Karstic morphology in northern Sinus Meridiani, Mars

Geosciences JournalDOI 10.1007/s12303-014-0003-0ⓒ The Association of Korean Geoscience Societies and Springer 2014

LETTER

Karstic morphology in northern Sinus Meridiani, Mars

ABSTRACT: This work describes karst landforms observed inthe northern Sinus Meridiani region located between 1°18'N to2°30'N latitude and 2°30'W to 0°13'W longitude, which coversapproximately 9,100 square kilometres, characterized by spectralsignatures of evaporite minerals. An integrated analysis of theReconnaissance Orbiter (MRO) High Resolution Imaging ScienceExperiment (HiRISE), High Resolution Stereo Camera (HRSC),and Mars Orbiter Camera (MOC) imagery were used to performa morphological and morphometric investigation of the study area.Results resolved three specific morpho-units, characterized by dif-ferent doline features, which displayed distinct types and degreesof karstification in the study area. The landforms indicated differentsolutional properties of the units, and suggested enough liquid waterin the recent past was present to form the karst landforms, empha-sizing climatic changes during the Amazonian period occurred inthe Sinus Meridiani area.

Key words: Sinus Meridiani, karst, doline, Mars

1. INTRODUCTION

Sinus Meridiani is part of a large area known as Terra Merid-iani, located near the southwestern margin of Arabia Terra,in the equatorial region of Mars (Fig. 1a). Sqyres et al. (2004)showed evidence of past aqueous activity preserved in sev-eral distinct units in this area, which were classified as lay-ered deposits with monohydrated and polyhydrated sulfatespectral signatures (Griffes et al., 2007). Wiseman (2009)performed subsequent spectral analyses, defined and mappedseveral distinct stratigraphic units, hydrate sulfates.

Karst landscapes in the Mars Meridiani region have beenhypothesised based on the potential for water, or some brine,to interact with water soluble rock produced by earlier fluidsedimentation (Bèrczi, 2005). Bèrczi (2005) observed sur-face morphology images from the MER-2 probe Opportunitysent from its landing site, which highlighted chains of pitsforming a trench, and named this landscape Anatolia. Theobserved closed depressions were interpreted as possibledolines, and the entire surface trench pattern of the Anatoliaarea as a solution groove. Wiseman et al. (2010) examinedthe geomorphic expression of materials in the northern areaof Sinus Meridiani, and reported the presence of different

surfaces characterised by erosional pits and grooves at smallto intermediate spatial scales.

Following previous research, the objectives of the presentstudy were to verify the possibility of karst morphology inthe Meridiani planum region, the existence of karst land-forms in other regions of Mars (Kargel et al., 2004; Baioniet al., 2009), and to investigate the presence of karst land-forms in the northern Sinus Meridiani. In particular, the aimsof this study were identify and describe the morphologiesthat can be interpreted as karst landforms, and where possible,interpret the morphogenetic processes involved in landformformation and shaping; and generate data regarding the mor-phological history and processes that affected this specificMars region.

The region chosen for our study is located between1°18'N to 2°30'N latitude and 2°30'W to 0°13'W longitude(Fig. 1b). The area extends approximately 130 km in an east-west direction, and about 70 km in a north-south direction,covering an area of about 9,100 square kilometres.

A detailed morphological analysis of the available Marsimages was performed to conduct our analyses. The morpho-logical landform features were investigated through an inte-grated analysis of Reconnaissance Orbiter (MRO) HighResolution Imaging Science Experiment (HiRISE), High Res-olution Stereo Camera (HRSC), Mars Orbiter Camera (MOC),and Thermal Emission Imaging System (THEMIS) data.

2. STUDY AREA SETTING

The Sinus Meridiani region (Fig. 1a) is located near thesouthwestern margin of Arabia Terra in the equatorial regionof Mars, centred at 5°S, 0°E. The study area was establishedin northern Sinus Meridiani between 1°18'–2°30'N and2°30'W–0°13'W (Fig. 1b), extending about 130 km east-westand 700 km north-south, covering an area of about 9,100 km2.

