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Introduction

I didn't set out to do planetary science. Hazards research dominated my work for a couple of decades. In the late 1990s, I did a hazards project on how the Internet was being used to organize opposition to the Cassini-Huygens mission to Saturn because of the plutonium dioxide power sources and heating units on the spacecraft (Rodrigue 2001). In 2001, NASA invited me to present this work at a five-center teleconference. Discussion after the paper turned to how NASA could better manage risk communication for a proposed mission they expected to be controversial: the Mars Sample Return Lander.

I was asked to follow the already emerging opposition to the mission, which was organizing online over the use of radioisotope thermal generators and over the prospect of possibly bringing martian microörganisms to Earth and triggering an "Andromeda strain" pandemic. I agreed to do so and began to "bone up" on Mars to understand the mission before the then-planned 2008 launch date. As I got into the research on Mars, the mission itself was repeatedly delayed and then cancelled due to Bush's vision for human missions to the Moon and Mars, which affected the budget for robotic missions, such as the Mars Sample Return Mission. After a real struggle to absorb Mars research, I found myself without a project! Rather than forget it all, I developed a Geography of Mars class for CSULB.

Since 1997, there have been ten successful missions to Mars out of fourteen sent. The result is huge amounts of data and images: There is a lot to do, and I hope to encourage geography students to consider helping out. Indeed, there is a long tradition of geographers researching other planets (Pike 1974; Rodrigue 2013), including Dr. Judith Tyner in the audience, who wrote a master's thesis (1963) and article on lunar cartography (1969)!

A: First "slide tray"

Acquiring a Mental Map of Mars

It is important to develop a mental map of Mars to place various people's work in its regional context. That is a bit challenging, due to unfamiliar nomenclature for landscape features, rigorous systems for place naming, and different types of geographic grid. Martian feature names are decided by the International Astronomical Union (IAU), and they have a particular system for assigning names. Names are generally linked to albedo features, areas of light and dark, that were first identified in the 19th century maps of Schiaparelli (1877-1886) and Antoniadi, who extended Sciaparelli's toponymy in the early twentieth century (1900- 1930).

There are certain standards for name assignment that vary with the scale of the feature being proposed for a name. Large craters are named for dead scientists and authors who made some contribution to Mars study (e.g., Antoniadi Crater). Small craters are named for towns and villages on Earth, such as Bonneville Crater examined by the Spirit Rover. Large "valles" are named for Mars in one of the languages of Earth, so Ma'adim Vallis comes from the Hebrew name for Mars and Kasei Valles are named for the Japanese name for Mars. Small valles are named for Earth rivers in classical or contemporary languages, such as Allegheny Vallis.

There is a whole new vocabulary to master. A valley is not called a valley, but a "vallis"; a mesa is called a "mensa"; a mountain is called a "mons." An area or region is called a "terra," while a plateau is a "planum" and a plain is a "planitia." This vocabulary is designed to forestall glib generalizations about the kinds of processes that can create one or another feature. A valley implies erosion, transportation, and deposition by running water, which could be a misleading parallel on another planet. Maybe there was running water, or maybe another fluid entirely was involved, such as lava or brines.

One resource for learning the regional organization of Mars is the IAU/USGS/NASA 1:5,000,000 quadrangles. There are 30 of them, each with a number, such as MC-14 (Mars Chart 14) and a name associated with an albedo feature in the quadrangle, such as Amenthes Quadrangle. That quadrangle is named for Amenthes Planum, but it's not very evocative if you are unfamiliar with Amenthes Planum.

Google Mars débuted in 2009. You can type in a feature name and it will put a mark on the map locating it. Unfortunately, you can't point at a spot on Mars and find out what it's called or what region it falls in. Google Earth Mars is an interactive mapping program and, as with Google Mars, it lets you do a search by feature name and it'll take you there. But you still can't point to a feature or region and ask for its name.

A very helpful resource is the Gazetteer of Planetary Nomenclature by the IAU and the USGS. It allows you to pick from a list of feature types, which brings up an alphabetical list you can scroll through. So, for example, I picked "Chaos/chaoses" from the feature list and then, fron the alphabetical list, I could pick "Aram Chaos" and a map comes up showing the outlines of that feature on a background map that you can zoom into and out of and pan around in. You can also click on the plus sign to access a menu that lets you change the background imagery for the map to help you visualize the location better.

