180° E

0° E

30° N

75° N

60° N

270° E SIM 3177 90° E

2012

U.S. Department of the Interior

U.S. Geological Survey

Prepared for the National Aeronautics and Space Administration

Geologic Map of the North Polar Region of Mars

By Kenneth L. Tanaka and Corey M. Fortezzo

Pamphlet to accompany

Scientific Investigations Map 3177
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Contents

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

Geography ......................................................................................................................................................1

Base Map and Data ......................................................................................................................................2

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

Age Determinations ......................................................................................................................................4

Geologic Summary ........................................................................................................................................4

Hesperian Period .................................................................................................................................4

Early Amazonian Epoch ......................................................................................................................5

Middle Amazonian Epoch (?) .............................................................................................................5

Late Amazonian Epoch .......................................................................................................................6

References Cited ...........................................................................................................................................7

Tables

1. Map units in the Planum Boreum region of Mars: Areas, crater densities, relative and model 

absolute ages, and superposition relations. ...........................................................................10

2. Mapped features. ...................................................................................................................................11

3. Crater types distinguished in figure 7. ................................................................................................11

Figures

1. Shaded-relief view of Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) 

digital elevation model (DEM) of the map region (230 m/pixel; also version at 115 m/pixel 

used in mapping), which forms the primary map base (map sheet) 

2. Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) image mosaic (constructed at 6.2 

m/pixel; version provided here is 24 m/pixel due to size) of the Planum Boreum region of 

Mars in Polar Stereographic projection (map sheet)

3. Mars Odyssey (ODY) mission Thermal Emission Imaging System (THEMIS) visual-range image 

mosaics of the Planum Boreum region in Polar Stereographic projection (courtesy of 

P.R. Christensen, Arizona State University) (map sheet)

4. Plot showing locations of Mars Global Surveyor (MOC) (>3 m/pixel) and Mars 

Reconnaissance Orbiter (HiRISE) (25 cm/pixel) images in the map region obtained 

during all seasons and released as of May 2009, on a background showing the 

topography in shaded relief based on the MOLA DEM (map sheet)

5. Composite mosaic of three color images acquired by the Mars Reconnaissance Orbiter Mars 

Color Imager (MARCI) on October 16th of 2006 during Martian northern summer, solar 

longitude, Ls = 121° (map sheet)

6. Map of unconformities (red lines) and grabens (black lines with balls in box) in Planum 

Boreum 1 unit and Planum Boreum 3 unit at full map scale to show detail (map sheet)

7. Plot showing the distribution of impact craters > 2 km in diameter in the map region (see table 

3 regarding morphologic and other characteristics) (map sheet)

8. Graphs showing crater size-frequency distributions of impact craters in the map region for 

the Planum Boreum rupēs unit (red data) (map sheet)
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1

Introduction

The north polar region of Mars occurs within the central and lowest part of the vast northern plains of Mars and is 

dominated by the roughly circular north polar plateau, Planum 

Boreum. The northern plains formed very early in Martian time 

(Nimmo and Tanaka, 2005; Frey, 2006) and have collected 

volcanic flows and sedimentary materials shed from highland 

sources (Tanaka and others, 2005b). Planum Boreum has 

resulted from the accumulation of water ice and dust particles 

(Thomas and others, 1992; Byrne, 2009). Extensive, uncratered 

dune fields adjacent to Planum Boreum attest to the active and 

recent transport and accumulation of sand (Bourke and others, 

2008).

Earlier geologic maps of the north polar region are largely 

based on Viking data and vary in scale from local 1:500,000-

scale quadrangles (Dial and Dohm, 1994; Herkenhoff, 2003) 

to regional 1:5,000,000- (Dial, 1984) and global 1:15,000,000-

scale products (Tanaka and Scott, 1987). The most recent formal 

map covering the region is at 1:15,000,000 scale and used 

topography and image data from the Mars Global Surveyor and 

Mars Odyssey missions (Tanaka and others, 2005b), and part of 

Planum Boreum, Mars Transverse Mercator quadrangle 85200 

(Skinner and Herkenhoff, map in preparation) was prepared 

coincident with this map at 1:500,000 scale using the same datasets herein. In addition, dune forms and migration directions 

derived from them were mapped by Tsoar and others (1979) 

using Viking images. Tanaka and Hayward (2008) remapped 

migration directions and measured local dune spatial densities 

based on higher resolution post-Viking images. A preliminary, 

page-size version of our map was presented in Tanaka and 

others (2008).

Our geologic map of Planum Boreum is the first to record 

this feature’s entire observable stratigraphic record using the 

various post-Viking image and topography datasets released up 

until 2009. We also provide much more detail in the map over 

what was presented by Tanaka and others (2008), including 

some substantial revisions based on new data and observations. 

The available data have increased and improved immensely in 

quantity, resolution, coverage, positional accuracy, and spectral 

range, enabling us to resolve previously unrecognized geomorphic features, stratigraphic relations, and compositional 

information. We also employ more carefully prescribed and 

effective mapping methodologies and digital techniques as well 

as formatting guidelines effective at submission (Tanaka and 

others, 2010). The foremost aspect to our mapping approach is 

how geologic units are discriminated based primarily on their 

temporal relations with other units as expressed in unit contacts 

by unconformities or by gradational relations. Whereas timing 

constraints of such activity in the north polar region are now 

better defined stratigraphically, they remain poorly constrained 

chronologically. The end result is a new reconstruction of the 

sedimentary, erosional, and structural histories of the north 

polar region, and how they may have been driven by climate 

conditions, available geologic materials, and eolian, periglacial, 

impact, magmatic, hydrologic, and tectonic activity (see Geologic Summary).