In this area, nearly horizontal morphologic surfaces areexposed above distinct stratigraphic units of Noachian–lowerHesperian age. A heavily cratered terrain (CT) of Noachianage, sloping regionally to the northwest is exposed at the baseof the stratigraphic section (Andrews-Hanna et al., 2009).This unit, generally displays a low albedo, intermediate ther-

Davide Baioni*

Alessio MuranaNadja Zupan Hajna

Dipartimento di Scienze della Terra, della Vita e Ambiente, Universita degli studi di Urbino “Carlo Bo”, Campus Scientifico Enrico Mattei , 61029 Urbino, ItalyInternational Research School of Planetary Sciences, University "G. D'Annunzio", 65127 Pescara, ItalyKarst Research Institute, ZRC SAZU, Titov trg 2, 6230 Postojna, Slovenia

*Corresponding author: [email protected]

Davide Baioni, Alessio Murana, and Nadja Zupan Hajna

mal inertia, and basaltic spectral signatures where it is notcovered by dust (Hynek, 2004; Poulet et al., 2008). Somecraters have degraded rims, whereas other impact craters arepartially filled with intracrater deposits, which exhibit highthermal inertia and are completely buried, or have flat floorsthat may have resulted from fluvial erosion and deposition(Craddock and Howard, 2002). Intercrater surfaces in thisunit have predominately low albedos, are relatively smooth,retain small impact craters, and in some areas display polyg-onal fractures (Wiseman, 2009).

Unconformably light-coloured, indurated sedimentary rocks

comprising an etched terrain (ET) overlays the Noachian CT(Hynek et al., 2002). Hynek et al. (2002) suggested this unitformed in the late Noachian–early Hesperian. The ET is later-ally extensive across Sinus Meridiani (Fig. 1c), is up to 300m thick, and includes sub-horizontal surfaces dipping 0.01–1°to the north-west, in the direction of the regional slope (Hynekand Phillips, 2008). Intermediate albedos, and relatively highthermal inertia (Hynek, 2004; Wiseman, 2009) character-ise the ET, which show multiple layering (Edgett, 2005) anderosional mesas. Parts of the sedimentary sequence of thisunit have been eroded away. The remains of the upper lay-ers form bright ridges and mesas outlined by steep escarp-ments separated by partly dark, low, flatter areas partiallycovered by dunes. The mesas generally have stair-steppedslopes due to differences in erosional resistance among layers.Carr (2006) reported almost all bedding appears horizontal,and some individual beds extend for several kilometres. Arecent study of geomorphic expression, and super positionalrelationships (Wiseman et al., 2010) subdivided the ET unitinto four laterally continuous subunits (ET1A-D). Accordingto this study, the lowest subunit (ET1A) exhibits variablethickness controlled by the underlying CT topography, andthe two units above it were 10–30 m (ET1B) and 40–60 m(ET1C) thick. Multiple strata are considered the composi-tion of the upper unit (ET1D), with a combined thickness ofapproximately 70 m, where exposed in the study area (Wise-man, 2009; Wiseman et al., 2010). Mineralogically, the sub-unit spectra are consistent with the presence of Fe/Mg smectite(ET1C); multiple hydrated phases, including hydrous hydrox-ides and hydrated ferric sulphates (ET1B, ET1D); and mono-and poly-hydrated sulphates (Wiseman et al., 2010).

Hematite-bearing plains (Ph) occur stratigraphically abovethe ET1 unit (Christensen et al., 2001; Hynek et al., 2002).This unit, which formed in the late Noachian–early Hespe-rian (Arvidson et al., 2006), is a few hundred metres thickin some areas, but is an order of magnitude thinner near itsnorthern margin. The Ph is a composite unit consisting oflayered, indurated rock called etched terrain 2 (ET2), and isoverlain by an unconsolidated veneer called the hematite-bearing plains mantle (Pm) (Wiseman et al., 2010). The hema-tite Ph signature results from hematitic concretions weath-ered out of the bedrock, and concentrated at the surface byaeolian processes (Arvidson et al., 2006). The ET2 unit isalmost completely covered by Pm, which protects the surfaceand slows deflation (Sullivan et al., 2005). Counts of small(<250 m diameter) craters suggest a Late Amazonian craterretention age for the Pm (Wiseman, 2009).