Orders of Relief Taxonomy

To create a regional framework for my class, which débuted in Spring 2007, I decided to create a nested system, based on the "orders of relief" scheme often seen in introductory physical geography and world regional geography textbooks. Each textbook has its own scheme, with anywhere from three to seven orders mentioned. Because of these disparities, I became interested in the intellectual history of the idea and found that it traced back to a 1916 article by Nevin Fenneman in the Annals of the Association of American Geographers, entitled, "The physiographic subdivision of the United States." The scheme in this article had three nested subdivisions:

• Physiographic divisions (e.g., the Atlantic Plain or the Pacific Mountain System)
• Geomorphic provinces (e.g., the Pacific Border Province within the Pacific Mountain System
• Sections (e.g., the California Coast Ranges within the Pacific Mountain System)

Textbook schemes have expanded upward and downward. Commonly the first order is the division of Earth into oceanic surfaces and continental ones. The second order is often comparable to Fenneman's physiographic province. The third order may comprise or be comparable to Fenneman's geomorphic province. If present, the fourth order may be roughly comparable to Fenneman's sections. There may be a fifth and even a sixth and seventh order.

The scheme I developed for Mars is a kind of two dimensional taxonomy. One dimension is spatial scale and the other is planetary conspicuousness. The result is a five orders of relief system. The spatial dimension is nested: The fifth order nests within the fourth order, which nests within the third order, which nests within, not the second order, but the first order. The second order is a series of large and conspicuous planetary features that, together with the first order, can be used as a framework within which to situate and refer to lower order features. So, for instance, Noachis Terra could be described as in the Southern Highlands (first order) between Argyre Planitia and Hellas Planitia (gigantic craters described as second order objects). Meridiani Planum (where the Opportunity rover still roams) could be described as lying within Arabia Terra (third order) east of Valles Marineris (second order) to the northeast of Argyre Planitia and northwest of Hellas Planitia (second order).

First Order of Relief

The first order of relief comprises huge divisions of the martian surface, covering at least a quarter of the planet. There are two such divisions: the crustal dichotomy and the Tharsis Rise.

The crustal dichotomy is a conspicuous division of the planet into its badly cratered, mostly high elevation southern two thirds and the much smoother, generally low elevation northern third. The crust under the Southern Highlands averages about 58 km in thickness, while the crust under the Northern Lowlands is thinner, averaging about 32 km.

The Southern Highlands are visually dominated by a profusion of craters at a wide range of sizes. Some landscapes are nearly saturated with craters, such that any new crater would necessarily obliterate all traces of another, older crater. Depending on the spatial resolution of imagery, craters may be as tiny as 10 m across ... or as huge as 2,300 km across (Hellas Planitia). In some places, craters take on a very distinctive martian appearance, the rampart crater, with a conspicuous raised blanket of ejecta surrounding it, sort of a "wet splat" effect. These distinctive craters are found in high latitude areas where there is evidence of subsurface ice.

Another common feature in many areas of the Southern Highlands is something that looks like a small channel system. Some of these have a densely branched, dendritic pattern like many precipitation-fed stream networks in humid regions on Earth. Others show single main trunks with very few, quite short tributaries that begin abruptly in a kind of alcove. These are similar in appearance to groundwater-fed stream systems seen on Earth in arid regions. Other potential signs of surface or groundwater include layering on the sides of cliffs and crater rims.

The Northern Lowlands convey an entirely different appearance. Most of the area north of the crustal dichotomy is low in elevation and much smoother in appearance, with very few craters and most of them small. These are clearly much younger surfaces, which have had less time to accumulate crater damage. Surfaces here are smooth or slightly hummocky and, in higher latitudes, it is common to find polygon patterned ground. On Earth, such polygons form above permafrost as soil water expands and freezes and then thaws. Larger rocks are squeezed toward the surface as smaller rocks and soil ooze under them. At the surface, they are squeezed into stripes and clusters, forming the polygonal patterning.

