To complete the work of this thesis, I developed a geographical database of the polar regions of Mars to enable comparisons of different data sets spread over different missions. I investigated geomorphological evidence for flow processes at the margin of the South Polar stratified deposits.
Introduction
Rationale for studying the Martian polar regions
In Chapter 2 I describe the discovery of a clear division within the stratigraphy of the arctic layered deposits. In Chapter 3, I describe modeling the erosion of the southern residual CO2 cap by expanding flat-floor wells.
Description of the Martian polar regions
They are sparsely cratered (Koutnik et al., 2002; Herkenhoff and Plaut, 2000) being one of the youngest surfaces on Mars. Herkenhoff and Murray (1990a) examined the color and albedo of the southern layered deposits and found that they closely matched bright reddish dust seen elsewhere on Mars.
Tools and data used in this thesis
- Mars Global Surveyor and Mars Odyssey
- Development of Martian Geographic Information Sys- tems and other toolstems and other tools
Knowledge of the Martian geoid (Aeroid) is necessary to convert this planetary radius to topography. This led to an order of magnitude reduction in the position errors for MOC narrow-angle images.
The relevance of this work to the larger picture
- North polar stratigraphy
- Southern residual cap geomorphology
- Flow vs. brittle processes at the margins of the southern layered depositslayered deposits
In Chapter 4, I examine fringe features of the southern stratified deposits for evidence for and against flow. A series of craters near the mapped rim of the stratified deposits at the South Pole contain large mounds of stratified sedimentary material.
Abstract
Introduction
In many recent publications, the ensemble structure of the polar layered deposits and the thin residual shell has been referred to simply as 'the cap', which is the convention we will adopt for this paper as well. Derived shaded relief image of the region containing the head of Chasma Boreale is shown.
Data preparation
This allows the narrow-angle MOC image to be projected onto the map in a best-fit manner to ensure that those MOLA points project to the correct position on the MOLA-derived basemap. Radiometric calibration of the MOC narrow-angle images was performed with the USGS ISIS software package.
Change in layering style
The altitude of the contact (approximately -4200 m) closely matches the altitudes recorded elsewhere in this region (see Figure 2.3). Here the platy unit protrudes about 1km from the bottom of the escarpment; a slope fracture is also visible at the contact.
Correlation with duneforms
You can see in Figure 2.11(a) that the contact height in this image is about -4600 meters. The chasm contains two distinct heads (see Figure 2.1(b)); the eastern frontal scarp of the chasm at a higher elevation contains the contact (Figure 2.11(a)).
Discussion and conclusions
This would significantly increase the minimum flat unit size originally estimated in section 3. As a result, no flat unit exposures are expected at the lid margin in this region. If Figure 2.12 is an accurate representation of the current state, the volume of material per unit plate can be roughly estimated.
Acknowledgements
Thus, this dark, sand-sized material is not expected to have spread over much of the planet again. The fact that the icy part of the northern polar cap was absent at some point indicates a much warmer polar environment than the current one. It is possible that the water was concentrated at lower latitudes filling craters and is responsible for the apparent sediment exposure reported by Malin and Edgett (2000).
Abstract
This fact, together with the observed shallow depths of these depressions, limits the contribution of the entire residual mantle to the global CO2 pool. Size distribution analysis combined with growth process modeling provides information on the age of these features. We find a narrow size distribution that indicates the total formation time and the final period during which the new features formed.
Introduction
The thickness of this residual CO2 layer is directly related to the demand for the global supply of this important volatile substance. Most of the notable features of the Swiss cheese characteristics are visible here, including steep walls, flat floors and a slight asymmetry in the north-south direction. Finally, we select a region of interest on the southern residual shell (see Figure 3.21) and use our model to infer the population history of Swiss cheese traits found there.
Feature description
This combination of characteristics has not been observed in Swiss cheese traits elsewhere on the residual cap. The second type (see Figure 3.2) occurs more widely on the remaining hood and is not restricted to any particular region. The fact that these depressions are strictly confined to the remnant cap indicates that the medium in which they form is dominated by CO2 ice and also that the CO2 coverage of the remnant is at least as deep as the relief of these depressions (ca . most cases). 2001) showed, using images separated by 1 full Martian year, that the walls of these depressions are retreating at a rate of 1-3 meters per year (see Figure 3.6, taken from that article).
Model description
These features occur throughout the residual canopy, in most cases right up to the edge of the residual CO2 deposit as can be seen in Figure 3.5. Sublimation of the water ice itself is assumed to be negligible at these temperatures and time scales. The model steps forward in time, changing the shape of the depression and updating subsurface temperatures in the water ice at the end of each step.
Model results
- CO 2 only cases
- Cases with a water ice base
We experimented with various albedo gradients and also varied the thickness of the overlying low-albedo CO2 sheet. The length of time the depressions spend in the ramp-up phase is directly related to the thickness of the low-albedo CO2 ice sheet. The darkest albedo is the albedo of the CO2 ice where it is in contact with the clean CO2 substrate.
Thermal observations from TES and THEMIS
The blue (cold) area at the bottom is the edge of the remaining CO2 ice sheet. In most of the remaining cover, only isolated remnants of an 8 m thick layer in the process of erosion with Swiss characteristics are visible. The warmer areas correspond to the floor of the depressions along the walls (trenches).
