Abstract

I taught the first geography of Mars course at California State University, Long Beach, in Spring 2007. Among its planned student learning outcomes was student acquisition of a sense of where on Mars features under discussion were located and their spatial relationships with one another. It became necessary to create a regional geography scheme for the planet to achieve this. I adapted the "orders of relief" scheme used in basic geography texts, inherited from Fenneman's 1916 physiography article. The outcome was a nested five level scheme. The first order is the divide between the smooth northern lowlands and the rough southern highlands. The second order comprises the 1,000 km + impact, rifting, volcanic, glacial, and fluvial features. The third order is that of the terrae and plana that define geological epochs on Mars, such as Noachis Terra, Hesperia Planum, and Amazonis Planitia. The fourth order is made up of landscape level features, such as individual landslides, craters, yardangs, dune fields, drainage systems, and the like. The fifth order describes the very local features at the scale of the rovers' and landers' activities, such as trenches, rock abrasion pits, and "blueberries." The paper will outline the bases for the ordering scheme.

[ Viewgraph 1 ]

Introduction

I taught the first geography of Mars course at California State University, Long Beach, in Spring 2007. How that class came to be was explained in the Geography of Mars panel earlier in the conference. [ Viewgraph 2 ] In this paper, I will concentrate on one of the student learning outcomes for that class, which was student acquisition of a mental of Mars features and their spatial relationships with one another. It became necessary to create a regional geography scheme for the planet to achieve this. I adapted the "orders of relief" scheme used in basic geography texts, inherited from Fenneman's 1916 physiography article. The result was a nested five level scheme that created a vivid mental map of Mars as a geographical landscape for the students in the class.

The First Order of Relief

[ Viewgraph 3 ] All the order of relief schemes in textbook use today agree on the division of Earth's surface into continents and oceans (c.f. Bridges 1990; Christopherson 2003; Garrard 1988; and Ritter 2006). On Mars, a spatially comparable dichotomy consists of the division into the low elevation northern third of the planet's surface and the high elevation southern two thirds (MOLA Science Team 2000). The Northern Lowlands range from the mean geoid to 5 km below, while the Southern Highlands typically range from 1 km to 5 km above the mean geoid. [ Viewgraph 4 ] The transition between them is quite abrupt, typically forming a 1 km high scarp (MOLA Science Team 1999).

[ Viewgraph 5 ] The elevation dichotomy coïncides with a contrast in cratering rates, with the Northern Lowlands having a smooth surface of lava flows and sediments overlying an ancient cratered terrain. The Southern Highlands are in most places highly cratered, preserving a record of the age of bombardment from the time of the planet's accretion to about 3.5 or 3.9 billion years ago. Lava surfaces in the Northern Highlands are typically andesitic in composition, while the Southern Highland lavas are predominantly basalts (Wyatt and McSween 2006). [ Viewgraph 6 ]

The first order dichotomy is again echoed, though not as perfectly, in differences in crustal thickness, with the Northern Lowlands quite thin and much of the Southern Highlands thicker and thickening irregularly towards the South Pole (Zuber et al. 2000; Science Visualization Studio no date).

The origins of this first order dichotomy remain contentious. The smoothness and low elevation of the northern lowlands have long suggested sedimentary deposits, perhaps from an ocean. Tim Parker traced out two sets of circumboreal shoreline-like features, which remain at least partially credible under examination of high resolution MOLA data (Parker et al. 1993; Head et al. 1999: Fig. 1). [ Viewgraph 7 ]

Countering the impression of a primordial ocean/continent dichotomy, however, are problems both of lithology and of shoreline elevation. On Earth, basalt dominates the ocean floors, while more highly fractionated igneous rocks, such as granites and andesites, and their sedimentary and metamorphic derivatives dominate the continents.

Another problem is the near failure, until recently, to find carbonates, which the interaction of the carbon dioxide-dominated martian atmosphere with water could be expected to create in abundance. Late in 2008, however, a layer of magnesium carbonates was found in association with clays in rock units dominated by olivines in Nili Fossæ near Isidis Planitia (Ehlmann et al. 2008). Olivines can be altered into these carbonates in a non-acidic aqueous environment. The northern ocean idea is, thus, not too far-fetched.

