U.S. Department of the Interior

U.S. Geological Survey

2018

Prepared for the National Aeronautics and Space Administration

Geological Map of the Nepenthes Planum Region, Mars

By James A. Skinner, Jr., and Kenneth L. Tanaka

Pamphlet to accompany

Scientific Investigations Map 3389

Map showing the location of the Nepenthes Planum region (Mars Transverse Mercator (MTM) 10237, 10242, 10247, 15237, 15242, 

15247, 20237, 20242, and 20247 quadrangles; hachured region) and major physiographic features. Image is Mars Orbiter Laser 

Altimeter (MOLA) artificial hillshade overlain on a Thermal Emission Imaging System (THEMIS) daytime infrared (IR) mosaic. 

Orthographic projection centered at long 90° E.

0° E

0° 90° E 180° E0°

30°

-30°

-60°

60°

30°

-30°

-60°

60°

VASTITAS BOREALIS

UTOPIA PLANITIA

ISIDIS

PLANITIA

SYRTIS

MAJOR

PLANUM

HELLAS

PLANITIA

HESPERIA

PLANUM

 ARABIA

TERRA

NOACHIS

 TERRA

 TERRA

CIMMERIA

TERRA

TYRRHENA

Elysium

Mons

E L Y S I U M P L A N I T I A

N i l o s y r t is M e n s a e

N e p e n t h e s M e n s a e

L i b y a M o n t e s

Skinner and Tanaka - Envelope Cover

Contents

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

Geography.......................................................................................................................................................3

Base Map and Data.......................................................................................................................................3

Methodology...................................................................................................................................................3

Unit Groups, Names, and Labels.................................................................................................................3

Contact Types ........................................................................................................................................5

Feature Types ........................................................................................................................................5

Digital Drafting Parameters ................................................................................................................5

Geomorphology..............................................................................................................................................6

Ridges.....................................................................................................................................................6

Depressions...........................................................................................................................................6

Scarps.....................................................................................................................................................6

Pitted Cones and Mounds ...................................................................................................................7

Surface Texture.....................................................................................................................................7

Age Determinations.......................................................................................................................................7

Geologic Summary.........................................................................................................................................8

Noachian Period ...................................................................................................................................8

Early Hesperian Epoch.........................................................................................................................8

Late Hesperian Epoch..........................................................................................................................9

Amazonian Period.................................................................................................................................9

References Cited .........................................................................................................................................10

Table

1. Characteristics of geologic units in the mapped Nepenthes Planum region, Mars: areas, 

crater densities, and superposition relations ..........................................................................4

i

1

Introduction

One of the most dominant topographic features on the 

surface of Mars is the highland-lowland transition, a globeencircling feature, which separates geologically ancient, 

higher-standing and densely cratered plateaus from the relatively younger, lower-lying and sparsely cratered plains by 1–5 

kilometers (km). The morphologic expression of this feature is 

laterally variable, ranging from a very shallow (<0.1°), northsloping, cratered surface in northern Arabia Terra (lat 30.0 to 

46.0° N, long –8.0 to 20.0° E) to a topographic scarp bounded 

by eroded remnants of highland plateaus and knobs in Nilosyrtis 

Mensae (lat 28.0 to 45.0° N, long 32.0 to 80.0° E). Intermediate within this morphologic continuum is the highland-lowland 

transitional zone, which separates the cratered highlands of 

Terra Cimmeria in the south from the low-lying plains of Utopia 

Planitia in the north. This region has been interpreted to be 

underlain by deeply seated crustal faults associated with the 

formation of the giant Utopia multi-ring impact basin (McGill, 

1989; Frey and Schultz, 1990; Tanaka and others, 2003a, 2005; 

Nimmo and Tanaka, 2005), which may have controlled not 

only the regional accumulation of rocks and sediments but 

also the trapping and transport of subsurface water and (or) 

ice reservoirs over a long period of Mars’ geologic history 

(Tanaka and others, 2003b; Skinner and Tanaka, 2007). A more 

complete understanding of the range of geologic processes that 

have operated within the highland-lowland transitional zone 

is now possible because of modern (post-Viking Orbiter) data 

sets, in particular higher-resolution images, which support 

more detailed geologic observations that are presented in this 

map. In turn, material units and modificational features can be 

dated with more precision based on stratigraphic relations and 

crater-density data. The map results permit new interpretations 

of the origin and age of geologic materials and features and a 

reconstruction of the geologic history that provides an improved 

understanding of the development of the highland-lowland transition zone and northern plains in the Nepenthes Planum region. 

The map also provides geologic and stratigraphic context for 

future studies within the Nepenthes Planum region, as well as 

comparative areas along the highland-lowland transitional zone.

The highland-lowland transitional zone located between 

Terra Cimmeria and Utopia Planitia was first mapped as part 

of the Amenthes quadrangle by Hiller (1979) using Mariner 9 

images at 1:5,000,000 scale. Hiller (1979) identified two main 

physiographic and geologic subdivisions in the region: the 

higher-standing, densely cratered plateau located to the south 

and sparsely to moderately cratered plains located to the north. 

Therein, he subdivided the densely cratered plateaus into “hilly 

and cratered material” and “cratered plateau material,” representing ancient Martian crustal materials or lava plains burying such materials. Moderately cratered plains were similarly 

subdivided into “cratered plains materials” and “smooth plains 

materials” (Hiller, 1979). The occurrence of discontinuous 

lobate scarps within the “smooth plains” unit prompted Hiller 

(1979) to interpret that unit as volcanic plains consisting of 

low-viscosity lava flows, geologically and morphologically 

comparable to lunar maria. The prominence of impact craters 

with diverse geomorphological expression was interpreted as 

evidence of various levels of erosional modification and, thus, 

increasingly older unit age from north to south (Hiller, 1979).

A composite of the 1:5,000,000-scale quadrangles—

including an early version of the Amenthes quadrangle geologic 

map of Hiller (1979)—resulted in a 1:25,000,000-scale global 

geologic map of Mars based on Mariner 9 images (Scott and 

Carr, 1978). This map was the first to assign chronostratigraphic 

ages to geologic units on Mars. Therein, Scott and Carr (1978) 

grouped the smooth, generally lower standing cratered plains 

material into the Amazonian System, the knobby terrains that 

morphologically define the northern margin of the cratered 

highlands into the Noachian and Hesperian Systems, and the 

rugged, generally higher standing cratered plains to the Noachian System.

Applying the stratigraphic framework of Tanaka (1986), 

Greeley and Guest (1987) used the improved areal coverage 

and spatial resolution of Viking Orbiter images to produce 

a 1:15,000,000-scale geologic and geomorphologic map of 

the eastern equatorial region of Mars that includes the Terra 

Cimmeria-Utopia Planitia highland-lowland transitional zone. 

These mapping geologists refined the occurrence of the “smooth 

plains” unit of Scott and Carr (1978) by differentiating an eastwest-oriented outcrop of Amazonian “knobby plains material,” 

which they interpreted as sedimentary or volcanic plains surrounding eroded vestiges of crustal massifs and (or) localized 

volcanic vents. Greeley and Guest (1987) mapped Hesperian 

“ridged plains material” directly south of the knobby plains, 

interpreting that unit as low-viscosity lava plains deformed by 

thrust faults related to global contraction. Though only partly 

documented by the Mariner 9-based geologic maps, Greeley 

and Guest (1987) discriminated an east-west swath of isolated 

and coalesced shallow depressions within the ridged plains 

material in the Terra Cimmeria-Utopia Planitia transitional 

zone. They described these as Amazonian to Hesperian “etched 

plains material” and interpreted them as accumulations of aeolian sediments that were subsequently scoured by wind. South 

of the ridged and etched plains materials, Greeley and Guest 

(1987) mapped the rugged plateaus, mesas, and knobs that make 

up the areal bulk of the regional highland-lowland transition 

scarp as the Hesperian and Noachian “undivided material”, 

interpreted as irresolvable collections of eroded highland rocks. 

