3628 - Pedersen, G. B., and J. W. Head

Icarus 211 (2011) 316–329
Contents lists available at ScienceDirect
Icarus
journal homepage: www.elsevier.com/locate/icarus
Chaos formation by sublimation of volatile-rich substrate: Evidence from
Galaxias Chaos, Mars
G.B.M. Pedersen a,⇑, J.W. Head III b
a
b
Department of Earth Sciences, University of Aarhus, Hoegh-Guldberggade 2, 8000 Aarhus C, Denmark
Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, RI 02912, USA
a r t i c l e
i n f o
Article history:
Received 1 May 2010
Revised 12 August 2010
Accepted 3 September 2010
Available online 16 September 2010
Keyword:
Mars
Galaxias Chaos
Elysium Volcanic Province
Vastitas Borealis Formation
a b s t r a c t
Galaxias Chaos deviates significantly from other chaotic regions due to the lack of associated outflow
channels, lack of big elevation differences between the chaos and the surrounding terrain and due to
gradual trough formation.
A sequence of troughs in different stages is observed, and examples of closed troughs within blocks
suggest that the trough formation is governed by a local stress field rather than a regional stress field.
Moreover, geomorphic evidence suggests that Galaxias Chaos is capped by Elysium lavas, which superpose an unstable subsurface layer that causes chaotic tilting of blocks and trough formation.
Based on regional mapping we suggest a formation model, where Vastitas Borealis Formation embedded between Elysium lavas is the unstable subsurface material, because gradual volatile loss causes
shrinkage and differential substrate movement. This process undermines the lava cap, depressions form
and gradually troughs develop producing a jigsaw puzzle of blocks due to trough coalescence.
Observations of chaos west of Elysium Rise indicate that this process might have been widespread
along the contact between Vastitas Borealis Formation and Elysium lavas. However, the chaotic regions
have probably been superposed by Elysium/Utopia flows to the NW of Elysium Rise, and partly submerged with younger lavas to the west.
Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction
Chaotic terrains are common on the martian surface, however
the origin of this landform is enigmatic and many different formation models have been proposed.
Chaotic terrain was thoroughly described by Sharp (1973) as
irregular jumbles of angular blocks of various size (kilometers to
tens of kilometers) many preserving remnants of the upland surface and sometimes controlled by linear features. Generally the
blocks are flat topped, their sizes decrease away from the bounding
head scarp and the terrain consists of alcove-like depressions that
commonly have a subcircular shape and can be divided into cells
(Nummedal and Prior, 1981; Chapman and Tanaka, 2002; Rodriguez et al., 2006). Usually, the chaotic terrain lies several hundreds
of meters up to a couple of kilometers below the surrounding plateau and thus, the topographic characteristics are very distinctive
for chaotic terrain. Meresse et al. (2008) used topographic profiles
from the Xanthe and Margaritifer Terra to resolve different stages
of chaos formation, showing an initial stage of shallow ground subsidence of a few tens to hundreds of meters, where the ancient plateau is heavily fractured and collapse is limited. During the
⇑ Corresponding author. Tel.: +45 89429539; fax: +45 89429406.
E-mail address: [email protected] (G.B.M. Pedersen).
URL: http://www.marslab.dk/ (G.B.M. Pedersen).
0019-1035/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.icarus.2010.09.005
collapse, the highland material is fractured into mesas making
the chaos margins well defined and resulting in a vertical displacement of 1000–3000 m. Equally, Rodriguez et al. (2005) has delineated different stages of chaos formation based on morphologic
and topographic observations of the Xanthe Terra region.
Chaotic terrain is found widespread and is generally geographically associated with huge equatorial troughs like Valles Marineris, with outflow channels like Chryse outflow channels and with
the dichotomy boundary (e.g., Soderblom and Wenner, 1978).
Several different geologic scenarios have been proposed as
chaos forming processes, often involving H2O as an important
agent. Sharp (1973) suggested both degradation of ground ice
and excavation of magma as plausible formation processes and
Soderblom and Wenner (1978) elaborated on a ground ice model
suggesting that erosion of strata originating from different zones
of H2O stability would form chaotic regions. Recently, Zegers
et al. (2010) proposed a new variant of this hypothesis suggesting
that water ice is a part of the rock sequence. As the rock overburden reaches a thickness of 1–3 km the ice begins to melt due to the
thermal insulation caused by overlying sediment, and thus, the
overlying material destabilizes and water escapes creating chaos
terrain.
The close spatial connection between chaotic terrain and outflow channels has resulted in suggestions that chaotic terrains
are source regions for channel formation. Carr (1978, 1979) argued
G.B.M. Pedersen, J.W. Head III / Icarus 211 (2011) 316–329
that outflow channels emerging full born from chaotic regions are
strongly suggestive for catastrophic flooding by rapid release of a
confined aquifer under great pressure resulting in collapse in adjacent areas. Triggering mechanisms could either be impact cratering
or pore pressure reaching lithostatic pressure. Magma–ground ice
interactions is another possible triggering mechanism, both as a
heat source for catastrophic melting of ground ice, as well as disrupting the cryosphere releasing a confined aquifer under pressure
(Chapman and Tanaka, 2002; Head and Wilson, 2002; Meresse
et al., 2008). This model has been supported by Glotch and
Christensen (2005), who observed grey, crystalline hematite along
with phyllosilicates and possible sulfate in Aram Chaos providing
evidence for episodic pulses of water release as a chaos forming
process. Cabrol et al. (1997) stress the importance of volcano tectonic strains as another triggering mechanism based on a survey
of Shalbatana Vallis, where crossing fault systems are suggested
to allow hydrothermal drainage of confined aquifers.
Rodriguez et al. (2005) suggest that the different stages of chaos
formation result from multiple episodes of reactivation of the
hydraulic head and propose a suite of processes such as: renewed
magmatic activity, topographic lowering with respect to the
groundwater table, subsiding cavern roofs or thickening of a permafrost seal, as mechanisms for increased hydrostatic pressure.
Analysis by Wang et al. (2006) show that freezing-induced pressurization is a possible mechanism for releasing groundwater,
however not on the order of big outflow channel systems.
Debris flow mechanisms, triggered by large scale failures of
subsurface material, have also been proposed to account for the
development of chaos (Tanaka, 1999; Nummedal and Prior,
1981). Based on observations of the Simud/Tiu deposits, Tanaka
(1999) proposed that seismic activity could liquefy water-rich sediments causing a collapse and lowering of the chaos surface versus
the adjacent plateau, resulting from subsurface removal rather
than rotational slumping or lobate landslide masses. Wang et al.
(2005) support this idea by suggesting impacts as a quaking agent
and find evidence for liquefaction in chaotic regions by checkerboard patterns of gaps between blocks of chaotic terrain, which
is similar to liquefaction patterns on Earth.
Dissociation of clathrates as a mechanism of subsurface removal has also been put forward in several papers (e.g., Milton,
1974; Hoffmann, 2000; Komatsu et al., 2000; Rodriguez et al.,
2006) starting with Milton (1974) who proposed carbon dioxide
hydrate as a possible phase, under which explosive dissociation
would result in a catastrophic dewatering. Komatsu et al. (2000)
argue that the stress from decomposed clathrates releases a large
quantity of CO2 and CH4, which would subject water-saturated
sediments to so much stress that the sediment would liquefy. A
similar model, including a series of runaway degassing events for
Ganges Chaos, has been proposed by Rodriguez et al. (2006), who
mention deep fracture propagation, magmatic intrusions and climatic thawing and thinning of the cryosphere as plausible trigger
mechanisms.