Whether or not the Northern Lowlands ever contained an ocean, the origins of the topographic low remain a topic of debate. Among ideas proposed for this dichotomy are the initiation of one-plume convection with crustal thinning by ablation from below and crustal thickening by compression (Roberts and Zhong 2006). This could have gone so far as mantle overturn by gravitational instability due to mantle crystalization and reaction placing iron-enriched, denser, and colder material in the upper mantle (Watters et al. 2007; Elkins-Tanton et al. 2004). Plate tectonics may have initiated and, though it did not persist, may have altered the crust in ways helpful to understanding the dichotomy. Another accounting for the dichotomy lately gaining support is the idea of a large impact early in Mars' history, as was known to have happened to the early Earth (Marinova et al. 2008; Andrews-Hanna et al. 2008). If so, the Northern Lowlands may simply be the biggest impact crater in the solar system. [ Viewgraph 8 ]

The Second Order of Relief

The second order of relief would be the very large planetary features, many of a scale as to be visible from Earth. They range in size from about 1,000 km to 8,000 km and include features created by impact, volcanism, rifting, glaciation, æolian, and fluvial processes. [ Viewgraph 9 ]

Craters

There are four stand-out great craters on Mars: Hellas Planitia, Argyre Planitia, Isidis Planitia, and Utopia Planitia. These range in size from ~1,500 km in diameter up to 3,300 km. They date to the bombardment of the Noachian Epoch but to a time after the planetary magnetic field had collapsed and, therefore, could not "stamp" the (re)solidifying basalts with iron alignment (Frey et al. 2007).

Volcanic Provinces

Volcanism on Mars, while present in many places, is overwhelmingly concentrated in two gigantic volcanic rises: Tharsis and Elysium. Confirmation that Earth-observed albedo features were, in fact, volcanoes and that they were so stupendously huge was one of the actual shocks provided by the Mariner 9 orbiter in 1972 as it waited out a planetary dust storm and then began to image Tharsis' Olympus Mons emerging from the settling dust.

[ Viewgraph 10 ] Elysium, centered at 25° north lat. and 146° east long., is massive: some 2,000 km in diameter, rising 5 km above the northern plains, and its tallest volcano, Elysium Mons, at nearly 13 km in elevation, would dwarf anything on Earth (Science Visualization Studio 2001). Compared with Tharsis, however, it becomes relatively puny.

[ Viewgraph 11 ] Tharsis, centered along the equator around 245° E. long., spans roughly 8,000 km and rises roughly 7 km. On its northwest edge lies Olympus Mons, towering some 27 km above the geoid, and three other gigantic volcanoes rise in a southwest to northeast line: Arsia Mons, Pavonis Mons, and Ascræus Mons (MOC 1999; Short no date). There is energetic debate about the internal forces concentrating magmatic activity in just two major areas for so long, the processes enabling the lithosphere to support such massive edifices, the effects that these volcanic rises have had on Mars, and whether any of their volcanoes remain active today (Grott 2008).

Rift Zone

[ Viewgraph 12 ] Valles Marineris proved yet another shock coming from Mariner 9, for which the feature was eventually named: a system of canyons stretching over 4,000 km just south of the equator (USGS no date). Its individual troughs are usually about 50 to 200 km wide, though they come together in the center of the formation to form a feature nearly 700 km wide. In places, Valles Marineris reaches close to 6 km below the geoid, creating local relief of as much as 7-10 km (USGS 2008).

Mechanisms proposed for the origins and development of Valles Marineris are quite varied. The consensus of the community has largely drifted away from incipient plate tectonics explanations, though these have gained a cautious second inspection in light of the possibility that the "missing" transform faults (Solomon 1978; Golombek 1985) defining a true plate tectonic rift zone may exist (Connerney et al. 2005). Work over the last couple of decades emphasizes extensional rifting associated with the buildup of Tharsis (Schultz 1991), landsliding (Montgomery et al. 2009), subsidence associated with subterranean removal of fluids (Tanaka and Golombek 1989; Rodríguez et al. 2006), and debate about the nature and geomorphic effects of such candidate fluids as magma (Schonfeld 1979), water, brine, carbon dioxide (Hoffman 2000), or magma intrusions as they affect subterranean fluids or ices (McKenzie and Nimmo 1999).

The Polar Ice Caps

[ Viewgraph 13 ] The smallest but arguably the most conspicuous and variable of the second order relief landforms are the two polar ice caps. These became visible from Earth with the earliest telescopes.