Greeley and Guest (1987) identified the cratered highlands of 

northern Terra Cimmeria as the Noachian “dissected unit” of the 

plateau sequence. The bulk of the units in the cratered highland 

were interpreted as laterally varying proportions of impact breccias and intercalated volcanic and (or) sedimentary rocks, some 

of which were subsequently dissected by fluvial processes.

Tanaka and others (2005) used topographic and image 

data sets from instruments onboard the Mars Global Surveyor 

(MGS) and Mars Odyssey (ODY) orbiting spacecraft to refine 

the geologic and stratigraphic context of the Martian northern 

plains at 1:15,000,000 scale. Therein, units were differentiated, 

where possible, based on depositional and (or) erosional hiatuses (unconformities). This approach resulted in the identification of two discrete units associated with the cratered highlands 

(the Libya Montes and Noachis Terra units), three units associated with the highland-lowland transitional zone (the Nepenthes Mensae and Utopia Planitia 1 and 2 units), and two units 
2

associated with the broader lowland plains (the Vastitas Borealis 

marginal and interior units) (Tanaka and others, 2005). Southern 

Utopia Planitia was used as the type locality for highland-lowland boundary units (Tanaka and others, 2005).

The northern plains map served as a precursor to a new 

global geologic map of Mars at 1:20,000,000 scale, which also 

used an increased volume of data obtained by MGS and ODY as 

well as image and other data from the Mars Express (MEX) and 

Mars Reconnaissance Orbiter (MRO) spacecraft (Tanaka and 

others, 2014). However, because of a revised unit grouping and 

naming scheme and the reduced scale, the global map employs 

different units. Globally occurring units that are depicted by 

Tanaka and others (2014) are geologically and stratigraphically comparable to hemispherically occurring units of Tanaka 

and others (2005). These include Middle Noachian massif unit 

(comparable to (~) the Libya Montes unit of Tanaka and others 

(2005)), Early Noachian and Middle Noachian highland units 

(~Noachis Terra unit), Hesperian and Noachian transition unit 

(~Nepenthes Mensae unit), Early and Late Hesperian transition units (~Utopia 1 and 2 units), and Late Hesperian lowland 

unit (~Vastitas Borealis marginal and interior units). Both maps 

include the north-trending wrinkle ridges and scarps named 

Hephaestus Rupēs, which occur in Amazonian and Hesperian 

geologic units.

The Nepenthes Planum region has been targeted as a favorable landing site for several landed missions to Mars because 

of its low-latitude position, pervasive planarity, and relatively 

low elevation. The backup landing site for Viking Lander 1 was 

the (informally named) Tritonis Lacus albedo region within 

Nepenthes Planum (lat 20.0° N, long 108.0° E) (Masursky and 

Crabill, 1981). Nepenthes Planum was again discussed as a final 

candidate landing site for the 1997 Mars Pathfinder mission 

(lat 20.0° N, long 108.0° E; Golombek and others, 1997), the 

2001 European Space Agency’s Beagle 2 mission (lat 20.0° N, 

long 114.0° E; Bridges and others, 1999), and the 2004 Mars 

Exploration Rover mission (lat 11.7° N, long 124.0° E; Tanaka 

and others, 2003b). Though the region met all data and mission safety requirements, the overarching perception in each of 

these instances was that the geologic character of the Nepenthes 

Planum region remained poorly understood (relative to other 

candidate landing sites), making potential science discovery 

difficult to assess.

Despite nonselection, the targeted examination of subregions within the Nepenthes Planum region resulted in an 

inordinate volume of specialized observations and topical science investigative results, which are useful for unraveling the 

regional geologic story. These results are particularly relevant 

to deciphering not only the largely planar units that make up the 

southern Utopia Planitia and, by extension, the broader northern 

plains of Mars, but also the undulating terrains that makeup the 

highland-lowland transition in the Nepenthes Planum geologic 

map region. Regarding the planar units within southern Utopia, Clifford and Parker (2001) notably used original landform 

mapping by Parker and others (1993) to postulate that the 

Martian northern plains are broadly encircled by two laterally 

continuous topographic benches, high albedo lobes, and arcuate 

ridges. These researchers interpreted these features as vestiges 

of shorelines formed by highstands of a northern ocean filled by 

water and sediment sourced through the circum-Chryse Planitia 

outflow channel (Parker and others, 1993; Clifford and Parker, 

2001). The lower of these two features, termed the “Deuteronilus shoreline” (Clifford and Parker, 2001), coincides with 

the southernmost margins of the Vastitas Borealis Formation 

of Greeley and Guest (1987) and the Vastitas Borealis marginal unit of Tanaka and others (2005). This putative shoreline 

transects the northernmost part of the Nepenthes Planum map 

area. Despite multiple mapping and topical science investigations conducted subsequent to the original, pioneering work by 

Parker and others (1993; for example, Head and others, 1998; 

Kreslavsky and Head, 2000; Kreslavsky and Head, 2002; Carr 

and Head, 2003; Tanaka and others 2005; Ghatan and Zimbelman, 2006; Ivanov and others, 2014), the existence, origin, 

and fate of a putative ocean in the Martian northern plains, the 

margins of which transect the Nepenthes Planum map region, 

remain mostly unresolved.

Regarding the origin of Nepenthes Planum, which represents the regional physiographic and topographic transition 

from the cratered highlands of Terra Cimmeria to the lowland 

plains of Utopia Planitia, Skinner and Tanaka (2007) mapped, 

along the southern margin of Utopia Planitia, Hesperian fractured rises, elliptical mounds, pitted cones, and cavi depressions 

proposed to be a suite of features potentially formed through the 

ascent of sedimentary diapirs and associated subsidence of adjacent planar sedimentary plains, similar to terrestrial mud volcanic processes (for example, Kopf, 2002). These researchers 

interpreted that these features collectively attested to possible 

accumulations of water-saturated sediments within structural 

rings of the Utopia basin. Brož and Hauber (2013) presented an 

alternative interpretation to the mud volcano-like hypotheses 

of Skinner and Tanaka (2007). These researchers studied the 

morphologic and morphometric shapes of pitted cones in and 

around Nepenthes Planum (and other regions) and speculated 

that these features are very similar to terrestrial hydrovolcanic 

tuff rings and cones, implying a juvenile magamatic origin. The 

two contrary perspectives offered by Skinner and Tanaka (2007) 

and Brož and Hauber (2013) illustrate the ambiguity inherent to remote-based interpretations of Martian landforms and 

underscore a need for objective context afforded by scale-based 

geologic mapping.

As summarized above, the systematic observation, mapping, and topical investigation of the geology in the highlandlowland transitional province on Mars result in complex, varied, 

and often contradictory interpretations of the evolutionary history for this region. The local to regional evolution of the rocks 

and sediments of the Nepenthes Planum region, particularly 

those of intermediate (Hesperian) to young (Amazonian) age, 

remain poorly understood. Herein, we discriminate and describe 

the geologic and stratigraphic units of Nepenthes Planum as 

well as the bounding cratered southern highlands and low-lying 

northern plains and provide an evolutionary history based on 

geologic mapping, which can be compared and contrasted with 

other sections of the Martian highland-lowland transitional 

province.
3

Geography

Our mapping efforts focused on the geomorphic terrains 

and geologic units contained within Mars Transverse Mercator 

(MTM) 10237, 10242, 10247, 15237, 15242, 15247, 20237, 

20242, and 20247 quadrangles. These quadrangles are located 

in the eastern hemisphere of Mars and span 15° of latitude (lat 

7.5 to 22.5° N) and 15° degrees of longitude (long 110.0 to 

125.0° E). The map region, defined by the above quadrangles, 

is centered on Nepenthes Planum (fig. 1), a smooth to rolling, slightly north-sloping plain that forms the physiographic 

transition from higher-standing cratered highlands of northern 

Terra Cimmeria to the south and the lower-standing, relatively 

sparsely cratered lowlands of southern Utopia Planitia to the 

north. Nepenthes Mensae are located to the south of Nepenthes 

Planum and are defined by east-west-oriented collections of 

closely spaced plateaus, mesas, and knobs as well as their intervening plains. The Amenthes Cavi are located to the north of 

Nepenthes Planum and are defined by east-west-oriented arrays 

of isolated and coalesced depressions with arcuate, rounded 

to scarp-like margins. Amenthes Cavi demarcate the southern 

margin of Utopia Planitia, a >3,000-km-diameter, plate-shaped 

depression centered north of the map area near lat 46.4° N, long 

113.1° E. North-south-oriented scarps and ridges of the Hephaestus Rupēs occur in the central and northern part of the map 

region. Northwest-southeast-oriented fractures of Hephaestus 

Fossae cross the map region in the northeast. The Nepenthes 

Planum region contains multiple large-diameter impact craters, 

including Ehden (57.4 km diameter), Canillo (33.9 km), Tavua 

(31.6 km), Zaranj (27.4 km), Doba (25.9 km), Marbach (24.7 

km), Phedra (20.3 km), and Linpu (18.2 km). As a candidate for 

the Viking missions landing site, focus on the region resulted 

in the naming of several small-diameter impact craters in the 

northwest part of the map region. Additionally, several darkrayed craters—suggestive of relatively young, rocky ejecta—are 

named in the eastern part of the map region.