2. Geologic setting
Galaxias Chaos is situated in a highly complex region in the
transition zone between Elysium Rise to the south, Utopia Basin
to the north, and is bounded by Hecates Tholus to the east and
by Elysium/Utopia flows to the west (Fig. 1). Galaxias Chaos was
first recognized by Schaber and Carr (1977) and later mapped by
Scott and Carr (1978) (1:25,000,000) as a knobby material in a zone
between the lowland surface of Vastitas Borealis and Noachian
Terrain. Based on Viking imagery, Mouginis-Mark (1985) and
Mouginis-Mark et al. (1984) suggested that the upland remnants
rather were a part of the compound lava plains extending from
317
Elysium Mons than Noachian terrain and emphasized that structural weakness seems to have played an important role. Likewise,
Greeley and Guest (1987) suggested that the material of isolated
patches of grooved terrain was lava and ascribed the formation
to Late Hesperian. However, Tanaka et al. (1992) proposed an Early
Amazonian origin related to Elysium lavas grooved due to flowage
of subsurface material, possibly as a result of ground ice melting
induced by volcanism, and Tanaka et al. (2005) later classified Galaxias Chaos as a part of the Vastitas Borealis Marginal unit.
Since the first observations of the huge Chryse outflow channels
draining into the northern lowlands, including Utopia Basin, studies investigating a volatile-rich past have been carried out and enhanced H2O content in Utopia Basin has been observed by the Mars
Odyssey Neutron Spectrometer (Feldman et al., 2002). This result
confirmed several proposals for a volatile-rich geologic history
based on a suite of morphologies (e.g., Cave, 1993), who observed
a high frequency of fluidized ejecta in Utopia Planitia.
Other observations have suggested either an ocean hypothesis
or a glaciation hypothesis and Kargel and Strom (1992) and Kargel
et al. (1995) have been in favor of the latter. They suggested an ancient continental glaciation in the martian northern plains based
on observations of thumbprint terrain in association with sinuous
troughs that contain medial ridges, resembling terrestrial glacial
features like moraines, tunnel channels and eskers.
On the other hand, Parker et al. (1993) argued for an ocean
hypothesis and mapped two potential shorelines, contact 1 and
contact 2, based on a variety of features like cliffs, terraces and
abrupt termination of outflow channels. Based on data from MOLA
onboard Mars Global Surveyor, Head et al. (1999) checked whether
the proposed shorelines were reflecting an equipotential line, finding that contact 2 was the best approximation to an equipotential
line having a surface elevation of 3760 m with a standard deviation
of 0.56 km. The discrepancies vary nonrandomly and deviations occur primarily in regions of Tharsis, Elysium and Isidis, where later
modification of topography is likely. However, as Carr and Head
(2003) pointed out, though the standard deviation is small, individual sections of contact 2 have significant ranges in elevation and by
assessing the observational evidence for shorelines they found little
evidence for that interpretation. Chapman (1994) and Chapman and
Tanaka (2002) proposed that in absence of classical coastal morphologies, volcano–ice interactions might provide morphological evidence of an ice sheet. Like Allen (1979), who found evidence for
analogs to terrestrial subglacial volcanism close to the Elysium volcanic region, she found examples of three hyaloclastite-like ridges
on the NW flank of Elysium Rise indicating the existence of an ice
sheet with a height at least slightly higher than the ridges, which
reach an absolute elevation of 3753 m.
The preferred explanation by Carr and Head (2003) is that the
Vastitas Borealis Formation (VBF) supports the former presence
of water from large floods better than the proposed shorelines.
The outer boundary of VBF lies in approximately 3660 m elevation and VBF is generally smooth with gentle slopes and has a distinctive 3 km scale background surface topography, which
suggests a non-volcanic origin (Kreslavsky and Head, 2000). The
homogeneous and fairly smooth nature of VBF at 100 m and longer
wavelengths, its location in a basin and in the lower end of the outflow channels as well as the similarity in age between the outflow
channels and VBF fit the hypothesis for VBF representing a sublimation residue from a standing body of H2O created by the outflow
channel effluents. Modeling the evolution of outflow effluents, Kreslavsky and Head (2002) show that freezing in a convection state
under current martian conditions would occur over a period of
104 years and hereafter sublime. Thus, the presence of both an
ocean and an ice sheet is a plausible result of a catastrophic water
release from Chryse and the fast freeze up of the ocean might explain why a clear coastal morphology has not developed.
318
G.B.M. Pedersen, J.W. Head III / Icarus 211 (2011) 316–329
Fig. 1. A close up of the Elysium Volcanic Province, displaying Elysium Mons, the two flank volcanoes, Hecates Tholus and Albor Tholus, and the transition between Elysium
Rise and the Utopia impact basin. Galaxias Chaos is situated in a highly complex region in the transition zone between Elysium Rise to the south, Utopia Basin to the north,
and is bounded by Hecates Tholus to the east and by Elysium/Utopia flows to the west.
The Utopia/Elysium deposits west of Galaxias Chaos extend
more than 1500 km from the base of Elysium Rise forming a lobate
tongue of relatively smooth and flat channelized materials associated with broad levees (Christiansen and Greeley, 1981; Tanaka
et al., 1992). Many of them originate from linear fracture or graben
systems, which are aligned NW–SE suggesting influence of a regional stress field (Plescia, 1986; Hall et al., 1986).
The deposits have been interpreted as lahar flows based on
observations of their lobate outline, steep snouts, smooth medial
channels, their wet-looking nature and their association with volcanism. Different models for generating lahars have been suggested, including volcano–ice interactions, volcano–ground ice
interactions and/or dike disruption of the cryosphere and groundwater release (Christiansen, 1989; Christiansen and Greeley, 1981;
Christiansen and Hopler, 1986; Skinner and Tanaka, 2001; Russell
and Head, 2001a,b, 2002, 2003; Tanaka et al., 1992).
Most recently, Russell and Head (2001a,b) investigated the Elysium/Utopia flows using MOLA data, including detrended and gradient topography, and they were able to resolve two main types of
flows; lava flows and debris flows. They also interpreted the debris
flows as lahar deposits because of their morphology, stratigraphic
relationships and because of observed dendritic ridges, suggesting
dewatering flow after the water-rich lahar deposits came to rest
(Russell and Head, 2002). The distribution of the lahar deposits below elevations of 3100 m to 4300 m correlates with the distribution of fluvial channels, and led Russell and Head (2001b,
2003) to suggest that the configuration of deep radial fossaes, lava
flows and lahar deposits can be explained by propagating dikes
disrupting a confined aquifer facilitating large amounts of water
responsible for the lahar deposits. Thus, the clustering of channel
deposits may reflect the minimum elevation of a subsurface water
table at the time of dike emplacement. This evidence led Russell
and Head (2007) to suggest that the global groundwater system
proposed by Clifford (1993) should be elaborated on to incorporate
multiple recharge centers at volcanic provinces including contribution of snow melt to outflow channel activity.
Hrad Fossae is the easternmost outflow channel, and it modifies the western part of Galaxias Chaos and continues as Hrad Vallis further to the west. Several investigations of this area have
been carried out, some suggesting karst or thermokarst processes
responsible for depressions and channel incisions, implying the
presence of either carbonates, volcanoclastic materials or porous
clastic material saturated with water/ice (De Hon, 1992). Mapping
performed by De Hon et al. (1999) strongly suggests interplay of
volcanism, near-surface volatiles and surface run off. Wilson and
Mouginis-Mark (2003) propose that Hrad Vallis has a phreatomagmatic explosive origin generated by shallow sill intrusion interacting with ice-rich rock layers creating mudflows. This hypothesis is
supported by Morris and Mouginis-Mark (2006) who outline
observations of thermally distinct craters within the deposits
and argue that the distribution of the craters and their morphology are a result of interactions between hot mud flows and ground
ice.
Hecates Tholus, situated east of Galaxias Chaos, is one of the
few martian volcanoes, which has been proposed to have an explosive volcanic origin, deviating from many other volcanic, Hesperian
edifices by the presence of channel-incised flanks (e.g., Reimers
and Komar, 1979; Fassett and Head, 2006). Recently, remarkable,
smooth, lineated deposits with similar characteristics as lineated
valley fill in a flank depressions at the base of Hecates Tholus were
documented, and Hauber et al. (2005) interpreted them as evidence of young glacial deposits from recent climate changes. If this
interpretation is correct, the close relation to Galaxias Chaos suggests that recent climate changes might have been a plausible
modifying agent obscuring the primary processes that generated
the chaos.
