Our units are shown in the geologic map (Figure 2), and summaries of their physical characteristics, stratigraphic information, and crater densities and distributions are provided in Tables 1 and 2 and Figure 3. The units are grouped geographically into highland, lobate, plains, and polar materials (Figure 2b). The following stratigraphy (from oldest to youngest units) includes some significant modifications to previous mapping [Scott et al., 1987].
Figure 3. Northern hemisphere of Mars showing distribution of relatively pristine craters (gray dots), ghost craters (red dots), and pedestal craters (yellow dots) greater than 5 km in diameter overlying the geologic mapping shown in Figure 2 (unit colors are subdued).
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Table 1. Characteristics of Map Units in the Northern Plains of Marsa
|Unit Symbol||Unit Name||Stratigraphic Relations||MOLA Topography (500 m Resolution DEM)||Cratering||Albedo||Structure, Other|
|Api||Polar residual ice||<Apl2||Smooth||None||V. High|| |
|Ad||Dune material||<AHvh, At, Apl2||Nearly flat with pebbly texture||None||Low||Dunes|
|Apl2||Polar layered deposits 2||<AHvh, Ah, >Api, Ad||Smooth surfaces, trough scarps >1–5° S-facing, 1–2°N-facing||None||Mod. high||Thin layers and terraces|
|Ah||Hummocky material||<AHvh, >Apl2||Smooth, broadly hummocky||Low|| || |
|Als2||Smooth lobate unit 2||<Als1, Am||Very smooth, nearly flat, lobes 10's of m thick||Embayed craters||Low||Crenulate margins; subtle channels|
|Aa||Apron material||<HNu, Hb2, Als1||Slopes of 1–3°, local relief up to 1 km; rims highstanding highland material||Very low||Mod. high||Concentric ridges in MOC|
|Als1||Smooth lobate unit 1||<AHvh/m, AHl, Am, >Als2||Moderately sloping to flat; lobes 10's of m thick||Low||Low||Crenulate margins; subtle channels|
|Alc||Coarse lobate material||<AHvh/m, AHl, Am||Moderately sloping to flat; lobes 10's to 100's of m thick||Low|| ||Hummocks, pits, channels and/or ridge structures.|
|Am||Medusae Fossae Formation||<HNu, Hb2, AHl, >Als2||Broad, irregular plateaus 100's to >1000 m thick||Low||High||Cut by sets of lineations; cones along Als2 contact|
|At||Tholi unit||<AHvh, ∼Act, >Ad||Pancake mounds, complexes up to several hundred m high; associated depressions||A few large craters|| ||Peaks, depressions, scarps, aprons, narrow ridges|
|Act||Younger chaos material||<Hch, Hb2, AHvh||Below −3800 m elevation; local relief up to 1000 m; mesas and knobs within depressions||Low||Varies||Locally includes polygonal troughs|
|Apl1||Polar layered deposits 1||<AHvh, >Apl2||Up to a km thick with abrupt scarp west of Chasma Boreale; gently sloping surface||A few large craters||Moderate?||Cross bedding|
|AHvm||Vastitas Borealis Fm, marginal member||<Hb2, ∼AHvm||Smooth with some outcrops bounded by scarps; networks of troughs (antfarm terrain)||Moderate|| ||Narrow medial ridges within troughs|
|AHvh||Vastitas Borealis Fm, hummocky member||<Hb2, Hch, >At, Als1, Alc, Apl1–2||Below −3100 m elevation, low kilometer-scale roughness||Moderate, with ghost and pedestal craters||Mottled in places||Systems of ridges and scarps, subkilometer bright cones in arcuate chains (thumbprint terrain)|
|Hb2||Boundary plains unit 2||<Hb1, HNu, HNk; >AHvh||Smooth with some ridges and hummocky topography; below −2700 m elevation||Moderate, with ghost craters|| ||Scarps common (100–200 m relief)|
|Hs||Scandia unit||<AHvh, Hb2||20 to 200 m thick, planar to knobby||Some pedestal craters|| || |
|Hct||Older chaos material||<HNu, HNk, ∼Hch||Local relief up to 1000 m; mesas and angular knobs within depressions||Low|| ||Fracture networks amongst mesas in places|
|Hch||Channeled plains material||<Hb2, ∼Hct, >AHvm/h||Smooth, gently sloping, bounded by scarps||Moderate|| ||Ridges, streamlined bars up to 200 m high|
|AHl||Older lobate material||<HNu||Moderate slopes and km-scale roughness||Moderate|| ||Some vent structures; lobate scarps 10's of m high|
|Hb1||Boundary plains unit 1||>Hb2, <HNu, HNk||Below −2000 m elevation; hummocky, particularly basin-ward where slopes commonly exceed 0.5°;||Moderate with ghost craters|| ||Ridges, scattered knobs up to few 100's of m high|
|HNk||Knobby unit||∼HNu, Hb1||High relief, large knobs and intermediate, sloping plains, local steep slopes, depressions||Moderate|| ||Local deep fractures|
|HNu||Highland material, undivided||<Hb1||High relief and elevations, slopes >1° common||Heavy|| ||Ridges, scarps, channels|
Table 2. Crater Densities of Map Units in the Northern Plains of Marsa
|Unit Symbol||Unit Name||Area (106 km2)||N(5)b|
|Api||Polar residual ice||0.60||–|
|Apl2||Polar layered deposits 2||0.33||–|
|Als2||Smooth lobate unit 2||1.31||61 ± 7d|
|Aa||Apron material||0.62||86 ± 12d|
|Als1||Smooth lobate unit 1||1.51||35 ± 5|
|Alc||Coarse lobate material||3.46||49 ± 4c|
|Am||Medusae Fossae Formation||0.80||42 ± 7|
|At||Tholi material||0.18||61 ± 18|
|Act||Younger chaos material||0.29||100 ± 18c|
|Apl1||Polar layered deposits 1||0.09||77 ± 29|
|AHvm||Marginal member of the Vastitas Borealis Formation||0.47||47 ± 10|
|AHvh||Hummocky member of the Vastitas Borealis Formation||17.12||73 ± 2|
|Hb2||Boundary plains unit 2||4.65||103 ± 5|
|Hs||Scandia unit||0.12||99 ± 29|
|Hct||Older chaos material||0.23||126 ± 23|
|Hch||Channel material||2.16||88 ± 6|
|AHl||Older lobate material||5.50||104 ± 4|
|Hb1||Boundary plains unit 1||2.14||177 ± 9|
|HNk||Knobby unit||2.51||223 ± 9|
|HNu||Highland material, undivided||7.46||393 ± 7|
 The crater-density data in Table 2 must be applied with caution. The more significant considerations include the following: (1) The craters were located using Viking 1:2,000,000-scale photomosaics produced both digitally and by hand, resulting in misregistrations with our MOLA-based geologic map commonly on order of 10 to 20 km. Thus craters occurring near contacts may be included in the incorrect unit. This will produce the largest errors for units having small areas, irregular contacts, and low crater densities. In particular, crater counts for the polar layered deposits (units Apl1–2) were most sensitive to this error, and so we performed our own counts for those units. (2) Embayed craters are included in counts of their surrounding units, making the count too high for those units. Thinner units having lower superposed crater densities overlying more densely cratered surfaces have the greatest error. Units most affected by this include smooth lobate unit 2 (unit Als2), whose count is perhaps entirely made up of embayed craters, coarse lobate material (unit Alc), apron material (unit Aa), and younger chaos material (unit Act). (3) A unit's crater retention age may not reflect the unit's emplacement age because of various possible resurfacing scenarios including crater obliteration and deep burial and later exhumation.
