characterization of mars’ scandia tholi...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. ???, XXXX, DOI:10.1029/,

Characterization of Mars’ Scandia Tholi moated1

domes: possible mud volcanism2

Edwin S. Kite,1,2

Niels Hovius,1

Jonathan Besserer3, John K. Hillier

1, Susan

J. Conway4, Adriano Mazzini

5, Talfan Barnie

6, and Stephane LeMouelic

3

D R A F T September 30, 2009, 8:22pm D R A F T

X - 2 KITE ET AL.: MARS POLAR MOATED DOMES

* Correspondence author. Email: [email protected]

1Department of Earth Sciences,

Cambridge University, Cambridge, United

Kingdom.

2Earth and Planetary Science, University

of California, Berkeley, Berkeley, USA.

3Laboratoire de Planetologie et

Geodynamique, Universite de Nantes,

Nantes, France.

4Open University, Milton Keynes, United

Kingdom.

5Physics of Geological Processes,

University of Oslo, Oslo, Norway.

6Department of Geography, Cambridge

University, Cambridge, United Kingdom.


KITE ET AL.: MARS POLAR MOATED DOMES X - 3

Abstract. Large moated domes near the North Pole of Mars have been4

proposed as the source of the polar gypsum. We carried out a detailed study5

of the domes, showing that they have the following properties:- elliptical, with6

long axes oriented ∼ E-W; 430 ± 90 m high, with average diameter 38 km;7

combined area of 5.4 x 104 km2, and combined volume of 3.6 x 103 km3. The8

dome surfaces appear rough in MOLA and CTX data, with multiple pits and9

peaks. These sometimes define annular crests and slumps, concentric about10

a central peak. High resolution images show mantling and, rarely, boulder-11

rich layers. Azimuthally averaged radial profiles show mean slopes ∼ 1◦. The12

domes region has an excess of relatively steep, S-facing slopes, which is ab-13

sent in a control region mirrored through the pole. This may be related to14

summertime melting of ice-rich, N-facing slopes at high obliquity. Analysis15

of OMEGA and CRISM spectra shows pyroxene signatures, but no unam-16

biguous evidence for hydrated minerals at the optical surface of the domes.17

Explosive igneous volcanism, or mud volcanism, could account for these fea-18

tures. Uniform elevations, subdued relief, geologic context, and Earth analogs19

point to mud volcanism. In particular, large uplifts with jumbled interior to-20

pography and well-defined margins, resembling the domes, are found in the21

offshore S Barbados mud volcano province. On Mars, ice sheet emplacement22

or removal can generate overpressure and is a potential trigger for mud volcanism.23

These results favor the mud volcanism hypothesis of Tanaka [2005]. How-24

ever, it is not known whether gypsum sand could survive saltation over the25



distance (> 100 km) from the moated domes to the area of peak gypsum con-26

centration.27



1. Introduction

What lies beneath the northern plains of Mars? The basement of the northern depres-28

sion is Early Noachian in age [Frey, 2006], and much of the subsequent infill is compactible29

sediment [Buczkowski, 2007]. The topmost unit mapped from orbit, the Vastitas Borealis30

Formation, is thought to be periglacially reworked [Tanaka, 2005]. Phoenix has confirmed31

[Smith et al., 2009] the indication from GRS data [Boynton et al., 2002] that at least the32

topmost meter poleward of 60N is largely water-ice, probably deposited during ice ages33

≤ 5 Ma [Schorghofer, 2007]. Yet a central question remains unanswered [Tanaka et al.,34

2003]: What process was most significant in filling the basin? Possibilities include basaltic35

flood volcanism [Head & Kreslavsky, 2002], ocean sedimentation and catastrophic flood36

deposits [Fairen et al., 2003], debris flows [Tanaka et al., 2001], ash, impact ejecta, and37

aeolian processes. Near the center of the northern depression, a field of large, moated38

domes scars the otherwise smooth surface of the plains [Tanaka et al., 2008]. The dome39

field is 450 km from the Phoenix landing site at 68.2N, 125.7W (Figure 1). Because40

these moated domes (formally named Scandia Tholi) were formed of subsurface materials41

[Tanaka et al., 2003, 2005], they should hold clues to the nature of the basin fill. For42

example, a phreatomagmatic origin would suggest the presence of an aquifer at depth.43

This paper is the first to focus exclusively on the Scandia Tholi. However, the moated44

domes have previously been described and interpreted during broader mapping efforts: –45

Fishbaugh & Head [2000] interpret the moated domes, and the adjacent ridge-bounded46

depressions, as proglacial deposits formed during asymmetric retreat of Planum Boreum.47

Garvin et al. [2000] interpret the same features (which they termed Martian Pitted Domes,48



MPD) as igneous volcanos, an interpretation followed by Kneissl & Neukum [2008]. The49

lack of clearly-defined craters associated with the domes, together with their blocky, dis-50

rupted surfaces, led Skinner & Mazzini [2009] to propose that violent hydrothermal erup-51

tions (triggered by dyke or sill intrusions) formed the domes. Tanaka et al. [2003] was52

the first to suggest that the moated domes were the result of mud volcanism. This is also53

our preferred interpretation. Other hypotheses, particularly explosive igneous volcanism,54

cannot be ruled out. Throughout this paper, we define mud volcanism to be the spatially55

concentrated surface release of sediment, driven by overpressure associated with a fluid56

or vapor phase or phases. The release may be extrusive or explosive in character. This57

broad definition is designed to include stratified sediment mobilization processes, which58

are associated with mud volcanoes and thought to be driven by the intrusion of mud plugs59

at depth [Deville et al., 2006].60

The purpose of this paper is to give additional constraints on the nature and origin of61

the domes, as a step toward understanding the subsurface stratigraphy of the Northern62

Plains. We show that the domes have a distinctive, jumbled/blocky morphology that has63

not been found outside the northern plains, and identify key morphological characteris-64

tics: moats, central peaks with adjacent pits, disrupted ring structures, overall elliptical65

shape, and subdued relief. We also show that moats and nested, disrupted rings appear66

even in azimuthally-averaged radial profiles of the domes. We show that steep, N-facing67

slopes are underrepresented in the domes region. We suggest this is related to insolation-68

dependent modification. Spectral analysis shows that the optical surface at visible and69

near-infrared wavelengths is similar to that of the background plains, with absorptions70

characteristic of ferric oxide and pyroxene. Our assessment is that mud volcanism is the71



least unsatisfactory explanation of the domes. In particular, offshore mud volcanism in72

the Barbados Accretionary Prism and Gulf of Cadiz shows all of the structural elements73

we identify for the domes. We show that ice sheet emplacement can lead to near-surface74

overpressures sufficient to trigger mud volcanism. A summary of our evaluation is given75

in Table 2. Because mud volcanism is sourced from overpressured, deep-buried sediments,76

our work supports the inference that the Northern Plains stratigraphic column contains77

a significant percentage of sediments [Buczkowski, 2007].78

2. Description of moated dome field

The features of interest in this paper are rough-topped ovoid rises, with summits at79

least 100 m in elevation above surrounding topography, and which are not centered on80

a crater. Using Mars Orbiter Laser Altimeter (MOLA) 256 pixels-per-degree (ppd) and81

512 ppd gridded topography, we have found 29 domes fitting this definition in the region82

72-80N, 180-215E (Table 1; Figure 2). This count is conservative; for example, dome83

12 may be a composite of five structures. Rises less than 10 km in size are not always84

well resolved by MOLA topography, so our catalogue probably excludes several rises that85

meet the definition, but which are too small for diagnostic features to be visible in the86

gridded topographic data. The amplitude of roughness on the dome tops exceeds that on87

the surrounding plains [Kreslavsky & Head, 2000] at 2.4 km scale and especially at 600 m88

scale, making the domes resemble welts. Welt-like features have only been found poleward89

of 70N on Mars, and Kneissl & Neukum [2008] have reported rare welt-like features in90

the 70-100E sector. The domes have a combined area of 5.4 x 104 km2. Referenced to91

the regionally detrended elevation of the Scandia plains, they have a combined volume92

of 3.6 x 103 km3. Some of the larger, more northerly domes have large central collapse93



cavities. Their morphology is transitional to that of two large ridge-bounded depressions94

(formally named Scandia Cavi) that lie adjacent to the domes (Figure 1). This suggests95

that the ridge-bounded depressions and the domes may have formed by the same process.96

The Scandia Cavi region also includes sinuous ridges, often terminating in craters, which97

we interpret to be either eskers, or (perhaps more likely) moraines. If this interpretation98

is correct, it would imply that the polar ice sheet was once more extensive in this sector99

- a history supported by the ice-rich composition of the Olympia Dome [Phillips et al.,100

2008], and the asymmetric distribution of the north polar plateau [Fishbaugh & Head,101