In terms of composition, the North Polar Cap is mainly composed of water ice, which dominates the residual ice that persists through all seasons. Carbon dioxide will sublime as frost out of the atmosphere whenever temperatures drop below 150 K, so winter in the northern hemisphere results in the development of a carbon dioxide veneer on top of the water ice core of the North Polar Cap (Zuber et al. 1998: 2055). During summer, the carbon dioxide sublimates into vapor and then the water ice exposed below it does the same, shrinking the ice cap noticeably.

The residual South Polar Cap is considerably smaller than the North Polar Cap at roughly 350 km diameter, versus ~1,000 km, respectively (Boyce 2002: 187). As with the North Polar Cap, the South Polar Cap expands with winter, to roughly -45° lat., and shrinks in summer. The South Polar Cap has until recently been believed to be completely different in composition from the North Polar Cap because of its elevation 6 km higher than its northern counterpart (Albee 2000). Carbon dioxide ice mantles the South Polar Ice cap year round, but it has recently been established that water ice lies below the carbon dioxide ice surface at 8 m in depth (Byrne and Ingersoll 2003; Kieffer et al. 2006).

Syrtis Major

[ Viewgraph 14 ] Wind is the process responsible for exposing the large, roughly triangular area of dark basalt in Syrtis Major. This feature, the "blue scorpion," was the first martian landform recorded in a sketch map drawn by Christiaan Huygens in 1659 (and, debatably, as early as 1636 by Francisco Fontana). The shape of the Syrtis Major dark areas change over time, which was the basis for early speculations about Mars having vegetation that responded to seasonality. Wind moves light colored dust over the borders of the Syrtis region but keeps its core essentially swept clean, exposing the dark basalts (Mustard et al. 1993).

The Chryse Trough

A large arc of locally depressed topography loosely rings the Tharsis Rise, most likely the result of the loading of lava on the lithosphere below the Tharsis volcanoes. This effect is particularly obvious to the east of Tharsis and to the west of the annular ejecta ringing Hellas Planitia, punctuated by the great Argyre Planitia basin. Timothy Parker in 1985 suggested that this depression east of Tharsis, dubbed the Chryse Trough, might have housed an actual channel for catastrophic flooding, comprising several tributary channels flowing from near the South Polar Ice Cap into Argyre. From a presumed lake in Argyre, the flow would move through Uzboi Vallis into a chain of smaller craters linked by channels that flowed into Margaritifer Terra east of Valles Marineris. From there, drainage would move into Chryse Planitia and the presumed northern lowlands ocean (Parker 1985, cited in Parker et al. 2003). The Mars Orbiter Laser Altimeter (MOLA), Mars Orbital Camera (MOC) instruments, and the Thermal Emission Spectrometer (THEMIS) have provided high resolution topographical information that does seem to confirm the existence of an 8,000 km drainage system (Parker et al. 2003), though not without criticism in one or another segment of the proposed system (e.g., Hiesinger et al. 2007). If, in fact, this system did move water or other fluids from the area around the South Polar Cap to Chryse Planitia, even as a sporadic and perhaps not always continuously connected drainage, at some 8,000 km in length, the Chryse Trough would constitute the longest fluvial network in the solar system.

The Third Order of Relief

The third order marks broad regions (terræ, plana, and planitiæ) [ Viewgraph 16 ] differentiated largely by crater density and, thus, relative age. I used this level of the hierarchy to present the tripartite epochs dividing Mars' geologic history. The Noachian Epoch dates from planetary formation to somewhere between 3.9 and 3.5 billion years ago (Hartmann 2004; Golombek 2006). This was a time of repeated and often substantial impacts, sometimes referred to as the "age of bombardment." This was also the time when the planet had a magnetic field and a denser atmosphere and, thus, the time when liquid water was most likely to have formed fluvial networks and pooled on the surface of the planet (Hartmann 2003: Part II). The early part of the Noachian (to about 4 billion years ago) thus generated phyllosilicate clay geochemistry (Bibring et al. 2006). Some of the buildup of Tharsis goes back to this epoch. [ Viewgraph 17 ] Noachian landscapes include Noachis Terra, Arabia Terra, Sirenum Terra, and Terra Cimmeria.

[ Viewgraph 18 ] The Hesperian Epoch runs from the end of the "age of bombardment" 3.9 to 3.5 billion years ago in some usages to 3.5 to 1.8 billion years ago in others (Hartmann 2004), a time of extensive volcanic flows, dominance by sulfate surface chemistry (Bibring et al. 2006), loss of the bulk of the planet's atmosphere and surface water, desiccation and freezing, and possibly some sudden and truly massive outflows of subterranean water or other fluids. [ Viewgraph 19 ] It includes such regions as Hesperia Planum, Terra Tyrrhena, Aonia Terra, Margaritifer Terra, and Gusev Crater.

