accumulation (thick lithosphere), 

whereas the distinct AMSA provinces occur as outlined above. 

The SPITTER hypothesis accounts for complete local crater 

destruction and for pristine craters (1, 2), yet it accommodates 

the occurrence of thin, rather than thick, lava flows (3). It 

predicts that craters should not record tessera deformation (4), 

and it allows for the preservation of stages of crater degradation 

(5) and the occurrence of distinct AMSA provinces (6), with 

specific geologic relations (7–10).

The SPITTER hypothesis embodies the spirit of the 

equilibrium volcanic hypothesis (Phillips, 1993), accommodates broad aspects of the view that volcanotectonic processes 

occurred across Venus through time, though styles and locations 

have changed (Guest and Stofan, 1999), and incorporates a 

secular change in resurfacing and dynamic processes (Solomon, 

1993; Grimm, 1994; Solomatov and Moresi, 1996; Schubert and 

others, 1997; Phillips and Hansen, 1998), which could address 

some aspects of a broadly defined geologic history that might 

accommodate some eras highlighted by the global stratigraphy 

hypothesis (Basilevsky and Head, 1995, 1998, 2002).

Summary

In summary, shield terrain comprises the most prominent 

unit across the map area. Shield terrain forms a thin lace-like 

veneer of shields and low viscosity shield paint; it occurs across 

a range of elevations; it postdates and lies directly upon ribbon 

terrain in the eastern part of the map area and on fracture terrain 

in the western part of the map area. In both cases, shield terrain locally covers or veils the units that serve as its basement. 

Shield terrain apparently formed by local point-source eruption 

across the map area. Shield paint coalesced into a mechanically coherent layer that was able to fracture and to deform into 

wrinkle ridges and polygonal fabrics.

The interactions among fractures, wrinkle ridges, and 

shield-terrain emplacement imply diachronous wrinkle ridge 

evolution, fracture reactivation, and shield-terrain evolution. 

Locally north-trending ridges represent inversion structures 

formed as a result of the filling of north-striking fractures with 

shield-terrain material and subsequent regional contraction and 

structural inversion of the fill into north-trending ridges. The 

interaction of shield terrain, wrinkle ridges, and fractures also 

indicates that shield terrain material forms a thin veneer, rather 

than a thick layer.

Ribbon-tessera terrain is divisible into two or three distinct 

packages based on fabric orientation and elevation. Ribbontessera terrain of Ovda Regio likely formed temporally separate 

from ribbon terrain of Haasttse-baad-Gegute Tesserae, which 

formed distinct from Thetis Regio ribbon-tessera terrain to the 

south in V–36. Undivided ribbon-tessera terrain displays patterns in its tectonic fabric that might indicate a genetic relation 

with Haasttse-baad-Gegute Tesserae. Circular lows, a distinctive 

class of coronae features marked by circular basins and concentric fractures zones, differ from many coronae, notably lacking radial fractures, long lava flows, and positive topography. 

Circular lows might represent a different class and perhaps a 

different origin from other coronae. Circular lows may represent 

impact craters formed late during the evolution of local ribbontessera terrain. Such a proposal will require testing by additional 

geologic mapping across the planet. Geologic relations across 

the map area do not support the occurrence of equilibrium volcanic resurfacing, catastrophic global resurfacing, or near-global 

(global stratigraphy) resurfacing, all of which require burial of 

ancient impact craters.
17

Figure 12. Illustration showing proposed evolution of Venusian surface. A. Time-step cartoons (t1–t9) illustrating the evolution of the SPITTER hypothesis for Venus resurfacing. t1, Initially the surface accumulates impact craters. t2, As a crustal plateau forms on thin lithosphere, 

impact craters within the plateau aureole are destroyed; t3, characteristic ribbon-terrain fabrics mark the plateau, which can accumulate new 

impact craters. t4, A new, spatially separate plateau forms on thin lithosphere, erasing impact craters within its aureole. t5, As the second 

plateau evolves and becomes able to preserve newly formed impact craters, the first formed plateau could topographically decay. But ribbontessera fabrics survive, except where buried by local thin volcanic material; impact craters are not buried. t6–t7, As new plateaus form on 

the thin lithosphere, impact craters are removed locally and earlier formed plateaus are variably covered with thin flows. t8, These processes 

continue until a secular change results in globally thick lithosphere. Crustal plateaus that formed just prior to the secular change stand high. 