3. KARST LANDFORMS

Surface morphology analysis highlights the presence ofclosed rimless depressions, displaying different shapes andsizes that are surrounded by unbroken plains. Their locationand shape seem to be unrelated to the surface slope or bed-

Fig. 1. (a) Location of Sinus Meridiani area on Mars. Image NASA/USGS/ESA/DLR/FU Berlin (G.Neukum) taken from Google Mars;(b) Study area in northern Sinus Meridiani (white dashed box),with location of the portion of HiRISE image (black box) showedin Figure 1c (white box). Image NASA/USGS/ESA/DLR/FU Berlin(G.Neukum) taken from Google Mars; (c) Northern part of HiRISEimage PSP_011721_1820 showing the three morpho-units (MU)that display different karstification (north toward up).


ding plane. These landforms, which lack evidence of windaction and erosional features associated with the evolutionof impact craters, can be interpreted as dolines of polyge-netic origin sharing the characteristics with those describedon the Earth by Ford and Williams (2007) such as, plan formranging from circular to subcircular or elongate downslope,sides that range from gently sloping to vertical, and bottomsthat vary from flat to concave. Moreover, these landformsstrongly resemble dolines that develop in all types of evaporitekarst terrains on Earth, and karst morphologies previouslyidentified in other regions of Mars (Baioni et al., 2009; Baioniet al., 2011). In particular, they display deep morphometric(sizes) and morphologic (shapes, bottoms, walls) similarities

with the dolines observed in the evaporite terrains located inother regions of Mars (Baioni et al., 2009; Baioni and Wezel,2010, Baioni, 2013).

The depressions in the study area were not created by otherprocesses, such as wind deflation (because they lack a pre-ferred orientation) or impact craters (because they lack rimsand ejecta). In fact, depressions shaped by wind action onthe Earth are very elongate along wind flow direction, dis-play mostly arcuate sides and thick accumulation of sedimentsat the foot of the wall facing the wind. The observed depres-sions in Sinus Meridiani do not display any of these features.

The study area on hydrated sulfates primarily displaysthree distinct surfaces termed morpho-units (MUs), charac-

Fig. 2. Upper morfo-unit (MU1); (a) Dolines in the MU1 showing the location of the unit at the top of the mesas relief located withinthe study area (HiRISE image ESP_011277_1825; north toward up); (b) Dolines located along alignment connected by channels thatdisplay sinuous trend (HiRISE image ESP_011277_1825; north toward right); (c) Doline located along straight lineament (HiRISE imageESP_011277_1825; north toward up). (d) Doline located along straight channels in the outcrop of limestone and dolomite in the northernOntario (Canada). Image (modified) taken by Google Earth (www.earth.google.com); (e) Rounded doline connected by sinuous channelsin the outcrop of limestone and dolomite in the northern Ontario (Canada). Image (modified) taken by Google Earth (www.earth.goo-gle.com); (f) Doline displaying elongate shapes in the karst terrain of Southampton island (Canada). Image (modified) taken by GoogleEarth (www.earth.google.com).


terised by what appears to be different karstification levels(Fig. 1c). The three MUs, which are nearly exposed in theentire area, can be classified stratigraphically as upper, mid-dle, and lower MUs.

These three MUs identified in the study area are describedin detail below.

3.1. Upper Morpho-unit (MU1)

The upper morpho-unit (MU1) (Fig. 1c) was often observedat the top of mesas, representing the upper relief, and waslocated within the study area (Fig. 2). MU1 is characterized

by various sized depressions, mainly with an elongate shape,often located along alignments, and displaying light browncolour in HiRISE images (Fig. 2).

MU1 dolines typically occur scattered and isolated, andprimarily exhibit an ellipse shape. The dolines are often dis-tributed along joints and are connected, or occur near chan-nels, which in some cases display a sinuous trend (Fig. 2b)or along straight lineaments (Fig. 2c). The depressions dis-play different sizes, and range in length from 10 m to anexcess of 80 m, and in width from 5 m to 50 m. The depres-sions exhibit steep or vertical walls, and where observed,display flat floor geometry. At MU1, most depressions appear

Fig. 3. Middle morfo-unit (MU2); (a) Drop-like shaped doline displaying wide shape in a side and narrow in the opposite side (center), withpits, interpreted as rock-holes (HiRISE image PSP_010222_1815; north toward up); (b) Bowl shaped doline displaying steep walls andconcave-up floor geometry (HiRISE image PSP_010222_1815; north toward up); (c) Rounded shaped doline displaying steep walls andflat floor geometry (HiRISE image ESP_003379_1835; north toward up); (d) Drop-like shaped doline displaying one side straight whilethe opposite is arcuate, steep walls and flat floor in the evaporite terrain of New Mexico (U.S.A.). Image taken by Google Earth(www.earth.google.com); (e) Circular shaped doline in the Burren National Park (Ireland). Image (modified) by Slieve Carran taken fromthe Burren National Park website (http://www.burrennationalpark.ie).


covered by dark dust or sediments, which often display dunemorphology; some are step sided with levelled bottoms, andan expressed deepest point.

