8.1. Cratering Studies
 Impact craters are ubiquitous on Mars, ranging from the Hellas basin (∼1800 km diameter) down to a crater only 10 cm diameter imaged by MER Opportunity (http://www.nasa.gov/vision/universe/solarsystem/mer-04272005.html) [Grant et al., 2006]. HiRISE observations of impact craters can achieve several science objectives by measuring meter-scale structures. While fresh impact craters often expose the bedrock of the region, useful for regional stratigraphic studies, and provide traps or settings for sand dunes, gullies, etc., here we focus on studies more directly related to cratering. Below we discuss three broad topics: what craters might be able to tell us about subsurface water, how they reflect impact processes and target material properties, and how they can be used to constrain the ages of planetary surfaces.
 Craters with fluidized ejecta morphologies have long been of interest because they are thought by many investigators to indicate the presence of ground ice [e.g., Carr et al., 1977a; Stewart et al., 2004], and crater morphometry has been used to map out variations in depth to ground ice and to constrain ice abundance over time [e.g., Mouginis-Mark, 1979; Costard, 1989; Barlow, 2004]. However, evidence for fluids in ejecta emplacement, such as boulders transported by water or channels suggestive of dewatering of the ejecta flow, has not been confirmed in MOC images according to Williams and Edgett . It is possible that the resolution, SNR, and topographic information of HiRISE images will lead to detection of such features.
 Subsurface ice abundances can also be investigated from viscous relaxation of topography [Squyres, 1989; Squyres and Carr, 1986]. Small craters are convenient for such studies because we have some idea of their original form [e.g., Melosh, 1989]. Ice-rich permafrost is quite mobile under current Martian conditions; for example, E. M. Turtle and A. V. Pathare (Numerical modeling of crater relaxation on Mars: Implications for sub-surface ice content, submitted to Geophysical Research Letters, 2006) predict the complete relaxation of a 2-km diameter crater in ice-rich ground near Mars' equator in just 104 to 105 yrs, whereas a 20 m diameter crater would require 108 yrs to relax. HiRISE DEMs of multiple small craters on potentially ice-rich terrains could be used to constrain the subsurface ice abundance, stratigraphy, and history, complementary to observations from the Shallow Radar (SHARAD) experiment on MRO [Seu et al., 2004, 2007]. Studies of viscous relaxation of craters in icy terrains may also provide constraints on age estimates [Pathare et al., 2005].
 The size-frequency and spatial distributions of blocks in impact ejecta and of secondary craters are important to understanding crater excavation processes and properties of the target material [Melosh, 1989], including tests of the hypothesis that Martian meteorites originate from spallation [Melosh, 1984]. Bart and Melosh  measured blocks and boulders around lunar and Martian craters, and found that the size-frequency distribution (SFD) is approximated by a cumulative power law slope of –4. This means that a factor of five increase in spatial resolution (roughly the HiRISE to MOC ratio) would result in resolution of 625 times as many boulders per unit area. However, most rock distributions roll over and become shallower (smaller negative power law exponent) at smaller sizes [Golombek et al., 2003b]. Given the number of blocks (≥5 m diameter) resolved by MOC, we can expect HiRISE to resolve much greater numbers of 1–5 m diameter blocks and boulders. The SFD of boulders, for example in a stratigraphic layer, may enable us to deduce whether or not the layer was emplaced as impact ejecta. The observed abundance and distribution of large ejecta blocks can also reflect the amount of gradation a crater has undergone since formation as well as providing clues (e.g., burial by aeolian drift) to responsible processes [Grant et al., 2006].
 Another potential use for small craters is for estimating the ages of planetary surfaces or their modification [e.g., Hartmann and Neukum, 2001]. However, it has been suggested that the majority of small (<300 m diameter) craters on Mars are secondary craters, and that the production function for small primary craters is poorly known [McEwen et al., 2005; McEwen and Bierhaus, 2006]. Millions of secondaries form essentially simultaneously and cannot be assumed to be random independent events like primary craters, so crater counts for age dating may be meaningless if secondaries dominate the statistics. Although some secondaries have distinctive morphologies, distant secondaries are expected to be more circular and isolated, thus difficult to distinguish from primaries. Furthermore, attempts to distinguish primaries from secondaries (which are generally shallower) are complicated by aeolian erosion and infilling. Nevertheless, there may be morphologic differences that can be recognized in higher-resolution images and DEMs. At a minimum, HiRISE images should reveal whether or not a crater is pristine by detecting the rock distribution of the ejecta blanket [Grant et al., 2006] (see section 8.8) or distinctive color properties of the ejecta, as is the case for the image in Figure 17. We can then study the morphologies of pristine secondary craters (using DEMs whenever possible) as a function of range from the primary (where identifiable), to develop quantitative criteria for identification of secondary craters. Counts of small primary craters, if we can confidently identify them as such, would be of great value for dating young surfaces where large craters are infrequent. There are issues of great interest to understanding the geologic evolution of Mars, such as whether there has been any recent volcanic and/or fluvial activity and how much Amazonian climate change may have occurred, for which chronology is key. HiRISE data may also show a clear “rollover” in the abundances of craters smaller than a few meters diameter, from the influence of the atmosphere [Chappelow and Sharpton, 2005].
 Mars is fundamentally a volcanic planet. It is home to the largest volcanoes in the Solar System and the majority of the surrounding plains were once covered with lava flows [e.g., Carr, 1973; Carr et al., 1977b; Greeley and Spudis, 1981; Greeley and Schneid, 1991; Keszthelyi and McEwen, 2007]. All the Martian meteorites are igneous rocks and the surface is dominated by mafic (basaltic to andesitic) compositions [e.g., Nyquist et al., 2001; Christensen et al., 2001], but more evolved compositions may be present locally [Christensen et al., 2005]. Volcanic exhalations would have entered the Martian hydrologic cycle with a mix of sulfur, halides, and other volcanic volatiles [e.g., Settle, 1979; Craddock and Greeley, 1995]. The heat from magmatic activity is likely to have caused major (local or regional) perturbations of the cryosphere, perhaps even triggering catastrophic aqueous floods [e.g., McKenzie and Nimmo, 1999; Head et al., 2003a]. Volcanological studies conducted with HiRISE will aim to (1) better understand both effusive and explosive examples of this fundamental geologic process and (2) investigate the interactions between volcanism and water.
