The seasonal behaviour of ice and features in craters at the Northern Polar Region of Mars

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MASTER’S THESIS

2010:056 CIV

MASTER’S THESIS

Universitetstryckeriet, Luleå

2010:056 CIV

Mitra Hajigholi

The Seasonal Behaviour of Ice and Features in Craters at the

Northern Polar Region of Mars

MASTER OF SCIENCE PROGRAMME Space Engineering

Luleå University of Technology

Department of Applied Physics and Mechanical Engineering Division of Physics

MASTER’S THESIS

2010:056 CIV

Mitra Hajigholi

The Seasonal Behaviour of Ice and Features in Craters at the

Northern Polar Region of Mars

MASTER OF SCIENCE PROGRAMME Space Engineering

Luleå University of Technology

Department of Applied Physics and Mechanical Engineering Division of Physics

MASTER’S THESIS

2010:056 CIV

Mitra Hajigholi

The Seasonal Behaviour of Ice and Features in Craters at the

Northern Polar Region of Mars

MASTER OF SCIENCE PROGRAMME Space Engineering

Luleå University of Technology

Department of Applied Physics and Mechanical Engineering Division of Physics

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Preface

This report is the diploma thesis for a Master of Science in the Space Engineering Pro- gramme at Luleå University of Technology. It is the result of an internship at NASA Ames Research Center in California between October and January 2009/2010. It was possible due to a collaboration initiated by Luleå University of Technology.

This thesis project was directed under Dr. Chris P. McKay (scientist at NASA Ames Research Center) and supervised by Dr. Adrian Brown (post-doc at SETI), who presented the idea upon which this project is based.

I and my fellow student, Angelique Bertilsson, from Luleå University of Technology have worked side by side on this project. The resulting two thesis works are both about the geological characteristics of craters, on the Martian Northern Polar Region (NPR), related to seasonal ice coverage and features.

I would like to begin by thanking Dr. Chris P. McKay for his time with us. He has supported the project with invaluable inputs, ideas and inspiration. He welcomed us with great hospitality to California and to NASA Ames Research Center.

I would like to show my gratitude to Dr. Adrian Brown for his time, knowledge and encouraging enthusiasm for this project. He provided us with essential knowledge about craters and the NPR of Mars.

Further, I would like to thank my examiner Prof. Sverker Fredriksson, for reading the report and giving constructive suggestions on it, and for giving me the opportunity to start with this thesis in collaboration with the NASA Ames Research Center.

I would like to thank Angelique Bertilsson for helpful discussions and creative ideas, and for our unforgettable memories together in California. I also would like to thank Mi- kaela Appel, Robin Ramstad and everybody who made the stay at NASA Ames an unforgettable time.

Last but not least I would like to send my huge gratitude to my family and beloved friends for supporting, inspiring and being there for me.

Mitra Hajigholi April 9, 2010

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Abstract

This Master Thesis work in Space Engineering was conducted at NASA Ames Research Center. It is a pre-study to understand how the Martian weather conditions affect water and carbon dioxide ice annually in craters, located in the Northern Polar Region (NPR) of Mars.

Different (known and unknown) features have been investigated when observing the 87 craters included in this work and located poleward of 60° in latitude. These craters are studied with images from CTX and HiRISE mainly, but also from CRISM.

There are many images taken by both CTX and HiRISE of the craters over seasons, acquired from 2006 to 2008. Over 500 possible crater images were examined. To make a good scientific observation and a satisfactory description of the ice amount and features covering the crater, a sort out of some insignificant data was needed. Only images where the crater is visible and easily identifiable have been used i.e., not obscured by any clouds, dust storms or even glaring from the Sun to the camera.

The 87 craters chosen to be investigated on the NPR have been carefully selected.

Only craters larger than 10 km in were monitored, unless they have a given name. Since large craters are created by high energy impacts and cause a more complex crater, they are more interesting for this study. All craters were localized on a virtual spherical map over Mars and pinned to be saved in a folder, so that the observed place/surface/image is easily found again.

To organize the entire collection of image data from varying solar longitude, of every 87 craters, a database called Information on Craters in the Martian Northern Polar Region was created by the writer and Angelique Bertilsson. The database is in addition to organize all the collected data, also designed to better, faster and easier use the information collected.

For future scientific study of this work, Information on Craters in the Martian NPR will probably in the future be a public tool on a website easily accessible for scientist and students to use.

Through this work both expected and unexpected seasonal variations have been observed. Theories of why the ice behaves, as it does, and how the features change seasonally are explained and discussed in this work.

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Nomenclature

DDS Dark Dune Spot

GRS/NS Gamma Ray Spectrometer/Neutron Spectrometer

LS Solar longitude

MEP Mars Exploration Program

MGS-TES Mars Global Surveyor -Thermal Emission Spectrometer MRO Mars Reconnaissance Orbiter

NPLD North Polar Layered Deposit NPR Northern Polar Region NPRC Northern Polar Residual Cap PLD Polar Layered Deposit SPC Southern Polar Cap

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List of Contents

1 Introduction ... 1

1.1 NASA Ames Research Center ... 1

1.2 SETI ... 2

2 Mars... 3

2.1 Mars orbit and seasons ... 3

2.2 Crater formation ... 5

2.3 Climate and atmosphere ... 5

2.4 The polar regions ... 7

3 Tools used to study the Martian craters ... 13

3.1 Cameras on MRO ... 13

3.2 IAS viewer ... 16

3.3 Google Earth ... 17

3.4 Database ... 19

3.5 Criteria used for image selection ... 19

4 The features that craters can contain in the Martian Northern Polar Region... 21

4.1 Dunes ... 21

4.2 Dust Devils ... 22

4.3 Defrosting features ... 24

4.4 Dark dune spots ... 26

4.5 Polygons ... 27

5 Crater characteristics, on Martian Northern Polar Region, related to seasonal ice coverage and other features ... 31

6 Results ... 67

6.1 Ice amount in all craters ... 67

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6.2 Dunes ...75

6.3 Dust Devils ...76

6.4 Defrosting ...78

6.5 Dark dune spots ...79

7 Discussion ... 81

7.1 Ice amount in all craters ...81

7.2 Dunes ...82

7.3 Dust Devils ...84

7.4 Defrosting ...85

7.5 Dark Dune Spots ...86

8 Future work ... 89

References ... 91

Appendix A ... 95

Appendix B ...102

Appendix C ...103

Attachment ...104

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1 Introduction

As early as the eighteenth century the Martian polar caps were discovered. In the late nine- teenth and early twentieth century, telescopic drawings showed “canals” on the surface of Mars. The ice caps, canals and seasonal changes observed on Mars, were factors which suggested to scientists that life might exist on Mars. One essential factor for the existence of life is water. Some scientists suggest that lakes of water may exist several meters below Martian surface. Today we know that the surface of the red planet is probably uninhabited, but the question is whether life is underground or has been in the past.

