Planning for the Scientific Exploration of Mars by Humans By the mepag human Exploration of Mars Science Analysis Group (hem-sag)
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Polar Processes and Formation of the Polar CapMEPAG 2006 Goal II-Objective B Investigation 5 lists four objectives for polar cap measurements: (i) the relative and absolute ages of the layers, (ii) their thickness, extent and continuity, (iii) their petrologic/geochemical characteristics (including both isotopic and chemical composition), and (iv) the environmental conditions and processes that were necessary to produce them. Progress on (iv) is very much tied to progress on global atmospheric dynamics (section 1.1) as the poles comprise a key element in the global annual cycle of volatiles and dust. Orbital geophysical sounding is currently proving information at ~100 m resolution (Figure 2) in the vertical to address (ii), and early results indicate the existence of non-continuous layers in the Polar Layered Deposits (PLD) suggesting periods of retreat and advance of the residualcaps over climate time scales (Figure 3). The Phoenix mission will provide the first information on Mars ice grain and dust characteristics using microscopy, wet chemistry and thermal evolved gas analysis (Smith, 2006), and will provide meteorological information from its landing site on Vastitas Borealis, the vast plains which surround the north pole. The return of a sample from within the PLD could provide an important snapshot of absolute age, isotopic and chemical composition for that segment of the PLD record, but unraveling the complex climate history evidenced in the PLD would require access to the full stratigraphic record through deep drilling or sustained sampling through exposed scarps. Understanding the asymmetries between north and south polar caps would require access to the stratigraphic record at both poles. In 2030, it is anticipated that access to the climate record held in the PLD would remain one of the most exciting and important challenges in the scientific exploration of Mars. 
Figure 2. Cross section of the north polar cap showing extent of cap and layering. Vertical depth is ~2km (Putzig et al, 2007, 7th Int Conference, Abstract #3295) Red box indicates a target region for a drill site where SHARAD shows homogenous layering on a macro scale. 
Figure 3. Detail of complex cross-bedding. Horizontal scale of image is ~210 m (Tanaka, 2007, LPSC). Planum Boreum baseline recent climate record and constraint on maximum obliquityEstimates of the age of the North Polar Layered Deposits based on crater counts vary significantly from 3 x 105 – 10 x 106 years, with the South Polar Layered Deposits appearing significantly older, 7-15 x 106 years (Fishbaugh and Head, 2001 and references therein). The North Polar Layered deposits show evidence of at least one retreat due to topographical features in the Vastitas Borealis (Fishbaugh and Head, 2001) and complex cross-bedding (Tanaka, 2007) — see Figure 3. Chaotic solutions for Mars climate history, based on uncertainities in orbital parameters, can show obliquity constrained between 15 and 35 degrees for the past 100M years (Folkner et al., 1997) or a significant change of state 5M years ago towards a high obliquity state of ~60 degrees (Laskar et al, 2002; Laskar et al, 2004). Models have shown that a change to a high obliquity state of >40 degrees can cause the loss of North Polar Dome ice at a rate of 10cm/year with preferential deposition in the equatorial region (Haberle et al, 2003; Mischna et al, 2003). Another approach to understanding the age and formation processes of the North Polar Layered Deposits has been to associate the layered structure with precession-related (51,000 years) or obliquity-related (120,000 years) polar insolation cycles with varying amounts of ice and dust deposited in each cycle. The dustier layers are hypothesized as lag deposits associated with higher insolation phase (e.g., Levrard et al, 2007). From Fourier transform analysis of imaged stratigraphy in north polar dome troughs in the upper 800 m, a dominant wavelength is found for the layered structure of ~30 m (Laskar et al, 2004; Milkovitch and Head, 2005). A paradox arises with the combination of insolation information, observed layer thickness and the North Polar Dome depth of ~2 km observed by MARSIS and SHARAD (Picardi et al, 2005 (check), Putzig et la, 2007). Modelled polar insolation is clearly driven by obliquity rather than precession (eg. Levrard et al, 2007) with insolation values first reaching the modeled threshold value (300 Wm-2) that would produce net loss of water vapour and formation of lag deposits around 0.5M years ago, and