Though this example is the closest evidence of late Amazonian
permafrost and periglacial modification to the study area, lots of
morphologies related to recent climate in Utopia have been reported including gullies, scalloped terrain, pingos and small size
polygonal patterned ground (e.g., De Pablo and Komatsu, 2007;
Soare and Osinski, 2007; Soare and Kargel, 2007).
G.B.M. Pedersen, J.W. Head III / Icarus 211 (2011) 316–329
In addition, Utopia Basin, among other regions, has been muted
by mantling material that has been proposed to be an ice-rich deposit emplaced symmetrically down to latitudes around 30° during
periods of high obliquity. At high obliquities water could migrate
atmospherically from the poles to mid-latitudes where it was
deposited as snow and frost. Today, this deposit is undergoing
reworking, degradation and retreat in response to instability due
to climate changes (Carr and Schaber, 1977; Mustard et al., 2001;
Head et al., 2003; Morgenstern et al., 2007).
3. Galaxias Chaos
Since the last investigations of Galaxias Chaos, a lot of new satellite data has been provided by Mars Global Surveyor (MGS),
Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter
319
(MRO) allowing more thorough and detailed studies of chaos forming processes.
3.1. Description of data
The region of Galaxias Chaos is covered by several different data
sets allowing a description of the area on a regional scale as well as
extracting information on small scale processes. Topographic data
like HRSC DEMs and MOLA data provide a new topographic perspective on Galaxias Chaos. The resolution of HRSC DEM varies between 75 m/pixel and 100 m/pixel and covers most of the region,
with the westernmost part as an exception. On the other hand the
MOLA DEM covers the whole region with a resolution of 460 m/
pixel, but the distribution of MOLA tracks reveals that there is up
to 13 km spacing across individual tracks.
Fig. 2. (A) Topography of Galaxias Chaos displayed by MOLA digital elevation model draped over a THEMIS IR mosaic. The color code ranges from 3550 m (red) to 4000 m
(dark blue). (B) Geologic map of Galaxias Chaos and surroundings. The chaos itself consists of the grooved unit (dark orange), sub-grooved unit (light orange) and trough unit
(dark brown). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
320
G.B.M. Pedersen, J.W. Head III / Icarus 211 (2011) 316–329
Several different data sets from cameras sensitive in the visible
to the infrared region have been released since the last examination of Galaxias Chaos. Viking (230 m/pixel), THEMIS IR (100 m/
pixel) and HRSC (12.5–25 m/pixel) provide a regional overview of
Galaxias Chaos and THEMIS VIS (18–80 m/pixel) covers a great extent of especially the eastern part of Galaxias Chaos. MOC, CTX and
HiRISE provide more sporadic, but detailed images with a resolution of 1.5–7 m/pixel, 6 m/pixel and 0.3–0.6 m/pixel, respectively.
3.2. Geomorphology of Galaxias Chaos
The region of Galaxias Chaos (142–150°E, 33–38°N) was
mapped in 1:100,000, and in several places to greater detail where
data allowed it. Galaxias Chaos is a mosaic of angular mesas, which
covers an area of approximately 14,000 km2 and the elevation varies from approximately 3550 m to 4000 m (Fig. 2A). Galaxias
Chaos is bounded to the east by Hecates Tholus and to the west
by the Elysium/Utopia flows and it is situated at the foot along
the edge of Elysium Rise as a complex jigsaw of polygonal blocks,
which are mapped as the dark orange, grooved unit and as light orange, sub-grooved unit on the geologic map on Fig. 2B.
The grooved unit of Galaxias Chaos is characterized by well-defined blocks, which vary in size from 0.25 km2 to approximately
100 km2 and there is no clear size progression away from Elysium
Rise. Both rough hummocky knobs and smooth surface texture are
observed on top of the blocks, but no relationship between the
troughs and the surface texture is observed.
Some larger, topographically less pronounced blocks, which are
several tens of kilometers across, are mapped as a sub-grooved
unit. Due to their less distinct relief they are more difficult to
map, and they are mainly distributed in the eastern part of
Galaxias Chaos further away from the edge of Elysium. Many of
the troughs do not penetrate this unit, resulting in blocks with a
size up to 40 km 70 km and in some places the unit seems to
be submerged by material from the Utopia Basin. Like the grooved
unit, rough and smooth surface textures are observed, which suggests that the surface material of both the grooved and subgrooved unit is the same. The transition between the two units is
abrupt and is displayed in Fig. 3A and D.
There is a huge morphological difference between the eastern
and western part of the grooved unit. To the east, the unit is well
defined and can be divided into subcircular cells, while the chaotic
terrain in the western part has been modified by Galaxias Fossae
and related outflow activity, which seems to submerge existing
troughs (Fig. 3B).
Numerous smaller, angular to subangular blocks up to approximately 2–10 km across are observed in Utopia Basin and they exist
both in the lobate plains material and plains material and are
marked as purple on Fig. 2B and in addition are displayed in the
top of Fig. 3D. At first sight it looks like these smaller blocks originate from Galaxias Chaos due to their close spatial relationship,
their shape and their elevation. However, these blocks can be
traced all the way to Phlegra Montes more than 1000 km NE of Galaxias Chaos as well as to the Isidis basin, more than 3000 km to the
southeast. Furthermore, they do not display the same jigsaw-like
pattern as the grooved and sub-grooved unit and are therefore
not considered to be a part of Galaxias Chaos, consistent with the
geologic map from Tanaka et al. (1992).
The main morphologic difference between the lobate plains
material and the plains material is the characteristic lobate plateau
features with a distinct rim, which are distributed between the
blocks (Fig. 3E) and both units contain unusual crater morphologies,
Fig. 3. Close-up of four parts of the geologic map of Galaxias Chaos and surroundings. (A) Geologic map as displayed on Fig. 2B. The black boxes show the location of three
different parts of Galaxias Chaos displayed in (B–D) and a zoom in on the lobate plateau material in (E). (B) Zoom in on the westernmost part of Galaxias Chaos. As it can be
seen from (A) this part of Galaxias Chaos consists of the grooved unit (polygonal blocks marked with dark orange) and trough unit (dark brown), and has been modified by
later NW–SE-wards fracturing and infilling of the trough unit. The chaos is bordered by the Elysium Rise unit (dark red) to the south and by flood plain deposits (light blue) to
the north. The image is a mosaic of CTX P14_006604_2140 and P15_006749_ 2149 and THEMIS VIS V025040061, V04389003, V11878007 and V14037006. (C) Zoom in on the
central part of Galaxias Chaos. As displayed in (A), this part of Galaxias Chaos consist of the well defined jigsaw puzzle of polygonal blocks (dark orange grooved unit) and
trough unit (dark brown) and borders the Elysium Rise unit to the south. Towards the north the blocks are less distinct and the troughs are absent. The topographically less
pronounced blocks are marked as the sub-grooved unit in light orange. Moreover, the blocks to the north are surrounded by plains material (light blue), which is a smooth
material draping some of the blocks. CTX P16_007395_2148. (D) The easternmost part of Galaxias Chaos has a cell-like nature consisting of distinct blocks and troughs (dark
orange and dark brown unit). Towards the north a big block partly incised by troughs is marked as a sub-grooved unit bordering lobate plateau material (dark blue) and
remnant hills (small mesas in dark purple). The image consists of THEMIS VIS V02429006 and V12427008. (E) Image displaying a typical morphology in lobate plains
material. The black arrows mark the distinct rims, which delineate the flat, lobate plateaus and the white arrows mark subdued knobs, which occasionally seems to penetrate
the plains material. THEMIS VIS V19191012. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
G.B.M. Pedersen, J.W. Head III / Icarus 211 (2011) 316–329
which have been described by Pedersen and Head (2009). The distinct rim associated with the plateaus in lobate plains material is
in some places draping the flanks of angular blocks, and within the
lobate plains material subdued knobs are observed and they seem
occasionally to penetrate the lobate plains material (Fig. 3E).
For resolving the formation process of Galaxias Chaos, extra
emphasis has been put on the data from the eastern and central
part of Galaxias Chaos, where the chaos is best preserved.