2000; Kite et al., 2009]. Near the limit of MOLA resolution, isolated to clustered conical102

rises are found occasionally within the moated domes region. These rises are especially103

concentrated near 76N, 103W, and have been interpreted as either erosional remnants of104

the formerly more extensive Rupes Tenuis plateau [Warner & Farmer, 2008], or volcanic105

edifices [Fagan & Sakimoto, 2009].106

To the east of 215E, the dome field is mantled by Scandia formation materials, and107

it is difficult to determine whether or not the domes were erosionally modified prior to108

this mantling. Outside of this area, dome outlines are less coherent, and their surfaces109

more pitted, in the north of the domefield. This implies that either: (1) the degree of110

exhumation increases to the N; (2) the more northerly domes were formed by a different111

mechanism than the southerly domes; or (3) that the degree of modification by ice varies112

across the study region. Incomplete preservation of the ring structures present in many the113

domes (Figure 2, Figure 4a) suggests that the domes have been erosionally modified. This114

indicates that the domes are composed of material that is susceptible to either sublimation115

or aeolian erosion. We can exclude glacial erosion, because there is little evidence for116



extensive wet-based glaciation in the north polar region. Periglacial modification can also117

be excluded as it tends to soften hillslopes, but not reduce base level.118

The Scandia Tholi overly the Interior Unit of the Vastitas Borealis Formation, which is119

Earliest Amazonian by definition [Tanaka, 2005] (3200–2000 Ma using the chronology of120

[Hartmann, 2005]). To the N of the domes, the Olympia Undae are underlain by up to121

800 m of horizontally-layered, relatively light-toned deposits forming the Olympia Planum122

rise (Figure 1; Byrne & Murray [2002]; Tanaka [2005]). Contour lines on the slope of the123

Olympia rise are truncated by some of the Scandia Tholi, implying that emplacement124

of these domes postdated deposition of the materials making up the Olympia Planum125

rise. However, the dark, gypsum- or bassanite-bearing [Langevin et al., 2005] dunes of126

the Olympia Undae embay and so postdate the moated domes.127

High-resolution images show that the domes have been mantled, confirming that they128

predate the last major glaciation 5 – 0.3 Ma. Pedestal (excess-ejecta) craters are found129

throughout the Scandia region. They are very likely to be erosionally-modified impact130

craters [Kadish et al., 2008], and imply at least one cycle of deposition, and incomplete131

removal, of a volatile-rich mantling unit. Because craters formed in the mantling unit may132

have been removed, crater-count chronologies are unlikely to be a good guide to absolute133

ages in Scandia. Context Camera (CTX) images show that the dome surfaces have far134

fewer pedestal craters than immediately adjacent plains. The simplest interpretation135

is that the domes formed after the erosional unconformity that created the pedestals.136

From this we infer that the moated domes are most likely Early Amazonian to Middle137

Amazonian in age (3200–200 Ma using the chronology of Hartmann [2005]). Alternatively,138



the domes may have been emplaced into a unit that has since been stripped back (J.A.139

Skinner, via e-mail).140

We now address the detailed morphology of individual domes.141

3. Characterization of moated domes

3.1. MOLA, CTX and HiRISE observations

Dome diameter ranges from 20 – 50 km (Table 1). Many have annular ridges and rings,142

and moats with a constant width (2.3 ± 0.37 km). MOLA topography shows that dome143

margins have gentle slopes (1 – 5◦). Steeper slopes are found on central peaks and peaks144

in annular ridges: these are almost always accompanied by a pit marginal to the peak145

(‘peak-marginal pit’), and have flank slopes of 5 – 9◦. Dome interiors are rough and146

blocky at the kilometer scale. MOLA topography shows narrow v-profile traces adjacent147

to some of the domes, but CTX resolves these apparent channels as the margins of scabby,148

pedestal bearing terrain; they are not fluvial channels.149

In CTX images, the dome interiors appear as trench-bounded zones of kilometer-sized,150

uplifted and jumbled ridges and blocks, set within scabby, layered plains bearing pedestal151

craters (refer to annotated Figure 4a). In some cases (e.g., MOC NA frame E0302376)152

flows from domes run into the moats. Alignments of blocks sometimes define arcs or153

broken rings. Lineations contouring around blocks and traced by frost or changes in tone154

resemble layers. Alternatively, their ragged planforms are consistent with shallow-seated155

gelifluction of the moderately steep slopes of blocks [Davis, 2001]. If the lineations do156

define layers internal to the blocks, this implies the absence of a mantle. Elsewhere, block157

topography appears softened, and HiRISE (High Resolution Imaging Science Experiment)158

shows boulder clusters (Figure 4c). Mantling by a geologically recent ice-rich material159



[Mustard et al., 2001] has obscured the detailed morphology of the underlying domes.160

There are exceptions: in the relatively steep E margin of one dome (#13 in Table 1), CTX161

shows approximately horizontal lineations that we interpret as layers. Both blocky and162

smooth appearing layers are present, which may correspond to boulder-rich and boulder-163

poor flows (Figure 4b).164

The domes are elliptical, with long axes preferentially oriented E-W (Figure 3), although165

with considerable scatter.166

3.2. Radial profiles

To quantify the morphometry of the domes, we have generated radial profiles for167

each dome using 256 ppd MOLA topography, using a method outlined in Appendix A1.168

Azimuthally-averaged profiles are shown in overview in Figure 5, and in more detail in169

Figure 6. Moats are present in the averaged profiles for many of the domes, even though170

radial averaging tends to mute and obscure the moats because it includes areas that have171

been modified subsequent to emplacement (for example, by impact ejecta). Many of the172

rings are located off-center within the domes; nevertheless, these are also present in many173

profiles. However, CTX shows that some of the ring structures that appear well-preserved174

in MOLA data are actually dissected/blocky. This is because the MOLA gridded dataset175

contains many interpolated pixels, and so does not capture the true roughness of the dome176

interiors. The vertical extent and average slope of the domes is remarkably consistent.177

The elevation of the summits above the plains ∆z = 432 ± 92 m (1 σ) (Table 1). The best-178

fitting simple cone has an aspect ratio (width:height) ∼ 100:1, confirming the remarkably179

low relief of the domes. Central peaks, which are definitely present in 17 domes, possibly180

present in 4, and absent in 8, have slopes in the range 5 - 9◦ (Table 1). Typical dimensions181



for these central peaks are 7 km (long axis) x 4 km (short axis). Peak-marginal pits or182

annular troughs are definitely present in 10 domes, possibly present in 2, and absent in the183

remaining 5 domes that have central peaks. Ridges defining concentric rings are present184

in 16 domes and possibly present in an additional 4. Inner ridges are always circular, even185

though parent domes are always elliptical. Outer ridges track elliptical dome flanks.186

3.3. Slope-aspect correlations

Many surface processes on Mars are affected by slope orientation. For example, gullies187

form preferentially on pole-facing slopes in the mid-latitudes [Head et al., 2008], and pole-188

facing slopes within spiral troughs on the NPLD are shallower than equator-facing slopes.189

To determine if the Scandia Tholi underwent aspect-dependent modification, we examined190

gridded MOLA data to look for correlations between the probability-density distribution191

of local topographic slope and aspect. We have defined a test region to include as many192

moated domes and ridge-bounded depressions as possible, but at the same time excluding193

areas of high albedo in the MOC WA Atlas of Mars, areas mapped as ice outliers on the194

USGS map of the Northern Plains, moated domes that are embayed by dunes (domes 23-195

25 and 27), and dune fields visible in CTX images and MOLA gridded topography (Figure196

7). A more conservative test region also excluded the ridge-bounded depressions. We also197

defined a control region, at the same latitude but 180◦ away in longitude (Figure 7). The198

region was defined by mirroring the outline of the test region through the 135E - 315E199

line, and translating the resulting polygon a short (< 100 km) distance to avoid the ejecta200

blankets of major craters.The control region has the same dimensions and is located at the201

same distance from the pole as the test region, but does not have dome-like features. The202

length scale of our analysis is 230 m (one MOLA pixel). We did not regionally detrend the203



data, because our assessment is that the regional slope was established by basin-forming204

impacts, before the Scandia Tholi were emplaced. We used the 256 ppd MOLA polar205

gridded topography for all our measurements, because the 512 ppd grid has only partial206

coverage of the dome field (and a greater fraction of interpolated pixels).207

The test region shows a statistically significant fractional excess of slopes steeper than208

4◦ that are S-facing, and a deficit of slopes steeper than 4◦ that are N-facing (Figure 8a).209

The control region shows no such anomaly; the orientation of steep slopes is consistent210

with a random distribution (Figure 8b). An analysis with smaller aspect bins (not shown)211

indicated that shallow slopes in the control region tend to be E-facing, consistent with212

the regional tilt. Only N-S tilting would affect the slope statistics used here, and the tilt213

values (∼ 0.05◦) are insufficient to change the fractional differences significantly.214

The MOLA ‘count’ file shows that many of the pixels in the 256 ppd dataset are215

interpolated, and, even in pixels that contain a MOLA shot, the data have been migrated216

to the center of the pixel. This begs the question of whether the slope-aspect anomaly is217

the result of the processing steps which translate raw MOLA data to gridded topography.218

To address this concern, we turned to the underlying PEDR (Precision Experimental Data219

