Icarus 285 (2017) 185–194

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Icarus

journal homepage: www.elsevier.com/locate/icarus

Candidate volcanic and impact-induced ice depressions on Mars

Joseph S. Levy

a , ∗, Timothy A. Goudge

b , James W. Head

c , Caleb I. Fassett d , 1

a University of Texas Institute for Geophysics, Austin, TX, 78758, USA b University of Texas Jackson School of Geosciences, Austin, TX, 78712, USA c Brown University Department of Earth, Environmental and Planetary Sciences, Providence, RI, 02912 USA d Mount Holyoke College, South Hadley, MA, 01075, USA

a r t i c l e i n f o

Article history:

Received 20 May 2016

Revised 17 October 2016

Accepted 20 October 2016

Available online 2 November 2016

Keywords:

Mars

Mars surface

Impact processes

Volcanism

a b s t r a c t

We present an analysis of two concentrically-fractured depressions on Mars, one in northern Hellas and

the second in Galaxias Fossae. Volumetric measurements indicate that ∼2.4 km

3 and ∼0.2 km

3 of mate-

rial was removed in order to form the North Hellas and Galaxias depressions. The removed material is

inferred to be predominantly water ice. Calorimetric estimates suggest that up to ∼10 3 –10 5 m

3 of magma

would have been required to melt/sublimate such a volume of ice under an ice/magma interaction sce-

nario. This process would lead to subsidence and cracking of the surface, which could produce the ob-

served concentric fracture (crevasse-like) morphology. While the Galaxias Fossae landform morphology is

consistent with an impact origin, the large volume of removed material in North Hellas is less consistent

with an impact origin and is interpreted to have resulted from volcanic melting of ice. The possibility of

liquid water formation during or subsequent to volcanism or an impact could generate locally-enhanced

habitable conditions, making these features tantalizing geological and astrobiological exploration targets.

© 2016 Elsevier Inc. All rights reserved.

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. Introduction

Magma-ice interactions are thought to be a major component

f martian geological and climate history. Several examples of

agma-ice interactions on Mars have been suggested, including

agmatic fracturing of the cryosphere, subglacial volcanism pro-

ucing dikes and moberg-like ridges, synglacial volcanic deposits,

upraglacial volcanic deposits, pseudo-crater formation, volcano-

ank melt channels, tindar-like features, and possible ice depres-

ions (sometimes called ice cauldrons) ( Payne and Farmer, 2001;

ead and Wilson, 2002; Fagents et al., 2002; Carr and Head, 2003;

hatan, 2003; Fassett and Head, 2006; 2007; Head and Wilson,

0 07; Keszthelyi et al., 20 09; Pedersen et al., 2010; Cassanelli and

ead, 2016; Mouginis-Mark and Wilson, 2016 ).

On Earth, depressions resulting from volcanic melting of glacier

ce can lead to the formation of concentric crevasses at the glacier

urface due to brittle failure of ice resulting from downsag flexure

nto an evacuated meltwater chamber ( Gudmundsson et al. 1997 ,

004 ). In glaciovolcanic settings, entrainment of ice blocks by la-

ars or in tephra can lead to the formation of meter- to decameter-

cale collapse pits (kettles) in the lahar/tephra deposits. These

orm when the ice blocks melt and have similar concentric-ring

∗ Corresponding author.

E-mail address: joe.levy@utexas.edu (J.S. Levy). 1 Now at NASA Marshall Space Flight Center, Huntsville, Alabama, 35812, USA.

b

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ttp://dx.doi.org/10.1016/j.icarus.2016.10.021

019-1035/© 2016 Elsevier Inc. All rights reserved.

orphologies to glacial and rock-supported calderas, however, they

re generally limited in size by entrained ice block dimensions

typically no more than a few meters) and preservability, as they

orm in soft sediments ( Branney and Gilbert, 1995, Smellie et al.,

016 ). Concentric fractures can also form around supraglacial melt

akes on glaciers with surface melt during rapid drainage into sub-

lacial channels ( Das et al., 2008; McMillan et al., 2015 ). Complexi-

ies of meltwater routing during chaotic subglacial drainage events

an result in concentrically-fractured depression formation over an

rea defined not by water infiltration, but by the location of tem-

orary subglacial reservoirs—a process that can be pinpointed via

tereo satellite image DEM generation ( Willis et al., 2015 ).