 For effusive volcanism, a prime focus will be on imaging locations where lava flows can be seen in cross-section. HiRISE images hold out the promise of being able to detect vertical variations in lava flow structures. Of particular interest would be morphologies related to variations in vesicularity, which can be related to the style of emplacement (Figure 20). While HiRISE cannot resolve vesicles from orbit, terrestrial experience shows that the more vesicular portions of lava flows erode more easily. The increased porosity usually results in higher permeability, perhaps leading to color variations due to hydrothermal alteration being visibly more pervasive in the vesicular portions of lava flows. The layering seen in MOC images from some canyon walls are similar to the stacks of lava flows seen in flood basalt provinces [McEwen et al., 1999]. However, MOC resolution has been inadequate to allow quantitative measurements of parameters such as the thickness of the upper vesicular crust and dense core of individual lava flows. These kinds of measurements should allow estimates of eruption duration and eruption rates, via the method used to study terrestrial flood basalt eruptions [Thordarson, 1995; Thordarson and Self, 1998]. Variations in the style of eruptions with time and location could help constrain the thermal and chemical evolution of the mantle. Sampling cliffs cutting lava flows of diverse ages in a wide variety of locations with the full HiRISE spatial resolution (and color) will be a high priority.
Figure 20. Idealized stratigraphic sections through the three main types of lava flows expected on Mars. Sections produced using terrestrial observations of active flows, older flows exposed in outcrop, and drill cores [e.g., Macdonald, 1953; Hon et al., 1994; Thordarson and Self, 1998; Coffin et al., 2000; Keszthelyi, 2002]. Note that pahoehoe and aa flows have three major sections, while rubbly pahoehoe has four. The relative thicknesses of these sections provide key constraints on eruption duration and other emplacement parameters [e.g., Thordarson, 1995; Keszthelyi et al., 2004].
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 Another focus for effusive volcanism will be lava surface features that are near or below the MOC resolution limit. Examples would include lava balls in open channels, inflation cracks in tumuli, and plates on lava lakes. These meter-scale features may be obscured by thick mantles of dust, so imaging will strongly favor young (or recently exhumed) surfaces. The plan is to begin by sampling lava surfaces with a variety of morphologies seen at the MOC/THEMIS scales. The best examples of each type of meter-scale and larger features, as seen in the early HiRISE images, will be targeted for stereo observations, to allow detailed quantitative comparisons with terrestrial features.
 The role of explosive eruptions in mafic volcanism is often under-appreciated. However, both terrestrial experience and physical models indicate that Martian pyroclastics are likely to be very voluminous. For example, the largest historical basaltic eruption on Earth, the 1783–1784 Laki eruption in Iceland, produced about 15 km3 of lava flows and close to 1 km3 of pyroclastics that were detected as far as Venice, Italy [Thordarson and Self, 1993]. On Earth, the basaltic explosive eruptions are most often relatively mild (i.e., “hawaiian” fountains or “strombolian” bubble bursts) [e.g., Wilson and Head, 1994]. However, the lower atmospheric density on Mars should enhance the production of pyroclastics [e.g., Wilson and Head, 1994; Fagents and Wilson, 1996]. In fact, some models predict explosivities similar to plinian eruptions on Earth [e.g., Hort and Weitz, 2001; Hynek et al., 2003]. Indeed, some of the older volcanoes, including Hadriaca and Tyrrhena Paterae, are thought to have large volumes of pyroclastics within the edifices [e.g., Crown and Greeley, 1993; Gregg and Williams, 1996]. The enigmatic Medusae Fossae Formation may be a more recent example of a large deposit of pyroclastic materials [e.g., Edgett et al., 1997; Keszthelyi et al., 2000]. While we will study these hypothesized pyroclastic deposits, it is likely that they have been significantly modified (altered, eroded and/or redeposited). The study of these deposits may provide more information on sedimentary processes than volcanic eruption processes. As such, these deposits are discussed in more detail under “Layering and Stratigraphy”.
 A focus for HiRISE studies of explosive volcanic eruptions will be to compare the size and shape of the best-preserved vents to predictions from theoretical models. Key model parameters that should be observable by HiRISE include (1) radius of welding/agglutination of large pyroclastics, (2) ellipticity of the vent nozzle, (3) flare of the vent nozzle, and (4) constraints on the spatial distribution of pyroclast sizes. The most promising locations will be identified using all the current Mars data sets [e.g., Mouginis-Mark and Christensen, 2005]. Those that look the best preserved in the initial HiRISE imaging will be retargeted to obtain stereo elevation data.
 Interactions between water and hot lava are of interest; there are many well-documented features that are diagnostic of water-lava interaction on Earth. These include pillow lavas, hyaloclastites, entablature jointing, rootless cones, and maar craters. We plan to use HiRISE to search for each of these on Mars. However, HiRISE is not particularly well suited to examine entablature jointing (10-cm scale features) or in the search for maar craters (kilometer scale features).
 On Earth, pillow basalts are considered diagnostic of subaqueous eruptions. They will be near the limits of HiRISE resolution, unless pillows are larger on Mars because of the lower gravity and/or higher viscosity lavas. Except under water pressures corresponding to the Earth's seafloor, pillow lavas are commonly associated with hyaloclastite deltas, where the rapidly cooling lava fractures. These deltas are often characterized by extensive palagonitization and steep bedding. When the lava pile emerges from the water, it can be capped by subaerial lava flows, forming a tuya or “table mountain” (Figure 21). Several mesas on Mars have been interpreted to be tuyas [e.g., Hodges and Moore, 1978; Allen, 1979; Lucchitta et al., 1994; Chapman, 2003; Chapman and Tanaka, 2001]. Testing these interpretations with HiRISE's combination of high spatial resolution, stereo, and color imaging is a top priority.
Figure 21. Idealized stratigraphic section through a tuya formed by eruption of lava under water or ice. Pillow lavas typically grade into steeply dipping hyaloclastites as water depth decreases. If the lavas build out of the water, the stack will be capped by resistant subaerial lavas.