Record) data. We used the Mars Orbital Data Explorer (http://ode.rsl.wustl.edu/mars/)220

to generate a shapefile containing all PEDR records in the vicinity of the test region and221

control region. We used ArcGIS to clip the shapefile to the polygons bounding the region222

of interest, and extracted tracks from the resulting database. In subsequent analysis, we223

used only track segments (lines joining two MOLA spot points) of length < 450 m and224

whose 2 neighbouring track segments also had length < 450 m. Because in the absence225

of clouds and instrument problems MOLA spots are seperated by no more than ∼ 300226



m, this procedure excluded areas where MOLA had difficulty gathering data. We then227

converted the latitude and longitude values from the track segments to x and y coordinates228

(approximating Mars as a sphere), and found the slope for each track segment. Because229

the Mars Global Surveyor ground tracks run approximately N-S in both the test and230

control regions, we were only able to compare the steepness distributions of N-facing to231

S-facing slopes. Shallow slopes are equally likely to face N as S (Figure 9). In the test232

region, but not in the control region, slopes steeper than 5◦ are more likely to face S. This233

excess is particularly noticeable for slopes steeper than 6◦ (Figure 9). When the ridge-234

bounded depressions are masked out of the control region, the excess of steep, S-facing235

slopes is less striking but still significant, and is noticeable for slopes steeper than 8◦.236

3.4. Mineralogical analysis using visible and near-infrared imaging spectroscopy

We have investigated the mineralogical composition of the mud volcanoes area using237

OMEGA (Observatoire pour la Mineralogie, l’Eau, les Glaces, et l’Activite, Bibring [2007])238

and CRISM (Compact Reconnaissance Imaging Spectrometer for Mars, Murchie et al.239

[2009]) hyperspectral data. OMEGA has 352 contiguous channels in the wavelength range240

0.38 - 5.1 µm, giving near laboratory-like spectra for each pixel of images acquired with241

a spatial resolution ranging from <350 m to 5 km per pixel. CRISM has 2 overlapping242

detectors with 6.6 nm spectral resolution, a VIS channel in the wavelength range 0.36 - 1.1243

µm, and an IR detector in the wavelength range 1.0 - 3.9 µm. It has a maximum spatial244

resolution of 18m. All images were corrected for atmospheric effects using the standard245

empirical method (the ‘volcano-scan’ correction, using the spectral ratio of Olympus Mons’246

summit and base scaled to the depth of the 2 µm CO2 band; Langevin et al. [2005]). Table247

3 gives details of the analyzed images.248



Since most mineralogical spectral information is contained in the 0.9 – 2.5 µm wave-249

length range, we focused our analysis on this domain. Figure 10a shows one of the studied250

OMEGA cubes, acquired with a spatial resolution of 4 km-per-pixel. Figure 10b shows251

the average spectra taken above the Scandia Tholi. These spectra appear at first order252

very homogeneous. In order to emphasize subtle and diagnostic spectral signatures, these253

spectra have also been ratioed to a neutral area taken outside the mud volcanoes. This254

technique proved useful in prior searches for phyllosilicates [Poulet et al., 2005; Mustard255

et al., 2008]. However, in our case the ratioed spectra on OMEGA data do not reveal256

any diagnostic band above the noise level. We performed a similar investigation at higher257

spatial resolution using CRISM FRT (Full Resolution Targeted Observation) and HRL258

(Half Resolution Long Observation) data. Three cubes cover welt # 13, a relatively well-259

preserved dome. Two of these CRISM cubes (000093F9 and 0009B30, acquired in January260

2008) display very strong water ice bands, which precludes the analysis of the underlying261

mineralogy. Only one cube (000BACF, Figure 10c) is free of ice. However, as was the case262

for the OMEGA data at lower spatial resolution, no diagnostic absorption bands showed263

up in the spectral ratio analysis of this cube (Figure 10d).264

We have also performed a spectral mixture analysis through by using an improved265

iterative linear unmixing model [Combe, 2005; Combe et al., 2008]. Details of the approach266

are given in Appendix A2. Some spectra could be fit by a mixture of pyroxenes and267

sometimes clays, but our assessment is that the clay components were probably model268

artifacts, as spectral ratio and band depth analysis did not show corresponding diagnostic269

bands above the noise level.270



The question of the bulk mineralogical composition of the welts therefore remains open.271

One possibility is that the domes are made entirely of the material of which we see the272

spectral properties in Figure 10b, with no particular diagnostic band appearing above the273

noise level. Another more probable explanation is that a pervasive dust cover prevents us274

from seeing the underlying materials in this area.275

4. Origin scenarios

4.1. What needs to be explained?

The salient features of the large, moated domes are: internal collapse features; central276

peaks; nested rises and slumps; jumbled interiors with multiple pits and peaks; overall277

elliptical shape with a preferred E-W long-axis orientation (Figure 3); subdued relief278

(summits 432 ± 92 m (n = 29) above the background plains, with central peak slopes279

typically 4 - 7◦); and circumferential moats (Table 1; Figure 4). Spectral analysis shows280

pyroxenes, but strong hydration bands or phyllosilicate signatures are not found. Steep281

south-facing slopes are over-represented (Figure 9), consistent with insolation-dependent282

modification of slopes (e.g., Kreslavsky & Head [2003]). A successful formation hypothesis283

must account for all of these features.284

Alternative explanations for the origin of the domes (Table 2) include that they are285

glacial features (pingos, kettles or kames), igneous features (effusive, viscous ‘pancake’286

igneous domes; tuyas; or the result of explosive igneous volcanism), or the result of mud287

volcanism (mud volcanoes, mud diapirs, or stratified sedimentary uplifts). We now address288

each mechanism in turn.289



4.2. Glaciation alone?

Pingos (ice-cored domes) and kames (till accumulations against the edge of glaciers) have290

some attributes that overlap with the moated domes, but do not share all characteristics291

(Table 2). They are composed of volatile material; they are common in deglacial and292

proglacial settings; and they form flat-topped or hemispherical structures, often with a293

central pit, consistent with the appearance of the ridge-bounded depressions [Fishbaugh294

& Head, 2001]. However, if the domes are pingos, then their interiors would be primarily295

water ice, and ice-rice deposits of this size would have been detected by MARSIS (Watters296

et al. [2006]) or SHARAD (SHAllow RADar, Holt et al. [2008]) mapping. In addition,297

pingos of the scale of the large, moated domes are absent on Earth. Kames (irregular,298

stratified proglacial sediment wedges) and kettles (depressions formed by the melting of299

calved blocks of ice) are ice-marginal features that form when the ice sheet is losing mass300

by melting. Sustained low surface temperatures (e.g., Shuster & Weiss [2005]) indicate301

that sublimation ablation is likely to have been the dominant loss term for ice sheets302

on Mars throughout the Amazonian. Nested rises and slumps defining large-scale ring303

structures are not expected outcomes of pingo formation, but they arise naturally from304

caldera subsidence during igneous volcanism and mud volcanism. Finally, the consistent305

E-W elongation direction of the moated domes (Figure 3) is not easily explained if the306

moated domes are pingos, but can be understood if the domes are extrusive constructs307

sourced from dykes. Elsewhere on Mars, pingos proposed on the basis of low-resolution308

imagery are – with few exceptions – not supported by (ongoing) HiRISE imaging [Dundas309

& McEwen, 2009; Burr et al., 2009].310



4.3. Igneous volcanism?

Every object in the solar system with a silicate crust and diameter > 1000 km shows311

magmatic volcanism, whereas mud volcanism has only been confirmed on Earth. There-312

fore, when assessing an extraterrestrial construct, a high prior probability should be as-313

signed to igneous volcanism. Andesitic to rhyolitic caldera-forming eruptions on Earth’s314

continents leave large collapse cavities that are often partly filled by resurgent domes. Off-315

shore, basaltic caldera-forming eruptions leave moated depressions [Mueller et al., 2008].316

We do not rule out explosive igneous volcanism. However, the uniform and low peak eleva-317

tion of the moated domes, and their low overall relief with nested rises and slumps, argues318

against caldera-forming magmatic eruptions (Table 2). Insolation-dependent modification319

of steep slopes (Figure 9) suggests that the domes are volatile-rich, which is inconsistent320

with igneous volcanism. Geologically recent igneous volcanism near the North Pole of321

Mars would be surprising (though not impossible) given that the lithosphere was thick322

and cold (Te > 300 km) at the time of Planum Boreum loading [Phillips et al., 2008]. Polar323

cones proposed to be young volcanoes early in the Mars Global Surveyor mission [Garvin324

et al., 2000] have recently been re-interpreted as erosional outliers [Warner & Farmer,325

2008]. Effusive volcanism on the terrestrial planets produces smooth shields [Basaltic326

Volcanism Study Project, 1981], in contrast to the blocky, disrupted dome interiors with327

their multiple pinnacles and pits. Pancake domes on Venus have no good Earth analogs,328

but are interpreted to be basaltic (less likely, rhyolitic) extrusions [Stofan et al., 2000].329

Unlike the moated domes, they have steep sides and cracked interiors, and lack multiple330

peaks and pits. Tuyas (subglacial volcanoes) tend to be flat-topped, ridge-like structures,331

and these are in fact seen in the Hesperian-aged Dorsa Argentea Formation near the South332