After constraining the extent of the chaotic terrain (grooved,
sub-grooved and trough unit) and the extent of later modifying
processes (deposits from outflow activity; flood plain deposits, levee deposits and smooth flow) an analysis of the surface material
was carried out by tracing layering and rough surface texture.
3.3. Surface material of Galaxias Chaos
MOC data reveal a dark surface layer observed in trough walls in
Galaxias Chaos that can be traced from troughs within Elysium
Rise into Galaxias Chaos (Fig. 4). The two images are 80 km apart
321
and are from the eastern and central region of Galaxias Chaos,
and they display a distinct dark, competent layer that can be
mapped out across the troughs (marked by black arrows). This
indicates that the same layer traced from Elysium Rise caps the
polygonal blocks within Galaxias Chaos.
Similarly, distinct rough texture and associated flow fronts can
be traced from Elysium Rise all the way to Galaxias Chaos (Fig. 5).
Both these observations suggest that Galaxias Chaos is capped by
Elysium lavas implying that Galaxias Chaos was a part of Elysium
Rise before trough formation.
3.4. Trough development
The trough formation itself can be studied by investigating the
variety of troughs, their geometry, and their spatial relationship to
the blocks.
The troughs are generally round-headed, with a width around
600–800 m and they are approximately 100 m deep; they vary
between being flat-floored and v-shaped. As displayed in Fig. 6,
Fig. 4. Close-up of the surface layer in the central and eastern part of Galaxias Chaos. Two MOC images from the central and eastern part (m0304232 and r13044670,
respectively) are located 80 km apart and both images display a dark surface layer that can be traced across troughs from the Elysium Rise (C and H) into the blocks of
Galaxias Chaos (A, B and E–G) as shown by black arrows. This indicates that Galaxias Chaos is capped by the same material as the Elysium Rise unit that consists of lavas. Both
MOC images are 3.8 km across.
322
G.B.M. Pedersen, J.W. Head III / Icarus 211 (2011) 316–329
different stages of trough formation can be found. The early trough
formation is displayed in Fig. 6A and B, and Fig. 6A shows a roundheaded depression forming within a block marked by breakage of
the competent surface layer. Likewise, Fig. 6B shows initial trough
development forming within the block, but with a close relation to
the edge of the block, which indicates that the trough formed due
to instability of the slope of the block.
The troughs either continue to grow within the blocks, forming
kilometers-long troughs or they incise the block from the edges.
Fig. 6C shows an example of a trough which curves subparallel to
the edge of the block and which to the northwest is abutting another trough separated by less than 10 m of block material. Moreover, the southwestern end of the trough seems to grow along a
linear depression marked with black arrows. Similarly, two troughs
on each side of a block seems to incise a block along a 70 m wide
depression, dividing the block into two parts, if the trough incision
process continues (Fig. 6D). An example of how the troughs coalesce and form cell-like trough systems making a jigsaw puzzle
of mesas is displayed in Fig. 6E, and it is observed that minor or
no rotation of the blocks has taken place. This indicates that chaos
formation was not a catastrophic process, but a gradual development allowing different stages of trough formation to be preserved.
Likewise the cell-like nature suggests that the trough formation
has been ongoing in several places along the contact as multiple
events of trough formation and collapse.
Moreover, the fact that the troughs form as depressions within
blocks also implies that the trough formation is not governed by
regional stresses, but rather local stress within and along block
edges.
3.5. Topography of Galaxias Chaos
The mesas in Galaxias Chaos are developed at 3.6 km to
3.7 km elevation and individual blocks are 100–200 m high,
reaching a maximum height around 3.5 km. In this manner, the
topographic characteristics of Galaxias Chaos deviate from other
described chaotic regions, where the blocks lie 1–2 km below the
surrounding surface. In contrast, some of the blocks in Galaxias
Chaos are higher than the adjacent slope (Carr, 1979; Rodriguez
et al., 2005; Meresse et al., 2008). This is displayed in Fig. 7, where
four selected MOLA profiles, going from the east to the west, cross
the eastern part of Galaxias Chaos and the black arrows delineate
the grooved unit. Profile 19875 (blue) and 13377 (red) show that
individual blocks within Galaxias Chaos are slightly elevated with
respect to the surrounding plateau.
At some places a plateau is developed in front of the chaos, as
shown in profile 10660 (yellow), but generally the elevation of
the blocks decrease away from Elysium Rise.
From THEMIS- and CTX-images it is observed that some of the
blocks display chaotic tilting, resulting in a well-defined relief in
Fig. 5. Rough flow texture and flow fronts can be traced from Elysium Rise into the chaos, displayed here in THEMIS image V11541008, which is 23 km across. The black
arrows point out flow fronts, which delineate the extent of the rough texture. North is up.
G.B.M. Pedersen, J.W. Head III / Icarus 211 (2011) 316–329
323
difference of 120 m, however this approximation is more uncertain
since the flanks of the block seem to have slid downhill, making the
distance between the flanks greater.
The calculated subsidence is within the same elevation range as
the depths of the troughs, and is similar to the topographic difference of the elevated blocks compared to the adjacent slope. Thus, if
these observations of differential subsidence up to 160 m can apply
to the rest of Galaxias Chaos, elevated blocks can be explained by
the adjacent slope subsiding a bit more than some of the mesas
in the chaos.
4. Model for formation of Galaxias Chaos
Fig. 6. Different stages of trough development marked with black arrows can be
traced within Galaxias Chaos indicating that chaos formation is a gradual process.
(A) Incipient trough development within a block. Fractures are highlighted by black
arrows. MOC m0304232. (B) Trough formation along the edge of a block. Section of
CTX-image P04_002714_2144. (C) A closed trough within a block. In the bottom of
the image a weakly developed linear depression indicates the direction of further
trough propagation (marked by black arrows). To the left of the image a black arrow
marks a bridge of material, which bounds the closed trough from coalescing with
other troughs. Section of CTX-image P04_002714_2144. (D) A block incised by
troughs from two sides. The arrow points to a linear depression, which indicates the
direction of trough propagation. Section of CTX-image P04_002714_2144. (E)
Coalescing troughs creates polygonal and very well-defined blocks. Section of CTXimage P04_002714_2144.
some parts of a mesa, while other parts are more subdued indicating that the surface material of Galaxias Chaos superposes an
unstable material and that they have experienced differential vertical displacement (Fig. 8A and B).
Furthermore some blocks partly superpose a competent material causing breakage of the superposed blocks due to unequal subsidence. In the case of Fig. 8C it seems that the central part of the
two yellow, highlighted blocks stalled on a stable subsurface material, while the flanks have experienced subsidence causing breakage of the central part of the block as the surface material could
not sustain further differential stress. The two topographic profiles
A–B and A0 –B0 (Fig. 8) are based on HRSC DEM with a resolution of
75 m/pixel and show clearly that the flanks are tilted in opposite
directions, having a local minimum, where the blocks break. If it
is assumed that the blocks originally were horizontal and that
the maximum elevation represents the initial elevation, an estimate of the subsidence can be calculated. Profile A–B reaches a
maximum height of 3720 m, while the minimum elevation of
the flanks is 3880 m resulting in an elevation difference of
160 m. The equivalent calculation for A0 –B0 gives an elevation
Galaxias Chaos is significantly different from other chaotic regions. First of all the topography is very different and in Galaxias
Chaos the elevation of mesas is in the same range as the surroundings, whereas other chaotic terrains have a significant elevation
difference up to several thousand meters between blocks and plateau. Moreover, Galaxias Chaos is not associated with huge outflow
channels, and Hrad Vallis is modifying rather than fed by the western part of Galaxias Chaos. The different stages of trough formation clearly indicate a gradual trough formation with minor
vertical movement, which contrasts to a catastrophic formation
process. Thus, proposed formation models for chaotic regions
around Chryse Planitia do not appear to be directly applicable to
Galaxias Chaos.