Outside of volcanic areas, other mechanisms can also remove

ubsurface ice, leading to brittle failure and surface subsidence, in-

luding melting via contact with surface water (rivers, lakes, or

ceans) to form thermokarst depressions ( French, 2007 ), or, in

rinciple, impacts into ice-rich materials ( Kawakami et al., 1983 ).

n addition, it has been reported that concentric fracture features

ay form on Earth through salt removal processes in marine set-

ings ( Underhill, 2009 ), although impacts may also be responsi-

le for forming such structures in marine sediments ( Stewart and

llen, 2002 ). Here, we explore the impact and volcanic endmem-

er processes for identifying the cause of concentrically fractured

epressions on Mars ( Fig. 1 ).

Previous studies provide a set of criteria for distinguishing

oncentrically-fractured depressions formed by impacts from those

186 J.S. Levy et al. / Icarus 285 (2017) 185–194

Fig. 1. Location map showing sites studied in this investigation: NH is the North

Hellas depression, GF is the Galaxias Fossae depression, PM indicates depressions

on Pavonis Mons (Fig. 2), and S1-A, S1-B, S1-C, and S1-D are depressions shown in

the supplementary materials.

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formed by volcanic processes ( Fig. 2 ). Key morphological indica-

tors associated with impacts into ice-rich substrates include: 1)

“pie-crust,” “ring-mold,” or “oyster shell” craters with ramparts or

ridges on the depression floor ( Mangold, 2003; Kress and Head,

2008 ), 2) a raised rim along the innermost concentric lineation,

3) decameter-scale roughness that decreases from the rim to a

distance of 1–2 rim diameters, associated with modification of

ice-rich substrates by ejecta ( Head and Weiss, 2014 ), 4) radial

lineations characterized by decameter-scale roughness, associated

with ejecta rays modifying the initial substrate ( Head and Weiss,

2014 ), 5) stippled textures on the depression interior associated

with compression resulting from viscous relaxation ( Scott, 1967 ),

6) approximately level terrain surrounding the depression ( Head

and Weiss, 2014 ), and 7) one or more depression-concentric frac-

tures that can be wide (extensional ridges) or narrow (flexural

cracking) ( Head and Weiss, 2014 ).

In contrast, depressions that form in terrestrial glaciers via

bottom-up volcanic melting are characterized by: 1) broadly slop-

ing circular or elongate depressions surrounding a central pit

( Gudmundsson, 2014 ), 2) concentric crevassing with near-vertical

walls ( Gudmundsson, 2014 ), 3) spatially-compact tephra deposits,

much of which remains in the depression ( Jude-Eton et al., 2012 ),

4) the presence of meltwater stream channels and/or depression-

Fig. 2. Ice-related depressions on Earth and Mars. (Left) An ice depression at the Bár ðarbu

1997). Data courtesy National Land Survey of Iceland. North to image top. (Right) An imp

2014 ). Portion of CTX image D02_028015_1751.

osted lake deposits ( Gudmundsson et al., 2004 ). More broadly,

olcanic glacial depressions are a subset of caldera structures char-

cterized by concentric fracturing, dilation of fractures due to

ownsag flexure, and the formation of inward-dipping rotational

locks that generate upwards-flaring crevasse margins ( Branney,

995 ).

In this manuscript, we present geomorphic measurements of

wo concentrically fractured depressions on Mars ( Figs. 3–5 )—one

reviously-noted feature in Galaxias Fossae (35 ̊N, 141 ̊E, Fig. 4 —

he examined feature is the largest depression in a field of sim-

lar depressions identified by Pedersen et al., 2010 ), and one iso-

ated depression north of the Hellas Basin (“North Hellas,” 28 ̊S,

3 ̊E, Fig. 3 ). The landforms are well-covered by HiRISE and CTX

tereo pairs, permitting detailed analysis of both the morphological

nd morphometric properties of the two depressions. We test two

andidate formation mechanisms for these features, volcanism and

mpact cratering, both of which can form round depressions with

omplex morphologies. We make calorimetric estimates of the en-

rgy required to form such features by melting or sublimation of

pure water-ice substrate and compare the cavity dimensions and

olumes to regional crater characteristics. Identification of charac-

eristics consistent with either impact into or volcanism beneath

mazonian ice-rich deposits would suggest that these depressions,

nd landforms similar to them, should be considered sites of geo-

ogical, glaciological, and astrobiological interest on Mars.