Geography1

The north polar region lies within the northern lowlands 

of Mars, which include three regional topographic basins. The 

largest and deepest of the lowland basins has an irregular shape, 

contains the north polar region, and has been referred to as 

either the “north polar basin” (Head and others, 1999, 2002) or 

“Borealis basin” (Tanaka and others, 2003, 2005b). The plains 

surrounding Planum Boreum form Vastitas Borealis, which displays hummocky textures at sub-kilometer and kilometer scales 

(Kreslavsky and Head, 2000; Tanaka and others, 2003) and scattered crater forms with surrounding ejecta that stand above the 

surrounding plains, commonly referred to as “pedestal craters” 

(for example, Mouginis-Mark, 1979).

The circumpolar areas of Vastitas Borealis within the map 

area (north of lat 75° N.) include morphologically and topographically diverse landforms. Scandia Cavi form irregularly 

shaped depressions tens of kilometers across and hundreds of 

meters deep in the northernmost part of the Scandia region, in 

among complex, rounded plateaus of Scandia Tholi that are 

tens to hundreds of kilometers across and tens to hundreds of 

meters high. Between the Scandia region and Planum Boreum, 

Olympia Planum forms a kidney-shaped rise that gently slopes 

overall at 0.10° to 0.15° up to where it peaks before abutting 

Planum Boreum along the higher standing Olympia Rupēs scarp 

system at ~lat 85° N. Most of lower Olympia Planum is covered 

by the broad Olympia Undae dune field, which extends locally 

into adjacent plains and depressions. Other isolated but concentrated fields of dunes east of Olympia Planum and into Chasma 

Boreale make up Hyperboreae, Siton, and Abalos Undae. In 

this region, Abalos Colles form five prominent, flat-topped or 

cratered mounds <700 m high and <17 km in diameter. East 

of these mounds and south of Chasma Boreale, Escorial crater 

tops an isolated plateau that stands 500 to 700 m high. West of 

Olympia Planum, the irregularly shaped Olympia Mensae rise 

<200 m and extend 180 km north-to-south and 400 km east-towest.

Planum Boreum makes up the ~1000 km diameter 

near-circular, north polar plateau at the center and lowest 

part of Borealis basin. The planum is slightly offset from 

the geographic north pole along the prime meridian, where 

planum extends south of lat 79° N. Mars’ north pole in the 

center of Planum Boreum is at an elevation of ~–2570 m, 

and some parts of the planum rise ~300 m higher. The topographic base of the north polar plateau ranges from –5200 

m (between long 300°–315° E.) to –4500 m (between long 

60°–110° E.). Thus, the height of the north polar plateau 

reaches ~2500 to 3000 m above the surrounding plains. The 

volume of Planum Boreum above the surrounding plains 

surface amounts to about 1.1 x 106 km3 (Smith and others, 

2001).

Planum Boreum has a complex physiographic character. 

Chasma Boreale forms a 450-km-long canyon that opens up to 

1Formal geographic names used in this map have been adopted by the 

International Astronomical Union (IAU)
2

a width of ~120 km at its mouth and divides a tongue-shaped 

lobe, Gemina Lingula, from the main part of Planum Boreum. 

The planum is carved by spiraling systems of troughs each 

generally between 10 and 300 km long, <20 km wide, commonly hundreds of meters deep, and mostly asymmetric in 

north-south profile, with steeper, taller south-facing slopes. 

The troughs encircling the north pole are named Boreales 

Scopuli, whereas troughs that incise Gemina Lingula and the 

margin of Planum Boreum east of the lingula (to long 110° 

E.) are named Gemini Scopuli. In some areas, gently sloping, 

arcuate undulations tens of meters deep and roughly parallel 

with the trend of nearby spiral troughs also mark the planum 

surface (for example, Blasius and others, 1982; Rodriguez and 

others, 2007).

The margins of Planum Boreum vary in morphologic 

character. Troughs and scarps of Gemina Lingula, commonly 

overlapping, open out to Vastitas Borealis. The Olympia 

Rupēs scarp system makes up the margin of Planum Boreum 

along Olympia Planum and makes up the equator-facing 

slopes of most of the elongate depressions of Olympia Cavi. 

These cavi are generally 10 to 20 km wide, hundreds to more 

than a thousand meters deep, and tens of kilometers long. 

At their eastern margin, Olympia Cavi gradually deepen, 

lengthen, and coalesce to form Rupes Tenuis, an irregular 

scarp a few hundred meters to a thousand meters high that 

defines the outer margin of Planum Boreum between 250° 

and 300° E. Adjacent to and south of Rupes Tenuis is Abalos 

Mensa, a domical, triangular plateau ~180 km across. West of 

Abalos Mensa and below and parallel to Rupes Tenuis is an 

elongate, low-elevation, southward-sloping plateau known as 

Tenuis Mensa.

Boola crater (17 km in diameter and centered at lat 81.1° 

N., long 254.2° E.) occurs near the western termination of 

Rupes Tenuis. Crotone crater (6.4 km in diameter and centered 

at lat 82.2° N., long 290.0° E.) is along the narrow trough that 

separates Rupes Tenuis from Abalos Mensa. The largest crater 

associated with Planum Boreum is Udzha, a partly buried, 

45-km-diameter impact crater that is centered at lat 81.8° N., 

long 77.2° E. in eastern Gemini Scopuli.

Chasma Boreale displays a complex geographic character. 