Pole of Mars (Sisyphi Montes; Ghatan & Head [2002]). The eroded appearance of the333

moated domes in MOLA topography is in contrast to the relatively uneroded Sisyphi334

Montes, which are older and at comparable latitudes. The simplest interpretation is that335

the moated domes are composed of material that is more susceptible to sublimation and336

aeolian erosion than that making up the Sisyphi Montes. (Alternatively, the higher at-337

mospheric pressure in the northern lowlands favors transient liquid water, which is an338

effective erosional agent in periglacial environments on Earth [Davis, 2001]). We con-339

clude that the degraded state of the moated domes argues against the hypothesis that the340

moated domes are tuyas.341

4.4. Mud volcanism hypothesis

1. Morphological features of the large, moated domes are hard to explain other than by342

mud volcanism343

Mud volcanoes develop when ‘fluid-rich, fine-grained sediments ascend within a litholog-344

ical section because of their buoyancy’ [Kopf, 2002]. Common attributes of mud volcanoes345

on Earth include collapse cavities, subdued relief, and multiple pinnacles and peaks. Like346

igneous volcanoes, many mud volcanoes develop calderas [Evans et al., 2008], but even347

mature mud volcanoes can lack central vents (e.g., Figure 11).348

The best terrestrial analogs we have found to the large, moated domes are the offshore349

mud volcano provinces of the Norwegian continental margin [Akhmetzhanov et al., 2008],350

Gulf of Cadiz [Somoza et al., 2003; Akhmetzhanov et al., 2008], Eastern Mediterranean Sea351

[Limonov et al., 1994] (Figure 11), and especially the South Barbados Accretionary Prism352

[Deville et al., 2006] (Figure 12). The S Barbados Accretionary Prism shows elliptical353

uplifts (preferentially oriented N-S) with central collapses, dotted with mud volcanoes.354



The uplifts reach elevations of up to 200 m above the adjacent seabed, and have blocky,355

irregular interiors. Seismic imaging confirms that the uplifts are stratified, rather than356

chaotic [Deville et al., 2006]. Their long axes are up to 20 km long, within a factor of 2 of357

the size of the Martian moated domes. The Gulf of Cadiz lacks the elliptical uplifts of the358

Barbados province. However, its mud volcanoes show central peaks, peak-marginal pits,359

moats, and (in the largest structure, 5 km in diameter) hummocky, km-scale roughness360

in the dome interior. The Cadiz mud volcanoes are probably related to elongate diapiric361

ridges of shale and marl [Fernandez-Puga et al., 2007]. These two mud volcano provinces362

are unusually well-imaged, and it is possible that other mud volcano provinces (e.g., the363

South Caspian Sea) will show a similarly close resemblance to the Scandia Tholi once364

multibeam bathymetry becomes publicly available.365

We now turn to individual mud volcanoes that for which high-resolution bathymetry is366

available. The TREDMAR mud volcano in the Eastern Mediterranean Sea (Figure 11)367

show multiple pinnacles and pits within a very rough dome interior, bounded by a well-368

defined moat. These attributes make the TREDMAR volcano resemble the Scandia Tholi369

(Figure 4a). High-resolution bathymetry of the Hakon Mosby mud volcano in the Barents370

Sea (not shown; see Beyer et al. [2005] and Feseker et al. [2008]) exhibits nested rises and371

slumps, jumbled, fractured blocks, and a circumferential moat. Like the TREDMAR mud372

volcano, it is morphologically similar to (although significantly smaller than) the Martian373

moated domes shown in Figure 4a. There is no central peak. Interestingly, with the374

increase in resolution from multibeam to ROV maps, nested annular crests and slumps375

are dissected into a complex patchwork of disaggregated blocks. A similar increase in376

complexity is found going from MOLA to CTX maps of the large moated domes.377



No single mud volcano on Earth reaches the size of the large moated domes. The largest378

known [Davies & Stewart, 2005] is 10 km in diameter, with annular structures traced to379

depth by 3D seismic investigations [Stewart & Davies, 2006]. The sizes and slopes of380

subaerial mud volcanoes on Earth are, however, consistent with the central and marginal381

peaks of the large moated domes.382

Mud volcanism can account for all of the morphometric features seen at the Scandia383

Tholi. In particular, moats and peak-marginal pits are ubiquitous near mud volcanoes on384

Earth and are interpreted as subsidence features. Annular troughs are probable ring-fault385

graben, similar to those seen in high-resolution seismic bathymetry of mud volcanoes in386

the Gulf of Cadiz [Akhmetzhanov et al., 2008]. Central and marginal peaks may be mud387

mounds above localized centers of activity, and outer ridges that track track elliptical388

dome flanks may be flow features. The low relief of the Scandia Tholi is also consistent389

with mud volcanism.390

Circumferential moats may be formed by flexure, extension, sublimation of a volatile-391

rich debris blanket [Hauber et al., 2008], mining by katabatic winds [Warner & Farmer,392

2008], thermal erosion of ice-rich material by magma or lava [McInnis et al., 2007], or393

subsidence after removal of material from an underlying reservoir. Subsidence moats are394

very common around submarine mud volcanoes on Earth. If the moats are caused by395

subsidence, as would be the case for mud volcanoes, then they would be expected to396

have a constant width controlled by the depth to that reservoir and overburden flexural397

rigidity (unless they are faulted). If, instead, the moats are a flexural response to loading398

by the domes, moat depth would be proportional to the weight of dome material within399

an annulus of width ∼ 2.5α inboard of the moat, where the flexural parameter α =400



(4D/∆ρg)0.25 (in which D is the flexural rigidity, ∆ρ is the density contrast, and g is401

gravity) is similar to the moat width [Watson, 2001]. We measured moat width and depth402

for the 13 domes with well-defined moats. Neither showed any correlation with dome403

height. If the density of dome-forming material does not vary between domes, which404

is reasonable, this excludes a flexural origin for the moats. Excluding one apparently405

atypical dome, moat width is (2.3 ± 0.37) km (n=12). This narrow range of values406

suggests that moat width is related to the depth to a shallow reservoir, consistent with407

the mud volcanism hypothesis. If the moats resulted from mining by katabatic winds, we408

would expect the margins of ejecta blankets in the study region to also show moats. The409

ejecta blanket margins are steeper and taller than the moat margins and hence should410

produce stronger katabatic winds. Because they do not, we reject mining by katabatic411

winds as an explanation for the moats.412

Next we consider dome relief. On Earth, offshore mud volcanoes never show more than413

1 km relief, but basaltic volcanoes can be up to 10 km above surrounding topography (Big414

Island, Hawaii). These differences may be related to the depth of the source reservoir: the415

magma making up basaltic volcanoes is sourced from greater depths than the fluidized416

sediments making up mud volcanoes. Therefore, isostatic balance sets only a weak upper417

limit on the height of basaltic volcanoes, but a strong upper limit on the height of mud418

volcanoes. (This assumes that a free connection exists between the source reservoir and419

surface; see Wilson et al. [1992] for an alternative view). Onshore, mud breccia and clay420

are more easily eroded than basaltic lava, so basaltic volcanoes would show more relief421

than onshore mud volcanoes even if the depths to their source reservoirs were equal.422



In conclusion, mud volcano provinces on Earth offer close morphological analogs to the423

Scandia Tholi; all of the key features of the Scandia Tholi (Table 2) are characteristic424

of mud volcanism on Earth and taken together the suite of features is suggestive of mud425

volcanism. We speculate that the Martian moated domes more closely resemble subma-426

rine mud volcanoes on Earth than they do terrestrial mud volcanoes on Earth because427

terrestrial mud volcanoes are rapidly eroded by rain.428

2. The Scandia Tholi region has undergone insolation-dependent modification429

Mud volcanism brings water to the surface, which under Martian conditions would430

rapidly evaporate or freeze. Icy slopes on Mars are subject to aspect-dependent modifi-431

cation. Aspect-dependent modification generally works through insolation/temperature-432

dependent rates of ablation, accumulation, or ice creep, and produces anomalies centered433

on north or south. If dome composition includes a significant ice component, then aspect-434

dependent processing could cause a slope-frequency anomaly centered on N or S. Such435

an observation would support the hypothesis that the moated domes are the result of436

mud volcanism, because rhyolitic or basaltic volcanoes would not be expected to show an437

anomaly.438

Because the slope anomaly occurs for slopes far shallower than the angle of repose for dry439

materials, dry landslides cannot explain the anomaly. Creep in ice has an Arrhenius-law440

dependence on temperature, so warm ice would flow more quickly than cold ice [Parsons441

& Nimmo, 2009]. Belts of north-south slope asymmetry found at 40-50◦ latitude in both442

hemispheres have been attributed to preferential melting of ground ice on pole-facing443

slopes during periods of high mean obliquity [Kreslavsky & Head, 2003]. At the latitude444



of the moated domes, the tendency for icy pole-facing slopes to suffer melting at high445

obliquity is even greater [Costard et al., 2002].446

If ice were preferentially deposited on cold, N-facing slopes at present-day obliquity447

(∼25◦), this would provide an alternative mechanism for suppressing steep N-facing slopes.448