Water ice is often evident on the surface in the form of small glaciers (which may be dust covered) and frost deposits. The presence of soil ice can create interesting patterns when a crater does hit the area, with the weaker ice-rich surface layers being blown into wide craters and the more resistant bedrock underneath generating smaller cratering, creating weird bull's eye craters. Water ice is evident above the surface, too, in clouds that form and sometimes organize themselves into systems that look like our own mid-latitude wave cyclones and even Arctic hurricanes. These are quite common in the Northern Hemisphere winter in the Northern Lowlands surrounding the North Polar Ice Cap.

Tharsis Rise is a massive volcanic province that covers over a quarter of the planet's surface, forming a huge "lump" positioned astride the crustal dichotomy, centered roughly around the equator. It contains five gigantic volcanoes and seven smaller ones.

Olympus Mons is the tallest volcano in the solar system. It lies on the western edge of Tharsis. At over 21 km high, it dwarfs the nearly 9 km tall Mount Everest here on Earth and the 4 km tall Mauna Kea on the big island of Hawai'i. The Mauna Kea comparison may be a bit much: If we measured Mauna Kea from the sea floor that is its actual base, it would be nearly 10 km, or nearly half the height of Olympus Mons. If we measured the base of Olympus Mons along its very distinctive, kilometers high escarpment, it would be roughly 600 km wide, covering most of France or Arizona. It would stretch from Los Angeles to San Francisco, and you'd be able to see it from Las Vegas!

Another gigantic mountain is Alba Mons, on the northwestern end of Tharsis. It is "only" 6.8 km tall, one third the height of Olympus Mons. It is far wider, however, its lava flows stretching 3,000 km east-west and 2,000 km north-south. It is the highest volume volcano in the solar system, with about 2.5 million cubic km of lava. Its shape is quite distinctive, a very low angle shield, with a large caldera complex, Alba Patera, on top. It is also badly incised with fossæ, which trend in different directions, marking shifts in the tensional stresses coming up from under the Tharsis Rise.

There are another three massive mountains, strung out in a straight line along the crest of Tharsis Rise. These are called the Tharsis Montes. From southwest to northeast, these are Arsia Mons, Pavonis Mons, and Ascræus Mons. Note also, that there are three additional, smaller volcanoes further northeast: Uranius Tholus, with Ceraunius Tholus and Uranius Patera in that same group. Their alignment, resembling Earth island arcs has fueled discussions about plate tectonics and speculations about movement of the entire martian lithosphere as a single unit to keep the Tharsis mass centered on the equator, perhaps thereby moving the lithosphere over a massive hot spot, rather like the Hawai'ian-Emperor chain over the Pacific hot spot.

Very interestingly, there are signs of fairly recent volcanism on Tharsis. First, much of the surface between and among the Tharsis Montes and Olympus Mon is Amazonian in age, that is, less than about 3 billion years old (what passes for "young" on Mars). Some of these materials, especially in the calderas and in some flows on the sides and base of these volcanoes, are less than 200 million years old and, in the case of some flows associated with Olympus Mons, less than two million years old. That is "nothing" to a geologist and it suggests that some of the Tharsis volcanoes may still be active. Wouldn't that be something to see?

B: Second "slide tray"

Second Order of Relief

The second order groups together very large and pretty conspicuous features on the surface of Mars. They do not nest tidily within the first order but often cross the crustal dichotomy or Tharsis Rise. Together with the first order features, however, they provide a prominent and easily memorized framework for situating other features on Mars.

The Great Craters

Mars has four great craters: Hellas Planitia and Argyre Planitia in the Southern Lowlands, Isidis Planitia right on the crustal dichtomy, and Utopia Planitia buried under the Northern Lowlands. There are other large craters, too, but these four are visually obvious. They date from the Late Heavy Bombardment (LHB).