Different features have been investigated when observing the 87 craters located poleward of 60° in latitude, throughout this work. The reason for examining specifically 87 craters was due to time limits but also trying to include as many large craters as possible.

These craters monitored, are studied with images mainly from CTX and HiRISE, but also from CRISM. To organize the entire collection of image data from varying solar longitude, of every 87 craters, a database called Information on Craters in the Martian Northern Polar Region (NPR) was created throughout this thesis work by the writer and Angelique Bertils- son.

Life on Mars is a topic both NASA Ames Research Center and SETI are working with today, in preparation of future space exploration of Mars. This Master Thesis work in Space Engineering, conducted at NASA Ames Research Center, is a pre-study to better understand how the Martian weather conditions affect water and carbon dioxide ice annually in craters, located in the NPR of Mars.

1.1 NASA Ames Research Center

NASA is the foremost federal agency of the United States driving the advancement of space science and aeronautics. Founded in 1958, NASA now has over 16000 employees (FedScope, OPM, 2010) and runs on a budget of close to $20 billion dollars (NASA, 2010) per year. NASA aims to pioneer the future in space exploration, scientific discovery and aeronautics research (Wilson, 2010)

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NASA Centers and facilities are all located in the United States. Some of the more commonly known are the Kennedy Space Center in Florida and the Johnson Space Center in Houston. Some of NASA’s current missions include Spirit and Opportunity, exploring the surface of Mars, and The Mars Reconnaissance Orbiter carrying the most powerful camera ever flown on a planetary exploration mission. (Jonas, 2008)

Formerly a part of NACA, the Ames Research Center became a part of NASA upon its founding. Located at Moffet field in Mountain View, California, the Ames Research Center has over 2300 research personnel and a $600 million annual budget. Ames is a part of most NASA missions and thus has a wide area of expertise. Its research fields include Information technology, Nanotechnology, Biotechnology and Astrobiology. (Jonas, 2008)

1.2 SETI

Initially, when SETI was founded on February 1, 1985, the project’s purpose was the Search for Extra Terrestrial Intelligence (SETI). In part sponsored by the Ames Research Center, the SETI institute has now expanded to explore, understand and explain the origin, nature and prevalence of life in the universe. (SETI, 2010)

The SETI institute is a private, non-profit organization with over 150 employed scientists, teachers and support staff. The institute is physically represented by three centres;

Center for SETI Research, the Carl Sagan Center for the Study of Life in the Universe, and the Center for Education and Public Outreach. (SETI, 2010)

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2 Mars

2.1 Mars orbit and seasons

Mars is a planet with many similarities to our own, but also with some important differences. To get an intuitive understanding of conditions on Mars, a comparison between the two planets can be made. One very important factor is its orbit. Mars orbits the Sun with an axial tilt of 25.19°, similar to that of the Earth (23.44°), giving rise to seasons during its orbit. The orbit is more elliptical than that of the Earth, with an eccentricity of 0.0935 compared Earth’s 0.0167. (Grayzeck, 2007) This gives rise to a different behaviour of seasons on Mars compared to the Earth.

Figure 1. Orbits of Mars and the Earth. Image source: NASA, Jet Propulsion Laboratory (2010)

A solar day on Mars is 24.66 hrs. At its perihelion Mars is at a distance of 2.0662∙10⁸- km from the centre of the Sun, and at its aphelion at a distance of 2.4923∙10⁸ km.

(Grayzeck, 2007) In Figure 1 it is clearly visible that the south pole of Mars is closer to Earth at Mars perihelion, and is therefore until today better explored due to the fact that we can observe it with instruments from Earth.

As mentioned before, seasons on Mars are different from those on Earth. As Earth’s orbit is less eccentric than Mars, each season will be of almost the same duration. Because

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of Mars ellipticity and Kepler’s second law, the planet will move faster at perigee and slower at apogee. This of course generates a long summer and short winter. Spring and autumn of Mars are the same length of time. Because of Mars larger orbit the seasons will have a longer duration than those on Earth. (de Pater, 2001)

Figure 2. The seasonal change in degrees of solar longitude for the northern and southern polar region. The Martian months are defined as spanning 30° in solar longitude and its seasons 90°. The first month of the Martian year starts at spring at 0° in solar longitude. The Northern and Southern polar hemisphere summer is from 90° to 180° and from 270° to 360° in solar longitude, respectively. Image source: The Mars Climate Database (2007).

It is a common practice to use the parameter solar longitude, LS, when expressing the time of year on Mars. Solar longitude is defined as the number of degrees from its Mars- Sun line at vernal equinox to its current Mars-Sun line. When Mars is at its vernal equinox LS = 0°. At the orbital aphelion, summer solstice, the solar longitude is 90°. At perihelion, winter solstice, it is 270°, see Figure 2. The seasons refer to those on the northern hemisphere of Mars. Mars northern and southern polar caps experience elevation change over Mars season, which is illustrated in Appendix C, in a graph showing how the Martian elevation (in meters) changes as a function of solar longitude, in degrees.

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2.2 Crater formation

There are no planets or satellites in our solar system that have a more diverse impact re- cording than Mars, in terms of crater morphology and crater density. Mars has greater variations than observed on any other solar system. Its unique morphology has probably been influenced by subsurface volatiles, and also by the atmosphere reacting with the ejecta blanket emplacement. (Strom, 1992)

The surface of Mars can be divided into two hemispherical regions based on the crater population, which is further described below.

The southern hemisphere region, spanning from 30°N to 85°S, represents the period of late heavy bombardment, possibly from accretion remnants left over from the formation of the terrestrial planets. This region contains high crater density with a large range in crater diameters. (Strom, 1992) The northern region is dominated by a younger surface with a low density distribution of craters, which differs significantly from the southern, heavily cratered, terrain, especially in size. The craters have accumulated since the end of the heavy bombardment, from asteroids and comets, with asteroids dominating over comets. (Strom, 1992)

Based on these two populations and their crater densities, the Martian surface units are assigned ages relative to the period of late heavy bombardment. Absolute ages may range from 4.2 Gyr for ancient cratered terrain to as young as 0.3 Gyr for Olympus Mons.

The polar deposits are younger. (Strom, 1992)

2.3 Climate and atmosphere

The primary difference in climate between Mars and Earth may be a result of the smaller size of Mars. The largest difference is the absence of liquid water on Mars, which will be explained further in Section 2.4. Even though liquid water cannot exist on the Martian surface today, numerous channels on the planet are evidence of running water in the past.

Some of these channels are tens of kilometres wide, several kilometres deep and hundreds of kilometres long. This, and the presence of tear drop-shaped islands in the outflow channels, suggests that vast flows of water must have been present, flooding the plains.