On its northeastern end, the chasma begins with the two arcuate 

depressions of Boreum and Tenuis Cavi, which display steeply 

sloping, high-standing southwest-facing walls. At the mouth 

of Chasma Boreale, the canyon floor becomes a tongue-shaped 

plateau, Hyperborea Lingula, which rises 200 to 350 m above 

adjacent Vastitas Borealis. Eastern Hyperborea Lingula is cut 

by several elongate depressions called Hyperborei Cavi. Pronounced polygonal troughs, ridges, and plateaus tens of meters 

in relief and kilometers in length below southwestern Hyperborea Lingula form Hyperboreus Labyrinthus.

The polar ice caps of Mars form high-albedo features that 

undergo seasonal changes, particularly as carbon-dioxide frost 

waxes and wanes (for example, James and others, 1992). Following maximum seasonal retreat in the summer, a water-ice 

cap remains over much of Planum Boreum and some outlying 

features, such as Olympia Mensae and parts of the Scandia 

region--thus defining a residual ice cap (for example, Tanaka 

and Scott, 1987; Martin and others, 1992).

Base Map and Data

The map is in Polar Stereographic projection from lat 75° 

to 90° N. with a central meridian of long 0° E., which runs 

vertically from the bottom edge of the map to the pole. Longitude increases to the east and latitude is planetocentric (Seidelmann and others, 2002) in accordance with current NASA and 

U.S. Geological Survey (USGS) cartographic standards. The 

topography (fig. 1) is derived from millions of measurements 

acquired by the Mars Orbiter Laser Altimeter (MOLA) instrument onboard NASA’s Mars Global Surveyor (MGS) spacecraft 

(Smith and others, 2001). The measurements were converted 

into digital elevation models (DEMs) that include a 230 m/pixel 

DEM for the entire map region as well as a 115 m/pixel DEM 

from ~lat 78° to 87° N. produced by the MOLA team. Because 

only sparse, less-reliable data were acquired above lat 87° N., 

the DEM in this area is entirely interpolated. A shaded-relief 

base was generated from the DEM with an incidence sun angle 

of 45° from horizontal, and an artificial, uniform sun azimuth 

from the upper left (northward along long 225° E. and southward along long 45° E.), and no vertical exaggeration. The 

DEM was then scaled to 1:2,000,000 at the north pole with a 

resolution of 300 dots per inch.

Post-Viking Orbiter remote-sensing missions to Mars 

include Mars Global Surveyor (MGS), Mars Odyssey (ODY), 

Mars Express (MEX), and Mars Reconnaissance Orbiter 

(MRO). The near-polar orbits of these spacecraft have resulted 

in a dense concentration of data in the map area.

The Mars Reconnaissance Orbiter (MRO) Context Camera 

(CTX) image mosaic that we produced as our primary image 

mapping base has a resolution of 6.2 m/pixel (fig. 2). Mosaic 

production involved geometrically and photometrically stretching the images using simple and relatively rapid image processing techniques, which we were limited to given the available 

resources and the size of the task.

Gaps in CTX coverage of the plains south of Planum 

Boreum were mostly filled in by ODY Thermal Emission 

Imaging System (THEMIS) spring and summer visual (VIS) 

image mosaics (18–36 m/pixel) (fig. 3). The positioning of the 

THEMIS VIS and HRSC images generally was not sufficiently 

accurate for mapping, however. Thus the mapping of contacts 

and features appearing in these data sets required inferring their 

locations in the accurately positioned MOLA topographic base. 

Consequently, location errors of the mapping in these gaps then 

are generally higher than where CTX image data were available.

Important mapping observations were obtained in places 

using higher resolution MRO High Resolution Imaging Science 

Experiment (HiRISE) images (25 cm/pixel) and MGS Mars 

Orbiter Camera (MOC) narrow-angle images (mostly >3 m/

pixel) (fig. 4). However, these data were of insufficient areal 

extent to serve as mapping bases.

We also consulted derived thematic maps of surface composition, including water-ice maps produced from spectral data 

acquired by the MEX-borne Observatoire pour la Minéralogie, 

l’Eau, les Glaces, et l’Activité (OMEGA) (Langevin and others, 

2005, Rodriguez and others, 2007) and the MRO-borne Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) 

(Seelos and others, 2007), and a color map created using Mars 
3

Color Imager (MARCI) data (Malin and others, 2008). However, the surface composition of the Planum Boreum region is 

highly variable temporally due to waxing and waning of the 

residual water ice cap and the seasonal carbon dioxide frost 

cap, along with redistribution of dust that can occur from storm 

activity (Malin and others, 2008). The last product was thus the 

simplest product to use for the mapping of residual ice, once we 

georeferenced it (fig. 5), because it represented the extent of the 

ice cap on one summer Martian day.

Methodology

Effectively organizing, viewing, and manipulating the 

available datasets for geologic mapping routinely included 

detailed, high-resolution analysis of key locations. To facilitate the complicated data management involved considering 

the diverse types, sizes, and resolutions of the datasets used, 

we georegistered the raster datasets into a common projection 

using the geographic information system (GIS) software package ArcMap 9.2 and 9.3® along with custom plug-ins. Some 

datasets had to be processed and (or) reprojected using U.S. 

Geological Survey (USGS) Integrated Software for Imagers and 

Spectrometers software. The GIS digital environment enabled 

us to navigate, enhance, and manipulate the data efficiently, as 

well as digitize map features within a defined and controlled 

spatial context. We stream digitized our original line work with 

a vertex spacing of ~500 m in most cases.

We mostly adopt geologic mapping methods used in and 

partly developed by Tanaka and others (2008). Consistent with 

the approach that Hansen (2000) applied to Venus mapping, we 

identify and map rock units and sedimentary cover on the basis 

of their apparent geologic uniqueness as defined by their primary 

(formed during emplacement) physical features, areal extent, 

relative age, and geologic associations. In many cases, distinguishing primary from secondary (formed after emplacement) 

features requires careful observation and may be difficult and 

uncertain where overprinting of secondary features has masked, 

obliterated, or mixed with primary features. Additional general 

formatting standards and guidelines for planetary geologic maps 

that we followed are described by Tanaka and others (2010).

