The Northern Polar Cap of Mars

By Sarah Hunyadi

Ge151 Paper

3-16-01

By Sarah Hunyadi

The northern polar cap of Mars is a very interesting geologic feature. There is a large variation between the north and the south poles. The southern cap is composed of CO₂ while the northern cap is has no permanent CO₂ deposits. The northern cap is composed of a large portion of permanent water ice. There are distinct layers that are visible in images received from Mars. The composition of these layers is debated, but there are many theories as to their distribution.

The knowledge about the cap comes mostly from images and radar. Because of a lack of surface measurements, the composition of has not been directly determined. There are many different ways to measure the composition of the cap. Radar, spectroscopy and sample analyses are all possible modes of exploration. There are many new technologies that could be used to explore Mars in the future. However, with each of these technologies, there are also difficulties the must be overcome. An idealized mission will incorporate these methods to obtain a comprehensive understanding of the Martian North Polar Cap.

Introduction

In the news recently, there have been many stories about the possibility of water on Mars, and the implications for this type of finding have been avidly discussed because water is the key to life. The presence of water on Mars would mean that the possibility of having life. These life would probably not be little green men, but very small organisms and while the possibility of life is still very unlikely, having water available would make it much more possible. Water on Mars is more important than just the search for life. The search for life is the motivation for much interest in Mars, but the Northern Polar Cap and the water contained there are of particular interest for many other reasons. Because water is so important, there has been a lot of press given to the potential for stream-beds in the mid-latitudes. In a recent Science article, Malin and Edgett discuss the fact that recent (on a geologic time scale) features show evidence of water flow. Runoff and seepage could have been the cause for certain features that are seen in the southern basin. These findings were obtained by examining Mars Orbiting Camera (MOC) images from the Mars Global Surveyor (MGS) spacecraft in detail. What few people actually realize is that water already exists on Mars. It cannot be found at the surface or in liquid form, but exists in the form of water ice on the Northern Polar Cap. This ice is important, not just because water exists there, but also because of the layers. The ice is layered in this region and the layering contains valuable information. The impact history, volcanic history and tectonic history are all stored there. By observing the layers, much can be determined about the past on Mars[i]. There are also other reasons why the ice is important. If there was an ocean on Mars, the last of the sediments would reside in the area surrounding or in the actual cap. More on the ocean hypothesis will be discussed later.

The Martian North Polar Cap is also relevant to Earth in many ways. The impact history on Mars that could be stored in the layers, would give information that was not only pertinent to Mars, but pertinent to the entire solar system. By knowing what happened on Mars, we could gain information about what was happening on Earth for that same time scale. The sediments, or lack thereof, could tell scientists more about the necessary requirements for life. If there was an ocean and evidence is found to support this theory, but no life is found, then it has been determined that more than water is necessary for life to evolve. The water on the cap is also important for Earth missions. In order for a mission to go to Mars, they need to be able to find a way to get fuel, survive and get back. Water would also be the key to a permanent settlement. Thus the ice in the cap plays a large role in the human future on Mars.

These are some of the reasons why the Northern Polar Cap of Mars is an interesting feature. The cap is also interesting for several other reasons. It has a strange spiral shape with many pits and valleys. There are many theories as to how the cap formed and why water ice exists there. Some of these theories will be addressed later. The cap is important, so it is important to understand how it was formed, what it is made of and how we can gain more information about it. The composition can be determined both from ground and landed missions. Such information will help us to understand the history of Mars and what was going on in the Solar System in the past. This in turn can give valuable information about the Earth’s past. These important features will be further explored in this paper.

Body

Because there is so much interest in life for the Northern Polar Cap, it had been investigated as far as possible. This investigation leaves many theories about its formation and appearance. The cap has three basic parts. These parts include the layered terrains, the residual cap and the seasonal cap. The layered terrains lie around the residual cap. They also could continue underneath the residual cap, but no evidence exists to support or rebut this hypothesis. The layers stretch for tens of kilometers and are basically horizontal These terrains are believed to contain many different types of materials. Early images from telescopes showed that there were layers, but now the more recent MOC images have showed that the deposits have at least 20 layers. Since they lie all of the way around the residual cap, it is widely suggested that the residual cap lies directly on top of the layered terrains. The residual cap is a highly reflective surface that is visible in both the summer and winter seasons. The Chasma Boreal is also present throughout all seasons. It is one of the major features that appear on the cap. It is a large valley where there is low albedo with respect to the surrounding surfaces and it can be seen through a telescope that is based on Earth. Inc contrast, the seasonal cap can only be seen at certain times of the year. It is deposited in northern winter and sublimates in northern summer. This seasonal cap puts constraints on landing on the cap because if the CO₂ has not all sublimated, a space craft will freeze very quickly.

