MC–21

MC–5

MC–6

MC–7

MC–12 MC–13 MC–14 MC–15

MC–1

MC–20 MC–22 MC–23

MC–27

MC–28

MC–29

MC–30

60°

60° 120°

120°

0°

30°

65°

0° 45° 90° 135° 180°

–30°

–65°

0°

30°

65°

–30°

–65°

Prepared for the National Aeronautics and Space Administration

Geologic Map of MTM –40277, –45277, –40272, and –45272 Quadrangles, 

Eastern Hellas Planitia Region of Mars

By Leslie F. Bleamaster III and David A. Crown

Pamphlet to accompany

Scientific Investigations Map 3096

2010

U.S. Department of the Interior

U.S. Geological Survey

i

Contents

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

Regional Geology and History .....................................................................................................................1

Data and Methods ........................................................................................................................................2

Crater Counting Methodology ...........................................................................................................3

Stratigraphy ...................................................................................................................................................3

Hellas Rim Assemblage ...............................................................................................................................3

Hellas Floor Assemblage .............................................................................................................................5

Vallis and Channel Materials ......................................................................................................................5

Crater Materials ............................................................................................................................................6

Discussion ......................................................................................................................................................6

Fluvial Systems that dissect the Hellas Rim ....................................................................................6

Extensions of Dao and Harmakhis Valles onto the Basin Floor and Contributions to Hellas 

Planitia from the Rim ..............................................................................................................7

Boundary between the East Hellas Basin Rim and Hellas Planitia .............................................7

Geologic History ............................................................................................................................................8

Acknowledgments ........................................................................................................................................8

References Cited ...........................................................................................................................................8

Table

1. Areas, crater densities, relative ages, and contact relations for map units in the eastern 

Hellas Planitia region of Mars .....................................................................................................11

1

Introduction

Hellas Planitia comprises the floor deposits of the Hellas 

basin, more than 2,000 km across and 8 km deep, which is 

located in the southern hemisphere’s cratered highlands and is 

the largest well-preserved impact structure on the Martian surface (fig. 1). The circum-Hellas highlands represent a significant 

percentage of the southern hemisphere of Mars and have served 

as a locus for volcanic and sedimentary activity throughout 

Martian geologic time. Hellas basin topography has had a longlasting influence, acting as Mars’ deepest and second largest 

depositional sink, as a source for global dust storms (Greeley 

and others, 1992), and as a forcing agent on southern hemisphere atmospheric circulation (Colaprete and others, 2004, 

2005). The region lies in the Martian mid-latitude zone where 

geomorphic indicators of past, and possibly contemporary, 

ground ice are prominent (Greeley and Guest, 1987; Crown and 

others, 1992; Squyres and others, 1992; Leonard and Tanaka, 

2001; Mest and Crown, 2001; Mustard and others, 2001; Crown 

and others, 2005; Berman and others, 2009). The highlands 

north of the basin show concentrations of Noachian valley networks (Carr, 1996), and those to the east show prominent lobate 

debris aprons that are considered to be geomorphic indicators of 

ground ice (Squyres, 1979, 1989; Lucchitta, 1984; Squyres and 

Carr, 1986; Pierce and Crown, 2003; Chuang and Crown, 2005). 

Several studies have proposed that Hellas itself was the site 

of extensive glacial and lacustrine activity (Kargel and Strom, 

1992; Moore and Wilhelms, 2001, 2007; Crown and others, 

2005; Wilson and others, 2007). Recent analyses of mineralogical information from Mars Express’ OMEGA (Observatoire 

pour la Minéralogie, l’Eau les Glaces et l’Activité) and Mars 

Reconnaissance Orbiter’s CRISM (Compact Reconnaissance 

Imaging Spectrometer for Mars) reveal outcrops of hydrated 

phyllosilicates in the region (Mustard and others, 2007; Pelkey 

and others, 2007), strengthening an already strong case for past 

aqueous activity in and around Hellas basin.

Our mapping and evaluation of landforms and materials of 

the Hellas region from basin rim to floor provides further insight 

into Martian global climate regimes and into the abundance, 

distribution, and flux of volatiles through history. Mars Transverse Mercator (MTM) quadrangles –40277, –45277, –45272, 

and –40272 (lat 37.5° S.–47.5° S., long 270° W.–280° W.) cover 

the eastern portion of the Hellas basin including the boundary 

between its floor and rim, the distal portions of Dao and Harmakhis Valles, and the deposits of eastern Hellas Planitia. The 

geologic mapping, at 1:1,000,000-scale from Viking Orbiter, 

Thermal Emission Imaging System (THEMIS) infrared (IR) 

and visible (VIS) wavelength, and Mars Orbiter Camera (MOC) 

narrow-angle images, combined with Mars Orbiter Laser 

Altimeter (MOLA) topographic data, characterizes the geologic 

materials and processes that have shaped this region. In particular, the mapping helps to evaluate landforms and deposits resulting from modification of highland terrains by volatile-driven 

degradation. This mapping study builds on previous mapping in 

Hellas Planitia and to the east (Crown and others, 1992; Tanaka 

and Leonard, 1995; Greeley and Guest, 1987; Price, 1998; 

Leonard and Tanaka, 2001; Mest and Crown, 2001, 2002, 2003, 

2006; Crown and Greeley, 2007) and facilitates comparisons 

between the geologic history of the east rim, the remainder of 

the rim, and Hellas Planitia. Specific objectives of our mapping 

are (1) to reconstruct fluvial systems that dissect the Hellas rim, 

(2) to characterize the extensions of Dao and Harmakhis Valles 

onto the basin floor and to identify, if present, sediments these 

canyons contributed to Hellas Planitia from the rim, and (3) to 

investigate the mode of origin, age, and history of modification 

of the boundary between the east rim and Hellas Planitia.

Regional Geology and History

The general geology of the Hellas region is known from 

1:15,000,000-scale, Viking-based geologic mapping of the 

eastern equatorial region of Mars (Greeley and Guest, 1987) 

and earlier Mariner 9-based mapping efforts (Potter, 1976; Scott 

and Carr, 1978). Later studies of Hellas have included updated 

mapping of the entire basin at 1:5,000,000 scale (Tanaka and 

Leonard, 1995; Leonard and Tanaka, 2001). Hellas basin and the 

surrounding highlands exhibit landforms shaped by a diversity of 

geologic processes, preserve exposures of Noachian, Hesperian, 

and Amazonian units, and extend across a wide range in latitude 

and elevation. Formation of Hellas basin by impact in the Early 

Noachian Epoch resulted in widely distributed ejecta materials 

and upheaved crust, which provided the precursor materials/

surfaces of the rugged rim terrain (Leonard and Tanaka, 2001). 

