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90° 120° 150° 180° 210° 240° 270°

–25°

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–75° –75°

QUADRANGLE LOCATION

Photomosaic showing location of map area. An outline of 

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

V–1

V–26 V–27 V–25

V–37

V–62

V–38

V–24

V–23

V–12

V–11

V–13

V–4

V–36 V–39

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V–49 V–50

V–5

V–58 V–59

V–3 V–6

V–57 V–60

V–16

V–47 V–52

V–28

V–35 V–40

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Prepared for the

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

GEOLOGIC INVESTIGATIONS SERIES I–2792

ATLAS OF VENUS: ATALANTA PLANITIA QUADRANGLE (V–4)

SCALE 1:4 711 886 (1 mm = 4.712 km) AT 25° LATITUDE

1:5 000 000 AT 34° and 73° LATITUDES

LAMBERT PROJECTION

Prepared on behalf of the Planetary Geology and Geophysics Program, Solar 

System Exploration Division, Office of Space Science, National Aeronautics and 

Space Administration

Edited by Derrick D. Hirsch; cartography by Darlene A. Ryan and Beatrice C. 

Landavazo

Manuscript approved for publication May 7, 2003

GEOLOGIC MAP OF THE ATALANTA PLANITIA QUADRANGLE (V–4), VENUS

By

Mikhail Ivanov and James W. Head

2004

50°

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PLAINS MATERIALS CRATER MATERIALS BELT

MATERIALS

TESSERA

MATERIALS

CORRELATION OF MAP UNITS

Table 1. List of large volcanoes in Atalanta Planitia quadrangle

(from Crumpler and Aubele, 2000).

 Volcano Latitude Longitude Diameter

 Melia Mons 63° N. 120° E. 300 km

 Jael Mons 51° N. 121° E. <300 km

Table 2. List of impact craters in Atalanta Planitia quadrangle

(from Schaber and others, 1998).

 Crater Latitude Longitude Diameter (km)

 Dickinson 74.6° N. 177.2° E. 67.5

 Sayligul 73.6° N. 172.9° E. 4.3

 Monika 72.3° N. 122.4° E. 25.5

 Erika 72.0° N. 175.4° E. 10.5

 Radmila 69.1° N. 167.0° E. 5.2

 Selma 68.5° N. 155.9° E. 11.4

 Unnamed 67.6° N. 157.2° E. 1.5

 Zhilova 66.3° N. 125.7° E. 53.0

 Hapei 66.1° N. 178.0° E. 4.2

 Jex-Blake 65.4° N. 169.3° E. 31.6

 Tsvetayeva 64.6° N. 147.4° E. 42.9

 Koidula 64.2° N. 139.6° E. 67.0

 Rani 64.1° N. 160.4° E. 10.7

 Rafiga 62.9° N. 175.6° E. 5.7

 Kumudu 61.3° N. 154.1° E. 4.4

 Unnamed 60.4° N. 137.2° E. 1.7

 Ermolova 60.3° N. 154.4° E. 60.9

 Sandugach 59.9° N. 143.5° E. 10.0

 Bineta 57.3° N. 144.1° E. 10.7

 Unnamed 55.0° N. 124.5° E. 3.2

 Khadako 54.2° N. 139.3° E. 7.4

 Unay 53.5° N. 172.7° E. 11.4

 Unnamed 52.1° N. 123.2° E. 3.2

 Cochran 51.9° N. 143.4° E. 100.0

 Irinuca 51.4° N. 121.9° E. 8.0

 Rampyari 50.6° N. 179.3° E. 7.7

DESCRIPTION OF MAP UNITS

[Map units are defined, characterized, and interpreted on the basis of radar backscatter; surface texture and morphology; associated features of apparent volcanic and tectonic origin; and roughness, 

reflectivity, emissivity, and topography. Although most map units are rock materials, some are 

defined on the basis of pervasive tectonic structure that obscure the underlying materials and become 

part of the characteristics of the surface morphology. Type localities are indicated for units previously defined (for example, Aubele, 1994, 1995; Basilevsky and Head, 1995b; Head and Ivanov, 

1996) and reference localities are indicated for units in the quadrangle]

PLAINS MATERIALS

Lobate plains material—Homogeneous plains material having internal elements arranged in parallel to sinuous to lobate or digitate radar bright and 

dark strips and patches; unit boundaries are typically lobate. Type locality: 

5.5° N., 196.0° E.; reference locality: 60.0° N., 127.0° E. Interpretation: 

Complexes of lava flows undisturbed by subsequent deformation

Smooth plains material—Smooth plains material of uniform, generally low 

to intermediate backscatter. Type locality: 3.5° N., 198.2° E.; reference 

locality: 66.5° N., 135.0° E. Interpretation: Volcanic plains material 

unmodified by subsequent deformation; wind-blown material in places

Wrinkle-ridged plains material—Homogeneous plains material of intermediate dark to intermediate bright radar backscatter complicated by narrow linear to anastomosing wrinkle ridges in parallel lines or intersecting 

networks. Type locality: 8.0° N., 177.0° E. Interpretation: Regional plains 

material of volcanic origin deformed by wrinkle ridges; sources of plains 

not obvious. Some small shield-shaped features appear to be kipukas of 

shield plains material (unit psh)

Upper unit—Plains material of generally high radar backscatter commonly 

with lobate boundaries. Embays and is modified by wrinkle ridges. Reference locality: 51.5° N., 158.5° E. Interpretation: Lava flows subsequently 

deformed by wrinkle ridges

Lower unit—Plains material of generally low radar backscatter; commonly 

mottled locally. Modified by wrinkle ridges. Reference locality: 64.0° N., 

168.0° E. Interpretation: Lava flows subsequently deformed by wrinkle 

ridges

Shield plains material—Plains material of high to low radar backscatter characterized by abundant small shield-shaped features (a few up to 10–20 km 

diameter) commonly having summit pits. Shields occur in clusters, giving 

unit a locally hilly texture, and as isolated outcrops in relatively smooth 

plains. Unit commonly occurs on densely lineated plains material; in places crossed by wrinkle ridges. Reference locality: 59.5° N., 158.0° E. 