. Analysis of concentrically-fractured depressions

.1. DEM generation

Digital elevation models (DEMs) of each site were constructed

rom HiRISE and CTX stereo pairs using the open source NASA

mes Stereo Pipeline (ASP), which generates high-resolution DEMs

sing targeted stereo pair images and stereo photogrammetry al-

orithms ( Broxton and Edwards, 2008; Moratto et al., 2010 ; Beyer

t al., 2014; Shean et al., 2016 ). During processing, HiRISE DEMs

re co-registered with CTX DEMs, which have been tied directly

o Mars Orbiter Laser Altimeter (MOLA; Smith et al., 2001 ) point

hot topography using the ASP pc_align function ( Beyer et al.,

014; Shean et al., 2016 ). This correction reduces any errors in re-

ional slope by translating and rotating the input DEM in three-

imensional space to minimize the error between the input and

eference elevation point cloud.

nga volcano, Iceland. Composite air photo (N-8375 and N-8376 collected 12 August

act crater in the Pavonis Mons tropical mountain glacier deposit ( Head and Weiss,

J.S. Levy et al. / Icarus 285 (2017) 185–194 187

Fig. 3. The North Hellas depression. (A) Regional view; arrows denote channel locations that cross-cut both the crater fill and the concentric fracture system. CTX and HiRISE

image composite of ESP_028535_1515, G04_019634_1509, B18_016720_1532, D21_035418_1514, and D17_033783_1509. The depression is located in fill material within the

( Robbins and Hynek, 2012) crater #21-0 0 0294. (B) Location of the study site in northeast Hellas. Arrow and dot show location. Base map is MOLA shaded relief. (C) Close-up

view of the depression. (D) Multiple, closely-spaced fractures located west of the main depression (white arrows).

Fig. 4. The Galaxias depression. (A) Regional view; CTX and HiRISE image composite of PSP_005813_2150, B18_016533_2152, G20_026093_2145, and B16_016045_2150. The

depression constitutes the ( Robbins and Hynek, 2012) crater #07-003974. (B) Location of the study site in Galaxias Fosse. Arrow and dot show location. Base map is MOLA

shaded relief. (C) Close-up view of the depression.

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For the North Hellas site, CTX images G04_019634_1522 and

17_033783_1509 were used to generate an 18 m/pixel DEM, and

iRISE images ESP_028535_1515 and ESP_033783_1515 were used

o generate a 2 m/pixel DEM. Because the HiRISE DEM cover-

ge does not fully overlap the North Hellas depression, volume

easurements described below were made using the CTX DEM

nly. For the Galaxias site, CTX images B18_016533_2152 and

17_016467_2154 were used to generate a ∼18 m/pixel DEM, and

iRISE images PSP_005813_2150 and PSP_005879_2150 were used

o generate a 1 m/pixel DEM. RMS differences between MOLA point

hot elevations and CTX stereo DEM values at shot point centers

re 7.9 m for B18_016533_2152_B17_016467_2154 and 27.1 m

or G04_019634_1522_D17_033783_1509 (see supplementary

aterials).

188 J.S. Levy et al. / Icarus 285 (2017) 185–194

Fig. 5. (Left) Topographic transect across the North Hellas structure. A-A ′ profile is marked in the DEM. North to image top. “Brain terrain” surface textures are present in

the southeast portion of the depression (arrow). (Right) Topographic transect across the Galaxias depression. B-B ′ profile is marked on the DEM. North to image top. Note the

presence of steep, extensional fractures in both DEMs. North Hellas map composite HiRISE DEM and imagery over CTX DEM and imagery. Galaxias map is HiRISE imagery

and HiRISE DEM colorized topography DEM.

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2.2. Morphological analysis

Both features are topographic depressions surrounded by

sharply-defined, concentric, extensional fractures ( Figs. 3 and 4 ).

Both features are largely rimless. The depressions are wide and

low-slope near the surface, and constrict down towards a narrower,

steeper cavity at depth, with steps or terraces marking the lo-

cation of rotational blocks present between extensional fractures.

The North Hellas depression is ∼400 m deep and ∼5300 m wide,

while the Galaxias depression is ∼150 m deep and ∼2600 m wide

( Fig. 5 ). For comparison, stereo topography for a fresh impact crater

of similar size to the North Hellas depression, and located just

∼35 km away, is shown in Fig. S2. It has a clearly-defined rim and

uniformly-sloping walls that do not have a funnel-like inflection

point below which the cavity abruptly becomes narrower.

The Galaxias depression is located close to several landforms

interpreted by Pedersen et al. (2010) and Mouginis-Mark and Wil-

son (2016) as evidence for previous volcanic and glacial activ-

ity associated with the Elysium Mons volcanic province, includ-

ing dike swarms, tindar-like ridges, and ice depressions. Cuspate

pits surround the Galaxias depression which locally increases sur-

face roughness. They are arranged both radial to and concentric

with the fractures ( Fig. 4 ). The morphology of the pits is consis-

tent with both impact ejecta (e.g., secondaries) ( Pedersen et al.,

2010 ) or ballistically-emplaced volcanoclastic deposits typical of

phreatic explosions associated with subglacial volcanic eruptions

( Gudmundsson et al., 1997; Gudmundsson et al., 2004; Pedersen

et al., 2010 ). Sediments within, and in proximity to the Galaxias

depression range in size from boulders to fine materials that have

been sculpted into dunes ( Fig. 4 ).