 Highland material, undivided (unit HNu; Figures 4 and 5) forms the rugged ancient highland terrain surrounding much of the northern lowlands, as well as part of Acidalia Mensa. The unit is marked in places by large craters, hills, valley networks, isolated depressions, and linear and sinuous ridges, scarps, and troughs. Interpretation: Mixture of volcanic and sedimentary material, gardened by impacts. A long history of resurfacing and deformation accounts for secondary erosional, depositional, and tectonic structures.
Figure 4. Transect of the HLB in southern Utopia Planitia (centered at 15.6°N, 115.9°E) showing characteristics of highland and lowland units, including highland material, undivided (unit HNu), the knobby unit (unit HNk), boundary plains units 1 and 2 (units Hb1 and Hb2, respectively) and the hummocky member of the VBF (unit AHvh). Top frame shows the distinctive elevation ranges and morphologies of the units. Bottom frame shows differences in slope character in the units (slopes measured from the MOLA DEM); note scalloped appearance of lower part of unit Hb1, flat surfaces between ridges in unit Hb2, and greater fine-scale roughness of unit AHvh compared to that of unit Hb2.
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Figure 5. Part of the margin of northwestern Arabia Terra and adjacent plains of Vastitas Borealis and of Cydonia in eastern Acidalia Planitia. Cratered highland material (unit HNu) is marked by irregular and crater-floor depressions (d) and by the fretted troughs and apron-covered plains (unit Aa) surrounding Deuteronilus Mensae. Boundary plains unit 2 (unit Hb2) occurs below highland material as a series of terraces that include ghost crater rims and locally is dissected by a narrow sinuous channel (ch) and by fretted troughs. In turn, the hummocky member of the Vastitas Borealis Formation (unit AHvh) forms lower plains that appear to result from burial or destruction of unit Hb2. In some cases, unit AHvh is bounded by lowland-facing scarps (arrows). See Figure 15 for a closer look at the Cydonia area, which includes the knobby unit (unit HNk) and younger chaos material (unit Act). MOLA topographic shaded-relief base at 1/128° resolution in Polar Stereographic projection centered near 40°N, 3°E.
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 The knobby unit (unit HNk; Figure 4) consists of knobs and mesas of highland rocks and interposed slope and plains materials. The unit forms much of the highland-lowland boundary (HLB), which underwent degradation starting at the end of the Noachian and extending into the Early Hesperian [Tanaka, 1986; Frey et al., 1988; Maxwell and McGill, 1988; McGill, 2000]. Interpretation: Highland rocks fractured and collapsed due to basal sapping of volatiles and mass-wasted debris.
 The oldest exposed northern plains unit, boundary plains unit 1 (unit Hb1, Figure 4), occurs adjacent to older highland and plateau materials including the knobby unit along the HLB of Utopia, Amazonis, Chryse, and Acidalia Planitiae and Arabia Terra. The unit appears to be missing along some parts of the HLB including northwestern Arabia Terra (Figure 5), Tempe Terra, and Isidis Planitia. The unit slopes gently downward away from the HLB. Irregular depressions and scarps many tens of kilometers in length and 200–300 m in relief locally give the unit a broadly scalloped appearance (Figure 4). The superposed crater density (Table 2) indicates an Early Hesperian age. Interpretation: Because this unit mostly occurs just below the knobby unit (unit HNk) and highland material (unit HNu), the unit likely results in most places from collapse, comminution, erosion, transport, and deposition of highland clastic material, perhaps by volatile-assisted slope processes. Previously, plains materials in the northern lowlands have been generally viewed as volcanic plains [e.g., Greeley and Spudis, 1981; Scott et al., 1987]; also, the VBF has been interpreted to be sediments underlain by Early Hesperian volcanic ridged plains [Head et al., 2001]. However, we find no clear evidence for volcanic vents or flows in boundary plains unit 1.
 Older lobate material (unit AHl) consists of extensive tongue-shaped flows and forms the gently sloping shields of the Elysium and Tharsis rises and Syrtis Major Planum (Figure 1). Dissected outcrops of the unit exhibit layered sequences. The flows predate the Vastitas Borealis Formation and both overlie and underlie boundary plains unit 2 (unit Hb2). Locally, the unit is marked by various vent and tectonic structures. The unit has a Late Hesperian mean crater age (Table 2). Interpretation: Lava flows and possibly other volcanic materials. Ages of flows range from Early Hesperian to Early Amazonian [Scott et al., 1987].
 Channel material (unit Hch) covers the floors of outflow channels tens to hundreds of kilometers wide and hundreds of kilometers long crossing highland and boundary plains units in Xanthe Terra and Chryse Planitia (Figure 6). Channel dissection ceased near the end of the Hesperian (Table 2). The northern extent of the channel material has been modified by the VBF, and in southern Chryse Planitia, some channel areas appear partly disintegrated and transitional with older chaos material (unit Hct). Interpretation: Fluvial sediments and debris flows sourced by catastrophic discharges from older chaos material (unit Hct) [e.g., Baker and Milton, 1974; Nummedal and Prior, 1981]; exposed deposits generally Late Hesperian, but some older, buried deposits likely [e.g., Rotto and Tanaka, 1995; Tanaka, 1997; Nelson and Greeley, 1999]. Chryse outflow-channel sediments may have been restricted to Borealis basin, given (1) the paucity of ghost craters (which would have been buried by the younger sediments) in the channel-ward part of the basin (Figure 3), and (2) the lack of an obvious outflow-channel connection in the saddle between Borealis and Utopia basins (Figure 7).