Base Map and Data

The extensive spatial coverage, high resolution, and 

diversity of post-Viking Orbiter data sets allow more detailed 

discrimination and analysis of the geologic materials and landforms that occur within and around Nepenthes Planum. Moreover, these data sets offer an improved context for interpreting 

the formational history of this and other highland-lowland 

transitional zones. Though photogeology and geomorphology 

continue to define the cornerstone of planetary geologic mapping (for example, Greeley and Batson, 1990), new techniques 

are required to most appropriately convey stratigraphic and 

structural relations, particularly at scales that adapt and merge 

multiple MTM quadrangles. Below, we describe the (1) characteristics of the primary geologic base map and (2) relevance and 

use of supplemental (secondary) data sets.

The primary base map used for this geologic map is 

composed of daytime thermal infrared (IR) images acquired by 

the ODY-borne Thermal Emission Imaging System (THEMIS) 

camera (Christensen and others, 2004) and compiled into a 

100-meter-per-pixel (m/px) mosaic by the U.S. Geological Survey Astrogeology Science Center. We augmented the THEMIS 

daytime IR base map with a 128 pixels/degree (~463 meters/

pixel) digital elevation model (DEM) interpolated from 300-m 

footprint surface altimetry measurements acquired by the MGSborne Mars Orbiter Laser Altimeter (MOLA) instrument (Smith 

and others, 2001). The geologic units that are mapped and 

described herein are geodetically tied to the THEMIS base map 

and MOLA DEM due to their full areal coverage of the selected 

quadrangles. We used Transverse Mercator projection with a 

center longitude of 110° E for all base data.

Though primary map bases establish the most critical 

contextual information needed to correlate geologic units and 

consistently map their bounding contacts and characteristics, 

we corroborated mapping results by integrating other data sets, 

including MOLA-derived products (in particular shaded-relief 

maps with color elevation scales) as well as the full range of 

ODY THEMIS visual, MGS Mars Orbiter Camera (MOC), 

MRO High Resolution Imaging Science Experiment (HiRISE), 

and MRO Context Camera (CTX) images via web-linked image 

footprints in a geographic information system (GIS) project. 

Though these data sets provided important information regarding unit texture and stratigraphic relations, they were considered 

supplemental because they provided neither complete areal coverage nor resolution compatible for discriminating discrete units 

at the publication map scale. Supplemental data sets were used 

to augment the accompanying Description of Map Units where 

clear and consistent textural and (or) stratigraphic characteristics were observed (table 1).

Methodology

We constructed the geologic map of the Nepenthes Planum 

region using methods that both follow and deviate from the 

approaches described by Tanaka and others (2005) for the 

hemisphere-scale geologic map of the northern plains of Mars. 

Below, we describe (1) unit groups, names, and symbols, (2) 

types of geologic contacts, (3) types of feature symbols, and (4) 

digital drafting parameters used to compile information.

Unit Groups, Names, and Labels

We name the geologic units that occur within the Nepenthes Planum region using terms that are either based on a 

pervasive landform or a particular genesis, where possible, in 

order to preserve objectivity (Salvatore, 1994; Hansen, 2000; 

Tanaka and others, 2005). Tanaka and others (2005) subdivided 

the Martian northern plains into specific geographic divisions 

and uniquely named and labeled the geologic units that occurred 

within these provinces as a means to partition the regional 

geologic histories and simplify description. In that map, the 

Nepenthes Planum region extends from highland terrains 

defined as “widespread materials” by Tanaka and others (2005) 

into the “Borealis province” of the northern plains. In contrast, 
4

Unit Name Unit label

Unit Area 

(Count Area)1

103 km2

N(1) N(5) N(16) Crater-density age2 Superposition relations3

Lowland Province/Utopia Region Units

Utopia 2 unit AHlu2 152.3 (154.8) 1,524.7±99.3 64.6±20.4 — lH–eA <Hlu1, Htnf, AHc; >AHc

Utopia 1 unit Hlu1 155.9 (168.0) 1,423.0±92.0 83.4±22.3 23.8 ±11.9 lH–eA <Htnv, Htnf, Nhc1, AHc, AHcea, 

AHceb; >AHlu2, AHc, AHceb

Transitional Province/Nepenthes Region Units

Nepenthes vent 

unit

Htnv 2.5 (2.5) 3221.6±1,139.0 — — — ~Htnf; >Hlu1, AHc

Nepenthes flow 

unit

Htnf 228.3 (292.8) 2,994.9±101.1 170.7±24.1 17.1±7.6 eH <HNhc3, Nhc1, AHc; ~Htnv; 

>Hlu1, AHc, AHcea, AHceb

Highland Province/Cimmeria Region Units

Cimmeria 3 unit HNhc3 24.0 (31.2) 2,983.8±309.4 224.6±84.9 96.3±55.6 lN–eH <Nhc2, Nhc1; >AHc, AHcea, 

AHceb

Cimmeria 2 unit Nhc2 20.8 (21.6) 3,048.9±375.3 231.0±103.3 92.4±65.3 lN–eH <Nhc1; >Nhc2,, AHc, AHceb

Cimmeria 1 unit Nhc1 42.1 (68.9) 2,958.7±207.1 290.1±64.9 43.5±25.1 lN >Hlu1, Htnf, HNhc3, Nhc2, AHc, 

AHcea, AHceb

Widely Occurring/Impact Crater Units

Crater material, 

undivided

AHc 38.6 — — — — <AHlu2, AHc, Hlu1, Htnv, Htnf, 

HNhc3, Nhc2, Nhc1, AHcea, 

AHceb, AHcw, AHcf, AHcp; 

>AHlu2, Hlu1, AHc, Htnf, 

AHcea AHceb

Crater ejecta, 

facies a (inner)

AHcea 36.0 — — — — ~AHceb, AHcr; >Hlu1, Htnf, 

Nhc3

Crater ejecta,

 facies b (outer)

AHceb 49.5 — — — — < Hlu1, Htnf, HNhc3, Nhc2, 

Nhc1, AHcea, AHceb; ~ 

AHcea, AHcr, AHcw, AHcf 

AHcp; >Hlu1, Htnf, AHceb, 

AHc

Crater rim AHcr 5.1 — — — — ~AHcea, AHcw, AHcf; >AHc

Crater wall AHcw 3.3 — — — — ~AHcr, AHcf, AHcp; >AHc

Crater floor AHcf 2.1 — — — — ~AHcw, AHcp, AHcr; >AHc

Crater peak AHcp 0.5 — — — — ~AHcw, AHcf; >AHc

1

 Count Area represents the unit area combined with the overlying impact crater units. See Age Determinations for details.

2

 Based on crater-density boundaries as determined by Werner and Tanaka (2011). A, Amazonian; H, Hesperian; N, Noachian; e, Early; m, Middle; l, Late. 

Table 1. Characteristics of geologic units in the mapped Nepenthes Planum region, Mars: areas, crater densities, and superposition 

relations.

