UNIVERSITY OF GOTHENBURG
Department of Earth Sciences
Geovetarcentrum/Earth Science Centre
Observations of Putative
Freeze-thaw Landforms on
Mars´ Northern Hemisphere
Distribution and Aspect Dependence of
Small-Scale Lobes on Interior Crater Walls
Elin Nyström
ISSN 1400-3821
Mailing address
Geovetarcentrum
S 405 30 Göteborg
Address
Geovetarcentrum
Guldhedsgatan 5A
B848
Bachelor of Science thesis
Göteborg 2015
Telephone
031-786 19 56
Telefax
031-786 19 86
Geovetarcentrum
Göteborg University
S-405 30 Göteborg
SWEDEN
Abstract
Discoveries within the last decade have shown that the orbital and rotational motions of Mars
strongly impact the Martian climate. This knowledge has led to the interpretation that the
present hyper arid and cold conditions are not necessarily representative of the late
Amazonian climate. Evidence for recent water related activity on Mars has been revealed by a
number of missions such as Mars Global Surveyor (MGS) and Mars Reconnaissance Orbiter
(MRO). In addition, recent studies have found numerous putative freeze-thaw landforms
called small-scale lobes on the Martian surface. These lobes show striking geomorphologic
resemblance to solifluction lobes on Earth. If these findings are correct it would imply an
active layer formation in recent geologic history on Mars.
In this thesis images acquired by the High Resolution Imaging Science Experiment (HiRISE)
onboard MRO have been surveyed for small-scale lobes in craters located in the latitude-band
of 55°–80° north. The study shows that small-scale lobes are even more widespread in the
northern hemisphere of Mars than previously thought. Furthermore, the possibility of aspect
dependence as a function of latitude has been investigated. The results reveal that small-scale
lobes on the northern mid-latitudes show a preference for the pole-facing crater slope,
whereas the lobes in the northern high-latitude craters show a preference for the equator- and
west-facing slopes. In addition, the study reveals that small scale lobes exhibit a close spatial
relationship to polygonal pattern, gullies and stripe-like patterns.
Keywords: Mars, Mars climate, Periglacial, Active layer, Solifluction
Sammanfattning
En planets omloppbana och rotationella rörelser har stor påverkan på dess klimat. Mars är
utsatt för kraftiga variationer i axial lutning och excentricitet och som följd tror man inte att
det nuvarande torra och kalla klimatförhållandet på Mars är representativ för planetens
klimatologiska historia. Till exempel har ett stort antal studier visat på omfattande spår av
hydrologisk aktivitet på Mars. I ett antal nya geomorfologiska studier har man dessutom
funnit landformer som indikerar att det i dagsläget finns eller nyligen har funnits ett aktivt
lager på Mars.
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I denna kandidatavhandling har högupplösta bilder tagna av High Resolution Imaging Science
Experiment (HiRISE) studerats. Småskaliga lobstrukturer på kratersluttningar mellan 55-80°
N, har inventerats och studerats. Studien visar att småskaliga lober är mer utbredda på Mars
norra halvklot än man tidigare trott. Vidare har lobernas sluttningsriktnings-beroende som en
funktion av latitud undersökts. Resultatet visar att loberna på mellanliggande breddgrader (5565° N) företrädelsevis är lokaliserade på pol-vända kraterslutnningar, medan loberna på högre
latituder (65-80° N) till största del är lokaliserade på ekvator-vända sluttningar samt
sluttningar vända mot väst. Dessutom visar studien att småskaliga lober förekommer i nära
rumslig relation till polygoner, raviner och stripes (linjära mönster).
Nyckelord: Mars, Mars klimat, Periglacial, Aktivt lager, Solifluction
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Table of Contents
1 Introduction to Thesis.............................................................................................................. 6
1.1 Thesis Outline ................................................................................................................... 6
1.2 Terrestrial Analogs ........................................................................................................... 7
1.3 Aim and Objectives .......................................................................................................... 8
2 Introduction to Mars ................................................................................................................ 8
2.1 General Facts of Mars....................................................................................................... 8
2.2 Martian Geologic History ............................................................................................... 10
2.3 Climate Change on Mars ................................................................................................ 11
2.3.1 Orbit Eccentricity..................................................................................................... 11
2.3.2 Obliquity .................................................................................................................. 11
2.4 Water and Ice on Mars.................................................................................................... 12
3 Periglacial Environment on Earth and Mars ......................................................................... 15
3.1 Active Layer, Permafrost and Ground Ice ...................................................................... 15
3.2 Mass Wasting Landforms ............................................................................................... 17
3.2.1 Solifluction ............................................................................................................... 17
3.2.2 Gullies ...................................................................................................................... 21
3.3 Patterned Ground ............................................................................................................ 22
3.3.1 Thermal Contraction Polygons ................................................................................ 22
3.3.2 Stripes and Nets ....................................................................................................... 24
3.4 Rock glaciers .................................................................................................................. 25
4 Method and Resources .......................................................................................................... 27
4.1 Method ............................................................................................................................ 27
4.2 Resources ........................................................................................................................ 27
5 Result and Observations ........................................................................................................ 28
5.1 Inventory......................................................................................................................... 28
5.2 Aspect Dependence ........................................................................................................ 41
6 Discussion ............................................................................................................................. 44
6.1 Method and Uncertainties ............................................................................................... 44
6.2 Small-scale lobes and their spatial and temporal relationship to polygons, stripe-like
patterns and gullies. .............................................................................................................. 44
6.3 Aspect dependence of small-scale lobes on Mars as a function of latitude. .................. 48
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7 Conclusions ........................................................................................................................... 49
8. Further investigations ........................................................................................................... 50
Acknowledgement .................................................................................................................... 50
References ................................................................................................................................ 51
Appendix .................................................................................................................................. 55
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1 Introduction to Thesis
Mars is the most Earth-like planet in our Solar system and it likely had global hydrologic
conditions in its ancient past. Early evidence of past liquid water comes from observations of
dried-out river beds and paleolakes by the Viking Orbiters (Carr, 2006). More recently the
Mars Global Surveyor and Mars Reconnaissance Orbiter missions found ample evidence for
recent fluvial activity in the form of gullies (Malin and Edgett, 2000) and spectral evidence of
ancient clay minerals (Poulet et al., 2005). Furthermore, the Curiosity rover has found
evidence that early Mars was habitable (Grotzinger et al., 2014). Taken together these
findings bear important implications for possible extant or past life on Mars. The discovery of
extremophiles on Earth; bacteria that lives and thrives in extreme environments, has
broadened our understanding of the prerequisites for life. Permafrost environments on Earth
are one of the extreme environments known to host life (e.g. Gilichinsky, 2002; Steven et al.,
2007). Mars may be considered a permafrost planet and possibly shares some characteristics
with the cold regions on Earth. Mars is therefore a prime target for the search for possible
exobiological life. The search for life on Mars includes getting an understanding of the
climate and geology. Topics of study have therefore come to include the orbital and rotational
motions of the planet, the atmospheric composition and evolution, and surface mineralogy.
Additionally, the study of the planets geomorphology may help to constrain the number of
environments with a likelihood of recurrent liquid water and thus the probability of
exobiological life.
In this bachelor thesis I present a study of the geomorphologic landscape on Mars' Northern
hemisphere with a special focus on landforms indicative of freeze-thaw and melt-water
conditions.
1.1 Thesis Outline
In subsection 1.2 the use of terrestrial analogs for planetary science is introduced and a brief
description of those analogs referred to in this thesis is made. The aim and objectives of the
thesis are introduced in subsection 1.3. Section 2 and 3 contain background information,
starting with an introduction to Mars. The general facts are in addition summarized in Table
1. Furthermore, in section 2, the geologic history of Mars and the orbital and eccentricity
properties are described and in the last subsection the current and past water situation on Mars
is briefly mentioned. In section 3 a number of periglacial processes and geomorphologic
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landforms related to this study is presented, terrestrial processes and landforms as well as the
Martian counterparts are listed and described.
The method and resources used in this study is described in Section 4. In section 5 the
results and observations are presented, here only descriptive terms are used and no genesis of
the landforms is discussed. In the following section, section 6, the structures are compared to
terrestrial similar structures and the genesis is discussed. In Section 7 conclusions connected
to the aim and objectives are made based upon the research in this thesis. Finally in section 8
a number of suggestions for further investigations are made.
1.2 Terrestrial Analogs
A common strategy in planetary science, in addition to theoretical modeling, is to study
terrestrial analogs. This approach builds on the premise that the landforms of interest on Mars
share some properties (e.g. form) with known landforms on Earth (Hauber et al., 2011b).