Figure 6. The Chryse outflow channel system of Mars showing major geographic features. The extent of channel scouring observed in the lowlands is mapped as channel material (unit Hch) and outlined by blue lines (dashed where uncertain) and includes streamlined islands (blue features). The channels originate from older chaos material (unit Hct) and dissect highland material (unit HNu) and boundary plains units (units Hb1–2). Note that the scoured features do not end as the outflow channels enter Chryse Planitia, but they continue northward into Acidalia Mensa where their gradual disappearance may be in part due to development of the Vastitas Borealis Formation (unit AHv; unit margin shown in green). MOLA topographic shaded-relief base at 1/128° resolution in Polar Stereographic projection with a 5° grid, centered near 25°N, 320°E.
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Figure 7. MOLA DEM's showing topographic saddles between Utopia and Isidis (left) and Borealis and Utopia (right) basins. Ghost craters, knobs, scarps, and ridges mark the saddle areas, but no obvious channel systems connect the basins. Also note possible outliers of polar layered deposits 1 and 2 (units Apl1,2?) near Planum Boreum. Other units include boundary plains units 1 and 2 (units Hb1,2), hummocky member of the Vastitas Borealis Formation (unit AHvh), coarse lobate material (unit Alc), polar residual ice (unit Api), and dune material (unit Ad). MOLA topographic shaded-relief views at 1/128° resolution (left, centered near 20°N, 95°E, north at top; right, centered near 65°N, 130°E, north at top left).
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 Older chaos material (unit Hct) forms irregular depressions tens to hundreds of kilometers across and hundreds of meters to more than a kilometer deep filled with irregular knobs and mesas at the heads of Chryse outflow channels (unit Hch). Outcrops of the unit postdate adjacent highland material (unit HNu) and both predate and postdate various outflow channels [Rotto and Tanaka, 1995]. Some of the older chaos material modifies the youngest outflow channels, Simud and Tius Valles. Interpretation: Highland and plains materials disrupted and eroded by volatile discharge and subsequent collapse in association with outflow-channel dissection [e.g., Sharp, 1973; Carr, 1979; Hoffman, 2000].
 Outcropping along the northwest periphery of the Alba Patera shield, the Scandia unit (unit Hs, Figure 8) forms the discontinuous plateaus and knobs of Scandia Colles that range from 20 to 200 m in height. Some northern outcrops of the unit are broken up by depressions filled with younger chaos material (unit Act). In one case, a plateau made up of the Scandia unit may be armored by superposed crater ejecta (Figure 8). In Viking images, broad, thin lobate flows form an apron surrounding the southern margin of the unit. Interpretation: Because of the modification of the unit, its relative age is highly uncertain. Previously, the unit was interpreted to be remnants of ancient Noachian material because of degraded crater forms within the unit [Scott et al., 1987]. However, we suggest that the unit is Hesperian, because it doesn't have the more rugged relief and high crater density typical of the Noachian knobby unit. The degraded, planar nature of the unit suggests that it may have originated as either polar dust deposits and/or aeolian deposits of fines produced by Alba Patera volcanism [cf. Tanaka, 2000] or by erosion of Noachian highland material (and thus the Scandia unit might be a fine-grained facies of the boundary plains units (units Hb1 and Hb2)). Mass wasting of the unit may account for its partial lobate apron.
Figure 8. Part of Scandia Colles showing how the Scandia unit (unit Hs) apparently has been degraded into knobs and plains materials (units Hb2 and AHvh); contacts between the units are gradational. Note that crater ejecta (c) may have served to armor the unit. (MOLA shaded relief image, 1/128° resolution centered near 61°N, 214°E).
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 Boundary plains unit 2 (unit Hb2, Figures 4–7) is chiefly identified as smooth plains material marked by wrinkle ridges that occurs at the base of the lower, rugged part of boundary plains unit 1 (unit Hb1) in Utopia, Chryse, Acidalia, and Amazonis Planitiae. Elsewhere, the unit embays highland material (unit HNu) in northwestern Arabia Terra and southern Isidis Planitia and the older lobate material (unit AHl) of Alba Patera. The coalesced, fretted trough floors of Deuteronilus Mensae, while partly buried by apron material (unit Aa), forms a continuous surface with the unit Hb2 surface in northwestern Arabia Terra (Figure 5) and has a density of craters, both partly buried and superposed, similar to that of unit Hb2 (Table 2); thus the fretted floors may consist of unit Hb2. In turn, the unit is overlain by flows from Elysium and Tharsis (units AHl and Als1–2) in Arcadia and Amazonis Planitiae, cut by outflow channels in Chryse Planitia (unit Hch; Figure 6), and modified by the VBF (Figures 4–6). The unit's crater density indicates a Late Hesperian age (Table 2). The unit slopes gently away from the highland margin and appears relatively smooth at subkilometer scale [Kreslavsky and Head, 2000] but somewhat rougher at longer length scales (Figure 4). The unit also may form the smooth, lower tier of Acidalia Mensa, embaying highland material and rising tens of meters above the adjacent plains (Figure 9). Ghost craters are common in some outcrops of the unit (Figure 3). Interpretation: Where the unit occurs adjacent to boundary plains unit 1, it likely results from reworking of that member by collapse, erosion, transport, and deposition of clastic material from scalloped terrain. Elsewhere, the unit also may be made up of sediments eroded from highland materials (units HNu and HNk) and perhaps some volcanic flows.
Figure 9. Shaded-relief view of Acidalia Mensa and Colles (from MOLA 1/128° DEM centered near 49°N, 335°E). Note that Acidalia Mensa forms a highstanding outcrop of highland material (unit HNu) embayed by smooth, plateau-forming material (unit Hb2?) that in turn is surrounded by the hummocky member of the VBF (unit AHvh). Acidalia Colles consist of knobs as much as several hundred meters high within a depression as much as a few hundred meters deep; also note moat on south edge of Acidalia Mensa cutting into unit AHvh.
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 The Vastitas Borealis Formation (VBF) covers the majority of the Borealis, Utopia, and Isidis basin floors (Figure 2a). Previously, the VBF was divided into grooved, ridged, mottled, and knobby members [Scott et al., 1987]. These members largely reflect differences in surface character rather than distinctions in lithology or age. Parker et al. [1989, 1993] mapped a plains unit (outlined by their Contact 2) that in places coincides with the VBF contact. Because the more widespread, subtler morphologic features characterizing the VBF commonly are more detectable with high-resolution (1/128°) MOLA gridded topography than with Viking images, we have been able to map VBF “contacts” more completely than previously possible. We substitute “margin” for “contact” in describing the edge of the VBF, because we interpret that the unit is not stratigraphic but results from in situ reworking of preexisting materials that has destroyed pre-VBF landforms including craters and depositional, erosional, and tectonic landforms. Parker , who believes that the same feature represents a possible shoreline and not necessarily a material-unit boundary, also recognized a semantics problem with using “contact.”