This unit might resemble the thermo-karst landscape withcollapsed dolines found in cold regions on Earth that exhibitseasonal permafrost melting, and common karst solutiondepressions developed along master joints on pavement. Inparticular, viewed from above, the depressions and channelsresemble water-filled dolines in limestone or dolomite karstlandscapes in cold regions on Earth, such as in the northernOntario, Canada (Figs. 2d–e), and in the karst terrain ofSouthampton island, Canada (Fig. 2f).

3.2. Middle Morpho-unit (MU2)

The middle morpho-unit (MU2) (Fig. 1c) is characterizedby minor sized depressions, generally with bowl or roundshaped, small pits. Polygonal cracking and light bright colourwere displayed at the HiRISE scale (Fig. 3).

MU2 dolines are typically bowl or round shaped (Figs.3b and c), or in some cases, exhibit an elongate or teardrop-like shape, wide on one end, and narrow on the oppo-site end (Fig. 3a). The doline diameters are generally up to20 m but not exceeding 50 m, and display asymmetrical orsymmetrical steep walls and concave-up (Fig. 3b) or flatfloor geometries (Figs. 3a–c). The depression floors showthe absence (Figs. 3b and c) or little (Fig. 3a) accumulationof dust or sediments.

On the surface of MU2, small pits, interpreted as rock-holes (Neuendorf et al., 2005) are also observed (Fig. 3a).These rounded, shallow depressions have diameters rangingfrom one to a few meters, and located primarily in the west-ern part of the study area. Location and shape appear unre-lated to surface slope. These rock-holes are smaller in size thanother closed depression landforms observed in the entire area,with high density, displaying a field-like distribution (Fig. 3a).

These landforms strongly resemble similar terrestrial fea-tures, displaying close morphological similarities with solu-tion dolines developed in all types of evaporite and carbonatekarst terrains, such as, in the evaporite terrain of New Mex-ico (U.S.A.) (Fig. 3d) and/or large solution pans formed bysheet wash water flow during ice melt in mountain areas asin the Burren National Park (Ireland) (Fig. 3e).

3.3. Lower Morpho-unit (MU3)

The highest depression density and deeper karstificationlevels characterise the lower morpho-unit (MU3) (Fig. 1c).Here, the depressions appear deeper and of greater size thanthose located in the other MUs. Depressions often displaymultilevel bottoms, and the unit appears light grey at theHiRISE scale (Fig. 4).

Dolines display different shapes at MU3. MU3 dolinesrange from rounded elongate and elliptical (Fig. 4a) to bowl

or round (Figs. 4b–d) and polygon shaped (Fig. 4c). Dolinesdisplay well-defined shapes characterised by sharp dividesand flat bottoms. The elongate shaped depressions generallyhave broader sizes, ranging from 50 m to 150 m in length, and30 m to 100 m in width (Fig. 4a), while the rounded andpolygon shaped depressions generally display a diameterfrom 20 m to more than 50 m (Figs. 4b–d). In these areas,shallow depressions where several individual dolines havedeveloped can be observed (Fig. 4d). Here, step-sided dolinesare very common, indicating different ground water levels(dropping water table), where separate bottoms were formedof different heights. The highest steps represent the remainsof older bottoms of individual depressions. Dust or sedimentaccumulation completely buried some depression bottoms;dark material in some cases also displays dune morpholo-gies, while other doline bottoms are totally bare, withoutany cover or accumulation of dust and/or sediments. Thisdifference can also be observed in depressions located closeto one another (Fig. 4d).

These landforms strongly resemble similar terrestrial fea-tures, displaying close morphological similarities with dolinesdeveloped in the evaporite terrains located in the arid regionson the Earth, such as, in Dead sea region (Figs. 4e–g).

4. DISCUSSION AND CONCLUSIONS

The karst landforms observed in the study area appear tobe formed as a result of flowing water, similar to evaporiterock landforms on Earth. Doline presence, a karst indexlandform (Ford and Williams, 2007) indicates intense sur-face dissolution and runoff along the entire area.

MU1 might be interpreted as a result of underground waterpercolation that caused the enlargement of existing jointsdue to dissolution in the evaporite material. In fact, missingkarren landforms or gullies on the surface suggests waterpenetrated the ground very fast at the points of maximumvertical permeability (along joints), and formed the observeddolines.

In MU2, the observed landforms suggest water remainedon the surface for longer periods of time, perhaps even inthe form of a water-film, before fully penetrating underground,to form younger dolines, and many small pits. Moreover,the landforms seem to indicate an unfrozen subsurface witha developed karst drainage system was formed, and the sur-face water layer was thick enough to allow dissolution pro-cesses. Here, it is also possible that the depressions beganas large solution pans, and developed successively as solu-tion dolines.