However, caution has to be taken since a similar appearance is not necessary an evidence for
the same genesis as similar looking landforms can derive from completely different sets of
processes. This is also known as the problem of equifinality (Beven, 1996; Zimbelman,
2001). Common analog environments to Mars are e.g. Antarctica (e.g. Marchant & Head III,
2007) and Svalbard (Norway) (e.g. Hauber et al., 2011b; Johnsson et al., 2012) as these
landscapes both exhibit a dry and cold climate similar to the condition on Mars. The
landscape of Svalbard is characterized by glaciers, permafrost and ground-ice related features
(Hauber et al., 2011b). In addition Svalbard is easily accessible and the landforms of interest
occur in close spatial proximity to each other. The possibility to study terrestrial landform
assemblages which are located in a similar manner as on Mars is an important factor why it
functions as a good analog to Mars. It also helps to decrease the level of equifinality (Hauber
et al., 2011a).
Studies of high-resolution images of Mars have revealed landforms similar to those in
terrestrial cold climate areas. Many of these landforms are latitude dependent and produced
by freeze-thaw activity, meaning that they may serve as climate indicators. Among the
studied landforms on Mars are small-scale lobate landforms on hill slopes and crater slopes in
mid-to high-latitudes. Based partly on the knowledge obtained from studies of similar
landforms on Svalbard and Alaska, these landforms have been interpreted as solifluction
lobes, indicating freeze-thaw cycles and consequently the existence of an active layer
(Mangold, 2005; Gallagher & Balme, 2011; Hauber et al., 2011a; Johnsson et al., 2012).
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1.3 Aim and Objectives
In this bachelor thesis I have investigated all craters between 55° N to 80° N that are covered
by recently released HiRISE data (data acquisition period from 2012-01-01 to 2013-12-31).
The aim is to add insight into the extent which slopes are modified by freeze-thaw processes
on the northern hemisphere and more firmly establish the latitudinal distribution that was
made by Johnsson et al. (2012). The study area of 55°N to 80°N were chosen based on the
belief that small-scale lobes are latitude dependent and more common on the mid- to highlatitudes (Johnsson et al., 2012). The aim is also to establish if an aspect dependence of the
small-scale lobate landforms exists.
The objectives of the thesis are listed below.

Catalogue craters which show evidence of slow mass wasting (small-scale lobes).

Investigate and determine the distribution of small-scale lobes on crater slopes.

Investigate if a spatial relationship of lobes to gullies, polygons and stripes, such as
superposition and cross-cutting relationships exists.

Determine if small-scale lobes, on the mid-to high-latitude on the northern hemisphere
of Mars, display aspect dependence as a function of latitude.
2 Introduction to Mars
The general facts presented in section 2.1 are summarized in Table 1, where it is compared to
the corresponding terrestrial properties. For comparison between Earth's and Mars' geologic
history see Figure 2.
2.1 General Facts of Mars
Mars is the fourth planet from our sun and located outside Earth's orbit. The planet exhibit an
Earth-like structure, differentiated into crust, mantle and core. Indicated by the absence of a
magnetic field the core is most likely solid, though paleo-magnetic remnants in the crust
indicate that it was once molten (Acuna et al., 1999; Connerney et al., 2001; Carr, 2006). The
composition of the core is probably more sulfur-rich compared to Earth's core and the crust is
mainly basaltic (Carr, 2006).
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Table 1. Comparison between Mars and Earth
Mars
Earth
Size
3389.5 km
6378 km
Atmospheric pressure
6.1 mbar
1000 mbar
23
Mass
6.42x10 kg
5.97x1024 kg
Gravity
3.72 ms-2
9.81 ms-2
Mean annual temperature (MAT) -58°C
Atmospheric composition
15°C
C02: 95.3 %, N: 2.7 %, Ar: CO2: 0.04% N: 78 %, Ar:
1,6 %, O: 0.13%
0.93 %, O: 20.9%
Eccentricity
0.093
0.0167
Obliquity
25.19°
23.44°
Duration of a day
24h 39m 35s
24h
Duration of a year
687 Earh days
365.25 Earth days
Moons
Phobos, Deimos
The Moon
Highest topograpic point
21.229 km
8.848 km
Lowest topographic point
-8.2 km
-10.911 km
Mars has a mean radius of 3400 km compared to Earths equatorial radius of 6400 km (Carr,
2006) and it exhibits a very distinctive dichotomy where most of the high southern
hemisphere is separated by elevation from the lower northern plains. Thus, the crust in the
southern hemisphere is much thicker compared to the north, which in turn results in much
greater topographic variations on Mars than on Earth (Figure 1). Since no natural reference
datum exist on Mars, similar to the sea level on Earth, it has been decided that the triple point
of water, where the atmospheric pressure is 6.1 mbar, will function as the Martian
topographic reference datum (Carr, 2006). The lowest topographic point, by that reference, is
located at Hellas basin at -8.2 km altitude and the highest topographic point is the 21.2 km
high Olympus Mons summit. The formation of the dichotomy is under discussion, one
candidate hypothesis is the formation by an oblique impact during the pre-Noachian time
period, thus it could be the oldest recorded geologic event on Mars (Carr & Head, 2010).
The lack of a magnetic field and the low gravity (3.72 ms-2) on Mars results in a thin
atmosphere which is subjected to solar wind erosion (Forget et al., 2007). The surface
pressure is consequently very low, only 6.1 mbar, which is approximately 200 times lower
than at Earth's surface (Balme et al., 2013).
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The atmosphere of Mars consists of 95 % CO2, and what characterizes the present day
climate on Mars is a low mean annual temperature (MAT) of -58° C at the equator (Owen et
al., 1977; Carr, 2006). During winter at high-latitudes the temperature drops below the frost
point for CO2 which result in condensation and formation of dry-ice caps (Balme et al., 2013).
Figure 1. Map showing the topography of Mars and the transition of highland to lowland. For more
information see the original map by Tanaka et al. (2014).
2.2 Martian Geologic History
The geologic history of Mars is divided into four major time periods (Figure 2). This division
is based on crater density calculations and calibration to lunar rock samples and stratigraphy
(Forget et al., 2007). The current geologic time period is the Amazonian; this is the longest
time period in the Martian history. Prior to the Amazonian the Hesperian time period took
place, the boundary between the Hesperian and the Amazonian is not well established but
defined to be between ~2.9 Ga and ~3.0 Ga. Prior to the Hesperian, the Noachian (~4.1 Ga to
~3.7 Ga before present) and pre Noachian time period took place. Throughout the preNoachian time period Mars most likely exhibited a magnetic field and was heavily
bombarded by massive impacts. During the Noachian time period the Martian surface
underwent dramatic changes due to volcanism and impact cratering. In addition, like Earth
Mars most likely acquired a substantial amount of water during this time period. The
formation of valley networks and lakes bear evidence of a warmer climate and a denser
atmosphere during the Noachian. The Hesperian time period was, just like the Noachian,
characterized by volcanism and formation of flooding and lakes as well as the formation of
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the great Valles Marineris rift valley. The Amazonian on the other hand has a distinctly lower
rate of geologic activity and is mostly characterized by accumulation and movements of
glaciers and other ice deposits as well as lobate debris aprons, gullies and a high aeolian
activity (Carr & Head, 2010).
Figure 2. Geologic timescale, top: Earth, bottom: Mars.
2.3 Climate Change on Mars
2.3.1 Orbit Eccentricity
The orbit of Mars is distinctly eccentric (0.093), which means that it deviates from a perfect
circle and has an elliptical form which results in a variation of its distance from the Sun over
time. This also leads to an extreme variation in incoming sunlight, during perihelion (when
Mars is closest to the sun) the distance from the sun is 1.381 Astronomical Units (AU) and
during aphelion (when Mars is the most distant from the sun) the distance is 1.666 AU. This
means that 45 percent more sunlight reaches Mars surface at perihelion compared to aphelion
(Carr, 2006). The orbit eccentricity of Mars oscillates in two cycles, a 95-99 thousand year
(kyr) cycle and a 2.4 million year (myr) cycle (Figure 3). The consequences are variations in
length and intensity of the seasons and therefore great differences in surface temperatures and
atmospheric circulations (Carr, 2006).
2.3.2 Obliquity
In addition to the eccentricity variations, the obliquity variations, meaning the tilt of the
planets polar axis, is an especially important factor for climate variations on Mars. The
obliquity oscillates with a period of 120 kyr (Figure 3). At present the obliquity of Mars is
25.19° but it may have varied greatly during the past Martian history. Over the last 0.3 Myr
the obliquity varied between 23° and 27°. Looking at the scale of 10 Myr the obliquity of
Mars has varied in the range of 14° to 48° and on a larger time scale the variations has likely
11
been even greater (Laskar et al., 2004). The obliquity of the Earth however, is relatively stable
at 23.44° due to the presence of our Moon (Laskar et al., 1993). High obliquity affects the
atmospheric pressure and ground ice stability which in turn result in a transportation of waterice from higher to lower latitudes. Thus, the obliquity variations have been interpreted as a
possible driving mechanism for ice ages on Mars (e.g. Head et al., 2003; Forget et al., 2006;
Madeleine et al., 2009; Schon et al., 2009). Furthermore it has the implication that highlatitude pole-facing slopes may be subjected to thawing of the ground to a depth of
approximately 0.5 meter (Costard et al., 2002).