The observations from Galaxias Chaos suggest that the surface
material is Elysium lavas based on the distinct surface layer and
the rough surface texture associated with flow fronts that have
been traced from Elysium Rise into Galaxias Chaos. Moreover,
the topography reveals that a differential amount of subsidence
has caused different stages of trough formation indicating an
unstable subsurface layer. The tilting of mesas and the fact that
some blocks have elevations equivalent or slightly higher than
the surrounding plateau also support a differential vertical displacement. This implies that some parts of Elysium Rise have experienced a slightly greater downward movement without causing
breakage in the lava cap than the few elevated mesas within Galaxias Chaos. Additionally, the fact that troughs develop within
the blocks indicate that the trough formation is not governed by
regional stress fields, but by local stress fields, which fits an explanation of differential subsurface degradation and/or flow. Thus, a
consistent formation model for Galaxias Chaos should include a
substrate, which due to shrinkage/movement can cause gradual
trough formation induced by local stresses.
All these observations exclude proposed chaos formation models such as magma excavation, clathrate dissociation, catastrophic
groundwater release and melting of ice due to thermal insulation
caused by overlying sediment. However, degradation of ground
ice is a plausible mechanism of differential downward movement
of a substrate creating gradual trough formation, tilt of blocks
and variable local stresses.
Considering the close spatial relationship with lobate plains
material, which Tanaka et al. (2005) assigned to be a part of Vastitas Borealis Formation, this unit would be a good candidate as a
subsurface material from a stratigraphic point of view. Moreover,
the observed morphologies of lobate plateau material draping
angular blocks and subdued knobs that occasionally seems to penetrate the lobate plains material indicate degradation and shrinkage revealing the subsurface topography (Fig. 3E).
Additionally, the VBF has been considered to be a residue from a
H2O-rich material deposited by huge floods, as earlier mentioned
(e.g., Kreslavsky and Head, 2002; Carr and Head, 2003). The thickness and the elevation of the outline of VBF fit both the depth and
the elevation of the troughs (Carr and Head, 2003) and the
324
G.B.M. Pedersen, J.W. Head III / Icarus 211 (2011) 316–329
Fig. 7. Four topographic profiles and their location in the eastern part of Galaxias Chaos. From the top the four profiles are selections of MOLA tracks; ap19875, ap10660,
ap13377 and ap19863 starting from the east to the west. Black arrows point out the extent of the grooved unit.
G.B.M. Pedersen, J.W. Head III / Icarus 211 (2011) 316–329
325
Fig. 8. Chaotic tilting of blocks within Galaxias Chaos shows that Elysium lavas are emplaced on an unstable material making the blocks tilt. (A) Two blocks tilt towards the
center of the image displaying well-defined relief to the east and to the north, while the outline of the blocks are weakly developed in the central part of the image. (B) The big
elongated block tilts towards the northwest resulting in well-defined relief in the southeastern part of the image compared to the northwestern part, where plains materials
partly surround the block. (C) Breakage of two blocks marked in yellow indicates that the area has subsided unequally making the blocks break, where they stalled on more
stable subsurface material. The topographic profiles A–B and A0 –B0 are based on HRSC DEM and show that the blocks break in the middle and slide to each side of a
topographic high. If it is assumed that the blocks were horizontal before subsidence, a minimum estimate of the subsidence can be calculated. The image is a section of
THEMIS VIS V124270081.
estimated age of VBF is Early Amazonian, compared to the latest
activity on the Elysium Rise (Tanaka et al., 1992, 2005), making it
a plausible scenario that Vastitas Borealis Formation is sandwiched
between Elysium lavas.
The suggested formation process for Galaxias Chaos is shown in
Fig. 9, where a block diagram shows the stratigraphic relationships
between Elysium lavas and VBF. VBF is displayed in variable light
blue colors indicating variable volatile content and it is sandwiched between Elysium lavas in grey. The lava cap on top of
VBF inhibits volatile sublimation, but in areas where there is no
lava or where troughs have formed, volatiles sublime (indicated
by wavy, orange arrows). Sublimation of volatiles within the VBF
results in shrinkage and degradation of the unit and thereby facilitates subsurface movement. Where the VBF wedges out between
Elysium lavas, VBF is so thin that mass movement and vertical displacement is so small that no trough formation occurs because of
the strength of the lava cap itself. As the VBF increase in thickness
the lateral transport of sediment and the potential vertical displacement increase, undermining the lava cap. Eventually the lava
cap experiences higher stresses than it can support, creating
depressions and later troughs as sublimation through the broken
lava cap occurs. The most intense chaos formation will therefore
occur in the parts of Elysium Rise where the thickness of the VBF
is sufficient, and where the slope is steep enough to facilitate and
concentrate subsurface movement. Where particularly intense
and continued mass movements occur, tilted blocks as well as subsurface material submerging and superposing low lying blocks
might result.
This model explains several observations. First of all, the
troughs with highest elevation occur along an equipotential line
having an altitude around 3.6 km to 3.7 km. This fits with a
stratigraphy where the VBF was deposited with a surface elevation
around 3660 m and later capped by Elysium lava flows. Thus, if
the VBF was deposited as an equipotential surface, it would be expected that the occurrences of the most elevated troughs should be
equipotential and it would have been a problem if chaos formed
well above the suggested altitude of the VBF.
Secondly, the extent of VBF may very well explain the cell-like
nature of the chaos development. Before the VBF was deposited the
topography of Elysium Rise was controlled by its fingering flows
creating a wavy topography. Thus, as the VBF was emplaced as
an equipotential surface its outline and its thickness were controlled by this pattern filling up lows between lava flows with
thicker VBF deposits than elsewhere along Elysium Rise. Therefore,
as the volatile loss initiated the movement of the substrate, it was
controlled by local topography under the volatile-rich unit, facilitating trough formation in the former lows between lava flows creating the cell-like nature of Galaxias Chaos. This is illustrated in
Fig. 9 where a thicker deposit of the VBF occurs in the lows of
the oldest Elysium lavas (dark grey).
Thirdly, this model explains the gradual development of
troughs creating a jigsaw pattern, because Galaxias Chaos basically
was a part of Elysium Rise, which was incised by troughs causing
well preserved mesa surfaces, minor lateral movement of the
blocks and no huge topographic difference between the mesas
and the plateau itself.
Moreover, the fact that the mass movement is limited by the
thickness of the VBF as well as by the dip of slope, might explain
why the well defined grooved unit occurs within a belt. The subgrooved unit is, like the grooved unit, capped by lava and the
326
G.B.M. Pedersen, J.W. Head III / Icarus 211 (2011) 316–329
Fig. 9. A schematic drawing of the chaos formation process, showing a bluish volatile-rich sediment (VBF) sandwiched between grey Elysium lavas and a deeper substrate.
The lava cap on top of the VBF inhibits volatiles from sublimating, but sublimation and shrinkage occur outside the lava capped region causing lateral transport in the VBF,
undermining the lava cap. Because of the wavy topography of the underlying Elysium lavas, the VBF varies in thickness. In areas with a thick volatile-rich substrate the
shrinkage and lateral movement will be greater in areas with a thinner volatile-rich substrate, causing differential mass movement and variable local stresses. Thus, cells of
chaos will develop in areas with a thick VBF package, because the subsurface movement is so significant that the lava cap cannot sustain the extension. The lava cap breaks
and creates a trough, from which volatiles can evaporate causing headwards growth of the troughs. Sometimes a linear depression indicates the direction of trough
propagation as a precursor of the headwards growth (e.g., Fig. 6C and D). The volatile content is illustrated by the intensity of the blue color showing a lowering in volatile
content, where the sediment is in contact with the atmosphere allowing evaporation; the red arrows show the direction of mass movement. Because of differential substrate
movement some blocks tilt (left block diagram), whereas in other areas blocks are elevated due to the existence of a competent subsurface layer (marked by the hatched
signature in the right block diagram) and thus the lava cap stalls, whereas the surroundings subside. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
deposits of the VBF must at least have been as thick as in the area
of the grooved unit. However, the region of the sub-grooved unit
has a significantly smaller dip (Fig. 7) resulting in smaller subsurface mass movement, which can explain less distinct trough development and mesa formation.