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 The other hydrovolcanic landform that has been claimed to exist on Mars are rootless cones [e.g., Allen, 1979; Greeley and Fagents, 2001; Lanagan et al., 2001]. Rootless cones (aka pseudocraters) form when a lava flow moves over a wet substrate and the resulting steam creates a sustained explosive vent [e.g., Thordarson, 2000]. The gentle escape of steam can produce spiracles, which are much more subtle in their surface expression. Some features that could be rootless cones have also been suggested to be sedimentary or periglacial features such as pingos and kettle holes [Rice et al., 2002; Gaidos and Marion, 2003; Burr et al., 2005]. The increased spatial resolution of HiRISE may provide new evidence to help resolve this debate: rootless cones would be constructed of agglutinated spatter capable of standing at well beyond the angle of repose, mud volcanoes would be dominated by fluid flows, and pingos would have no emanations except perhaps water along the margins.
 The surface of Mars is not sculpted by active plate tectonics. Instead, a different style of tectonism operates, one dominated by volcanic loading of the lithosphere and by a hemispheric dichotomy of thicker crust in the south and thinner crust in the north [e.g., Zuber et al., 2000]. In the absence of a thin mobile lithosphere, Martian volcanoes such as Olympus Mons have grown to enormous proportions. Tharsis, the largest volcanic province on Mars, is so massive that its formation likely shifted the spin axis of the planet [Schultz and Lutz, 1988; Zuber and Smith, 1997]. Such tremendous loading of the lithosphere produces a tectonic signature with effects seen across much of the globe. The tectonism associated with lithospheric loading is manifest both directly as graben, wrinkle-ridges, pit-crater chains, fissures, etc. and indirectly as a major factor in controlling the locations and orientations of other landforms (e.g., Valles Marineris). Tharsis circumferential compressive stresses may have decreased in the Late Hesperian to Early Amazonian, producing strike-slip faulting [Okubo and Schultz, 2006]. Tectonism and volcanism are linked in another manner: buoyant magma can exert a force (cf. hydrostatic head) large enough to fracture overlying rock, and many of the fractures that crisscross Mars are suspected of having had such a volcanic component to their origin.
 Nonvolcanic processes influence Martian tectonism as well. The hemispheric dichotomy between the relatively young northern lowlands and the old and cratered southern highlands is the most notable of these. The dichotomy is demarcated by a topographic scarp that reflects a decrease in crustal thickness from the southern to the northern hemisphere. Counter-intuitively, tectonic features associated with the dichotomy are observed to be compressive on the southern side and tensile on the northern side. It is postulated that these features may have formed in response to northward lower-crustal flow as the boundary readjusts toward a more equilibrium state [Nimmo, 2005].
 Faults in porous sedimentary rocks are of particular interest as they play an important role in controlling volatile migration pathways, acting as either barriers or conduits [Okubo and Schultz, 2005]. The high-resolution images and topographic measurements of HiRISE will allow predictions of volatile pathways, which may help focus future surface exploration [Okubo, 2005].
 Although much is known about global- and regional-scale tectonics on Mars, many questions remain. Comparatively little is known about local-scale tectonics; these may hold the key to answering some outstanding questions. One of the basic axioms of structural geology is Pumpelly's Rule, which can be summarized as “a cumulative look at small-scale structures of a given generation and over a given region can reveal the large-scale deformation history.” HiRISE will image structures at the meter to decameter scale with the intent of unraveling tectonic processes that may have remained ambiguous when studied at larger scales. Another focus will be to build upon the capability born from MOC and MOLA for using the elevations of thin marker beds on opposite sides of a structure to measure the offset [Beyer and McEwen, 2005]. HiRISE stereo observations will provide new precision in Mars elevation maps, allowing fault offset and other forms of local strain to be evaluated.
8.4. Fluvial and Hydrothermal Processes
 The major issues involving the formation of fluvial landforms on Mars relate to the water source, the erosional mechanism, and the ultimate fate of the water. Which mechanisms and processes combined to erode and modify the surface, ultimately forming deep integrated valley systems, catastrophic flood channels and recent gullies? While similar fluvial features form on Earth, Mars is a planet whose present climate is not conducive to rainfall and in which liquid water is highly unstable. Although the spatial distribution, overall morphology, and source regions of these landforms differ from their terrestrial counterparts [Carr, 1996; Gulick, 1993, 2001a], water was available periodically throughout Mars' geological history in sufficient quantities over the required duration to provide a full suite of fluvial landforms. Many of the features employed by terrestrial geomorphologists to study and characterize fluvial processes are at the meter scale and finer and thus have been unreachable by previous orbital imagery of Mars.
 Perhaps the most puzzling fluvial features on the planet are the geologically young gullies eroding into the flanks of midlatitude craters and valley walls [Malin and Edgett, 2000a]. Numerous formation processes have been hypothesized including surface snowmelt, eruptions of liquid CO2, wind erosion, seepage from subsurface aquifers [see Heldmann and Mellon, 2004, and references therein], or even hydrothermal outflow from smaller localized heat sources such as a small dike, sill intrusion, a recent impact, or tectonic movement along a fault or fracture [Gulick, 2001b]. (There are also dry mass-wasting features on angle-of-repose slopes on Mars that have been referred to as “gullies. ”) While liquid water erosion or wet debris flows remain the most likely explanations, the source of water remains a mystery. Indeed, the diverse geologic contexts in which gullies are found suggests more than one type of source may be likely. Because of their young geologic age, the prospects of recently active surface water on the surface is of key importance to the exploration of Mars.