Crustal plateaus cannot form on thick lithosphere by any of the three hypothesized mechanisms; however, volcanic rises could form above a 

large thermal mantle diapir (dashed circle). During volcanic rise formation, impact craters are not erased, because the lithosphere is too thick 

for crater destruction via subsurface processes. Volcanic rise formation raises the local altitude of the surface but does not affect impact crater 

density, except by local volcanotectonic processes. The entire global surface becomes one of impact crater accumulation (t8, t9), except 

local areas of concentrated volcanotectonic activity where impact craters are buried by surface lava flows, such as the Beta-Alta-Themis and 

Lada regions (outside of map area). B. A composite cartoon surface schematically illustrating three average model surfacing age (AMSA) 

provinces that would emerge from surface evolution described in A. Old AMSA provinces would include lowland regions where ancient 

crustal plateaus formed (tessera-terrain inliers) or no crustal plateaus form (black stipple). Young AMSA provinces would include areas 

affected by recent volcanotectonic activity (dark gray shaded area) that buried both impact craters and their halos (white stipple), such as 

Beta-Atla-Themis and Lada regions. Intermediate AMSA provinces would include high-standing crustal plateaus and lowland regions with 

tessera-terrain inliers marking ancient crustal plateaus. Boundaries between AMSA provinces would not be sharp as illustrated. Note that 

volcanic rises could correlate with any of the AMSA provinces, because volcanic rise formation would only affect the local AMSA if rise 

formation included extensive volcanotectonic burial of impact craters. In general, rise formation simply elevates a pre-existing surface but 

does not otherwise affect impact crater density or morphology. Rectangle outlines area conceptually similar to the surface history of the map 

area. Figure modified from Hansen and Young (2007).

lithosphere

thickness transition

crustal plateaus

before; volcanic

rises after

 equilibrium

resurfacing

shut-down

A

B

INTERMEDIATE OLD

t1 t2 t3 t4 t5

t6 t7 t8 t9

YOUNG
18

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Table 1. Numbers and densities of definite shields (DS) and combined shields 

(CS), which consist of definite and potential shields, in the Niobe Planitia 

quadrangle (V–23).

Centered at

lat/long

DS CS DS/106 km2 CS/106 km2

23° N./103° E. 402 626 10,500 15,650

21° N./113° E. 167 1,080 4,175 27,000

13° N./111° E. 142 1,347 3,550 33,675

23° N./117° E. 251 1,005 6,275 25,125

13° N./119° E. 271 631 7,078 15,775
22

Table 2. V–23 impact craters.

Name

Latitude

(deg N.)

Longitude

(deg E.)

Diameter

(km)

C

density*

Unit

Location

Ejecta

blanket

Impact

halo

Central

peak Rim

Inte

floo

Millay 24.4 111.2 48.0 2.86479 st Y N N Y Y 

LiQuingzhao 23.7 94.6 22.8 2.54648 st Y Y Y Y YCaldwell 23.6 112.4 51.0 3.1831 st Y Y N Y N Horner 23.4 97.8 25.2 2.86479 st Y N Y Y Y crater a 22.6 94.1 3.0 2.22817 st Y N N Y N Kiris 20.9 98.8 13.3 2.22817 st Y N N Y Y Zulfiya 18.4 101.9 12.9 2.54648 st Y N N Y N Hannah 17.9 102.6 19.8 2.54648 st Y N Y Y Y Barrera 16.7 109.4 26.8 3.1831 st Y N Y Y Y Ferrier 15.7 111.3 19.1 3.1831 st Y N Y Y Y crater b 15.1 116.8 4.5 3.50141 st Y N N Y ? 

crater c 13.3 112.8 4.2 1.90986 st Y N N Y N Adzoba 12.8 117.0 10.0 3.1831 st Y N N Y Y Merit Ptah 11.4 115.6 16.5 2.86479 st Y N N Y N Gregory 7.1 95.8 18.0 3.1831 st Y N N Y Y Icheko 6.6 97.9 5.9 2.86479 st Y N Y Y N Chapelle 6.4 298.3 20.8 2.22817 st Y N Y Y Y Abra 6.2 282.91 7.2 2.86479 st ? ? ? ? ? Susanna 6.0 287.75 13.3 3.50141 st, rttO Y N N Y N

Hepworth 5.1 289.95 62.6 3.50141 rttO, itbO Y N Y Y Y 

Tsiala 2.9 280.3 16.5 3.81972 rttO partial N Y Y Y 

Doris 2.3 277.0 14.5 3.50141 rttO minor N N Y Y 

deBeauvoir 2.0 283.8 52.5 3.81972 rttO Y N Y Y Y 

Estella 1.1 285.27 18.8 3.50141 rttO Y N Y Y Y 

*Crater density (values from Herrick and others, 1997) at a crater’s location (Rho). Value is the density of craters in the neighborhood of the specified crat

by counting the number of craters (including the specified crater) within a 1000 km radius circle, and normalizing to give the number of craters per 1 x 10