Introduction
In 2001, due to my earlier work in how the then-new Internet was being used to protest and interfere with the Cassini-Huygens mission to Saturn, NASA asked me if I'd be interested in following a similar controversy involving the Mars Sample Return Lander Mission, then scheduled for a 2008 launch. I agreed and started "boning up" on Mars and then ... the mission was delayed several times and then canceled (and only recently reïncarnated as a proposed mission for launch in 2027 and return in 2033. Rather than lose all this hard-earned familiarity with Mars, I decided in 2007 to offer it to our students. Thus was launched the world's only Geography of Mars class!
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One of the key student learning objectives was development of a mental map of Mars, so that Mars would be a real place for students.
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To accomplish this, I dusted off an old scheme often presented in introductory geography textbooks: the "orders of relief".
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This is based on Nevin Fenneman's 1916 article dividing the physical geography of the United States into divisions, geomorphic provinces, and sections, each level a finer scale than the previous one and nested into it.
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Various textbooks use different extensions of Fenneman's idea. The first, global level is the distinction between continents and oceans, while the second refers to major regions sharing similar topographic relief and underlying geology, such as the Pacific Mountain System or the Coastal Plain of the Atlantic and Gulf coasts. The third level is that of Fenneman's geomorphic province, with the Sierra Nevada, for example, nesting within the Pacific Mountain Systems division. The fourth level takes it down further to such features as the Morongo Basin section, say, nested within the Mojave Desert geomorphic province, itself nested within the Intermontane Plateaux division. Some authors may take it down even further, with such local features as the Parlett Mountains nested in the Morongo Basin!
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My variant on this idea for Mars has two dimensions. The first is spatial, just like the Earth schemes, with the fifth order nesting in the fourth and the fourth order nesting within the third. The third, however, nests within the first order. I use the second order for another dimension: conspicuousness. These are prominent features on Mars that create a vivid visual framework for organizing one's mental map of Mars but which may themselves cross the first order features or which may be confined within a large third order region. So, spatial nesting AND conspicuousness.
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I'll spend the rest of this talk on the first, second, and third orders of relief on Mars.
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Mars' First Order of Relief
There are two huge features of Mars that I've put at the first order of relief.
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The first of these is the great crustal dichotomy separating the Southern Highlands, with their ancient battered surfaces and strikingly high elevations (and, mostly, thick crust) from the Northern Lowlands.
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The Northern Lowlands refer to the northern third of the planet with its strikingly low elevations, thin crust, and relative paucity of cratering. This is a reworked and resurfaced younger terrain.
The origins of this approximately 6 kilometer (nearly 4 miles) contrast in elevations has been hotly debated but the most common, if not universal, view today is that it is exogenic, that is, the result of a terrific impact very early in Mars' existence as a planet. If this is true, the Northern Lowlands may be the largest impact crater in the solar system! The Lowland basin is very ancient and there are ghost craters buried below that smooth surface that resemble the Southern Highlands in number and diversity of sizes. The ancient basin then was resurfaced by lava flows, massive water outflows, and the presence of oceans at various points in Mars' deep past, covering over the craters and producing that deceptively young surface.
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The other exceptionally massive feature is the Tharsis Rise. This is a vast platform of lava about 10 km thick extending some 5,000 by 8,000 km (more than 6 miles thick and 3,000 by 5,000 miles across). It sports five unimaginably large shield volcanoes, the three Tharsis Montes across its spine (Arsia, Pavonis, and Ascræus), the extremely sprawling Alba Mons to the north, and the extremely tall Olympus Mons to the northwest). There are another seven large volcanoes up there, too! This first order feature is endogenous, driven by a massive and astoundingly persistent mantle plume, which has shifted locations over time, starting at Claritas Fossæ, moving to Alba Mons, and presently near Pavonis Mons. The stress of its initiation and maintenance shows up in huge graben (extensional faulting), the rifting in Valles Marineris, and the seeming compressional wrinkling in the Thaumasia block (see second order, below). It is amazing that the crust of Mars can even support something this heavy this long. Tharsis began building about 4.1 billion years ago and, on a planet without plate tectonics, it's stayed pretty much in the same general area since. It is so massive that it may have caused the entire single plate of Mars crust to shift to keep Tharsis balanced on the equator. Its weight is so great that this feature acts much like our own Moon does to stabilize the rotational axis of Mars.