Broad-scale, plan-form asymmetry of Hellas basin has been 

recognized, and oblique impact remains a popular speculative 

hypothesis for its general shape (Leonard and Tanaka, 2001 and 

references therein); however, asymmetry also may result from 

subsequent, localized basin modification unrelated to the initial 

impact. The Argyre impact partly blanketed the western rim 

of Hellas with ejecta and may have been the source of several 

medium-sized secondary impacts in the region between Hellas 

and Argyre basin and across Hellas Planitia (Morrison and Frey, 

2007). Subsequent impacts on the Hellas basin rim, and within 

Hellas Planitia, resurfaced local regions with impact breccias and 

may have controlled the locations of later volcanic and tectonic 

centers (Schultz, 1984; Schultz and Frey, 1990).

Once broad-scale basin topography stabilized, Hellas was 

subject to infilling by aeolian, fluvial, glacial, and volcanic 

materials. Pyroclastic volcanism in the Late Noachian and Early 

Hesperian Epochs formed Tyrrhena and Hadriaca Paterae on the 

northeast Hellas basin rim, as well as Amphitrites and Peneus 

Paterae on the south rim (Peterson, 1977, 1978; Greeley and 

Spudis, 1981; Greeley and Crown, 1990; Crown and Greeley, 

1993). Subsequent effusive volcanism created lava flows southwest of Tyrrhena Patera and may have triggered early development of Dao and Harmakhis Valles along Hellas’ northeast rim 

(Squyres and others, 1987; Crown and others, 1992; Tanaka 

and others, 2002). Wrinkle ridges deform lavas and sedimentary 

materials that were emplaced within Hellas basin, in Malea 

Planum, and in Hesperia Planum. The Amazonian Period is 

characterized by morphologies and deposits indicative of (1) 

widespread aeolian processes that eroded the Hellas rim and 

deposited smooth materials in local catchments in central Hellas 
2

Planitia, (2) mass wasting and debris flows that form large 

aprons around rim massifs and along steep interior basin scarps, 

and (3) dune fields, yardangs, dust-devil tracks, and transient 

splotches; interior basin fines are actively being moved and 

removed by aeolian transport mechanisms (Moore and Edgett, 

1993; Leonard and Tanaka, 2001). In southern regions (> lat 40° 

S.), the terrain is blanketed by mid-latitude mantling deposits considered to be a vast contemporary (last few millions of 

years) reservoir of volatiles (Mustard and others, 2001), which 

could play a major role in the formation of youthful gullies on 

crater and canyon walls (Bleamaster and Crown, 2005) and 

overall crater degradation (Berman and others, 2005, 2009).

The basin-floor materials in the map area are included 

in widespread plains material previously mapped across the 

entire basin and generally attributed to a combination (typically 

unspecified) of volcanic, sedimentary, and aeolian materials dissected and redistributed by fluvial activity and modified by the 

wind (Potter, 1976; Greeley and Guest, 1987; Crown and others, 

1992; Moore and Edgett, 1993; Tanaka and Leonard, 1995; 

Price, 1998; Leonard and Tanaka, 2001). Additional interpretations of the geology of the basin interior from older datasets 

include widespread glaciation (Kargel and Strom, 1992; see 

also Baker and others, 1991). In Viking Orbiter images, wrinkle 

ridges, mesas, and channels are evident in the basin interior, and 

high-resolution images reveal layering in several local scarps 

and benches within the hummocky floor terrain. Tanaka and 

Leonard (1995) favor an aeolian-dominated environment for 

the basin interior. More recently, Moore and Wilhelms (2001, 

2007), citing evidence from MOC images primarily of the 

deepest parts of western Hellas Planitia (below –6,900 m), suggested that Hellas basin contained ice-covered lakes. Accordingly, they attributed layered sediments and polygonal cavities 

to a lacustrine environment. Viking and MOLA data suggest 

shorelines at –6,900 m, –5,800 m, and –3,100 m (fig. 2) (Moore 

and Wilhelms, 2001, 2007). Crown and others (2005), citing 

highland crater preservation, proposed the –1,800 m contour as 

a potential maximum high-stand for a Hellas paleolake. Leverington and Ghent (2004) modeled crustal rebound in response to 

the removal of large bodies of water from the surface of Mars; 

Hellas basin was one of their candidate sites. This rebound may 

have induced sufficient lithospheric stress to deform the Hellas 

floor materials, giving the central plateau the ridged and hummocky appearance preserved today.

Maps of Dao/Niger, Harmakhis, and Reull Valles (Price, 

1998; Mest and Crown, 2002, 2003) show evidence for small, 

young, integrated watersheds adjacent to remnants of the 

cratered highlands, large expanses of sedimentary plains surrounding the canyons, and a concentration of Amazonian debris 

aprons shed from massifs, crater rims, and canyons walls. 

Viking image analyses showed numerous small channels in 

the plains adjacent to Dao/Niger and Harmakhis Valles and a 

sequence of plains adjacent to Reull Vallis that was attributed 

to progressive degradation of the adjacent highlands and (or) 

flooding and subsequent erosion from Reull Vallis. Several of 

these previously defined units extend into the current map area. 

Regional syntheses of the geology of eastern Hellas based on 

Viking MTM mapping provide constraints on the nature and 

sequences of volatile-driven processes from the Noachian to 

the Amazonian Period (Greeley and Crown, 1990; Crown and 

others, 1992; Mest and Crown, 2001; Crown and others, 2005).

Data and Methods

Digital files in raster and vector format produced by the 

U.S. Geological Survey (USGS) were used in the geological 

mapping of the eastern Hellas Planitia region in conjunction 

with Geographic Information Systems (GIS) software. Datasets 

used include a mixed-resolution (>231 m/pixel) Viking Orbiter 

Mars Transverse Mercator (MTM) base mosaic, full-resolution 

THEMIS day- and night-time IR (~100 m/pixel) and VIS (~20 

m/pixel) wavelength image strips (data through March 2005), 

128 pixel/degree (~463 m/pixel) resolution MOLA derived 

digital elevation model (DEM), grayscale synthetic hillshades, 

surface slope maps, and a 200-m-interval contour shapefile. 