[N(1), number of craters ≥1km in diameter per million square kilometers; N(5), number of craters ≥5km in diameter per million square kilometers; N(16), number of 

craters ≥16km in diameter per million square kilometers; —, not applicable or no data]
5

we denote our map regions as parts of global-scale geographic 

provinces (highland, transitional, lowland, and widely occurring) as indicated in the Correlation of Map Units. To assist with 

geographic, geologic, and stratigraphic description, we group 

geologic units of this map into three discrete geographic regions 

based on nomenclature approved by the International Astronomical Union (IAU): the Cimmeria, Nepenthes, and Utopia region 

units. The Cimmeria region units are those that comprise the 

rugged, high-standing massifs and large-diameter impact craters 

and basins that extend from approximately –300 m elevation 

in the southern part of the map region to approximately –1,300 

m elevation along the northern margin of Nepenthes Mensae 

(fig. 1). The Nepenthes region units are those that comprise the 

undulating, locally smooth to rugged plains that extend from 

approximately –1,300 m elevation along the northern margin 

of Nepenthes Mensae to approximately –3,300 m along the 

northern margin of Amenthes Cavi. The Utopia region units are 

those that comprise the nearly horizontal, low-lying plains that 

extend from approximately –3,300 m along the northern margin 

of Amenthes Cavi to approximately –3,900 m in the northern 

part of the map region.

The approach summarized above refines organizational 

efforts by Tanaka and others (2005) in two fundamental ways. 

First, we intentionally partition the geologic units that define 

the highland-lowland transitional zone, which allows for their 

description as geologically and stratigraphically unique entities. Second, we assign each geographic region names that 

correspond to their nearest (and largest) regional feature, which 

allows for comparison of geographically, and perhaps geologically, discrete regions along the globe-encircling highlandlowland transitional scarp. These groupings are shown in the 

COMU.

We name the units of the Nepenthes Planum map region 

based on their geographic occurrence as well as their stratigraphic relations to one another. We identify fourteen geologic 

units in the map region, as follows: two units of the Utopia 

region of the lowland province, two units of the Nepenthes 

region of the highland-lowland transitional province, three units 

of the Cimmeria region of the highland province, and seven 

impact crater units. The Utopia region units consist of the Utopia lowland 1 and 2 units. The Nepenthes region units consist 

of the Nepenthes transitional vent and flow units. The Cimmeria 

region units consist of the Nepenthes highland 1, 2, and 3 units. 

Impact crater units consist of the crater undivided, ejecta facies 

a (inner), ejecta facies b (outer), rim, wall, floor, and peak units.

We assign each mapped unit a label that uniquely identifies 

the chronologic period(s) in capital form (H, Hesperian Period). 

Next, for non-crater units, the geographic province in lowercase form (l, lowland province), followed by the geographic 

region in lower-case form (u, Utopia region). Crater units are 

indicated by a lower-case c. An additional descriptor is used as 

a subscript when it is necessary to identify either the interpreted 

stratigraphic order for closely related units (1, oldest of two 

or more sequentially emplaced units) or primary emplacement 

morphology (v, eruptive vent unit). Units that have morphologic 

descriptors are interpreted to have been emplaced contemporaneously. As examples, under this scheme, the geologic label 

“Nhc1” refers to the Noachian-age, stratigraphically lowest unit 

that occurs in the Cimmeria region of the Martian highlands, 

and the geologic label “Htnv” refers to the Hesperian-age eruptive vent unit that occurs in the Nepenthes region of the Martian 

highland-lowland transition. Finally, materials of craters >16 

km in rim diameter are divided into structural components (rim, 

wall, floor, peak, and ejecta) that require an additional small 

letter symbol, and the ejecta are subdivided into two morphologic facies—facies a (inner) and facies b (outer)—indicated as 

subscripts.

Contact Types

We delineate geologic units within the map region using 

contacts that are attributed with their interpreted level of certainty in identification and spatial placement as well as their 

geologic occurrence. A certain contact denotes the most precise 

contact between well-characterized geologic units that contrast in appearance and (or) interpreted relative age. In some 

instances, certain contacts have hachures to denote unit margins 

that are defined by lobate scarps (hachures on downslope side 

of contact). These are always assigned to certain contacts. An 

approximate geologic contact is regarded as less precise in its 

identification and (or) spatial placement due to data quality, subtlety of contact, and (or) obscuration by secondary processes.

Feature Types

The map includes a variety of morphologic feature symbols, following precedents established in previous terrestrial 

and planetary geologic maps. We subdivide our mapping of 

some features as indicated in the Explanation of Map Symbols. Although consistent mapping of features is desired, it is 

generally not possible or practical due to complications that can 

arise from (1) locally rugged terrain; (2) variation in feature 

character, length, and (or) relief; (3) feature orientation relative to illumination (for smaller features not resolved in MOLA 

data); and (4) areas of low data resolution and (or) incomplete 

data coverage. The features we mapped commonly grade in 

appearance and overlap in places and should be considered cartographic representations rather than exact in regard to spatial 

location and abundance. Linear features include ridges, troughs, 

scarps, and crater rims. Point features include pitted cones (discriminated by size) and mounds. A single polygon feature was 

mapped on unit Htnf to indicate rugged surface textures, which 

grade across this unit in the map region.

Digital Drafting Parameters

We used the Esri (v. 10.1, 1982–2013, Redlands, CA) ArcGIS to co-register and analyze available datasets. We digitized 

points, lines, and polygons using the digital streaming capability 

in ArcGIS. We digitized vector linework at a consistent scale 

of 1:250,000 (600 percent of the publication map scale), which 

allowed for substantially detailed lines for use in both hard-copy 

map publication, as well as a digital product, while remaining 

true to the publication map scale of 1:1,500,000. Vertex spacing 
6

was set to place points within linework at 250 m (1 vertex per 1 

millimeter (mm) at 1:250,000 digitizing map scale).

Linework was streamed using a WACOM Intuos 3 digital 

mapping tablet into an ArcGIS digital geodatabase in Transverse Mercator projection (center longitude of 110° E). The 

geodatabase housed the attribute information for each digitized 

feature, which was assigned iteratively using attribute domain 

settings stored within the geodatabase. Geologic map symbols 

are based on Federal Geographic Data Committee Digital Cartographic Standards for Geologic Map Symbolization (Federal 

Geographic Data Committee, 2006). Iterations of the digitized 

linework allowed for refinement of contact placement and unit 

descriptions based on cross-comparison between the map base 

and supplemental data sets. Contact linework was cleaned 

(dangles removed) and used to build unit polygons. To balance 

base map resolution and clear, consistent cartographic representation, all unit outcrops are >7 square kilometers (km2

) and line 

features are >3 km long, effectively representing a 2 mm lower 

limit at map scale for polygon and line features. Points features 

are based on the resolution of the THEMIS and MOLA base 

maps and are considered to be >500 m in diameter.

Geomorphology

The spatial and morphologic characteristics of landforms 

that occur within the geologic units of the Nepenthes Planum 

region provide information that aids in discerning the regional 

geologic history.

Ridges

We identify two types of ridges in the map region. A crenulated (wrinkle) ridge is a sub-linear landform whose crest is 

defined by singular or en echelon sets of prominent to subdued 

crenulations (that is, “wrinkles”). Crenulated ridges are commonly asymmetric in topographic profile and have dominant 

east-west and north-south orientations, though directional trends 

vary across the map region. North-south oriented crenulated 

ridges are most prominent within and north of Nepenthes 

Planum, where they define the axes of elongated topographic 

arches and parallel north-south outcrops of the Nepenthes flow 

unit (Htnf). These make up the southern extents of the Hephaestus Rupēs (fig. 1). Crenulated ridges generally rise 50 to 100 m 

above surrounding plains, though taller features occur locally. 

These features are generally between 3 and 5 km wide, with 

the widest portion occurring near the middle of the observable 

length. These types of ridges form a notable reticulate pattern in 

the Cimmeria 3 unit (HNhc3) located east of long 120° E. We 

mapped 434 wrinkle ridges within the map region ranging in 

length from 3.1 to 52.1 km (13.1 km mean).