 The hummocky member (unit AHvh) accounts for the majority of the VBF. This unit is marked by numerous low hillocks and arcuate ridges (in places organized into thumbprint terrain); by patches of grooves hundreds of meters wide forming networks of polygons several kilometers across; by local systems of wrinkle-ridge-like, mostly asymmetric, low ridges; by dozens of unevenly scattered circular depressions (ghost craters) up to tens of kilometers in diameter (Figure 3); and by many superposed, relatively high-albedo impact craters. In the MOLA data, the hummocky morphology appears to be pervasive along the unit margin (e.g., Figure 4), whereas the grooved and mottled appearances delineated by Scott et al.  and others are additional or dominant features within the interior of the unit. MOC images show that thumbprint ridges generally are made up of continuous and discontinuous chains of pitted domes; also, the domes locally appear unorganized (Figure 10). The unit has an overall Late Hesperian to Early Amazonian crater density (Table 2). The unit is surrounded by Hesperian boundary plains units and channel material and overlain by Amazonian polar and lobate materials. In the Deuteronilus region, the unit's margin occurs along a subtle scarp (Figure 5). Interpretation: We propose that the VBF is a mixture of undifferentiated Noachian to Hesperian materials and local outflow-channel sediments that have been highly and pervasively altered by in situ permafrost-related processes such as cryoturbation, desiccation, and thermokarst involving near-surface deformation and mass-movements aided by subsurface volatiles as previous workers have suggested for northern plains landforms [e.g., Carr and Schaber, 1977; Lucchitta, 1981; Rossbacher and Judson, 1981; McGill and Hills, 1992; Kargel et al., 1995]. The various surface textures and bright crater ejecta may be more an indication of locally varying physical properties, volatile content, and/or subsequent modification than lithology or pervasive structural character, so we refrain from mapping additional units where these features occur. Previously, Tanaka  assigned the VBF as the Late Hesperian stratigraphic referent, from which crater-density boundaries were derived to define chronologic boundaries for the end of the Hesperian. However, we feel that crater densities provide a less ambiguous means of defining relative age, considering that many regional geologic units on Mars including the VBF may be significantly time transgressive. Thus, we rely on the N(5) = 67 crater density to define the Hesperian/Amazonian boundary [Tanaka, 1986], which, considering unit AHvh's crater density (Table 2) and that development of the unit may have been lengthy in time, places this unit within the Late Hesperian and possibly the Early Amazonian.
Figure 10. Examples of pitted domes and bright hillocks in the northern plains of Mars in MOC images (all at same scale). The features appear steeper and taller at lower latitudes. Note that some form chains that in places occur along narrow troughs. (a) MOC image 0402307 (centered at 13.11°N, 84.18°E; 5.91 m/pixel). (b) MOC image 0201213 (centered at 33.60°N, 86.93° E; 3.59 m/pixel). (c) MOC image 23023053 (centered at 32.88°N, 87.69°E; 6.01 m/pixel). (d) MOC image 0203364; 45.69°N, 8.94°E; 5.43 m/pixel).
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 The marginal member (unit AHvm) of the VBF forms low plateaus dissected by linear to sinuous, 2- to 5-km-wide troughs that extend for tens of kilometers and resemble the burrows of ant colonies (hence, we informally call this “ant-farm terrain”; see Figure 11). Locally, the troughs form rectilinear or irregular networks and some terminate at and/or connect to large knobs. The plateaus are bounded by linear, lobate, or sinuous scarps similar in trace to the troughs within the plateaus. In places, the troughs have angular corners. In higher resolution Viking and MOC images, low, narrow medial ridges occur within the trough floors (Figures 11 and 12). Also, the troughs appear to dissect ridges of boundary plains unit 2 (Figure 11) and streamlined islands in channel material (Figure 12). The troughs of the marginal member dissect boundary plains units 1 and 2 and channel material. Interpretation: The unit's unusual morphologic character defies comparison with common terrestrial analogs such as glacial, periglacial, lacustrine, or tectonic terrains, but shows evidence for both brittle and viscous deformation that may be related to the presence of near-surface volatiles [e.g., Scott, 1982; Tanaka, 1997]. The unit's crater density (Table 2) places it in the Early Amazonian, but its gradational appearance with the hummocky member of the VBF (unit AHvh; Figure 11) suggests that it formed contemporaneously with unit AHvh, perhaps as a late-stage, incipient form of degradation at the margin of the VBF.
Figure 11. The marginal member of the VBF (unit AHvm, also called “ant-farm terrain”) and adjacent units in Arcadia Planitia. (a) MOLA DEM showing in color the detailed topography of the unit. The unit AHvm surface is flat with small hillocks and knobs tens to >100 m high and several kilometers wide and sinuous troughs tens of meters deep, a few km wide and tens of meters deep. The unit AHvm and boundary plains unit 2 (unit Hb2) surfaces are continuous. Note ridges marking unit Hb2 are dissected by the troughs of unit AHvm. In turn, the troughs of unit AHvm appear to coalesce into plains made up of the hummocky member of the VBF (unit AHvh). Note that the floor of unit AHvh consists of higher and lower plains. MOLA 1/128° DEM (centered at 44°N, 176°E). (b) Shaded-relief view of area in (a). Notes that some topographic features are more difficult to detect in this view than in (a). (c) MOC image 2001466 (centered at 45.74°N, 175.95°E; 5.99 m/pixel; north at left) shows a close up of one of the troughs (see location in b). Note subtle, discontinuous medial ridge within trough. Illumination from bottom.
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Figure 12. Views of features in southwestern Acidalia Planitia. (a) Shaded-relief context view from MOLA 1/128° DEM (centered at 36°N, 322°E) of marginal member of the VBF (unit AHvm) modifying channel material (unit Hch). Note streamlined bars (marked b) and knobby-rimmed, flat-floored craters (marked c). Location of MOC image shown by arrow. (b) MOLA altimetry shot locations (dots) and profile across trough with medial ridge in MOC image 0702810 (centered at 37.03°N, 321.2°E; 4.43 m/pixel; north at top). (c) Close-up of part of MOC image showing crater rim deformed by medial ridge.