Additional datasets used in our mapping and crater counts 

include a publicly released THEMIS daytime IR mosaic in 

simple cylindrical projection resampled to a resolution of 256 

pixel/degree (~231 m/pixel) and individually processed MOC 

narrow-angle images at resolutions better than 12 m/pixel. The 

publicly released IR mosaic contains image data released subsequent to production of the USGS base materials. MOC images 

were used in selected areas to supply detailed morphologic 

information and to discern the distribution of finely layered 

deposits, which are resolved only in higher-resolution images.

The Viking, THEMIS, and MOLA DEM data were all 

processed using the Integrated Software for Imagers and 

Spectrometers (ISIS) software produced and distributed by the 

USGS Astrogeology Team (http://isis.astrogeology.usgs.gov). 

Map-projected THEMIS images were produced from individual 

raw, radiometrically calibrated data acquired from Arizona State 

University (http://themis-data.asu.edu). All of the digital data, 

with the exception of THEMIS IR mosaic, are in Transverse 

Mercator map projection with a center longitude of 270° and a 

center latitude of 0° using the International Astronomical Union 

(IAU) 2000 shape model of Mars (Seidelmann and others, 

2002). All of the USGS digital data were produced in a format 

that could easily be imported into a GIS.

This project took advantage of advances in digital technologies utilizing Environmental Systems Research Institute 

(ESRI) ArcGIS ArcView 9.0 software for georeferencing and 

3D analysis of datasets; however, geological mapping was conducted primarily on a merged data product including the Viking 

MTM mosaic/colored MOLA DEM with superposed THEMIS 

VIS images in Adobe Illustrator. The THEMIS IR mosaic, 

although not in the same projection, proved invaluable where 

image quality of the Viking mosaic is degraded. Contacts were 

transferred into ArcGIS to calculate surface areas for major 

material units, facilitating compilation of crater size-frequency 

statistics. Mapping methods herein draw on historical techniques (Gilbert, 1886; Wilhelms, 1972, 1990), as well as new 

mapping precedents set forth by geologic mappers in the terrestrial and planetary communities (Hansen, 1991; Tanaka and 

others, 1994; Hansen, 2000; Skinner and Tanaka, 2003; Tanaka 

and others, 2005).
3

The map area includes a variety of linear depressions and 

channel forms interpreted to be both the direct and indirect result 

of fluvial activity. Thus, terms like valley, channel, and canyon 

are all used extensively throughout this text. In general, the term 

“valley” typically applies to small elongate troughs, or systems 

of troughs, that appear to have formed by fluid flow but lack an 

obvious suite of bed forms on their floors. The term “channel” 

is in turn restricted to Martian troughs that display evidence of 

fluid flow (sinuosity, bed forms). “Canyon” is reserved for larger 

troughs that have relatively steep, scarp-like margins that may 

have been formed directly by fluid flow. For our purposes, we 

refer to Dao and Harmakhis Valles’ upper reaches, or heads, as 

canyons because they are relatively large in scale and do not preserve bed forms, nor do they resemble fluvial systems; however, 

the lower reaches and termini of Dao, Harmakhis, and Sungari 

Valles display varying degrees of fluvial landform preservation 

such that both channel and valley are used when discussing different locations of these complex systems.

Crater Counting Methodology

We derived relative age constraints for the geologic units 

in the eastern Hellas region in part by counting the number and 

size distribution of superposed impact craters within the areas 

of each major geologic unit (Tanaka, 1986; Tanaka and others, 

1992). To examine statistics for craters ≥5 km in diameter, we 

imported the Martian crater database compiled by Barlow and 

others (2000) into ArcGIS. Evaluation of the Barlow crater database allowed us to (1) identify craters larger than 5 km within 

each mapped unit, (2) eliminate ridge rings, and (3) determine 

superposition on the surfaces of interest. We transferred unit 

boundaries into GIS using the Viking MTM and MOLA DEM 

bases, and surface areas were auto-calculated by ArcGIS 9.0 

software; summing individual unit exposures provided cumulative unit areas. Table 1 summarizes the crater size-frequency 

distribution data for each major geologic unit; N(5) and N(16) 

represent the cumulative number of craters ≥ 5 and 16 km in 

diameter normalized to 1 million km2. The crater size-frequency 

error is ±((N0.5)/A) x 106 km2, where A = area. See figure 3 

for a plot of the N(5) and N(16) crater data for major geologic 

units. We did not derive statistics for small areas or elongate 

exposures (vallis and channel units).

Stratigraphy

The four-MTM map area includes the terminal portions of 

Dao and Harmakhis Valles and their floor deposits, dissected 

plains materials, and deposits associated with the Hellas basin 

floor. We defined geologic units on our map to build from previous mapping (Price, 1998; Leonard and Tanaka, 2001; Mest 

and Crown, 2002) and on the results of our recent work on east 

Hellas rim geology (Crown and others, 2005). Following the 

mapping precedents of Leonard and Tanaka (2001) and Price 

(1998), materials in MTM quadrangles –40277, –45277, and 

–45272 (and –40272, previously mapped by Price, (1998)) are 

divided here into two major geologic assemblages: the Hellas 

rim assemblage (three units) and the Hellas floor assemblage 

(three units). For most of the map area, the contact between 

these assemblages occurs at a prominent topographic break 

near the –5,800 m contour (fig. 2, red contour). At 1:5,000,000 

scale, Leonard and Tanaka (2001) mapped materials within the 

region above –5,800 m as the Hesperian/Noachian dissected 

unit, member 1 (unit HNd1), described as a rugged and heavily 

cratered material with relatively broad (several kilometer wide) 

smooth-floored channels and interpreted as older rim terrain that 

was extensively eroded and resurfaced by fluvial dissection and 

sedimentation. Below –5,800 m, they mapped materials as the 

Hesperian smooth interior material (unit His), described as an 

annular band of plains along the east margin of Hellas marked 

by smooth, locally undulating deposits that bury wrinkle ridges 

and have gradational boundaries to other interior floor materials. Mapping at 1:1,000,000 scale by Price (1998) subdivided 

the dissected material (rim assemblage) into the Hesperian/

Noachian hummocky plains (unit HNh), which she described 

as irregular to highly irregular surfaces with a mottled appearance, and the Hesperian smooth plains (unit Hps), described as 

plains of slight to moderate relief cut by sinuous channels. She 

interpreted the hummocky plains as volcanic and sedimentary 

materials underlying the smooth plains, which are sedimentary 

deposits modified by fluvial and possibly periglacial and aeolian 

processes. The Price (1998) map area falls east of the rim/floor 

assemblage boundary, thus there was no evaluation of the floor 

assemblage in that study.