Interpretation: Shields are interpreted to be of volcanic origin and are 

likely to be the sources of adjacent plains material

Ridged and grooved plains material—Characterized by uniform and intermediate radar backscatter plains having generally densely spaced, sinuous 

ridges up to 5–10 km wide and several tens of kilometers long. Arranged 

in linear belts (250–300 km long and 50–100 km wide). Type locality: 

37.7° N., 396.6° E.; reference locality: 54.5° N., 179.3° E. Interpretation: 

Volcanic plains material deformed into ridgelike belts by compression

Densely lineated plains material—Predominantly flat plainslike material 

intensely lineated by closely spaced narrow parallel lineaments 10-20 km 

long and less than 1 km wide, anastomosing patterns in places; with a few 

exceptions, typically radar bright due to dense fractures. Type locality: 

48.5° N., 15.0° E.; reference locality: 53.5° N., 143.5° E. Interpretation: 

Volcanic plains material very highly modified by fractures of probable 

extensional origin. Local changes in fracture density suggest some resurfacing during formation

BELT MATERIAL

Groove belt material—Characterized by numerous short and long curvilinear 

subparallel lineaments that are typically wide enough to be resolved as 

fractures and grabens. Forms curvilinear belts up to 500 km long and 

150–200 km wide that are characterized by generally high topography but 

with associated linear depressions. Remnants of preexisting plains can be 

seen at higher resolution but are not mappable at this scale. Distinguished 

from densely lineated plains material by type of fractures and beltlike 

form. Type locality: 70. 0° N., 133.0° E. Interpretation: Various materials 

deformed into broad belts; fractures formed largely by extension

TESSERA MATERIAL

Tessera material—Radar bright and dominated by closely spaced ridges and 

grooves oriented in at least two directions, commonly orthogonally; terrain typically elevated. Embayed by plains units. Type locality: 67.5° N., 

20.0° E.; reference locality: 53.0° N., 138.5° E. Interpretation: Material 

intensely deformed by compression and extension

CRATER MATERIALS

Crater material, undivided—Craterform material of various diameters; 

includes central peaks, floor, walls, rim, and ejecta but lacks dark paraboloid. Reference locality: 60.0° N., 155.0° E. Interpretation: Impact crater 

and associated ejecta that has lost parabola through erosion or formed 

without one

Crater flow material—Characterized by high radar backscatter; lobate margins common. Reference locality: 52.0° N.; 142.0° E. Interpretation: Outflows from impact craters

Contact—Dashed where approximately located, queried where inferred

Ridge

Wrinkle ridge

Sharp groove

Subdued groove

Flow front—Arrow indicates flow direction

Channel

Corona, coronalike feature

Rim of impact crater (>10 km diameter)

Central peak of impact crater

Rim of impact crater (<10 km diameter)

The Magellan Mission

The Magellan spacecraft orbited Venus from August 10, 1990, until it plunged into the 

venusian atmosphere on October 12, 1994. Magellan had the objectives of (1) improving 

knowledge of the geologic processes, surface properties, and geologic history of Venus by 

analysis of surface radar characteristics, topography, and morphology and (2) improving 

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 datasets: synthetic aperture 

radar (SAR) images of the surface, passive microwave thermal emission observations, and 

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 done in mission cycles 1, 2, and 3, from September 1990 until 

September 1992. Ninety-eight percent of the surface was mapped with radar resolution of 

approximately 120 meters. The SAR observations were projected to a 75-m nominal horizontal resolution; 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 from about 20° to 45°.

High-resolution Doppler tracking of the spacecraft was done from September 1992 

through October 1994 (mission cycles 4, 5, 6). High-resolution gravity observations from 

about 950 orbits were obtained between September 1992 and May 1993, while Magellan 

was in an elliptical orbit with a periapsis near 175 kilometers and an apoapsis near 8,000 

kilometers. Observations from an additional 1,500 orbits were obtained following orbitcircularization in mid-1993. These data exist as a 75° by 75° harmonic field.

Magellan Radar Data

Radar backscatter power is determined by the morphology of the surface at a broad 

range of scales and by the intrinsic reflectivity, or dielectric constant, of the material. Topography at scales of several meters and larger can produce quasi-specular echoes, with the 

strength of the return 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 

thermodynamically stable. This effect 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. As such, 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 topography data produced by this technique have horizontal footprint sizes of about 

10 km near periapsis and a vertical resolution of approximately 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.

INTRODUCTION

The Atalanta Planitia quadrangle (V–4) is in the northern hemisphere of Venus and 

extends from 50° to 75° north latitude and from 120° to 180° east longitude. It covers the 

central and western part of Atalanta Planitia and parts of its margins. Atalanta Planitia consists of a centralized deformed lowland flooded by volcanic deposits and surrounded by 

Nightingale and Earhart Coronae in Tethus Regio to the west (Pronin and Stofan, 1990; 

Stofan and Head, 1990), Ananke Tessera (Ivanov and Head, 1996) and Vellamo Planitia 

(Aubele, 1994, 1995) to the southwest, an extensive zone of ridge belt to the east (Kryuchkov, 1990; Frank and Head, 1990), Vellamo and Ganiki Planitiae to the south (Aubele, 

1994, 1995), Louhi Planitia to the northwest, and Lukelong Dorsa to the northeast 

(Kryuchkov, 1990; Frank and Head, 1990). In contrast to more elongate lowland areas on 

Venus, the Atalanta Planitia area is one of several large, somewhat equidimensional lowlands (basins) and as such is an important region for the analysis of processes of basin formation and volcanic flooding.

Before the Magellan mission, Atalanta Planitia was known on the basis of Pioneer 

Venus altimetry to be a lowland area (Pettengill and others, 1980); Venera 15 and 16 radar 

images showed that it was surrounded by several coronae and coronalike features from the 

west and by numerous ridge belts parallel to the basin margin to the east (Barsukov and 

others, 1986; Sukhanov and others, 1989). Venera 15 and 16 data also revealed that the 

interior was populated with complex patterns of deformational features in the form of belts 

of ridges and volcanic plains, and several regions along the margins were seen to be the 

sources of large radar-bright areas, interpreted to be lava flows (Barsukov and others, 

1986; Sukhanov and others, 1989). Early Magellan results showed that the ridge belts are 

composed of complex deformational structures interpreted as predominantly of contractional origin (Squyres and others, 1992; Solomon and others, 1992) and that the complex 

lava flows along the margins emanated from coronae at western and southwestern edges of 

Atalanta Planitia (Head and others, 1992). In addition, global analysis of the distribution 

of volcanic features revealed that the interior of Atalanta Planitia is somewhat deficient in 

distinctive volcanic sources and coronalike features (Head and others, 1992; Crumpler and 

others, 1993). Atalanta Planitia gravity and geoid data show that the lowland is characterized by a –35 mGal gravity anomaly and a –55 m geoid anomaly, centered on eastern Atalanta (Bindschadler and others, 1992b; Konopliv and Sjogren, 1994). Indeed, the characteristics and configuration of Atalanta Planitia have been cited as evidence for the region 

being the site of large-scale mantle downwelling (Bindschadler and others, 1992b). Thus, 

this region is a laboratory for the study of the formation of lowlands, the emplacement of 

volcanic plains, the formation of associated tectonic features, and their relation to mantle 

processes. These questions and issues are the basis for our geologic mapping analysis.