When the solar system first began to organize back around 4.6 billion years ago (Ga), dust grains and, farther out, dust grains with various ices, began to accrete through chemical interactions and gravitation into clumps, many of which became progressively larger. The larger ones became planetesimals of varying sizes from asteroid size to planet size. There were so many that there were many impacts among them, basically how they grew. With less gravitational competition from the early sun, those in the ice-rich outer reaches of the solar system grew into the gas giants. Gravitational interactions among them caused the two smaller ones, Uranus and Neptune, to move outward. There, they destabilized the orbits of smaller objects, many of which headed into the inner solar system. This resulted in the Late Heavy Bombardment, which peaked around 4 Ga, tapering off by 3.8 or 3.7 Ga.

The entire inner solar system got hammered, including Earth. Earth was hit by a Mars sized object, Theia, which blew much of its mantle into orbit, where it eventually gathered into our Moon. This probably happened within 100,000 years of Earth's formation. Mars, too, was hit by a large impactor about the same time, and that impact is now believed to have created the crustal dichotomy. Mars also captured two asteroids that now orbit the planet as its moons, Phobos and Deimos.

All inner solar systems were pummelled and the signs of that disaster are highly apparent on the surfaces of Mars and its moons, our Moon, and Mercury, all of which show the intense cratering of the Late Heavy Bombardment. Earth does not have cratered landscapes from the Hadean eon or the Archæan eon's Eoarchæan era, when the LHB was going on. Our planet is geologically hyperactive, with plate tectonics recycling crust and intense gradational forces operating. For that matter, Venus doesn't show the LHB, either, having experienced an intense volcanic resurfacing less than half a billion years ago and relatively little cratering.

Hellas Planitia is a gigantic hole in the Southern Highlands, punched some 8 km below the geoid and containing the deepest spot on Mars. The crater is some 2,300 km across. It forms a bright albedo spot, the result of dust and frost activity. In the image shown, you can see the albedo spot, frost from the South Polar Ice Cap in the Southern Hemisphere winter and two of the early spring dust storms.

Argyre Planitia is distinctive with its radially and circumferentially broken rim structure. It shows signs of having been occupied by a water body at some time in the past, with valles entering on the south side and Uzboi Vallis carrying flows out the north rim. It is part of another second order feature, the Chryse Trough.

Isidis Planitia is perched on the edge of the crustal dichotomy and features a very smooth, nearly level floor. The southern rim and hillslopes are well-defined, made up of materials with high thermal inertia (dense, tightly consolidated), while the northern rim is nearly completely missing, perhaps due to erosion from an ocean or burial by volcanic ash. The floor of Isidis Planitia, with its low thermal inertia, shows similarities with the ash materials covering the Sytris Major volcanic province to the west.

Utopia Planitia is a large lowland basin within the Northern Lowlands. It apparently formed after the great impact that probably created the Northern Lowlands but before resurfacing by volcanic and/or ocean materials. In fact, radar has revealed a lot of subsurface craters in the Northern Lowlands, under that smooth younger surface. The image shown here is a surface view from the Viking 2 lander and this image was noteworthy for showing frost forming on the surrounding rocks and soil, despite the low atmospheric density and low water vapor content.

The Elysium Rise

Tharsis isn't the only volcanic rise on Mars. There is another large rise northeast of Hellas and Isidis, adjacent to Utopia Planitia. On Earth, this 2,000 km wide, 6 km high volcanic province would be humongous but, on Mars, it is dwarfed by comparison with Tharsis. It features three large volcanoes, Albor Tholus to the southeast, Elysium Mons to the west, and Hecates Tholus to the northeast.

As with Tharsis, there are signs of rather recent volcanic activity on Elysium. Flows and calderas have been dated within the last 200 million years and Elysium Mons may have erupted 20 million years ago. Mars is, apparently, not a dead planet.

Valles Marineris

One of the great surprises of the Mariner 9 orbiter mission was the finding of a vast system of rifts and canyons gashing across a fifth of Mars' circumference. Mariner 9 arrived at Mars in 1971, right in the middle of a great planetary dust storm. It had to wait out the storm and, then, as the dust settled, first Olympus Mons began to stick out of the dust. Then, as the dust settled all over the planet, there was still this huge white streak that people began to realize was a tremendous canyon, still filled with airborne dust from the great storm. This massive canyon system, stretching about 4,000 km, covers about one fifth the circumference of the planet. It is about 7 km deep and about 200 km wide. It originates in the west at Noctis Labyrinthus, a complex terrain of collapsed and broken blocks, and it opens out on the east into the Chryse Trough drainages in Margaritifer Terra. There are several subsidiary chasmata, from Ius and Tithonius in the west, through Melas, Candor, and Ophir in the middle, to Coprates Chasma on the east, which divides into Eos and Capri branches at the end. There are also several related chasmata to the north that are not connected into the main system: Echus, Hebes, Juventæ, and Ganges chasmata.