In addition, obliquity variations are believed to be responsible for the formation of the
latitude dependent mantle that drapes mid-to high-latitudes on both north and south
hemispheres on Mars. (Head et al., 2003; Kreslavsky et al., 2008). This mantle terrain is
young (~0.4–2.1 Ma) and considered to be an air-fall deposit that has been subsequently
cemented by atmospherically deposited ice.
Figure 3. This image shows the variation in obliquity, eccentricity and insolation at the North Pole
surface, on two different time scales. (a-c) Over 1 Myr; (d-f) Over 10 Myr. Modified image from
Laskar et al. (2002).
2.4 Water and Ice on Mars
The Amazonian is in comparison to the earlier time periods a modest phase with relatively
few impacts and limited fluvial and geological reworking of the surface. For instance,
observations made from high-resolution images, indicate an extensive activity in the
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periglacial environment during the Hesperian and Noachian time periods. It is likely that
surface water in these time periods was an important agent to shape the Martian surface
(Balme et al., 2013). However, at present the cold climate in combination with a low
atmospheric surface pressure results in a dry surface where water cannot exist in liquid form
and water ice is unstable at the surface. The present surface of Mars is therefore permanently
frozen and all the available water on Mars is stored in frozen reservoirs. Examples of
currently known frozen reservoirs are the polar caps, polar layered deposits, ground ice in
mid- to high-latitudes and debris covered glacier ice. However, rare conditions exist on
present day Mars that allow for water to exist at the surface. This includes e.g. low areas with
an atmospheric pressure above 6.1 mbar, low permeability soils with sufficiently high
temperatures and finally, salt-rich soils. Although the recent climate on Mars is cold and dry,
the existence of valley networks (Figure 4c) and paleo-lakes serves as evidence that liquid
water was once more widespread than today (Carr, 2006).
Figure 4. (a) Outflow channels located at 18° N, 55°W. The area shown is 225 km across. (b)
Landscape altered by water flowing past two 8-10 km diameter craters leaving behind two streamlined
islands at 20°N, 31°W (c) Warrego Vallis valley network at 42°S, 92°W may be an indication that
Mars was once warmer and wetter than it is today. The image shows an area of ~200 kilometers
across. Image credit: Water Kiefer (a) and Brian Fessler (b-c), Lunar and Planetary Institute.
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Both the north and south poles exhibit polar caps where the south cap is larger than the north
cap. The seasonal polar caps are composed of solid CO2 and both polar caps are underlain by
water ice caps. The northern water ice cap is revealed during summer when the CO2 has
sublimated (Figure 5). The South Pole however is presently, all year round covered by a CO2
cap. In addition, both poles exhibit layered deposits at their base, which have intrigued
scientist for a long time. Speculations are that the layered deposits can be used as a record for
recent climatic changes (Carr, 2006).
Figure 5. The image shows the evolution of the north polar cap from October 1996 to Mars 1997.
Hubble Space Telescope image STScI-PR97-15b. Modified image from ESA Space telescope
webpage. Image credit: P. James (Univ. Toledo), T Clancy (Space Science Institute), S. Lee (Univ.
Colorado) and NASA/ESA.
Outflow channels, valley networks, and gullies are examples of what is considered to be water
related features on Mars (Figure 4 and Figure 12). At high obliquity Mars's atmosphere is far
more water-rich than at present. Modeling has shown that, at obliquities higher than 30°,
water ice may accumulate close to the surface in the regolith. On pole-facing slopes in mid-to
high-latitudes the temperature is lower, which results in accumulation of water ice even closer
to the surface (Mellon & Jakosky, 1995). During late spring and summer, at high obliquity,
temperatures may exceed the melting point and thus release the near surface water. If this
process is quick, the soil water content may not have time to completely sublimate and may
result in the formation of gullies (Costard et al., 2002).
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3 Periglacial Environment on Earth and Mars
In this section a number of periglacial landforms and processes on Earth are briefly described.
Furthermore in each subsection possible Martian counterparts are introduced. Note that the
Martian counterparts are given descriptive terms and the genesis are still under debate,
whereas the terrestrial landforms are given genetic terms which are well established. The
terrestrial landforms and the Martian counterparts are usually very similar in appearance.
However due to the problem of equifinality, the similar appearance is not necessary an
evidence for the same origin (Beven, 1996; Zimbelman, 2001).
The term periglacial is defined as “a range of conditions, processes and landforms associated
with cold, non-glacial environments” (French, 2007b). It should in addition be subjected to
ground freeze-thaw activity and display the presence of permafrost. Topography, aspect and
slope angles are important factors concerning the existence of periglacial landforms on Earth.
This is due to the difference in incoming solar radiation with elevation and aspect, that
consequently influences air and ground temperatures, precipitation in the form of rain or
snow, and the distribution and duration of snow coverage (Ridefelt, 2009).
3.1 Active Layer, Permafrost and Ground Ice
The active layer is the soil layer that overlay the permafrost table and undergoes seasonal
thawing and freezing with temperatures that exceed 0 °C (Figure 6) (van Everdingen, 2005).
The thickness of terrestrial active layers vary in the range of a few centimeters to a few
meters, it mainly depends on the ground surface temperature, vegetation, soil moisture, snow
cover, altitude and sun radiation among other things (French, 2007d).
On Mars, it has been suggested that the existence of an active layer would not have been
possible on a global scale within the last 5 Ma. However, according to Kreslavsky et al.
(2008) an active layer could form at high-latitudes and on pole-facing slopes at mid-latitudes
during high obliquity. The presence of an active layer is indicated by geomorphologic
observations of similar looking landforms to terrestrial active layer landforms. Thus, an active
layer may have been present in the recent past.
For ground to be defined as permafrost the temperature should be 0°C or below for a
minimum of two subsequent years. The water content however is not important (van
Everdingen, 2005). Since the presence of salts or higher pressure can cause the freezing point
of water to be lower than 0 °C, the definition of permafrost does not necessary require that the
15
ground is frozen (Hauber et al., 2011b). Ground ice on the other hand refers to the ice within
frozen ground. On Mars, a typical landscape feature is scalloped terrain which is linked to
thawing or sublimation of ground ice (Lefort et al., 2009) and to thermal contraction cracking
(Zanetti et al., 2010). Scalloped terrain is often characterized by a steep pole-facing slope
(scarp) and a more gentle equator-facing slope (Lefort et al., 2009) and show close
resemblance to some thermokarst landforms on Earth (Ulrich et al., 2010).
When a freezing front propagates down toward the permafrost table through the active layer,
in some cases it reaches the table and in other situations it does not reach all the way down. It
results in a liquid water soil in between the freezing front and the permafrost table, which is
called talik (Figure 6d). Different types of permafrost and active layers are presented in Figure
6. Selected landforms and processes related to active layer, permafrost and ground ice are
listed and described in the following sections.
Figure 6. Illustration of different permafrost and active layer configurations. (a) No active layer is
present and temperature is below the freezing point of water. (b) An active layer is present. Surface
temperatures can exceed the freezing point of water resulting in a thawing front that propagates down.
(c) Surface temperature is below the freezing point of water and a freezing front propagates
downward. Meanwhile a freezing front propagates upward from the permafrost table resulting in a
frozen active layer. (d) An active layer with talik present. Image credit: Kreslavsky et al. (2008).
16
3.2 Mass Wasting Landforms
3.2.1 Solifluction
Solifluction is a result of repeated freeze-thaw activity and occur within the active layer. The
term usually refers to both slow mass wasting (generally less than 1 m year-1) including freeze
and thaw conditions, and saturated soil movement on slopes (Matsuoka, 2001). The process is
widespread in cold and periglacial environments and has a great effect on shaping the
landscape in these areas on Earth (Matsuoka, 2001) and most likely also on Mars (Matsuoka,
2001; Balme et al., 2013; Barrett et al., 2013).
For solifluction to occur, the presence of liquid water at grain boundaries is necessary to
activate the movement. Thus, it is common in soils with a high sand and silt content.
Solifluction can be categorized into needle ice creep, frost creep, gelifluction and plug-like
flow (Figure 7).
Figure 7. (a) Illustration of needle ice creep, a shallow and relatively rapid process that occurs during
thawing. (b) Diurnal frost creep, also shallow but not equally as rapid as needle ice creep, (c) Annual
frost creep or gelifluction affects a deeper layer, (d) Plug-like flow affects to a greater depth and
generally move equally fast in the upper and lower part. Modified image from Matsuoka (2001).