Finally, variable VBF thicknesses as well as heterogeneities
within VBF will cause differential vertical displacement during
the downwasting of the VBF. This explains why some mesas within
Galaxias Chaos are higher than the plateau and why some blocks
seem to have stalled due to a competent subsurface layer. Differential downwasting will make the surface layer bend and tilt until the
stress is so high that the surface layer breaks (Fig. 8C). This is illustrated in the right part of the block diagram in Fig. 9, where the
hatched signature symbolizes the competent subsurface layer.
The geologic history of Galaxias Chaos is summarized in Fig. 10.
First Elysium lavas were emplaced in the Late Hesperian period in
stage A (Tanaka et al., 1992, 2005). In the Early Amazonian period
the Vastitas Borealis Formation was deposited and was later superposed by younger lavas due to continued volcanic activity (stage
B). Because of sublimation of volatiles within VBF, either due to enhanced geothermal activity or climate changes, the VBF degraded,
causing differential movement and undermining of the lava cap,
resulting in local extension and subsiding troughs. Along the
trough walls enhanced volatile evaporation caused the trough to
grow headwards, producing a jigsaw puzzle of blocks as the
troughs coalesce (stage C).
Finally after formation of the chaos, continued volcanic activity
along the NW–SE trending fractures caused modification of the western part of Galaxias Chaos, producing outflow deposits (stage D).
5. Discussion
This proposed formation model raises several important questions considering the nature of VBF; what mechanisms controlled
the sublimation of volatiles? And do other examples of similar
stratigraphic relationships exist both in the region of Elysium
and elsewhere?
The role of the VBF in the history of the martian hydrologic cycle is crucial, especially with respect to resolving mechanisms of
deposition and later modification, linking the present day morphology to its origin. This work stresses the importance in estimating the amount of H2O deposited and sublimated as well as
understanding the nature of the VBF. This is particularly important
in this study, because of the morphologies that developed during
volatile loss, not only within the VBF, but also in the marginal
areas, where the VBF might be superposed by other units. Kreslavsky and Head (2002) modeled the evolution of outflow channel
effluents and proposed that after emplacement, intense sublimation and cooling, outflow effluents would freeze up solid, sublime
and in the end a residual ice-rich sediment producing VBF would
be left for later modification.
The porosity on Mars is considered to decay significantly slower
than on Earth and Clifford (1993) calculated that a Moon-like crust
scaled to martian conditions would have a porosity decay constant
of 2.82 km. Thus, assuming that the lithostatic pressure is the only
controlling factor on the porosity in the VBF, the porosity would be
97%, 93% and 90% of its initial porosity in 100 m, 200 m and 300 m
depth, respectively. Basically, this means that if the VBF is an ocean
deposit containing lots of very fine grained material, and with H2O
G.B.M. Pedersen, J.W. Head III / Icarus 211 (2011) 316–329
Fig. 10. Formation model for Galaxias Chaos. (A) Emplacement of Elysium lava. (B)
Deposition of Vastitas Borealis Formation (beige unit) superposed by new lava flows
from Elysium. (C) Substrate movement due to volatile loss causes local extension in
the overlaying lava cap, creating subsiding troughs. The trough formation enhances
the volatile loss along the trough walls and therefore the troughs grow and coalesce
producing a jigsaw puzzle of blocks. (D) Later volcanic activity along the NW–SE
trending fractures caused modification of the western part of Galaxias Chaos.
filling up the pore space, immense amounts of volatiles would have
been present in the deposits. Moreover, freezing such a fine
grained material would certainly result in cryosuction, for which
reason ice segregation would produce massive ice lenses of ground
ice producing stratified or veined sequences of sediment and ice
(Clifford and Parker, 2001). Therefore, it is reasonably to believe
that the volatile content in the VBF would be sufficient for shrinking the VBF to a degree, where it would undermine a lava cap producing troughs; moreover, the heterogeneous distribution of ice
lenses would facilitate differential subsidence.
Another interesting question is: How was the degradation of the
VBF below the lava cap initiated? Obviously, where the VBF was in
contact with the atmosphere, an ice-rich sediment under current
martian condition would sublime and cause shrinkage. The question, however, is whether this downwasting was significant enough to cause downward and lateral movement within the lavacapped part of the VBF initiating the trough formation, which
would facilitate further volatile escape, or whether enhanced geothermal flux was essential. Gulick (1998) showed that in volcanic
regions with intrusions greater than several hundreds of cubic kilometers, hydrothermally derived fluids can melt through 1–2 km
thick permafrost in several thousand years, and since Galaxias
Chaos is associated with Elysium Mons and Hecates Tholus, it is
clear that a heat source for melting ground ice is available in the
region. However, it is very likely that there will be no geomorphic
difference, whether Galaxias Chaos originates purely due to climatically sublimed volatiles or whether enhanced geothermal flux existed, and it is questionable if mineralogical data from orbit would
provide any evidence. Thus, the most plausible way to constrain
327
this problem would be to estimate the amount of volatiles deposited in VBF and the amount of sublimed volatiles estimating that
the vertical displacement of the exposed VBF would be sufficient
to cause a gravitational subsurface flow in the region of Galaxias
Chaos.
If one suggests that the stratigraphic relationship between the
VBF and Elysium lavas is controlling the formation of Galaxias
Chaos, an import question to raise is whether chaos formation occurs elsewhere on the edge of Elysium Rise. From the proposed formation model, one would expect to find chaos, where the VBF and
Elysium lavas are interbedded, and from the geologic map made by
Tanaka et al. (1992, 2005) this contact between VBF and Elysium
lavas can be traced from west of Hecates Tholus along the boundary of Elysium Rise to the western part of Elysium Rise. However,
the later Amazonian outflow channels have modified this contact
heavily to the northwest of Elysium Rise, and therefore the western
part of Elysium Rise is the only area where there might be remnants of chaos formation.
Fig. 11 displays chaotic terrains found on the western flank of
Elysium and the similarities with Galaxias Chaos are striking. Skinner and Tanaka (2007) ascribed these terrains to fractured rises
originating from laccolith-like intrusions, but because of the similar morphology and because they are situated approximately at the
same elevation, they are here suggested to have formed the same
way as Galaxias Chaos. The chaotic terrains can be seen on
Fig. 11A as red areas, where the contour lines 3500 m, 3600 m
and 3700 m are marked by orange, blue and green, respectively.
Some chaotic terrains on the western flank are isolated features
and seem to have been partly submerged by later lava flows
(Fig. 11B and C), which might explain why some of the chaotic terrains are situated at 3400 m height. A similar stratigraphic relationship has been reported by Ivanov and Head (2003), where a
zone of plateau breakup of young lava flows that have been superimposed onto the surface of a volatile-rich substratum has been reported along the contact between Syrtis Major and VBF in the
transition zone between Syrtis to Isidis. Therefore, it reasonable
to believe that the formation process of Galaxias Chaos is relevant
to several transition zones producing chaotic terrains significantly
different from the chaos regions in Valles Marineris and in the region of Chryse outflow channels.
6. Conclusion
Galaxias Chaos deviates significantly from other chaotic regions
due to the lack of associated outflow channels, lack of large elevation differences between the chaos and the surrounding terrain,
and due to gradual trough formation. A sequence of troughs in different stages is detected, and examples of closed troughs within
blocks suggest that the trough formation is governed by a local
stress field rather than a regional stress field. The geomorphic evidence suggests that Galaxias Chaos is capped by Elysium lavas,
which superpose an unstable subsurface layer that causes chaotic
tilting of the blocks as well as trough formation. Based on regional
mapping we here suggest a formation model where the unstable
subsurface material, which is interbedded with Elysium lavas, is
the Vastitas Borealis Formation, and which due to gradual volatile
loss, causes shrinkage and differential substrate movement. This
process would result in local extension and depression formation,
ending with trough development and trough coalescence producing a jigsaw puzzle of blocks. Observations of chaos regions to
the west of Elysium Rise indicate that this process might have been
widespread along the contact between the Vastitas Borealis Formation and Elysium lavas, but probably has been covered to the NW
of the Elysium Rise by Elysium/Utopia flows, and has partly been
submerged by younger lavas to the west.
