Layers in the Martian polar deposits and the climate connection

Details Layers in the Martian polar deposits and the climate connection Layers in the Martian polar deposits and the climate connection

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..307f&link_type=abstract

European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a

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Abstract Since the polar layered deposits (PLD) likely contain the most complete record of relatively recent climate change on Mars, it is crucially important to obtain a realistic understanding of what a "layer" is and what its characteristics and stratigraphic position tell us about the contemporary martian climate. Using data from the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter, we reassess the methods by which layers within the NPLD could be defined and delineated. From these analyses, we find that the appearance of layers depends to a large degree on the distribution of younger, mantling deposits and on the shape and location of the particular outcrop. We have measured the thicknesses of several layers found to be similar to famous Marker Bed discovered by [1] using a high resolution DEM created from a HiRISE stereo pair. The thicknesses and separation distances of these layers and their observed morphologic characteristics hint at several possible but disparate climate connections. Defining "Layer" HiRISE has confirmed that apparent layer brightness is not necessarily indicative of the bulk composition of the layer [2]. Therefore, the bright and dark striping of the polar layers cannot be used alone to delineate layer boundaries. Additionally, there can exist several types of layers, defined by the means used to detect them: radar, visible images, thermal images, spectroscopic measurements, etc. Compositional layering, for example, will not necessarily exactly match visible layering. Different scales of layering also certainly exist. For example, Viking scale layers actually consist of packages of MOC scale layers, and annual layering cannot be observed from orbit. In this study, we define a "layer" as a stratum evident in images and topography that cannot be broken-up into thinner strata at the best data resolution available. As discussed below, we delineate the layers based on their morphologic appearance, topographic expression, and, to a lesser extent, on their apparent brightness. Controls on Layer Brightness Qualitatively, our observations indicate that apparent brightness depends largely on the presence of younger, mantling deposits of frost and dust. The distribution of this mantle in turn is controlled by trough shape (e.g., bends in the trough affecting wind direction), trough location (wind and illumination patterns), and roughness on the several meters scale. Fig. 1 illustrates that the small, meter-scale physical properties of the layers (aspect, roughness, and slope) have little effect on apparent brightness. The Marker Bed and other marker beds Malin and Edgett [1] easily traced a particular layer in three different images and thus named it the Marker Bed. Our observations of HiRISE images reveal other layers similar in appearance to the original Marker Bed; we suggest that these layers have a similar origin and name them "marker beds" as well. The marker beds are hummocky, exhibit linear erosional fluting on their upper edges, and, compared to the surrounding layers, are generally smoother, protrusive, and covered with less ice and frost. These beds correspond to layers previously identified by [3] in MOC images (but not recognized at the lower resolution to be marker beds) and correlated across the PLD. We have not, thus far, found any evidence of finer scale layering within the marker beds, indicating that they were either deposited quickly, as massive beds, or that the younger, mantling deposits are shrouding evidence of fine layering. Fig. 2 shows examples of the marker beds. We have also observed sets of thin (~1m and less) layers between the marker beds in all HiRISE images examined thus far, but erosion and the presence of the younger mantling deposits makes it difficult to count these layers and correlate them from place to place. Future work with more images to be taken during the upcoming northern summer season and with more DEMs should make that process easier and allow comparison with layering elsewhere on the planet. Delineating Layers Layer boundaries are not sharp at the HiRISE scale, partly because we are observing a gradual transition (vs. abrupt) of depositional style and partly because of the presence of the younger mantling deposits. Thus, detailed analysis of high resolution topography is necessary. Even this is not entirely straight forward, as the topographic expression of morphologically defined layers is highly variable. To measure layer thicknesses, we use several averaged sets of two profiles each and look for breaks in slope that occur in each profile and correspond closely to morphologic these data, we find that the six marker beds measured range in thickness from 5 to 10 m, and their separation distances are all ~20 m or ~ 30 m, similar to the dominant brightness wavelengths discovered by [4]. Climate Connection Interestingly, the ratio of the separation distances between the marker beds and the thinner layers (20,30:1) is similar to the ratio of the orbital inclination period to the climate precession period (23:1). Based on models of polar layer formation as controlled by changing orbital parameters [5], our observations of layer thicknesses and separations indicate that when a low orbital inclination coincides with a low obliquity, marker beds are formed, and when precession takes control (during short, warm northern winters) thinner layers are formed. This scenario might explain the layer thicknesses and separations, but it does not explain the apparent erosional resistance of the marker beds. The erosional resistance is most easily explained by formation during times of high obliquity. Each of the following (or a combination) could occur during higher obliquities and could produce erosionallyresistant layers: layer ablation and consequent buildup of a dusty lag, increased atmospheric dust content creating dustier layers, or ice grain metamorphism and the growth of (or annealing of grains to form) larger ice crystals. References [1] Malin, M. and K. Edgett (2001), JGR 106, 23429. [2] Herkenhoff, K. et al. (2007), Science 317, 1711. [3] Fishbaugh, K. and C. Hvidberg (2006), JGR 111 (E06012). [4] Milkovich, S. and J. Head (2006), JGR 110 (E5). [5] Cutts, J. and B. Lewis (1982), Icarus 50, 216.

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