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If water was present, then the Mars atmosphere must have been denser and warmer in the past. The channels are observed to be restricted to the old and heavily cratered terrain, and therefore the warm Martian climate did not extend beyond the end of the heavy bombardment era, about 3.8 billion years ago. (de Pater, 2001)

Scientists’ estimations of Mars early atmosphere suggest a mean surface pressure of 1 bar and temperatures close to 300 K. The large source of CO2 and H2O at that time must have been supplied by widespread volcanism, impacts by planetesimals and tectonic activity. Impacts of large planetesimals may also have caused a loss of atmospheric gases through impact erosion. (de Pater, 2001)

In a relatively small region called Tharsis, numerous volcanoes have been created, which appear to be from the same age. “The eruptions from these volcanoes must have enhanced the atmospheric pressure and, via the greenhouse effect, its temperature.” states de Pater (2001). The rarity of impact craters in this area implies though that the volcanic eruptions occurred well after the formation of the runoff channels on Mars highlands. (de Pater, 2001)

Mars does not show any tectonic activity at present, which on Earth is one of the natural sources of carbon dioxide to the atmosphere. Consequently a large amount of CO2 is

presently lost via adsorption onto regolith, condensation on the surface and carbonaceous weathering processes (the breaking down of rocks, soils and minerals through the direct contact with the planet's atmosphere).

Without liquid water on the surface, weathering came to an end and the small fraction of carbon dioxide was retained. (de Pater, 2001) The present amount of H2O on Mars is mainly unknown. Most might have escaped, but recent studies show the existence of large amounts of subsurface water-ice on the NPRs of the planet.

Table 1, Basic atmospheric parameters for Mars compared with the Earth. Table source: de Pater (2001)

Parameter Mars Earth

Mean heliocentric distance (AU) 1.524 1.00

Surface temperature (K) 215 288

Surface pressure (bar) 0.0056 1.013

Equilibrium temperature (K) 222 263

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The average surface pressure on Mars is 6 mbar, and the mean temperature 215 K.

However, due to the planet’s low atmospheric pressure and therefore low thermal inertia, axial tilt and eccentric orbit (Milankovic cycles) (Byrne, 2009), the surface temperature shows large latitudinal, diurnal and seasonal variations (de Pater, 2001). These parameters may have caused large climate changes on Mars. On Earth, these parameters change every 10⁴ years, and may be responsible for the series of ice ages and ice-free epochs during the past million years. For Mars these parameters have periods about ten times longer than for Earth. When Mars has a large obliquity the polar regions receive more sunlight, and large eccentricities increase the amount of sunlight falling on the summer hemisphere at perihelion. Periodic changes, taken place on Mars, can be observed in the layered deposits in Mars polar region.

The Mars Pathfinder (in 2007) and the Viking space craft (in 1976), both measured significant variations of the atmospheric temperature, ranging from 0 to 70 km in altitude, on timescales of months to years. These variations are strongly correlated with the amount of dust carried along in the atmosphere. Noticeable pressure variations are caused by condensation of the important fraction of Mars carbon dioxide-dominated atmosphere onto the planet’s polar caps. Like Venus, Mars does not have a stratosphere, but a thermosphere at an altitude of 120 km, with a temperature nearly constant at about 140 K, compared with 250 K at the Earth. The low temperature is due to the cooling characteristics of CO2 in the lower atmosphere. (de Pater, 2001)

The H2O and CO2 clouds on Mars can modify the surface temperature by changing the radiative energy balance. Due to their high reflectivity they can decrease the amount of incoming sunlight, by cooling the surface. However, clouds can block the outgoing infrared radiation and increase the greenhouse effect, and play a major role in the formation of storms on the planet. (de Pater, 2001) At the equator of Mars, the surface temperature can drop to 200 K at night and peak up to 300 K during the day. The temperature in the polar region at winter is 148 K, while the summer pole temperature is 190 K.

2.4 The polar regions

Mars is an Earth-like planet with respect to its polar regions. They both have large kilome- tre-thick sheets of water ice that interacts with the planetary atmosphere and records cli-

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matic variations in their stratigraphy (layers of accretion/deposit over time). (Byrne, 2009) The polar caps have the role of a summer time source and a winter time sink for water, with dynamic equilibrium determining the amount of water vapour in the atmosphere. (Jakosky, 1992)

At the Martian poles water ice is permanently frozen. In the winter when the temperature drops below the freezing point of CO2, it will condensate to form a seasonal polar cap of dry ice. The ice sheet will then extend down to 50-55° of latitude. (de Pater, 2001) Under the ice layers dust is present, which becomes visible during the summer time when ice sublimes. Over time layering structures of dust and ice have been produced, and are clearly visible in images taken by satellites orbiting Mars, such Mars Reconnaissance Orbiter (MRO) with its high resolution camera HiRISE. (Byrne, 2009)

Due to periodic variations in the orbital eccentricity, obliquity and season of perihelion of Mars, there are differences between the northern and the southern poles. In the Northern Polar Ice Cap (NPIC), the seasonal cap, i.e., the dry ice, sublimes completely away during the summer, leaving behind the 1000 km in diameter permanent ice cap of water.

In the Southern Polar Cap (SPC), however, the carbon dioxide never completely sublimates away, leaving a permanent southern cap 350 km in diameter. This southern residual cap is not just a simple residue of CO2. Images obtained by MGS (Mars Global Surveyor) and MRO (The Mars Reconnaissance Orbiter) show geological features sugges- tive of depositional events unique to Mars south pole. Unique to the NPR is the huge field of dunes surrounding it, which is an indicative of the differences in dust storms between the north and the south poles. (de Pater, 2001)

Water Ice

Water is presently known to exist in the residual ice caps, at shallow depths in the regolith, on the surface and in the atmosphere. Major unresolved questions are the exchange of water between the north and the south polar reservoirs, i.e., what amounts and timescales are involved. (Titus, 2008)

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Due to the low Martian atmospheric pressure, 5.6 mbar, water is present only as frost, ice or as vapour. As Figure 3 shows, pure liquid water may only exist at temperatures above 273.16 K (0.01°C) in correlation with a pressure above 6.12 mbar (611.73 Pa). According to Table 1, it is currently impossible on the surface of Mars.

The north polar hood shows patterns from year to year that water-ice clouds form.

(Titus, 2008) It can be seen as far south as 48°N and obscure the residual cap as early as LS

167°. When the temperature at night gets low enough, clouds of water-ice can be created near an altitude of 10 km, in the equatorial regions. (de Pater, 2001)

With consistent values over two Martian northern summers, the peak atmospheric water vapour was observed in the north at LS 120°, with MGS-TES (Mars Global Surveyor- Thermal Emission Spectrometer) (Calvin, 2008). In contrast, the water over the southern cap is observed to be highly variable. This implies that a water cap underlies the residual carbon dioxide ice in the south, with a history of highly variable exposure and sublimation.