17
Needle ice creep occurs when surface debris is lifted by ice needles and later topples during
thaw; it is a shallow and relatively rapid process. The process known as frost creep include
frost heaving acting perpendicular to a slope in combination with vertical or near-vertical
thaw consolidation, which result in net movement of soil particles downslope. Frost creep can
be subcategorized into diurnal or annual, which represents the duration of a freeze-thaw cycle.
Diurnal frost creep often affects the upper few centimeters and the annual frost creep usually
affects a few decimeters of soil. The term gelifluction represent the elasto-plastic deformation
of saturated or near-saturated soil that occurs due to thawing of seasonally frozen ground
and/or additional inflow of water from snowmelt or rainfall. Plug-like flow represents the
movement of the whole active layer during thawing due to the formation of ice lenses near the
base of the active layer. This type of solifluction occurs in cold permafrost areas (Matsuoka,
2001).
Figure 8. Schematic image of slope influenced by solifluction. Image credit: Ballantyne and Harris
(1993)
The process of solifluction often results in characteristic lobate or sheet-like landforms
(Figure 8Figure 9). The lobes consist of a lobe tread and a front riser (Figure 10) that can be
either sorted or non-sorted. Sorted lobes refers to those that display vertical or lateral sorting
of clasts, a product of a faster up-freeze and lateral sorting of coarse sediment than the
downslope movement (Johnsson et al., 2012). This sorting often results in a riser with a
18
higher concentration of larger clasts and a tread with finer material (Figure 9), which is why
these lobes on Earth are sometimes referred to as stone-banked. Stone-banked lobes are most
common in Antarctica and high mountainous areas (Johnsson et al., 2012). Non-sorted lobes,
more often referred to as turf-banked lobes, lack a vertical frost sorting and are usually found
in soils with high moisture content. They are believed to develop where the rate of the
downslope movement are decreased with a subsequent thickening of the soil. In addition the
non-sorted type of lobes are common in vegetated soils, due to the restraining effect of the
vegetation (Benedict, 1976).
As a result of the widespread distribution of solifluction lobes and its dependence of climatic
fluctuations, they can be used to reconstruct paleoclimatic conditions (Matsuoka, 2001).
Figure 9. Example of solifluction lobes on Svalbard. Image credit: Johnsson et al. (2012).
Figure 10. Schematic image demonstrating the geometry of a typical solifluction lobe. Modified image
from (Matsuoka & Ikeda, 2005).
19
Figure 11. (A-B) Examples of non-sorted small-scale lobes and (C-D) sorted small-scale lobes on the
northern hemsiphere of Mars. Modified image from Johnsson et al. (2012). Image credit:
NASA/JPL/University of Arizona.
On Mars, small-scale lobes, found mainly on crater slopes, have been interpreted to be the
result of solifluction. They are believed to be formed due to frost creep, plug-like flow and/or
gelifluction, rather then by permafrost creep, another process that may generate lobate
landforms (e.g. Matsuoka & Ikeda, 2005; Gallagher et al., 2011; Johnsson et al., 2012). This
interpretation is based partly by comparison to terrestrial solifluction lobes on Svalbard
(Figure 9) (e.g. Hauber et al., 2011a; Johnsson et al., 2012). Furthermore, the lobes are
belived to be very young landforms (~0.5-2 Ma) which contradict the previous notion of Mars
not having an active layer in the last 5 Ma (Gallagher et al., 2011; Johnsson et al., 2012). Two
differents classes of small-scale lobes are found on Mars, similar to the sorted and non-sorted
types on Earth, examples of these can be seen in Figure 11. One specific crater, located at
71.9º N 344.5º E, have been used as an example crater where small-scale lobes are found on
all slope directions, even the cold northern slope (e.g. Johnsson et al., 2012). In a recent study
this is explanied by the prescence of dark dune material in the crater, which during high
20
obliquity paints the slopes dark, resulting in a lower albedo and consequently higher
temperatures that may have created a local active layer (Kreslavsky & Head, 2014). Smallscale lobes have mainly been found on mid-to-high latitudes on Mars and as a result believed
to be latitude dependent (Johnsson et al., 2012).
3.2.2 Gullies
The definition of gullies are a "degrading - aggrading system on a slope consisting of a source
area (the alcove), an erosional/transport area (channel), and a depositional area (gully fan)"
(Reiss et al., 2011). Thus, "gully" is simply a descriptive term and say nothing about the
genesis. Terrestrial gullies are usually formed by a combination of processes which typically
depends on a variety of parameters such as water content by rainfall or melting of snow, the
presence of steep slopes and finer material and debris (Reiss et al., 2011).
The discovery of gullies on Mars in 2000 led to a hypothesis on recent water-related
activities (Malin & Edgett, 2000), they may in addition be among the youngest known
features on Mars, dated to ~200 ka in a recent study (Johnsson et al., 2014). Currently, there
seems to be consensus regarding the water as the medium involved in the formation of gullies
on Mars (e.g. Johnsson et al., 2014), however, the source of the water is under debate, as is
the process by which the gullies are formed. Some researchers state that the Martian gullies
may be formed by debris-flows (e.g. Lanza et al., 2010; Mangold et al., 2010; Conway et al.,
2011; Johnsson et al., 2014) whereas others argue that they are more likely to be formed by
fluvial deposition (e.g. Reiss et al., 2011). A proposed origin of the water is the release of
water from underground aquifers, another proposed origin is from the melting of ground ice
or top down melting of snow (Johnsson et al., 2014). The Martian gullies are mainly
distributed between 30-60° in both hemispheres though they are more widespread on the
south hemisphere. They are found on a variety of slopes where inner crater walls are the most
common; in addition they seem to show a preference of aspect and are mainly found on polefacing slopes (Costard et al., 2002; Balme et al., 2006; Kneissl et al., 2010). According to a
study on gully orientation on the southern hemisphere, gullies show an increased pole-facing
preference toward the equator, though the high-latitude gullies still show a significant
preference for pole-facing slopes (Costard et al., 2002). Additionally it seems to exist a
strong spatial correlation between gullies and the latitude dependent mantle terrain found on
mid-to high-latitudes, as well as to small-scale lobes and polygons (Johnsson et al., 2012;
Johnsson et al., 2014). Example of both terrestrial and Martian gullies can be seen in Figure
12.
21
Figure 12. (A) Gullies on crater slope on Mars' southern hemisphere. (B) Terrestrial gullies on
Greenland. Image credit: Costard et al. (2002).
3.3 Patterned Ground
Landforms associated with permafrost and ground ice often falls under the term patterned
ground. Patterned ground is mainly a result of cryoturbation, that is freeze-thaw processes
which may produce both sorted and non-sorted patterns (French, 2007c). Common patterned
ground landforms are e.g. stone circles, polygons, stripes and nets (Figure 13).
3.3.1 Thermal Contraction Polygons
Cold climate polygons are widespread both on Earth and Mars; they are products of thermal
contraction cracking, which occur due to annual temperature variations. These polygons are a
type of non-sorted patterned ground and should therefore not be confused with the type of
sorted polygons that arise from sorting by freeze and thaw processes. The formation of
thermal contraction polygons is strongly dependent on the properties of the subsurface, the
active layer and ground moisture, thus polygons may be useful as paleoclimatic indicators.
Classifications of terrestrial thermal contraction polygons are usually made on the type of
wedge produced in the subsurface. Most common are ice-wedge polygons (Figure 13b),
during winter the ground are subject to cooling and cracking and during warmer periods the
cracks are filled with liquids which subsequently refreeze during cooler periods and cause
cracking (Hauber et al., 2011b).
22
Figure 13. (a) Polygons on Mars and (b) ice-wedge polygons on Svalbard. Image credit: Hauber et al.
(2011b). (c) Stripes on Mars. Image credit Balme et al. (2013). (d) Sorted stripes on Svalbard
(Alternating coarser unvegetated stripes and finer vegetated stripes). Image credit: Hauber et al.
(2011a).
Other types are sand-wedge polygons, composite-wedge polygons and sublimation polygons
(Figure 14). Sublimation polygons are formed without an active layer when finer material
collects in buried cracks, leaving coarser material at the surface and subsequently make the
ground more susceptible to sublimation (Marchant & Head III, 2007). The mentioned classes
of polygons are all non-sorted although sorting can occur as a secondary effect when grains
fall into the created depressions.
Cold climate polygons on Mars should not be confused with similar polygons caused by
tectonics or rock jointing; these polygons differ in size from thermal contraction polygons in
cold climates. Thermal contraction polygons however can be hard to distinguish from
desiccation (tensile stress drying) cracks. Polygons found on high-latitude crater floors on
Mars are similar to terrestrial polygons, thus the polygons on the surface of Mars are an
example of discovered features that contribute to the interpretation that Mars may currently
23
have or used to have an active layer in the recent past (Hauber et al., 2011b). The Martian
polygons match the terrestrial sublimation-, sand-wedge- and composite polygons, whereas
ice-wedge polygons are not believed to form under present conditions (Levy et al., 2009).