328
G.B.M. Pedersen, J.W. Head III / Icarus 211 (2011) 316–329
Fig. 11. (A) Distribution of chaos terrain in the Elysium Volcanic Province. Chaos is marked as red areas and the elevations; 3700 m, 3600 m and 3500 m are marked by
the green, blue and orange contour lines, respectively. Chaotic terrain is observed approximately in the same elevation along the northern transition zone and the western
transition zone between Elysium Rise and the Utopia Basin. (B) Zoom in on the chaotic terrain on the western flank of Elysium, which seems to be partly submerged by later
lava flows. THEMIS IR mosaic. (C) Chaotic terrain in the western part of Elysium with a similar polygonal jigsaw pattern as observed in Galaxias Chaos. The rough surface
texture is associated with flow fronts and the troughs show rounded head scarps like Galaxias Chaos. CTX-image P010013431967. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
Acknowledgments
Thanks are extended to James Dickson and Caleb I. Fassett from
Brown Planetary Geoscience Group for their very valuable help in
GIS mapping and data processing. Moreover, we thankfully
acknowledge Per Noernberg and the rest of the Mars Simulation
Laboratory staff at Aarhus University as well as Sandra Schumacher
and an anonymous reviewer for their improvement of the manuscript. Furthermore, we appreciate the efforts of the HiRISE, CTX,
HRSC, MOLA, MOC and THEMIS teams to make their data available.
References
Allen, C.C., 1979. Volcano–ice interactions on Mars. J. Geophys. Res. 84, 8048–8059.
Cabrol, N.A., Grin, E.A., Dawidowicz, G., 1997. A model of outflow generation by
hydrothermal underpressure drainage in volcano-tectonic environment,
Shalbatana Vallis (Mars). Icarus 125, 455–464.
Carr, M., 1978. Formation of martian flood features by release of water from
confined aquifers. NASA Technical Memorandum 79729, 260–262.
Carr, M., 1979. Formation of martian flood features by release of water from
confined aquifers. J. Geophys. Res. 84, 2995–3007.
Carr, M., Schaber, G.G., 1977. Martian permafrost features. J. Geophys. Res. 82 (28),
4039–4054.
Carr, M., Head, J.W., 2003. Oceans on Mars: An assessment of the observational
evidence and possible fate. J. Geophys. Res. 108 (E5). doi:10.1029/
2002JE001963.
Cave, J.A., 1993. Ice in the northern lowlands and southern highlands of Mars and its
enrichments beneath the Elysium lavas. J. Geophys. Res. 98, 11079–11097.
Chapman, M.G., 1994. Evidence, age, and thickness of a frozen Paleolake in Utopia
Planitia, Mars. Icarus 109, 393–406.
Chapman, M.G., Tanaka, K.L., 2002. Related magma–ice interactions: Possible
origins of Chasmata Chaos, and surface materials in Xanthe, Margaritifer, and
Merdiani Terrae, Mars. Icarus 155, 324–339. doi:10.1006/icar.2001.6735.
Christiansen, E.H., 1989. Lahars in the Elysium region of Mars. Icarus 17, 203–206.
Christiansen, E.H., Greeley, R., 1981. Mega-lahars in the Elysium region, Mars. Lunar
Planet. Sci. XII, 138–140 (abstract).
Christiansen, E.H., Hopler, J.A., 1986. Geomorphic evidence for subsurface volatile
reservoirs in the Elysium Region of Mars. Lunar Planet. Sci. XVII, 125–126
(abstract).
Clifford, S.M., 1993. A model for the hydrologic and climatic behavior of water on
Mars. J. Geophys. Res. 98 (E6), 10973–11016.
Clifford, S.M., Parker, T.J., 2001. The evolution of the martian hydrosphere and its
implications for the fate of a primordial ocean. Icarus 154, 40–79. doi:10.1006/
icar.2001.6671.
De Hon, R.A., 1992. Polygenetic origin of Hrad Vallis region of Mars. Lunar Planet.
Sci. XXII, 45–51 (abstract).
De Hon, R.A., Mouginis-Mark, P.J., Brick, E.E., 1999. Geologic map of the
Galaxias Quadrangle, MTM 35217 of Mars. Atlas of Mars, US Geological
Survey.
De Pablo, M.A., Komatsu, G., 2007. Pingo fields in the Utopia Basin, Mars: Geological
and climatic implications. Lunar Planet. Sci. XXXVIII, 1278 (abstract).
Fassett, C.I., Head, J.W., 2006. Valleys on Hecates Tholus, Mars: Origin by basal
melting of summit snowpack. Planet. Space Sci. 54, 370–378. doi:10.1016/
j.pss.2005.12.011.
Feldman, W.C. et al., 2002. Global distribution of neutrons from Mars: Results from
Mars Odyssey. Science 297, 75–78. doi:10.1126/science.1073541.
Glotch, T.D., Christensen, P.R., 2005. Geologic and mineralogical mapping of Aram
Chaos: Evidence for water-rich history. J. Geophys. Res. 110. doi:10.1029/
2004JE002389.
Greeley, R., Guest, J.E., 1987. Geologic map of the eastern equatorial region of Mars.
US Geol. Surv. Misc. Invest. Ser. Map I-1802-B.
Gulick, V.C., 1998. Magmatic intrusions and a hydrothermal origin for fluvial valleys
on Mars. J. Geophys. Res. 103, 19365–19387.
Hall, J.L., Solomon, S.C., Head, J.W., 1986. Elysium region, Mars: Tests of lithospheric
loading models for the formation of tectonic features. J. Geophys. Res. 91 (B11),
11377–11392.
Hauber, E., Gasselt, S.V., Ivanov, B.A., Werner, S.C., Head, J.W., Neukum, G., Jaumann,
R., Greeley, R., Mitchell, K.L., Muller, J.P., and The HRSC Co-Investigator Team,
2005. Discovery of flank caldera and very young glacial activity at Hecates
Tholus, Mars. Nature 434, 356–361. doi:10.1038/nature03423.
Head, J.W., Wilson, L., 2002. Mars: A review and synthesis of general environments
and geological settings of magma–H2O interactions. In: Smellie, J.L., Chapman,
M.G. (Eds.), Volcano–Ice Interaction on Earth and Mars. Geological Society,
London, pp. 27–57.
Head, J.W., Hiesinger, H., Ivanov, M.A., Kreslavsky, M., Prat, S., Thomson, B.J., 1999.
Possible ancient oceans on Mars: Evidence from Mars Orbiter Laser Altimeter
Data. Science 286, 2134–2137. doi:10.1126/science.286.5447.2134.
G.B.M. Pedersen, J.W. Head III / Icarus 211 (2011) 316–329
Head, J.W., Mustard, J.W., Kreslavsky, M., Milliken, R.E., Marchant, D.R., 2003. Recent
ice ages on Mars. Nature 426, 797–802. doi:10.1038/nature02114.
Hoffmann, H., 2000. White Mars: A new model for Mars’ surface and atmosphere
based CO2. Icarus 146, 326–342. doi:10.1006/icar.2000.6398.
Ivanov, A.M., Head, J.W., 2003. Syrtis Major and Isidis Basin contact: Morphological
and topographic characteristics of Syrtis Major lava flows and material of the
Vastitas Borealis Formation. J. Geophys. Res. 108 (E6), 5063. doi:10.1029/
2002JE001994.
Kargel, J.S., Strom, R.G., 1992. Ancient glaciation on Mars. Geology 20, 3–7.
Kargel, J.S., Baker, V.R., Begét, J.E., Lockwood, J.F., Péwé, T.L., Shaw, J.S., Strom, R.G.,
1995. Evidence of ancient continental glaciation in the martian northern plains.
J. Geophys. Res. 100 (E3), 5351–5368.
Komatsu, G., Kargel, J.S., Baker, V.R., Strom, R.G., Ori, G.G., Mosangini, C., Tanaka, K.L.,
2000. A chaotic terrain formation hypothesis: Explosive outgas and outflow by
dissociation of clathrate on Mars. Lunar Planet. Sci. XXXI, 1434 (abstract).
