The Phaethontis quadrangle is dominated by the cratered highlands of Terra Sirenum, which display prominent, marginal basins and tectonic structures, reaching thousands of kilometers in length. Except for the interiors of larger craters, elevations are generally 1–3 km above datum. Tharsis lava-flow materials inundate and partially cover the rugged, ancient terrain in the northeast corner of the quadrangle. Some of the structural basins in the northwestern part of the quadrangle display disrupted floors, referred to as chaotic terrain, most notably Atlantis Chaos and Gorgonum Chaos. The segmented, narrow graben systems of Sirenum Fossae and Icaria Fossae extend southwestward from the Tharsis rise, northeast of the quadrangle, cutting both ancient cratered highland materials and some of the older Tharsis lava flows.

The Iapygia quadrangle consists almost entirely of heavily cratered highlands, as high as 3 km above datum, descending to the northern basin rim (0–3 km below datum) and floor (3 to over 5 km below datum) of Hellas, and a piece of the southwestern rim of Isidis basin. Terra Sabaea makes up the western two-thirds of the quadrangle, whereas Tyrrhena Terra makes up the third that is east of a topographic divide, at ~75° E. An arcuate, north-facing series of scarps, Oenotria Scopuli, crosses this divide and appears to be concentric to Isidis basin. Huygens forms a prominent impact basin, with an outer rim of ~470 km in diameter, and it has an inner (250-km-diameter) and partial intermediate (350-km) ring.

Eridania quadrangle is composed almost entirely of the ancient cratered highland terrain of Terra Cimmeria, at 0–2 km elevation. The largest crater, Kepler, is about 230 km in diameter. Less-cratered, relatively low-lying plains are scattered throughout the quadrangle, including Eridania Planitia in the northwest corner and Planum Chronium in the southwest part of the quadrangle. Ridge systems occur throughout the quadrangle, with northeast-trending Eridania Scopulus forming the most prominent ridge.

Mars has attracted study ever since its motions were first apparent to ancient skywatchers. Summaries of early observations and ideas are listed in, e.g., Collins, 1971; Hartmann and Raper, 1974; Moore, 1977; Kieffer et al., 1992a; Martin et al., 1992; Sheehan, 1996; Morton, 2002. Hubbard (2011) gives an interesting example of the planning of Mars missions.

The Mare Acidalium quadrangle is dominated by the gently northward-dipping lowland plain of Acidalia Planitia and its contiguous southern neighbor, Chryse Planitia, both lying between 4,000 and 5,000 m below datum. These plains are partly bordered by cratered highlands that rise as much as 3,000 m above the plains – Arabia Terra to the southeast and Tempe Terra to the southwest. The outer margins of these terrae define the mouths of the largest fluvial-type channels anywhere on Mars – the circum-Chryse outflow channels (named for Chryse Planitia, into which the channels collectively emerge, see also MC-10 and MC-11). The regional planitiae contain the generally north–south trending ridge systems of Xanthe Dorsa. Acidalia Mensa is located in the center of the quadrangle and forms an irregularly shaped plateau that rises from 100 to >500 m above the surrounding plains.

Mars is the fourth planet from the Sun and the outermost of the rocky, terrestrial planets that make up the inner solar system. Mars is the second smallest planet; only Mercury is smaller. Surface gravity on Mars is 3.71 m s–2, which is 37.6% that of the Earth. The present atmospheric pressure is low (~0.6 kPa) relative to Earth’s (101 kPa), and the atmosphere is mostly carbon dioxide (95%). The obliquity of Mars (tilt of the axis of rotation relative to the plane of orbit) is presently 25 degrees and may have varied by tens of degrees over the past tens of millions of years and longer (Laskar et al., 2004).

Elysium Mons rises 14 km above the surrounding plains, while nearby Albor Tholus is 4 km high. Much of the quadrangle consists of plains near datum to –3,000 m, but in the east, Tartarus Montes, Tartarus Colles, and the rimmed depression, Orcus Patera, constitute a more rugged region, largely made up of knobs and low plateaus and ridges that separate Elysium Planitia from Amazonis Planitia to the east (MC-8). To the south of Orcus, Marte Vallis extends from Elysium Planitia into the Amazonis basin. Elysium Planitia includes the landing site of the InSight mission, which is exploring the interior of Mars using geophysical measurements.

The Amenthes quadrangle contains parts of the Martian southern highlands and northern lowlands, as well as the transition between the two. In the southern part of the quadrangle, Cimmeria and Tyrrhena Terrae form rugged, cratered plateaus as high as 1 km above datum, which are gouged by the long, linear depressions of Amenthes Fossae. Highland terrae in this quad are separated by Amenthes Planum, an elongate, topographic basin that is located as much as 1 km below the highlands. The northern part of the Amenthes quadrangle consists of southern Utopia Planitia, over 4 km below datum. The western part of the quadrangle is made up of eastern Isidis Planitia, which is nearly as low in elevation. Both Utopia and Isidis Planitiae – which are centered outside of the Amenthes quadrangle – are plains of sediments that fill very ancient impact basins. Various scarps and depressions mark the surface of these lowland planitiae. From south to north, the highland–lowland boundary is defined by distributed fields of knobs and intervening plains of Nepenthes Mensae, rolling plains of Nepenthes Planum, and isolated and coalesced depressions of Amenthes Cavi.