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 The polar layered deposits 1 (unit Apl1) form the base of part of Planum Boreum and possibly highstanding knobs and mesas south of Chasma Boreale and underlie the evenly bedded polar layered deposits 2 (unit Apl2; Figure 13). Unit Apl1 may be up to a kilometer thick along the margins of Chasma Boreale and thins out away from there; it presumably underlies much of Planum Boreum, particularly near Olympia Planitia. MOC images and MOLA data reveal that this unit has irregular bedding, locally steep scarps, a dark color, and some large, superposed craters (Figure 13) [Edgett and Malin, 2000; Kolb and Tanaka, 2001; Byrne and Murray, 2002]. Previously we thought that some of the layered deposits on the margin of Planum Boreum opposite Chasma Boreale were underlain by the ejecta of a 24-km-diameter crater and thus consisted of unit Apl1 (Figure 14, inset), but a higher-resolution DEM (Figure 14) shows the opposite relation and thus the deposits are more likely the upper unit (unit Apl2). Interpretation: Possibly an eroded sand sea [Byrne and Murray, 2002] or modified polar layered deposits. Formerly, unit Apl1 likely was more extensive in places, perhaps accounting for some of the material underlying the dunes of Olympia Planitia [Zuber et al., 1998; Fishbaugh and Head, 2000] and the pitted and flat-topped knobs in the plains surrounding Planum Boreum that rise to similar elevations of unit Apl1 at the margin of Planum Boreum (Figure 13). Alternatively, those knobs have been interpreted to be volcanic cones [Hodges and Moore, 1994; Sakimoto et al., 2001].
Figure 13. Shaded-relief view of region near mouth of Chasma Boreale (from MOLA 1/256° DEM centered at 80°N, 302°E). The hummocky member of the VBF (unit AHvh) forms the plains floor and appears overlain (in sequence) by polar layered deposits 1 (unit Apl1), polar layered deposits 2 (unit Apl2), hummocky plains material (unit Ah), and dune material (unit Ad). Note tongue of unit Apl1? a few hundred meters thick forms floor of Chasma Boreale; a few impact craters <7 km across mark the outcrop (c). West of the chasma, Planum Boreum displays a kilometer-thick scarp also made up of unit Apl1 that is superposed by two impact craters, 6 and 24 km across (c). South of Chasma Boreale, a 600–700 m high plateau is superposed by the 23-km-diameter crater Escorial and may be an outlier of unit Apl1. Pitted cone features a few hundred meters high forming Abalos Colles may be either remnants of pedestal craters underlain by unit Apl1 or volcanoes.
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Figure 14. Shaded-relief view of margin of Planum Boreum on opposite side from mouth of Chasma Boreale (from MOLA 1/256° DEM centered at 79.7°N, 61.1°E; illuminated from upper left). The hummocky member of the VBF (unit AHvh) consists of plains material marked by polygonal troughs and is overlain by polar layered deposits 2 (unit Apl2), which has a subdued form of the underlying hummocky surface of unit AHvh. Unit Apl2 also mantles the ejecta rampart (arrows) of a 24-km-diameter crater. Inset (illuminated from upper right) shows the crater in the MOLA 1/128° DEM; the superposition relation between the crater blanket and polar layered deposits is unclear at this lower resolution.
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 Younger chaos material (unit Act) occurs within depressions tens to hundreds of kilometers across and tens to a few hundreds of meters deep in the Acidalia Mensa, Cydonia, Isidis, and Scandia regions (Figures 9, 15, and 16). The unit locally includes polygonal fractures, knobs, and irregular scarps. Younger chaos material postdates channel material (unit Hch) and the VBF (unit AHvh), and may be coeval with tholi material (unit At). Interpretation: Mostly VBF and perhaps other materials that have been disrupted and eroded by volatile discharge and subsequent collapse.
Figure 15. (a) Shaded-relief view of Cydonia Labyrinthus (polygonal troughs), Colles (knobs), and Mensae (mesas) in eastern Acidalia Planitia (MOLA 1/128° DEM centered at 40°N, 348°E). Geologic units include highland material (unit HNu), knobby unit (unit HNk), boundary plains unit 1 (unit Hb1), the hummocky member of the VBF (unit AHvh), and younger chaos material (unit Act). The polygonal troughs in unit Act form within a broad basin 100–300 m deep that partly surrounds large knobs and mesas of unit HNk. The largest mesa rises ∼600 m above the VBF surface and may have been uplifted as shown by its surface elevation and the domical form defined by its surface and that of nearby mesas. MOC images located at b and c. (b) MOC image 0400576 (centered at 11.10°N, 317.73°E; 4.47 m/pixel) showing knob surrounded by bench, a trough, and pitted domes. (c) MOC image 1901441 (centered at 9.56°N, 318.31°E; 5.93 m/pixel) showing irregular mesas surrounded by smooth bench and apron.
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Figure 16. Scandia Tholi and Cavi region of Mars south of Olympia Planitia (MOLA 1/256° shaded-relief view centered at 77°N, 204°E). The hummocky member of VBF (unit AHvh) is marked by polygonal troughs and is superposed by lumpy pancake features and broad, rugged hills mapped as the tholi unit (unit At). The largest and most irregular outcrops contain depressions (Scandia Cavi; mapped partly as unit Act) a few hundred meters deep and tens of kilometers across. Many of the smaller lumps are partly ringed by shallow moats (arrows). Narrow sinuous ridges (r) ring parts of the large outcrops of unit At and in some cases demarcate areas of differing ruggedness. Pebbly texture of Olympia Planitia reflects dunes of unit Ad.
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 Tholi material (unit At) forms circular to irregular domical hills and rugged complexes tens to a few hundred kilometers across and tens to hundreds of meters high south of Olympia Planitia (Figure 16). The larger complexes have interior and surrounding depressions of collapsed terrain (unit Act) tens to hundreds of meters deep and a few narrow sinuous ridges a couple kilometers wide and tens of kilometers long. Many of the hills are bounded by shallow moats. Interpretation: Lava, mud, or ice extrusions and/or pingo-like intrusions formed the domes and perhaps phreatic or cryoclastic eruptions and discharge-related collapse produced the depressions and moats. Alternatively, the deposits and associated depressions have been interpreted as glacial kames and kettles by Fishbaugh and Head [2000, 2001].