In our analysis we have focused on the geologic mapping of the Atalanta Planitia 

quadrangle using traditional methods of geologic unit definition and characterization for 

Earth (for example, American Commission on Stratigraphic Nomenclature, 1961) and 

planets (for example, Wilhelms, 1990) appropriately modified for radar data (Tanaka, 

1994). We defined units and mapped key relations using the full-resolution Magellan synthetic aperture radar (SAR) data (mosaicked full-resolution basic image data records, C1-

MIDR’s, F-MIDR’s, and FMAP’s) and transferred these results to the base map compiled 

at a scale of 1:5 million. In addition to the SAR image data, we incorporated into our analyses digital versions of Magellan altimetry, emissivity, Fresnel reflectivity, and roughness 

data (root mean square, rms, slope) to characterize the surface properties of the units mapped. The background for our unit definition and characterization is described in Tanaka 

(1994), Basilevsky and Head (1995a, b), and Basilevsky and others (1996).

Acknowledgments. Thanks are extended to Alexander Basilevsky and Jayne Aubele 

for detailed discussion about regional and global units and relations. We particularly 

appreciate the helpful discussions that occurred during the annual Venus Mappers meetings and during an informal mapping workshop held at Brown University in the summer 

of 1995 and attended by George McGill, Steve Saunders, Jim Zimbelman, Alexander Basilevsky, Elizabeth Rosenberg, and Nathan Bridges. Their comments and discussions were 

most helpful in this effort. Jayne Aubele and Nathan Bridges provided detailed and very 

helpful reviews of the map and map text. Financial support from the NASA Planetary 

Geology and Geophysics Program (Grant NAGW-5023) is gratefully acknowledged.

MAGELLAN SAR AND RELATED DATA

The synthetic aperture radar (SAR) instrument flown on the Magellan spacecraft (12.6 

cm, S-band) provided the image data used in this mapping and interpretation. SAR images 

are a record of the echo (radar energy returned to the antenna), which is influenced by surface composition, slope, and wavelength-scale surface roughness. Viewing and illumination geometry also will influence the appearance of surface features in SAR images. 

Guidelines for geologic mapping using Magellan SAR images and detailed background to 

aid in their interpretation can be found in Elachi (1987), Saunders and others (1992), Ford 

and Pettengill (1992), Tyler and others (1992), and Tanaka (1994). In the area of the Atalanta Planitia quadrangle, incidence angles are such that backscatter is dominated by variations in surface roughness at wavelength scales. Rough surfaces appear bright, whereas 

smooth surfaces appear dark. Variations also occur depending on the orientation of features relative to the incident radiation (illumination direction), with features normal to the 

illumination direction being more prominent than those oriented parallel to it. Full-resolution images have a pixel size of 75 m; C1-MIDR’s contain the SAR data displayed at ~225 

m/pixel. Altimetry data and stereo images were of extreme importance in establishing geologic and stratigraphic relations between units. Also essential in the analysis of the geology of the surface are data obtained by Magellan on the emissivity (passive thermal 

radiation), reflectivity (surface electrical properties), and rms slope (distribution of radar 

wavelength scale slopes). Aspects of these measurements were used in unit characterization and interpretation; background on the characteristics of these data and their interpretation can be found in Saunders and others (1992), Ford and Pettengill (1992), Tyler and 

others (1992), Tanaka (1994), and Campbell (1995).

GENERAL GEOLOGY

Several different geologic processes have influenced the Atalanta Planitia region and 

have combined to form its geologic record. Volcanism is the dominant process of crustal 

formation on Venus (Head and others, 1992) and production of the observed geologic units 

in this map area. Tectonic activity modified some of these basic crustal materials (for 

example, Solomon and others, 1992; Squyres and others, 1992) in a variety of modes (extension, contraction, and shear), and in places deformation is so extensive, as in the case of 

tessera terrain, that the deformational features become part of the definition of the material 

unit, as has been done for Mars (see also Tanaka, 1994; Scott and Tanaka, 1986). Impact 

cratering also has locally influenced regions in the quadrangle, most notably in the area 

dominated by Monika and Dickinson craters, but in general has not been an influential 

process over the quadrangle as a whole. Eolian processes require a source of sediment to 

produce deposits, and thus deposits are concentrated around impact craters and localized 

around tectonic fractures and scarps (for example, Greeley and others, 1992).

The eastern and northern flanks of two large volcanic edifices, Melia Mons and Jael 

Mons (table 1; Head and others, 1992; Crumpler and others, 1993), are visible at the very 

western edge and in the southwestern corner of the Atalanta Planitia quadrangle, respectively. Small shield volcanoes less than about 15 km diameter (Guest and others, 1992; 

Aubele and Slyuta, 1990; Crumpler and others, 1997) are common. The small shields are 

low in elevation, commonly have a summit pit, do not appear to have distinct associated 

flows, and are commonly embayed by subsequent regional plains deposits. Although not 

continuous, concentrations of small shield volcanoes occur over all parts of the quadrangle. No steep-sided domes (Pavri and others, 1992) have been found in the Atalanta Planitia quadrangle. Coronae are not well developed or abundant in the lowlands of Atalanta 

Planitia, but four coronae—Earhart, Nightingale, Mari, and Holde—and one coronalike 

feature—Ciuacoatl Mons—are present (fig. 1) in the western and southern topographically 

higher part of the quadrangle. The 370-km-diameter Earhart Corona (71.0° N., 136.0° E.) 

and 560-km by 480-km Nightingale Corona (63.0° N., 130.0° E.) are exposed in the western part of the quadrangle and are two of several making up a cluster of coronae and coronalike features in Tethus Regio outside the map area (Barsukov and others, 1986; Stofan 

and Head, 1990; Stofan and others, 1992). Extensive lobate lava flows emanate from these 

features and dominate the northwestern part of the quadrangle. Other lobate lava flows 

enter the quadrangle from its southern edge, near one intermediate volcano and three small 

coronae. Sources along ridge belts in the Pandrosos Dorsa area also produce lobate lava 

flows, which enter the quadrangle from its eastern margin. Most of the quadrangle comprises relatively homogeneous plains interpreted to be of volcanic origin and modified by 

tectonic structures to varying, but usually low, degrees. The source vents for these plains 

are not well known. Some of these plains display radar backscatter variations and apparent 

flow fronts that permit stratigraphic distinctions among subunits. The composition of these 

volcanic plains is not known from data in this quadrangle, although Venera 9 and 10 

(northeastern and southeastern slope of Beta Regio, respectively) and Vega 2 (Rusalka 

Planitia) lander geochemical analyses of sites in similar terrains suggest compositions 

similar to terrestrial tholeiitic basalt (Basilevsky and others, 1992).