There are many hypotheses about how the Chasma formed and how the Northern Polar Cap formed. The chasma could have formed from a large outflow of water from the cap. This would have come from a pool of water underneath or within the cap and washed out through the ice to create the large feature. This theory goes along with the theory that the polar cap was once a large ocean in the Northern Hemisphere of Mars. The planet slopes from the south to the north. Due to this elevation gradient, if there had been an ocean, there would last have been water in the Northern Hemisphere by the northern polar cap. With an old ocean by the cap, sediments could be trapped in the cap. The northern polar cap could also harbor remnants of that ocean in the ice that it contains. This is only one of the many hypotheses that indicate the significance of the cap.

Other hypotheses are based on surface features or the swirling nature of the cap. The surface of the cap shows variation that is sometimes described as looking like a sponge. Some scientists hypothesize that 2-m tall ice cones exist on the surface of the ice. They are formed when the sun hits dark particles in the ice and melts the darker material preferentially, thus leaving a crater. Such conditions occur on Earth in the Himalayas. The swirling nature of the cap is also interesting to many glaciologists. One theory for the structure is the sublimation theory. In this explanation the ice ablates, due to sublimation. This sublimation is solely due to the thermal temperatures that change on a yearly basis. The grooves are curved due to basic flow and rebound of the structure. This produces the large, curved grooves on the cap[ii]. Another explanation has been coined accublation. This model combines the ideas of Ivanov and Muhleman with other processes. The accublation model is an ideal of the polar cap in which the white areas of the cap accumulate and the dark areas ablate. In this case accumulation occurs when ice is deposited either by precipitation or re-freezing. The grooves, or scarps, are mobile in the accublation model. They rotate, much like the planet, to produce a swirling pattern and the accublation creates visible records of their motion[iii]. Other models by other authors suggest that the ice cap is not totally solid. With this in mind, they suggest that the ice cap flows and creates the swirling pattern. This is independent of both accumulation and ablation. None of these theories has any hard proof. The models all produce a structure that resembles the visible structure, which means they all carry equal weight.

Fig 4. The Mars North Polar cap in the summer season. The residual cap is the white material. The dark dune material can clearly be seen as a dark ring surrounding and intersecting part of the cap.

http://oposite.stsci.edu/pubinfo/PR/97/15/B.html

However, these processes do not indicate the composition of the cap. Little is known about the actual composition of the cap because there have been no in-situ measurements. The residual cap is highly reflective surface. This can, in part, be used to determine the composition. By measuring the albedo of the cap and comparing that to the albedo of snow or ice on Earth or snow or ice covered with dust, you can determine that it is not composed of pure ice or snow. It is not reflective enough for this so there must either be a thin layer of dust or larger particles mixed in with the ice. The composition of the mixing agent is unknown although it is assumed that they are mostly particles of Martian dust. The residual cap is composed mostly of water ice. This is evidenced due to thermal mapping in the northern summer. The carbon dioxide that resides on the cap sublimates during the summer leaving just the water ice. The layered terrain appears to be composed of a combination of water ice and dust or small particles in different layered proportions. This would explain some of the color differences. The dust is believed to contain at least two kinds of material. One material is similar to the bright red dust that characterizes Mars. The other material is similar to the dark dune material that usually comes from the low and mid-latitudinal regions. In the summer, the dunes are very visible. The composition of the dark dune material is unknown. Since there are no surface missions to either cap area on Mars, the compositions of such structures have not been determined.

In lieu of direct evidence spectral analysis and thermal analysis have been implemented to try to determine the composition of some of the largest features. Spectral analysis and thermal analysis are done by using an orbiting instruments. One such instrument was aboard Mariner 7. These observations taken by Mariner 7 confirmed that the seasonal cap that disappears in the spring is composed of CO₂. When Viking arrived in 1976 the Viking orbiter measured the summertime brightness over the northern polar cap to be much higher than the temperatures in the south. These temperatures were higher than the sublimation temperature of CO₂, proving that CO₂ could not exist on the northern cap year round[iv]. Further mapping and analysis by Paige et. al. gave clues not only to surface composition, but also to volume composition as well[v]. Based on these models the residual cap consists of dense, coarse grained or compacted snow or ice that extends from the surface to at least 10 cm below the surface. It most likely extends at least 1 m below the surface based on other measurements. The layered terrains show high thermal inertia. This means that these deposits are not composed just of dust or sand. There could be some weathering agent or bonding agent that holds the particles together to make it so energetically high.