Figure 14. Image showing an illustration of different polygon morphologies. Modified image from
Levy et al. (2009).
3.3.2 Stripes and Nets
Closely related to solifluction are stripes (Figure 13d) that may occur as sorted or non-sorted
on slopes in periglacial environment. Terrestrial stripes found on Svalbard are usually in the
order of 50-100 cm between the fine and course domain. The Martian counterparts are mostly
separated in the range of 50 cm to 15 m (Hauber et al., 2011b). The non-sorted stripes on
Earth are usually controlled by vegetated versus non-vegetated ground; this is of course not
the case on Mars, where the non-sorted stripes are seen as straight lines with different albedo,
stretching downslope. In more flat areas, as at the foot of slopes where for example craters
slopes transition into crater floor, the stripes may transform into nets. Stripes and nets may be
formed as a result of frost heaving and sorting (Mangold, 2005). A common occurrence is
polygons that due to an increase in slope inclination are transformed into stripes (fFigure 15),
or where stripes are transformed into lobes (Figure 13c).
24
Figure 15. Polygons transforming into stripes due to an increase in slope inclination. Image credit:
Birkeland and Larson (1989).
3.4 Rock glaciers
Another type of terrestrial landforms besides solifluction lobes that may display a lobate
appearance are rock glaciers. Both active and relict rock glaciers exist in a vast number in
cold climate areas on Earth. The definition of active rock glaciers is "lobate or tongue-shaped
bodies of frozen debris with interstitial ice and ice lenses which move downslope by
deformation of ice contained within" (French, 2007c). The process by which rock glaciers are
formed is not well understood but is commonly attributed to the process called permafrost
creep that refers to a gravity-driven deformation of frozen ground (e.g. Matsuoka & Ikeda,
2005; French, 2007a; Dobinski, 2011). The term permafrost creep is however highly debated
since permafrost refers to a temperature state and could consequently not creep, on the other
hand the term creep very well describes the downslope deformation of rock glaciers
(Dobinski, 2011).
On Earth rock glaciers can be confused with solifluction lobes since both display a lobate
form that consist of a tread and a riser. However, the dimension, some descriptive features
and the morphological settings differ. Rock glaciers are generally much larger and thicker
than solifluction lobes, they normally exhibit treads that are hundreds of meters in length and
width and risers that are tens of meters. Solifluction lobes on the other hand often have treads
in the range of 2-50 m and risers around one meter (Matsuoka & Ikeda, 2005). Another
difference is that rock glaciers do not require temperatures above freezing to form;
solifluction however, requires a seasonal average temperature above the freezing point. The
shape of the down flow profiles is another difference; rock glaciers often display a convex
profile whereas solifluction produce a concave profile (Matsuoka & Ikeda, 2005). In addition,
rock glaciers often display compression ridges and furrows which are not seen on solifluction
lobes (Johnsson et al., 2012).
25
Figure 16. (A, C) Possible rock glaciers on Mars compared to (B, D-E) showing tongue shaped rock
glaciers on Svalbard. North is up. Image credit: Hauber et al. (2011b).
Although the formation of small-scale lobes on Mars has not been attributed to permafrost
creep, other features on Mars have been interpreted as the counterparts of terrestrial rock
glaciers. They are known as lobate debris aprons (Squyres & Carr, 1986) and viscous flow
features (Milliken et al., 2003). The lobate debris aprons are found in two latitudinal bands,
one band at 40° on the northern hemisphere and one band at 45° on the southern hemisphere.
This latitude dependence indicates a climate control on the lobate debris aprons similar to
terrestrial rock glaciers (Squyres & Carr, 1986). See Figure 16 for a comparison between
terrestrial rock glaciers and the Martian counterparts.
26
4 Method and Resources
Sub-section 4.1 consist of a description of the method used to assess the frequency of
periglacial landforms on crater slopes as well as the possibility of an aspect dependence of
small-scale lobes. In 4.2 a brief description is given on the HiRISE-camera and CTX-images.
4.1 Method
The approach is to study high-resolution images acquired by the High Resolution Imaging
Science Experiment (HiRISE). The HiRISE images have a resolution of approximately 25 cm
/pixel (McEwen et al., 2007). Only images in the latitudinal band of 55˚-80˚ in the northern
hemisphere, acquired between January 2012 and December 2013 were studied. A spreadsheet
of the images that meets the requirements was compiled using NASA’s planetary data system
archive (PDS) Mars Orbital Data Explorer (ODE). 942 images fulfilled the requirements
though only images that contain a full perimeter crater or a part of a crater were of interest in
this study; hence after each image was studied the images of interest were selected. This
resulted in a total of 245 images that were visualized and analyzed using the software
HiView, which has been developed by the HiRISE team for this specific purpose (University
of Arizona HiView).
Google Mars was used for a first general measurement of the crater diameter, as well as a
tool for finding other images. Other image data sets were used when necessary to improve
data quality, and to get a comprehensive view, within craters that exhibit interesting
landforms (e.g. small-scale lobes) and aspect dependence. Other images used are those
acquired by the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) or HiRISE
images acquired before January 2012.
Images that meet the requirements, containing a full perimeter crater or part of a crater, were
compiled into spreadsheets, further categorized and presented in tables and diagrams using
Microsoft Excel. Distribution maps were made in the software ESRI ArcGIS 10.2.
4.2 Resources
The HiRISE camera is one of a number of high technological instruments situated on board
the spacecraft Mars Reconnaissance Orbiter (MRO) managed by the Jet Propulsion
Laboratory (JPL). The MRO reached Martian orbit on Mars 10, 2006 and the first highresolution image was taken September 29, 2006. The high resolution of 25 cm/pixel is
revolutionary in this field and an important step to detailed study of the surface of Mars.
Some of the main objectives of the HiRISE mission are to locate potentially future landing
27
sites on Mars and investigate surface processes and properties including craters, volcanism,
tectonics, hydrology, sedimentary processes, aeolian processes, mass wasting, landscape
evolution, seasonal processes, climate change, glacial and periglacial processes etc. (McEwen
et al., 2007). For a more detailed description of the HiRISE Mission see McEwen et al. (2007)
or the HiRISE homepage, (https://hirise.lpl.arizona.edu/) (University of Arizona HiRISE).
The MRO Context Camera (CTX) generates images with a resolution of approximately 6
m/pixel. The CTX images have in this thesis mainly been used to get a context for the HiRISE
images and when a full perimeter crater image in high-resolution is lacking. For more
information on the CTX see Malin et al. (2007).
5 Result and Observations
5.1 Inventory
The resulting 245 images that met the requirements listed in section 4 are distributed on 154
different craters, of which 135 images and 65 craters display small-scale lobes. The crater
diameter distribution is presented in Figure 17. The distribution of these craters is presented in
Figure 18. The majority of the studied craters with small-scale lobes are in the range of one to
five km in diameter followed by those craters with a diameter of 10-15 km. Only two craters
have diameters greater than 25 km. Example images of observed small-scale lobes can be
seen in Figure 21, 23-27 and 31.
30
25
Counts
20
15
10
5
0
1-5
5-10
10-15
15-20
20-25
25+
Crater Diameter [km]
Figure 17. Distribution by size of craters with observed small-scale lobes.
28
50+
Figure 18. Map of the northern hemisphere of Mars. The white triangles represent the craters with observed small-scale lobes. Horizontal axis represents
longitude and vertical axis represents latitude.
29
An inventory of the 65 craters that display small-scale lobes was made. The lobes were closer
investigated in HiView and a classification was made based upon the observable sorting
degree of the lobes (sorted or non-sorted). Of the 65 craters, 44 displayed only sorted lobes,
10 displayed only non-sorted lobes and 11 craters displayed both sorted and non-sorted lobes
(Figure 19). Other structures that may indicate cycles of freeze and thaw, like polygons,
gullies and stripes were inventoried, 89 % of the craters exhibit polygons, 48 % exhibit gullies
and 65 % display stripes. The result of this inventory is presented in Figure 20 where it is
divided into the three above mentions classes as well as the total percentage for all craters. No
significant difference in polygon quantity can be seen between the classes. Craters with nonsorted lobes display a significant lower percentage of gullies, and a higher percentage of
stripes compared to the other classes. The distribution of the respective landforms is presented
in maps in Appendix.
Non-sorted small
scale lobes; 10
Sorted and nonsorted small scale
lobes; 11
Sorted small scale
lobes; 44
Figure 19. Pie chart that display the distribution between sorted and non-sorted small-scale lobes in the
studied craters.
30
100
90
80
70
%
60
50
40
30
20
10
0
All craters
craters with sorted
lobes
craters with nonsorted lobes
craters with both
sorted and nonsorted lobes
polygons [%]
89
91
90
83
gullies [%]
48
49
30
58
stripes [%]
65
67
80
50
Figure 20. Diagram displaying the distribution of polygons, gullies and stripes between craters
exhibiting the different lobe-classes.