[ Viewgraph 20 ] The Amazonian Epoch is "modern" Mars, dating back 1.8 billion years or as much as 3.5 billion years (Hartmann 2004). This is a time in which the dominant geomorphic processes seem to be æolian, though there are tantalizing hints of volcanic eruptions and outflows, mass wasting, glacial, and fluvial activity perhaps driven by groundwater eruptions or seeps. Surface alteration transitions to dominance by anhydrous ferric oxides (Bibring et al. 2006). [ Viewgraph 21 ] These would include the landscapes of Amazonis Planitia, Chryse Planitia, Acidalia Planitia, and Utopia Planitia.

The Fourth Order of Relief

The fourth order is made up of smaller features, "landscape" scale features. [ Viewgraph 22 ] Nearly ubiquitous at this scale, if spatially varying in density, are the impact features: craters at every size, level of modification, and degree of superposition. There are pristine, bowl-shaped craters with sharp rims, such as Zunil Crater [ Mars Odyssey THEMIS ] and craters so new they still have debris blown around them, such as this one in Arabia Terra that formed between April 2001 and December 2003 [ MGS MOC ]). There are craters perched on top of pedestals or ramparts of their own consolidated ejecta blankets, which then become elevated above the landscape as erosion removes materials around them, as seen on the frozen plains of Acidalia Planitia [MGS MOC ]. There are craters with erosion-softened edges and floors nearly flat with lava, sedimentary, glacial, snow, or æolian fill, such as this glaciated pair of craters on the east side of Hellas Planitia [ HRSC ], the snow-covered Udzha Crater near the North Polar Cap [ Mars Odyssey THEMIS ], Victoria Crater (made famous by the Opportunity Rover's activities) with its dunes [ MRO HiRISE ], and the seemingly sedimentary fill in Schiaparelli Crater since etched out by the wind [ MGS MOC ]. Many craters are completely buried by newer surfaces, as revealed by these subsurface radar soundings by the Mars Express MARSIS sensor.

[ Viewgraph 23 ] Some of the fourth order landscape features are tectonic in origin, such as the huge volcanic montes (e.g., Olympus Mons [ Viking ]) and smaller tholi (e.g., Uranius Tholi, Tharsis [Viking ]), landscapes criss- crossed by what look like lava tubes (e.g.,Pavonis Mons' south slope [ HRSC ]), and lava flows themselves (e.g., in this partially buried crater in the area between Dadalia Planum and Terra Sirenum [MGS MOC ]. Faulting is suggested by the fossa or graben-like structures that may indicate tensional stress and failure or, alternatively, the loss of surface support as subterranean lavas or other fluids are extracted (e.g., Claritas Fossæ along the Thaumasia Montes boundary with Solis Planum [ HRSC ] and Cerberus Fossæ southeast of Elysium Planitia [ MRO HiRISE ]). There is, however, very little if, arguably, any folding and faulting like those associated with Earth plate tectonics. A possible exception is the Thaumasia Montes, which look the part (Anguita et al.) and may represent an area of incipient and stalled subduction [ map from Technische Universität Berlin using MOLA ].

[ Viewgraph 24 ] The fourth order landscapes include such fluvial features as deltas (e.g., Eberswalde Delta in Eberswalde Crater in the northeast of Holden Crater, in the second order fluvial system [MGS MOC]), valles that look like fluvial networks (e.g., Ma'adim Vallis, seeming to debouche into Gusev Crater and a dendritic network on the Southern Highlands [ Viking ]), and gullies in the walls of south polar pits that resemble sapping outflow features on Devon Island in the Canadian Arctic [MGS MOC]).

[ Viewgraph 25 ] Massive fluvial outflow events , or jökulhlaups, are evident in several places on Mars, particularly to the east and west of the Tharsis Rise. These entail sudden flows of volumes of water one or two magnitudes beyond the Missoula Flood events that created the channelled scablands of eastern Washington at the Pleistocene-Holocene boundary. Valles Marineris may have experienced these (though the "mouth" is at higher elevation than the middle of the system), as did Kasei Vallis [ MOLA map ], Athabasca Vallis [ MRO HiRISE ], Ganges Chasma [ Mars Odyssey THEMIS ], Nanedi Vallis, Ares Vallis, Uzboi Vallis, and dozens of others.