A narrow, arcuate ridge is a <500-m wide, sub-linear to 

curving landform that most commonly occurs in nested sets 

with ridges of similar shape and orientation. These features 

appear to be <10 m tall, though the accuracy of these values are 

difficult to ascertain because feature width is comparable to the 

spatial resolution of topographic data. Narrow, arcuate ridges 

are composed of and (or) occur in close proximity to small 

diameter pitted cones (described below). Narrow, arcuate ridges 

occur exclusively in the Utopia 2 unit (AHlu2), typically within 

100 km of its contact with the Utopia 1 unit (Hlu1). Therein, 

features commonly link low-relief knobs and plateaus of the 

Nepenthes flow unit (Htnf; fig. 2). We mapped 828 narrow, arcuate ridges within the map region ranging in length from 3.0 to 

16.2 km (4.9 km mean).

Depressions

We identify three types of negative relief landforms in the 

map region. A subdued groove is a linear to sub-linear, shallowwalled depression. These features occur in the Nepenthes flow 

unit (Htnf) in Nepenthes Planum as well as around the central 

depressions of Amenthes Cavi. Troughs also occur in plateaus 

and mesas of the Cimmeria 1 unit (Nhc1) in northern Terra 

Cimmeria and Nepenthes Mensae. Troughs are generally <1 km 

wide (though locally wider features are common in highland 

units) and <10 m deep, though these features may locally be 

several tens of meters deep. We mapped 325 troughs in the map 

region ranging in length from 3.0 to 40.0 km (6.1 km mean).

A trough is a linear to sub-linear, steep-walled depression 

that ranges from several tens of meters to up to several hundreds 

of meters in depth. These features occur in the northeast part of 

the map region and collectively form the Hephaestus Fossae. 

Troughs trend northwest-southeast and northeast-southwest and 

are oriented orthogonal to one another, forming a characteristic 

polygonal pattern. These features are located entirely in the 

Utopia 1 and 2 units (AHlu2 and Hlu1, respectively). Grooves 

are both continuous and discontinuous, the latter occurring as 

linear segments connected by narrow septa (fig. 3). These types 

of troughs occur between –3506 and –3908 m elevation, with 

discontinuous segments becoming more frequent to the northwest and at lower elevation. We mapped 322 grooves in the map 

region ranging in length from 3.0 to 134.4 km (11.0 km mean).

A channel is mapped herein as the axis of a valley that has 

a sinuous to anastomosing planimetric shape and that tends to 

merge with like features in the downslope direction. Channels 

occur in shallow highland basins in the Cimmera 2 and 3 units 

(Nhc2 and HNhc3, respectively) along the highland margin of 

the highland-lowland transitional zone (fig. 4). Channels can 

locally be positioned on positive relief segments (fig. 5). We 

mapped 42 channel troughs in the map region ranging in length 

from 3.5 to 23.3 km (8.1 km mean).

Scarps

 We identify two types of topographic scarps in the Nepenthes Planum region. A base of scarp identifies the basal topographic inflection of a linear to curving, sharp to subdued scarp. 

In northern Terra Cimmeria, scarp bases occur individually and 

as sets that define the margins of east-west trending massifs 

(roughly parallel to the highland-lowland boundary scarp). 

Scarp bases between Amenthes Cavi and southern Utopia Planitia occur parallel and in close spatial association with northsouth trending crenulated ridges and outcrops of the Nepenthes 
7

flow unit (Htnf). In the plains of southern Utopia Planitia, scarp 

bases occur in arcuate segments that outline shallow, circular 

depressions. We mapped 155 scarp bases in the map region 

ranging in length from 4.0 to 38.5 km (11.9 km mean).

A lobate scarp is a topographically subtle, curving scarp 

that defines the margins and terminations of lobes that are 

generally <10 m tall. These features occur predominantly in the 

Nepenthes flow (Htnf) as well as in the Utopia 2 unit (AHlu2) 

near the contact with the Utopia 1 unit (Hlu1). Lobate scarps 

also locally define geologic contacts throughout the map region. 

We mapped 198 lobate scarps in the map region ranging in 

length from 4.5 to 67.8 km (14.3 km mean).

Pitted Cones and Mounds

We identify two types of positive relief landforms within 

the map region. A pitted construct is a conical landform that 

contains a central pit or depression. We subdivide the regional 

population of pitted constructs based on the basal diameter of 

observed features. A small pitted construct is generally between 

500 m and 1.0 km in diameter, has sloping flanks, and appears 

to have a central pit that is near the limit of the THEMIS base 

map resolution (fig. 2). Small pitted constructs are either isolated or aligned and occur almost entirely in the Utopia 2 unit 

(AHlu2), often in close association with narrow, arcuate ridges 

(see Scarps). A few of these features are mapped within the Utopia 1 unit (Hlu1

) within the Amenthes Cavi. We mapped >3300 

of the small pitted constructs in the map region.

Large pitted constructs are from 1.9 to 9.1 km in diameter 

(4.8 km mean), have sloping or rounded flanks, and have a 

central pit with a diameter that is commonly >50 percent of the 

feature’s total basal diameter (fig. 6). Large pitted constructs are 

either isolated or coalesced and either circular or crescentic in 

planimetric shape. They occur almost entirely in the Nepenthes 

flow unit (Htnf) west of long 119° E. Several features occur in 

the depressions of Amenthes Cavi where they are surrounded 

by the smooth plains of the Utopia 1 unit (Hlu1). We mapped 75 

large pitted constructs in the map region.

Small tholi (that is, mounds) range from 2.0 to 6.6 km 

in diameter (4.7 km mean) and have marginal scarps, subtle 

interior slopes, circular to ovoid planimetric shape, and in some 

cases central peaks (fig. 7). Mounds are most often isolated 

rather than coalesced and occur entirely in the Nepenthes flow 

unit (Htnf), south of the Amenthes Cavi, often adjacent to 

wrinkle ridges. We mapped 109 mounds in the map region.

Surface Texture

We identify a single texture—rugged surface—within 

the map region, which is confined entirely to the transitional 

Nepenthes flow unit (Htnf). These surfaces are pitted and hummocky at THEMIS base map resolution (fig. 6), grade laterally 

into smoother surfaces within unit Htnf, and occur entirely west 

of long 118° E in Nepenthes Planum. We mapped 22 discrete 

occurrences of the rugged surface in unit Htnf, with areas ranging from approximately 20 to >38,000 km2

 (45,716 km2 total).

Age Determinations

The cumulative densities of impact craters greater than 

specified diameters provide some stratigraphic control and 

temporal constraint (table 1), as do crosscutting and superposition relations. Crater densities were derived for non-crater units 

using computed map areas and a project-specific catalog of all 

craters ≥1 km in diameter. (Crater units were not counted since 

each crater represents an isolated event, and the occurrences are 

generally too small in areal extent for reliable statistical results.) 

Craters within this size range were catalogued using a threepoint tool to measure impact crater rim diameters. Crater rims 

were identified on the THEMIS base map and verified/refined 

where necessary using MOLA topography and derived products. Map units were assigned to the eight Martian epochs on 

the basis of the scheme of Tanaka (1986) as updated by Werner 

and Tanaka (2011) for the number of craters ≥1, 5, and 16 km in 

diameter per million square kilometers (N(1), N(5), and N(16), 

respectively), as well as on stratigraphic relations. Unit areas, 

crater densities, crater-density ages, and superposition relations 

with contiguous units are presented in table 1.

Our unit age assignments take into account first and 

foremost unit superposition relations observed and interpreted 

from mapping observations, which define the sequence of unit 

emplacement. Crater-density ages, primarily from our results 

(table 1), but also in consideration of regional crater counts having less statistical error but more geologic uncertainty (Tanaka 

and others, 2005, 2014; Werner and Tanaka, 2011), provide 

some calibration to the age assignments. The crater-density 

ages provide a mean surface age, even though some units may 

have been emplaced over a considerable time span. In addition, 

smaller craters can be destroyed by later modification, yielding 

younger surface ages for smaller craters (see Platz and others, 

2013). For the Nepenthes Planum region, the N(1) crater densities consistently yield younger ages than N(5) ages and thus do 

not appear to be reliable for assigning unit emplacement ages.