 HiRISE images will enable us to examine as yet unresolved characteristics of gully morphology that may shed light on their history and the specific source of erosion. Alcove regions at the head of the gullies tend to be associated with exposed subsurface layers. High-resolution morphology and color variations within these layers may implicate a particular layer as the source. Mantled deposits have been eroded by many gullies and have been suggested to be dirty snow deposits [Mustard et al., 2001; Christensen, 2003]. These mantles can be examined for subsurface layering and evidence of sublimation or melting. Gully channels are the most suggestive of liquid water as the eroding agent [Malin and Edgett, 2000a]. HiRISE will enable close inspection of channel levees, as well as tributary and cross cutting relationships indicative of multiple flood episodes. HiRISE stereo will aid in interpretive modeling of fluid flow by providing morphological dimensions and slopes [e.g., Mangold et al., 2003]. Topographic data may also prove key to testing hypotheses of gully formation by snowmelt [e.g., Clow, 1987; Christensen, 2003]. Deposition aprons at the base of each gully system can be examined for distributary characteristics indicative of sediment load, color variations suggestive of salt deposition from brine flows, boulders suggesting dense debris flow, and evidence of subaqueous or subaerial depositional environments.
 The formation of the outflow channels remains an enigma. Although the formation by catastrophic floods continues to be the leading hypothesis [Baker et al., 1992], debate persists [e.g., Leovy, 2000; Leverington, 2004] and it is still not clear exactly how such vast deluges of groundwater required to form the channels (on the order of 107 m3/s for both the Channeled Scablands and Altai floods terrestrial analogs [Baker et al., 1993]) were abruptly released onto the surface. Other scenarios for Mars such as glacial erosion, mudflows, debris flows, mass wasting, ice rafting, etc., have been proposed in an effort to reduce the magnitude of flood volumes and discharges required to erode the surface and carve the enormous channels. Detailed imaging of the stratigraphy, boulders and bedforms within the channel, including channel bars, mega current ripples (a.k.a., gravel wave trains, diluvial dunes and antidunes, etc.), areas of scour, and potential high-water indicators should enable a clearer understanding of outflow channel formation.
 MOC was expected to help resolve debates about the origin(s) of outflow channels; Malin et al. [1992, p. 7702] wrote the following:
MOC will provide a means of testing these alternative ideas and, perhaps more importantly, allow quantitative evaluation of fluvial models. These tests derive principally from differences between the hydraulic relationships for ice, air, lava, and water. These differences are most obviously manifested in the quantity and size of the debris transported by each of these fluids. Ice can carry very large loads, both in size and quantity, and these are usually angular and unsorted. Lava rarely carries debris, and when it does, the debris is often coated with spatter and not recognizable as a separate rock type. Debris flows transport considerable material, unsorted, but with distinctive size/frequency characteristics that depend on the rheology of the fluid phase. Water transports less debris, but is much more efficient in sorting and rounding; bedforms are created that reflect the flow regime of the fluid. Wind is most effective at sorting, but cannot carry large particles. Examination of the beds, banks, and mouths of the major Martian channel systems at meter-scales will provide a means of judging the nature of the debris transported by the channel-forming process and an evaluation of the mechanisms responsible.
 However, basic questions about formation of outflow channels remain unanswered. Perhaps MOC images are too low in spatial resolution to identify fluvially transported boulders. MOC detects many blocks that are larger than 5 m diameter, usually located very close to the source region such as impact craters or exposed bedrock layers just uphill from the blocks. It is rare on Earth for a block larger than 5 m to be transported for any significant distance (kilometers); there are a few examples in the Ephrata fan of the channeled scabland, but would just look like isolated bumps in a MOC image, not a clear boulder deposit. Many of the rocks on Mars originate from flood lavas; such lavas on Earth tend to have a joint spacing of 1–2 meters due to cooling and contraction, which break into boulders ∼1–2 m diameter when transported for a significant distance by water, mud, ice, or even just gravity. This idea is supported by observations in the outer slopes of Valles Marineris, where blocks are resolved by MOC only close to the outcrops or on landslide lobes, but not in the talus several km from the nearest apparent source region [Beyer and McEwen, 2005]. HiRISE will target those locations where transported debris is suspected, and where we expect the sediment load to be deposited such as upstream of channel constrictions. For example, HiRISE will target the only location known so far where a type of fluvial bedform known as mega-current ripples or subaqueous dunes has perhaps been detected, in Athabasca Valles [Burr et al., 2002, 2004].
 Since they were first recognized on Mariner 9 images, the Martian valley networks have continued to engender substantial debate. To this day, the formation mechanism(s) for valleys formed throughout Mars' geologic history, from those on the ancient heavily cratered terrain to those on the flanks of young volcanoes, remains uncertain. As with the fluvial features seen at other scales, the principle questions surround the water source(s) and cycling mechanism(s), the prevailing climate in which the valleys formed, the role of overland vs. subsurface flow and other flow processes, and the ultimate fate of the fluid. Unlike the outflow channels that could form under current climatic conditions, the fluvial valleys may have required a warmer, denser atmosphere and thus may provide a record of past climatic conditions [Carr, 1979; Mars Channel Working Group, 1983; Gulick and Baker, 1989, 1990; Baker et al., 1992; Craddock and Howard, 2002]. The extensive valley network development on the ancient heavily cratered terrains, Hesperian-age terrains such as in Valles Marineris [Quantin et al., 2005; Williams et al., 2005], and on the flanks of some younger volcanoes implies that water was cycled through the surface and atmosphere over prolonged periods [Gulick and Baker, 1989, 1990; Gulick, 1998, 2001a]. An Earth-like hydrologic cycle has often been invoked to provide a recycling mechanism for the ancient valley networks. However, regional or localized hydrothermal groundwater outflow [e.g., Brakenridge et al., 1985; Gulick and Baker, 1989, 1990; Gulick, 1998; Squyres, 1989; Squyres and Kasting, 1994] or combined snow accumulation and subsequent melting in hydrothermal areas have also been suggested for the younger valleys [Gulick, 1997, 1998, 2001a].
 Distinguishing between these different formation mechanisms may be aided by high-resolution imagery and stereo coverage of the headwater regions and valley walls. Do the headwater regions of the valleys start abruptly full-borne, transition into incipient piping or gully development, undercut certain layers or do they taper and blend in gradually with the surrounding terrain? Are there zones of mineral alteration or color changes associated with valley heads or walls? HiRISE's color and stereo capability combined with CRISM data may provide additional clues. However, the valley networks are old and heavily modified, so finding exposures that reveal primary meter-scale structures will be a major challenge [Carr and Malin, 2000].