Tectonic features in the Atalanta Planitia quadrangle include individual narrow (less 

than about 1–2 km) grabens hundreds of kilometers long, concentrations of parallel narrow grabens into belts, and some concentrations in such high density that the underlying 

terrain is completely obscured.

Wrinkle ridges are widespread throughout the plains. These structures are sinuous and 

narrow (about a few kilometers wide). Their typical length is several tens of kilometers. 

Ridges of longer wavelength are known as arches and ridge belts. Wrinkle ridges, arches, 

and ridge belts manifest compressional deformation (anticline and thrust faults).

No evidence for shear deformation exists within the quadrangle. In some places, several tectonic styles have operated together or in sequence to produce terrain more heavily 

deformed than typical regional plains. The interior and annulus of Nightingale Corona, 

where fractures and grabens coexist, is an example of this. An extreme example is in 

Ananke Tessera, in the southwest part of the quadrangle, where the deformation is so 

intense that it becomes a major part of the unit definition (for example, Bindschadler and 

others, 1992a).

Twenty-six impact craters are mapped in the quadrangle (fig. 1, table 2). The craters 

range in diameter from 1.5 km to 100 km (Schaber and others, 1998). No splotches (surface markings and deposits interpreted to be formed from air blasts from projectiles traversing the atmosphere; for example, Schaber and others, 1992; Ivanov and others, 1992) 

were detected in the quadrangle. Extended surface deposits emplaced during the cratering 

event (both outflow deposits and remnants of dark haloes; Schaber and others, 1992; Phillips and others, 1992; Campbell and others, 1992; Schultz, 1992) were noted and are particularly concentrated in the central part of the quadrangle around crater Ermolova (60.9 

km diameter) and in the northwestern (around crater Monika, 25.5 km diameter), southwestern (at unnamed crater, 52.1° N., 123.2° E., 3.2 km diameter and Irinuca crater, 8.0 

km in diameter), and southeastern (around crater Rampyari, 7.7 km diameter) corners of 

the quadrangle. Areas to the west of crater Dickinson and southwest of it, at crater Erika 

(10.5 km in diameter), appear to have fragmental surface materials that have been redistributed by eolian processes. Evidence for this includes east-west-oriented wind streaks 

and diffuse patches of dark and bright materials to the west of Dickinson (between about 

160° and 165° E.) and around Erika.

STRATIGRAPHY

On the basis of an analysis of the global size-frequency distribution of impact craters, 

a crater retention age of 800 Ma (McKinnon and others, 1997), 500 Ma (Schaber and others, 1992; Phillips and others, 1992), or 300 Ma (Strom and others, 1994) for the present 

surface of Venus was proposed. The crater areal distribution cannot be distinguished from 

a spatially random population, which, together with the small total number of craters, 

means that crater size-frequency distributions cannot be used to date stratigraphic units for 

an area the size of the Atalanta Planitia quadrangle. Therefore, attention must be focused 

on the definition of geologic units and structures and on analysis of crosscutting, embayment, and superposition relations among structures and units in order to establish the 

regional geologic history.

Although we have mapped tectonic structure independent of geologic units, in many 

cases tectonic features are such a pervasive part of the morphology of the terrain that it 

becomes part of the definition of a unit. For example, our tessera unit is similar to several 

members of the Olympus Mons Formation on Mars (aureole members 1–4), which are 

defined on the basis of tectonic structure (“...corrugated, cut by numerous faults that 

formed scarps and deep troughs and grabens...,” Scott and Tanaka, 1986). Our plains unit 

with wrinkle ridges is analogous to member 1 of the Arcadia Formation on Mars (“Lowlying plains; mare-type (wrinkle) ridges common”). In other cases, the approach depends 

on scale and density. For example, where the structures are more discrete and separated, 

we map them separately and not as a specific unit, whereas in other cases, where they are 

very dense and tend to obscure the underlying terrain, we chose to map them as a unit. 

Here we summarize the stratigraphic units and structures and their relations.

TESSERA MATERIAL

The unit interpreted to be stratigraphically oldest in the quadrangle is tessera material 

(unit t), which is embayed by most of the other units within this quadrangle. Tessera terrain is radar bright, consists of at least two sets of intersecting ridges and grooves, and is a 

result of tectonic deformation of some precursor terrain (Barsukov and others, 1986; Basilevsky and others, 1986; Bindschadler and Head, 1991; Sukhanov, 1992; sometimes referred to as complex ridged terrain, Solomon and others, 1992). Arches, ridges, grooves, and 

grabens are tectonic features, so structure is an essential component of the tessera terrain 

and a key aspect of the unit definition, similar to the situation in the aureole members of 

the Olympus Mons Formation (Scott and Tanaka, 1986). Globally, tessera occupies about 

8% of the surface of Venus (Ivanov and Head, 1996) and occurs as large blocks and small 

islands standing above and embayed by adjacent plains. These types of occurrences are 

mirrored in this quadrangle, where the northeastern portion of a large tessera block about 

1,500 x 650 km across (Ananke Tessera) and the easternmost part of a smaller tessera 

massif (Meskhent Tessera) are exposed, and several small patches are seen elsewhere in 

the quadrangle.

PLAINS MATERIALS

Densely Lineated Plains Material

The stratigraphically oldest plains unit is densely lineated plains material (unit pdl), 

which is characterized by somewhat flat surfaces on a regional scale and by swarms of 

parallel and subparallel lineaments (resolved as fractures if they are wide enough) having 

typical spacing of less than 1 km. Although the unmodified precursor terrain for the 

densely lineated plains material is not observed, the flatness suggests that it was plains. 

Although fractures are structural elements, they are such a pervasive part of the morphology of this terrain that it becomes a key aspect of the definition of the unit, as in several of 

the Mars examples cited above. Densely lineated plains predominantly occur in areas surrounding Ananke and Meskhent Tesserae, and small patches of the plains occur in the central and northern part of the quadrangle, in places in close association with shield plains. 

Material of the plains, where it is in contact with tessera, embays tessera ridges and penetrates into grooves on the tessera surface. In turn, unit pdl appears to be embayed by 

younger units.

Ridged and Grooved Plains Material

Ridged and grooved plains material (unit prg) is commonly deformed by somewhat 

broad (5–10 km wide) ridges tens of kilometers long. Although ridged and grooved plains 

do not occur in direct contact with densely lineated plains, patches of these two units often 

occur spatially close to each other. The typical structural pattern of densely lineated plains 

is not seen within nearby occurrences of ridged and grooved plains. Thus, the emplacement of the later unit took place after the deformation of densely lineated plains. In the 

Atalanta Planitia quadrangle, this unit is arranged in linear outcrops or belts 50–100 km 

wide and 250–300 km long and is largely equivalent to the ridge belts of Squyres and others (1992). These belts are formed from preexisting flat plains. Ridged and grooved plains 

are interpreted to be volcanic plains materials deformed into ridgelike belts by compression. Ridged and grooved plains occur primarily in the southeastern and eastern parts of 

the quadrangle and represent westernmost extension of the ridge belt concentration in the 

Pandrosos Dorsa area (east of the map area). Occurrences are oriented predominantly 

NNE–SSW in the southern part of the Atalanta Planitia quadrangle, but in the northeastern 

corner of the quadrangle the strike of the ridge belts is NNW–SSE (Lukelong Dorsa).