Our map units mostly form unconformity-bounded units 

(Salvatore, 1994), also known as allostratigraphic units (North 

American Commission on Stratigraphic Nomenclature, 1983), 

where a significant hiatus can be inferred between overlying 

and adjacent units given the unconformity between them. The 

types of unconformities that bound the map units include (a) 

unit-wide angular unconformities at the base of a unit, which we 

interpret as indicative of erosion of underlying strata followed 

by deposition of the new unit having a noticeably different 

bedding attitude; (b) disconformities (parallel unconformities) where, in the absence of a distinct, observable erosional 

signature, other indicators of hiatus exist, such as a surface 

that collected impact craters and (or) a major change in material properties and inferred, sustained deviation in emplacement style, and (c) combinations of angular unconformities 

and inferred disconformities arising from layered materials 

emplaced by sedimentation over a mixed flat and sloping, 

eroded surface.

The result is a set of units that are each distinctive in their 

geologic character and (or) relative age. Because units are 

mapped based on complex characteristics and relations difficult 

to summarize in brief descriptive terms, we chose unit names 

and symbols based on geographic locality and stratigraphic 

position, as previously used in Tanaka and others (2008) and 

following the approach of Tanaka and others (2005b). However, 

significant releases of CTX and HiRISE images and additional 

observations since those studies have resulted in some additional changes in map unit names.

Circumpolar plains materials above middle latitudes on 

Mars, except for dunes, are almost entirely mantled by a thin 

deposit (1–10 m thick at ~lat 40° N.) that may be composed of 

dust and water ice (Mustard and others, 2001). The geomorphic 

mapping coincides with inferred near-surface water-equivalent 

concentrations of 20% to 100% by mass north of lat 50° N. 

(Feldman and others, 2004). The material may be what the 

Phoenix lander rests on and investigated; this material forms an 

icy soil that may be continuous and includes patterned ground 

(Smith and others, 2009). This same material and patterned 

ground obscures the fine-scale surface morphology and spectral 

characteristics of the Early Amazonian materials that underlie it. 

However, larger features subdued by this deposit such as impact 

craters kilometers across appear well preserved at MOLA 

resolution (Garvin and others, 2000). The mantle seems to be an 

ice-rich, stratigraphic equivalent to our Planum Boreum 2 unit, 

which appears to be thin but lithic rich. However, as with previous mapping (Tanaka and others, 2005b), we chose not to map 

this mantle, but instead we map the underlying units in which 

the larger landforms appear to be mainly formed.

We group the map units into three geographic and geomorphic categories: Planum Boreum units, circumpolar plains 

units, and crater material. Unit symbols signify the Amazonian 

and Hesperian Periods (“A” and “H”, respectively), those units 

belonging to the Planum Boreum group (small capital “B”), 

unit name (one or two small letters), and subunit or member 

(subscripted letter or number).

We map three types of contacts: certain, approximate, 

and gradational. A certain contact denotes the most precise and 

best-expressed contacts between well-characterized material 

units as observed in the mapping datasets. In comparison, an 

approximate contact is less precisely located due to lower data 

quality, subtlety or complexity of the contact, and (or) secondary surface modification; the existence of the contact, however, 

is generally not in question. A gradational contact is used for 

complex boundaries where units are intimately mixed together 

at a scale that cannot be displayed precisely in the map; this 

includes dispersed dune forms or erosional remnants of one unit 

that are surrounded by exposures of another unit. Gradational 

contacts are drawn where the concentration of each bounding 

unit accounts for ~50% of the area. We also use gradational contacts in a few places where the Planum Boreum 2 unit gradually 

thins out to where most kilometer-scale morphologic features 

of underlying materials were evident; however, as mentioned 

previously, the unit may also be grading into a widespread, 

middle- to high-latitude mantle.
4

We show polar residual ice on and around Planum Boreum 

as a white stipple pattern, based on a mosaic of MARCI summertime images (fig. 5). The ice appears to be relatively ephemeral given that it may largely be less than a meter thick, displays 

ablated surface textures (pits, cracks, ridges, and knobs), shows 

discontinuous coverage in some HiRISE images, and varies in 

extent from year to year (Thomas and others, 2000; Malin and 

Edgett, 2001; Herkenhoff and others, 2002; Tanaka and others, 

2008; Byrne, 2009).

Nine mapped grabens are all >5 km in length and generally 

are made up of several echelon segments as viewed in CTX and 

HiRISE images, which also display collapse pits along the fault 

scarps (Nunes and others, 2007; Tanaka and others, 2008). A 

total of 308 unconformities 659 m to 114 km long were mapped 

within Planum Boreum 1 and 3 units (fig. 6; 113 are >15 km 

long in the printed map). We limited our mapping of non-crater 

material outcrops to those >~4 km in width (2 mm at map scale) 

and those of crater material >10 km across (5 mm at map scale). 

Thus, smaller outcrops are absorbed in one or more surrounding 

units.

Age Determinations

The Planum Boreum region includes some of the youngest sedimentary sequences on Mars. These include the polar 

layered deposits, which have long been recognized as important 

materials formed and shaped by polar processes related to the 

recent climate of Mars. Moreover, the high quality and volume 

of image and topographic data available and the relatively pristine exposures in this region makes it exceptional for defining 

stratigraphic relations.

Previous workers determined crater densities and important cross-cutting relationships with adjacent units for some of 

the polar units described herein at regional scales, including 

the Vastitas Borealis interior and Scandia region units (Tanaka 

and others, 2005b) and what amount to be equivalents to our 

Planum Boreum 1 and 3 units (Herkenhoff and Plaut, 2000; 

Tanaka, 2005; Banks and others, 2010).