with around 34 insolation cycles crossing the threshold value in the last 5M years. As has been pointed out by both Levrard et al (2007) and Milkovich and Head (2005), there are many more layers observed in PLD troughs, with various interpretations: more than one layer is laid down each obliquity cycle with unknown process, the polar layered deposits survived the large obliquity change at ~5M years, or the obliquity range has not exceeded 40 degrees over a period significantly longer than 5M years. A key scientific goal to address this paradox is to access the stratigraphic record from the current residual cap through to the base of the PLD. Dome sites are chosen on Earth to give baseline climate records via an extracted ice core. Figure 4 shows that most deep drill sites in Antarctica (Vostok aside) are associated with dome features. Typically dome sites are selected as they represent regions of net accumulation, close to a flow divide, and where flow distortion of the ice sheet is minimal and well understood (Morse et al, 2002). An alternative approach to accessing the ancient ice at the base of the PLD is to sample from the scarp face, using mobility to access scarps of different ages, with shallow core segments pieced together using stratigraphic markers and flow modeling to provide the ancient climate history. As the north polar dome has been hypothesized to have undergone at least one retreat (eg. Fishbaugh and Head, 2001), and major dust lag unconformities can be predicted from climate modeling (Levrard et al, 2007), sampling from the base may be difficult to interpret in terms of chronology and local versus regional effects. 
Figure 4. Deep drill sites in Antarctica mainly associated with dome features. (ICWG, 2003). Horizontal Sampling of the NPDHorizontal sampling (shallow drilling at several sites along a transect) of the NPD could be complementary to deep drilling as a way of investigating heterogeneity across a polar dome (Mayewski et al, 2005), or trying to understand specific local episodes of cap retreat evidenced by e.g., cross-bedding. Sampling overlapping stratigraphies along a descending transect may be an alternative means of reconstructing the long-term climate record. Stratigraphic Markers and DatingDue to the expectation of uncomformities described above, a visual dating of stratigraphy would best be supported by an absolute dating method. The nuclear-decay modulated Ar40/Ar39, or atmospheric loss modulated N15/N14 have been suggested as Martian chronometers (Doran et al, 2005) with values sampled from atmospheric inclusions and calibration of N from absolute dating of rocks which contain inclusions by other methods. Cosmogenic nuclides in trapped dust may also be used for dating (eg. 53Mn (half life 3.7My), 10Be (1.5 My), 26Al (0.705 My) and 14C (5730 years) (Doran et al, 2005)) but due to high background cosmogenic radiation it may be necessary to return selected samples of dust to Earth for sensitive analysis. 26Al is deposited on a planet directly from supernovae ejecta and can be used also to identify pre-solar condensates (Cole et al, 2006). On the basis of a model of Mars ice reservoirs and isotope fractionisation, Fisher (2006) has suggested that the D/H ratio may provide a very effective chronometer for the polar cap, with obliquity cycle variations (120,000 years) appearing as a variation of 1000 to 2000 per mil, and an additional small component of variability at 2500 years detectable with precision of 1 per mil. Exchange with subpolar reservoirs is suggested to appear as a trend in D/H superimposed on the above mentioned avrialbility. Lacelle et al (in preparation) have proposed that a measure of molar ratios of the set of occluded gases CO2, O2/Ar, N2/Ar and N2/O2 could additionally discriminate between vapour and water deposited ice and perhaps indicate the presence of microbes. Composition (atmospheric gases and dust) and dating of lag deposits found would be especially interesting, to understand extremes of global dust and volatile cycles. Measurements of ice composition, mass concentrations of ionic species and electrical conductivity, associated with stratigraphy could help with the detection and interpretation of recent impacts or volcanic events. Visual stratigraphic markers could include gas bubbles as well as dust. On Mars, dust is expected to dominate. Expected grain and crystal sizes would determine imaging needs. Desired precursor measurements
Investigations for the human era of explorationDeep core and baseline chronology
Polar cap mass and energy balance for current climate state and seasonal cap formation processes
Geophysical investigation
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