The Viking and Mariner 7 data came after many different types of instruments were used to try to determine the composition and the structure of the northern polar cap. Some of these methods involve ground based observations. Some involve space based missions. Both methods of observation are very useful to the understanding of the Martian polar cap. Ground based observation can be made through radio spectroscopy and telescopic observations. These were the first type of observations to be considered. Ground based telescopic information was used to determine information about Mars as early as 1609. This date was when Galileo determined that Mars existed and had phases. After this time many more observations were achieved. In 1784, Herschel discovered that the planet had a small atmosphere due to occultation data. This means that by observing Mars when it passed between the Earth and another star, Herschel could see that there was no effect on the starlight. This meant that there was a small atmosphere. Thirty years later another astronomer observed that the caps melted in the spring and predicted that they were made of layers of ice and snow. It wasn’t until 1909 that spectroscopy was first used on Mars[vi]. This spectroscopic analysis was done from on top of Mount Wilson and it was determined that the atmosphere of Mars didn’t contain water vapor to an appreciable level and was much more arid than the climate of Earth. This finding was not confirmed until 1926. Also around this time, Mars was observed through filters of other wavelengths of light (not visible). Using these data Donald Menzel calculated that the pressure was lower than 1/15^th that of Earth. Large day to night temperature differences were also measured by reflected light and gave further evidence of a thin atmosphere. Carbon dioxide was first detected in 1947 through use of infrared light and spectroscopy in this range. All of these measurements were taken from the ground. The Martian atmosphere can be studied through reflected light and analyzing data obtained from measuring the planet at different wavelengths of light. It is much harder to study the surface, specifically the surface by the north pole.

There are some surface measurements that can be deduced without space missions. The seasonal changes in the size and shape of cap can be observed as well as the seasonal change in atmospheric CO₂ levels. Telescopic observations can also give clues as to the composition of the ice. The albedo (reflectivity) of the cap can be observed from the ground as well. By comparing the albedo of the ice cap to the albedo of known earth materials, we can get a rough idea of what the cap surface is made of, or, at least, what the surface would look like using Earth analogues.

Obtaining data from the cap is not an easy process. The atmosphere obscures some readings and at the very least, it creates noise in the data. Because the atmosphere is composed mostly of CO₂, it is difficult to get readings from the cap because the winter cap is composed of CO₂ as well. Clouds and dust storms are also a problem. They hide the cap from view and do not allow for the reflected light to be visible to telescopic observation or to some spectroscopic observation.

Dust storms and clouds would also be a problem for a mission sent to Mars, but missions can obtain information that is difficult or impossible to obtain from Earth. Space missions can be accomplished by orbiter and landed mission elements. Orbiters can make many measurements over a large area. They can use radio data as well as thermal data and have a much better resolution than telescopic measurements. A landed mission would provide invaluable information. In situ measurements of the surface composition, surface features and local variations would serve to provide a more cohesive view of the Northern Polar Cap.

Orbiting missions are useful for many reasons, including the mapping of the planet. Orbiters can use photographs to show the surface structure of the planet and can use lasers to determine surface stratigraphy. MGS employed both of the aforementioned two processes. When the MGS images are matched with the laser data, images like the 3-D picture of the cap are produced. This gives a comprehensive view of the cap for surface and volume measurements. The resolution obtained by such mapping is much greater than the best earth images. Besides topographic information, orbiters can provide many other useful measurements. Thermal inertia and atmospheric chemistry was measured by Mars orbiters as well. Thermal inertia determines the density and viscosity of the surface cap materials. This thermal mapping was accomplished by using Viking orbiters. They determined surface temperatures to a greater degree of precision and accuracy. Then
these temperatures were compared with images of the surface to obtain viscosity data. TES (Thermal Emission Spectrometer) also measures the surface. TES can watch for dust storms and measure heat coming from the planet. This is valuable in the same way that the Viking data was useful. The atmospheric chemistry was done by MGS in the form of radio occultation. By combining radar reflectances with pressure information obtained from NASA’s planetary data system, weather profiles could be generated. Throughout the MGS mission, such measurements were taken. These measurements can be found at: http://nova.stanford.edu/projects/mgs/late.html.