In North Hellas, the depression is located in a ∼25 km di-

ameter impact crater, which is inferred to be of Noachian or

Noachian/Hesperian age because its ejecta is extensively cross-cut

by valley networks ( Figs. 3 and 6 ). The host crater rim and inner

wall are partially cut by concentric scarps that are inferred to be

related to crater formation (notable in the northwest quadrant of

he crater) ( Fig. 3 ). The depression is located in a unit covering

he crater floor. The North Hellas depression lacks radial or con-

entric pitting outside of the fractured depression, and is primarily

elineated by concentric fracture-bounded terraces that step

ownwards towards a central depression, surrounded by a larger

nnulus of closely-spaced fractures located to the west of the main

epression ( Fig. 3 D). Fractures and/or scarps are also present in the

orthwest periphery of the crater floor unit, which have a simi-

ar appearance to scarps found at the boundary between concen-

ric crater fill (CCF) and host crater walls ( Levy et al., 2010 ). Sin-

ous channels extend from the crater rim towards the depression,

nd are cross-cut by the outermost fracture, suggesting the follow-

ng order of events: emplacement of the crater floor unit, followed

y incision of channels, followed by formation of the concentric

ractures ( Fig. 3 , black arrows; Fig. 6 ). Contacts between the crater

ll and crater walls are abrupt, with complexly-fractured fill units

opographically and stratigraphically higher than the crater wall

nd floor. To the west of the crater central peak, the floor-filling

nit is elevated ∼70 m above the surrounding fill. This elevated

ection is fractured, but does not have down-dropped segments

Fig. 3 D), suggesting the possibility of either inflation of the surface

rom intrusion of material beneath the fill, or preferential preser-

ation of the fill unit and vertical lowering of the surrounding fill

urface.

The crater-floor unit is morphologically similar to “low-

efinition concentric crater fill” and contains exposures of “brain

errain” ( Fig. 3 ) ( Levy et al., 2010 ). “Brain terrain” has been inter-

reted to be of Amazonian glacial origin ( Levy et al., 2009; Dick-

on et al., 2012 ), suggesting that a buried ice substrate may fill the

rater. It is notable that the crater floor unit is smoother and less-

ratered immediately surrounding the depression than in sections

f the crater far from the depression, suggesting that the forma-

ion of the depression involved reworking or overprinting of the

ll unit over a ∼1–2 km radius from the outermost fracture.

The North Hellas depression is located along the extended rim

f the Hellas Basin, a region containing abundant examples of

J.S. Levy et al. / Icarus 285 (2017) 185–194 189

Fig. 6. Cross-cutting relationships between channel-like landforms (dashed lines) and concentric fractures in North Hellas. Fractures cross-cut channel-like landforms (ar-

rows).

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lacial activity (e.g., Head et al., 2005; Holt et al., 2008; Levy

t al., 2014 ), ancient flood volcanism ( Rogers and Nazarian, 2013 ),

nd Hesperian-Amazonian-aged phreato-magmatic eruptions (e.g.,

assanelli and Head, 2016 ; Hadriacus Patera is < 500 km to the

outheast). Intriguingly, a second concentric fracture system ( Fig. 8 )

s located in a crater floor unit ∼80 km south of the North Hellas

epression. Although stereo coverage is not presently available for

his site to compare the depth structure of this feature, the frac-

ure system shares several characteristics with the North Hellas

epression, including formation in a crater-floor deposit, smooth-

ng of the crater-floor unit immediately surrounding the fractures,

diameter of several kilometers ( ∼2.3 km vs. ∼5.3 at North Hellas),

nd wide fractures bounding detached blocks that appear to step

own towards a central depression, which are in turn surrounded

y narrower fractures ( Fig. 8 ).

The presence of boulders, dunes, and blocky debris atop the

orth Hellas surface ( Fig. 3 ) suggests that the material that cracks

ormed in was debris-covered at the time of failure. Debris could

ave been transported over the North Hellas crater-filling material

y erosion from crater walls and central peaks (consistent with

he presence of boulder tracks and gullies on the central peak).

oulders and dunes/ripples at the North Hellas (and the Galax-

as site) site suggest that any ice involved in depression forma-

ion was debris-covered at the time of fracture formation (boul-

ers are unlikely to have rolled across multi-meter high fractures)