(Titus, 2008)

The average water ice/frost precipitation in the north is 100 µm and 50 µm in the south. Models suggest that the amount of water sublimated from the northern residual cap

Figure 3, Phase diagram for water, where the triple point temperature (T.Pt.) is the point where all the phases can occur. Stability fields for the solid, liquid and gas phases are indicated. At typical Martian temperatures and pressure, liquid water is not stable. Also shown, as the dashed line, is the vapour pressure of super cooled liquid water. Figure source: Chaplin (2009).

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is insufficient to account for the peak amounts of water in the atmosphere, and that the regolith exchange must also contribute to the observed atmospheric reservoir in the northern summer (Titus, 2008).

The Mars Odyssey Gamma Ray Spectrometer/Neutron Spectrometer (GRS/NS) shows large amounts of subsurface ice in both the north and the south reservoir (mid- to

high-latitude ice-permeated ground). Due to the fact that the southern hemisphere lacks a large water-vapour peak it means that the ground ice in the southern hemisphere is not in exchange with the atmosphere and may therefore be more deeply buried, as inferred from thermal inertia data. (Titus, 2008)

As stated earlier the polar caps have the role of a summertime source and a wintertime sink for water. The seasonal variations of atmospheric water content may also depend on

Figure 4. Chart describing the principal events affecting the Martian water cycle over a year. The compo- nents of the water cycle are illustrated, including the migration of water ice along of the retreating seasonal caps. The cap at the NPR reaches 55°N in latitude, end of the northern fall and the southern spring. The cap at the SPR reaches 55°S in latitude, end of the southern fall and the northern spring. NPCS stands for North Polar Cap Sublimation; SCR stands for Seasonal Cap Recession. Figure source: Titus (2008).

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the exchange with the regolith. (Jakosky, 1992) To better understand the water cycle on Mars, especially the role of clouds, general circulation models have been used, as the one in Figure 4. (Titus, 2008)

Carbon dioxide ice

The Martian atmosphere is composed mainly of carbon dioxide. Because of the shape of the Martian orbit, which is more elliptic than Earth, Mars will come closer to the Sun during its southern hemisphere summer and farther away during southern hemisphere winter.

Hence the Martian seasons are more extreme compared to the seasons on Earth. As a result these extremes will cause seasonal change in the pressure and carbon dioxide content of the atmosphere. (Gardiner, 2010) As carbon dioxide needs five times the atmospheric pressure on Earth at sea level to become liquid, the carbon dioxide on Mars will go directly from solid ice to gas (sublimation).

Carbon dioxide ice, also referred to as “dry ice”, is a non-polar molecule with a dipole moment of zero. It has a low thermal and electrical conductivity, where intermolecular Van der Waals forces act.

In the northern Martian hemisphere the temperature will drop so much that the carbon dioxide gas, either condensates directly onto the surface or into the air on condensation nuclei, such as dust grains. These then fall down to the surface adding a coating of dry ice to the polar caps. In the meantime the southern hemisphere has the summer and the frozen carbon dioxide in the polar cap sublimes into carbon dioxide gas. As the southern summer ends and the northern summer begins the whole process reverses. (Titus, 2008) 25% of the atmosphere, of which 95% is carbon dioxide, will cycle seasonally between the northern and the southern polar caps annually. This is why the carbon dioxide cycle dominates the atmospheric circulation. The current Martian climate is driven by this process, where carbon dioxide freezes out of the atmosphere in autumn and the winter on the surface, and then sublimes back to the atmosphere during the spring.

At the ambient Martian pressure of 6 mbar carbon dioxide and water ice will sublimate and condense at a temperature of 150 K and 200 K (Xie, 2008), respectively. Since carbon dioxide is more volatile than water, the surface will act as an efficient cold trap for water at low temperatures, when carbon dioxide frost is present.

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Observing carbon dioxide and water ice in images, the reflectance of fresh carbon dioxide and water ice is similar to each other, which make them difficult to distinguish in monochrome or multiband reflectance imaging, unless coverage extends long-ward of about 1 µm. (Bennett, 2002) All frozen carbon dioxide will sublime during the northern summer, leaving a residual polar cap made of water ice mixed with Martian dust, which will last throughout the summer. In contrast, at the southern hemisphere the frozen carbon dioxide will remain frozen throughout the Martian year.

Small amounts of water or dust will have a large effect on the reflectance, as pure carbon dioxide has a low absorption coefficient. (Titus, 2008) For example the reflectance will be 25% less, with 0.1% fine dust or 1% water, in a region of 1.5-2.5 µm bands. In visible wavelengths only dust can darken carbon dioxide. Looking with thermal IR even the grain size of carbon dioxide will have an important effect on emissivity. The variation of albedo can tell the size of the carbon dioxide grains. Seasonal frost with grain size less than 100 µm will be brighter than permanent ice, with grain size about 1 mm in midsummer.

According to the work of James et al. (2003) pure carbon dioxide is bright with small variations in wavelength in the visible part of the spectrum. Visible albedo is then weakly dependent on the grain size of pure carbon dioxide. However, the emissivity of the surface carbon dioxide deposits and the albedo (which is wavelength dependent) control the process of deposition and sublimation in the Martian caps.

The knowledge of the seasonal polar cap and the understanding of condensation and sublimation of carbon dioxide and water will allow us to understand the past, current and future Martian climate. Craters located at the seasonal polar cap regions provide a great opportunity to study condensation and sublimation of water and carbon dioxide, especially those with high albedo deposits of frost and/or ice. It is therefore important to understand how carbon dioxide ice changes and interacts with the Martian surface and atmosphere, exploring the craters located in the NPR.

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3 Tools used to study the Martian craters

3.1 Cameras on MRO

The Mars Reconnaissance Orbiter (MRO) is one of the satellites orbiting Mars as part of the NASA Mars Exploration Program (MEP). Some of the scientific objectives of MEP, advised to NASA by scientific communities, are: the search for evidence of past or present life; to understand the climate and volatile history of Mars; to understand geological processes and their role in shaping the surface and sub surface; and to assess the nature and inventory of resources on Mars in preparation for human exploration.

Figure 5. Mars Reconnaissance Orbiter (MRO) monitors the present water cycle in the Mars atmosphere and the associated deposition and sublimation of water ice on the surface. The instruments involved are the shallow radar SHARAD, the CRISM spectrometer, the MARCI weather camera, the HiRISE high- resolution camera, the CTX context camera and the Mars Climate Sounder (MCS). Figure source:

Watanabe (2005).

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The MRO has mapped Mars since 2006, in its nearly polar, circular and low-altitude orbit (320 km above the surface). The spacecraft carries and operates six scientific instruments (shown in bold text in the Table below) for global mapping, regional surveying, and target observations and eight scientific investigation tools (Malin, 2007; Zurek, 2007) :

1. ACCEL, Upper Atmosphere Structure Investigation

2. CRISM, Compact Reconnaissance Imaging Spectrometer for Mars 3. CTX , Context imager

4. GRAVITY, Radio Science Investigation

5. HiRISE, High Resolution Imaging Science Experiment 6. MARCI, Mars Color Imager

7. MCS, Mars Climate Sounder

8. SHARAD, Shallow Subsurface RADAR

The mission has a much higher data return than any previous planetary mission, with 96 Tb (Terra bit) returned so far.