We performed additional crater counts that specifically 

apply to the unit outcrops in the present map and used the 

Robbins-Hynek crater database to gather crater statistics for 

the oldest Planum Boreum rupēs, Vastitas Borealis interior and 

Scandia region units (Robbins and Hynek, 2009). This database for the map region is based on THEMIS infrared images 

(100 m/pixel) and the MOLA DEM (at 260 m/pixel), resulting 

in a fairly complete catalog of craters 1.5 to 63 km in diameter. From this database, 320 were selected of those > 2 km in 

diameter for crater statistics (fig. 7). Some craters in the original 

database were omitted from our survey, because they were (1) 

mantled by younger units, which prevented confident determination of the unit in which they were impacted; (2) in locations 

where the identity of the impacted unit was not obvious; (3) 

so degraded that the preserved crater form did not accurately 

represent the original diameter of the crater (most of these are 

in the Planum Boreum rupēs unit); and (4) higher resolution 

data showed that some of the cataloged craters were more likely 

erosional or collapse features rather than impact craters (plotted 

in black on fig. 7).

For the purpose of crater statistics, we produced a map 

product in which crater material (unit AHc) outcrops were 

removed and buried contacts were added as needed to produce a 

geologic map without crater material (see table 1 for the revised 

areas for the crater counts). This modified map enabled more 

accurate determination of the unit surface areas.

Crater statistics are summarized in table 1 relative to 

the Martian stratigraphic scheme based on cumulative craters 

densities > 2 and 5 km diameter (Tanaka, 1986). The full list 

of craters with their locations, diameters, and map units is 

provided in Digital Supplement 1. The crater size-frequency 

distributions are shown in figure 8 in cumulative, incremental, 

relative, and production-function fit graphical forms (see Crater 

Analysis Techniques Working Group (1979) for a description of 

the types of plots). The previous and new crater statistical data 

are valuable for establishing relative ages of the surfaces and 

inferring model absolute ages of the material units. For eroded, 

exhumed, or partly buried units, the crater density provides a 

lower age limit for emplacement age, which may be much older 

than the surface age. In addition, crater ages provide a surface 

mean age. For geologic units whose surface histories are not 

well understood and complex, calculated surface mean ages 

may be misleading as many resurfacing events may be averaged together, including those that might have locally obliterated craters over differing time scales, whereas other craters on 

surfaces were largely preserved. The Vastitas Borealis interior 

unit in particular shows an unusual paucity of craters >12 km in 

diameter; this paucity is not understood and may reflect either 

an unrecognized deviation from the crater production function 

possibly caused by target property effects (Werner and others, 

2004) or perhaps the true crater production function for Mars 

(Tanaka and others, 2005a). Given such uncertainties in the 

statistical samplings of crater populations, the interpretation of 

the crater-density data, and the form of the crater production 

distribution, model-based absolute ages are not well constrained 

for Mars, especially for the Amazonian Period in which such 

age estimates may be incorrect by as much as a factor of 2 

(Hartmann, 2005).

Geologic Summary

Here we discuss the geologic history of the north polar 

region of Mars as determined by geologic mapping results and 

crater size/frequency statistics. This summary is also portrayed 

graphically in the Correlation of Map Units. Details of the 

stratigraphic relations and crater statistics are provided in table 

1. Helpful cross sections and a graphical stratigraphic section of 

north polar region units appear in Tanaka and others (2008).

Hesperian Period

The recorded geologic history of the north polar region 

begins with the Hesperian during which the Vastitas Borealis 

interior, Scandia region, and Planum Boreum rupēs units were 
5

formed as indicated by N(5) crater densities (table 1). The 

N(5) values are higher than those given by Tanaka and others 

(2005b), which may in part be due to the higher latitude focus 

of the current map plus analysis with more comprehensive data 

(crater diameters from 2 to 5 km). The greater inferred ages 

also likely reflect older formational ages and longer durations 

of formation. These units display complex geologic relations 

and reworked surfaces according to their crater size-frequency 

distributions, thus reconstructions of their histories are poorly 

constrained and allow for multiple geologic scenarios.

The Vastitas Borealis interior unit includes troughed, 

ridged, knobby, and mantled plains materials (for example, 

Tanaka and Scott, 1987) thought to be composed largely of 

fluvial and lacustrine (or even paleo-ocean) sediments emplaced 

during the Early to Late Hesperian and perhaps earlier resulting 

from the dissection of highland materials by outflow channels 

that debouched into the northern plains (Lucchitta and others, 

1986; Parker and others, 1989). Subsequent degradation of the 

unit resulted in obliteration of the craters until near the end of 

the Late Hesperian. South of the map area, the surface age is at 

the beginning of the Amazonian (Tanaka and others, 2005b).

When emplaced, the Vastitas Borealis interior unit is 

likely to have contained interstitial water and ice, which may 

have led to sedimentary diapirism and mud volcanism to form 

the Scandia region unit made up of knobby and hummocky 

plains and dome-forming materials and surfaces that superpose, 

intrude, and otherwise appear to modify the Vastitas Borealis interior unit (Tanaka and others, 2003, 2005b). Given the 

higher density of craters in the Scandia region unit, it appears 

that resurfacing, including crater degradation, may have continued in the Vastitas Borealis interior unit after the Scandia 

region unit formed, at similar times but at higher rates in the 

Vastitas Borealis interior unit, and (or) that some of the craters 

within the Scandia region unit were exhumed. Such differing 

degradational histories may have been controlled in part by differing volatile contents.