Several of the studied high-latitude craters exhibit clearly observable small-scale lobes at the
rim but not on the slopes, where the dominant landforms instead are sorted patterned ground.
The same phenomenon can be seen in a few craters at lower latitudes though it seems to be
more common on higher latitudes. Two of these high-latitude craters, in close spatial
proximity at 74.8˚ N and 74.9˚ N respectively, exhibit an interesting rill-like pattern in front
of and between non-sorted small-scale lobes on the south-facing rim (Figure 21). In both
craters, the opposite rim exhibits more diffuse non-sorted small-scale lobes and in addition a
distinct stripe like pattern on the slopes. The stripes are polygonized and alternating dark and
light with a wavelength of approximately 30 meter. Both craters have an approximate
diameter of 11.5 km and are, in these images, partly snow or frost covered at the rims.
Another example of observed stripe-like pattern can be seen in a crater located at 57.97˚ N,
59.45 ˚ E (Figure 22). These stripes are alternating dark and light with no observable clasts,
and a wavelength of approximately 17 meter.
Figure 23 shows a crater with a diameter of 12.3 km, located at 69.27˚ N, 274.0˚ E, with
scalloped terrain and a central mound. The crater exhibits gullies on all slopes, many of which
display lobate sorted fans. On the north-east-facing slope, sorted lobes are superposing the
gully alcoves. This same relationship, where small-scale lobes superpose gullies, can be seen
31
in several other studied craters. The south-west-facing slope exhibits fewer lobes on gully
alcoves but is more polygonized than the other slopes.
Figure 24d shows a crater with polygons, stripe-like pattern as well as features that looks
like gullies superposed by possible small-scale lobes. In addition, the crater displays the
reverse relationship between gullies and lobes, gullies that crosscut lobes (Figure 24b). In this
crater it is demonstrated that different sized lobes could exist in the same crater provided that
the features observed on the gully debris apron are small-scale lobes and not polygons. The
crater has a diameter of 24.3 km and is located at 59.54˚ N, 302.34˚ E.
The crater in Figure 25 is characterized by mass wasting in all directions and detectable
polygons on pole-facing slopes. The polygons on pole-facing and west-facing slopes are
transitioned into stripes downslope and furthermore transitioned into clast-banked small-scale
lobes. The equator-facing slopes are more characterized by possibly juvenile gullies and rock
falls. The crater is located at 61.69˚ N, 228.71˚ E and has a diameter of 18.8 km.
Figure 26 shows a fresh-looking crater located at 65.77˚ N, 334.8˚ E, It has a diameter of 43
km and is characterized by sorted small-scale lobes on the equator-, west- and pole-facing
slopes and abundant gullies on the northeast-facing slope. A few lobes are also present
between the gullies on the north-east-facing slope and adjacent to the gullies. The lobes on the
equator-facing slope as well as the gullies are superposed by different regimes of polygons. A
stripe-like pattern, in which sorted clasts are feeding the clast-banked lobe fronts, can mainly
be seen on the equator-facing slopes. Although all sides, to some extent, show this tendency
of sorting. A number of the observed small-scale lobes in this crater exhibit the same
phenomena as the crater in Figure 25, a sheet-like appearance where sorted stripes feed the
clast-banked lobe fronts. Another example of this phenomenon can be seen on the west-facing
slope in a 13.8 km diameter crater, located at 64.9˚ N, 155.7˚ E (Figure 27). In addition, this
specific crater displays a dissected, scalloped terrain on the crater floor. Several other craters
display scalloped terrain in addition to gullies, sorted small-scale lobes, polygons and stripes,
examples of these craters can be seen in Figure 28.
32
Figure 21. (a) Crater with a diameter of 11.8km, located at 74.873˚ N, 14.874˚E. The crater is
characterized by a stripe-like pattern and small-scale lobes on the slopes and dunes on the crater floor.
(b) Close-up of south crater slope that display a polygonized stripe-like pattern with a wavelength of
~30 m. (c) Close-up of north-west side that display clast-free lobes close to crater rim with adjacent
rills. HiRISE image: ESP_026888_2550. Image credit: NASA/JPL/University of Arizona.
33
Figure 22. Crater with a diameter of 50 km, located at 57.97˚ N, 59.45 ˚ E. The image shows only a small part of the eastern rim crest and slope. It displays
polygons that transform into a stripe-like pattern (white arrows) with a wavelength of ~17 m. Further down the stripes transform back into polygons. The
black arrow indicates the downslope direction. HiRISE image: ESP_018184_2385. Image credit: NASA/JPL/University of Arizona.
34
Figure 23. (a) The crater has a diameter of 12.3 km and is located at 69.27˚ N and 274.0˚ E. The crater
exhibit gullies on all slopes. CTX image: G04_019732_2494_XN_69N086W. (b) On the pole-facing
slope more developed gullies are displayed. (c) In addition the gully alcoves are superposed by sorted
small-scale lobes (white arrows). (b-c) HiRISE image: ESP_034527_2495. Image credit: NASA/JPL/
University of Arizona/Caltech.
35
Figure 24. (a) Crater with a diameter of 24.3 km located at 59.54˚ N, 302.34˚ E. CTX image: G21_026311_2396_XN_59N057W. (b)-(c) Close-ups of
southeast-facing slope shows sorted lobes (white arrows) being crosscut by gullies. (c) In addition the crater display sorted stripe-like pattern (black arrows).
(d) Gully debris apron superposed by what could be small-scale lobes or draped polygons (white arrows). (b-d) HiRISE image: ESP_034381_2400. Image
credit: NASA/JPL/University of Arizona/Caltech.
36
Figure 25. (a) Crater with terraces and a central mound. The crater has a diameter of 18.8 km located at 61.69˚ N, 228.71˚ E. CTX image:
G01_018758_2419_XN_61N131W. (b) Close-up of the southeast-facing crater side showing sorted lobes (white arrow) on the northwest-facing slope but not
on the interior facing slopes. (c)-(d) Close-ups of the northwest-facing slope showing polygonized stripes (black arrows) that are transitioned into clast-banked
lobes (white arrows) on the interior-facing slopes. The stripes have a wavelength of ~20 m. (e)-(f) Close-up of the northeast-facing slope showing sorted lobes
and rill-like features that may imply partially liquefied fine-grained flows (white arrows). HiRISE image: ESP_026828_2420 and ESP_027197_2420. Image
credit: NASA/JPL/University of Arizona/Caltech.
37
Figure 26. Crater with a diameter of 4.3 km located at 65.77˚ N, 334.8˚ E. (a) HiRISE image: ESP_026455_2460. (b) Southeast-facing slope of the crater
display sorted small-scale lobes and gullies. (c) Northwest-facing slope display sorted lobes and sheets (white arrows) overriding polygons. In addition it
displays stripe-like pattern (black arrows) and gullies. (d) This image is a close up of (b), it displays protracted and sorted small-scale lobes (white arrows) and
sorted stripe-like pattern (black arrows). (b-d) HiRISE image: ESP_025901_2460. Image credit: NASA/JPL/University of Arizona.
38
Figure 27. (a) The image shows a crater with scalloped terrain. The crater has a diameter of 13.8 km
and is located at 64.9˚ N, 155.7˚ E. (b) Close-up of northwest-facing slope display sorted stripes
feeding a clast-rich sheet (white arrows). The black arrow indicates the downslope direction. Image:
ESP_028545_2450. Image credit: NASA/JPL/University of Arizona.
39
Figure 28. Example images of typical scalloped crater terrain. (a) Crater with a diameter of 11.3 km, located at 55.32˚ N, 253.57˚ E. HiRISE image:
ESP_026959_2355. (b) Crater with a diameter of 3.8 km, located at 59.35˚ N, 245.04˚ E. HiRISE image: ESP_025878_2395. (c) Crater with a diameter of
18.6 km located at 60.56˚ N, 89.44˚ E. HiRISE image: ESP_034613_2410. Image credit: NASA/JPL/University of Arizona.
40
5.2 Aspect Dependence
For the aspect study, the following criteria had to be fulfilled; (1) Images that cover the full
crater have to exist, (2) the images should be of sufficient resolution and quality, (3) the
craters have to display small-scale lobes. Based on these criteria 30 craters were selected for a
detailed analysis on aspect preference. 22 craters exhibit clear aspect dependence, with a
difference in lobe distribution on the crater slopes. Eight craters display no preferred aspect.
The distribution of the preferred aspect of small-scale lobes is presented as two separate
latitudinal bins, 55-65˚ N and 65-80˚ N in Figure 30. The dark grey area in the chart
represents craters located at 55-65˚ N and display the crater slopes where small-scale lobes
are present. The majority of these craters display small-scale lobes on the south and east
slopes, which in the future is referred to as the pole-facing and west-facing slopes. The light
grey area in the chart represents craters located at 65-80˚ N. The majority of these craters
display small-scale lobes on the north and east slopes, which in the future is referred to as the
equator-facing and west-facing slopes. Figure 31 shows two examples of the craters used in
the aspect study, one crater in the high-latitude bin (a-c) and one crater in the mid-latitude bin
(d-f).