 HiRISE will also search for evidence of hydrothermal outflow associated with fluvial and other geologic features. Alteration zones that commonly accompany regions of terrestrial hydrothermal outflow may be recognizable in HiRISE color imagery. Plausible hydrothermal regions, particularly in the headwaters of valleys, channels and perhaps even gullies that originate near volcanoes, vents, intrusions, large impact craters, maar-like craters, collapse zones and linear structures (e.g., faults and other tectonic features), will be high-priority targets for coordinated observations with CRISM. However, some morphologies associated with hydrothermal systems tend to be fragile and may not be well preserved.
 Perhaps the largest-scale controversy about the history of water on Mars is whether or not there have been oceans, and if so, when. Baker et al.  proposed this controversial hypothesis to explain a variety of geomorphological features including the formation of valley networks, glacial features, possible ice sheets and other features that on Earth would require long-term hydrological cycling. These “oceans,” Baker et al.  argued, would have inevitably formed in the Northern Plains from the large volumes of water required to form the outflow channels. In addition, Parker et al. [1989, 1993] presented evidence for the existence of what he interpreted as shorelines delineating such oceans. This controversy was reviewed by Carr and Head , who concluded that the best evidence for the presence of a large water body on the northern plains may come from the Vastitas Borealis Formation (VBF), which they interpreted as an Upper Hesperian sublimation residue from the ponded outflow channel effluents. However, Tanaka et al.  interpreted the origin of the VBF as from a variety of processes involving ground volatiles, postdating potential oceans. Hopefully HiRISE data can better constrain the origin of the VBF, perhaps by revealing distinctive distributions of boulders or small-scale morphologies. Geomorphic evidence for shorelines has also been controversial, with several workers concluding that Noachian-age features would no longer be recognizable. However, geomorphic evidence persists for a Noachian-age paleolake at the headwaters of Ma'adim Vallis [Irwin et al., 2004].
 Persistent surface water is important to habitable environments; the discovery of meandering channels in a fan-shaped sequence provided important geomorphic evidence for persistent water [Malin and Edgett, 2003; Moore and Howard, 2005; Bhattacharya et al., 2005]. HiRISE will follow up on this discovery and perhaps find additional evidence for persistent surface water. The evaporite sequences discovered by MER [Squyres et al., 2004a, 2004b] and Mars Express [Bibring et al., 2005] will also be studied in greater detail by HiRISE and CRISM, as discussed in the next section.
8.5. Layering and Stratigraphy
 Interior layered deposits in the canyons of Valles Marineris [McCauley, 1978; Lucchitta et al., 1992] were identified from Viking and Mariner images as alternating light-dark banding on the scale of hundreds of meters (Figure 22). MOC images showed that this apparent layering was due to relatively flat surfaces covered by dark debris and steeper slopes that appeared brighter due to no accumulation of this dark debris [Weitz et al., 2001]. However, a finer, meter-scale layering became apparent in MOC images of the interior layered deposits (Figure 22b). The two favored origins for the layering are deposition of material in standing bodies of water [e.g., Nedell et al., 1987; Malin and Edgett, 2000b] or volcanism [e.g., Chapman and Tanaka, 2001; Komatsu et al., 2004]. Light-toned meter-scale layering is seen in many impact craters (Figure 23) and terrains elsewhere on Mars [Malin and Edgett, 2000b], suggesting that whatever process(es) emplaced the layered terrain occurred widely in the Martian past.
Figure 22. (a) Viking mosaic of a portion of Hebes Chasma and the interior layered deposit unit contained within the chasma. (b) Portion of MOC image M0900284 at 4.3 m/pixel scale showing a cliff face along the interior layered deposits. The kilometer-scale light and dark banding which had been interpreted as layers in the Viking image actually reflects differences in the amount of dark talus accumulated along the slopes of the unit. The MOC image does reveal meter-scale layering that was not resolved in the Viking images.
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Figure 23. Portion of MOC image R1800383 of layers in Becquerel crater (21.6°N, 8.3°W) at 3.15 m/pixel scale. Some layers appear to be only a few meters thick and are rhythmic, while other layers appear more resistant and massive in appearance. Image is 1.61 km across.
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 Even higher-resolution images from HiRISE could reveal finer-scale layering in these light-toned layered units, as well as geologic contacts that could help decipher the processes that emplaced the layered rocks. The close association between sulfates as detected by the OMEGA instrument for several of these layered deposits [Gendrin et al., 2005; Bibring et al., 2005] suggests that both CRISM and HiRISE may further reveal why some light-toned layered deposits have sulfate signatures while others do not. Color images taken by HiRISE could also reveal compositional differences in the layering that could help us decipher their origin. Stereo images could be used to determine the thicknesses and absolute heights of different layers, which will show if layers follow a specific elevation and were therefore likely formed in a lacustrine environment, or if the layers form a blanket that covers preexisting uneven topography, which would favor an airfall (volcanic or aeolian) origin. HiRISE may be able to detect large-scale depositional structures such as cross-bedding, which has already been suggested from MOC images [Komatsu et al., 2004].
 Results from the Opportunity Rover at Meridiani Planum also shed new insight into layering and stratigraphy on Mars. Images returned from the rover show cm-scale cross-laminations interpreted as evidence for flowing water at the site [Squyres et al., 2004b] as well as stratigraphic exposures of multiple layers of sulfur-rich sedimentary rocks along the walls of larger impact craters [Grotzinger et al., 2005]. Orbital images and OMEGA spectra of the Meridiani Planum region suggest the light-toned layered units may extend over a large area [Hynek, 2004; Arvidson et al., 2005]. The ability to tie the rover images from the ground to orbital images has greatly aided in deciphering the geologic history of this particular area on Mars. HiRISE images of the Opportunity landing site will provide an intermediate resolution between the rover images and the current orbital data sets to further elucidate how layering and stratigraphy appear at different scales.