The Cimmeria 1 unit has a Late Noachian crater density 

that is even lower than that of the Cimmeria 2 unit, but in the 

map region it is composed of remnants that we interpret were 

being emplaced and modified during the Hesperian. As such, we 

suggest that the age for the Cimmeria 1 unit is better reflected 

in ages determined from regional and global mapping (Tanaka 

and others, 2014), which are Early to Middle Noachian. The 

Cimmeria 2 unit is assigned to the Middle to Late Noachian, 

which is consistent with N(5) and N(16) crater densities and 

superposition relations. The Cimmeria 3 unit overlies the Cimmeria 2 unit and its larger crater densities are consistent with 

a Late Noachian to Early Hesperian age. The Nepenthes flow 

unit has an N(5) age and stratigraphic relations consistent with 

an Early Hesperian age. The Nepenthes vent unit is coeval with 

the Nepenthes flow unit. The Utopia 1 unit has a Late Hesperian 

age based on both crater density and superposition relations 

with regional to global units (Tanaka and others, 2014). The 

Utopia 2 unit has a mean N(5) age at the Hesperian/Amazonian 

boundary. The mapped crater units all represent cases of craters 

with well-preserved rims and ejecta, which indicate that they 

post-date globally high rates of erosion that terminated at the 

end of the Noachian Period. In some cases, particular craters 
8

have superposition relations with map units that provide some 

age constraint. Units derived from Ehden crater appear to be 

embayed by the Cimmeria 3 unit and are thus likely to have 

been emplaced during the Early Hesperian. Units derived from 

Marbach, Phedra, and Tavua craters superpose the Nepenthes 

flow unit and are embayed by the Utopia 1 unit, indicating that 

they were likely emplaced during the Early to Late Hesperian. 

Units derived from Linpu crater show no clear indication of 

embayment by the Utopia 1 unit, implying emplacement during 

the Late Hesperian or Early Amazonian (or perhaps later).

Geologic Summary

In this section we summarize the geologic history of the 

Nepenthes Planum highland-lowland transitional province of 

Mars based on the geologic unit and landform mapping and 

stratigraphic determinations presented herein. This summary is 

portrayed graphically in the accompanying Correlation of Map 

Units.

Noachian Period

The recorded geologic history of the Nepenthes Planum 

region of Mars begins in the Early to Middle Noachian during 

which the Cimmeria 1 unit (Nhc1) was emplaced. The Cimmeria 1 unit likely represents regolith composed of impact 

breccia and melt, volcanic rocks, and clastic sedimentary rocks 

of aeolian, alluvial, mass wasting, and other geologic materials of undetermined origins. Crater densities indicate the unit 

Nhc1 post-dates the formation of the Utopia multi-ring impact 

basin, which impacted into rocks that likely underlie the oldest 

exposed Noachian rocks within the map region. The Cimmeria 

1 unit outcrops are oriented sub-parallel to the regional eastwest orientation of the highland-lowland transitional scarp, raising the possibility that some of these outcrops are impact-modified massifs uplifted by the crater- and basin-forming impact 

events, specifically the Utopia impact. Lithologic and (or) depositional characteristics that may indicate the direct origin of the 

rocks and sediments within the unit Nhc1 are altogether absent. 

They are generally interpreted to represent degraded crustal 

rocks and highland regolith that were perhaps subsequently 

modified by mass-wasting processes along the highland-lowland transitional scarp. Unit Nhc1

 is separated from the Cimmeria 2 unit (Nhc2

) based on topographic inflections, wherein 

the older unit is generally characterized by high -relief, rugged 

outcrops. The stratigraphic nature of the contact between units 

Nhc1 and Nhc2 is likely of embayment given its general topographic form, though high-resolution images demonstrate that 

clear evidence of embayment is obscured. We interpret this lack 

of local unit embayment as evidence that the emplacement and 

subsequent degradation of the first two highland units continued 

throughout much of the Noachian Period because of the high 

rate of surface impacts. The low-lying, basin-filling character of 

the Cimmeria 2 unit—relative to the Cimmeria 1 unit—suggests 

that the unit accumulated through the erosion of higher-standing 

units, likely as the result of fluvial, colluvial, and (or) lacustrine 

processes. Large-diameter impact craters are abundant in units 

Nhc1 and Nhc2, suggesting these are made up of significant 

amounts of impact-related ejecta and brecciated highland rocks 

(that is, Martian “regolith”). Local inclusions of volcanic rocks 

are possible, though lithologic and (or) depositional characteristics that definitively outline such components are sparse.

The occurrence of wrinkle ridges in the Cimmeria 2 unit 

indicates crustal shortening in both the east-west and northsouth direction during the Middle to Late Noachian. Though 

younger units obscure the actual extent of unit Nhc2 across the 

highland-lowland boundary scarp, it is likely that unit extended 

onto part of the proto-boundary plain. Local pitting and isolated 

knobs and mesas within unit Nhc2 suggest it was modified by 

erosional deflation and (or) localized subsidence after emplacement during the Middle to Late Noachian.

Early Hesperian Epoch

The timing of events in this epoch is perhaps the most 

tightly controlled for the geologic units mapped within the 

Nepenthes Planum region owing to excellent exposure and 

well-constrained crater counts. We infer the existence of a 

depositional hiatus between the Cimmeria 2 and 3 units during 

the Late Noachian. Later, during the Late Noachian to Early 

Hesperian, the Cimmeria 3 unit (HNhc3) was emplaced as 

irregularly shaped, isolated patches of bright, sloping, and hummocky surfaces on the lower reaches of the regional highlandlowland boundary scarp, particularly in impact crater basins and 

low-relief depressions bounded by the Cimmeria 2 unit. The 

unit also surrounds low-relief outliers of Cimmeria 1 unit. The 

Cimmeria 3 unit’s northern extent is obscured because of burial 

by younger units. The unit’s southern margins are commonly 

lobate in form. The occurrence of narrow, braided valleys and 

ridges (fig. 5) indicate the role of fluids in the emplacement 

history of the Cimmeria 3 unit. Outcrops in the southeastern 

map region contain rugged lobate margins and small depressions. These provide some evidence of their emplacement and 

modification through fluid processes, perhaps owing to eruption 

of lava or liquefied sediment followed by local subsidence and 

(or) collapse. The unit appears to pre-date all large (>16 km) 

diameter impact craters, with the exception of Ehden.

The Nepenthes vent and flow units (Htnv and Htnfv, respectively) dominate the central part of the map region and compose 

the areal bulk of Nepenthes Planum. The Nepenthes flow unit 

was emplaced as a series of overlapping lobes, evidenced by the 

lobate margin that forms the southern boundary, which overlies 

the Cimmeria 3 unit, as well as the occurrence of abundant 

lobes within the unit itself (fig. 6). The unit is interpreted to 

be composed of either erupted sheet lavas, fluidized sediments 

(mud flows), or perhaps a combination of both (Tanaka and 

others, 2003b). Source regions for the Nepenthes flow unit are 

likely to be the (large diameter) pitted cones and mounds that 

dot the region, as well as perhaps fissure-like troughs. Some 

flows are directly traceable to large-diameter pitted cones 

(Skinner and Tanaka, 2007) mapped as the Nepenthes vent unit, 

though others appear to emerge from the overall plains. The 

Nepenthes flow unit includes a characteristic rugged textural 

facies that we identify using a hachured pattern, whereas the 
9

remainder has a smoother surface. The rugged facies sometimes forms lobate margins but is more often associated with a 

lateral gradation to the smoother facies. The range of textures 

of the Nepenthes flow unit likely results from differing eruption 

characteristics and (or) post-eruption modification (for example, 

desiccation, deflation, and (or) aeolian scour and mantling).

The Nepenthes vent unit identifies constructional edifices 

formed through the eruption of pyroclasts (Brož and Hauber, 2013) or fluidized sediment (Skinner and Tanaka, 2007). 