In addition to the rim and floor assemblages, we defined 

vallis and channel materials (seven units) and crater materials 

(three units). Leonard and Tanaka (2001) placed all vallis materials into one unit (AHv), which includes the heads, cavi, long 

sinuous depressions, and smooth floor deposits of Dao, Niger, 

Harmakhis, and Reull Valles, and interpreted it to be the result 

of catastrophic breakouts of groundwater and fluidized debris, 

followed by sapping and collapse. Price (1998) delineated several vallis units, for Dao Vallis in particular, recognizing multiple cut-off segments (units AHv1 and AHv2) and several units 

related to the continued evolution of the vallis systems (units 

AHvi, AHk, and AHv). Recent reevaluations of the geology of 

the circum-Hellas canyons suggest that at least the later stages 

in their evolution have been dominated by collapse, with the 

possibility that significant, catastrophic flooding may have been 

limited in scope and (or) been confined to early stages, evidence 

of which (streamlined islands, terraces) is not preserved in their 

current morphologies (Bleamaster and Crown, 2004; Crown and 

others, 2005).

Hellas Rim Assemblage

From THEMIS IR and VIS data, we have evaluated the 

contacts of units HNh and Hps mapped by Price (1998) and 

have extended these unit boundaries westward from MTM 

quadrangle –40272 to the boundary between the rim and 

floor assemblages. These two areally expansive units, which 

we redesignate units Nph and HNps, respectively, surround 

the distal portions of the Dao and Harmakhis Valles canyon 
4

systems. The hummocky plains material (Nph) displays tens to 

hundreds of meters of relief and is located in the western part 

of the rim assemblage near its boundary with the floor assemblage and within high-standing inselbergs on the basin floor. 

Both THEMIS daytime and nighttime infrared images show a 

mottled and highly variable thermo-physical characteristic for 

unit Nph surfaces (fig. 4); visible data sets show that unit Nph

has a lower albedo than the smooth plains of both the rim and 

floor assemblages. Unit Nph follows the regional slope of the 

eastern rim and is approximately bound by elevations –5,200 m 

and –6,000 m. Outcrops of unit Nph on the basin floor west of 

the rim/floor assemblage boundary express a similar elevation 

range, suggesting that these outcrops were once contiguous with 

the rim materials and have been locally preserved. Some outcrops of unit Nph display exposures over 400 m of continuous 

elevation change, providing a local thickness estimate of ~400 

m; but the unit could be significantly thicker because its lower 

contact is not observed.

In MOC narrow-angle images, outcrops of finely layered 

materials are exposed atop local high-standing mesas, knobs, 

and surfaces near –5,700±100 m (fig. 5a); all identified exposures of layered outcrops (dot symbol on the map) are located 

within the boundaries of the hummocky plains material, based 

on a thorough search of all MOC narrow-angle images within 

a 20° by 20° area (lat 30°–50° S., long 270°–290° W.) (Crown 

and others, 2005). Layers appear as a fingerprint pattern 

in which successive layers make multiple-inset curvilinear 

benches whose outlines occur at similar topographic levels, 

indicating that they are most likely flat lying (fig. 5b,c). Correlation of layered beds from outcrop to outcrop was not possible 

without distinguishing marker beds, but the exposure, relation 

with elevation, and spatial concentration of the layered deposits 

within unit Nph suggest that the isolated layered outcrops most 

likely represent remnants of a once laterally extensive, and perhaps continuous, deposit that covered much of the east Hellas 

rim. These layers could represent the uppermost member of 

unit Nph, remnants of a more laterally extensive unit HNps, 

and (or) younger atmospherically derived, lacustrine, or fluvial 

sedimentary deposits. Because of the scale of the outcrops and 

the paucity of images that reveal the layers, the exact nature 

of the layered deposit is unclear. It probably is not dominated 

by young atmospheric mid-latitude mantling deposits, which 

would conform to local topography. More likely it is waterlaid, either at the termini of Dao, Harmakhis, and other fluvial 

channels or in a Hellas-wide lake (Moore and Wilhelms, 2001, 

2007; Crown and others, 2005; Wilson and others, 2007). 

Many additional scarps are present along the basin floor to the 

west; new high-resolution HRSC and HiRISE image data may 

help to identify additional layered outcrops and better constrain 

the regional distribution.

The smooth plains material (unit HNps) most likely consists of sedimentary deposits because it lacks visible lava-flow 

structures and displays a consistent smooth surface across a 

range of image resolution, with a few layered outcrops occurring at prominent scarps. The smooth plains superpose unit Nph

and surround and contain several narrow, moderately sinuous, 

surface channels. In several places, these small channels appear 

as spaced and coalesced pits (indicative of immature development) and (or) irregularly shaped depressions (possibly the 

result of modification of the original channel form). Some channel traces in unit Nph can be interpolated across gaps (probable 

windows) in unit HNps (lat 44° S., long 274° W.). This suggests 

that the channels were once through-going and that more progressive erosion has resulted in a transition into large, scarpbound depressions present throughout unit Nph (lat 45° S., long 

275° W.; lat 39.5° S., long 274° W.; and lat 40.0° S., long 272° 

W.). Although it is unclear if the smooth plains were deposited 

from the channels (bank-full deposits) or if the channels simply 

cut or undermine the plains, most likely both circumstances 

exist, and the small channels within the smooth plains likely 

represent the first stage in canyon development and hummockyplains exposure (Crown and others, 2005). This is seen from 

truncation of the small channels by later channel and canyon 

development. Mobilization of the upper smooth plains materials and enlargement of the small channels into scarp-bound 

canyons would have occurred by downcutting from channels, 

scarp retreat and aeolian erosion, collapse from loss of volatiles, 

or a combination of these processes. Several zones within the 

smooth plains are in an initial stage of degradation and preserve 

original surface characteristics of the adjacent undisturbed 

plains (unit HNps); these areas include downdropped and 

rotated slump blocks and are shown with a stipple pattern, notably in the northeast quadrant of the map area adjacent to Dao 

Vallis (and along several sections of Dao, Niger, and Harmakhis 

Valles in the surrounding region). The small channels within 

unit HNps may have played an important role in the initial 

transportation of volatiles and volatile-rich sediments from the 

eastern Hellas highlands and in their redeposition as units HNh

and HNps.