CTX

The Context Camera (CTX) is a camera providing black and white context images of the Martian surface, with high resolution imaging. These CTX images are used as a comple-

Figure 6. This is a CTX image (P15_006803 _2505_XN_ 70N257W) taken of the 39 km crater Louth, during Martian northern spring, at LS = 4.6°. North is up in this image.

10 km

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-ment for the High spatial Resolution Imaging Science Experiment (HiRISE) camera. CTX has a spatial resolution of 6 m/pixel and a swath width of 30 km. (Malin, 2007)

HiRISE

The High Resolution Imaging Science Experiment, HiRISE, is a 0.5 m long reflecting telescope, which provides coloured (red, green, blue and IR) images with a detailed resolution of 0.25 to 1.3 m/pixel. (McEwen, 2007) HiRISE combines this capability with re- markably high signal-to-noise and can also acquire stereo images (though it requires target- ing the same site on different orbits). With HiRISE there are capabilities to provide incredi- ble detail and insight into Mars history, as represented by the surface morphology. (Zurek, 2007)

CRISM

The Compact Reconnaissance Imaging Spectrometer for Mars, CRISM, can cover most of the planet at resolutions of 200 m/pixel in more than 70 bands covering wavelengths from 0.4 to 3.96 µm.

Figure 7. This is a HiRISE image (PSP_ 006869_2505) taken during Martian northern spring at LS = 17.0°, of dark dune spots, in average 10 m in diameter, that emerged at the bottom of the Louth crater. In this image is north upwards.

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Figure 8.This is a CRISM image (FRT000092AB_ 07_IF187L_TRR2 ) taken during the northern spring LS = 12.13°, of the dune formation located at the bottom of the Louth crater. North is upward in this image.

It can achieve full spectral resolution over a swath 11 km wide by slowing down the apparent ground imaging speed with its articulating instrument, to a spatial resolution of 20 m/pixel. It can isolate the surface compositional signature by its ability to remove atmospheric features from the sunlight reflected by both the surface and the atmosphere. These measurements can provide key data about atmospheric thermal structure, dust loading and water vapour column abundance. (Zurek, 2007)

3.2 IAS viewer

Full resolution crater images were investigated from CTX, HiRISE and CRISM, by downloading them from the public internet web site provided by Arizona State University (2008). The images are stored in JPEG2000 (JP2) format, a relatively new format that provides potential to efficiently handle large images. One of the resources that help display the images is the free application IAS (Image Access Solutions) viewer. Some of the offers

5 km

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IAS viewer gives to the user are features as zooming, panning, magnifications, chipping from display and band selection. (HiRISE, 2007)

3.3 Google Earth

Imagine a virtual global map that can browse Mars as it looks now. This is reality with the free software programme Google Earth. (Google, 2010) Google Earth has several global maps over the Martian surface, built up with images from different satellites orbiting Mars.

Visible imagery, colourized terrain, day-time/night-time infrared imagery and Viking col- our imagery, are some of the named maps covering Mars in 3D. With its zooming-function the Martian surface can easily be viewed to some extent in detail, and even more by downloading the full sized image available as a link from Google Earth to the image home- page.

Interesting places can be marked with a pin and saved as a folder to share or store it.

A ruler tool can be used to measure the scale length in km of an object found on the map.

Above all, additional information about the observed place/surface/image can easily be found.

The tools and features Google Earth provides were perfect to locate the craters chosen to be monitored for our project. The approximated crater diameter could be measured and the image data and source for the craters chosen could be directly found on the map.

Figure 9. IAS Viewer is a user friendly and a fast software programme to view jpg2000 images with.

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Figure 10. With Google Earth, Mars can be studied in detail with the data provided from Mars Global Data Access. (University, 2008)

Figure 11. This is a zoomed in image with Google Earth at the crater called Louth. The amount of image data the crater is covered by and provided to any user is observed.

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3.4 Database

To organize all the collected data of images acquired between 2006 and 2008 for the 75 craters monitored, a database called Information on Craters in the Martian Northern Polar Region was created by the author and fellow student Angelique Bertilsson.

Every crater has a set of images with data, such as image ID; camera used; image location; acquisition date; solar longitude; and the website the image can be downloaded from. They are all recorded in the database. In addition to image observation, the amount of ice, features, crater diameter, location and a description of the area are recorded. Every crater has a unique crater ID based on its location on Mars. The ice coverage is denoted by none, less than 50%, more than 50% and full.

The database was created and assembled with Microsoft Access and a search engine function, called Query, made the search for specific requests from the Information on Cra- ters in the Martian Northern Polar Region possible. The search engine was used to create statistical observations, to achieve a faster scientific result of craters.

The database was made to organize all the data collected, to better, faster and easier use the information. Information on Craters in the Martian Northern Polar Region will probably be a public tool on a website, easily accessible for scientists and students to use.

3.5 Criteria used for image selection

There are many craters on the NPR, located polward of 60° in latitude. In this project, craters with larger diameter than 10 km have been selected to be monitored, unless they have a given name. Large craters are created by high energy impacts and cause a more complex crater, with interesting features.

Over 500 possible crater images from all Martian season, 0-360° in solar longitude, have been examined. Firstly, the image must match with the right crater, selected to be monitored. Secondly, in all studied images, selected to the database, the monitored crater has to be visible and easily identifiable, i.e., no clouds or dust storms should obscure the crater. This is important, in order to better make a good description of how the ice amounts and the features in the craters vary with solar longitude.

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4 The features that craters can contain in the Martian Northern Polar Region

4.1 Dunes

In geosystems a dune is defined as a hill of sand that has been created by aeolian processes.

The dunes are formed by interactions with the wind, giving them different shapes and sizes.

Based on the various types of dune formation they are categorized differently. By observing the changing pattern of the sand dunes, the interaction between the Martian surface and the atmosphere better can be understood. The observation of the dune can determine the activity of the Martian winds, but also how and with what rate the Martian winds move the sediment around.

Sand grains are capable to move with the wind in two distinct ways, either by surface creep or by saltation, where saltation is the primary method. (Mangimeli, 2010) As the wind picks up the sand grains from the surface, the wind will give them a forward momentum. Depending on the weight of the grains, they will be carried away by the wind over different distances. Bigger grains will fall to the ground after a short distance. If the surface is composed by coarse sand grains, they will bounce up in the air and the wind will, again, provide the grain a forward momentum, while lighter grains will be moved longer distances by the wind.

Figure 12. The wind will continue to move the sand up to the top and create a pile of sand. When the pile will become too steep, it will collapse under its own weight. When the right steepness is reached the dune will be stable. Depending on the properties of the material, the angle of the steepness will be different.