Though each hypothesis offers a markedly different perspective on the geologic evolution of Nepenthes Planum—as well 

as the underlying tectonic environment—the determination of 

one hypothesis over another is complicated by the dominance 

of dust and a general lack of compositional signatures. Brož 

and Hauber (2013) favored a phreatomagmatic origin for the 

large pitted cones of Nepenthes Planum, which also occur in 

adjacent highland-lowland transitional zones to the west and 

northwest, based on their morphometric similarity to terrestrial 

tuff rings and cones. Skinner and Tanaka (2007), however, 

favored a mud volcano-like origin of the large pitted cones and 

small tholi based on their presumed occurrence within structurally annular compartments of the Utopia multi-ring impact 

basin (for example, McGill, 1989) as well as their extended 

distance from the large-scale Elysium volcanic center known 

to have been active during this time (Tanaka and others, 1992, 

2014). The shape of the vents that make up the transitional vent 

unit changes across the map region; the eastern vents are small 

tholi and the western vents are large pitted cones with a central 

depression. Lobate materials appear to emanate from the latter 

in multiple instances. Small tholi that are located in the eastern 

map region are more commonly associated with wrinkle ridges, 

which Skinner and Tanaka (2007) cited as further evidence of a 

mud volcano-like genesis, resulting from subsurface pressures 

and the ascent and eruption of buoyant sediments. Brož and 

Hauber (2013) suggested that morphometry implicates an origin 

more akin to terrestrial lava coulees.

Large, north-south-oriented wrinkle ridges and scarps form 

the Hephaestus Rupēs, which occur in unit Htnv of Nepenthes 

Planum. These ridge and scarp systems rarely extend into the 

Cimmeria 3 unit (HNhc3) to the south, suggesting that they 

formed within a discrete sequence of materials. Older, Cimmeria 1 unit (Nhc1

) outcrops are the most dominant high-relief 

features in Nepenthes Planum and are erosional vestiges of geologic units associated with the cratered highlands. Well-defined 

ejecta blankets with rampart margins surround impact craters in 

this region. Past studies suggest that these crater-related landforms form as the result of the incorporation of subsurface water 

or ice into the impact-brecciated and ejected target materials, 

which form mobile, ground-hugging debris flows (for example, 

Barlow, 2006, 2010). Depth-diameter ratios of impact craters 

within this region are smaller than global averages (Boyce and 

others, 2005), implicating a lack of rheological strength in target 

materials such as may occur from weak (unconsolidated) rock 

sequences and (or) the presence of subsurface water and (or) ice 

during the Noachian and Hesperian.

Late Hesperian Epoch

The northern margin of Nepenthes Planum is defined by 

isolated and coalesced depressions of Amenthes Cavi, which 

are characterized by gently sloping margins and are often ringed 

by narrow, arcuate troughs. The Amenthes Cavi are filled with 

the Utopia 1 unit (Hlu1), which is characterized by a smooth 

surface. Unit Hlu1 is traceable to narrow troughs that bound 

Amenthes Cavi (fig. 8), which we interpret as evidence that 

these cavi-bounding linear shallow troughs were the source 

of unit Hlu1. Small, streamlined islands and shallow channels 

emanate from these narrow, arcuate troughs, suggesting that the 

material that occurs within—and apparently fills—the Amenthes Cavi had low viscosity (fig. 8). Given the association of 

arcuate fractures, channels and islands, and smooth cavi infill, 

we believe that the eruption of material to form the Utopia 1 

unit (Hlu1) was contemporaneous with the fracturing of the 

Nepenthes flow unit (Htnf) formation of Amenthes Cavi through 

subsidence and (or) collapse. Evacuation of subsurface reservoirs of interstitial water presumably led to focused subsidence 

and marginal fracturing of the existing plains. A lack of lobes or 

channels within the Utopia 1 unit suggests that the material was 

made up of fine-grained clastic material emplaced largely by 

sheet flow. The Utopia 1 unit extends to the north 100 to 200 km 

and forms the first of two nearly horizontal lowland sequences 

within the map region.

Phedra, Tavua, Linpu, and Marbach craters are each 

located partly to wholly within Amenthes Cavi and are surrounded by ejecta that have been partly embayed by the Utopia 

1 unit. The formation of Amenthes Cavi and eruption (or extrusion) of fluid material from the subsurface appears contemporaneous. Close association with large-diameter impact craters and 

their embayment by this younger unit raise the possibility that 

these impacts (separately or collectively) may have provided the 

triggering mechanism for the formation of parts of Amenthes 

Cavi and the contemporaneous eruption or extrusion of pressurized, fluidized subsurface material. The Utopia 1 unit embays 

and surrounds north-south ridges of the Nepenthes flow unit.

Amazonian Period

The materials that occur in the northern part of the map 

region are among the southernmost extension of materials that 

cover vast portions of the northern plains (Tanaka and others, 

2005, 2014). Within the map region, this unit is mapped as the 

Utopia 2 unit (AHlu2) because of close spatial association and 

physical appearance with the Utopia 1 unit. The Utopia 2 unit 

was emplaced at the end of the Hesperian to the beginning 

of the Amazonian. A series of south-facing lobes define the 

Utopia 2 unit’s southern margin. These lobes characteristically 

surround and embay tens-of-meters-high knobs and plateaus 

(fig. 2), which are mapped herein as outliers of the Nepenthes 

flow unit because their heights are consistent with those of 

larger, clearly identifiable outcrops of that unit to the south. The 

south-facing—and notably upslope—orientation of the Utopia 

2 unit’s southern margin implies fluid emplacement from the 

north. These features are similar in shape and orientation to 

others documented elsewhere along the northern plains margin 
10

(Tanaka and others, 2005). Narrow, arcuate ridges that parallel 

the unit’s lobate margin are common and are everywhere associated with sub-kilometer-diameter pitted cones. This ridge-cone 

pattern has been mapped and described as thumbprint terrain 

in several locations of the Martian northern plains (Tanaka and 

others, 2005).

References Cited 

Barlow, N.G., 2006, Impact craters in the northern hemisphere 

of Mars—Layered ejecta and central pit characteristics: 

Meteoritics and Planetary Science, v. 41, no. 10, p. 1,425–

1,436, doi: 10.1111/j.1945-5100.2006.tb00427.x.

Barlow, N.G., 2010, What we know about Mars from its impact 

craters: Geological Society of America Bulletin, v. 122, no. 

5–6, p. 644–657, doi:10.1130/B30182.1.

Boyce, J.M., Mouginis-Mark, P., and Garbeil, H., 2005, 

Ancient oceans in the northern lowlands of Mars—Evidence from impact crater depth/diameter relationships: 

Journal of Geophysical Research—Planets, v. 109, no. E3, 

doi:10.1029/2004JE002328.

Bridges, J.C., Grady, M.M., Pillinger, C.T., and Sims, M.R., 

1999, Landing site selection for the Beagle 2 mission to 

Mars: Meteoritics and Planetary Science, v. 34, no. 4 (supplement), p. A19.

Brož, P., and Hauber, E., 2013, Hydrovolcanic tuff rings and 

cones as indicators for phreatomagmatic explosive eruptions 

on Mars: Journal of Geophysical Research—Planets, v. 118, 

no. 8, p. 1,656–1,675, doi: 10.1012/jgre.20120.

Carr, M.H., and Head, J.W., III, 2003, Oceans on Mars—An 

assessment of the observational evidence and possible fate: 

Journal of Geophysical Research—Planets, v. 108, no. E5, p. 

8–1 to 8–28, doi:10.1029/2002JE001963.

Christensen, P.R., Jakosky, B.M., Kieffer, H.H., Malin, M.C., 

McSween, H.Y., Nealson, K., Mehall, G.L., Silverman, S.H., 

Ferry, S., Caplinger, M., and Ravine, M., 2004, The Thermal 

Emission Imaging System (THEMIS) for the Mars 2001 

Odyssey Mission: Space Science Reviews, v. 110, p. 85–130.

Clifford, S.M., and Parker, T.J., 2001, The evolution of the Martian hydrosphere—Implications for the fate of a primordial 

ocean and the current state of the northern plains: Icarus, v. 