Mars' Second Order of Relief
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The second order of relief is made up of other pretty conspicuous features. One is the persistent dark wedge of the
Syrtis Major "blue scorpion." This was the first feature observed and recorded from Earth, by Christiaan Huygens back in 1659, who used its nightly movement to create a good estimate of Mars' day length.
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This dark object is now understood to be the result of winds in the martian general circulation consistently sweeping the ubiquitous martian dust off the dark basaltic surface of Syrtis Major Planum and nearby areas.
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The second feature observed from Earth was the ice covering the north and south poles. These ice caps were recorded by Jean Dominique Cassini in 1672 and his nephew, Giacomo Maraldi, used their alternating growth and shrinkage to infer Mars had seasons.
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The ice caps lose most (south) or all (north) of their carbon dioxide ice in summer, creating a drastic growth in atmospheric pressure then. The carbon dioxide eventually freezes onto the opposite pole, reducing air pressure and creating a large seasonal hood of CO2 frost that can extend 30 to 40 degrees away from the poles!
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A third conspicuous feature is the result of four very large impacts somewhere between 4.1 and 3.7 billion years ago, when all kinds of space debris was flung into the inner solar system by gravitational interactions among the four large gas and ice giant planets. Utopia Planitia may be the oldest, hitting the newly formed Northern Lowlands and then getting resurfaced. Hellas was probably next, followed by Argyre, and Isidis Planitia formed after Hellas Planitia, as it cuts into the ejecta debris from that impact.
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A fourth conspicuous feature is another large volcanic platform: Elysium Rise. This thing is huge, some 6 km or 4 miles thick and about 2,000 km or 1,250 miles in diameter, but it suffers by comparison with Tharsis! It contains three large volcanic edifices: Elysium Mons, a large shield, and two steeper-sided tholi, Albor Tholus to the southeast and Hecates Tholus to the northeast.
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A fifth highly conspicuous feature is Valles Marineris, a gigantic rift system associated with the terrific stress of Tharsis' uplift. It stretches over 1/6th of the planet's circumference, 3,000 km or over 2,000 mi. It starts in Noctis Labyrinthus, a chaotic terrain on the west end, through parallel great chasmata to a central area of massive landslides and terminating in forked chasmata on its far east end. This feature was discovered by Mariner 9, which got to Mars in late 1971, just in time for a massive planet-covering dust storm that made observation pretty much impossible. Mariner waited out the storm for a few months before beginning imaging. It then watched as the dust settled down and exposed the gigantic volcanoes of Tharsis and after that watched the air in Valles Marineris slowly empty of dust. The feature is named for this first successful orbiter!
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The sixth big feature is the Chryse Trough. There's an area of lower elevation surrounding Tharsis, kind of like how your mattress has an area of lower elevation surrounding your own sleeping self! The eastern part of this ring is occupied by a series of interconnected fluvial channels, punctuated here and there by craters of various sizes, including Argyre Planitia. CSULB alumnus Timothy Parker argued that these, in fact, are some kind of fluvial network, transporting water from under the South Polar Ice Cap through four channels into southern Argyre Planitia, which he thought might have been occupied by a large lake or small sea. The northern rim of the massive crater is breached by another channel, Uzboi Vallis, that then winds northward, collecting waters from a few tributaries, and pours into a series of craters and basins, presumably lakes. One of these is Eberswalde Crater, which features a classic river delta. The flow eventually got into Margaritifer Terra and linked up with a very long outwash channel called Ares Valles. The waters eventually ended up in the Northern Lowlands.
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Speaking of outflows, these are signs of just enormous and sudden outpourings of water. On Earth, they are sometimes referred to as jökulhlaup. These sudden and massive outflows on Earth can happen from the failure of an ice dam, as at the end of the Pleistocene, creating landscapes, such as the Channeled Scablands of the Pacific Northwest. On Mars, they may reflect subterranean contact between magma moving underground and permafrost. The ice sublimates suddenly and explosively, the terrain above it collapses chaotically, and an incomprehensibly huge outflow results. The most spectacular such outflow on Mars is Kasei Valles, fed by chaos terrains in Echus Chasma to the northwest of Valles Marineris and Lunæ Planum along the edges of the channels. It is about 3,000 km long and 300 km wide in places and descends some 3 km from Tharsis into the Northern Lowlands!