Often aspect dependence seems to exist where different lobe characters are more prominent
on various sides of the craters, like sorted versus non-sorted and lobes versus sheets and
different degradation stage.
N
NW
NE
W
E
SW
55-65
65-80
SE
S
Figure 29. Radar chart displaying the preferred crater wall of small-scale lobes in mid- and highlatitudes. The dark grey represents craters located in the mid-latitudinal band of 55-65˚ N, and the light
grey represents craters located in the high-latitudinal band of 65-80˚ N. The mid-latitude lobes show a
preference for the pole-and west-facing crater walls and the high-latitude lobes show a preference for
the equator- and west-facing crater walls.
41
Figure 30. Map of the northern hemisphere of Mars. The white triangles represent the craters with observed small-scale lobes. The red triangles represent the
craters used for the aspect study. Horizontal axis represents longitude and vertical axis represents latitude.
42
Figure 31. Image showing two examples of craters used in the investigation of aspect dependence of smallscale lobes (a-c) Crater with a diameter of 2.5 km, located at 72.44˚ N, 126.47˚ E. The crater displays
sorted small-scale lobes on the equator-facing and west-facing slopes. (d-f) Crater with a diameter of 3.1
km, located at 60.20˚ N and 236.28˚ E. The crater displays sorted small-scale lobes on the pole-facing
slope. (a) CTX image: D02_027860_2526_XN_72N233W. (b-c) HiRISE image: ESP_027768_2525 (d)
CTX image: D02_028186_2405_XN_60N123W. (e-f) HiRISE image: ESP_026564_2405. Image credit:
NASA/JPL/University of Arizona.
43
6 Discussion
6.1 Method and Uncertainties
The HiRISE Images used for this study differ to some extent in quality, where sometimes
clouds, dust or shadows reduces the quality of images. The main implication for this is that
some craters that may exhibit lobes have been disregarded, and that the classification of some
lobes has been difficult to make. Only the interior of the craters has been investigated,
meaning the exterior crater slopes have been excluded in this study. This may have the
implication that features in those settings have been disregarded. In addition, no closer
investigation or classification of the gullies have been made. The problem of equifinality is
difficult to avoid, however by comparing the observed landforms to previous observations on
the northern hemisphere of Mars, this problem has been minimized. All studies which involve
cataloguing landforms on a regional scale are bound to be affected by instrument targeting.
This bias is difficult to avoid. However, the high number of images used in this study covers
most of the areas of interest and therefore may represent the distribution sufficiently.
One issue that arose during the analysis of the images was that a few of the HiRISE images
were not accurately map-projected, in some cases the images were tilted 180 degrees, with the
consequence that the aspect study could be affected. This was solved by comparing all
available images and using fix points in the surrounding terrain in Google Mars.
6.2 Small-scale lobes and their spatial and temporal relationship to
polygons, stripe-like patterns and gullies.
A substantial part of the investigated craters in this study display small-scale lobes. The large
number further confirms previous statements that small-scale lobes are widespread on crater
slopes in the northern hemisphere of Mars (e.g. Gallagher et al., 2011; Johnsson et al., 2012).
In addition, it shows that small-scale lobes are even more widespread than previously thought.
Similar lobes on Mars (Figure 11) have previously been compared to solifluction induced
landforms on Earth (Figure 9) which display a striking resemblance to the observed lobes on
Mars (e.g. Gallagher et al., 2011; Hauber et al., 2011a; Johnsson et al., 2012). The small-scale
lobes in the study area (55-80˚ N) seem to be relatively evenly distributed except for a few
areas with no craters with small-scale lobes (Figure 18). These areas can be explained by a
lack of craters covered by HiRISE images rather than a lack of small-scale lobes.
44
The majority of craters that display small-scale lobes also exhibit polygons and stripe-like
pattern (Figure 20Figure 32-Figure 33), which is in agreement with the previous observations
(e.g. Gallagher et al., 2011; Johnsson et al., 2012). The distribution of gullies in the
investigated craters is not as widespread as polygons and stripes (Figure 20 Figure 34),
nevertheless they are still regarded as closely related to small-scale lobes.
The stripe-like pattern that can be observed in 65 % of the craters with small-scale lobes
shows a similarity to stripes in terrestrial periglacial environment. For example, the above
mentioned relationship between polygons, lobes and stripe-like pattern is corresponding to
terrestrial relationship. On Earth an increase in slope inclination may consequently transform
polygons into stripes (Figure 15). This same relationship can be seen in a number of the
investigated craters in the study area (e.g. Figure 25d). An example of dissimilarity between
the terrestrial stripes and the observed stripe-like pattern in the study area is the wavelength.
Measured wavelength of a few selected stripe-like patterns reveals a higher wavelength (1530 m) than those generally seen on Earth (50-100 cm).
The stripe-like pattern in Figure 22 shows no signs of clasts which suggest that they are nonsorted stripes. However, the lack of observable clast can also be explained by clast-sizes being
smaller than the pixel-size (~25 cm). As a consequence it is difficult to make any
characterization regarding the sorting degree.
The polygons in the study area have previously been interpreted as thermal contraction
polygons (e.g. Mangold, 2005; Levy et al., 2009). Polygons in close proximity to scalloped
terrain are often believed to be caused by stress induced by subsidence or desiccation.
However, scalloped terrain has been linked to thermal contraction cracking (Zanetti et al.,
2010), for that reason thermal contraction may also be regarded as a possible origin for
polygons close to scalloped terrain. Furthermore, scalloped terrain indicates that the craters
have been subject to sublimation of ground ice (Lefort et al., 2009). It is common on the ~55˚
latitude but exists between ~45˚ and ~65˚ on both hemispheres (Zanetti et al., 2010). In the
study area scalloped terrain are observed in craters located between 55˚ N and 65˚N.
Examples of scalloped terrain within craters can be seen in Figure 27-Figure 28. They display
the typical appearance with a steep pole-facing slope (scarp) and a more gentle equator-facing
slope (Lefort et al., 2009). In addition, the crater in Figure 27 displays polygons, lobes and
stripes. The polygons in combination with other patterned ground features, like stripes and
lobes indicate that freeze and thaw, and thereby thermal contraction, may be the most likely
formation process of the polygons in this scalloped terrain.
45
As seen in several of the example images; stripes, polygons and gullies often exists in close
spatial and temporal relationship with small-scale lobes, where superposition is common and
indicate a relatively younger origin for the landform being superposed. The crater in Figure
23 for example, displays gully alcoves being superposed by sorted small-scale lobes; this
indicates that the lobes are a secondary landform in this assemblage. In addition, the lack of
polygons may suggest that it is a young assemblage. The crater in Figure 24 display a more
complex relationship where small-scale lobes are both superposing gullies and being
superposed by gullies, which suggests a temporally close and dynamic linkage between the
two landforms. The assemblage in this crater has previously been studied by Gallagher and
Balme (2011) who noticed that the larger sheet-like lobes, in places, have been depleted of
fine sediment by the braided gully system leaving a noticeable coarse slope. It was interpreted
that the morphogenesis initially were driven by gelifluction followed by fluid erosion.
At high latitudes, in these otherwise very cold areas, crater rim crests receive more solar
insolation than low interior crater slopes. This may explain why small-scale lobes exist close
to the rim crest but not on the lower interior slopes in many high-latitude craters. The
observed patterns in front of the small-scale lobes in two of these high latitude craters, at
74.8˚ N and 74.9˚ N respectively (Figure 21), gives the impression of being water-related rills.
If so, a study on its origin would be interesting. This is a common sight on terrestrial slopes
that have been modified by solifluction. On Earth this is explained by supersaturation of the
lobes and rapid release of water to form this type of rills (Price, 1974). The similar appearance
may imply the same process on Mars. Another explanation may be that the lobes are younger
than the rills and therefore superposing them. The rills may in turn have been formed when
fast seasonal melting due to higher insolation, are acting on the frost-covered rims resulting in
liquid water saturating the topmost soil, which in turn induce failure of the soil and
consequently producing the rills down slope, before the water sublimates into the atmosphere.
Although a more thorough study of these observations are required to make more firm
interpretations.