Kreslavsky, M., Head, J.W., 2000. Kilometer-scale roughness of Mars: Results from
MOLA data analysis. J. Geophys. Res. 105 (E11), 26695–26712. doi:10.1029/
2000JE001259.
Kreslavsky, M., Head, J.W., 2002. Fate of outflow channel effluent in the northern
lowlands of Mars: The Vastitas Borealis Formation as a sublimation residue
from frozen pounded bodies of water. J. Geophys. Res. 107 (E12). doi:10.1029/
2001JE001831.
Meresse, S., Costard, F., Mangold, N., Masson, P., Neukum, G., 2008. Formation and
evolution of the chaotic terrains by subsidence and magmatism: Hydraotes
Chaos, Mars. Icarus 194 (2), 487–500. doi:10.1016/j.icarus.2007.10.023.
Milton, D.J., 1974. Carbon dioxide hydrate and floods on Mars. Science 183, 654–656.
Morgenstern, A., Hauber, E., Reiss, D., Gasselt, S.V., Grosse, G., Schirrmeister, L., 2007.
Deposition and degradation of a volatile-rich layer in Utopia Planitia and
implications for climate history on Mars. J. Geophys. Res. 112. doi:10.1029/
2006JE002869.
Morris, A.R., Mouginis-Mark, P.J., 2006. Thermally distinct craters near Hrad Vallis,
Elysium Planitia, Mars. Icarus 180, 335–347. doi:10.1016/j.icarus.2005.09.015.
Mouginis-Mark, P.J., 1985. Volcano/ground ice interactions in Elysium Planitia,
Mars. Icarus 64, 265–284.
Mouginis-Mark, P.J., Wilson, L., Head, J.W., Brown, S.H., Hall, J.L., Sullivan, K.D., 1984.
Elysium Planitia, Mars: Regional geology, volcanology and evidence for
volcano–ground ice interactions. Earth Planet. Sci. Lett. 30, 149–173.
Mustard, J.W., Cooper, C.D., Rifkin, M.K., 2001. Evidence for recent climate change
on Mars from the identification of youthful near-surface ground ice. Nature 412,
411–414. doi:10.1038/35086515.
Nummedal, D., Prior, D.B., 1981. Generation of martian chaos and channels by
debris flows. Icarus 45, 77–86.
Parker, T.J., Gorsline, D.S., Saunders, R.S., Pieri, D.C., Schneeberger, D.M., 1993.
Coastal geomorphology of the martian northern plains. J. Geophys. Res. 98,
11061–11078.
Pedersen, G.B.M., Head, J.W., 2009. Overview of possible ice-related morphologies in
the transition zone between Elysium and Utopia Basin, Mars. Lunar Planet. Sci.
XXXX, 2081 (abstract).
Plescia, J.B., 1986. Tectonics of the Elysium region, Mars. Lunar Planet. Sci. XVII,
672–673 (abstract).
Reimers, C.E., Komar, P.D., 1979. Evidence for explosive volcanic density currents on
certain martian volcanoes. Icarus 39, 88–110.
Rodriguez, J.A.P., Sasaki, S., Kuzmin, R.O., Dohm, J.M., Tanaka, K.L., Miyamoto, H.,
Kurita, K., Komatsu, G., Fairén, A.G., Ferris, J.C., 2005. Outflow channel sources,
reactivation, and chaos formation, Xanthe Terra, Mars. Icarus 175, 36–57.
doi:10.1016/j.icarus.2004.10.025.
329
Rodriguez, J.A.P., Kargel, J.S., Crown, D.A., Bleamaster III, L.F., Tanaka, K.L., Baker,
V.R., Miyamoto, H., Dohm, J.M., Sasaki, S., Komatsu, G., 2006. Headward
growth of chasmata by volatile outburst, collapse, and drainage: Evidence
from Ganges chaos, Mars. Geophys. Res. Lett. 33. doi:10.1029/
2006GL026275.
Russell, P.S., Head, J.W., 2001a. The Elysium/Utopia flows: Characteristics from
topography and a model of emplacement. Lunar Planet. Sci. XXXII. Abstract
1040.
Russell, P.S., Head, J.W., 2001b. The Elysium/Utopia flows: Evidence for release of
confined groundwater in the context of a global cryosphere–hydrosphere
system. In: Conference on the Geophysical Detection of Subsurface Water on
Mars. Abstract 7052.
Russell, P.S., Head, J.W., 2002. Unusual dendritic Ridged Terrain in Utopia Planitia:
Dewatering of Megalahars? Lunar Planet. Sci. XXXIII. Abstract 2032.
Russell, P.S., Head, J.W., 2003. Elysium–Utopia flows as mega-lahars: A model of
dike intrusion, cryosphere cracking, and water-sediment release. J. Geophys.
Res. 108, 18-11–18-28. doi:10.1029/2002JE001995.
Russell, P.S., Head, J.W., 2007. The martian hydrologic system: Multiple recharge
centers at the large volcanic provinces and the contribution of snowmelt to
outflow channel activity. Planet. Space Sci. 55, 315–332. doi:10.1016/
j.pss.2006.03.010.
Schaber, G.G., Carr, M., 1977. Martian permafrost features. J. Geophys. Res. 82,
4039–4054.
Scott, D.H., Carr, M., 1978. Geologic map of Mars. US Geol. Surv. Misc. Invest. Ser., M.
I-1083.
Sharp, R.P., 1973. Mars: Fretted and chaotic terrains. J. Geophys. Res. 78, 4073–
4083.
Skinner, J.A., Tanaka, K.L., 2001. Regional emplacement history of the Utopia and
Elysium Plains deposits, Mars. Lunar Planet. Sci. XXXII. Abstract 2154.
Skinner, J.A., Tanaka, K.L., 2007. Evidence for implications of sedimentary diapirism
and mud volcanism in the southern Utopia highland–lowland boundary. Icarus
186, 41–59. doi:10.1016/j.icarus.2006.08.013.
Soare, R.J., Kargel, J.S., 2007. Thermokarst processes and the origin of crater-rim
gullies in Utopia and western Elysium Planitia. Icarus 191 (1), 95–112.
doi:10.1016/j.icarus.2007.04.018.
Soare, R.J., Osinski, G.R., 2007. Periglacial evidence (Using HiRISE, MOC and THEMIS
Imagery) in Utopia and western Elysium Planitia, for a recent wet and warm
Mars. Lunar Planet. Sci. XXXVIII. Abstract 1440.
Soderblom, L.A., Wenner, D.B., 1978. Possible fossil H2O liquid–ice interfaces in the
martian crust. Icarus 34, 622–637.
Tanaka, K.L., 1999. Debris-flow origin for Simud/Tiu deposits on Mars. J. Geophys.
Res. 104, 8637–8652. doi:10.1029/98JE02552.
Tanaka, K.L., Chapman, M.G., Scott, D.H., 1992. Geologic map of the Elysium region
of Mars. US Geol. Surv. Misc. Invest. Ser., Map I-2147.
Tanaka, K.L., Skinner, J.A., Hare, T.M., 2005. Geologic map of the northern plains of
Mars. US Geol. Surv. Misc. Invest. Ser. Map I-2811.
Wang, C.Y., Manga, M., Wong, A., 2005. Floods on Mars released from groundwater
by impact. Icarus 175, 551–555. doi:10.1016/j.icarus.2004.12.003.
Wang, C.Y., Manga, M., Hanna, J.C., 2006. Can freezing cause floods on Mars?
Geophys. Res. Lett. 33, L20202. doi:10.1029/2006GL027471.
Wilson, L., Mouginis-Mark, P.J., 2003. Phreatomagmatic explosive origin of Hrad
Vallis, Mars. J. Geophys. Res. 108 (E8), 1–16. doi:10.1029/2002JE001927. 1-1.
Zegers, T.E., Oosthoek, J.H.P., Rossi, A.P., Blom, J.K., Schumacher, S., 2010. Melt and
collapse of buried water ice: An alternative hypothesis for the formation of
chaotic terrains on Mars. Earth Planet Sci. Lett. 297 (3–4), 496–504.
doi:10.1016/j.epsl.2010.06.049. ISSN 0012-821X.