 The Medusae Fossae Formation (unit Am; MFF) forms sequences hundreds to more than a kilometer thick extending discontinuously for thousands of kilometers along the highland-lowland boundary between the Elysium rise and Olympus Mons. In many places, the MFF displays systems of linear ridges and grooves. Interpretation: Various origins for the MFF have been proposed, including poorly indurated volcanic ash or aeolian sediment, carved into yardangs by the wind [e.g., Scott and Tanaka, 1982; Scott et al., 1987].
 Coarse lobate material (unit Alc) forms flows in Amazonis and Utopia Planitiae, in association with smooth lobate unit 1 (unit Als1, Figure 17). The proximal parts of flows in Amazonis Planitia appear to be buried by smooth lobate unit 1 and Olympus Mons aureole deposits (mapped as unit Aa). In Utopia Planitia, the unit emanates from mostly northwest-trending fissures and troughs in the northwest margin of the Elysium rise. The unit includes hummocky, pitted, and channeled surfaces. The flows typically are several tens of meters thick; in Utopia, the flows appear to embay and dissect ejecta blankets of large craters. Interpretation: Volcaniclastic flows formed from magma/volatile interactions; subsequent modification due to volatile escape and perhaps heating and activity of volatile-rich substrate [Christiansen, 1989; Skinner and Tanaka, 2001].
Figure 17. Geologic materials in south-central Utopia Planitia, including smooth lobate unit 1 (units Als1), coarse lobate material (unit Alc), and the hummocky member of the VBF (unit AHvh). (a) Shaded-relief view of area (from MOLA 1/128° DEM centered at 30°N, 122°E). Note hummocky surface of unit AHvh overlain by lobate flows of units Als1 and Alc. (b) Geologic map of a showing unit contacts (solid lines), flow scarps (hachured lines), flow directions (arrows), fault scarps (lines with triangles), channels in unit Alc (dashed lines), cross section line (A-A′) and MOC images (d, e). (c) Cross section A-A′ shows moderately rough unit AHvh on inner basin slope; thick, flat-topped flows of unit Als1; and thick, dissected flows of unit Alc. (d) MOC image 0200618 (centered at 32.49°N, 123.61°E; 4.46 m/pixel) shows fine-scale roughness contrast between units Als1 and Alc. (e) MOC image 1201669 (centered at 29.52°N, 118.52°E; 5.97 m/pixel) showing unit Als1 flowing into irregular set of en echelon grooves with raised rims in unit AHvh.
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 Smooth lobate unit 1 (unit Als1) consists of Early to Middle Amazonian tongue-shaped flows tens of kilometers wide that extend for hundreds of kilometers in Amazonis and Utopia Planitiae (Figures 17 and 18). The unit buries the VBF and other units of Noachian and Hesperian age. In eastern and southern Amazonis, the flows mostly appear to underlie the Olympus Mons aureole (unit Aa) and the Medusae Fossae Formation (unit Am). The flows are sparsely cratered in the MOLA DEM. The unit generally exhibits well-preserved flow morphologies. Interpretation: Lavas flows [Scott et al., 1987]; high effusion rates may account for their great extent across flat plains.
Figure 18. Southern Amazonis Planitia showing thick flows of smooth lobate unit 1 (unit Als1) overlying older lobate material (unit AHl), which in turn embays boundary plains unit 2 (unit Hb2) (shaded-relief view from MOLA 1/128° DEM centered near 19°N, 202°E). Note wrinkle ridges trending N50°W are crosscut by scarps (strike-slip faults?) trending N50°E. Younger, uncratered flow superposes structures and lightly cratered flows.
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 Apron material (unit Aa) includes smooth, sloping deposits along the base of highstanding scarps of Deuteronilus Mensae as well as the extremely rugged aureole materials of Olympus Mons. Interpretation: Deposits derived from ice-lubricated mass-wasting deformation processes [e.g., Tanaka, 1985; Mangold and Allemand, 2001].
 Smooth lobate unit 2 (unit Als2) consists of Late Amazonian tongue-shaped flows tens of kilometers wide that extend for hundreds of kilometers across Elysium Planitia. The unit buries smooth lobate unit 1 (unit Als1) and other adjacent units. The unit locally embays and encircles dozens of impact craters a few kilometers and larger in diameter, indicating that the unit is probably tens to a few hundred meters thick in these areas. The flows are sparsely cratered in MOC images and may be less than 10 million years old [Hartmann and Neukum, 2001]. The unit mostly displays crisp, platy flow morphologies; in southwestern Amazonis Planitia, the flows contain sinuous channels bounded by rounded scarps. Interpretation: Mostly lava flows [Plescia, 1990]; high effusion rates may account for their great extent across flat plains. Near Cerberus Rupes, the unit may include young channel floors and deposits [Burr et al., 2002]. Distal material in Amazonis may also be debris flows [e.g., Rice et al., 2002], perhaps co-mingled with fluvial deposits [Scott et al., 1987; Fuller and Head, 2002].
 Hummocky material (unit Ah) occurs in Borealis basin south of Chasma Boreale (Figure 13). The unit may be tens of meters thick, sufficient to bury and obscure the hummocky topography of the underlying VBF (unit AHvh). The unit may abut the ejecta ramparts of impact craters as seen in MOLA DEM's and thus may be uncratered at diameters of several kilometers and larger. Interpretation: Poorly indurated aeolian dust deposits, perhaps including reworked polar layered deposits or other polar region materials.
 Polar layered deposits 2 (unit Apl2) form the upper part of Planum Boreum (Figure 13) and show fine layering, unconformities, and minor deformation in MOC images [e.g., Edgett and Malin, 2000; Kolb and Tanaka, 2001; Malin and Edgett, 2001]. On part of the margin of Planum Boreum, the unit appears to be draped over underlying topography (Figure 14). These deposits display no impact craters larger than a few hundred meters across in Viking images and MOLA DEM's [Herkenhoff and Plaut, 2000]. Interpretation: Likely made up of water ice and dust; alternating layers may result from varying proportions of dust and ice.
 Dune material (unit Ad) forms broad, pebbly surfaced patches in MOLA DEM's covering Olympia Planitia (Figure 16) and other areas south of Chasma Boreale (Figure 13) and appears to be made up mostly of individual and chains of low-albedo barchans as seen in higher resolution Viking and MOC images [e.g., Breed et al., 1979]. Interpretation: Vast seas of wind-blown, sand-size particles, recently active and eroding from the dark polar layered deposits 1 (unit Apl1).