Implications for the solid CO 2 inventory
We have demonstrated two possible ways to produce this characteristic morphology with either a particularly pure CO2 or water ice substrate. Where we have THEMIS observations of sufficient quality, they show that water ice underlies these features. We believe it is a safe generalization that all Swiss cheese traits are subject to water ice.
Population statistics of Swiss-cheese features
We also measured the preferred pointing direction by drawing line segments from the cusp on the poleward wall (see Figure 3.1) through the center of the depression. Using the average diameter of the population (217m) and the range of expansion rates quoted above, we find that the age of this population could range anywhere from 43 to 217 Martian years (or 81-408 Earth years). We can choose the interquartile scale (40m in this case) to represent the width of the distribution.
Implications for environmental variability
Growth stops due to some climatic disturbance and the depression begins to fill with a CO2 ice and dust mixture (D). Alternatively, moats may just be a feature of large Swiss cheese features where the center no longer feels a significant radiating effect from the walls. In this picture, exposed water ice on flat surfaces would currently tend to be covered by CO2 year-round, but the extra radiation reflected by the walls of the depression could create a small zone where the opposite is true.
Acknowledgements
Finally, after an unknown period of time, the climate returns to its former state and the walls begin to move outward again, leaving the recent central deposit as an island surrounded by a moat. The width of the canals and their current rate of expansion indicate that this last event occurred on Mars several decades ago.
Abstract
The white circle within 87◦S represents no data, the dashed black line represents the defined boundary (Tanaka and Scott, 1987) of stratified deposits.
Introduction
Evidence of ice flow
- Compressional ridges
- Superposed mounds
- Terminal moraines
A common phenomenon where these hills are in contact with a crater wall are ridges at the edge of the deposit (see Figure 4.3). The arcuate fractures in slope seen at the edge of the layered deposits in Figure 4.5 indicate that. The black rectangle shows the location of the MOC narrow-angle frame in the left panel (E16/00609, illumination from lower right).
Evidence contrary to flow
- Large-scale slumping
- Landsliding
- Brittle faulting
Landslides are not observed at the margin of the south polar stratiform deposits in other locations (although they are suspected in parts of the north polar stratiform deposits (Byrne and Murray, 2002)). A slope break at the base of the scarp indicates the presence of talus mounds. Disturbed layer sections are common around the edge and near the bottom of layered deposits in the longitude range from 110◦E to 160◦E.
Conclusions and future direction
If the polar stratified deposits waxed and waned in thickness, then glacial activity at its margin would also respond to the increased pressure and thrust of the thicker ice sheet. The evidence supporting both flow and brittle processes indicates that the South Polar stratified deposits alternated between these two regimes. However, one would expect that movement of the ice would affect the larger craters more than the smaller ones.
Conclusions
Overview and accomplishments of this work
It was found that the nature of the lower unit was mainly sand-based, which is radically different from that of the overlying dust-ice mixture. If there was indeed a time when the icy Arctic cap was absent and a large Saharan-scale sand deposit existed, this leads to new insight into the variability of the Arctic environment. It is also a possibility that the stratified deposits are a recent phenomenon and did not exist at all before a few million years ago, although the inferred surface age of 30-100 Myr of the southern deposits (Koutnik et al., 2002) contradicts this.
Present shortcomings and future opportuni- ties of Martian polar scienceties of Martian polar science
Using the layers present in the Greenland ice sheet as an analogy, the Greenland ice sheet can tell us a little about Earth's history without actually going there and obtaining an ice core for analysis. Given that there are several main questions that can be answered in the short term using existing or soon-to-be-obtained data. The elusive holy grail that everyone continues to pursue, ie. deriving a detailed history from stratified sediments will probably have to wait much longer than that.
Application of future data to these issues
Detection of the shape of internal layers will be an important diagnosis of whether flow processes are occurring. And extracting the subsurface topography of the ice caps will go a long way in constraining models of the flow history. Even if all these future missions explode on the launch pad, there is more than enough data to keep Mars polar scientists busy for a long time to pursue just the research questions outlined in the previous section.
A Geographic Information System (GIS) for Mars
Interpolating gridded products
Using MOLA to register MOC images
This is unfortunate because the spacecraft time for each line is calculated by counting the number of lines and combining that with the exposure duration of the line. If the lines are completely missing, then this method of calculating the time of each line will fail and it will not be possible to match the image to the MOLA dataset. For most areas of the planet this is not serious; image locations can be corrected manually.
Radiation balance inside a depression com- posed of annular rings
- Scattering of radiation from one facet to another
- Scattering of radiation from one annular ring to an- otherother
- Calculating the energy balance of an annular band
Noting that δA1 =δl1r1δφwhere δl1 is the width of annulus 1, we integrate φ from 0 → 2π to sum the contribution of each part of annulus 1. The flux from annulus 1 to annulus element 2 is:. The limits of the original integral are 0 → 2π, so this integral is now a closed unit circle in the complex plane. For thermal radiation, the flux (Fi) leaving each square meter of the ith lane can be calculated by combining its thermal emission and the reflected thermal radiation incident from the other lanes.
Response to energy balance: Conduction and sublimationsublimation
Following depression geometry
- The outer edge
- The inner edge
Murray, High-resolution topography and albedo of layered sediments of the south pole on Mars, Journal of Geophysical Research b. Tanaka, Geological history of the polar regions of Mars based on Mars Global Surveyor data. Kolb, Geologic history of the polar regions of Mars based on Mars Global Surveyor data.