This introduces a possible problem for interpretation: that the volatile content implied449

by our results might belong to a thin drape compositionally unrelated to the material450

making up the bulk of the domes and ridge-bounded depressions. Some evidence against451

this possibility comes from the control region, which has basketball terrain indicating the452

presence of an ice mantle, but no slope anomaly. Given that it is not associated with a453

slope anomaly in the control region, it is unlikely that the formation of basketball terrain454

is responsible for the slope anomaly in the test region. We conclude that the moated455

domes and ridge-bounded depressions are likely to have a significant volatile content.456

These volatiles are (relatively) deep-seated, in contrast to the superficial seasonal frosts457

picked up by the spectral analysis.458

3. Geological and geophysical context is consistent with mud volcanism459

Mud volcanism requires a reservoir of overpressured, fine-grained sediments. In this460

section we outline indirect evidence that the Scandia Tholi region is underlain by fine-461

grained sediments, and in the next section we suggest mechanisms that could generate462

the required overpressure.463

Quasi-circular depressions visible in MOLA topography, but hard to discern in Viking464

images, are interpreted as buried Noachian impact craters [Frey et al., 2002]. The strong465

correlation between depression depth and depression diameter for quasi-circular depres-466

sions within a single province is interpreted as due to differential compaction of a cover467



material. Variability in the depth:diameter ratios between provinces is interpreted as468

due to variations in the thickness of that cover material [Buczkowski et al., 2005]. The469

depth:diameter ratio of quasi-circular depressions found in MOLA data for the Bore-470

alis back-basin (which includes the Scandia domefield; Figure 1) is among the lowest in471

the northern plains [Buczkowski, 2007]. This suggests that the thickness of compactible472

sediments is greater beneath the Scandia Tholi than elsewhere in the northern plains473

[Buczkowski, 2007]. However, because compaction is largely irreversible (Ingebritsen et474

al. [2006], p. 66), past overriding of the Borealis back-basin by a thick ice sheet is also475

consistent with the depth:diameter observations.476

After correction for North Polar Layered Deposits (NPLD) density, gravity maps based477

on Mars Global Surveyor (MGS) Radio Science data show a negative Bouguer anomaly478

centered on 77.5N, 192E [Neumann et al., 2004]. This is the largest-amplitude anomaly479

of this type north of 55N. Close to the center of the Borealis back-basin, the ∼ 100 Mgal480

negative anomaly could be generated by substituting 3km ice, or 6km sediments, for the481

same thickness of basaltic crust.482

These observations suggest that low-density, compactible sediments (for example clay-483

stone or mudstone) are unusually thick or unusually close to the surface in the Borealis484

back-basin. Geologic mapping supports this inference [Tanaka et al., 2008]. Because low-485

density fine-grained sediments are required for mud volcanism, this is consistent with the486

hypothesis that the Scandia Tholi are mud volcanoes. Large, moated domes with jumbled487

interiors are not found elsewhere on Mars, suggesting that unusual conditions – such as a488

thick basin fill, or a nearby ice sheet – are required for their genesis.489



4. Ice sheet emplacement and removal produces stresses sufficient to trigger mud490

volcanism491

On Earth, mud volcanism is usually triggered by compressional tectonism or rapid492

sedimentation [Kopf, 2002]. Mars has undergone only minor contractional strain in the493

Amazonian [Okubo & Schultz, 2006], making compressional tectonism an unlikely trigger.494

Although post-Noachian catastrophic outflows from the southern highlands would have495

caused rapid sedimentation, it is not clear that their deposits extend as far as the Borealis496

back-basin, and catastrophic outflow activity had sharply declined by the Amazonian,497

when the Scandia Tholi formed. Although rapid sedimentation from the southern high-498

lands during the Noachian could have generated overpressure at depth, we propose that499

mud volcanism was triggered later through ice sheet emplacement or removal.500

When pore fluids in a sedimentary deposit cannot be expelled fast enough in response501

to further loading, the pressure of the pore fluids increases - a condition known as dise-502

quilibrium compaction. This is one of the most effective ways of generating overpressure503

in basins [Swarbrick et al., 2002], alongside hydrocarbon generation at depth. Shale per-504

meability k in these basins on Earth ranges from 10−22 – 10−18 m2. If sediments were505

rapidly deposited in the Borealis back-basin, the basin fill has a permeability similar to506

shale permeability on Earth, and the annually-averaged surface temperature on Mars has507

been below freezing since deposition, then overpressure generated by disequilibrium com-508

paction can only be relieved by fracturing the cryosphere or lateral diffusion. The lateral509

diffusion timescale τ = L2/k over the radius L of the Borealis back-basin (∼ 500 km)510

is greater than the duration of the Amazonian, provided k ≤ 10−6 m2. The advance of511

the freezing front as the lithosphere cools could also contribute to overpressure [Wang et512



al., 2006]. Changes in ice loading near the North Pole, driven by chaotic shifts in mean513

obliquity (e.g., Putzig et al. [in press]), could trigger mud volcanism either during ice514

sheet emplacement or removal. Emplacement of an ice sheet in the area of the moated515

domes would cause differential stress (10.4 MPa for a 3km-thick ice sheet) within the over-516

pressured aquifer, favoring mud volcanism just beyond the ice margin. Through thermal517

insulation, it would also gradually thin the zone of frozen ground beneath it. If subsequent518

deglaciation outpaced freezing-front deepening, then the overpressured aquifer would be519

left close to the surface, favoring hydrofracturing of the permafrost during a transient520

period when it is unusually thin.521

This proposal has several attractive features: there are multiple lines of evidence that522

the ice-rich polar plateau once extended further south in the moated domes sector. These523

include sinuous ridges which we interpret as moraines or eskers, the Basal Unit [Fishbaugh524

& Head, 2001], the asymmetry of the present day Planum Boreum [Kite et al., 2009], and525

the presence of ice outliers that may mark the former outer edge of the ice-rich polar526

plateau [Zuber et al., 1998].527

Assuming that the eruption initiates by hydrofracturing, we would expect both the528

initiating dyke, and any resultant extrusion, to be elongated perpendicular to least prin-529

cipal strain. There is a clear preference for domes to be elongated E-W (Figure 3), and530

chains defined by alignments of nearby domes are also oriented E-W. This implies that the531

least principal stress was oriented N-S. A simple explanation is that the lithosphere was532

subjected to regionally extensive N-S stress associated with the Alba Patera dyke swarm533

[Tanaka, 2006]. However, there are two problems with this explanation. The first is that534

the preferred orientations do not line up with the continuation of mapped Alba Patera535



dykes. The second is that there is more scatter in the distribution of orientations than ex-536

pected for a dyke swarm at great distance from its center. Ice sheet emplacement/removal,537

which is often lobate or irregular, can explain both observations. Therefore, we interpret538

the dome elongation data as reflecting a stress field produced by expansion/contraction539

of an ice sheet. Flexural stress due to an ice sheet will depend on the exact distribution540

of ice, and the elastic thickness of the lithosphere. However, for a concentrated (line or541

point) load, there is in general an upper crust zone of extension 1 - 3 α from the load542

(where α is the flexural parameter; Watson [2001]), which would favor hydrofracturing.543

Alternatively, and similar to chaos terrain formation, the required overpressures could be544

generated by a magmatic intrusion at depth [McKenzie & Nimmo, 1999] driving explosive545

hydrothermal activity [Skinner & Mazzini, 2009]. If the gypsum adjacent to the Scandia546

Tholi is sourced from the Scandia Tholi, then hydrothermal activity could also account for547

the formation of the gypsum [Skinner & Mazzini, 2009]. Whatever the ultimate trigger,548

the multi-kilometer scale and obliteration of craters in the moated domes region suggests a549

more violent process than that usually associated with mud volcanism. We speculate that550

on a planet with a thick cryosphere, greater overpressures can accumulate before reservoir551

drainage than for similar tectonic settings on Earth. Therefore, when mud volcanism552

occurs, it has a more explosive character. Preliminary simulations of sediment eruptions553

at high overpressures are reported by Gisler [2009].554

We conclude that compressional tectonism is not required for Martian mud volcanism.555

Summary556

Key evidence in support of the mud volcanism hypothesis comes from (1) a comparison557

of morphological attributes of the large, moated domes to those of mud volcanism on558



Earth. We also show (2) that the Scandia Tholi region has undergone insolation-dependent559

modification, probably indicating a volatile-rich composition, (3) that the mud volcano560

hypothesis is supported by the geological context, and (4) the existence of at least one561

physical model for the triggering of mud volcanism in the Scandia region.562

The data are insufficient to show that the Scandia Tholi must be the result of mud563

volcanism. Rather, we assess mud volcanism to be the least unsatisfactory of a range of564

origin scenarios.565

5. Discussion

5.1. Relationship to other candidate Mars mud volcanoes

Mud volcano candidates have been reported from other sites on Mars [Farrand et al.,566

2005; Skinner & Tanaka, 2007; Kangi, 2007; Skinner & Mazzini, 2009]. All reported mud567

volcano fields are within the northern lowlands, and almost all are close to the Martian568

dichotomy boundary. However, the other mud volcano candidates have diameters of < 5569

km (compared to 30 - 50 km for the moated domes) and are (usually) morphologically570

simpler. These differences can be interpreted to mean (1) that the Scandia Tholi are571

not the result of mud volcanism, (2) that the triggering process differed between the572