The age designations of both rim units mapped by Price 

(1998) are older on our map, based on our N(5) and N(16) 

values. A significantly greater areal exposure of the hummocky 

plains material in this map area has (1) increased the counting 

area and, thus, the precision of the count and (2) decreased the 

likelihood that a cover by younger deposits might contaminate 

counts. In addition, age designations based on N(2) values by 

Price (1998) may have been shifted away from the Noachian 

production slope by Amazonian surface processes, which 

more easily bury, degrade, and erode smaller craters. The new 

Noachian designation for the hummocky plains is consistent 

with the mostly Noachian designation of dissected unit 1 of 

Leonard and Tanaka (2001). N(5) counts for the smooth plains 

material within this study area are ~ 33% greater than in Price 

(1998) and result in our inclusion of Noachian in the unit’s age 

designation; N(16) counts for the smooth plains (Hps) by Mest 

and Crown (2002) are in agreement with the older designation 

herein. The difference in counts, affecting only smaller crater 

diameters, may also be the result of Amazonian resurfacing 

(debris aprons, mid-latitude mantling), which is more concentrated east of the area mapped here.

In the southeast corner of the map area, the division 

between the rim and floor assemblages is a less distinct, more 

gradual transition than the sharp margin to the north; however, 

the –5,800 m contour still marks a significant morphologic 

change. The rim-floor assemblage contact manifests as a much 

less pronounced transition between the channeled plains (unit 
5

AHpc, an expansive unit to the east; mapped by Mest and 

Crown, 2002, 2003) and the smooth interior floor material 

(unit HNfis). Some ridge traces cross this gradational boundary, suggesting that deformation either postdates deposition 

of unit AHpc or that ridges were in place and buried by unit 

AHpc; the subdued and smooth topography of ridges and lack 

of sharp embayment contacts support the latter. The surface of 

unit AHpc is smooth and displays several sinuous superposed 

channels; whereas, to the west, young superposed channels are 

lacking and several small to medium-sized craters mark a distinct morphologic difference, as apparent in crater counts (table 

1). Channeled plains material is interpreted to be a sedimentary 

unit that formed following removal and redistribution of smooth 

plains material south of Reull Vallis located northeast of the 

map area. Unit AHpc may mark the westernmost floodplain 

deposits of Reull Vallis (Mest and Crown, 2002, 2003) or a 

possible older lacustrine deposit from a Hellas-wide lake; the 

present-day exposure likely represents the erosional retreat 

of materials that once continued farther west and into Hellas 

Planitia. Unit-age designation is consistent with counts of substantially larger areas to the east by Mest and Crown (2003) and 

suggests that significant aqueous reworking continued at least 

until the Late Hesperian.

Hellas Floor Assemblage

Hellas floor materials are moderately to heavily cratered, 

deformed by wrinkle ridges, and locally dissected or transected 

by the farthest extensions of Dao and Harmakhis Valles. Most 

of the floor assemblage in this map area is composed of smooth 

material (unit HNfis) with an undulating characteristic that 

forms clear and sharp embayment contacts and stratigraphic 

markers with unit Nph and ambiguous or gradational contacts 

when adjacent to rim assemblage unit AHpc and the various 

vallis units. 

In the southeast quadrant of the map area, near the contact 

between units HNfis and AHpc, an anomalous zone of semiexhumed craters exists (depicted on the map with a stipple 

pattern). Several of the crater rims within this zone merge with 

both the pronounced and the subdued morphologic features 

characteristic of Hellas Planitia regional deformation, namely 

wrinkle ridges. This spatial relation suggests that these craters predate the deformation rather than having coincidently 

formed by impact at wrinkle-ridge intersections. This cratered 

zone appears to be a subsurface connection of ancient material 

between the hummocky plains on the rim and inselbergs on the 

floor that has been covered and later stripped, leaving behind a 

thin veil of unit HNfis. This substrate may represent extensions 

of unit Nph itself; however, the floor outcrops lie ~200 m below 

those on the rim. The substrate material may be a lower member 

of unit Nph or the original post-Hellas impact basement draped 

by unit Nph.

Near the mouth of Harmakhis Vallis, several scarpbound plateaus preserve a wrinkle-ridge pattern like that in 

the anomalous crater zone and in high ground north and south 

of the sinuous depressions in unit HNfis. In this same region 

several relatively deep sinuous depressions extend onto the 

Hellas floor and meander around high-standing plateaus. These 

distal extensions of Harmakhis Vallis may or may not represent 

genetic surface connections with the Harmakhis head valley 

to the northeast, because they do not preserve primary channel 

features linking their origins (Bleamaster and Crown, 2004); 

however, the characteristic plan-form shapes including sinuosity 

and braiding appear likely to be of fluvial origin (see Discussion). No wrinkle ridges are visible in the depressions, so the 

channel-like features postdate wrinkle-ridge deformation of the 

plateaus and the surrounding plains, both of which are resurfaced by unit HNfis, which predates significant downcutting of 

the Harmakhis Vallis terminus.

Within these depressions at the Harmakhis Vallis terminus, we identify two geologic units: hummocky floor material (unit Hfh) and smooth fill material (unit AHfs). Unit Hfh

lies adjacent to and overlies the smooth interior floor material 

(unit HNfis) and broadens towards the basin center where the 

materials are dissected by several linear-curvilinear troughs. 

These troughs break the material into low-relief mesas and 

knobs (lat 42° S., long 278° W.). Materials of unit Hfh likely 

consist of Harmakhis paleochannel deposits, as well as materials mass wasted from the adjacent high-standing plateaus. The 

small-scale troughs may be remnants of paleodrainage or they 

may also be structural in nature, representing minor amounts 

of extension of a loosely consolidated sedimentary deposit. 

