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).

The rugged highland terrains of Noachis Terra and Terra Sabaea dominate the region. The higher-standing, tectonically deformed, and densely cratered Terra Sabaea contains Scylla and Charybdis Scopuli. Also present are Denning, Bouguer, Lambert, Dawes, Pollack, Schiaparelli, Tuscaloosa, and Bakhuysen craters. To the west, the relatively subdued, but still rugged highland region of Noachis Terra has Newcomb, Wislicenus, and Mädler craters; and Marikh and a portion of Evros Valles. Numerous other moderately to highly degraded craters are scattered throughout the area. Valley networks ranging from tens of kilometers to thousands of kilometers in length dissect much of the topography. Wide grabens scar parts of Terra Sabaea. The region slopes from close to 3,000 m above datum in Terra Sabaea to as low at –1,500 m in the northwest. The northeast region is a portion of the zone that occurs between highland terrains to the south and transition terrain of Arabia Terra to the north (MC-12).

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.

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.

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).

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.

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).

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.

In this quadrangle, the deepest canyon system on Mars and one of the most spectacular in the solar system, Valles Marineris, extends more than 2,500 km across the map, cutting the northern part of the Thaumasia plateau (informal name). Parts of the floor are more than 5,000 m below datum and as much as 10,000 m below the plateau rim. The system of canyons includes Melas, Candor, and Ophir Chasmata, prominent at its central part, Coprates along its eastern part, and Ius and Tithonium Chasmata, forming its western arm. Echus, Hebes, Juventae, and Ganges Chasmata form additional, separate canyons, north of the main canyon system. The canyons of Noctis Labyrinthus, west of the map (MC-17), join Valles Marineris, which connects eastward with the chaos-filled canyons of Capri and Eos Chasmata. Some of the other chasmata are linked to outflow channels north (MC-10) and east of the map (MC-19). Sinai, Solis, and Thaumasia Plana make up high plains that span the western to south–central parts of the Coprates quadrangle. Together they constitute much of the Thaumasia plateau, a dish-shaped, elevated region at 3,000–5,000 m. At their eastern edges, the Thaumasia plateau and Ophir Planum drop as much as 3,000 m down to variably cratered plains of Noachis and Xanthe Terrae, respectively. The informally named Coprates rise is a prominent north-trending mountain range, forming the eastern margin of the Thaumasia plateau, south of Coprates Chasma. Crossing this entire region are north- to northeast-trending wrinkle ridges as well as local sets of narrow grabens and pit chains.

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 Diacria quadrangle includes parts of Arcadia and Amazonis Planitiae on the planet’s northern plains (around –4000 m elevation), the northwestern flank of Alba Mons (reaching >3000 m elevation), and part of Vastitas Borealis to the north. Other prominent features include the northern part of the rugged Lycus Sulci terrain, the Acheron Fossae that cut a broad, arcuate ridge, and knobby and cratered plateaus of Erebus Montes.

The Margaritifer Sinus quadrangle comprises the distal eastern portions of the Valles Marineris canyon floor, including Eos, Ganges, and Capri Chasmata. Those features transition into the Iani, Margaritifer, Aurorae, and Aureum Chaoses that dominate much of the remaining northwestern and north–central region of the quadrangle. The heavily cratered, high-standing plains of Noachis Terra, at 1 km above datum, in the southern portion of the map, transition into the moderately cratered, middle-relief plains of Arabia Terra to the northeast. The elevation descends to 3,000 m below datum in parts of Xanthe and Margaritifer Terrae and drops another 1,000 m or more into eastern Valles Marineris. Located in a portion of Arabia Terra, Meridiani Planum was the hematite-rich landing site of the Mars Exploration Rover-B, Opportunity (Figure 19.A; Christensen and Ruff, 2004; Christensen et al., 2005). A portion of the Xanthe Terra highlands in the northwest shows how the formation of Valles Marineris isolated some of the highland terrains. The large channel and lake system of the Uzboi–Ladon–Morava (ULM) Valles runs diagonally from the south–southwest to the north–northeast, through several large ancient crater basins, thought to have held lakes, including Holden crater.

The highland–lowland boundary transects the central part of the Aeolis quadrangle. Elevations range from 3 km above datum in the highlands to 3 km below datum in the lowlands at the north edge of the quadrangle. Aeolis Mensae and de Vaucouleurs display the rugged, gradational nature of the boundary in this region. Along the boundary is Gusev crater, the landing site of the Mars Exploration Rover (MER-A), Spirit (Squyres et al., 2004; Arvidson et al., 2006), while near the west edge of the map is Gale crater, location of the Mars Science Laboratory (MSL) rover, Curiosity (Golombek et al., 2012; Grotzinger et al., 2012). Ma’adim Vallis originates from a complex of proposed paleolake basins and enters Gusev crater from the south (MC-29; Irwin et al., 2002). About 200 km directly north of Gusev is a prominent, 200-km-wide and 5-km-high shield volcano, Apollinaris Mons.

The Syrtis Major quadrangle is named after a prominent dark feature, discovered by Christiaan Huygens, in 1659, as the first surface feature recognized on another planet (Figure 13.A). The name “Syrtis Major,” however, originated more than 200 years later, with the astronomer Giovanni Schiaparelli. The quadrangle contains the volcanic province, defined by Syrtis Major Planum, a large but gently sloping dome that rises to almost 2,100 m in elevation. Centered on Syrtis Major Planum are two rimless depressions named Nili and Meroe Paterae. West of Syrtis Major Planum lie the ancient cratered highlands of Mars, named Terra Sabaea. This ancient terrain descends to near datum and is pockmarked with multiple ancient impact craters and basins, including Antoniadi (400 km in diameter), Baldet (181 km in diameter), and Flammarion (174 km in diameter). Isidis Planitia resides to the east of Syrtis Major Planum, and represents the smooth, infilled surface of a very ancient multi-ring impact basin that was at least 1,500 km in original diameter. The floor of Isidis Planitia is close to 4 km below datum. Large-scale landforms, characteristic of giant impact basins, are circumferential to Isidis Planitia. To the south, Libya Montes forms an arcuate ring of large, angular mountains, some of which rise >2 km above the surrounding plains. Northwestern Isidis Planitia is bounded by the elongate and arcuate troughs of Nili Fossae, which are up to 40 km wide and 1,300 m deep. Arena Colles forms the best-preserved highland–lowland transition zone within the Syrtis Major quadrangle, and it is composed of a field of rugged knobs as well as their intervening plains.