Shield Plains Material

Shield plains material (unit psh) is in contact with groove belt material (unit gb) and 

ridged and grooved plains material (unit prg) in the southeastern, northeastern, and northwestern parts of the quadrangle. At the contacts, the surface features of both groove belt 

and ridged and grooved plains materials are truncated and covered by material of shield 

plains. This relationship is the evidence for the young age of shield plains. This unit is 

characterized by abundant small shield-shaped features ranging from a few kilometers in 

diameter up to about 10–20 km, commonly with summit pits. Although small clusters of 

shields were recognized earlier planetwide (Head and others, 1992), they were thought to 

be localized occurrences possibly related to individual sources such as hot spots. Later 

work in Vellamo Planitia (Aubele, 1994, 1995) recognized that many of these occurrences 

represented a stratigraphic unit in this region, and subsequently this unit has been recognized in many areas on the planet (Basilevsky and Head, 1995b; Basilevsky and others, 

1997), including this quadrangle. Shields characterizing this unit occur in clusters, giving 

the unit a locally hilly texture, and as isolated outcrops in relatively smooth plains. The 

shields are interpreted to be of volcanic origin and are likely to be the sources of the adjacent smooth plains. A small occurrence of shield plains material is seen in the core area of 

Earhart Corona and is surrounded by flowlike features radiating away from the occurrence. This is the only example in the Atalanta Planitia quadrangle where specific flow 

units are associated with the small edifices of shield plains. The unit is widely distributed 

in the quadrangle especially in its central part and in the southwestern (around Ananke 

Tessera) and northwestern (around Meskhent Tessera) parts of the quadrangle. Almost no 

concentrations of shield plains material occur along the eastern margin of the quadrangle, 

where the plains are likely buried by younger plains units. Some isolated shields occur 

where subsequent plains units embay shield plains and form kipukas (flooding the bases 

of the shields and leaving the tops exposed). As the radar brightness of the two units typically is different, this permits an estimate to be made of the thickness (about 100–200 m) 

of the margins of the embaying unit, as information exists on the topography of the shields 

(Guest and others, 1992).

Wrinkle-Ridged Plains Material

After emplacement of the earlier plains units (and locally the formation of groove 

belts; see following section), the most widespread plains unit in the quadrangle, wrinkleridged plains material, was emplaced. Throughout the quadrangle, evidence exists for 

embayment of all previous units by regional wrinkle-ridged plains material. This unit is 

composed of morphologically smooth, homogeneous plains material of intermediate dark 

to intermediate bright radar albedo complicated by narrow linear to anastomosing wrinkle 

ridges (a structural element) in parallel lines or intersecting networks. This unit is analogous to the ridged unit of the plateau sequence on Mars (Scott and Tanaka, 1986), which is 

a plains unit defined by “long, linear to sinuous mare-type (wrinkle) ridges.” In the map 

area, the wrinkle ridges typically are less than 1 km wide and tens of kilometers long; in 

some areas, they may be smaller, whereas in others they are larger. Their trend typically 

varies locally even within one site. This material is interpreted to be regional plains of volcanic origin that were subsequently deformed by wrinkle ridges. Volcanic edifices and 

sources of the plains are not obvious although some small shield-shaped features are seen 

and appear to be kipukas of shield plains material (unit psh). Wrinkle-ridged plains material is subdivided into two units. The lower unit (unit pwr1) generally has a low radar 

albedo but can be mottled locally. The upper unit (unit pwr2) generally has a high radar 

albedo and lobate boundaries. In the southern portion of the quadrangle, unit pwr2 penetrates into areas occupied by unit pwr1, using local lows such as small-scale fractures and 

lava channels. This relationship is strong evidence that unit pwr1 is older than unit pwr2. 

Together these two units form more than 60% of the surface of the quadrangle and occur 

predominantly in low-lying regions between linear highs composed of preexisting units 

and ridge belts (for example, Bolotnitsa Fluctus between Frigg Dorsa and Vedma Dorsa in 

the southern part of the quadrangle). Unit pwr1 is more widespread than unit pwr2, as unit 

pwr2 forms less than about 30% of the combined area of the two units. Unit pwr2 is concentrated mostly in the southern part of the quadrangle.

Smooth Plains Material and Lobate Plains Material

The youngest plains units in the stratigraphic sequence are characterized by distinctive lobate and digitate shapes and margins and by morphologically smooth surfaces commonly unmodified by wrinkle ridges or other structural elements. Typically, wrinkle ridges 

and older tectonic structures are embayed by material of these plains as it is seen, for 

example, east of Earhart Corona and at Baltis Vallis in the southeastern part of the quadrangle. Two plains units are recognized: smooth plains material (unit ps) of uniform, either 

low or moderate albedo and lobate plains material (unit pl), having internal elements 

arranged in parallel to sinuous to lobate radar bright and dark strips and patches and unit 

boundaries that are typically lobate. Both of these units are interpreted as complexes of 

lava flows undisturbed by subsequent deformation. Smooth plains material may be composed to some extent by redistributed eolian material. In the Atalanta Planitia quadrangle, 

smooth plains material occurs predominantly in small patches along the northern, eastern, 

and southern margins and between Nightingale and Earhart Coronae. Lobate plains material occurs predominantly along the western and southwestern margins of the basin in the 

form of large flow deposits (for example, around Nightingale and Earhart Coronae) emanating from distinct sources such as coronae and volcanoes.

BELT MATERIAL

Swarms of narrow grooves are concentrated in the western and southern parts of the 

quadrangle. In some cases, grooves are so closely spaced that they tend to obscure the terrain they are superposed on and their presence takes on a defining character to the terrain. 

These concentrations are characterized by numerous short and long curvilinear, subparallel and anastomosing lineaments that are commonly wide enough to be resolved as fractures and grabens. In detailed mapping at the FMAP scale, remnants of preexisting plains 

can be seen between these tectonic structures. Stratigraphic relationships of groove belt 

material and ridged and grooved plains material are unclear, because these units do not 

occur in contact with each other. Both units, however, pre-date shield plains material and 

apparently post-date densely lineated plains material. Thus, the belts of narrow grooves 

(made up of groove belt material, unit gb) and ridged and grooved plains material (unit 

prg) were formed after the deformation of densely lineated plains material (unit pdl) and 

before the emplacement of shield plains material (unit psh). Within the Atalanta Planitia 

quadrangle, groove belts form part of the annulae of Nightingale and Earhart Coronae and 

a starlike pattern of deformation in the core of Ciuacoatl Mons. At these coronae, structures of groove belts are mostly embayed by material of both shield plains and regional 

plains with wrinkle ridges. Orientations of grooves in groove belts are shown within the 

unit and generally trend parallel to the strike of the groove belts as a whole.