Valles Marineris has its origins in the extensional stresses that would be created on the surface by the uplift of Tharsis. There are several radial fossæ pointing to the center of Tharsis, of which Valles Marineris is the vastest. Once the surface failed in a series of normal faults, the blocks in between were free to slip downward, creating the deep valley system. Rupture of the surface and subsurface heating of ground ice would lead to massive outflows of water, seen in the chaotic terrain of Noctis Labyrinthus. Valles Marineris may have carried huge jökulhlaup type outwashes, though much of the fluids could not exit on the east because the valley floor's elevation is higher there than in the middle. There probably was a lake in the center for some time as a result. There is also evidence of massive landslides along the walls of Valles Marineris, perhaps no more spectacularly than in Ophir, Candor, and Melas chasmata, as seen in these images.

The Chryse Trough

The sheer volume and mass of the Tharsis bulge exerts tremendous downward force on the crust below it and surrounding it. This depressive force is evident in a ring of relatively subdued topography surrounding Tharsis, most obviously to the east. This depression creates a potential hydrological trough, and Timothy Parker of JPL (and CSULB B.S. Geology alumnus!) argued in his 1985 master's thesis that that potential was realized back in Noachian times (4.6-3.8 Ga), when Mars had a much thicker atmosphere capable of allowing liquid water to exist on the surface. He traced several channels that seem to conduct flows from the base of the South Polar Ice Cap into Argyre Planitia, which would have been a huge lake or sea. North of Argyre, Uzboi Valles winds through a series of craters, such as Holden and Eberswalde, and collects flows from tributaries, such as Nirgal Vallis. Eberswalde Crater even sports a dramatic delta, implying that Uzboi flowed into a lake impounded in the crater. Ladon Valles carried flows down to the next major crater basin, Ladon, where other valles conducted fluids into Ares Valles.

Ares Valles apparently also received a massive outflow from the inside of Aram Crater with the sudden liquefaction or explosive evaporation of ground ice. This may have been the result of subsurface heating, as when magma warms country rock or permafrost. The event produced Aram Chaos, that blocky landscape that formed from the loss of subterranean support, the Aram Vallis channel carved through the rim of Aram Crater, and a large outflow in Ares Valles, seen in the massive teardrop shaped bars downwater from craters in the floor of the valles. Note the one crater in the image shown that does not feature the teardrop bar behind it: That was an impact that took place after the outwash flood.

If Parker's argument is sound, Chryse Trough may be the largest hydrological drainage in the solar system. This would be true whether it was filled with water or other fluids continuously through its whole length at one time or whether it functioned sporadically and discontinuously, depending on local or regional climates or subsurface conditions.

Kasei Valles

Kasei Valles is another massive drainage to the north of Valles Marineris and seeming to have its origin in the chaos terrains in Echus Chasma and along the eastern side of Kasei Valles. This may have been a single or at most a few episodes of epic outflow, the kind of unimaginable flood referred to as a jökulhlaup on Earth. It is easy to imagine the kind of subterranean heating that would melt/evaporate ground ice in this area, on the east side of Tharsis Rise. On Earth, a famous example is the failure of the Cordilleran ice dam that had supported the Pleistocene Lake Missoula in Montana. When the ice failed, this massive lake emptied in a single great flood, one of the greatest ever experienced on Earth. The force of the floodwaters was so great, it created the Channeled Scablands in Idaho, Washington, and Oregon in one fell swoop. The scale of the Kasei flood event dwarfs even the great Missoula flood.