The deepest parts of these depressions, although discontinuous, 

may represent segments of the paleothalweg of the Harmakhis 

Vallis terminal channels. Unit AHfs lies in these lowest regions 

and has a gradational surface texture that is very smooth in its 

interior and hummocky near the contact with unit Hfh. This 

suggests that the unit has achieved sufficient thickness to more 

completely bury the hummocks of unit Hfh near the center of 

the paleochannel than at its edges. Unit AHfs is locally the 

youngest unit and has a plan-form characteristic like that of 

Quaternary alluvium along terrestrial river channels (contacts 

mimic topographic contours and deposits fill the lowest elevations of the channel), although unit AHfs may not be deposited 

as alluvial sediment. Complicating the interpretation of these 

depressions and their deposits are their subdued surface character and topography resulting from burial by as much as tens 

of meters of material by aeolian settling and (or) atmospheric 

frost-mantling deposits.

Vallis and Channel Materials

Several dozen small channels dissect the rim and floor 

assemblages. Most are depicted on the map with a blue line. 

Channel materials (Hch1, AHch2, and AHch3), mapped in 

channels that have wide enough floors to warrant a unit designation, are composed of original channel deposits, eroded 

floor materials, materials mass-wasted from the walls, and any 

other subsequent fill materials. Crosscutting relations constrain 

the relative ages of the morphologic channel forms. On the 

basin rim, the small channels are located primarily within unit 

HNps. Most of these small channels are narrow, shallow, and 
6

in general represent immature dissection; however, they reflect 

some of the oldest fluvial dissection of the rim when water 

may have been more prevalent on, or available to, the surface. Several channels parallel the larger vallis systems; when 

spatially coincident, small channels are truncated by younger, 

wider, and more developed canyons and valles. We also see 

examples of narrow, sinuous channels that contain segments 

that have been widened into a trough or small canyon. This 

relation, the discontinuous nature of some small channels, and 

the disrupted blocks along the margins of Dao and Harmakhis 

Valles favor an extended period of collapse and modification 

over a catastrophic, fluvial interpretation of the observed morphologies (Crown and others, 2005). Where the canyons are 

sufficiently wide, Amazonian or Hesperian valley fill superposes their floors (units AHvD and AHvs). Floor materials are 

composed of remnants of surrounding plains materials mixed 

with younger aeolian, mass-wasted, and mantling materials. These materials display isolated pits of various sizes and 

lineations parallel to canyon walls. The pits do not appear to be 

impact craters (no ejecta), thus they must be related to collapse 

over void space. This suggests that the materials have been 

volatile-rich and (or) have significant porosity and that collapse 

occurs when volatiles migrate out of the sediments. Volatilerich material (sediments with interstitial ice) is also consistent 

with the floor lineations, which indicate some component of 

flow, in successive downcanyon and (or) across-canyon events. 

Mantling deposits on canyon walls flow into and merge with 

floor materials in some places, which is again consistent with 

both cross-canyon and downcanyon movement.

The largest, most mature, canyon segments of Dao/Niger 

and Harmakhis Valles, which also dissect the rim and floor 

assemblages, are thought to be more highly evolved features: 

zones where continued and possibly concentrated surface flow, 

subsurface flow and piracy, and volatile migration have served 

to widen the once-narrow surface channels. The lower reaches 

of Dao Vallis preserve several episodes of channel development 

showing abandoned truncated channels (units Hc1 and AHc2) 

(fig. 6) and provide evidence of a protracted history contrary to 

a single catastrophic mechanism for Dao and Harmakhis Valles 

development.

With the exception of a lobe extending from the mouth of 

Dao Vallis (unit AHvt), distinct depositional margins are lacking 

near channel and canyon mouths that cross the rim-floor assemblage boundary. Unit AHvt ranges upward from a few to tens of 

kilometers in width with subparallel margins and is entrenched 

by a channel nearly down the center of its length (fig. 7). The 

resultant deposit’s morphology is more consistent with emplacement in a single event rather than successive individual flows; 

channel entrenchment may represent waning of material transport or subsequent dissection.

Crater Materials

One hundred and twenty craters, ranging from a few 

kilometers to 40 km in diameter, buried and exposed, have 

been identified and marked with a crater-rim symbol. Of these, 

56 (mostly with diameters ≥5 km), including Bogia crater, 

have associated ejecta materials, floor materials, and (or) peak 

materials. Three unit designations are used to differentiate crater 

peak material (unit cp), crater undivided material (unit cu), and 

crater fill (unit cf). Bogia crater (lat 44.7° S., long 276.6° W.; 

39.4 km diameter) contains all three crater units and is the only 

crater in the region with a central peak typical of larger impact 

craters. Several craters have floor deposits (unit cf), which 

may be mass-wasted material from the crater rim, atmospheric 

deposits, lacustrine sediments, or a combination of all of these. 

Several crater walls are dissected by young hillside gullies; no 

deltaic landforms have been identified in any of these craters.

South of the Harmakhis Vallis terminal plateau region, several adjacent craters display commingled ejecta blankets, which 

have radial lineations, lobate margins, and dark ejecta (Viking 

and THEMIS IR datasets). A variety of degradation states is evident and consistent with various ages of the formative impacts; 

however, the exposed surface is dominated by armored ejecta 

blankets with few superposed pristine craters, suggesting that 

this surface was buried and has since been exhumed in a relatively uniform manner. Craters whose ejecta blankets include 

the dark material (cross-hatch pattern) are interpreted to have 

excavated rocks from unit Nph, which lies beneath unit HNfis

and also displays a low albedo.

Discussion

This geologic study has shed light on the role of water in 

the evolution of Hellas Planitia and the Martian surface, including the possibility of ancient lake deposits hypothesized for 

western Hellas. Geomorphic and geologic evidence reveals an 

extended history of widespread fluvial dissection, but evidence 

of large-scale catastrophic flooding, as in the circum-Chryse 

Planitia outflow channels, is lacking, either because it never 

occurred or it occurred earlier and has since been destroyed.

Fluvial Systems that dissect the Hellas Rim

The eastern Hellas rim has experienced fluvial activity 

throughout its long history. A progression from small ancient 

channels that dissect the highland terrains to massive canyon 

development and to contemporary gullies on the slopes of crater 

and canyon walls is preserved. Timing relations along segments 

of Dao and Harmakhis Valles place the major development of 

these valles in the middle of the protracted history.