CRATER MATERIALS

Impact craters and related deposits are observed in several places in the quadrangle 

(fig. 1). They are subdivided into crater material, undivided (unit c) and the proximal textured and outflow deposits—crater flow material (unit cf). Several craters are characterized by a surrounding of radar dark material that partly to wholly obscures the underlying 

terrain. Dark parabolas apparently related to three impact craters within the quadrangle 

(Monika, Ermolova, and Irinuca) are shown in fig. 1. We observed only one crater in the 

quadrangle that may be embayed by lava flows—Koidula (64.2° N., 139.6° E.; 76 km 

diameter)—of the lobate plains. No craters in the Atalanta Planitia quadrangle are cut by 

tectonic features.

Some surficial streaks and patches of apparently unconsolidated material that has 

been redistributed by eolian processes (for example, Greeley and others, 1992) are 

observed in the quadrangle but are not mapped as a specific unit. They are primarily in the 

northeastern corner of the quadrangle, where material from the crater Dickinson has been 

partially redistributed.

STRUCTURES

A variety of tectonic features is observed and mapped in the quadrangle. Long linear 

fractures and some paired and facing scarps interpreted to be grabens are seen in the eastern part of the quadrangle. They are 150–700 km long, oriented west-east, and spaced 

50–150 km apart, cutting all stratigraphic units older than the upper unit of wrinkle-ridged 

plains material (unit pwr2). In some cases, grabens are so closely spaced that they tend to 

obscure underlying terrain and their presence takes on a defining character to the terrain. 

Concentrations of grabens in groove belt material (unit gb) are distinguished from 

densely lineated plains material (unit pdl) by their beltlike form and the character of the 

fractures (more distinctly recognizable grabens in unit gb). Groove belt material appears 

to be distinctive stratigraphically. The structure of the belts crosscuts tessera material (unit 

t) in Nightingale Corona and, in turn, are mostly embayed by wrinkle-ridged plains material (unit pwr1). Some of the structures of groove belts cut the surface of this latest unit. 

The individual fractures cutting many younger units and mapped as separate structures 

commonly are oriented in the same direction and may represent the waning stages of continued deformation.

Several types and scales of features of compressional origin also are observed. Wrinkle ridges are seen throughout the quadrangle and are so important in the broad plains that 

they in part define and characterize wrinkle-ridged plains material. These features are 

mapped in a representative sense in terms of density and trend by individual wrinkle ridge 

symbols. Ridged and grooved plains material (unit prg) is dominated by ridges and arches, 

and the general trends of these are represented in the units by symbols. Occurrences of 

ridged and grooved plains form predominantly north-northeast- and northeast-trending 

bands or belts that are largely equivalent to the ridge belts of Squyres and others (1992). 

Basilevsky and Head (1995a, b) described a structure (ridge belts, RB) that was a belt consisting of a cluster of densely spaced ridges 5–10 km wide and a few tens of kilometers 

long; this unit was often transitional to ridged and grooved plains material (unit prg). 

Although we did not map ridge belts in this area, the belts of ridged and grooved plains 

are closely related.

Flanks of two large volcanoes are visible within the Atalanta Planitia quadrangle and 

several coronae are seen there (fig. 1), the largest of which is the 560-km x 480-km Nightingale at the west margin of the map area. This structure and Earhart Corona, about 800 

km north of Nightingale, is part of a corona cluster in Tethus Regio. Nightingale and Earhart Coronae are defined by an annulus of highly grooved and intensely deformed terrain. 

The coronae also are characterized by concentric and radial grabens and fractures both 

inside and along the margins of the annulus. In the core area of Nightingale Corona, both 

tessera material and densely lineated plains material occur. Nightingale and Earhart Coronae are surrounded by an extensive apron of flows of lobate plains material (unit pl) and 

smooth plains material (unit ps). Other coronae on Venus typically have an annulus made 

up of material resembling densely lineated plains broadly embayed by younger plains 

units (Basilevsky and Head, 1998; Ivanov and Head, 1998). The southernmost Mari 

Corona and the stellate fracture center of Ciuacoatl Mons (at about 53.5° N., 151.0° E.) 

are embayed by flows of lobate plains material, some of which appear to emanate from the 

coronae. On the basis of these observations, the structural deformation of the coronae 

apparently occurred early, with volcanism continuing in several cases until recently.

GEOLOGIC HISTORY

The Atalanta Planitia area is one of several large, somewhat equidimensional lowland 

areas of Venus and is an important region for analysis of processes of lowland formation 

and volcanic flooding. Major questions include: What is the sequence of events in the formation and evolution of large-scale equidimensional basins on Venus? What are the characteristics of the marginal areas surrounding these basins? How do the units and implied 

geologic histories in both the basin and the marginal areas compare within Atalanta? 

When did the Atalanta Planitia basin and its marginal region form and is there any evidence that the basin represents a stage in the evolution of other terrain types, such as tessera? How do the units in Atalanta Planitia compare with each other and what information 

do they provide concerning models for Venus global stratigraphy and tectonic history? 

Here we discuss stratigraphic positions of units, their temporal correlations, and the 

implied geologic history of the region. We also examine the sequence of tectonic deformation and its interpretation as well as the evolution of volcanic styles. We conclude with an 

assessment of the geologic history of Atalanta Planitia in relation to models for the global 

evolution of Venus.

The stratigraphically oldest unit in the map area is tessera material, a high-standing, 

complexly deformed unit of which the largest outcrop is exposed in Ananke Tessera. 