Thaumasia Block

There is a distinctive lozenge shaped block comprising the southeastern quadrant of Tharsis Rise: The Thaumasia block. This feature is dominated by Valles Marineris and Noctis Labyrinthus to the north, the extensional faulted zone of Claritas Fossæ to its west, and the folded and faulted mountains of Thaumasia Highlands to its south and the Coprates Rise range to the east. Between the extensional grabens of Claritas Fossæ and Valles Marineris, there is a large basin of lava flows crossed by wrinkle ridges, Solis Planum. These compressional features run at roughly right angles to the fossæ. There is also a smaller basin on the far east, Thaumasia Planum, also showing wrinkle ridges, as does Sinai Planum to the north, just south of Valles Marineris. Syria Planum, in the northwestern corner of the Thaumasia block, is the highest elevation of the block, surrounded by the extensional graben of Noctis Labyrinthus and Claritas Fossæ

This massive block invited speculation about plate tectonics on Mars, with Valles Marineris tempting as a nascent rift zone like Earth's own East Africa Rift or the Mid-Atlantic Rift. If that were a constructive zone in the making, then perhaps the Thaumasia Highlands would be the destructive zone, with its folded mountains, like our own Andes or Himalayas.

Valles Marineris as a zone of divergence does not square with evidence provided by An Yin of UCLA, who has analyzed a possible large partial crater in Melas Chasma and matched it with what looks like the rest of the same crater offset to the left about 150 km. He argues in a 2012 paper that Valles Marineris is more akin to our own San Andreas Fault but with left-lateral motion rather than the San Andreas' right-lateral motion.

Another explanation, one that can accommodate An Yin's findings, is that the whole Thaumasia block may be a megalandslide. This was argued by David Montgomery et al., in 2009. In this interpretation, the region had been a basin of some sort in early Noachian times, well before 4 Ga. It may have collected deep sediments laid down in briny water as we see in Earth playas. The sediments would have been loaded with salts. In the cold temperatures of Mars, much of the brine would have been turned into ground ice, with segregation out of the salts. So, underground, there would have been layers of ice and dirty salts.

Then, Tharsis began to build up in late Noachian times. This would have been accompanied by many volcanic eruptions, ash and pyroclastic falls, as well as lava flows. These would be draped over the underlying mechanically weak ice and salt layers. Uplift would have created extensional faulting close in to the uplifting center and radiating out from that center. It would also have caused compressional stresses on the periphery of the uplifted zone. The result would have been gravitational spreading, with left-lateral translational movement along Valles Marineris. Underground, the salt and ice stratigraphy would have resulted in shallow thrust faulting, allowing layers agove to be detached from those below, creating the wrinkle ridges of Solis Planum. It would also have produced the crumpling and folding seen in the Thaumasia Highlands and Coprates Rise. So, the Thaumasia block may well be the largest mass movement feature in the solar system, a megalandslide, produced by salt tectonics, not plate tectonics!

Syrtis Major "Blue Scorpion"

This consistently low albedo area north of Hellas Planitia and west of Isidis Planitia is the first feature made out on the surface of Mars after early experimentation with telescopes. Christian Huygens trained a telescope on Mars in 1659 and sketched what he saw, a triangular shaped dark area, producing the first map of Mars. This feature was sometimes referred to as the "Blue Scorpion" because it looks dark blue or greenish-blue against the bright orange dust-covered areas surrounding it. The color invited speculation that perhaps it was a sea or maybe an area of dense vegetation.

Albedo features on Mars often shift from year to year, darkening or lightening, moving subtly. The "Blue Scorpion" of Syrtis Major is an unusually stable low albedo area, which long encouraged belief in a water body or vegetation formation there. It turns out that this is an area with consistent surface winds, coming from the northeast. These are generated by the planet's Hadley cell circulation as distorted by the planet's topographic extremes and by the meridional contrasts in elevation across the crustal dichotomy. The result is a swath of basaltic lava and regolith that is swept clean of the ubiquitous martian dust, revealing its dark color in contrast with nearby areas that receive deposits of the bright reddish dust.

Polar Ice Caps

Finally, we come to the two polar ice caps. These were the second feature to be made out from Earth in the first century of telescopy. Jean Dominique Cassini observed the bright spots they form in the 1660s, and his nephew, Giacomo Maraldi, noticed in 1719 that these bright spots grew and shrank, concluding from this that Mars had seasons.