There is also a lot of value for in situ or landed missions. Although the orbiter can obtain more measurements over a wider area, a landed mission would provide more precise information on a smaller area. A landed mission could also provide valuable information about surface topography. This could be accomplished by having a camera to scan the landscape or by having a moving rover to scan. This scanning can be done to sub-meter accuracy which is something that cannot be done from orbit. Cameras sometimes have problems when looking at an ice landscape due to reflectivity and lack of contrast. A camera would need to be fitted with several filters over a large spectral range in order to ensure that information about all parts of the surface was obtained. A landed mission could also sample the much talked about surface. By using a spectrometer, different elements of the surface could be analyzed. Depending on the device, both large and small samples could be analyzed. MPL had a device that could dig in the surrounding ground and then deposit material in a measurement device. This would have told scientists about the composition of the area surrounding the south polar cap. There are also other spectrometer options. One of the options is Raman spectrometry. Raman is good for looking at small particles in the ice because it has a small spot size. There are also difficulties with looking at small particles. Raman has to reject a lot of data to obtain the signal that it returns. In just looking at small particles, there is not much data to reject. It presents engineering problems as well. It would need to be mounted on a robot arm and used in parallel with a scanning camera. A green or other color laser would have to be used as well because red lasers do not work as well on icy surfaces. Another

spectrometer option is the point spectrometer. It has a much larger spot size so it would not be able to look at small particles in the ice. It would be good for looking at rocks, even from meters away. However rocks might not even exist on the cap. The point spectrometer is better at analyzing material with large particles or cemented particles. It also doesn’t require a robot arm. This spectrometer must just be pointed at a target and can take data from large distances. Besides spectrometers, there are several ways to measure or obtain clues about the composition of the cap. One way is to measure the

sub-surface. Using ground penetrating radar is method. Ground penetrating radar (GPR) sends waves downward and waits to hear the echo. Just GPR alone will give a record of the layers, but neither the thickness nor the composition of the layers will be absolutely known. The one exception is if there is a layer of liquid water beneath the ice of the cap. If the GPR signal hits water, it will return a very strong signal, indicative only of liquid water. Ground Penetrating Radar coupled with good composition data or a stratigraphic section is a good way to determine the overall makeup of the north polar cap. Taking a GPR measurement where the layers are exposed (like by the layered terrain) would give a scale to the data. Taking the composition of the layers would give a complete characterization. Another method of characterizing the subsurface is drilling. Drilling gets to the inner layers, but presents many challenges. First of all, one must ensure that the rover or lander doesn’t rotate while drilling. The drill must be able to take a section without destroying the visual section. The drill must also have some way of analyzing the core unless it is sent back for sample return. In addition, the make-up of the ice is unknown. The ice could be very pliable or it could be lattice-like. Such conditions make it very difficult to drill. One other method of examining the surface would be the use of the cryobot. Cryobots are devices that are designed to melt through the ice and take pictures of the layers as they go down. There is still no spectrometry to go with the pictures, but the size of the layers could be obtained. The sizes of particles in the layers could also be determined. The cryobot can also detect pockets of water. This is a new technology so it requires lots of power and instrumentation. It can, however, provide lots of useful data.

One other advantage to having in situ measurements is having atmospheric sensors. Having sensors to measure water vapor, wind, CO₂ content, pressure and temperature would all provide valuable information. Water vapor and CO₂ sensors would tell what the weather was like at certain times on the cap. Pressure and temperature sensors could relate gaseous abundances to changes either locally or globally. By linking the measurements, one can obtain a comprehensive view of the weather on Mars, at least at the time when the mission is running.

Conclusions

Mars is indeed a very interesting place to study. The Northern Polar Cap is of particular interest because it harbors water in the form of ice. Although life on Mars is not very plausible, it is believed that where there is water there is a chance. The northern polar cap has many interesting geologic features as well. The Chasma Boreal is a scarp visible from Earth. The cap has three parts: the residual cap, layered terrains and the seasonal cap. The layered terrains give clues as to the planets past because the ancient dust is trapped in the layers. What the layers show is a mystery. The swirling nature of the cap is also a mystery. Theories of accumulation, flow and accublation have been discussed previously. These theories all show that it is possible for the cap to have formed in several different ways.

Very little is actually known about the composition of the cap. Scientists can remotely determine that the northern cap is composed of a permanent water ice component and a seasonal CO₂ frost. There are particles in the ice, but the composition of the particles is also unknown. Although there has been much observation remotely, landed missions are really the only way to determine the composition of these small particles. Orbiters can map the planet to a great degree and measure thermal inertia. They can also do certain weather measurements. There has been a long history of observation of Mars dating back to the early 17^th century. Since this time the methods and predictions have gotten much more accurate, but without a landed element, precision and accuracy can only go so far. There are many new technologies and instruments that could be used to form a comprehensive view of the cap. Knowing this composition would provide multitudes of new information and new understanding. All that is left to do is to go there.