MU3 displays an etched surface, showing karst processesexisted, and worked for the longest period of time. Corrodedsurface observations and doline characteristics indicate thecloseness of the water table or permafrost/ice layer. More-over, this suggests the following: water could not easily pen-etrate underground, multiple ice melting and/or permafrost,


or more likely inflow of incompletely saturated water fromoverlaying beds (from upper units). Observations suggestsediment presence in some doline bottoms or dipper depres-sions without bottom sediments are younger, or that bottoms,particularly in higher topography were more suitable forsedimentation of materials by wind, acting as sediment traps.

Karst development on Mars is presumably triggered byice melting and/or snow or permafrost. It has also been assumedthis process might explain karst landforms and topographyfound in some other regions of Mars (Baioni and Wezel2010; Grindrod and Balme 2010; Baioni et al., 2011).

Ice melting, probably formed during periods of ice-rich

deposition from the atmosphere, which might have occurredas the result of obliquity changes in Mars (Laskar et al.,2004). In addition, the tropical and equatorial regions (Megeand Bourgeois, 2011; Schon and Head, 2012) provided thenecessary liquid water for karst processes.

The different levels and types of apparent karstificationobserved, which characterize the three units identified, canbe caused by three primary factors, including different waterlevels that affected the surface; different topography con-ditions (e.g., surface morphology, slope, etc.); and differentresponses to solutional processes, due to diversity in min-eralogical composition; or a combination of these factors.

Fig. 4. Lower morfo-unit (MU3); (a) Area displaying rounded elongate and elliptical shape doline (HiRISE image PSP_010222_1815;north toward up); (b) Doline displaying rounded shape and a meandering outflow channel (HiRISE image PSP_010222_1815; northtoward up); (c) Polygonal shaped doline displaying steep walls, flat bottom and the presence of a step in the left side (HiRISE imagePSP_008152_1825; north toward up); (d) Several individual rounded dolines displaying bottom with and without accumulation of dustlocated within shallow bigger depressions (HiRISE image PSP_001427_1820; north toward up); (e) Doline in the Dead sea region(Israel). Image (modified) by Ilan Shacham taken from the website www.neiture.tumblr.com; (f) Doline in the Dead sea region (Israel)Image (modified) by Shmuel Browns (taken from the website http://www.trekearth.com/gallery/Middle_East/Israel/Yerushalayim/Jerus-alem/Dead_sea/photo1175756.htm); (g) Aerial view of doline in the southern Dead sea area (Israel). Image (modified) by Ofir Ben Tovtaken from the website www.latitudeimage.com.


Considering morphological and topographic characteristicsof the study area, the karst landform characteristics observed,and the water origins, we propose it is more likely the differ-ent karstification levels might be related to different responsesto material erosive processes, rather than other factors.

Considering evaporite karst characteristics, and its rapidformation on Earth (Klimchouk, 2004), our observationssuggest liquid water existed in the area, and persisted longenough for solution features to form, but there was only oneor a few episodes during which water was available. Afterthis period of karstification, water was no longer available,and hence no additional karstification occurred in the entirearea. Consequently, the landforms can be classified as relictkarst forms displayed on the Martian surface.

The analysis carried out in this study also suggests thefollowing:

i) The landforms observed indicate the presence of karstprocesses, inconsistent with other processes such as winderosion, or impact craters heavily eroded or reworked bygeomorphic processes. Moreover, the presence of evaporiterock, and the observed landforms and abundance indicatethe morphology of the study area is due to development viakarst processes rather than thermo-karst processes.

ii) In the study area, different evolution degrees of karstfeatures can be clearly distinguished, which we interpret asdue to different solutional properties for the varied units (e.g.,mineral chemistry and concentration, and/or sedimentary tex-ture and architecture), with important implications in theinterpretation of the rock unit compositions and origins.

iii) The karst landforms also suggest a rock response toclimatological change, requiring the presence of enough liq-uid water to form the observed landforms.

iv) The landform characteristics and their erosional ageallow us to infer the landforms have been affected by a sin-gle or few geologically “wet episodes” during their history,with a first phase characterised by sufficient water avail-ability, followed by a dry climatic period.

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Manuscript received February 1, 2013Manuscript accepted January 15, 2014

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Karstic morphology in northern Sinus Meridiani, Mars