 In addition to the fine-layered deposits, much of the upper Martian crust appears more coarsely layered. Such crustal layering is especially well exposed in the outer “walls” of Valles Marineris, where it has been interpreted as primarily volcanic lavas [McEwen et al., 1999], sedimentary rocks [Malin and Edgett, 2001], and/or layered intrusives [Williams et al., 2003]. The layers in Valles Marineris are not of uniform strength, but consist of rare strong layers, perhaps dense lavas, separated by thick stacks of much weaker layers, perhaps volcanic ash or other sedimentary deposits [Beyer and McEwen, 2005]. Some exposures of the Martian crust, especially in the ancient highlands and a few deep portions of Valles Marineris, appear to lack layering in MOC images, consistent with megaregolith. Higher-resolution images may help resolve these debates if we can locate structures diagnostic of origin such as the characteristic internal stratigraphy of lava flows (Figure 20), or the block and boulder size-frequency distributions characteristic of megaregolith.
8.6. Landscape Evolution
 Landscape evolution refers to the overall changes in a region's shape and elevation through time. As such, it reflects the imprint and heritage of all the geological processes combined that have operated on the surface since its formation. Often, separate endogenic and/or exogenic processes contribute to evolution of either degradational or aggradational landforms whose overall form is similar and difficult to distinguish [Easterbrook, 1993, Ritter et al., 1995]. In other cases, sequential or competing processes create “hybrid” landforms where deconvolving the processes responsible for their appearance may be difficult, especially where no one process may dominate. Nevertheless, the ability to resolve the specific signatures related to individual geomorphic processes is crucial for understanding the geologic evolution of a planetary surface.
 For Mars, existing image resolution is often sufficient to identify general processes contributing to large-scale components of landscapes, but leaves many details literally unresolved [Greeley and Guest, 1987; Scott and Tanaka, 1986]. For example, at the regional scale, it is obvious that volcanic activity has dominated the evolution of the Tharsis region, but any component of more silicic activity contributing to structures there and elsewhere often remains uncertain [Mouginis-Mark et al., 1992]. Similarly, impact cratering has played a large role in sculpting the surface [Strom et al., 1992], but it is unclear whether subtle differences in pristine morphology may reflect the influence of the atmosphere and/or subsurface volatiles during formation [Schultz and Gault, 1979; Schultz, 1992; Barlow, 2005]. The source(s) of water responsible for shaping portions of the surface at different times during Martian history is even less constrained and may include the subsurface, atmosphere, or both [Baker et al., 1992; Carr, 1996; Squyres et al., 2004a]. Comparison with landforms on the Earth suggests that the morphologies required for resolving such ambiguities in process often occur at scales too small to be resolved in existing orbital data sets for Mars.
 Similarly, the relative contributions of volcanic versus impact versus alluvial/lacustrine versus aeolian processes in formation of widespread deposits on Mars [Malin and Edgett, 2000b] are also not completely understood. In some instances, structure and stratigraphy (e.g., occurrence of aeolian cross-bedding or finely laminated beds extending for kilometers) that may hold the key to completely evaluating the role of these and other processes occur at scales too small to be resolved in existing imagery.
 The submeter to meter views of the surface coupled with meter-scale knowledge of surface relief and slope afforded by HiRISE images and derived DEMs of any location on Mars should resolve morphology, structure, and stratigraphy needed to quantitatively evaluate the role of various aqueous versus nonaqueous processes in past and ongoing modification of the surface. For example, evaluation of many morphometric parameters associated with basins encompassing gullies to those associated with large integrated valley systems on Mars requires detailed knowledge of across and down-slope relief. These data record important information regarding the relative role of fluvial denudation versus mass wasting versus impact cratering in shaping associated basin surfaces [Howard, 1994]. Detailed knowledge of gradient across depositional surfaces is also useful in determining whether mass-wasting or alluvial processes dominated emplacement [e.g., Grant, 1999] and can distinguish whether a deposit drapes relief and is a candidate for airfall or lacustrine deposition or is confined to local depressions and more likely the result of volcanic or fluvial infilling. Determining whether along valley profiles are concave up or more variably shaped can help distinguish associated valley segments and whether incisement was due to surface runoff versus subsurface discharge of water, whereas information on how the elevation of valley floors compares with adjacent deposits can help identify terraces, how base level may have changed over time, and constrain the duration, number and magnitude of valley forming events [Howard et al., 2005, Irwin et al., 2005a, 2005b]. A combination of high-resolution HiRISE image and DEM data and with the lower-resolution context imagery provided by CTX, mineralogic data from CRISM, and other existing image orbital data sets should enable better interpretation of how Martian landscapes evolved over time.
 The potential for HiRISE to provide new insight into the evolution of Martian landscapes is borne out by comparisons between orbital and surface views of Gusev crater and Meridiani Planum (Figure 24). Images from the Mars Exploration Opportunity Rover reveal plains mostly buried by a thin mantle of sediments that are swept into bedforms tens of centimeters high and meters to tens of meters wide and long [Sullivan et al., 2005]. The plains are punctuated by small craters of widely varying preservation state that are being stripped and buried by wind transported sediments from the surrounding plains [Grant et al., 2006]. The widespread aeolian bedforms and variable appearance of craters are not well detected in the best possible MOC images (Figure 24) and results in an incomplete understanding of the importance of aeolian processes in shaping the presently exposed surface [Golombek et al., 2003b]. At Gusev crater, the Mars Exploration Spirit Rover imaged a basaltic plain covered by relatively pristine (for diameters >100 m) mostly secondary craters dominated by limited aeolian and impact modification [Squyres et al., 2004b; Grant et al., 2006]. By contrast, the subdued appearance of the craters in MOC images (Figure 24) suggests a paucity of ejecta. Interpretation of gradation based only on the MOC images could lead to the erroneous conclusion that there has been significant modification by aeolian and mass-wasting processes [Grant et al., 2006]. HiRISE will provide color and stereo images at meter resolutions that can detect morphology visible to the rovers (e.g., aeolian bedforms, small craters, and meter-sized ejecta rocks) that is required for complete and accurate determination of gradation state and processes.