The geology of the eastern rim of Hellas basin, and in 

particular the sequence of plains materials surrounding Dao 

and Harmakhis Valles, suggest the formation of a depositional 

shelf that largely covers any remnant highland terrains previously preserved here. Widespread dissection along vallis and 

channel systems is evident on the basin rim. The following 

idealized stages can account for the observed geologic relations: 

(1) deposition of sedimentary layers possibly with interbedded 

volcanics, (2) strata enriched with volatiles either during deposition or subsequently, (3) small-scale surface and subsurface 

pathways established through rim materials to the basin floor, 
7

(4) plains strata fractured and lowered along these pathways, 

partly because of weakening by volatile migration, (5) blocks 

subsided and rotated by continued collapse and fracturing, (6) 

volatile infiltration facilitated by the resultant irregular topography, leading to growth of sapping valleys along fractures and 

over subsurface voids, (7) collapse of canyon walls and localized erosion of wall materials, and (8) resurfacing of canyon 

floors and mass-wasted deposits from canyon walls and lineated 

valley-fill deposits extending downcanyon and surrounding 

remnant hills and mounds of collapsed plains.

The canyons may have developed either in a regional episode of subsidence or in distinct episodes of initial subsidence 

and collapse at different times in different places. Topographic 

coincidence of canyon heads with the –1,800 m contour (see 

Crown and others, 2005) supports a common origin, possibly by 

the lowering of a lake level or by commonalities in the subsurface geology. Local factors may affect subsequent morphologic 

development in either scenario. Eventually the entire eastern 

Hellas landscape, as well as regions south and north of 40° 

latitude, preserves morphologies consistent with surface and 

subsurface ice-dominated environments. We propose that Dao 

and Harmakhis Valles are the most evolved channel features in a 

suite of channels on the rim; their floors are among the youngest 

of the channel and vallis deposits including the tongue of material that extends from Dao Vallis’ terminus onto the Hellas floor.

Extensions of Dao and Harmakhis Valles onto the 

Basin Floor and Contributions to Hellas Planitia 

from the Rim

Direct connections between the Hellas floor materials and 

their potential rim sources are difficult to identify (except for the 

small lobe at the mouth of Dao Vallis). Alternatively, the eastern 

Hellas floor materials (1) may be composed of far-reaching 

deposits from Dao and Harmakhis Valles, extending west of the 

map area, or (2) may consist of planitia-wide deposits mostly 

of local origin and not delivered by Dao and Harmakhis Valles. 

Amazonian resurfacing (aeolian infill, mid-latitude mantling) 

apparently has masked any original relations between canyonfloor and Hellas-floor materials. Direct comparison of vallis 

and canyon-cavity volumes (Dao, Niger, Harmakhis, and Reull 

Valles) (Mest, 1998; Tanaka and Leonard, 1995) to the estimated total volume of sediment contained in the eastern Hellas 

floor deposit showed that missing material from the four major 

tributaries accounts for only a small fraction of the material 

present on the floor (Rogeiro and others, 2003). In any case, 

the small channels and larger valles were likely significant 

paleopathways for the migration from the rim to the floor of 

liquid water, debris-rich slurries, or glacial flows. The volumes 

of volatiles and sediments that may have been transported from, 

or through, the rim to the basin floor remains uncertain (Crown 

and others, 2005). Similarly, marine canyons on Earth show 

little correlation between the size of the canyon, the sedimentary flux, and the final volume of their associated deposits (as 

high as 1 to 3 orders of magnitude greater sediment transport) 

(Normark and Carlson, 2003), thus it is not surprising for the 

Martian values to remain unconstrained.

Boundary between the East Hellas Basin Rim 

and Hellas Planitia

Both the Hellas rim and floor assemblages contain units 

of Noachian, Hesperian, and Amazonian age. In each case the 

Noachian materials appear to have been exposed by erosion 

following significant burial by younger materials. In each 

situation, Amazonian surface processes are masking critical 

relations between Noachian and Hesperian materials. Key to 

constraining the age of the rim-floor boundary to a minimum 

of Hesperian is that the large vallis filled by later materials 

(AHvD and AHvH) cut directly across the contact. However, 

the nature of the rim-assemblage materials, in particular unit 

Nph, may push the boundary’s age back into the Noachian. The 

characteristic of the rim/floor boundary located at –5,800 m is 

crucial to its interpretation. This topographic level in the eastern 

region marks a distinct variation in rim morphology. The –5,800 

m contour deflects inward toward the basin center precisely 

where Dao and Harmakhis Valles enter the Hellas floor. Several 

outcrops of the finely layered deposits of unit HNps also occur 

here. The deflection of the contour coupled with the presence 

of the layered materials strongly suggests that this area is a site 

of concentrated (relative to other regions around the basin) and 

significant deposition within the Hellas basin interior. After 

considerable reconnaissance and analysis of the Hellas rim (see 

Crown and others, 2005), we are confident that this deposit is 

unique to eastern Hellas and is most likely the result of concentrated aqueous activity (precipitation, recharge, storage and 

release of volatiles) related to persistent atmospheric circulation 

patterns that allow for migration of volatiles from the southern 

pole (models by Colaprete and others, 2004, 2005). The availability of volatiles in this location facilitates both weathering 

and mobilization of the highland materials from the rim to the 

floor of Hellas. We suggest that this present-day shelf is a volatile-rich accumulation zone (both ancient and contemporary). 

The margin preserved at –5,800 m may represent a lake highstand or the extent of an ice-covered body of water (Moore and 

Wilhelms, 2001, 2007). If unit Nph represents an early phase 

of this depositional shelf, then the rim materials were beginning 

to build out onto the floor of Hellas in the Noachian with later 

deposits of smooth plains in the Hesperian, which may or may 

not be associated with unit Nph; if a basin-wide lake did exist 

at this time, then materials being shed from the eastern rim may 

have been deposited subaqueously as deltaic sequences, which 

would be interbedded with lacustrine sediments from the basin 

as a whole. If the Hellas lake was ice covered, materials may 

have been deposited as they were shed from the eastern rim and 

abutted against an ice margin.

With respect to later modification of the material on the 

eastern rim (Dao and Harmakhis Valles), if the volatile-rich 

depositional shelf existed as described above, it would have 

facilitated collapse and growth of the large canyon systems by 

supplying unconsolidated materials with high infiltration capacities, sufficient volatile components, and pathways for easy 

removal. Additional factors that may have contributed to eastern 

Hellas’ unique geology are its proximity to both Hadriaca 

Patera, a potential heat source providing enhanced geothermal 

flow from magma bodies and surface flows, and southern hemi-
8

sphere atmospheric circulation, which carries volatile components directly off the southern polar cap and emplaces them east 

of Hellas basin.