Fig. 4 ). Further, the presence of linear arrangements of pits along

nter-fracture ridges (blocks) suggests that subsurface material loss

e.g., sublimation) and sediment infilling have continued after de-

ression formation ( Fig. 7 ). These fractured and pitted ridges are

imilar in appearance to ridges and furrows seen in equatorial

emnant ice landforms by Shean (2010) .

h

.3. Calorimetry: estimates of removed ice and required magma

olumes

In order to evaluate the subglacial volcanism model for these

eatures, calorimetric estimates were made for a hypothetical erup-

ion to determine the volume of magma that would have been

equired to remove a volume of ice equal to the volume of the de-

ressions (after Björnsson, 1983 ). Depression volumes were deter-

ined by calculating the volume between the DEM surface and a

lane defined by the average elevation around the depression rim.

verage elevations around the rim were determined by extracting

levation values at points spaced every 10 m around a depression-

ounding polygon and are −3947 ± 4 m and −1896 ± 65 m, for

alaxias and North Hellas, respectively (reported error is 1 stan-

ard deviation). Measured volumes of the depressions are 0.23 km

3

or Galaxias and 2.4 km

3 for North Hellas (measured on the HiRISE

nd CTX DEMs, respectively). We assume that all volume between

he depression floor and the surrounding terrain was lost during

epression formation, and that no pre-existing cavities were filled

uring the emplacement of the fractured substrate. Because topo-

raphic smoothing over steep surfaces and error from occlusion of

he fracture bottom due to imaging geometry are both included

n these measurements, and because some infilling has likely oc-

urred since the fractures formed, they should be considered min-

mum volumes.

Using the approach outlined in Gudmundsson et al. (2004) and

hatan (2003) , the volume of magma required to melt this volume

f material, assuming it is pure water ice, V m

, is:

m

= ( ρi L i V i ) ( ρm

c m

�T) −1 (1)

here i indicates ice, m indicates magma, ρ is density, L is latent

eat of fusion, V is volume, c is specific heat, and T is temper-

190 J.S. Levy et al. / Icarus 285 (2017) 185–194

Fig. 7. Linearly arranged pit chains in the North Hellas depression. The pits are arranged along fine-scale concentric fractures that dissect the block defined by the larger

concentric fractures. Pit formation suggests loss of subsurface material or further fracture opening and infalling of overlying sediments to form pits.

Table 1

Summary of depression geomorphic characteristics, calorimetry calculations, and

impact scaling predictions.

Galaxias North Hellas

Depression volume (km

3 ) 0 .23 2 .36

Depression diameter (km) 2 .6 5 .3

Depression depth (km) 0 .15 0 .4

Average fracture width (m) 50 m 110 m

Minimum magma to melt (km

3 ) 2.0 × 10 −2 2.0 × 10 −1

Minimum magma sublimate (km

3 ) 1.6 × 10 −1 1.7 × 10 °Impact predicted cavity (km

3 ) 1 .57 6 .98

Impact predicted depth (km) 0 .71 0 .61

Inferred process Impact Volcanism

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ature, provided heat transfer is from cooling of a magma with-

out crystallization. We assume 100% efficiency in heat transfer

between cooling magma and glacial ice in order to calculate a

minimum energy (magma volume) required to produce the de-

pressions. Typical efficiencies observed on Earth range from 10%

in pillow-forming eruptions to 70–80% in fragmenting magmas

( Gudmundsson, 2003 ). All material constants were taken from

Gudmundsson et al. (2004 ) and references therein, except ρm

(2700 kg/m

3 ) ( McSween, 2002 ) and �T (1200 °C, a reasonable

temperature drop from near-liquidus basaltic magma cooling to

mean martian global surface temperatures). ρ i is 910 kg/m

3 , L i is

3.35 × 10 5 J/kg, and c m

is 1100 J kg −1 K

−1 . We neglect heat loss to

warming of any debris within the depression because the sedi-

ment/ice mixing ratio is unknown as is the sediment temperature

gain.

Using Eq. (1) , the magma volume required to melt the Galax-

ias depression is 2 × 10 −2 km

3 and the magma volume required to

melt the North Hellas depression is 2 × 10 −1 km

3 . Because no as-

sumptions are made regarding the crystallization of magma, this

calculation yields a minimum estimate of the volume of magma re-

quired, as subglacial eruptions commonly result in magma quench-

ing to glass, limiting the latent heat of fusion released from the

melt ( Gudmundsson et al., 1997 ).

Alternatively, the case can be considered where subglacial mag-

matism results in the sublimation of ice to form a depression.