Scandia Tholi and the features at the dichotomy boundary, (3) that the Scandia Tholi573

are more deeply eroded than the dichotomy boundary dome fields, (4) that the source574

layer for the Scandia Tholi was deeper and/or thicker, or (5) that the Scandia Tholi575

include a component of stratified sediment uplift [Deville et al., 2006], or mud diapirism576

[Fernandez-Puga et al., 2007], superposed by mud volcanoes. It is possible that cryosphere577

thickness sets the scale of fluid outflow events; for obliquities < 54◦, equilibrium cryosphere578

thickness is greater at the poles than at the dichotomy boundary. We speculate that579



the differences between the Scandia Tholi and the dichotomy boundary mud volcano580

candidates may be related to ice sheet triggering for the Scandia Tholi, versus triggering581

by rapid sedimentation, volcanic destabiliation or tectonic shortening for the dichotomy582

boundary mud volcanoes [Skinner & Mazzini, 2009].583

On Earth, mud volcanism is most often driven by the production of methane that is584

produced by the decay of buried organic matter [Kopf, 2002]. Methane plumes have585

been detected on Mars [Mumma et al., 2009], although their reported location is not586

correlated with the reported mud volcanoes [Skinner & Mazzini, 2009]. The poor state587

of preservation of the Scandia Tholi indicates that they are very unlikely to be active.588

Nonetheless, mud volcanism on Mars would sample deeper levels in the northern lowlands589

basin fill, probably deposited at a time when rates of physical erosion and formation of590

aqueous minerals were higher than at present [Oehler & Allen, 2009].591

5.2. Gypsum and the Scandia Tholi

The Olympia Dunefield, adjacent to the large, moated domes, is gypsum-rich [Langevin592

et al., 2005], and hydration signatures extend across the dark polar dune belt [Horgan et593

al., 2009]. This detection is startling, because it does not conform to the emerging con-594

sensus on Mars’ aqueous history – that global conditions may have supported extensive595

aqueous alteration prior to > 3.5 Ga, but not since [Mustard et al., 2008]. Researchers596

responding to the discovery of gypsum have either (1) invoked jokulhlaups (glacial out-597

burst floods) discharging from beneath the present-day Planum Boreum [Fishbaugh et al.,598

2007], (2) suggested that sulfate minerals can form within the present-day, cold-based ice599

sheet [Niles & Michalski, 2009], or (3) hypothesized transport of gypsum from depth by600

mud volcanoes [Tanaka, 2006]. This work is important, because if gypsum can form in the601



present-day Martian environment, it breaks the link between sulfate deposition and past602

environments conducive to life [Catling et al., 2006]. Our results are consistent with the603

mud volcanism hypothesis. However, the closest of the Scandia Tholi is ∼380 km from604

the area of maximum gypsum concentration, and the center of the domefield is ∼500 km605

from the area of maximum gypsum concentration. As we shall now discuss, it is not clear606

whether this is close enough to source the dunes.607

The tight link between the gypsum signature and the girdle of dark dunes around608

Planum Boreum suggests, but does not require, that the gypsum is present in grains that609

are able to saltate. Because there is a minimum for grain mobilization by wind on Mars610

at ∼ 102 µm [Greeley & Iverson, 1985], and because Martian winds are only marginally611

able to move sand [Sullivan et al., 2008], it is possible for gypsum to be broken down612

to sizes too fine to be easily lofted by the wind, and which would remain in suspension613

once airborne. Assuming this to be the case, an important, unconstrained parameter614

is the length scale over which saltating gypsum sand is comminuted to fine grains that615

are unable to saltate. That length scale must be less for gypsum than for quartz sand,616

since gypsum is softer than quartz. This would define the ‘search radius’ for the gypsum617

source region. At White Sands National Monument, New Mexico, gypsum grain size618

data show rapid comminution (e-folding reduction length scale = 6.2 km, including some619

unpublished data supplied by R.P. Langford, via e-mail) with increasing distance from620

the shoreline of Pleistocene Lake Otero, whose deflation sources the dunes [Langford,621

2003]. However, it is not clear how to relate these data to present-day Mars, because the622

rapid reduction in grain-size at White Sands may be a transient associated with increasing623

roundness, and because gypsum grains partly recover from saltational abrasion through624



reprecipitation in wet interdunes (R.P. Langford, via e-mail). Wind-tunnel experiments625

on sulfate comminution at Mars atmospheric pressure have not been carried out. In the626

absence of these data, we cannot say whether the communition length scale for gypsum627

breakdown is:– tens of km, in which case the gypsum must have formed from precipitation628

out of solution very close to or within the dunefield [Fishbaugh et al., 2007]; hundreds of629

km, in which case the veneer atop the polar plateau [Horgan et al., 2009], or the Scandia630

Tholi (Tanaka [2006]; this paper) are both possible sources; or thousands of km, in which631

case the gypsum could have been reworked from ancient crust in the southern hemisphere.632

We conclude that wind tunnel experiments on sulfate comminution at Mars atmospheric633

pressure are highly desirable, because they could set tight constraints on the origin of634

hydrated minerals near the North Pole of Mars.635

5.3. Depth to source reservoir

Isostatic considerations indicate that, if the domes formed subaerially, then their source636

reservoir is unlikely to be the Vastitas Borealis Formation, because the Vastitas Borealis637

Formation is too thin [Kreslavsky & Head, 2002] to engender 400+ m of extrusional relief638

(Table 1). We assume that the maximum height of mud expulsion is set by isostatic639

balance:640

dsρb = (r + ds)ρd (1)

where ds is the depth to the source reservoir, ρb is the density of overburden, r is dome641

relief, and ρd is the density of dome-forming material. If ρd = 1.8 – 2.3 g cm−3 as expected642

for mud, and ρb = 3.0 g cm−3 (basalt), our measurements of r (Table 1) require ds 540-643



1370 m. This is a conservative estimate of source depth because the overburden density644

is likely to be less. Even if the domes are composed entirely of ice (ρd = 0.9 g cm−3), and645

the overburden is basalt sand with ice-filled pores, ds is still large (290m). This tends to646

support the proposal that ancient sediments are present at depth beneath the Northern647

Plains [Fairen et al., 2003].648

Alternatively, if the dome-forming material is silicate magma with a density of 2.65 –649

2.9 g cm−3, our results indicate a depth to the source layer for an overburden with the650

density of basalt of ∼3.5 km. This might represent a magma chamber at or near the top651

of the buried crater-bearing basement [Frey et al., 2002].652

5.4. Further tests of the mud volcanism hypothesis

Additional high-resolution data could help to constrain the origin of the domes. A653

HiRISE image in the area where CTX shows layers would help to determine whether the654

blocky appearance of some of the layers is due to boulders in a fine-grained matrix (which655

would be consistent with mud volcanism) or meter-scale cooling joints (which would not656

be consistent with mud volcanism). CRISM FRTs of young postglacial craters in the657

domes region might expose material from below the mantling layer. If mud volcanism or658

explosive hydrothermal activity is responsible for the moated domes, hydrated minerals659

would be expected.660

6. Conclusions

1. The Scandia Tholi are characterized by moats, internal collapse features, central661

peaks, nested rises and slumps, jumbled interiors with multiple pits and peaks, sub-662

dued, internally-consistent relief, elliptical margins showing a preferred E-W orientation,663



and susceptibility to sublimation or aeolian erosion. HiRISE shows their surfaces to be664

boulder-rich. A possible exposure of layers in the margin of one dome shows some boulder-665

rich layers.666

2. Azimuthally-averaged radial profiles confirm the existence of moats and indicate667

averaged slopes of ∼ 1 ◦.668

3. Slope-aspect relations show an excess of S-facing aspects (equivalently, a deficit of669

N-facing aspects) among steep slopes in the moated domes region. This indicates climatic670

modification of the moated domes region.671

4. OMEGA spectral analysis, supported by CRISM, shows the presence of pyroxene672

at the optical surface of the domes. Hydrated minerals, if present, are at or below the673

noise level. It is not clear what geological unit is the source of the detected minerals. The674

existence of a mantle suggests recent glaciation, in which case the minerals could form675

part of a lag deposit associated with windblown dust and need not be locally derived.676

5. The elevation of the moated dome peaks above the background plains has a narrow677

range, (432 ± 92)m (n = 29). If the height of the moated domes is related to the depth678

to the source reservoir, isostatic balance requires that the depth sampled by the moated679

domes, D ≥ 290 m. This rules out the Vastitas Borealis Formation as the source of the680

dome-forming materials.681

6. Because the length scale for comminution of saltating sulfate sand is not well con-682

strained, we cannot say whether the large moated domes are a possible source of the polar683

gypsum.684

7. Our assessment is that mud volcanism is a plausible scenario for the origin of the685

large moated domes. Some mud volcanic systems on Earth show all of the salient features686



of the large moated domes. Ice sheet emplacement or removal is a plausible trigger for687

mud volcanism on Mars. The geological history of the Scandia region is consistent with688

mud volcanism [Tanaka et al., 2008].689

8. However, the moated domes are, on average, 4 times larger than the largest mud vol-690

cano known on Earth, and the overall appearance of the moated domes is more disrupted691

than Earth mud volcanoes, suggesting explosive rather than purely effusive processes.692