Figure source: Nature (2000)

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When lighter grains strike the sandy surface they will more likely be buried selves, and the impact will eject a second grain into the air. (Mangimeli, 2010) As the wind picks up the grains, it will lose its force and velocity. Also, a small pile of sand can decrease the wind’s velocity and strength and cause even more sand to deposit and eventually create a large pile of sand, defined as a dune. However, since the gravitational force is three times weaker on Mars than on Earth, the sand grains will not be pushed downward by the gravity in the way that they are on Earth. They will therefore be able to stay in the air much longer before they strike the surface.

Martian dunes were discovered for the first time in 1972 by Mariner 9 (Jet Propulsion Laboratory, 2010) and they are still actively studied.

A major part of the observed craters on the Martian NPR contains dunes. The dunes are most likely located in the centre or in the middle part of the crater. Also, if the crater contains a central peak, some of them, contain dunes that are located on, or next to, the central peak. Some craters also have dunes located close to the crater wall, or have a large sea of dunes outside, around the crater.

4.2 Dust Devils

Figure 13. In contrast to tornados, dust devils are created through a different mechanism. When the Sun is heating up the dry surface, the air will start to produce convective rolls. Some of these rolls will get tilted upright with the wind, producing a dust devil. When dust and debris get caught inside the vortex, the dust devil will be visible. Weatherquestions (2003)

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Suggested to be effective for raising dust in the low-density atmosphere, dust devils are leaving tracks of dark lines on the Martian surface. Dust devils are described as convective vortices made by dust and sand, emerging from high rotating wind speeds, significant elec- trostatic fields and reduced pressure. (Balme, 2006)

On both Mars and Earth, dust devils are common atmospheric phenomena. On Mars, dust devils have been observed by the Mars Pathfinder, by the MGS camera and by both the Viking orbiters and landers (Thomas, 1992). On Earth dust devils can be observed on terrestrial dry lands and desert landscapes.

Dust devils are characterized by upward moving and spiralling flows that are caused by insulation that is heating up the near-surface air. When the ground is heated by the Sun, warm air will raise and interact with the surrounding wind. The air will move towards the centre of the updraft to spin, while attempting to conserve angular momentum. The friction of the surface will then reduce the angular momentum of the spinning air and disturb the balance between the centrifugal and pressure gradient forces (Thomas, 1992). When the centrifugal forces decrease, the warm, near-surface air will converge toward the centre of the vortex. In turn, the concentration of the ambient vorticity will increase by the radial inflow. If dust enters the rising vortex, a dust devil will appear.

Figure 14. This image, B02_010507_2456_XN_65N231W, is taken with the CTX camera at LS = 146.09°

(northern summer). At the bottom of this low latitude located crater (65N-128E), clear tracks of dust devils can be observed.

3.5 km

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By moving over nearby areas of hot air, the dust devils can be able to sustain them- selves for longer. When the dust devil enters a terrain where the surface temperature is lower, cooler air will be sucked in and disturb the balance and the dust devil will dissipate over seconds.

According to the work of Murphy et al. (2000), the typical temperature and pressure within the dust devils varies between 4 and 8 K and from 2.5 to 4.5 hPa.

Throughout this work, dust devils have been observed to be irregularly spread within the craters. Often the dust devils could be seen in the middle part of the crater, but also straight through the crater or behind and on the crater rim.

4.3 Defrosting features

As the Martian spring season begins, the atmospheric and surface temperature will gradually increase. Due to the fact that carbon dioxide ice has a lower sublimation point than water ice, so it will begin to sublimate back to the atmosphere and expose the water ice or regolith below. As the temperature increases it will become warm enough for the water ice to sublimate to the atmosphere. On average, the temperature in the NPR increases until the middle of the summer. This process is called defrosting. As defrosting occurs, interesting patterns and features can be observed, similar or dissimilar to what is observed on Earth.

One way of observing defrosting features in CTX and HiRISE images of craters containing ice/frost, is by observing its albedo (in scales between 0 and 1, were 0 is the lowest and 1 is the highest) and monitoring its seasonal change during one Martian year. The regolith on the crater floor has a low albedo and ice has a high albedo. In particular carbon dioxide ice has a much higher albedo compared with water ice. An increase in albedo is most likely due to condensation of ice onto the crater floor/wall. A decrease in albedo could be due to defrosting of the area, but also due to the fact that ice can age, or as a result of usual dust storms occurring in the NPR periodically.

Aged ice gives a lower albedo as a result of its larger ice/frost grain size. This is caused by accumulation of ice grains with time. An area of small-sized ice grains will have more surfaces to reflect the incoming light with, compared to the same area covered by larger sized ice grains. During the dust storm season, large amounts of dust from the large

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fields of dunes in the NPR will hence cover the ice/frost deposit. When dust settles down on ice it will decrease its albedo significantly.

Figure 15. During the end of the Martian northern spring, at LS = 88.6°, this crater at 77N-89E experienced defrosting. Frost and ice have most likely experienced high and low temperatures and created patterns of waves and streaks next to the crater rim. These patterns can be seen to the left of this high resolution image (PSP_008926_2575), where a large amount of ice has sublimed. To the right a partly bare crater rim is visible. North is upward in this image.

Another way of observing defrosting features when observing crater images is by searching for the patterns that remind of defrosting patterns occurring on Earth. When ice/snow/frost melts it starts to move. Movement of ice can get the shape of stripes or waves that appear darker than the area surrounding it. The ice melts most likely only during the day when sunlight heats up the surface, and freezes during the night. The melting process is much slower on Mars than on Earth. So, as the ice melts and freezes during day and night, patterns of ice layers can be observed. Generally, the defrosting patterns start to appear on the rim (the location that gains most sunlight during the day and has the highest incline will also show most defrosting) and then on the crater wall and last on the floor (which is usually deep and covered by the crater wall’s shadow).

2 km

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4.4 Dark dune spots

Dark Dune Spots (DDSs) (Kereszturi, 2009) seem to form only in craters containing dunes, and when defrosting patterns occur. In rare cases DDSs can be observed even when defrosting is not present. These dark albedo features can be located upon and next to thin ice/frost covered dune formation on the flat ground. DDSs seem to be formed under these ice sheets, unexposed to the atmosphere. The first signs when DDSs start to form are similar to spots, being a few meters in diameter. Under these sheets of ice, the spots develop by increasing radially in size until they become exposed to the atmosphere.

Figure 16. At a latitude of 74°N, during the middle of the summer (LS = 42.60°), DDSs have emerged on and next to the partly ice covered dunes located at the bottom of this unnamed crater (74N-13E). The spots are between 0.5 m and 1.25 m in diameter. This image (PSP_007584 _2550) was taken by HiRISE, and north is directed upwards in this image.