 Polar residual ice (unit Api) has high albedo and appears to be largely uncratered in Viking and MOC images and MOLA DEM's [Herkenhoff and Plaut, 2000]. It forms a veneer on plateau surfaces of Planum Boreale. The mapping of this unit shown in Figure 2 is based on the Viking Mars Digital Image Mosaic and MOC wide-angle images, which show different extents of ice in places. Occurrences of the unit in the Scandia region are not mapped in order to show the underlying geology evident in the MOLA DEM. Interpretation: Recently deposited carbon dioxide ice precipitated from the atmosphere.
2.2. Tectonic Structure
 Most of the extensive systems of subtle, linear ridges throughout the northern lowlands ring Tharsis rise and Utopia basin; also ridges oriented radial to Utopia occur within that basin (Figures 2, 4, and 7). Most ridges have asymmetric cross-sectional topographic profiles. Ridge heights are similar among the plains units (∼100 to 300 m), which also are similar to ridge heights in highland and Tharsis volcanic rocks elsewhere on Mars [Golombek et al., 2001]. The maximum slopes on the ridges derived from the MOLA DEM are ∼0.5°.
 Cross-cutting relations indicate a lengthy history of ridge development in the northern plains, beginning in the Early Hesperian in boundary plains unit 1 (unit Hb1). The areal density of wrinkle ridges locally appears to decrease with decreasing age of the boundary plains and VBF units (units Hb1–2, AHvh). Ridges are generally absent in outcrops of older lobate material bordering the lowlands, where tectonic structures are largely absent or extensional faulting tends to dominate. Younger Amazonian lobate units (units Alc, Als1–2) that extend into the plains mostly bury ridge structures in the VBF and the boundary plains units, with local exceptions (e.g., Figure 18). In southern Utopia, the unit AHvh margin does not deflect across the ridges (Figure 4), indicating that either the ridges postdate unit AHvh or the local development of unit AHvh was not surface elevation dependent. The marginal member of the VBF (unit AHvm) appears to destroy ridges in Arcadia Planitia (Figure 11). In Chryse and Acidalia, ridges formed both prior to and following Late Hesperian channel dissection. Within the VBF, ridges are locally absent, perhaps where buried by outflow channel sedimentation in Borealis basin and where obscured by tholi and younger chaos materials (units At and Act) and related resurfacing features. We therefore see the ridge systems as a partly preserved record of contractional strain primarily from the later part of the Early Hesperian into the initial part of the Early Amazonian. The nearly ubiquitous Hesperian and Amazonian resurfacing apparently has destroyed or subdued the record of possible earlier ridge development in the northern plains. Thus the dominant signature of plains contractional deformation extends to younger times than the Late Noachian to Early Hesperian ages assigned to most highland and Tharsis-related contractional ridges [e.g., Scott et al., 1987; Tanaka et al., 1991; Watters, 1993; Dohm et al., 2001b].
 The apparent subtle nature of the lowland ridges, compared with those observed in highland rocks in Viking images, has been attributed to burial beneath a blanket of ocean sediment [Kreslavsky and Head, 2001; Thomson and Head, 2001]. However, the ridges retain relief similar to that of their highland counterparts, suggesting that their generally poor detectability in Viking images may have resulted instead from (1) the greater atmospheric scattering in lowland images, and (2) the lack of steep, readily perceived crenulations on the lowland ridges. Wrinkle (crenulated) ridges on Mars typically form in lava sequences, but uncrenulated scarps are more common in highland rocks [Watters, 1993] that may include sediments and highly mechanically weathered material. Okubo and Schultz  indicate that crenulations result from secondary synthetic and antithetic faulting within mechanically heterogeneous material; thus the northern plains ridges may reflect formation in mechanically homogenous material materials that inhibit such secondary faulting., whereas stronger lava sequences, possibly interbedded with weak materials, may favor the development of pronounced secondary faults. Notably, a small medial ridge in unit AHvm in Chryse Planitia has crenulated shoulders (Figure 12), perhaps indicative of a shallow, mechanically heterogeneous stratigraphy.
 Most ridges appear to be controlled by the regional topography, with major systems concentric to the north Tharsis region and Utopia basin (Figures 2a and 7). Within Utopia basin, many of the ridges trend radial to the basin center. Ridges in Isidis Planitia follow NW, E, and NE trends largely independent of the basin structure but perhaps concentric and radial to the HLB, which coincides with the intersection of Isidis and Utopia basin rims on the NE margin of Isidis basin. Compressional stresses responsible for the ridges probably resulted from planetary contraction and Tharsis loading [e.g., Tanaka et al., 1991], with ridges orientations locally controlled by Utopia basin [Thomson and Head, 2001] and other buried structure.
 A few narrow grabens of Tantalus Fossae cut northeastern Alba Patera and adjacent plains materials (Figure 2). Many are buried by the VBF but a few cut this unit, indicating that Tantalus Fossae development likely extended from the Hesperian into the Amazonian, in agreement with previous results [Tanaka, 1990]. The tensional stresses imposed by rifting associated with Alba Patera that produced these grabens apparently also precluded the development of ridges in this region.
 In southern Amazonis Planitia, wrinkle ridges trending ∼N50°W (concentric to Olympus Mons) deform an older flow sequence of smooth lobate unit 1 (unit Als1) and other older units (units AHl and Hb2) and are buried by a lightly cratered flow sequence of unit Als1 (Figure 18). These relations place the deformation in about the Middle Amazonian. The ridges are crosscut by linear, locally en echelon scarps trending N60°E (Figure 18); vertical offsets across the scarp alternate between northwest and northeast facing. Possibly these scarps formed by strike-slip faulting given that (1) they trend oblique to the wrinkle ridges, (2) one scarp forms at the termination of a wrinkle ridge, (3) the geometry of the ridges and scarps are consistent with a possible maximum compressive principal stress radial to Olympus Mons, and, (4) in contrast, wrinkle-ridge interactions generally result in curved ridge traces rather than linear sets [cf. Schultz, 1989; Watters, 1993].
 Tectonic deformation may also involve broad zones of uplift and lowering. Tanaka et al. , for example, proposed deformation along the VBF contact northeast of Alba Patera, in the outflow channels, and across Isidis Planitia. In the present study, with significantly higher resolution MOLA DEM data, such deformation is not resolved. Northeast of Alba Patera, the VBF/Hb2 contact exhibits ∼800 m of elevation range, some of which could be due to tectonic warping. However, the older Hb2/AHl contact that borders Alba Patera shows <300 m of relief; this observation seems to contradict significant warping. The southward-sloping outflow-channel floors stretching across hundreds of kilometers (shorter ones may be controlled by local structures) appear to be restricted to the region of chaos development within southern Chryse Planitia and northern Xanthe Terra. Since some of the chaos within the outflow channels appear to postdate some of the channel system that coursed into Borealis basin, development of the chaos terrains may be responsible for altering drainage direction. While the southwest-trending dip of the Isidis basin floor could have a tectonic origin, it might instead be related to volcanic loading of Syrtis Major Planum and local collapse due to magmatic activity rather than to plains-wide tectonic deformation. Finally, the Chryse outflow channels and young flow sequences (units Als1–2 and Alc) in the northern plains consistently trend in the direction of regional slopes. Overall, we find no unambiguous evidence for regional tectonic tilting of the surface.