154, no. 1, p. 40–79, doi:10.1006/icar.2001.6671.

Federal Geographic Data Committee [prepared for the Federal 

Geographic Data Committee by the U.S. Geological Survey], 

2006, FGDC Digital cartographic standard for geologic map 

symbolization: Reston, Va., Federal Geographic Data Committee Document Number FGDC–STD–013–2006, 290 p., 2 

plates.

Frey, H.V., and Schultz, R.A., 1990, Speculations on the origin 

and evolution of the Utopia-Elysium lowlands of Mars: Journal of Geophysical Research—Solid Earth, v. 95, no. B9, p. 

14,203–14,213, doi:10.1029/JB095iB09p14203.

Ghatan, G.L., and Zimbelman, J.R., 2006, Paucity of candidate 

coastal constructional landforms along proposed shorelines on Mars—Implications for a northern lowlands-filling 

ocean: Icarus, v. 185, no. 1, p. 171–196, doi:10.1016/j.

icarus.2006.06.007.

Golombek, M.P., Cook, R.A., Moore, H.J., and Parker, T.J., 

1997, Selection of the Mars Pathfinder landing site: Journal 

of Geophysical Research—Planets, v. 102, no. E2, p. 3,967–

3,988, doi:10.1029/96JE03318.

Greeley, R., and Batson, R.M., eds., 1990, Planetary mapping: 

New York, NY, Cambridge University Press, 296 p.

Greeley, Ronald, and Guest, J.E., 1987, Geologic map of the 

eastern equatorial region of Mars: U.S. Geological Survey Geologic Investigations Series Map I–1802–B, scale 

1:15,000,000.

Hansen, V.L., 2000, Geologic mapping of tectonic planets: 

Earth and Planetary Science Letters, v. 176, no. 3–4, p. 

527–542, doi:10.1016/S0012-821X(00)00017-0.

Head, J.W., III, Kreslavsky, Mikhail, Hiesinger, Harald, 

Ivanov, Mikhail, Pratt, Stephen, and Seibert, Nicole, 1998, 

Oceans in the past history of Mars—Tests for their presence using Mars Orbiter Laser Altimeter (MOLA) data: 

Geophysical Research Letters, v. 25, no. 24, p. 4,401–4,404, 

doi:10.1029/1998GL900116.

Hiller, K.H., 1979, Geologic map of the Amenthes quadrangle 

of Mars: U.S. Geological Survey Miscellaneous Investigations Map I-1110, scale 1:5,000,000.

Ivanov, M.A., Hiesinger, H., Erkeling, G., and Reiss, D., 2014, 

Mud volcanism and morphology of impact craters in Utopia 

Planitia on Mars—Evidence for the ancient ocean: Icarus, v. 

228, p. 121–140, doi:10.1016/j.icarus.2013.09.018.

Kopf, A.J., 2002, Significance of mud volcanism: 

Reviews of Geophysics, v. 40, no. 2, p. 2-1 to 2-52, 

doi:10.1029/2000RG000093.

Kreslavsky, M.A., and Head, J.W., III, 2000, Kilometer-scale 

roughness of Mars—Results from MOLA data analysis: 

Journal of Geophysical Research—Planets, v. 105, no. E11, 

p. 26,695–26,711, doi:10.1029/2000JE001259.

Kreslavsky, M.A., and Head, J.W., 2002, Fate of outflow 

channel effluents in the northern lowlands of Mars—The 

Vastitas Borealis Formation as a sublimation residue 

from frozen ponded bodies of water: Journal of Geophysical Research—Planets, v. 107, no. E12, p. 4-1 to 4-25, 

doi:10.1029/2001JE001831.

Masursky, Harold, and Crabill, N.L., 1981, Viking site selection 

and certification: NASA Special Paper, no. 429. 
11

McGill, G.E., 1989, Buried topography of Utopia, Mars—Persistence of a giant impact depression: Journal of Geophysical 

Research—Solid Earth, v. 94, no. B3, p. 2,753–2,759.

Nimmo, Francis, and Tanaka, Ken, 2005, Early crustal 

evolution of Mars: Annual Review of Earth and Planetary Sciences, v. 33, p. 133–161, doi:10.1146/annurev.

earth.33.092203.122637.

Parker, T.J., Gorsline, D.S., Saunders, R.S., Pieri, D.C., 

and Schneeberger, D.M., 1993, Coastal geomorphology of the Martian northern plains: Journal of Geophysical Research—Planets, v. 98, no. E6, p. 11,061–11,078, 

doi:10.1029/93JE00618. 

Platz, Thomas, Michael, Gregory, Tanaka, K.L., Skinner, J.A., 

Jr., and Fortezzo, C.M., 2013, Crater-based dating of geological units on Mars—Methods and application for the new 

global geological map: Icarus, v. 225, no. 1, p. 806–827, 

doi:10.1016/j.icarus.2013.04.021.

Salvatore, A., ed., 1994, International stratigraphic guide—A 

guide to stratigraphic classification, terminology, and procedure (2d ed.): Boulder, Colo., The Geological Society of 

America, 214 p.

Scott, D.H., and Carr, M.H., 1978, Geologic map of Mars: U.S. 

Geological Survey Miscellaneous Investigations Map I-1083, 

scale 1:25,000,000.

Skinner, J.A., Jr., and Tanaka, K.L., 2007, Evidence for and 

implications of sedimentary diapirism and mud volcanism 

in the southern Utopia highland-lowland boundary plain, 

Mars: Icarus, v. 186, no. 1, p. 41–59, doi:10.1016/j.icarus.2006.08.013.

Skinner, J.A., Jr., and Mazzini, Adriano, 2009, Martian mud 

volcanism—Terrestrial analogs and implications for formational scenarios: Marine and Petroleum Geology, v. 26, no. 9, 

p. 1,866–1,878, doi:10.1016/j.marpetgeo.2009.02.006.

Smith, D.E., Zuber, M.T., Frey, H.V., and 21 others, 2001, Mars 

Orbiter Laser Altimeter—Experiment summary after the 

first year of global mapping of Mars: Journal of Geophysical Research—Planets, v. 106, no. E10, p. 23,689–23,722, 

doi:10.1029/2000JE001364.

Tanaka, K.L., 1986, The Stratigraphy of Mars, in Proceedings 

of the seventeenth Lunar and Planetary Science Conference, 

Part 1: Journal of Geophysical Research—Solid Earth, v. 91, 

no. B13, p. E139–158.

Tanaka, K.L., Carr, M.H., Skinner, J.A., Gilmore, M.S., and 

Hare, T.M., 2003b, Geology of the MER 2003 “Elysium” 

candidate landing site in southeastern Utopia Planitia, Mars: 

Journal of Geophysical Research—Planets, v. 108, no. E12, 

doi:10.1029/2003JE002054.

Tanaka, K.L., Chapman, M.G., and Scott, D.H., 1992, Geologic 

map of the Elysium region of Mars: U.S. Geological Survey 

Miscellaneous Investigations Map I-2147, scale 1:5,000,000.

Tanaka, K.L., Skinner, J.A., Jr., Dohm, J.M., and 6 others, 2014, 

Geologic map of Mars: U.S. Geological Survey Scientific 

Investigations Map 3292, scale 1:20,000,000.

Tanaka, K. L., Skinner, J. A., Jr., and Hare, T. M., 2005, 

Geologic map of the northern plains of Mars: U.S. Geological Survey Scientific Investigations Map 2888, scale 

1:15,000,000.

Tanaka, K.L., Skinner, J.A., Jr., Hare, T.M., Joyal, Taylor, and 

Wenker, Alisa, 2003a, Resurfacing history of the northern 

plains of Mars based on geologic mapping of Mars Global 

Surveyor data: Journal of Geophysical Research—Planets, v. 

108, no. E4, doi:10.1029/2002JE001908.

Werner, S.C., and Tanaka, K.L., 2011, Redefinition of the craterdensity and absolute-age boundaries for the chronostratigraphic systems of Mars: Icarus, v. 215, no. 2, p. 603–607, 

doi:10.1016/j.icarus.2011.07.024.