Taking a 100% sublimation case, Eq. (1) is modified such that L i is replaced with L is , the latent heat of sublimation for water ice

(2.83 × 10 6 J/kg). The resulting magma volume required to subli-

mate the ice within the developing depression is then calculated to

be 1.6 × 10 −1 km

3 for Galaxias and 1.7 km

3 for North Hellas. These

calculations are summarized in Table 1 . Compared to the volume

of terrestrial crustal magma chambers (10 s of km

3 ) and large lava

p

ows on Mars (10 s–100 s of km

3 ) ( Mouginis-Mark and Yoshioka,

998 ), these volumes are small, and may represent the combined

ction of small pulses of magma, such as the isolated, volcanic

epression-forming eruption at Gjalp, Iceland (e.g., 0.45 km

3 at,

udmundsson et al., 2004 ) or inefficient heat transfer from depth.

.4. Impact processes

An alternative to the subglacial eruption scenario outlined

bove is an impact mechanism: could the North Hellas or Galaxias

tructures have been formed by impacts? We compared the diam-

ter, volume, and depth of the Galaxias and North Hellas depres-

ions to regional best-fit power laws for simple, D < 10 km, mar-

ian impact craters ( Stewart and Valiant, 2006 ). Note that these

ower law relationships are empirical, regional best fits and in-

lude information about subsurface composition, ice distribution,

tc. Regional properties were selected to be analogous to surface

roperties in the sites where the depressions are found: i.e., mid-

J.S. Levy et al. / Icarus 285 (2017) 185–194 191

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atitude highland and lowland terrains with units interpreted as

ce-related fill. In particular parameters for the Stewart and Valiant

2006) Utopia Planitia study area (northern lowlands) were used

or Galaxias Fossae, and parameters for the crater population in Lu-

ae Planum study area (southern highlands) were used for North

ellas.

Based on the measured depression diameters (2.6 km for Galax-

as and 5.3 km for North Hellas), and constants for lowland and

ighland craters, respectively, we calculated fresh crater cavity vol-

me, V, as V = kD

n (where D is diameter, and k and n are 0.132 and

.59 and 1.78 and 2.2 for Galaxias and North Hellas, respectively).

he inferred volume for Galaxias is calculated to be 1.6 km

3 and for

orth Hellas is 7.0 km

3 . For comparison, the volume of the Galax-

as depression is ∼0.2 km

3 and for the North Hellas depression, the

easured volume is ∼2.4 km

3 . Fresh crater depth, d, was calcu-

ated as d = kD

n , where k and n are 0.492 and 0.39 for Galaxias

nd 0.469 and 0.16 for North Hellas. This yields a depth estimate

or Galaxias of 0.7 km and for North Hellas of 0.6 km. For compari-

on, the measured depth of the Galaxias depression is ∼150 m and

he depth of the North Hellas depression is ∼400 m. These calcu-

ations are summarized in Table 1 .

. Discussion

Both impact and volcanic mechanisms seem to account for

ome morphological characteristics of the observed depressions,

ut both interpretations also have shortcomings. Both mecha-

isms suggest different processes of heat transfer and depression

ormation: excavation, top-down heating from warm ejecta, and

ottom-up heating from compressed target material for the im-

act case, and bottom-up heating, through direct magma-ice con-

act or hydrothermal heating in the volcanic case. In the latter sce-

ario (bottom-up heating), evaporating meltwater or sublimated

ce would need to leave the growing cavity through conduits in

he ice cover made by fractures associated with a phase change

overpressurization) or eruption.

Considering the impact mechanism, in both locations, the im-

act calculations over-estimate both the volume and the depth of

he depression, compared to their measured diameter—however,

any craters are partially or completely in-filled or relaxed on

ars, resulting in reduced volumes for a given diameter ( Senft and

tewart, 2008 ). If the features formed by impact, either the con-

entric fractures have expanded the apparent diameter of the land-

orms from an initial rim diameter to a new, larger cavity size, or

n-filling and/or relaxation of the depressions occurred post-impact

n order to drive a reduction of cavity size and depth (e.g., Head

nd Weiss, 2014 ). This infilling or viscous relaxation would need

o be pronounced at these locations, as impacts into ice-over-basalt

argets produce deeper crater depths than impacts into basalt for

given crater diameter ( Senft and Stewart, 2008 ).

Impacts into icy targets on Mars are expected to produce a va-

iety of devolatilization morphologies that are similar to landforms

bserved at the North Hellas and Galaxias sites. The formation of a

hot plug” of ice > 270 K as a result of hypervelocity impacts into

cy targets is consistent with the removal of mass via sublimation

hat could account for the formation of depressions in icy targets

hat subsequently exceed the size of a smaller, initial crater cav-

ty ( Senft and Stewart, 2008 ). Melting resulting from the ejection

f warm target material (ice and/or basalt) could melt in situ ice

r ice in surrounding material, generating pitting, and potentially,

hannelized flow features in the ejecta immediately surrounding

mpact crater ( Senft and Stewart, 2008 ). Pits surround the Galaxias

epression and channels are present at the North Hellas site, how-

ver, cross-cutting relationships suggest that the concentric frac-

ures post-date the formation of the channels. This is consistent

ither with fracture formation entirely occurring after channel inci-

ion, or with fracture formation occurring during channel incision,

ith the outermost concentric fracture forming after the channels

ere emplaced.