Compared to the surrounding area the features appear to be dark/black. With time and increasing thermal heat they increase in number and grow in size. Depending on where the spots appear they will develop differently. If they appear on top of a dune peak they will in time stream down similar to how liquid stream downhill, looking like streaks featuring the same albedo as the spots.

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4.5 Polygons

In some craters, a structure of polygonal features can be observed on the crater floor, on the crater wall and on and beside a central peak within the crater. These land formations are similar to those patterned grounds that are common in periglacial environments on Earth (at the northern and southern polar regions). At extreme cold temperatures these regions surface soil and sediment freeze to a depth of up to 1500 m. This layer is called permafrost.

(Pidwirny, 2009)

Periglacial environments are defined as landforms created by processes associated with intense freeze-thaw actions that drastically modify the ground surface. A number of types of modification observed on Earth are migration of ground water, and the formation of unique landforms. Patterned ground features on Earth and on Mars arise on horizontal surfaces, but also on slopes. Their shapes range in size from a few cm up to 100 m in diameter, and look similar to mud beds. The polygonal cracking surface is usually the result of desiccation (the state of extreme dryness) or thermal contraction. (Pidwirny, 2009)

Figure. 17 CTX image (P15_007013_2437_XN_63N228W) of polygonal nets at the bottom of the crater, 64N-132E.

6 km

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There are a variety of patterned ground formations observed on Earth that includes frost-crack polygons, ice-wedge polygons, sorted and non-sorted circles, and stone or soil stripes. From thermal contractions in rock or frozen ground with ice content, steep fractures are formed, and called frost-cracks. (Humlun, 2004)

Figure 18. HiRISE image (PSP_007571_2490) of polygons at the bottom of the crater, 68N-13E.

Figure 19. This polygonal network, formed in dry and cold areas, is located in Svalbard but resembles polygons found in Antarctica and in Martian craters in the NPR. Figure source: Portal (2010).

470 m

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Ice wedge polygons are similarly formed as frost-cracking polygonals, but the cracks are gradually filled with ice. Sorted circle formations are often delineated with a border of stones surrounding a central area of finer material, varying in size from a few centimetres to over 3 m and can extend to depths of about 1 m. The stones are largest at the surface and decrease with depth. Unsorted rings usually 0.5 to 3 m in diameter, have a lack of definite ring of stones and can be found singly or in groups. Soil stripes are linear patterns of soil or vegetation on slopes without related lines of stones. (Price, 1972)

On Mars there are large-scale polygonal nets, identified commonly in craters and on level terrain. The small ridges or furrows from polygonals are sometimes observed to be covered by ice during late and early spring, which makes the structure more distinct from the surrounding crater floor.

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5 Crater characteristics, on Martian Northern Polar Re- gion, related to seasonal ice coverage and other fea- tures

In this Section follows short descriptions of 87 craters. A summary is included about the crater ice amount and features that are monitored and interpreted. This is done by observing images of the craters from all Martian seasons. Some craters have names. The nameless craters are numbered by degrees in centre latitude (North) and longitude (East).

The short description of every crater includes:

 The crater location

 The number and the type of images covering the crater during all Martian seasons referred in degrees of LS (solar longitude)

 The diameter in km

 The formation (complex or simple)

 The amount and location of the ice

 The observed features in the crater, described in the previous Section (as dunes, dust devils, defrosting features, dark dune spots (DDS) and polygons).

The craters listed below are in order of rising latitude. By looking at the map over the NPR where also the NPRC is visible, in Appendix B, one can see were the craters are located. The seasons of Mars in solar longitude are illustrated in a graph in Appendix C.

Crater: Kunowsky

Location: 56.50°N, 350.58°E

At Vastitas Borealis region, the 67 km crater is located. Kunowsky is covered by eight images taken between LS = 16.7° and 351.92° (i.e., at early spring, beginning and end of the summer and the winter season). Next to the circular group of peaks in the middle of the complex crater, dunes are observed. There is not much snow or ice visible in this crater,

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only thin layers are visible on the peaks, on the rough crater floor between the dunes and partly on the crater rim. The situation does not change for the ice amount during early summer, but there are tracks of dust devils visible over dune formations. Images from the winter season show a fully ice covered crater, with a thick ice layer on the rim and crater floor and a thin layer of ice in hollowed out areas and on the rough floor.

Crater: 60N-222E

Location: 60.12°N, 221.9°E

The 16 km crater is localized on the Vastitas Borealis area. Three out of four images covering the crater are taken by CTX and one by HiRISE. The images are all taken during Mar- tian spring season, LS = 20.29-60.56°, and all show a cloudy area partly obscuring the dunes in the centre of the crater. The eastern crater wall and rim have a thin ice layer that decreases fairly rapidly with time. Tracks of dust devils are visible at LS = 60.56°.

Location: 60.15°N, 280.83°E

In the Vastitas region is the 37 km crater located. The three CTX and two HiRISE images cover the crater over LS = 51-141.7°. The crater has several gullies formed on its rim, small dune formations at the bottom of the crater next to its large peak, and a mini crater. This crater experiences a thin ice layer covering the whole crater during the middle of the spring.

The dunes become darker with solar longitude, and dark dust is spread north by the wind.

Also some of the gullies, northwest of the crater, are observed to become darker in albedo.

Location: 60.25°N, 251.08°E

The 22 km sized crater is a complex one. It has only one CTX image covering the crater, showing a flat peak and a crater bottom with polygonal nets. This image covers the whole crater during the middle of the northern spring (at LS = 58.27°). No signature of an ice cover can be seen but tracks of dust devils on the eastern side of the crater floor are seen.

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33 Crater: 60N-313E

Location: 60.32°N, 313.47°E

On the region called Vastitas Borealis is this 20 km crater located. Three CTX images cover the crater during LS = 44.11-100.63°. A thin layer of ice is observed on the crater wall in the earliest image. The regolith is observed to have polygonal patterns over wave- formed hills and a peak in the middle of the crater. A couple of gullies are observed on the north-eastern crater wall.

At the end of the spring defrosting patterns start to show on the northern crater wall.

Dust devil tracks are visible in the southern part of the crater close to the dunes. They increase in amount during the beginning of the summer. With increasing solar longitude the dunes become darker and almost all ice sublimes.

Location: 60.36°N, 129.37°E

All of the images are taken by the CTX camera, starting from the middle of the northern spring and reaching the middle of the northern summer, from 44.81° to 140.42° in solar longitude. There are five images covering the 24 km crater. In these images, almost no change can be discerned. The crater is almost uncovered except for some ice on the eastern crater wall and on the crater rim. Structures of polygonal nets are visible in the middle region of the crater. Black traces of dust devils are visible behind and across the crater.

Location: 60.40°N, 101.25°E

The 21 km sized crater is located close to the area called Alba Fossae. This complex crater, with a peak in the middle, is covered by three images, two taken by CTX and one by HiRISE, during the northern spring and the summer, at LS = 58.27°, 68.6° and 128.7°.