The Planum Boreum rupēs unit may be a remnant polar 

ice deposit that was eroded into an irregular shape (Tanaka and 

others, 2008; Putzig and others, 2009). The unit’s crater density suggests an Early to Late Hesperian surface age, perhaps 

contemporaneous with outflow channel activity (Tanaka and 

others, 2005b) and (or) degradation of the Scandia region unit, 

which could have released volatiles and sand- and dust-sized 

lithic particles that were transported to the pole and cold trapped 

(Tanaka, 2005). Several craters of various sizes including Boola 

(17.0 km) and Crotone (6.4 km) form prominences on the 

unit above Rupes Tenuis and on Hyperborea Lingula in which 

material surrounding the crater ejecta blankets were preferentially eroded, forming pedestal craters. Similarly, cratered and 

flat-topped mounds of Abalos Colles may be degraded pedestal 

craters as well (Tanaka and others, 2008), although these have 

also been interpreted to be volcanoes (Fagan and Sakimoto, 

2009). Given that the surrounding Vastitas Borealis interior 

unit’s surface is younger than that of the Planum Boreum rupēs 

unit based on crater-density statistics (table 1), the former unit 

may be exhumed and (or) resurfaced by other processes such 

as cryoturbation. Possibly, the Planum Boreum rupēs unit may 

even represent a remnant of an older set of surfaces of the Vastitas Borealis interior unit, which would indicate that latter unit 

has had a long history of degradation.

Afterwards, large polygonal troughs formed in the Vastitas Borealis interior unit south of Chasma Boreale and around 

western Scandia Tholi (south of Scandia Cavi outside of map 

area) and in the lower part of the Planum Boreum rupēs unit 

exposed on the outer floor of Chasma Boreale. The troughs are 

apparently Late Hesperian given that they generally underlie 

impact craters, as previously noted by McGill (1986). Hyperboreus Labyrinthus represents a set of eroded trough structures 

in the lower part of the rupēs unit that display ridges indicative 

of hardening of the trough margins; these ridges may be made 

up of intruded material similar in origin to the Scandia region 

unit (Tanaka and others, 2008).

Early Amazonian Epoch

No extensive units are recognized from this epoch either 

due to lack of deposition or to erosion of deposits formed at this 

time. Many of the impact craters that produced crater material 

(unit AHc) in the map region were emplaced during this time 

(larger craters with preserved ejecta may have formational ages 

that range from the Late Hesperian to the Late Amazonian, 

whereas degraded craters may be Early Hesperian). Similarly, 

pedestal craters scattered across the map region may have 

formed at various times throughout the Late Hesperian to Amazonian (Rodriguez and others, 2010). Notably, the crater statistics indicate a paucity of craters less than ~3 km in diameter on 

the Hesperian units (fig 8). Possibly, many of the craters formed 

on thin, episodic, ice-rich mantles that when removed left 

behind the crater pedestals in some instances, but in other cases 

craters apparently were obliterated. In addition, many of the 

craters a few hundred to a few thousand meters across in higher 

resolution CTX and HiRISE images appear to be completely 

infilled and buried, leaving behind knobs and flat surfaces cut 

by circular cracks. These features may be widespread, and they 

may account for the bumpy texture at length scales of hundreds 

of meters to a few kilometers of the Vastitas Borealis interior 

unit’s surface in the MOLA DEMs (115 to 460 m/pixel).

Middle Amazonian Epoch (?)

The timing of events described in this epoch is highly 

uncertain. The Planum Boreum cavi unit appears to be made up 

of basaltic sand and ice-rich interbeds forming in some places a 

succession of frozen dunes (Byrne and Murray, 2002; Kocurek 

and Ewing, 2010). The cavi unit was emplaced onto the eroded 

margins of the Planum Boreum rupēs unit, during Middle and 

(or) the earlier part of the Late Amazonian, if two craters >5 km 

in diameter in Olympia Planum superpose the unit or the overlying Planum Boreum 1 unit as has been suggested by Tanaka 

and others (2008). These craters include 19-km-diameter Jojutla 

in the center of Olympia Planum and an unnamed 6.8-kmdiameter crater at lat 81.2° N., long 161.5° E. (the latter crater is 

ringed on its downslope side by a layered, light-toned pedestallike scarp probably made up of the Planum Boreum 1 unit 

underlying the crater’s ejecta and now obscured by dunes of the 
6

undae unit). The Planum Boreum cavi unit’s original extent is 

uncertain and remnants of it are observed in Olympia, Tenuis, 

and Boreum Cavi and a small depression on the northwest flank 

of Abalos Mensa. Possibly, the cavi unit results from saltating 

sand accumulations during high obliquity that alternate with 

episodes of ice accumulation during lower obliquity, until the 

sand supply is depleted and (or) is no longer transportable onto 

the growing deposit due to lower winds during lower obliquity 

(Herkenhoff and others, 2007).

The Planum Boreum cavi unit appears to grade into the 

Planum Boreum 1 unit vertically and perhaps horizontally in 

Boreum Cavus as the dark, sandy layers in the top of the cavi 

unit become thinner and less common in the unit’s upper section 

(Tanaka and others, 2008). Therefore, it may be that some lower 

sections of the Planum Boreum 1 unit also are similar in age to 

the cavi unit.

Late Amazonian Epoch

The Planum Boreum 1 unit developed during the Late 

Amazonian (and perhaps beginning in the Middle Amazonian as described above) as finely layered (as thin as 10 cm in 

HiRISE images; Herkenhoff and others, 2007), light-toned, 

ice-rich deposits hundreds of meters to more than a kilometer 

thick forming much of Planum Boreum as well as a few outlier 

plateaus including Abalos and Olympia Mensae. Generally, the 

layers thicken toward the north pole, and they do not show evidence of significant ice flow (Fishbaugh and Hvidberg, 2006). 