These differences may indicate that the moated domes formed by explosive magmatic or693

hydrothermal eruptions, not mud volcanism; or they may include stratified sedimentary694

uplifts as well as mud volcanoes; or mud volcanism on a planet with a thick cryosphere695

may have a more explosive character [Gisler, 2009].696

9. Ongoing CRISM and HiRISE imaging can further test the mud volcanism hypothesis.697

Mud volcanism, if confirmed, would provide near-surface access to fine-grained sediments698

probably deposited early in Mars’ history [Oehler & Allen, 2009].699

Appendix A: Analysis of MOLA and OMEGA data

A1. Radial profiles of moated domes

Our objective was to collapse a three-dimensional dataset (elevation as a function of700

latitude and longitude) into the two dimensions of elevation and radius from dome center.701

The moated domes are 100 - 200 times larger than the pixels making up the gridded702

MOLA data, so it is reasonable to use gridded data. Constructing radial profiles requires703

a) a reliable definition of the dome center; b) a definition of the dome edge; c) given704

that many domes show gross deviations from radial symmetry, a method of normalizing705

to varying distances between centre and edge. Ideally, dome edges and centers would be706

defined automatically, but automatic fitting of ellipses to dome margins produced poor707



(scattered, high inter-profile s.d.) results. A revised method uses a manually defined,708

irregular dome edge/mask, and gives much improved results. For each dome, the mask is709

drawn ∼ 1 moat width from the outer edge of the moat where a moat is present (Figure710

13a), or a constant distance (∼ 2 moat widths) from the dome margin where a moat is711

not present. The simplest approach is to take the center to be the centroid of the mask.712

A better alternative is to optimize the center position, minimizing scatter (Figure 13b).713

We find center coordinates (i, j) such that a penalty function A is minimized714

A = f(i, j) =∑Rn

Rnσ(Rn) (A1)

where Rn is normalized radius from the center, and σ(Rn) is the standard deviation of715

profile elevation over 200 profiles with equal angular spacing (Figure 13c). The factor of716

Rn within the sum accounts for increase in spacing between profiles, which is proportional717

to Rn. This procedure yields well-defined local minima in A near the visually-picked dome718

center, which are often significantly offset from the centroid of the mask. All domes show719

global minima for centers near the mask edge, because many of the profiles cross only a720

single DEM element here and so have zero standard deviation. Therefore, we scanned721

over a restricted range of (i, j) close to the visually-picked dome center. An alternative722

center-finding criterion is to minimize deviation from circularity (the range of centre –723

mask edge distances). This gives similar results to finding the areal centroid of the mask.724

Once the dome edge and dome center have been defined, n radial profiles are drawn725

out from the dome center, each with x points. We used n = 180 and x = 200. To keep x726

constant, the point seperation is scaled to the distance between the center and the mask727

edge. A second-order global polynomial fit to the regional elevation trend is subtracted728



from all DEMs before processing. Results are shown in Figures 5 and 6 and discussed in729

Section 3.2.730

A2. Analysis of OMEGA data

OMEGA Method 1: Band depths (BDs). BDs were calculated at 1.8 µm, sensitive to low731

calcium pyroxenes (LCP); 2.2 µm, sensitive to high calcium pyroxenes (HCP); and 1.5 µm732

BD, to mask out ice pixels, particularly important for analyses in those northern regions.733

In addition, BD at 1.45 and 1.94 µm was used to map hydrated minerals. Combined, for734

example, with 1.7 µm and 2.2 µm BD, the hydration bands can provide solid evidence for735

gypsum [Langevin et al., 2005]. In several cases, we calculated spectral ratios to enhance736

the spectral signatures. The JGR-Planets Special Section on OMEGA/Mars Express737

[Bibring, 2007] provides more details on these methods.738

OMEGA Method 2: Improved linear unmixing model. One of the strengths of this model739

[Combe, 2005; Combe et al., 2008] is its ability to eliminate unphysical negative component740

mixing coefficients (in case of inexact spectral library for the area observed or non-linear741

intimate mixtures implied by multiple scattering) by iterative processes. Another strength742

is the use of artificial photometric spectral endmembers to take into account, at first order,743

photometric or grain size effects : a flat spectrum to consider pure albedo differences and744

positive/negative slope spectra to account for continuum slope variations. Its important745

to note here that this model gives the contributing percentage of each library mineral746

to the observed spectrum, but this cannot be associated with absolute compositional747

percentages of minerals. Instead, it gives a first idea of the general minerals or mineral748

families which may be present in a given region and of their relative concentrations. The749

spectral library included pyroxenes (hypersthene and diopside), olivine (forsterite and750



fayalite), phyllosilicates (kaolinite, smectite, muscovite, montmorillonite, illite, nontronite751

and chlorite), serpentines (chrysotile and lizardite), sulfates (alunite, gypsum, jarosite,752

kieserite and epsomite), carbonates (calcite, dolomite and siderite), iron oxides (hematite,753

goethite, ferrihydrite and maghemite) and water ice (frost). The image fraction values754

obtained (raw coefficients given by the model) were normalised to the 1.1 µm albedo, in755

order to best represent the spectral contribution of each endmember. We examined both756

normalized image fractions obtained by the model and correlations between endmember757

maps and (simple or ratioed) BD maps. Root mean square (RMS) image fractions were758

used to separate the real endmember contributions (low RMS) from the artificial ones759

(high RMS). We also extracted mean spectra of domes 2-9, 11-13, and 19, and tried to760

model them with various linear unmixings of different numbers of spectral poles selected761

in the library described above to best fit these data. This permitted us to look for spectral762

variations both between different selected domes and between the domes and the plains.763

In this case, the unmixing analysis did not provide significant additional interesting results764

compared to band depth analysis.765

Acknowledgments. We thank M.A. Kreslavsky for his valuable advice on MOLA data766

analysis, and M. Manga for proposing ice sheet removal as a trigger for mud volcanism.767

We are grateful to Eric Deville, who supplied Figure 12. J.A. Skinner provided advice768

on geologic context, and R.P. Langford shared unpublished gypsum grain size analyses.769

Much of this work was carried out while E.S.K. was at the Department of Earth Sciences,770

Cambridge University, and he thanks the Department librarians, Libby Tilley and Sarah771

Humbert, for their kindness.772



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Table 1. Domes. n.m. = not measured993

994

Dome ID Summit coordinates Area (km2) Volume (km3) ∆z (m) Moat? Central peak? Peak-marginal pit? Concentric rings?

1 74.8N 167.1W 4.1 x 103 2.7 x 101 390 - Y Y Y

2 74.2N 163.6W 1.9 x 103 9.3 x 101 495 Y Y Y Y

3 73.6N 161.0W 1.8 x 103 1.1 x 102 395 - ? - -

4 73.9N 159.4W 1.6 x 103 1.1 x 102 420 - Y Y -

5 73.5N 157.8W 1.9 x 103 1.2 x 102 449 Y Y - Y

6 74.1N 157.4W 1.9 x 103 1.6 x 102 595 ? Y Y Y

7 75.4N 156.0W 1.3 x 103 1.4 x 102 511 ? Y Y ?

8 75.9N 155.0W 1.8 x 103 3.1 x 102 501 - ? - Y

9 76.1N 153.7W 2.4 x 103 3.2 x 102 581 - Y Y Y

10 75.6N 143.1W 5.8 x 103 4.6 x 102 604 Y - - Y

11 74.7N 151.2W 3.6 x 103 2.8 x 102 554 Y Y - Y

12 73.6N 152.2W 2.2 x 103 1.8 x 102 376 Y Y - Y

13 72.8N 151.9W 2.6 x 103 1.7 x 102 432 Y - - -

14 72.6N 150.9W 9.9 x 102 -1.3 x 101 335 Y - - Y

15 73.2N 149.0W 3.0 x 103 1.4 x 102 420 Y Y Y Y

16 72.4N 149.1W 1.2 x 103 1.9 x 101 394 Y Y - Y

17 73.3N 144.0W n.m. n.m. 404 Y - - -

18 75.0N 141.3W 2.5 x 103 2.4 x 102 523 Y - - -

19 71.1N 146.9W 6.2 x 103 1.1 x 101 203 Y Y - -

20 73.8N 169.1W 4.3 x 103 3.6 x 102 516 Y Y Y Y

21 72.1N 172.0W 1.9 x 103 5.2 x 101 407 Y ? - Y

22 79.1N 178.5W 1.5 x 103 3.4 x 101 342 - Y Y ?

23 78.7N 157.3W 1.1 x 103 8.5 x 101 374 Y Y Y Y

24 79.4N 154.3W 8.4 x 102 1.0 x 102 424 ? Y - -

25 79.1N 162.6W 7.7 x 102 6.9 x 101 304 ? - - ?