Figure 24. MOC c-proto images [Malin and Edgett, 2005] of (top) the approximately 150 m diameter Endurance crater in Meridiani Planum and surfaces to the west and (bottom) the 210 m diameter crater Bonneville and surfaces to the west. In Meridiani, MOC images do not easily distinguish widespread aeolian bedforms on the plains or the widely varying states of crater preservation that include the mistaken suggestion of widespread ejecta around Endurance crater. Images from the Opportunity rover highlight the dominant role played by aeolian processes in shaping the present landscape and confirm a paucity of ejecta around Endurance. In Gusev, MOC images of Bonneville crater reveal a subdued form and crenulations in the southeast wall that might be interpreted as debris chutes. Images from the Spirit rover, however, confirm the crater is a relatively pristine crater and that crenulations on southeast rim are due to superposition of small, younger craters. HiRISE images will be capable of resolving morphology at scales overlapping with that observed by the rovers and necessary for accurate interpretation of geologic setting. Top image is MOC R1602188; bottom image is MOC R1502643. In both cases the original data have a scale of ∼1.5 m/pixel, but with 3 times oversampling in the downtrack direction, so the images are assembled at 0.5 m/pixel to show all of the data at a correct aspect ratio.
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8.8. Glacial and Periglacial Processes
 The climate of Mars is presently cold and dry, with a surface layer dominated by permafrost [e.g., Leighton and Murray, 1966; Fanale et al., 1986; Mellon and Jakosky, 1993], but extensive glaciation may have occurred in the past [Kargel and Strom, 1992; Kargel et al., 1995; Kargel, 2004]. Ice-rich permafrost and surficial ice deposits can have a pronounced effect on the morphology of the Martian surface [e.g., Squyres, 1989, Carr, 1986; 1996; Kargel, 2004; Mangold, 2005] as they do on Earth [e.g., Péwé, 1974; Sugden and John, 1976; Washburn, 1980; Williams and Smith, 1989]. Indeed, large deposits of subsurface hydrogen (presumably water ice) have been observed in the Martian high latitudes [Boynton et al., 2002; Feldman et al., 2002]. While there are many similarities between the Martian cold climate and terrestrial polar deserts, differences in their relative climatic conditions raise questions about the dominant geologic processes and the characteristics of the resulting landforms. In the absence of freeze/thaw cycles, which landforms dominate the Martian surface and can these specific landforms provide us with evidence of liquid water? Can differences between short-lived and long-lived landforms provide clues about the Martian climate history with respect to water and temperature? A general understanding of glacial and periglacial landforms and their specific morphological characteristics will aid in developing a broader understanding of the climate history of Mars and the history of water.
 A wide range of landforms occur in terrestrial cold climates and polar deserts [Washburn, 1980; Sugden and John, 1976]. Active glaciers form characteristic topography, flow fronts, and streamlines as evidence of movement and direction under the influence of gravity. Rock glaciers and debris-covered glaciers form many diagnostic features without any expression of exposed surface ice. Past glacial flow can leave characteristic landforms such as moraines, eskers, and kettles long after the ice has been lost to melting or sublimation. The occurrence of such features and their morphological details (including boulder distributions) not only provide evidence for the existence of past ice deposits, but also the climate conditions under which the ice occurred and was eventually lost. Periglacial landforms are perhaps the most common in Earth's cold regions. Solifluction lobes and stone circles can be diagnostic of freeze thaw cycles. Larger polygonal patterns can indicate permanently frozen, ice-rich soil deposits (see Figure 25). Characteristics such as the spatial distribution of rocks, plan view patterns, and micro-topography can help distinguish process and history. Many of these periglacial and glacial landforms exhibit morphology on small, frequently submeter, length scales. HiRISE provides a unique opportunity to resolve micro-topographic characteristics diagnostic of the formation processes.
Figure 25. Polygonal patterns in the northern high latitudes. Such patterns on Earth form from seasonal thermal-contraction cracking of ice-rich permafrost. A wide range of sizes and shapes of polygonal patterns are observed on Mars down to and presumably below the resolution of existing images. Subframe of MGS MOC image E03-00299. Scene width is 3.2 km.
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8.10. Polar Geology
 It is widely believed that the Martian polar layered deposits (PLD) record climate variations over at least the last 10 to 100 million years [Murray et al., 1972; Cutts et al., 1976, 1979; Squyres, 1979; Toon et al., 1980; Carr, 1982; Howard et al., 1982b; Plaut et al., 1988; Herkenhoff and Plaut, 2000], but the details of the processes involved and their relative roles in layer formation and evolution remain obscure [Thomas et al., 1992]. Variations in axial obliquity and orbital eccentricity are thought to influence the climates of both Earth and Mars, but are of greater amplitude in the Martian case [Ward, 1974; W. R. Ward, 1979; Bills, 1990; Touma and Wisdom, 1993; Mellon and Phillips, 2001]. A common presumption among Mars researchers has been that the polar layered deposits (PLD) are the result of variations in the proportions of dust and water ice deposited over many climate cycles [Cutts et al., 1979; Squyres, 1979; Toon et al., 1980; Mellon and Jakosky, 1995; Tanaka, 2000; Laskar et al., 2002]. Results from the GRS experiment on Mars Odyssey are consistent with abundant water ice in the near subsurface of both polar regions [Boynton et al., 2003]. Aeolian erosion is likely the dominant process that has exposed the layers [Cutts, 1973; Howard, 2000]. Blasius et al.  presented evidence for both topographic and albedo variations between layers in the north polar layered deposits, based on analysis of springtime images [Howard et al., 1982a]. By combining stereophotogrammetry and photoclinometry, Herkenhoff and Murray  showed that the layered appearance of an exposure of the south polar deposits is due both to “staircase” topography and albedo variations caused by differential frost retention rather than compositional variations between layers. The expression of layers is highly variable [Malin and Edgett, 2001], further complicating separation of topographic and albedo effects on their appearance.
 HiRISE is capable of observing the topography and stratigraphy of the Martian PLD at meter-scale resolution. Such observations can be used to determine whether the PLD contain fine laminae at the limit of HiRISE resolution, which would indicate that even thinner layers may be present. Alternatively, HiRISE images of massive layering would suggest that the stratigraphy of the PLD is adequately resolved. In either case, HiRISE observations of the PLD during the winter and spring seasons, when the surface is covered by seasonal CO2 frost, may be used to derive topography with submeter resolution using two-dimensional photoclinometric techniques (Figure 26). When the resulting high-resolution topography is combined with HiRISE images of the same exposures of PLD taken during the summer, albedo and color variations between layers can be quantified at finer spatial scales than previously possible. By repeating these types of observations and analysis in various locations, regional stratigraphic correlations can be made. Thus the stratigraphy of the PLD can be measured with submeter precision and compared to theoretical models of PLD deposition and evolution. These types of studies will help to assess the importance of global climate changes in the geologic history of the PLD.