[ Viewgraph 26 ] Fourth order landscapes include features evocative of coastal processes, which were the basis for Tim Parker's delineation of two marine stands in the Northern Lowlands. At the fourth order level, these include step-like slopes on the north side of Alba Patera that suggest wave cut benches and wave built terraces. Also evoking a coastal impression are all the fluvial and outflow channels that seem to discharge into the Northern Lowlands, especially in and around Chryse Planitia.

[ Viewgraph 27 ] Other fourth order features reflect mass wasting on a grand scale, including landslides (e.g., Noctis Labyrinthus [ Mars Odyssey THEMIS ] and Aram Chaos [Mars Express HRSC]) and avalanches (e.g., the North Polar ice cap [MRO HiRISE]). The landslides may be responsible for the formation of chaos terrain, which looks like large blocks tumbled down from the extraction of subterranean support, perhaps the sudden liberation of warm water when ice faces fail, volcanic heating of permafrost or groundwater or even frozen carbon dioxide. Chaos terrains are often implicated as the source of the jökulhlaup outflows.

[ Viewgraph 28 ] Another type of fourth order landscape is dominated by æolian processes. Mars is, for the most part, an amazingly dusty place, with the wind homogenizing the chemical composition of the dust by distributing it nearly everywhere, sometimes in world-engulfing duststorms. Wind appears to have dominated the surface of Mars to the extent that fluvial processes dominate on Earth. There are landscapes of wind erosion, such as the second order feature Syrtis Major already mentioned. At the fourth order level, there are large expanses of yardangs, such as in Æolis Mensæ [ MGS MOC ] or an area south of Olympus Mons in the transition to Amazonis Planitia [ HRSC ]. In many areas, sandblasting appears to be sequentially exposing and etching out layers of volcanic or fluvial origin, creating complex "contour map" terrain, such as here in far southwestern Candor Chasma. Sand deposition is also abundantly in evidence, with landscapes dominated by dunes, such as these barchans in Nili Patera [ MGS MOC ], in Endurance Crater [ MER Opp PanCam ], and this field trapped in the bottom of a crater in northwest Argyre Planitia [ HRSC ].

[ Viewgraph 29 ] Fourth order landscapes also include glacial and periglacial phenomena at a smaller scale than the second order ice caps. Small glaciers have been found in craters, such as this one in Vastitas Borealis [ HRSC ], and the ice caps themselves present exposures of layers along their edges, sometimes showing glacio-fluvial features, such as the channels seen in this landscape in Planum Boreum near Olympia Undae under and adjacent to the residual ice cap [MRO HiRISE ]. There are periglacial features, too, such as polygon-patterned land, as here in Acidalia Planitia [ MGS MOC ] and at the tiny scale of the Phoenix landing site [ Phoenix ].

[ Viewgraph 30 ] There are also glacial geyser features that particularly affect the South Polar Cap. As spring approaches, some of the carbon dioxide that forms the upper 8 meters of the cap begins to sublimate explosively, blasting through layers of carbon dioxide ice and dust layers, taking some of the dust with it. This creates a distinctive pitted landscape, often accompanied by spider-like patterns of dust deposits around the pits.

The Fifth Order of Relief

[ Viewgraph 31 ] The fifth order of relief describes the small features and contrasts typically seen at the scale of the rovers' and landers' activities, but sometimes also from orbit-borne high-resolution sensors: individual boulders and rocks, frost deposits on them and on the ground, dust devil tracks, light-toned new gully flows and deposits, sand and dust streaks, the small-scale "blueberries," and "RAT holes" from the rovers' Rock Abrasion Tools.

Conclusion

To conclude, I believe Mars and other extraterrestrial objects are within the purview of geography, geographers can contribute meaningfully to Mars research, and the study of Mars can benefit geography. Involvement with Mars research could increase activity in æolian process research, which is relatively underdeveloped in physical geography today compared with fluvial or glacial geomorphology. Mars research and geographers can definitely benefit from the field-based analogues of Earth landscapes that can be used to generate hypotheses for testing with Mars data (Bishop 2009). Mars can be used to deepen Earth understanding among our students through the compare and contrast it affords. It is in the spirit of sparking this conversation that I offer this application of a canonical geographical idea: the Martian orders of relief.

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