The two ice caps are quite different in character, reflecting differences in composition having to do with their very different elevations and the differences in temperatures they imply. The Southern Ice Cap is the smaller, especially in the summer. The ice cap was believed for a long time to be comprised of carbon dioxide ice, due to the much colder temperatures at the South Pole than at the North Pole. It is now known to be largely water ice, like the North Polar Ice Cap but, because of the extreme cold, it retains a permanent veneer, about 8 m thick, of carbon dioxide ice even through the summer. During the winter, the cap grows spatially as carbon dioxide and water frost and snow creates a hood extending out to about 40° S. During the winter, the hood sublimates away, leaving the residual water ice cap with its carbon dioxide ice covering.

Note that the residual ice cap is oddly asymmetrical, not centered on the South Pole. It turns out that the global wind circulation blasts out of Hellas Planitia in the Eastern Hemisphere and creates a surface high on the east side of the ice cap and allows only the accumulation of frost on that side during the winter. The topographically distorted general circulation produces a companion low on the other side of the ice cap, which enables storms and snow as well as frost. Snow is more resistant to sublimation and, so, the permanent ice cap builds up west of the South Pole.

The ice cap is layered, with carbon dioxide, dust, and water ice. During the spring, as the cap warms, the carbon dioxide sublimates, sometimes quite explosively, like shaking a beer or champagne bottle before opening it. This creates geysers that pull up subsurface dust and deposits it in these spider-like patterns around the geyser made holes in the ice. Something akin to this sublimation and geysering may be going on in the permafrost of Siberia right now, where large holes are showing up on the surface, except the gas involved is methane rather than carbon dioxide.

The North Polar Ice Cap also shrinks to a residual ice cap in summer and expands in a hood of water and carbon dioxide frost and snow that reaches down to around 55° N. Unlike the South Polar Cap, the residual North Polar Cap is all water ice: Summer temperatures get well above carbon dioxide's triple point.

Both polar ice caps show deep crevasses, believed to be the result of katabatic winds racing down from the high pressure that develops over ice caps. The Northern Polar Ice Cap is particularly well carved and it contains one chasm that seems to cross-cut across the direction of the others: Chasma Borealis. That disparity is still not well understood.

Like its southern counterpart, the North Polar Ice Cap is intricately layered, with layers of relatively pure water ice alternating with layers of dirty, dusty ice. This alteration may reflect climate change on Mars, as changes in the planet's obliquity and the ellipticity of its orbit over millions of years create dryer and dustier conditions and then more humid conditions. Coring that stack of layers will, no doubt, be a goal of human exploration of Mars.

Second Order Summary

The second order of relief, then, takes in very large and conspicuous features on Mars' surface, independently of the two huge features that comprise the first order (the crustal dichotomy and Tharsis Rise). Each of these reflects a major geological or geomorphological process: the Late Heavy Bombardment (the four giant craters), volcanism (Elysium Rise), rifting (Valles Marineris), fluvial processes (Chryse Trough), massive outflows or jökulhlaup (Kasei Valles), megalandslide (Thaumasia block), æolian processes (Syrtis Major's "Blue Scorpion"), and glaciation (the polar ice caps). Together with the crustal dichotomy and the Tharsis Rise, the second order features provide an easy to remember framework to which finer scale areas and features can be referenced as one's mental map grows in detail.

C: Third "slide tray"

Third Order of Relief

The third order of relief describes large regions, many as large or even larger than second order features, but not as visually conspicuous. These regional features are differentiated from one another in terms of the density of craters they sport. Crater density and size distributions are the only way we have now to age the surface of Mars and then use the differences in age to analyze processes operating on the surface. Variations in crater densities have led to a rough geological time scale for Mars.

Three subdivisions of time are widely recognized. They are called "periods," though they are more akin to the time divisions we call "eons" here on Earth.

Noachian Period

The oldest of the three is the Noachian period. This covers the time between the formation of Mars about 4.6 Ga through the tapering off of the Late Heavy Bombardment, roughly 3.7 or 3.8 billion years ago. Noachian landscapes are very heavily cratered, and the craters are of a wide variety of sizes.