Geologic History

The following sequence of events is suggested from the 

geologic, geomorphic, and topographic relations in the mapped 

part of eastern Hellas basin:

1. Formation of Hellas basin and the circum-Hellas 

highlands during the Early Noachian Epoch provides the topographic framework for subsequent geologic activity. 

2. Degradation and removal of material throughout the 

Noachian from the basin rim results in a significant topographic 

depression in a northeastern corridor between Hellas Planitia 

and Hesperia Planum northeast of the map area; this represents 

a period of erosion and transport and a significant gap in the 

geologic record within the map area.

3. Materials shed from the Hellas rim are locally deposited around high-standing knobs of the degraded rim (unit Nph). 

This unit represents remnants of the ancient highlands and may 

include pre-Hellas crust, contributions from the Hellas impact 

(melt sheet, ejecta, shocked basement), and post-Hellas impacts 

(including Argyre impact ejecta) intermixed with younger sediments. The anomalous zone of craters (stipple pattern in unit 

HNfis) on the Hellas floor also represents a remnant Noachian 

surface that was buried and later exhumed.

4. Widespread deposition of plains on the rim and floor 

(units Nph, HNps, and HNfis). These materials may have 

been deposited subaqueously in a standing body of water or 

along the margin of an ice-covered lake (Moore and Wilhelms, 

2001, 2007; Crown and others, 2005). If the sequence of plains 

exposed in units Nph and HNps (above –5,800 m) is even partially lacustrine in nature, then extensive Hellas paleolakes may 

have existed into the Early Hesperian Epoch; however, if not, 

then Hellas paleolakes would have been more ancient and (or) 

constrained to the Hellas basin floor.

5. Fluvial dissection becomes prominent on the east rim 

of the Hellas basin in the Hesperian Period, perhaps in relation 

to recession of Hellas paleolakes. Small-scale channels develop 

within and dissect unit HNps’ surface and transport material 

toward Hellas Planitia. It is likely that the larger valles evolved 

from some of these smaller channels, thus the early development of Dao and Harmakhis Valles would have begun in the 

Early Hesperian Epoch.

6. Deformation in the form of northwest-trending wrinkle 

ridges crosscuts the deposits of Hellas Planitia (unit HNfis). 

This deformation postdates widespread Hellas paleolakes 

(stands near –5,800 m) and could have formed by rebound of 

the lithosphere in response to removal of the overburden of 

water or ice, but it predates the intermediate stages of Dao and 

Harmakhis development (especially at their termini: units Hfh

and AHvt), signifying that there was glacial or fluvial activity 

along the eastern rim well into the Late Hesperian and possibly 

Early Amazonian Epochs.

7. Widespread channel development transitions to localized enlargement of channel segments by sapping and collapse 

(stipple zone in unit HNps) facilitated by earlier deposited 

sedimentary and volcanic materials (units Nph and HNps).

8. Deposition of plains materials (unit AHpc) from Reull 

Vallis overflow (Mest and Crown, 2002, 2003) and (or) ice-rich 

surface mantles were followed by dissection by channels via 

sheet wash and scour in the southern portion of the map area.

9. Vallis floor materials (units AHfs, AHvD, AHvH, and 

AHvS) and channel deposits (units Hc1, AHc2, and AHc3) may 

span a significant amount of time depending on the specific 

source of the material. Downcanyon contributions sourced from 

the canyon heads may begin to collect concurrently with development of the canyons in the Early Hesperian Epoch, whereas 

wall contributions via localized mass wasting may have been 

supplied into the Late Amazonian Epoch.

10. Craters within this region likely formed throughout 

the preserved geologic record and exhibit a range of preservation states; the complex surface morphologies observed may be 

attributed to various combinations of crater size and age along 

with different degrees of crater burial and exhumation. Craters 

with the dark ejecta (within unit HNfis) are most likely the 

result of younger impact events that have excavated the underlying ancient basement material of unit Nph.

11. The Amazonian Period is characterized by the local 

exhumation and redistribution (aeolian dominated) of materials and continued exposure of hummocky plains on the rim 

(unit Nph) and floor (unit Hfh) of Hellas Planitia. Widespread 

deposition of mid-latitude mantling deposits in southernmost 

latitudes, the formation of gullies on crater and canyon walls, 

and the development of wind and slope streaks (mostly small 

components of vallis and channel materials) most likely formed 

over several episodes of activity in the recent geologic past; 

several of these small, young morphologic features are not 

mapped (see Bleamaster and Crown, 2005, for gully and mantle 

morphologies).

Acknowledgments

The authors wish to thank Katherine Price and Don Wilhelms for thorough and insightful reviews, Ken Tanaka for his 

continued service to the planetary geologic mapping community 

as map coordinator, Jenny Blue for her assistance with planetary 

nomenclature, and Jan Zigler and Darlene Ryan for their editing 

and graphical expertise, respectively. This work was made possible by funding from NASA’s Planetary Geology and Geophysics program (Geologic Investigations of the Martian Highlands, 

grant #NNG04GI85G). This is Planetary Science Institute 

contribution #448.

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Table 1. Areas, crater densities, relative ages, and contact relations for map units in the eastern Hellas Planitia regio

Unit Name Unit Area Count1 Count1 N(5)2 N(16)2 N(5) N(16) Designation3 

Symbol (km2) N(5) N(16) Epoch Epoch Younger 

range range than 

Channeled plains material AHpc 34,370 2 0 58±41 -- LH-EA LH-EA Nph, HNps

HNfis

Hummocky floor material Hfh 16,463 2 0 121±86 -- LN-EA H Nph, HNfis Smooth interior floor material HNfis 136,790 25 4 183±37 29±15 LN-EH LN-EH LN-EH Nph Smooth plains material HNps 68,463 12 0 175±33 -- LN-EH LN-EH Nph Hummocky plains material Nph 72,402 23 5 318±66 69±31 LN LN LN 1 Actual number of craters counted within the unit area.2 N(x), No. craters >x km in diameter per 106 km2; error, ± [(N1/2)/A] x 106 km2.3 Unit age designation determined by superposition relations and crater density analysis; N, Noachian; H, Hesperian; A, Amazonian; E, Early; L, Late (Tanaka, 1986).