Images show that the crater has a polygonal shaped ground. Defrosting patterns are visible during late northern spring, and thinner layers of ice are observed in the whole crater. How- ever, earlier in spring the crater has an ice/frost layer covering almost the whole crater.

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34 Crater: 60N-148E

Location: 60.43°N, 147.72°E

Three images are taken over the 22 km wide polar crater, during the middle of the northern spring, reaching from 41.57° to 58.85° in solar longitude. During this time the crater has a lot of ice, although, the middle part is almost ice free. Some ice is still left in small hollowed out areas within the polygonal structure, which cover the middle region of the crater.

In the latest image, the ice is still covering larger parts of the eastern and the southern regions of the crater, the crater walls and the crater rim. In the middle, which is uncovered, dust devils can be seen.

Crater: 60N-90E

Location: 60.59°N, 89.66°E

There are ten images taken of the 21 km wide crater, located in the southern part of the northern hemisphere. The images are taken from the beginning of the northern spring and reach to the middle of the northern summer, 28.59° to 142.01° in solar longitude.

At the western crater rim gully formations are visible. Some vague structures of polygonal nets are visible in the middle of the crater. Also some dunes are located in the north-western part of the crater. In the end of the spring and in the beginning of the summer, dust devils have appeared around the dunes. Some small amount of ice is visible on the eastern crater rim in the beginning of the spring. Except for that, the crater is more or less empty in all images taken during the spring and the summer.

Crater: 60N-88E

Location: 60.65°N, 87.74°E

The CTX camera has taken five images of the 61 km crater, located in the southern part of the NPR. The images are taken during the northern spring and up to the middle of the northern summer, 36.09° to 147.19° in solar longitude. The floor in the middle of the crater is composed of raised ridges and some vague structure of polygonal nets is visible as well.

The crater has more or less no ice during this period. From the middle of the spring, to the latest image, in summer, dust devils are visible around the dunes in the middle of the crater.

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35 Crater: 61N-312E

Location: 61.19°N, 311.61°E

The 30 km crater, covered by three images, is located at Vastitas Borealis. Two CTX images and one HiRISE image cover the crater between LS = 41.4° and 63.59°. All images show barely any ice, except in hollowed out areas at the peak region and some on the rim.

Some high peaks of the large dune formation at the bottom of the crater show some ice deposit. Data observations from CRISM during this time period does not show any water or carbon dioxide ice deposit.

Crater: 61N-22E

Location: 61.31°N, 21.51°E

The 13 km wide crater is located in the southern region of the northern hemisphere. Two images are taken by the CTX camera during the northern spring, 47.98° and 69.79° in solar longitude. On the earlier image, only small amounts of ice can be seen. Some of this ice is located on the inner side of the north-eastern crater rim, which becomes even smaller in later image. Some ice is located in small hollowed out areas on the crater floor, which is composed of a polygonal structure.

Crater: 61N-308E Location: 61.35°N, 307.7°E

The 22 km crater is located in the Vastitas Borealis. It has two CTX and two HiRISE images covering the crater in total between LS = 20.6° and 43.7°, which is the beginning and the middle of the spring. The crater is observed to have several gullies around the entire crater rim, which are partly covered in thin ice. A large field of dark dunes is visible at the bottom of the crater. Early in the spring, the southeast of the crater has a small part that has a thin ice layer.

Location: 61.46°N, 229.45°E

One CTX image covers this 22 km crater, located northwest on the Vastitas Borealis region, at LS = 68.63°. The rough terrain on the crater bottom shows spots of ice layering in

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hollowed out areas. A thicker ice layer is observed at the north-eastern rim with darker spots visible.

Crater: 62N-6E

Location: 61.7°N, 6.36°E

The crater is located in an area called Acidalia Planitia. There are three images taken by CTX during the northern hemisphere spring, reaching from solar longitude 41.29° to 68.46°. In the middle of the 24 km crater, dunes are seen, but also a structure of polygonal nets can be seen on the crater floor. The crater is almost completely empty of ice during this period. Some ice is still left from winter and can be seen on the edge of the crater rim, in the east of the crater image.

Location: 62.41°N, 221.76°E

The crater is located, in an area called Scandia Colles. The 16 km crater is covered by two images, both during the middle of the northern spring. The crater shows a thin ice layer on its western rim and wall. However, in the north of the crater, a thick ice layer is visible on top of the unusual hills created on the crater rim. Other features visible, are tracks of dust devils in the middle of the crater.

Location: 62.53°N, 186.81°E

On the area called Vastitas Borealis, the 35 km crater is located. Nine images cover the crater from the middle of the spring to the middle of the summer.

In the beginning of the Martian spring the crater is observed to have a thin ice layer on part of its rim and on its peak, which is surrounded by dunes. With time, more ice accu- mulates on the peak, and consequently in the end of the spring the peak has a thick ice layer covering it. The ice cover increases in area, reaching and surrounding the dunes next to the peak as well. However, the ice on its rim decreases with time.

At Martian summer the dunes in the crater become dark, and tracks of dust devils are visible during the middle and late summer. The rugged crater floor shows no sign of ice,

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but small valleys in the centre of the crater do, and the crater peak has a thin ice layer in the end of the summer.

Crater: 63N-239E Location: 62.8°N, 238.52°E

This 34 km crater has three CTX images in total cover it during the middle of the Martian Spring. 63N-239E has a rough and bubbly patterned crater floor. Dunes are visible next to a peak. The dunes become darker with time and the ice layer thinner as ice sublimates.

Crater: 63N-12E

Location: 63.4°N, 11.92°E

This crater is located in the southern part of the northern polar region. There are three images taken over the 41 km wide crater, where two are taken by the CTX camera and one by the HiRISE camera. All images are taken during the northern spring, reaching from 29.16°

to 64° solar longitude. Some kind of plateau formation can be seen in the middle of the crater. Also a structure of polygonal nets can be seen around this plateau on the crater floor.

A couple of dunes are located north-east of the crater. The crater is almost empty of ice during this period. However, the dunes are still covered, and some DDSs have appeared on their peaks. Some of the ice can also be seen in small hollowed out areas on the crater floor.

Aside from this, the only ice seen is located around the crater, on the crater rim.

Location: 63.46°N, 292.482°E

The 17 km crater is covered by seven images (four CTX and three HiRISE images). At LS = 37.2° the crater barely shows any sign of ice, as most is on the rim. Its many gully formations, that may be a proof of water flowing down the crater, are partly ice covered by a thin layer. The large dunes in the bottom of this crater are also partly covered by thin ice at this time and both are so until middle of the spring. During the beginning of the summer, the dunes have become much darker as all the ice has sublimated. Tracks of dust devils are observed to cross the dunes and the crater bottom. Still, there can be seen a thin layer of ice on the eastern crater rim.