2.3. Crater Modification
 Impact craters scattered throughout the northern plains provide a basis for analyzing the temporal and spatial history of resurfacing at vertical scales generally ranging from tens to hundreds of meters using MOLA data. Given that fresh impact craters generally have pronounced rims, distinct ejecta lobes, and deep, bowl-shaped floors, subdued crater rims and floors and missing ejecta features provide evidence for various sorts of resurfacing. Because the age and initial morphology of each crater are not precisely known, only qualitative and general statistical assessments of crater modification history can be made.
 Shallow circular depressions that have subtle, raised rims and lack distinct ejecta (“ghost craters”) are scattered throughout the northern plains (Figure 3) and occur within the VBF (units AHvh and AHvm) and boundary plains units (units Hb1–2). Those that we identified range from 13 to 188 km in rim diameter and have crater floors ∼105 ± 69 m deep (from our data for 295 craters) below the surrounding plains. When the density of ghost craters is added to that superposed on the VBF, an Early Hesperian crater density results [Kreslavsky and Head, 2001]. Furthermore, additional larger, but subtler “quasi-circular depressions” recognized in fine-scale topographic contouring indicate an Early Noachian density [Frey et al., 2002]. Some of the ghost craters on the boundary plains units have greater depth and rim height than those common in the VBF.
 Ghost craters (Figure 3) are unevenly distributed in the northern plains and likely were obliterated in places by Late Hesperian and Amazonian resurfacing. Such regions include: (1) central Utopia Planitia, where young lava and volcaniclastic flow deposits from Elysium Mons (units Als1 and Alc) have raised the plains floor by tens of meters (sufficient to bury ghost crater rims and floors) [Skinner and Tanaka, 2001]; (2) southern Utopia Planitia, where ring fractures in polygonal troughs occur rather than ghost craters, indicating that ring fractures may result from burial of ghost craters and subsequent contraction of the overlying sediments [McGill, 1989; McGill and Hills, 1992]; (3) Isidis Planitia, where young alluvium from Libya Montes and perhaps volcanic materials from Syrtis Major Planum may have been deposited [Crumpler et al., 2002]; (4) the lower part of the Acidalia side of Borealis basin, where sediments from the Chryse outflow channels must have collected (mapped as unit AHvh but probably originally unit Hch); (5) the Scandia region, where complex local resurfacing activity resulted in the Scandia, tholi, younger chaos material and polar units and features; and (6) Amazonis and Elysium Planitiae, where lavas, degraded flows, and local fluvial deposits from southeastern Elysium Mons, the western flank of the Tharsis rise, and northwestern Olympus Mons (units Als1–2 and Alc) have buried much of the plains [Scott et al., 1987; Fuller and Head, 2002].
 How were fresh craters with deep, bowl-shaped floors and prominent rims apparently transformed into ghost craters with subdued rims and flat floors of remarkably uniform depth? One hypothesis is that ghost craters represent Early Hesperian craters that have been buried by materials comprising the VBF [Kreslavsky and Head, 2001; Head et al., 2001]. However, we find that ghost craters also occur in the boundary plains units. Thus, processes other than burial may have contributed to the formation of the northern plains ghost craters. For example, at latitudes >30°N, many sub-kilometer and larger plains craters in Viking and MOC images appear heavily modified by terrain softening and perhaps aeolian infilling, in which the crater rim appears relaxed and the crater floor is nearly filled with layered and/or ridged debris [Squyres and Carr, 1986; Zimbelman et al., 1989]. Ghost craters are similar to virtually rimless, flat-floored craters prevalent in highland regions of Mars that have greater ranges in floor depth and rim height than their lowland counterparts and have been attributed to fluvial erosion and infilling, mass wasting, and perhaps other processes [e.g., Craddock et al., 1997]. In addition, other effects such as fluidization and smoothing of fluid-saturated crater-floor material due to shaking from impacts and other seismic events [Clifford, 1993] may also have contributed to ghost-crater development. Thus the actual dominant modificational process(es) that produced the lowland ghost craters remains uncertain and subject to further analysis.
 Other modified crater forms include (1) pedestal craters, which commonly have distinct rims and moderately deep floors, lack ejecta ramparts, and are perched on low plateaus tens of meters or more high and from ∼2 to 10 crater diameters wide [Mouginis-Mark, 1979], and (2) craters with distinct rims and ejecta but having shallow floors at or above the level of the adjacent surface in northern Arcadia Planitia and near Planum Boreum. Pedestal craters are locally abundant in the plains north of Alba Patera, in southern Amazonis Planitia, and especially on the northern rim of Utopia basin (Figure 3). The ejecta of pedestal craters seem to have armored a blanket of unconsolidated, perhaps ice-rich material that was later stripped away [e.g., Mouginis-Mark, 1979; Schultz and Lutz, 1988]. A particularly extreme example of the second crater type at 45.2°N, 184.6°E in Vastitas Borealis has a moderately to well-preserved rim 23 km in diameter and is infilled to an elevation 200 m above the surrounding plains. Large craters south of Olympia Planitia and Planum Boreum (e.g., Figures 14 and 16) appear partly filled by asymmetric, dome-shaped polar layered deposits 2 (unit Apl2) hundreds of meters thick. This second class of modified craters may have been filled by dust and ice entrapped only within the craters and/or by a broader mantle (or mantles) that was later stripped away except for within the craters. The deposit forms may reflect preferential erosion due to both local down-slope winds along the crater walls, producing the moats surrounding the domes, as well as regional prevailing winds, resulting in the off-centered dome crests. Collectively, the pedestal and infilled craters indicate that unconsolidated deposits had formed extensive blankets and isolated deposits within topographic traps in the northern plains; the blankets were preserved long enough to be impacted before eroding away. The deposits may have consisted of dust and ice related to volcanic and impact activity or the erosion and redistribution of material from elsewhere [cf. Schultz and Lutz, 1988; Tanaka, 2000; Malin and Edgett, 2001; Mustard et al., 2001].