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The last second-order conspicuous feature is the Thaumasia block, which forms the southeastern flank of Tharsis Rise. This distinctive lozenge-shaped slab contains much of the Valles Marineris rift zone along the north, and there is another area expressing extensional stress on the west side: Claritas Fossæ, a landscape of aligned graben. Both of these radiate from the Syria Planum region on the northwest, adjacent to Pavonis Mons, which seems to be the general area that the Tharsis uplift is centered on now. Across Solis Planum, there are all kinds of wrinkle ridges crumpling up the lava flows there and these compressional strain features are circumferential to that Syria Planum uplift zone. Both patterns, radial graben and circumferential wrinkle ridges, express the stresses associated with the uplift of Tharsis. The mountain ranges bounding the south and east of the Thaumasia block are the only place on Mars that has folded and faulted mountains somewhat akin to our own Coastal Ranges, as opposed to the volcanic edifices common on Mars and some possible basin and range structures spotted south and west of Tharsis. The combination of rifting in Valles Marineris and folding in the Thaumasia Highlands led to some early speculation about incipient plate tectonics on Mars, with motion from north to south across Thaumasia. Another argument is that Valles Marineris is more like a left-lateral fault zone (our San Andreas is a right-lateral fault). More recently, an argument was put forth that the Thaumasia block may be an enormous landslide, with detachment zones in Claritas Fossæ and Valles Marineris, which would account for putative left-lateral motion in Valles Marineris and frame the Thaumasia Highlands as, basically, the toe for the slide.
Mars' Third Order of Relief
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The third order of relief is up next, the order with units nesting inside the first order features. These are large regions differentiated from one another by the density and size distribution of craters, which have attached to them names dating from nineteenth century observations of albedo differences. Crater-counting is at present the only way we have for constraining the ages of surfaces on Mars. The greater the number of craters and the more diverse their sizes, the longer a surface has been exposed to bombs from outer space without getting resurfaced by any geomorphic process since then. The large impactors were common in the first 100 million years of the baby solar system, died down a bit, and then surged again around 4.1 to 3.7 billion years ago when the gravitational interactions among the four huge outer solar system planets caused all kinds of debris to be shot inward, pummeling all the planets of the inner solar system.
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Mars' geological history has been divided into three eons, which on Mars, rather inconsistently, are usually called periods. These are the Noachian, which dates from the formation of the planet 4.6 billion years ago to about 3.7 or 3.8 billion years ago, which marks the easing up of the Late Heavy Bombardment. These regions are very heavily hammered and there are some quite large craters in them. Some regions are "saturated" -- any new crater would necessarily obliterate an older one, meaning we can't assign ages to surfaces older than about 4.1 billion years. Following this is the Hesperian, from 3.8 or 3.7 billion years ago up to 3.0 or 2.9 billion years ago, when the flow of impactors eased, so that there are fewer craters and very few larger ones. These regions have had substantial resurfacing due to volcanism, massive outflow floods, and wind deposits. The last 3 billion years or so are considered "modern" Mars, called the Amazonian. These regions have been resurfaced extensively, obscuring the presence of craters. Very few craters have impacted the new surfaces and these are generally pretty small.
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I've created a timeline of the three periods illustrated by a typical region belonging to each time.