Planum Boreum 1 unit includes hundreds of unconformities, 

mostly within its lower sections as exposed in outer troughs and 

scarps of Planum Boreum. These features indicate that accumulation locally was commonly interrupted by erosion. In addition, the Planum Boreum cavi and 1 units subsequently eroded 

broadly and deeply, removing outer parts of proto-Planum 

Boreum and initiating the spiral trough topography and other 

large depressions including Olympia, Boreum, and Tenuis Cavi 

(Tanaka and others, 2008). The huge trough of Chasma Boreale 

developed in part by preferential accumulations of the Planum 

Boreum cavi and 1 units over eroded scarps made up of the 

Planum Boreum rupēs unit (Holt and others, 2010).

The eroded surface of the Planum Boreum 1 unit has a 3.6 

± 2.5 m.y. old crater-retention age (Tanaka, 2005), and thus accumulation of the unit predates that age. Moreover, the crater age 

for Planum Boreum exposures of the unit may signify when the 

rate of erosion decreased, possibly coincident with when obliquity decreased from a range of 25° to 45° to 15° to 35° about 4 to 

5 m.y. ago; this resulted in a decrease in insolation by about 40% 

(Laskar and others, 2002). Thus, in spite of attempts to correlate 

the layer stratigraphy of the unit to the calculated insolation 

history (Laskar and others, 2002; Milkovich and Head, 2005), 

the lack of a firm absolute age for the termination of the Planum 

Boreum 1 unit prevents a robust temporal reconstruction of these 

layered deposits. Prior to a few tens of millions of years ago, the 

obliquity and insolation cannot be calculated with precision due 

to chaotic variations (Laskar and others, 2004); thus, if the unit is 

mostly or entirely older than this, age reconstructions cannot be 

constrained to any helpful precision.

Following erosion of the Planum Boreum cavi and 1 units, 

deposition of the dark-toned, layered Planum Boreum 2 unit 

unconformably on these and on the Planum Boreum rupēs unit 

occurred extensively across the main lobe of Planum Boreum, 

especially on southern Olympia Planum where Olympia Undae 

now occurs and in the southern troughs of Olympia Cavi 

(Rodriguez and others, 2007). The lithic-rich character of this 

unit indicates that previous to its deposition and during erosion 

of the Planum Boreum cavi and 1 (and perhaps rupēs) units, 

the polar region underwent deflation thereby releasing lithic 

particles. Perhaps the Planum Boreum 2 unit originated as a 

sand sheet deposit (Rodriguez and others, 2007). The unit is 

mapped as grading into the Vastitas Borealis interior unit west 

of Olympia Undae, because a distinct contact between the units 

is not observed; possibly, the Planum Boreum 2 unit grades into 

and is stratigraphically equivalent to an icy mantle superposed 

on most craters marking the interior unit as seen in CTX and 

HiRISE images. This icy mantle may be the northern extension 

of one documented extensively in middle latitudes (Mustard and 

others, 2001).

Next, the dark circumpolar dune fields made up of basaltic 

sands arose primarily from erosion of the Planum Boreum cavi 

and 2 units (Byrne and Murray, 2002; Fishbaugh and Head, 

2005; Tanaka and others, 2008). Most dune formation and migration preceded emplacement of the Planum Boreum 3 unit, but 

locally such as in a section of Hyperboreae Undae, dunes appear 

to have been reactivated, leaving behind depressions within the 

Planum Boreum 3 unit (Mullins and others, 2006). The dunes 

generally migrated southward and southwestward from the 

margins of Planum Boreum and Abalos Mensa, presumably due 

to katabatic winds, then mostly westward (except eastward east 

of the mouth of Chasma Boreale) by circumpolar winds (Tsoar 

and others, 1979; Tanaka and Hayward, 2008). The largest dune 

sea forms Olympia Undae, which appear to originate from the 

Planum Boreum 2 unit that underlies these dunes. Spectral mapping from Thermal Emission Spectrometer (TES) and OMEGA 

data indicate that the dark sand making up the undae unit contain 

hydrated and weathered silicate minerals (for example, Bandfield, 2002; Wyatt and others, 2004; Horgan and others, 2009). 

Gypsum as mapped by OMEGA and CRISM hyperspectral data 

and originating predominantly at the east end of Olympia Undae 

(Langevin and others, 2005) may be formed within the dunes 

(Horgan and others, 2009), from fluvial discharges from beneath 

Planum Boreum (Fishbaugh and others, 2007), or released by 

aeolian deflation from underlying, gypsum-rich materials in the 

Vastitas Borealis interior unit perhaps produced by dike propagation and hydrothermal groundwater alteration resulting from 

Alba Mons magmatism (Tanaka and others, 2008).

The Planum Boreum 3 unit accumulated as 6 to 8 lighttoned layers on top of most of Planum Boreum as well as on: 

(a) parts of adjacent plains, especially below Rupes Tenuis 

(including Abalos and Tenuis Mensae) and along much of the 

margin of central Gemina Lingula; (b) Olympia Mensae; and (c) 

in some larger and deeper craters including Dokka, Jojutla, and 

an unnamed, 31-km-diameter crater at long 89.1° E., lat 77.1° 

N. The Planum Boreum 3 unit is observed to rest unconformably on Planum Boreum cavi and 1 and 2 units within troughs 

of Borealis and Gemini Scopuli and cavi depressions and 
7

mostly embays dunes of the undae unit, except it appears that 

some dunes continued to migrate after Planum Boreum 3 unit 

was emplaced, for example at Hyperborea Undae (Mullins and 

others, 2006). The unit was subsequently degraded and eroded, 

especially on equator-facing scarps. The scarps and adjacent 

surfaces of the Planum Boreum 1 and 3 units are darkened by 

veneers that appear to result from release of hydrated minerals originating from the Planum Boreum 2 unit (Rodriguez and 

others, 2007; Horgan and others, 2009). Craters on Planum 

Boreum are being resurfaced on ~104 year time scales (Tanaka, 

2005; Banks and others, 2010); thus emplacement ages of 

Planum Boreum 2 and 3 units are uncertain.