A weakness of the impact mechanism for forming the North

ellas structure in particular is the apparent paucity of impact-

rater-related landforms around the depression (e.g., rim deposits,

jecta, etc.). Although the crater central peak could obscure some

jecta, the North Hellas depression is rimless and lacks radial pit-

ing. Terrain on the north side of the depression is slightly rougher

han elsewhere in the crater and is composed of faint lineations

omposed of positive-relief terrain. To the southeast and south-

est, the terrain nearest the depression is smoother than else-

here in the crater. Although impacts into ice over basalt can pro-

uce muted rims associated with horizontal, non-ballistic flow of

all material, energetic ejecta is expected from impacts into ice

nd ice-over-basalt ( Senft and Stewart, 2008, 2011 ). On balance,

vidence for radially-oriented ejecta deposits are ambiguous at the

orth Hellas site.

Clearer geomorphic evidence at the North Hellas depression

onsists of dilated fracture blocks in the main depression, sur-

ounded by a network of fine, concentric fractures ∼3–4 km from

he depression ( Fig. 3 ) This suggests broad downsag of the pit floor

o form a steep topographic rim (the depression) surrounded by

ing fractures, similar to caldera formation in terrestrial settings

Branney, 1995 ), rather than piston-like collapse and hole forma-

ion driven by direct magma-ice interaction in terrestrial settings

e.g., Gudmundsson et al., 1997, 20 04; Smellie, 20 02 ). The general

rend of increasingly large fracture-spacing from narrow fractures

t the depression edge to wider fractures at the center of the de-

ression ( Figs. 3 and 4 ) is consistent with more removal of mate-

ial from the depression center than from the depression margins.

n such a case, the enhanced fracture widening in the center would

e inferred to result from higher driving stresses caused by greater

emoval of underlying ice at the center of the depression.

Since there is no evidence of flood discharge from the depres-

ion (e.g., Smellie, 2002 ) and because basal water escape is un-

ikely from a closed basin (the host crater), we infer that the ma-

ority of ice was removed largely by sublimation or evaporation

f meltwater. Inefficient heat transfer due to hydrothermal heat-

ng may also have occurred, suggesting that the magma volumes

equired for depression formation are a minimum and that lit-

le to no magma reached the surface to result in infilling of the

it. In this case, with low efficiency heat transfer (as low as 10%

or pillow-forming eruptions, or still lower for hydrothermal heat

ransfer), required magma volumes could be up to 10–20 times

arger, reaching 10 −1 to 10 1 km

3 , depending on whether the ice

elted or sublimated.

A weakness for the subglacial volcanic mechanism for the

alaxias structure is the presence of a number of depressions

n the vicinity of the landform studied here that display a mor-

hological continuum between the concentric, extensional fracture

tructures and bowl-shaped impact craters ( Fig. 3 ). Taken together,

hese calorimetric estimates, coupled with morphological observa-

ions of the structures suggest that the North Hellas depression is

stronger candidate for subglacial heating and ice depression for-

ation. Two candidate hypotheses for heat sources include sub-

lacial volcanism associated with Amazonian magmatic activity, or

ydrothermal activity associated with the central peak of the host

rater. However, the apparently old (Noachian/Hesperian) age of

he crater and the young (Amazonian) age of most crater-filling

lacial deposits suggests that interactions between ice and the host

rater are not likely ( Fassett et al., 2014 ). Instead, the lack of in-

lling from dust fall (e.g., Kahre et al., 2006 ) suggests that both

he North Hellas and Galaxias depressions are recent Amazonian

n age. Intriguingly, the presence of a second concentric fracture

192 J.S. Levy et al. / Icarus 285 (2017) 185–194

Fig. 8. An incomplete concentric fracture system in the margin of a crater floor-filling unit near the North Hellas depression. White arrow indicates fine-scale fractures

concentric to the depression and crater wall. Portion of CTX image D19_034495_1509. No stereo pair exists for this image, precluding topographic analysis. Inset shows the

location of this concentric fracture system, ∼80 km from the North Hellas depression (black arrow).

p

l

g

2

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b

a

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g

M

g

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2

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system near the North Hellas depression ( Fig. 8 ) suggests the pos-

sibility of regional volcanic activity at this time.