Smaller massifs of tessera within the map area are represented by Meskhent Tessera at the 

west margin of the quadrangle and small tessera patches preserved mostly in the southern 

and northern parts—Mago-Halmi Tesserae—of the map area. Tessera is consistently 

embayed by younger plains material of apparent volcanic origin. The principal massif of 

Ananke Tessera shows chaotically organized narrow and broad ridges 5–15 kilometers 

across cut by numerous narrow and shallow grabens that are mostly orthogonal to the 

ridges. The same basic set of structures occurs within Meskhent Tessera. Consistent with 

observations made on regional (Bindschadler and others, 1992a) and global scales (Ivanov 

and Head, 1996), tessera material in Ananke and Meskhent Tesserae was formed from 

some precursor material that was initially deformed by shortening, folding, and probably 

shear, which produced more highly deformed terrain than seen in any subsequent units on 

Venus, and then underwent extensional deformation (cut by fractures and grabens of various types) to produce the generally orthogonal structural fabric typical of much of the tessera planetwide. Other smaller outcrops are exposed as kipukas in the quadrangle, and on 

the basis of this distribution and the global distribution of similar small outliers, tessera 

terrain is thought to exist extensively in the subsurface beneath younger plains units (for 

example, Ivanov and Head, 1996). Whether this distribution is planetwide or regional or 

even local is unclear. No direct evidence exists in the map area for the duration of the formation with the characteristic structural pattern of tessera, but global crater studies (Ivanov and Basilevsky, 1993; Gilmore and others, 1996) suggest that it was of relatively short 

duration (for example, tens of millions of years). 

Marginal to the tessera terrain and embaying it are plains that have been densely fractured (unit pdl) and in some cases have a structural fabric orientation similar to the latest 

phase of deformation in tessera (for example, Basilevsky and Head, 1995a, fig. 4). Along 

the northern margin of Meskhent Tessera, an east-west-trending band of densely lineated 

plains material is characterized by a very dense set of north-south-oriented structures. Features of the plains appear to continue the structural trend of some of the sets of structures 

in tessera. The deformation patterns in densely lineated plains are very dense and unidirectional in contrast to the orthogonal patterns of the tessera. The morphology and planimetric shape of the structures of densely lineated plains suggest that both tensile and shear 

stresses have been involved in the deformation of the plains. This unit is interpreted to be 

volcanic plains material that embayed early tessera but which was deformed by the latest 

phases of tessera deformation. Densely lineated plains material is thus partly laterally 

equivalent to tessera material, and the northern margin of Meskhent Tessera and the east 

edge of Ananke Tessera are examples of such a transition. Densely lineated plains material 

makes up a small percentage of the area of our mapping but is widespread in the map area, 

suggesting an extensive presence in the subsurface.

Following the emplacement and deformation of densely lineated plains material (unit 

pdl), a less intensely deformed plains unit was emplaced (ridged and grooved plains material, unit prg). Although little evidence exists for sources, the smooth surface texture of the 

background material of the unit strongly suggests that it is of volcanic origin. The most 

important features of ridged and grooved plains material are ridges (usually curvilinear 

arches 5–10 km wide and comparable to lunar mare arches of compressional origin) that 

are very pervasive from place to place. Occurrences of the unit apparently make up a very 

broad arc running generally in the north-south direction along the east edge of the map 

area. The unit occurs as broad arches (as much as 75–100 km across) striking northeastsouthwest in the south-central part of the map area and northwest-southeast in the northern 

and eastern parts. The parallelism of the strike of the tectonic features and broad arches of 

this unit suggests that the deformation that produced the features is similar. Thus, the 

period during which this unit was formed and modified was characterized by emplacement 

of widespread volcanic plains and their subsequent deformation into broad arches with 

distinctive ridges and fractures. On the basis of its present topographic configuration, this 

unit and its deformation appear to have produced the second-order topography of the 

basin, that is, broad segmentation into local low regions defined by linear and curvilinear 

arches of ridged and grooved plains standing several hundred meters above the surrounding terrain.

The time of the formation of groove belts is poorly constrained in the map area. The 

belts occur almost exclusively in the annuli of Nightingale and Earhart Coronae. In Nightingale Corona, some evidence suggests structures of groove belts cut tessera material and 

are embayed by wrinkle-ridged plains material. Tight spatial association of groove belts 

and coronae suggests that the formation of the belts is related to the development of coronae and could continue through a significant time interval. Groove belts are a few hundred 

kilometers wide and hundreds of kilometers long and occur predominantly in the eastern 

and southern parts of the map area. Topographically the groove belts represent local highs 

of the large corona annuli. Formation of the belts of grabens mark an episode of extension 

that was radially oriented relative to the core areas of Nightingale and Earhart Coronae 

and apparently unevenly distributed throughout Atalanta Planitia. Groove belts appear to 

cut some portions of shield plains, yet shield plains material (unit psh) is seen, in places, 

embaying groove belt material and forming in the depressions within the groove belts (for 

example, in Ciuacoatl Mons).

Following emplacement and deformation of ridged and grooved plains material, 

another distinctly different plains unit (shield plains, unit psh) was emplaced in many 

parts of the map area and is now preserved as extensive occurrences preferentially in the 

central part of the map area and around Ananke and Meskhent Tesserae and as patches in 

high areas elsewhere in the quadrangle. The abundant shield volcanoes and intershield 

plains that are characteristic of this unit are noticeably different from the volcanic style of 

both ridged and grooved plains and subsequent plains with wrinkle ridges. The extent of 

these vents indicates widespread local and shallow magma sources during the emplacement of shield plains material. The close association of this unit with densely lineated 

plains material suggests that the widespread shields may be related to extensional deformation at least locally. The shield plains material (unit psh) embays ridged and grooved 

plains material (unit prg) and apparently was emplaced on horizontal surfaces, which is 

suggested by the lack of systematic asymmetry of individual edifices (Kreslavsky and 

Head, 1999). Such asymmetry could be introduced if structures of shield plains (edifices) 

were formed on a regionally tilted surface. On the basis of topographic distribution of outcrops of subsequent units (wrinkle-ridged plains material and younger units), the shield 

plains material underwent some tilting after its emplacement. The lack of extensive development of deformational features demonstrates that regional deformation had further 

waned in intensity. Small edifices of shield plains are typically embayed by wrinkle-ridged 

plains material, which strongly suggests that shield plains material is mostly an older unit. 

The only exception to this is in the core area of Earhart Corona where a small occurrence 

of shield plains material is apparently in a geometric center of a field of lava flows that 

radiate away from an outcrop of shield plains material. This relation could mean that the 

edifices of shield plains probably were the sources for the lava flows.

Subsequent to the emplacement of shield plains, the style of volcanism changed. 