Residual ice (mapped as a stipple pattern) forms a discontinuous blanket on much of Planum Boreum and parts 

of Scandia Cavi and Olympia Mensae. Its ridged and pitted 

texture indicates that it is likely undergoing deflation at present (Thomas and others, 2000). Whether or not residual ice 

relates to accumulation or other significant modification of 

the underlying Planum Boreum 3 unit, which it locally drapes 

over (Skinner and Herkenhoff, 2007), is unknown. Waxing and 

waning of seasonal carbon dioxide frost (Prettyman and others, 

2009), local slope failures and erosion that forms mass-wasting 

and talus features along the steep cavi cliffs (Herkenhoff and 

others, 2007), and the redistribution of lithic fines in the form of 

veneers as distributed by storm activity (Rodriguez and others, 

2007; Malin and others, 2008) characterize the continued, 

dynamic nature of the north polar region.

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Table 1. Map units in the Planum Boreum region of Mars: Areas, crater densities, relative and model absolute ages, and supe

Unit name Unit

symbol

1Relative

age

Unit area

(103 km2)

2Unit area

with crater

material

(106 km2)

3N(2)

(no. per 106

km2)

N(5)

(no. per 106

km2)

4Superposition relations 

Residual ice (mapped as stipple pattern)

--- LA 760 760 --- --- <all other units

Crater material AHc A 76 --- --- --- <HBs, HBvi, HBbr 

Planum Boreum 3 unit ABb3 LA 943 941 --- --- <ABu, ABbc,1,2, HBbr, HBvi, AHc 

Undae unit ABu LA 315 317 --- --- <ABbc,1,2, HBs, HBvi, HBbr, AHc; >

Planum Boreum 2 unit ABb2 LA 61 61 --- --- <ABbc,1, HBs, HBvi, HBbr, AHc; >A

Planum Boreum 1 unit ABb1 MA-LA 110 110 --- --- <ABbc,1, HBvi, HBbr; >ABb2,3, ABu 

Planum Boreum cavi unit ABbc MA-LA 1.8 1.8 --- ~30±207 <HBvi, HBbr; >ABb1,2,3, ABu

Scandia region unit HBs EH-LH 114 131 336±51 122±31 <HBvi; >ABb2, ABu, AHc 

Planum Boreum rupēs unit HBbr EH-LH 39 42 476±107 148±58 <HBvi; >ABbc,1,2,3, ABu 

Vastitas Borealis interior unit Hbvi EH-LH 822 877 295±18 87±10 >ABb1,2,3, HBs, HBbr, ABu, AHc 

1Relative age determined by superposition relations and crater density analysis; A, Amazonian; H, Hesperian; E, Early; M, Middle; L, Late (after Tanaka, 1986).2Includes unit and unit AHc outcrops superposed on unit.3N(x) = no. of total craters > x km in diameter per 106 km2; no data shown in some cases where <4 craters counted and for crater units. Crater locations and sizes adapted fr

Hynek (2009).

4<, younger than; >, older than.5Based on fits to crater chronology and production functions of Hartmann and Neukum (2001) and Ivanov (2001), as applied in the software tool of Michael and Neukum (26From Banks and others (2010)7From Tanaka and others (2008)
11

Table 2. Mapped features.

Feature type Description Interpretation

Residual ice High-albedo surface deposit mapped with MARCI data 

during Mars northern summer; meter-scale ridge forms 

observed in HiRISE images.

Water ice that remains through Martian summer; ridges 

may result from ablation due to insolation and (or) 

wind.

Unconformity Angular and parallel unconformities evident in sequences of ~meter-thick layers of Planum Boreum 1 

and 3 units.

Paleosurfaces eroded by insolation-driven sublimation 

and wind sculpture resulting from climate excursions 

and local topography.

Graben Narrow linear troughs in Planum Boreum above 

western Olympia Rūpes; mapped using CTX. Locally 

have meter-sized pits along trough walls as resolved in 

HiRISE images. Grabens mostly occur along or aligned 

with ridge tops.

Produced by gravity-driven extension of Planum 

Boreum 3 and perhaps 1 units due to local slopes and 

possible differential compaction over buried scarps in 

the Planum Boreum rupēs unit.

Table 3. Crater types distinguished in figure 7.

Type Characteristics

Rimmed Used in determining relative ages of unit surfaces. Includes three general morphologic 

categories: (1) Larger craters with sharp-crested rims >10 km in diameter with detectable 

circumferential ejecta and deep floors below elevation of adjacent plains. Interior floors subdued 

and partly infilled by younger mantle deposits. (2) Smaller craters with subdued, bowl-shaped 

forms 2 to 10 km diameter on top of conical mounds and ejecta mostly absent or obscured. 

Interior floors typically flat and substantially infilled. (3) Pedestal craters with subdued rims 2 to 

13 km across having some preserved ejecta resting on pedestals generally tens of meters above 

the adjacent plains. Some partly buried or embayed by younger map units (including units 

Abb1–3 and Abu).

Rimless Shallow circular, flat-floored depressions 4 to 72 km in diameter lacking raised rims and ejecta 

blankets indicative of deeply buried and (or) heavily modified crater forms.