The distribution between sorted and non-sorted small-scale lobes in the investigated craters
shows that sorted lobes are significantly more common than non-sorted lobes. 85 % of the
craters display sorted lobes and 32 % display non-sorted lobes. This is the opposite
relationship of what is found in terrestrial environments, where non-sorted solifluction lobes
are more common than sorted lobes (Johnsson et al., 2012). On Earth, however, the sorted,
stone-banked solifluction lobes are mainly found in mountainous areas and on Antarctica,
areas that exhibit sparse or no vegetation cover and a higher abundance of course debris. This
46
would more resemble the surface characteristics on Mars. The sorting effect is a product of a
faster up-freeze and lateral sorting of coarse sediment than the downslope movement
(Johnsson et al., 2012) and the non-sorted lobes require a high moisture content and is very
often associated with vegetated ground which is obviously not the case on Mars. In general a
sorted appearance of lobes are attributed to frost creep whereas a non-sorted appearance are
associated with gelifluction or plug-like flow (e.g. Gallagher et al., 2011; Johnsson et al.,
2012). Thus, the higher quantity of sorted lobes may imply that frost creep is the dominant
process acting on craters slopes in the study area. It is important to remember that the image
resolution may not be sufficient to detect sorting, which could mean that lobes characterized
as non-sorted in this study may in reality be sorted. This in turn adds even more assurance that
frost creep is the main process producing the lobes.
In addition to sorted versus non-sorted appearance, the observed lobes display a variety of
different morphologies such as size and geometry, meaning sheet-like or tongue-shaped lobes.
The most common is that the lobes within one crater display a similar appearance to those
lobes within craters in close spatial proximity. This may be an indication that the local
geology with similar grain size distribution and climate are important factors for the lobe
morphology as well as the sorting degree. Although this is true for most of the studied craters,
exceptions exist. The crater in Figure 24 is an example of such an exception, it displays a
range of different lobe types in close spatial proximity to each other which indicate that
different processes (both gelifluction and frost creep) may act in tandem in the same
environment.
Figure 25 shows a so-called complex crater, a crater that exhibit numerous terraces, hills and
a central mound (Carr, 2006). This specific crater serves as a good example for an aspect
study, since the same crater, due to the terraces, exhibit more than one slope in each direction.
The mass wasting and rock-falls on the interior-facing slope on the north side of the crater
appear to be more consistent with dry mass wasting than water-related gully formation, since
it is lacking the typical alcove, channel and gully apron. However, the appearance may also
exist due to aeolian draping of older preexisting gullies. The lack of polygons on these slopes
may be an indication that the draping is relatively young. The pole-facing and west-facing
slopes display the very typical transition of polygons into stripes and furthermore transitioned
into clast-banked sheets. This relationship between polygons, stripes and sheet-like lobes have
previously been described by Gallagher et al. (2011).
47
6.3 Aspect dependence of small-scale lobes on Mars as a function of
latitude.
In previous investigations on lobate features it is stated that no slope preference seem to occur
(Johnsson et al., 2012), contrary to the results observed in this study. The majority of the
craters used for this study exhibit small-scale lobes that show both a preferred aspect and a
latitudinal dependence. The craters were divided into two latitudinal bins, one mid-latitude
bin, for craters located between 55-65˚ N and one high-latitude bin for craters located between
65-80˚ N. The mid-latitude craters appear to mainly exhibit small-scale lobes on the polefacing and west-facing slopes, with slightly higher preference for the pole-facing slope. The
high-latitude craters on the other hand exhibit small-scale lobes on the equator-facing slope,
whereas the east-facing slopes rarely display lobate features at all. In addition, the majority of
the craters with small-scale lobes that do not display any aspect preference are located at
higher latitudes. A number of craters exhibit small-scale lobate features on all slopes,
however they still display some sort of aspect dependence, in that case the side with the most
prominent lobes was selected as the side of preferred aspect.
A reason why it is not easy to make this type of aspect dependence study is the lack of
images containing a full perimeter crater with sufficient resolution. It is in addition rare that
the entire craters have been documented by HiRISE, often a combination of resources are
necessary, which is why in this study, CTX-images were used to improve the coverage. The
CTX-images have a lower resolution but larger areal extent than the HiRISE-images, which
therefore leads to an uncertainty in the result.
The increase in pole-facing preference towards the mid-latitude and the lack of preference in
some craters at high-latitudes are corresponding to the same trend that gullies display in
craters at the Southern Hemisphere of Mars (Costard et al., 2002). Temperature calculations
for various latitudes, obliquities, surface slope angles and orientations have shown that
temperatures may have exceeded 0°C on pole-facing slopes at mid- and high-latitudes during
periods of high obliquity. In addition they have shown that temperatures above 0°C are
possible on both north and south facing slopes on latitudes above 60°. These results were used
to explain the gully presence on pole-facing slopes (Costard et al., 2002).
The results in this study show that the majority of the high-latitude craters, which display
aspect preference for small-scale lobes, display them on west-facing and equator-facing
slopes. In addition several of these craters exhibit pole-facing slopes covered by CO2-snow,
which may conceal the existence of lobes. Furthermore, several other of these craters display
48
gullies on the pole-facing slopes which may have disintegrated possible small-scale lobes that
could previously have existed. This would explain the apparent preference for equator-facing
slopes instead of the pole-facing slope, it also opens up for the possibility that a lack of
preference is more likely on high latitudes, which is more consistent with the studies on
aspect preference of gullies.
The strong similarity in aspect dependence as a function of latitude between gullies and
small-scale lobes may imply that they are genetically linked and form under similar climatic
conditions.
In a recent study, the existence of small-scale lobes on all sides of a particular crater located
at 71.9º N 344.5º E, were explained by the presence of dark dunes (Kreslavsky & Head,
2014). The authors argue that dark dune sediments may cover crater walls, decreasing albedo
and thus increasing the temperature to provoke the formation of a local active layer. This in
turn would lead to the formation of small-scale lobes by the process of solifluction. In the
study by Kreslavsky and Head (2014), no account for possible aspect dependence is made.
Since this is a relatively high-latitude crater the existence of small-scale lobes on all sides is
not surprising, it is consistent with the results in this study.
Taken together, the distribution and assemblages of various landforms similar to terrestrial
periglacial landforms, and the aspect dependence as a function of latitude implies that the
lobes found in this study may be of freeze and thaw origin which in turn supports the
possibility that the northern hemisphere of Mars had an active layer in the recent past.
7 Conclusions
1) Small-scale lobes which may be indicative of slow mass wasting due to freeze and thaw
activity have been catalogued in this thesis. The study, which expands on the work by
Gallagher et al., 2011 and Johnsson et al., 2012, shows that small-scale lobes are even more
widespread in the northern hemisphere of Mars than previously thought.
2) Small-scale lobes on the northern mid-to high-latitudes on Mars show preferred aspect
dependence as a function of latitude. The lobes in mid-latitude craters show a preference for
the pole- and west-facing slopes whereas the lobes in high-latitude craters show a preference
for equator- and west-facing slopes. The majority of the craters that exhibit lobes with no
preferred aspect are located at high-latitudes.
49
3) Polygons, stripes and gullies are spatially closely related to small-scale lobes.
Superposition and cross-cutting relationships further suggest that they may be temporally
closely related as well. This may imply that they are genetically linked and form under similar
climatic conditions. Thus representing a continuum of freeze-thaw and meltwater related
processes.
4) Altogether, the results in this thesis support the hypothesis that an active layer may have
been present on Mars in the recent past.
8. Further investigations
Suggestions for further investigations:
 More detailed studies of gullies and their association with small-scale lobes.
 Perform a more thorough investigation of the rills below the small-scale lobes in the
two craters at 74.8˚ N and 74.9˚ N respectively.
 Investigate aspect dependence of small-scale lobes on the southern hemisphere.
 Follow up the aspect study when more images over time are acquired.
 Include other factors in the aspect study such as slope angles, the presence of dark
dunes and geologic setting etc.
Acknowledgement
I want to thank my supervisor Dr. Andreas Johnsson for his never ending patience, support
and for literary introducing me to a whole new world. I would also like to thank the Olle
Engkvist Byggmästares Stiftelse for financial support to attend the European Planetary
Science Congress (EPSC 2014) in Portugal.
Furthermore I want to thank the High-Resolution Imaging Science Experiment (HiRISE) and
Context Camera (CTX) teams from NASA, JPL (Jet Propulsion Laboratory), University of
Arizona and Malin Space Science Systems respectively for making it possible for the public
to use their images and the HiView viewing tool.
50
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54
Appendix
Figure 32. Map of the northern hemisphere of Mars. The red triangles represent the craters with observed small-scale lobes and stripe-like pattern. The white
diamonds represents craters with observed small-scale lobes and no stripe-like pattern. Horizontal axis represents longitude and vertical axis represents
latitude.
55
Figure 33. Map of the northern hemisphere of Mars. The red boxes represent the craters with observed small-scale lobes and polygons. The white diamonds
represents the craters with observed small-scale lobes and no polygons. Horizontal axis represents longitude and vertical axis represents latitude.
56
Figure 34. Map of the northern hemisphere of Mars. The red circles represent the craters with observed small-scale lobes and gullies. The white diamonds
represents the craters with observed small-scale lobes and no gullies. Horizontal axis represents longitude and vertical axis represents latitude.
57