The Galaxias structure is a complicated landform in a complex

region of Mars. It meets many criteria of both impact and vol-

canic origin mechanisms. However, the impact calculations come

closer to predicting the actual attributes of the depression, sug-

gesting that impact into an icy substrate, followed by additional

ice loss and pit expansion during fracture formation could account

for the Galaxias feature. A volcanic origin for the feature (and its

neighbors) may be a possibility; however, impact processes appear

more likely in this location.

It should be noted that these two landforms are not the only

concentric fracture systems on Mars, however, they are two that

are particularly well-imaged. We identify four other sites (Fig. S1)

that contain smaller ( < 2 km diameter), less sharply defined, con-

centric fractures and a fourth with a comparably-sized depression;

however, none of these sites are covered by stereo imaging, mak-

ing it impossible to assess formation mechanisms using the ap-

proaches outlined here. It is interesting, however, that all five of

these sites are in the low-to-mid latitudes ( ∼30–40 ̊) where land-

forms interpreted as debris-covered glacier remnants are common

( Dickson et al., 2012; Levy et al., 2014 ). The four supplementary

sites are also located in regionally fractured terrain (graben), com-

mon to other candidate ice-volcano interaction sites on Mars (e.g.,

Pedersen et al., 2010 ), or in the ejecta of impact craters (or in one

case, both). Accordingly, concentrically fractured deposits may be

more widespread on Mars than this pilot study indicates and may

serve as a useful probe of subsurface conditions and/or ice loss.

They may also be sites suitable for astrobiological exploration, if

ice locally melted to produce short-lived ponds or lakes.

a

c

However, the overall rarity of concentrically fractured ice de-

ressions on Mars is striking given the abundance of glacial

andforms present at martian mid-latitudes ( Squyres, 1979; Man-

old, 2003; Head et al., 2006; Head et al., 2010; Levy et al.,

010; 2014 ) and their long emplacement history during the Ama-

onian ( Fassett et al., 2014 ). Although small, bowl-shaped craters

re common on Amazonian glacial deposits, large impacts capa-

le of removing subsurface ice to form concentric fracture patterns

nd depressions appear to be primarily found in broad, contin-

ous, ice-bearing deposits where low-frequency/large-magnitude

mpacts can be recorded. Indeed, most concentric fracture fea-

ures on Mars are confined to the glacier deposits on the flanks of

he Tharsis Montes ( Head and Marchant, 2003; Head et al., 2005;

hean et al., 20 05 , 20 07; Head and Weiss, 2014 ). Although both

lacial terrains and volcanic terrains are spatially widespread on

ars ( Head and Wilson, 2002 ), the temporal overlap of the two

eological processes appears to be the exception, rather than the

ule, in recent martian geological history (again, outside of the

harsis Montes, e.g., Neukum et al., 2004; Scanlon et al., 2014,

015 ). Accordingly, those unusual locales in which volcano-ice in-

eractions are observed on Mars should be considered sites of high

strobiological potential for future exploration. The North Hellas

tructure, and to a lesser extent, the Galaxias structure, may be

wo such sites.

. Conclusions

The morphological properties of the North Hellas and Galax-

as Fossae depressions are strongly suggestive of surface collapse

nd fracture due to removal of subsurface ice. Volumetric and

alorimetric estimates suggest that up to two cubic kilometers of

J.S. Levy et al. / Icarus 285 (2017) 185–194 193

i

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f

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B

B

B

B

C

C

D

D

F

F

F

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ce may have been removed in order to form these depressions

ejected, melted, and/or sublimated), and that an ice-rich substrate

ay have cracked in response to surface subsidence to produce the

bserved concentric fracture (crevasse) morphology. Volcanogenic

ce-cauldron formation is found to be the favored explanation for

he North Hellas depression, although impact remains a possible

xplanation. Impact processes are favored for the Galaxias depres-

ion. The possibility of geologically-recent ice melt in proximity to

n impact heat source or a potentially volcanic-gas enriched site

n Mars makes these features tantalizing astrobiological targets,

nd suggests the importance of in-situ and terrestrial-research in

olcano-ice systems.

cknowledgements

Special thanks to the HiRISE and CTX teams for access to the

igh-quality image datasets used in this analysis. Thanks to Sam

eel and Cassie Stuurman for helpful discussion and to Dr. John

mellie and Dr. David Shean for their constructive reviews. This

ork was supported in part by NASA Mars Data Analysis Program

ward NNX13AN50G to JSL and CIF.

upplementary materials

Supplementary material associated with this article can be

ound, in the online version, at doi:10.1016/j.icarus.2016.10.021 .

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