Figure 26. Oblique shaded relief view of digital elevation model of exposure of south polar layered deposits, generated from MOC image M0905983 using two-dimensional photoclinometry. Green plane is horizontal reference; note folding and truncation of layers, suggesting flow and faulting.
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8.11. Seasonal Processes
 The polar caps of Mars are a reservoir for Mars' volatiles CO2 and H2O. In 1966 Leighton and Murray published their theory that Mars' CO2 atmosphere is in vapor pressure equilibrium with its polar caps. Atmospheric pressure changes by ∼25% seasonally as frost condenses at the poles in the fall and sublimes in the spring. This global picture serves well as a high-level description of the state of Mars' volatiles, however in detail there are numerous questions to address regarding the condensation and sublimation processes.
 A number of exotic features have been observed in the seasonal polar caps that HiRISE is well suited to study. Although MOC coverage of the polar regions is substantial, HiRISE offers higher resolution and greater sensitivity. Improvements in sensitivity enable more detailed study of the inherently low light level regions at high latitudes. The MRO capability to frequently turn off nadir enables repeat coverage of specific sites to monitor change as well as stereo.
 The process of spring sublimation is particularly interesting on a local level. The seasonal CO2 frost exhibits a wide variety of enigmatic phenomena. One intriguing feature found in groups in some areas of the cryptic region in the southern hemisphere has been termed “spiders.” These features are hypothesized to form from geysers, which may develop as CO2 gas is trapped under pressure below seasonal ice, then released through cracks in the ice [Piqueux et al., 2003]. Geysers have not actually been observed on Mars, however analogies have been drawn to the geysers observed on Neptune's moon Triton by Voyager [Smith and the Voyager Imaging Team, 1989]. The brief observation period available to image Triton left more questions than answers on the actual eruption process. HiRISE observations may catch a geyser in the process of eruption, confirming their existence and increasing our understanding of geyser mechanics on both Mars and Triton.
 Other HiRISE images of the subliming seasonal cap may shed light on the evolution of the CO2 seasonal frost coverage. Does CO2 start as a bright frost from winter snows, then change to slabs of clear ice? Is this the reason for the cryptic terrain [e.g., Titus et al., 2001; Kieffer et al., 2000]? How do cracks in the ice affect the albedo? Figure 27 shows one example of a MOC image of a strange area on the southern seasonal cap in the process of defrosting in the spring.
Figure 27. This cropped version of MOC image M0305635 shows one of the more enigmatic areas of the southern seasonal cap in southern spring. Regions like this will be imaged repeatedly by HiRISE to study the sublimation process and its interaction with underlying terrain.
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 Polar terrain in the process of defrosting exhibits multiple dark spots that gradually grow. Apparently the positive feedback as the dark spot warms and grows enhances sublimation of frost in that spot. The loss of ice frees the loose dust originally entrained and deposited as the frost condensed, allowing it to be lofted by local winds and create a wind streak. The fields of streaks can be used as a snapshot of local wind conditions and compared to the Mars GCM for polar meteorology studies. The streaks imply a certain ease of lofting materials, which gives us insight into the dust available to form layers.
8.12. Nonaeolian Surface Changes and Potential Climate Change
 We will be keenly interested in dynamic processes and changing features on the Martian surface and implications for current processes. Aeolian and seasonal changes are well documented, but many nonaeolian changes have also been detected by MOC. The higher spatial resolution of MOC has enabled detection of new types of variable features, and we expect that trend to continue with HiRISE.
 MOC images of the south residual polar cap “Swiss cheese” terrain, composed of roughly circular pits, ridges and mounds, are suggestive of recent climate change. The escarpments in this unit have been observed by MOC to retreat 1–4 m over one Mars year [Malin et al., 2001], with older materials eroding more quickly than younger units [Thomas et al., 2005]. HiRISE images will extend the temporal coverage of these units and address whether the erosion is consistent with a longer-term trend in climate change (if they continue to retreat) or represents year-to-year variability. HiRISE stereo will enable accurate measurement of the mass flux.
 There has been recent interest in evidence for late Amazonian climate change on Mars, as evidenced by a midlatitude debris mantle that appears to be sublimating [Mustard et al., 2001] and possible young glacial activity [Hauber et al., 2005; Head et al., 2003b]. The mantle has been suggested to be less than 100,000 years old based on the lack of small impact craters [Mustard et al., 2001; Head et al., 2003b], but McEwen et al.  suggested that 10 Ma is a more realistic upper limit given the large uncertainties in the small-crater production function on Mars. Direct observation of changes in various features is probably our best hope for quantitative constraints on rates of change in very recent features and extrapolation to estimate absolute ages. Nevertheless, the features are geologically young and well preserved at the meter scale, so HiRISE images are sure to be interesting. High-resolution DEMs should enable detailed modeling of ice stability with topography [Russell et al., 2005].
 Dark slope streaks have been observed to be currently forming on Mars [Malin and Edgett, 2001; Sullivan et al., 2001]. These streaks form exclusively in regions of low thermal inertia and steep slopes, consistent with dust avalanches [Sullivan et al., 2001]. They also form only where peak temperatures exceed 275 K, so phase transition of small amounts of water may trigger the mass movements [Schorghofer et al., 2002]. In spite of the strong correlation with recent dust deposits, some workers have proposed that a few of the streaks could be due to currently active fluvial processes [Ferris et al., 2002; Miyamoto et al., 2004]. HiRISE will take a closer look at some of these features and may improve our understanding of the processes. Other forms of active mass wasting have been observed by MOC, including new rock slides and boulder tracks (see image releases at http://www.msss.com).
 Van Gasselt et al.  have documented evidence for changing and evolving patterned ground in a South Polar trough over three years, which they interpret as due to thermal contraction. These results suggest that major climate change may not be needed to explain the presence of patterned ground on Mars.