Instead of abundant small shield volcanoes, broad units of plains materials, now regionally 

deformed by wrinkle ridges (units pwr1 and pwr2), were emplaced from sources that are 

now rarely visible. The presence of sinuous channels (a small segment of the largest channel on Venus, Baltis Vallis, is in the very southeastern corner of the map area; Basilevsky 

and Head, 1996) and the wide extent of these units suggest emplacement in high-effusionrate eruptions from a few sources, in distinct contrast to the widespread and abundant 

shield volcanoes just preceding this phase. At least several occurrences of unit pwr2 occur 

along the eastern margin adjacent to later flows of lobate plains material (unit pl). One 

extensive occurrence of this unit in the southern part of the map area is associated with a 

large volcanolike source (48° N., 160° E.) just outside the southern margin of the Atalanta 

Planitia quadrangle. This suggests that regional deposition of the lower unit of wrinkle 

ridged plains material (unit pwr1) may have given way to deposits of the upper unit which 

were associated with specific sources. Deposits of wrinkle ridged plains material cover 

well over one-half of the map area and are concentrated in the lowland areas between tessera material outcrops and belts of ridged and grooved plains material. Their distribution 

is good evidence that most of the topography observed today was formed before the 

emplacement of the majority of wrinkle ridged plains material. However, the volcanic 

plains of units pwr1 and pwr2 were further deformed by the formation of wrinkle ridges 

subsequent to their emplacement; the central parts of deposits of these units are depressed 

up to a few hundred meters from surrounding marginal deposits of the same unit. Some 

wrinkle ridges in the eastern part of the map area are oriented generally consistently with 

the tectonic fabric and principle stress directions typical of ridged and grooved plains. 

Thus, the deformation recorded in units prg, pwr1, and pwr2 appears to be relatively consistent in structural trends but reflects a decrease in the intensity of shortening and deformation with time.

In a few cases some smooth plains material (unit ps) similar to the underlying plains 

materials (unit pwr1 and pwr2) was emplaced at the periphery of the area of the Atalanta 

basin and is apparently unmodified by wrinkle ridges. In the northeastern corner of the 

map area, a large occurrence of smooth plains material is close to impact crater Dickinson. 

The surface of the plains there is characterized by wind streaks, which are suggestive of 

the redistribution of loose material. Thus, the smooth plains material may be related, to 

some extent, to the deposition and redistribution of debris material that was formed after 

impact events.

Near the end of emplacement of wrinkle-ridged plains material, extensive lobate and 

digitate flows (unit pl) originated from coronae and volcanoes along the margins of the 

Atalanta Planitia basin. Some of the flows were radial relative to coronae whereas the others flowed down the regional slopes into the basin. The sources of lobate plains material in 

the map area are Nightingale, Earhart, and Mari Coronae and Ciuacoatl Mons and intermediate volcanoes Melia Mons and Jael Mons, from which numerous flows emanate and 

extend down into the basin, following preexisting topography. No evidence exists for the 

deformation of these flows by wrinkle ridges, which suggests that the flows are either very 

recent and as yet undeformed or that wrinkle ridge deformation as a general phenomena 

ceased by this time. A few wrinkle ridges, which are visible, in some places, within areas 

of the lobate plains material (unit pl), apparently represent kipukas of structures deforming 

material of plains with wrinkle ridges (unit pwr1). On the basis of the general decrease in 

tectonic deformation intensity as a function of time, we interpret the general absence of 

wrinkle ridges in lobate plains material to be the result of waning deformational forces. 

Despite the fact that these are the youngest flows in the map area, evidence from Nightingale and other coronae indicate that corona interiors and margins contain units equivalent 

to some of the earliest post-tessera plains units (for example, densely lineated plains material, unit pdl; Basilevsky and Head, 1995a, b) and that they thus started to form during this 

early period. In summary, volcanism at the time of the emplacement of lobate plains material (unit pl) switched from the basin interiors to the margins and from extensive floodlike 

plains units to digitate and lobate flows emanating from specific sources such as coronae 

and intermediate and large volcanoes. Coronae had apparently started to form at the same 

time as earlier activity in the basin interior.

Of the 26 impact craters seen in the map area (fig. 1, table 2), we observed only one 

example of an impact crater (Koidula, 64.2° N., 139.6° E., 67 km diameter) possibly 

embayed by volcanic activity (unit pl); no craters apparently are cut by tectonic structures. 

Thus, most impact craters appear to be younger than the regional plains materials. Rim 

heights of craters less than about 30 km in diameter should be <500 m (Sharpton, 1994). 

On the basis of the fact that no examples of craters embayed by pre-lobate plains units are 

observed and that the widespread deposits of wrinkle-ridged plains material are thought to 

be relatively thin, we interpret the lack of embayed craters to indicate that the plains units 

probably were emplaced in a relatively short period, although the small total number of 

craters makes this interpretation tentative.

In summary, the trends in the geologic history outlined here suggest that deformation 

decreased in intensity from an initially very high level associated with tessera formation to 

increasingly lower levels associated with deformation of densely lineated plains, ridged 

and grooved plains, wrinkle-ridged plains material, and finally, to essentially no deformation of the latest flows of lobate plains. Trends in principal stress orientation suggest a relatively consistent post-tessera northwest-southeast and northeast-southwest principal 

compression axis orientation in the eastern part of the basin from early post-tessera 

through the emplacement of plains with wrinkle ridges. The topography of the basin in 

which Atalanta Planitia lies initially formed at the time of deformation of ridged and 

grooved plains (producing a series of smaller basins bounded by belts of unit prg) and 

continued to subside following partial filling of plains with wrinkle ridges. Volcanism was 

initially widespread and partly coincident with tessera formation (for example, unit pdl), 

then became concentrated into widely distributed small individual sources (the small 

shields of unit psh) in Atalanta Planitia, then changed style and was focused in the basin 

interior, where individual basins were filled by possible large volume eruptions, and 

finally, changed style and location, erupting from coronae and related features along the 

basin margins into the basin interior, where volcanism had apparently ceased.

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Figure 1. Distribution of impact craters and coronae in Atalanta Planitia quadrangle.

180

75 120°

°

°

120° 50°

180°

75°

50°

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Descriptions of the nomenclature used on this map sheet are listed 

at http://astrogeology.usgs.gov/Projects/PlanetaryNomenclature/

Crater

Crater-related dark halo

Corona, coronalike feature

Monika

Zhilova

Koidula

Tsvetayeva

Kumudu

Rani

Sandugach

Bineta

Khadako

Irinuca

Ermolova

Cochran

Selma

Radmila

Jex-Blake

Rafiga

Unay

Rampyari

Hapei

Erika

Dickinson

Lukelong

Sinanevt

Sinanevt

Dorsa

Dorsa

Norwan

Dorsa

Dorsa

Sayligul

o

Monika

Zhilova

Koidula

Tsvetayeva

Kumudu

Sandugach

Bineta

Irinuc

Khadako

a

Ermolova

Cochran

Selma

Radmila

Rani

Jex-Blake

Rafiga

Unay

Rampyari

Hapei

Erika

Dickinson

Sayligul

Nightingale N

le ps

Corona

Earhart

Corona

Mari

Corona pdl

Ciuacoatl g

Monsps

Holde

Corona

e

Meskhent

Tessera

ATALANT A

PLANITIA

TILLI-HANUM

PLANITIA

Ananke 

Tes

sera

Frigg

Dorsa

Mago-Halmi

Tesserae

Vedma 

Dorsa

B a l t i s

Val l i s