26 78.0N 176.6W 7.2 x 102 9.8 x 101 418 Y Y ? Y

27 79.2N 174.9W n.m. n.m. 387 - ? ? ?

28 76.0N 172.7W 5.4 x 102 6.9 x 100 333 A A A A

29 75.6N 167.0W 9.3 x 102 6.1 x 101 435 A A A A

Total Σ = 5.4 x 104 Σ = 3.6 x 103 432 16(+4?)/29 17(+4?)/29 10(+2?)/17 16(+4?)/29

s.d. 92 n.a. n.a. n.a. n.a.

995



Figure 1. Regional context of the Scandia Tholi, all of which lie within the inner black

box shown in more detail in Figure 2. North polar stereographic projection. Elevation scale

saturates at -4000 m (red) and - 5000 m (white). The ‘Borealis back-basin’ is the lozenge-shaped

depression centered near 200E. White cross labelled PHX corresponds to the Phoenix landing site.

The orange line bounds the gypsum region of Fishbaugh et al. [2007]. It is almost coextensive

with the region of 10% or greater band depth at 1.9 µm mapped by Horgan et al. [2009]. High

gypsum concentrations are only found within the Olympia Undae erg.

Figure 2. The domefield located on Figure 1. Inner black box corresponds to region shown

in more detail in Figure 3. Topography has been detrended by subtracting a 2nd-order global

polynomial interpolation from the region shown. 256 ppd MOLA gridded data. Numbers cor-

respond to the domes discussed in the text and listed in Table 1. Domes 23-29 encircle a large

ridge-bounded depression (one of the Scandia Cavi).

Figure 3. Directions of elongation of all 29 domes in catalogue (blue), and of the subset of

domes with aspect ratios (long axis / short axis) > 1.2 (21 domes, red). Domes are elliptical,

and preferentially oriented E-W. However, some domes are oriented WSW-ESE. The preferred

direction of dome elongation does not correspond to the continuation of the strike of dykes

radiating from Alba Patera as mapped by Tanaka [2005].

a)

b)



c)

Figure 4. High-resolution images of the domes. a) Context for high-resolution images. Close-

up of Domes 12 and 13 (see Figure 2). Image is 100 km across. Background is MOLA shaded relief

over color topography, with color scale running from -4502m (red) to -4899m (purple). Grayscale

is mosaic of CTX images P22 9515 2535, P22 9581 2531, and P16 007142 2529. Illumination is

from the top, and arrowed features are discussed in the text. b) Mosaic of CTX frames showing

possible layers (black arrows) exposed in E flank of Dome 13. Image is 16.1 km across. Moat

is at lower left, dome interior at upper right. c) HiRISE subframe (PSP 006931 2530) showing

patterned ground, boulder clusters, and frost in a dome interior. Image is 2.1 km across.

Figure 5. Radially-averaged profiles of the large moated domes. Solid lines are the mean of

radially-averaged profiles. Asterisks joined by gray lines to profiles correspond to the highest

point anywhere within the dome, at their correct normalized radial distances. Similarly, open

circles joined by gray lines to profiles correspond to the lowest point anywhere in the dome.

Thick lines highlight moats where these are visible; domes 3 and 4 appear to have moats in the

averaged profiles, but not in the raw data. Profiles are offset from each other by 200 m, such that

2100 m should be subtracted from the highest point on the radially-averaged profile of Domes 1,

11, and 21 to give the true elevation above the background plains.

Figure 6. Azimuthally averaged (median) profiles of domes. The centers of the domes are

on the left, their edges at right. Each horizontal strip corresponds to the azimuthally-averaged

profile of one dome. Warm colors correspond to high elevations, cool colors to low. Elevations

are expressed as a percentage of the total elevation range on each profile. Asterisks correspond

to peak elevation on the averaged profile, and circles to the lowest elevation on the averaged

profiles. Vertical lines are at intervals of 5 % of the total elevation range on each profile.



Figure 7. Regions-of-interest for slope-aspect analysis (Section 3.2). Gray flat-toned areas are

ice outliers mapped by Tanaka [2005]. White cross corresponds to the Phoenix landing site. The

orange line surrounds the gypsum region of Fishbaugh et al. [2007]. Background is MOLA 256

ppd shaded relief.

Figure 8. a) Results of slope-aspect analysis for the test region. Blue lines correspond to ±1

standard deviation envelope, calculated for 1◦ bins. Moderately steep slopes are significantly more

likely to be S-facing. b) Results of slope-aspect analysis for the control region. Blue is standard

deviation calculated for 1◦ bins. There is no statistically significant correlation between slope and

S-facing versus N-facing aspect. More detailed analysis (not shown) shows an overabundance of

slopes facing ∼120◦ or ∼300◦ in the control region, consistent with the regional tectonic trend

(wrinkle ridges).

Figure 9. Results of PEDR analysis. Blue symbols correspond to the test region, red symbols

to the control region, and black symbols to the conservatively-defined test region (excluding

ridge-bounded depressions). The solid lines correspond to the number of N-facing slopes, and

the dot symbols correspond to the number of S-facing slopes. All plotted MOLA track segments

have length < 450m.



Figure 10. Summary of spectral analysis. OMEGA cube 0917-2: a) Regions selected for mean

spectra extraction of the domes, identified by colors, draped over a grayscale MOLA topographic

map (elevation range -5979 m, black, to -3546 m, white). The brown patch near 67◦ N is a control

region. (b) Corresponding mean spectra (colors link each spectrum to a dome), which are very

similar. The main differences can be explained by albedo variations. The spectral ratio of dome

13 to the background plains (brown patch in panel a) is also presented, showing no diagnostic

bands of hydrated minerals above the noise level. CRISM cube 000BACF: (c) 1.1 µm albedo,

cube location is box outlined in white on the OMEGA cube (a)). (d) Spectral ratio of an outcrop

(red) to a reference region (brown patch in panel c). CRISM’s higher resolution does not show

any diagnostic spectral features on the dome, confirming that the spectral homogeneity extends

to small spatial scale.

Figure 11. Sidescan sonar image of TREDMAR mud volcano, the Eastern Mediterranean,

TTR-3 (1993) data (slightly modified from Limonov et al. [1994]). A clearly defined collapse moat

frames the volcano. Note: multiple pinnacles and heights characterize the volcano similarly to

the multiple peaks and rugged surface seen on Mars images. Arrows 1 and 3 indicate the moat,

while arrows 2 and 4 indicate the pinnacles and displaced blocks.

Figure 12. Multibeam bathymetry from the Barbados accretionary prism, Earth. Mud

volcanoes are shown in pink, and stratified sedimentary uplifts are outlined with a white dotted

line. Yellow is shallow and blue is deep, and the stratified sedimentary uplifts have a relief of up

to 150m, measured from the surrounding seafloor. Water depth is ∼ 2.5 km. This is a slightly

modified and corrected version of Figure 5 in Deville et al. [2006]; we are grateful to Eric Deville

(Institut Francais du Petrole) for supplying this figure.

a)

b)



Table 2. Truth table.996

997

Observation Pingos /kames /kettles

Effusive‘pancake’domes

Tuyas Explosivesilicatevolcanism

Mudvolcanism

Moats possible flexural yes, thermalerosion

encirclingresurgentdomes

yes,subsidence

Internal collapsefeatures

yes no no unexpected yes

Central peaks sometimes no yes yes yesNested rises andslumps

sometimes no no rare yes

Jumbled interiorw/ multiple pitsand peaks

yes no no maybe yes

Subdued, consis-tent relief

maybe no no maybe yes

Elliptical shape w/preferred orienta-tion

yes yes maybe yes yes

Susceptible to sub-limation or aeolianerosion

yes not very not very yes, ifunweldedtuff

yes

Volatile content yes no no no yesConsistent withpresent day Te?

yes no no no yes

Formation condi-tions restricted toN Plains

requires pastice

no requires thickice

no yes

Only mafic miner-als in spectrum

maybe yes yes yes hydratedmineralswould beexpected

Boulder-rich layers no no yes yes possible

998

Te = elastic thickness of lithosphere.999



Table 3. OMEGA and CRISM data used.1000

1001

Instrument Image ID Date Ls (◦) Resolution (km/px) Image width (pixels)OMEGA 0917 2 6 Oct 2004 97.2 3.860 128

0928 1 9 Oct 2004 98.6 1.946 640991 0 27 Oct 2004 106.5 3.454 641007 1 31 Oct 2004 109.5 5.700 128

CRISM 000093F9 7 Jan 2008 14.0 ≥ 0.036 360x32000009B30 29 Jan 2008 24.6 ≥ 0.036 360x3200000BACF 21 Jul 2009 101.5 ≥ 0.018 540x640

1002



c)

Figure 13. To show how azimuthally-averaged profiles are generated for moated domes. A

mask is picked manually around the dome (a), and the center of extracted topography is found

by minimizing the weighted standard deviation of the resulting profiles (b). The long axis and

short axis of the dome are defined using the radial profiles. Finally the mean (solid line) and

median (dotted line) elevations are plotted, together with the 1 standard deviation envelope

(dash-dotted lines) (c).


characterization of mars’ scandia tholi...

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