The Argyre basin spans the west half of the quadrangle, while part of Noachis Terra, at 0–2 km elevation, lies to the east. Argyre, as deep as –3 km elevation, is the best preserved of the largest multi-ringed impact basins on Mars, and is comparable in size to the Orientale basin of the Moon. The size and number of rings in the basin, which are generally expressed by discontinuous, concentric ridges and basin-facing scarps, are debated (three to seven rings or more), owing to later modification. The most common diameter assigned to a prominent, inner ring is 800–900 km, while the entire structure may be 1800 km or more across. Valleys drain toward Argyre from the south and east, while large channels may connect Argyre to the Uzboi–Ladon–Morava (see MC-19) system to the north. Drainage into the northwestern flank of the basin from surrounding plains is blocked by concentric, broad ridges. The hummocky floor of Argyre is 3–4 km below the average terrain elevation beyond the rim (Hiesinger and Head, 2002) and includes a variety of landforms. Noachis Terra is typical of the southern cratered highlands of Mars and gives its name to the oldest period of geologic time on Mars (MC-27).

This map spans diverse highland to lowland terrains from west to east. The western smooth region is the margin of the Tharsis rise that is centered to the west of the map, descending from 2 km to near datum. The elevated Lunae Planum and southern part of Tempe Terra occupy the central third of the map area. Lunae Planum rises to 3,500 m elevation along the southern border of the map as it approaches the Valles Marineris canyon system (south of the map area, MC-18). East of the planum lies the heavily cratered Xanthe Terra and Xanthe Montes. Cutting into these highlands are the large outflow channels Maja Valles and Kasei Valles. The latter originates from the irregular Echus Chasma depression that lies near datum. In the northeastern corner is Chryse Planitia, part of the vast northern lowlands, more than 3000 m below datum, and the destination for channel outflow as well as the Viking 1 lander.

The Mare Australe quadrangle is dominated by the south polar plateau, Planum Australe (Figure 30.A), rising to about 4,800 m in elevation on its highest part, Australe Mensa (Figure 30.B). Planum Australe is roughly circular, with horizontal dimensions of 1,100 km by 1,400 km. The plateau is dissected by the large troughs Chasma Australe, Promethei Chasma, and Ultimum Chasma, which divide parts of the plateau into the tongue-shaped forms of Australe Lingula and Promethei Lingula. Also present are systems of lower relief, concentrically and obliquely trending troughs, and asymmetric ridges (Figure 30.A). The surrounding cratered highlands lie at 1–3-km elevations and include some rather exotic terrains, unique to the south polar region. Cavi Angusti form depressions tens of kilometers across and reaching depths of a kilometer. Branching ridges form Dorsa Argentea and other ridge systems that cover Argentea, Promethei, and Parva Plana, surrounding Planum Australe.

The global landscape of Mars is diverse, and this diversity tells a story of its surface history. Geographic regions and zones, each unique in character, are dominated by specific landforms and materials. These in turn express processes and histories, including geologic and climate-driven activity. Understanding the major geographic regions of Mars, as on Earth, then, is essential to unraveling the factors involved in global and regional geologic activity. It also provides context for study of local areas and individual features or sets of features. In addition, some of the major regions of Mars are indicative of heterogeneities in the crust and mantle (see Chapter 5).

Memnonia occurs on the western margin of the Tharsis volcanic province and on the southern margin of Amazonis Planitia. Its elevation ranges from a few kilometers above datum in Terra Sirenum to a few kilometers below datum along the northern edge of the map sheet. The cratered and faulted terrain of Terra Sirenum gives way to Tharsis lava flows to the east within Daedalia Planum. Valleys and channels mark the northern margin of the highlands, where they grade into low regions interrupted by elongate plateaus like Amazonis Mensa and the broader Lucus Planum.

Ismenius Lacus is located in the northern mid-latitudes of the eastern hemisphere of Mars. It includes sections of both the southern highlands and northern plains. The topographic transition is defined by gently sloping surfaces, steep scarps, linear to sinuous channels, and isolated knobs and plateaus. The southern part of the quadrangle is defined by the northernmost extent of the high-standing, ancient cratered highlands of Arabia Terra and Terra Sabaea, at elevations near datum to –3,000 m. The regional highlands in Ismenius Lacus contain large, ancient channel systems – Okavango and Mamers Valles (Figure 5.A; Mangold and Howard, 2013) – as well as networks of linear depressions – Ismeniae and Coloe Fossae. These physiographic features all record complex geologic processes that are associated with the long-term break-up and marginal erosion of the cratered highlands. From the north to the south, the highland–lowland transition in Ismenius Lacus is marked by the high-standing plateaus of Deuteronilus and Protonilus Mensae as well as the irregularly-shaped depressions of Deuteronilus Colles and Colles Nili. The lowlands in the north typically lie at –4,000 m or lower. The 236-km-diameter Lyot crater and its radial and lobate ejecta blanket dominate the center of the quadrangle.