Generalization of Special Regions and the Utility of Maps
Figure 47 of the SR-SAG2 report plots the global distribution of confirmed and unconfirmed recurring slope lineae (RSL) with 50 km buffer zones around them (Figure 5.1). These buffer zones, the report argues, “provide for adequate precautions for spacecraft landings in their proximity, including an allowance for the possibility of an off-target landing (Figure 47)” (p. 943 of the SR-SAG2 report). This could be misinterpreted in the sense that a landing outside the areas shown in Figure 47 would definitely avoid RSL-related Uncertain or Special Regions, although the review committee is aware that this is certainly not the intention of the authors of the SR-SAG2 report. As RSL studies are a very active field of Mars research, it is expected that the number of fully and partially confirmed RSL will increase from now to the near future, just as it has increased from their first detection (McEwen et al. 2011) up to now (Dundas et al. 2015). Hence, the map displayed in Figure 47 represents only a snapshot in time and will probably be outdated soon. While it is helpful to provide a general overview of regions that may be favorable for the formation of RSL, it is of limited use in the identification of Uncertain or Special Regions. The same applies to other maps that also may be updated soon (e.g., Figures 45 and 46 of the SR-SAG2 report; see Figure 5.2).
Another potential source of misinterpretation related to the use of maps in Special Region studies is the issue of scale. Identification of a Special Region needs a multiscale approach (see also the discussion in Chapter 2, “Detectability of Potential Small Scale Microbial Habitats,” and thus, as far as missions to Mars are concerned, conservatism demands that each landing ellipse be scrutinized on a case-by-case basis. Maps, which come necessarily at a fixed scale, can only provide information at that scale and are, therefore, generalizations. The review committee envisages that the case-by-case evaluation will follow a process analogous to that used to certify the landing sites for Schiaparelli—the entry, descent, and landing demonstrator on ESA’s ExoMars 2016—and for NASA’s InSight lander. In the case of Schiaparelli, for example, all available data for the proposed site in Meridiani Planum were analyzed to determine if Special Regions existed within the landing ellipse. In particular, all HiRISE images were inspected for the possible presence of RSL.
In general, the review committee contends that the use of maps to delineate regions with a lower or higher probability to host Special Regions is most useful if the maps are accompanied by cautionary remarks on their limitations. Maps that illustrate the distribution of specific relevant landforms or other surface features can only represent the current (and incomplete) state of knowledge for a specific time—knowledge that will certainly be subject to change or be updated as new information is obtained.
FIGURE 5.1 Locations of recurring slope lineae (RSL) on Mars, identified at the time of publication. Red circles indicate confirmed RSL, while yellow triangles represent partially confirmed RSL. RSL require high-resolution and time-series observations for initial identification. They may be the most significant candidate sites for Mars Special Regions. SOURCE: SR-SAG2 report (Rummel et al. 2014, Figure 47); courtesy of the Second MEPAG Special Regions Science Analysis Group.
FIGURE 5.2 Map depicting geological features relevant to characterizing Special Regions on Mars. Indicated units describe shallow ground ice or potential transient surface water in terms of their depth below the surface and spatial continuity. The map base is MOLA digital elevation model of Mars (~463 m/pixel; Neumann et al. 2001) in simple cylindrical projection. Purple is low in elevation, and grey is higher elevation. Red and blue lines delineating regions are approximately 50 km in width. SOURCE: SR-SAG2 report (Rummel et al. 2014, Figure 45); courtesy of the Second MEPAG Special Regions Science Analysis Group.
Recommendation: Maps should only be used to illustrate the general concept of Special Regions and should not be used to delineate their exact location. Uncertain Regions in planned landing ellipses should be evaluated on a case-by-case basis as part of the site selection process. The goal of such an evaluation is to determine whether or not the landing ellipse contains water, ice, or subsurface discontinuities with a potential to contain hydrated minerals that could be accessed via a landing malfunction or by the operation of subsurface-penetrating devices (e.g., drills). As an example, landing site analysis will likely include a geological analysis, drawing on the Mars geologic literature (covering a broad range of relevant topics, including ground truth at previous lander locations) as well as orbital imaging, infrared spectroscopy, gamma-ray spectroscopy, and ground-penetrating radar sounding of the specific region.