Polar Geology Theme Overview:
By Shane Byrne and Ken Herkenhoff
Explanation of the theme.
The deposits at both Martian poles can naturally be divided into the polar layered deposits and the residual ice caps which partially cover them. The polar layered deposits form a layered sedimentary stack up to 3 km thick at both poles. Although their exact compositions are unknown, both examples are thought to be primarily made up of dust and water ice and are roughly similar in volume to the Greenland ice sheet. Mars appears to have experienced recent climate changes, like "ice ages" on Earth. Scientists think that the Martian polar layered deposits record climate variations over at least the last 10 to 100 million years, but the details of how they formed and evolved remain obscure. Changes in tilt of the rotation axis and shape of the orbit are thought to influence the climates of both Earth and Mars, but are larger for Mars. Many Mars researchers think that the polar layered deposits are the result of variations in the proportions of dust and water ice deposited over many climate cycles. The layers themselves are therefore expected to contain an important summary of recent climate changes on Mars just as ice-cores from Greenland and Antarctica do for the Earth.
The residual ices on top of these layered deposits exert control over the current Martian climate (and vice versa). These ice caps are composed of clean, large-grained water ice at the north pole and carbon dioxide ice at the south. The northern residual ice may be the site of active formation of the polar layered deposits and controls the global distribution of atmospheric water vapor. The southern residual ice cap currently controls the pressure of the atmosphere by supplying extra carbon dioxide gas when it partially sublimates during the summer. The southern ice cap contains populations of geomorphic features, such as groups of pits and ridges, which change size and shape at rates of meters per year and are only a few decades old.
Major science questions for this theme.
Several important questions related to the residual ices are: Are the residual caps currently accumulating or losing material? How much exchangeable carbon dioxide and water is locked up within these deposits? How stable are these deposits over timescales of centuries to millennia? How do the residual caps control the formation of the polar layered deposits?
The relationship between layers within the polar layered deposits and the climate during their formation remains uncertain. The dust/ice ratio and how it varies from layer to layer is currently unknown; some layers may also contain other more minor components such as volcanic fallout and impact debris. Identification of these materials would help our understanding of recent Martian history significantly. HiRISE is capable of observing Martian layers at sub-meter resolution. Such observations can be used to determine whether these layered deposits contain even thinner layers than those that are resolved by MOC. An important goal of Mars polar science is the association of layers with independently derived Martian orbital changes. One of the more puzzling questions regarding the Martian polar regions is why the southern polar layered deposits appear to be so much older than those in the north. The difference in surface age is inferred by counting craters, and there are many more craters on the southern layered deposits than on the northern ones.
Relationship to other science themes.
In contrast to other science themes polar geology refers to a place rather than a process. Examination of these polar deposits draws upon many other research areas including glacial and periglacial processes, seasonal processes, stratigraphy, sedimentary geology and landscape evolution. Polar geology spans, and can be considered a subset of, all of these broader categories. This science theme should be used when the observational goal is the further understanding of the current polar deposits, whether it pertains to the stratigraphic record of the layered deposits (which represents at least millions of years of history) or the evolving geomorphology within the residual ice caps (which represents only a few centuries of history). Behavior of seasonal frosts would be covered by the seasonal processes theme despite their polar location.
Features of interest potentially visible at HiRISE scale.
Changes in the geomorphic features within the southern residual carbon dioxide ice will be observable over timescales substantially less than 1 year. The northern residual ice surface has an unusual texture at the limit of MOC resolution. HiRISE observations will allow characterization of these small-scale landforms and their evolution rates. Layers less than 1 meter thick are visible on the sides of mesas in the southern residual ice. These layers can be imaged by HiRISE and may hold the key to Mars' climatic history over the past few centuries.
High-resolution stereo imaging of exposed sequences of polar layered deposits will allow measurements of layer properties such as thickness and bedding attitudes. Observation of small craters on the layered deposits will allow us to put a more accurate constraint on the timescales of deposition and erosion operating in that area as well as providing information about the target material and crater modification processes. HiRISE views of how these crater shapes have relaxed and where the icy layers have broken in a brittle way will give us a better idea of the strength of the layered deposit material and whether it has flowed significantly (like ice caps on Earth do).
Observing the relationship between changes in the residual ices and the current climate will allow us to better understand how the layered deposits were formed. This information can be used as a 'Rosetta stone' for interpreting the climatic record within the layers themselves.
Examples of HiRISE targets
Polar geology offers a diverse set of features to observe and puzzles for us to try and solve. Some examples of polar data from the Mars Orbiter Camera (MOC) are shown below and serve as a guide to the kind of features we will be targeting with HiRISE.
Southern (top panel) and northern (bottom panel) layering have very different appearances. Layering in the southern polar layered deposits shows staircase topography indicating differing material strengths between layers. In contrast, layering in the north usually crops out on smooth slopes. Both these scarps are roughly equatorward facing with the downhill direction from top to bottom. Top panel is a sub-frame of MOC image E11/03053, illumination from the upper right. Bottom panel is a sub-frame of MOC image M18/01897, illumination from the lower right.
A steep scarp in the North polar layered deposits at the head of Chasma Boreale shows the two main stratigraphic units that comprise these deposits. The polar layered deposits (upper unit) are bright and show continuous layers. The lower basal unit is dark and has a jumbled appearance with some layering visible. The basal unit is thought to be comprised mostly of sand Elevation decreases from top to bottom. MOC image E03/00889, illumination from the lower right.
Two views of an 8m deep pit (dubbed a 'Swiss-cheese feature') separated by two Martian years. The gridlines are spaced every 50 m; expansion of the pit relative to this fixed coordinate system can be seen. The walls retreat about 5 meters every Martian year, HiRISE will allow us to monitor these changes over many years. The unchanging polygonal cracks on the surrounding terrain are a mystery but allow us to precisely co-locate these two MOC images M09/00609 (left) and R08/01050 (right). Illumination is from the lower left.
Craters are uncommon on the polar deposits. On the north polar layered deposits and residual cap there are only a few known examples shown above. All four of these rare craters exhibit raised rims, and to a lesser degree, circular outlines. The craters on the northern residual ice are highly degraded, consistent with a very high resurfacing rate of that unit which gives it its very young age.
Faults give us information about the strength of the material involved. Some examples of brittle processes on the bounding scarp of the south polar layered deposits are shown above. Arrows highlight offset layers indicating that faulting has taken place. Images are from left to right M04/02455, M10/03514, M15/02058 and E09/01926. Illumination is from the bottom right in each case and the downhill direction is from top to bottom (except in second from left).