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Noachian regions, besides being badly beat up, are actually the most common surfaces on Mars, making up about 45% of the planet, meaning a substantial part of Mars is actually a pretty pristine sample of conditions in the earliest solar system. Back then, Mars had a planetary magnetic field, which protected its atmosphere from the solar wind. The atmosphere, then, was much denser and that allowed water to exist on the surface in a liquid condition. That water was also pretty neutral or mildly alkaline in pH, allowing the formation of clay minerals. Such water also dissolves halogens, such as chlorine and bromine, and these then concentrate wherever water evaporates. The water was abundant enough and there was enough of a hydrological cycle to allow the formation of small and even larger valley networks, which are seen in many Noachian regions. There are signs of ocean, sea, and lake development. These are conditions that, on Earth, facilitated the evolution of life, so it is possible that life developed on Noachian Mars, too. But bad news was gathering. Volcanic activity was present in much of the Noachian but Tharsis began developing about 4.1 billion years ago and the scale of that volcanic activity was enough to induce changes not friendly to life. Another hostile development was the collapse of the planetary magnetic field, which had protected the atmosphere.
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Noachian regions include Noachis Terra, Aonia Terra, Terra Sirenum, Terra Cimmeria, Promethei Terra, Tyrrena Terra, Terra Sabæa, Arabia Terra, Margaritifer Terra, Xanthe Terra, and Tempe Terra.
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Hesperian regions show up as conspicuously less hammered landscapes and they are smaller than most Noachian regions. Though the Hesperian lasted as long as the Noachian, these landscapes only cover about 30% of Mars. The Hesperian experienced the steady loss of the atmosphere under the assault of the solar wind. Surface water more readily evaporated or solidified as pressure dropped. Oceans, seas, and lakes evaporated away or drained into the soil and froze. Water vapor was photodissociated in the atmosphere, separating hydrogen from oxygen. The freed hydrogen just richocheted into space, too light to be held by Mars' gravity. The freed oxygen, being heavier, was retained and, being super reactive, started oxidizing iron-rich rocks and minerals: Mars began to rust. Volcanoes remained highly active and the various sulfur species they emit pretty much swamped the dwindling atmosphere. Any remaining surface water and the groundwater became extremely acidic. Hydrochemistry began to be dominated by sulfates. Any ancient life very plausibly went extinct in these hostile conditions. Magma moving around underground encountered permafrost and eruptions became more explosive than effusive. This also sometimes created vast amounts of liquid, which burst out any available opening, resulting in massive outflows, like Kasei Valles, and the formation of chaos terrain, badly broken up and collapsed surfaces.
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Hesperian regions include Hesperia Planum, Malea Planum, Sisyphi Planum, Argentea Planum, the Thaumasia block's subsidiary plana, Lunæ Planum, Chryse Planitia, and Vastitas Borealis.
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Though the Amazonian period lasted the longest, 3 billion years, it covers the least surface on Mars, about 25%. The atmosphere dwindled to nearly nothing, less than 1% the density of Earth's. The planet dried out, other than ice caps and permafrost. Volcanism continued but at a much reduced and sporadic rate, even as recently as a few million years ago. Oddly enough, despite the thin atmosphere, wind became the dominant geomorphic agent. A low force wind operating for 3 billion years can create all sorts of ventifacts: fluted and chipped rock, yardangs, and dunes. Chemistry on Mars became dominated by anhydrous ferric oxides, basically rust. The rusting of Mars creates an enormous amount of fine dust, which is easily carried by that weak force wind (and is going to be a huge headache for any humans who get to Mars).
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Amazonian regions include Amazonis Planitia, Elysium Planitia, the surface materials overlying Utopia Planitia, Acidalia Planitia, the vast lavas surrounding the Tharsis Montes, Alba Mons, Olympus Mons, and areas among them, east of them, and Dædalia Planum southwest of them. Arcadia Planitia is largely Amazonian, as is Planum Boreum under and around the North Polar ice cap and Planum Australum, too.
Au Revoir!
I hope you've enjoyed this guided tour of Mars organized by the orders of relief, which may make the geography of Mars easy to remember. Below are some more resources, including two editions of my Geography of Mars class, complete with lectures, viewgraphs, and labs!
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For More Information
- Geography of Mars class: GEOG 441/541 at CSULB
- interactive martian orders of relief
- free NASA s/w for manipulating MOLA elevation data
- Google Mars
- Nevin M. Fenneman's 1916 article on the physiographic divisions of the United States
Acknowledgments
I would like to thank Prof. Dean Arvidson and Dr. Jim Melton for inviting me to speak to the Southern California chapter of The Mars Society and the Mars Ambassadors Program at the Desert Star Home.