During the Noachian, the planet's core dynamo organized and gave Mars a magnetic field. Remanent magnetism is found in many Noachian landscapes, the result of iron-rich magmas and lavas cooling and the iron in them orienting to the then prevailing planetary magnetic field. The magnetic field collapsed by the end of the Late Heavy Bombardment, though, since there is no remanent magnetism in the four gigantic craters. When the impactors hit, the collision heated the upper crust to the point of melting, but the iron-rich magmas did not align with anything.

The Noachian also had a much denser atmosphere, perhaps protected from the solar wind by the planetary magnetic field. Water could clearly exist, as pressures were above water's triple point. And there is a lot of evidence of surface water in Noachian Mars. There are indications that Mars had an ocean and seas in Noachian times, and there are hydrological channel systems. Some of these are dendritic, intricate branched valley networks of the type we see in humid landscapes on Earth, fed by precipitation. Others have a single main trunk and just a few short tributaries that head in amphitheatre-shaped alcoves. On Earth, we see those in groundwater-fed systems, especially in arid regions.

There is mineralogical evidence that Noachian Mars had largely neutral or alkaline hydrochemistry, espcially the existence of phyllosilicate clays that can only form in such water. This is, then, the time friendly to "life as we know it, Jim."

Later in the Noachian, volcanic activity started and then began to concentrate spatially, in Tharsis and Elysium. This introduced a lot of sulfur dioxide into the atmosphere, which then acidified water, and this created a different hydrochemistry, one dominated by sulfates.

Hesperian Period

The transition to Hesperian conditions started about 3.8 or 3.7 billion years ago and then gave way to "modern" Mars more ambiguously and arbitrarily, somewhere between 3.5 and 1.8 billion years ago. The most commonly used date for the end of the Hesperian is about 3 Ga.

Impacts still happened, but there are noticeably fewer craters in Hesperian landscapes and they occur in a narrower range of sizes, with many fewer large craters.

With loss of the planetary magnetic field, the atmosphere was slowly ablated by the solar wind. Air pressure at the surface began to drop. As it declined, water fell below its triple point. That is, it can only alternate between ice and water vapor, no longer able to exist at the surface for very long as a liquid. Surface waters, then, began to evaporate faster than they accumulated. The water then photodissociated and the hydrogen was lost to space, while oxygen quickly combined with rock materials. Mars began to lose its ocean to the atmosphere and space or to subterranean deposits of permafrost.

Volcanic activity was at its heyday, much of it now focussed on Tharsis and Elysium. The thinner atmosphere contained a lot of carbon dioxide but also a lot of sulfur dioxide. These gasses interacted with the dwindling surface waters to create very acid water. One would expect a lot of carbonate deposition in waters enriched in carbonic acid, but that seems to have been largely precluded by the amount of sulfur dioxide in the air and sulfuric acid in the water, resulting instead in various sulfate minerals and evaporites.

Every once in a while, subterranean magma would heat country rock -- and permafrost. Ground ice would explode out onto the surface in massive outwash floods or jökulhlaup, like those in Kasei Valles or in Ares Valles, creating temporary oceans until they evaporated.

Amazonian Period

The Amazonian is "modern" Mars, beginning somewhere around 3 Ga and continuing through today. Cratering is much rarer and generally involves smaller objects. Amazonian surfaces are, therefore, much smoother than Hesperian or Noachian surfaces.

Air pressure is generally below 1 hPa, where on Earth, it averages 1,013 hPa at sea level. There are a few places in Hellas Planitia and Valles Marineris that are at such a low elevation that they transiently experience air pressures somewhat higher but not for long, especially in the spring evaporation of carbon dioxide off one hemisphere's ice cap and transit through the atmosphere to freeze out in the other hemisphere's ice cap. Mars completely lost all its surface waters for good, the planet desiccating. Occasionally, there are debatable signs of soils dampening seasonally because of groundwater seeps. Geochemistry shifts to oxidation: Oxygen unites especially with iron-bearing minerals to form anhydrous oxides (a large part of the ubiquitous martian dust).

Volcanic eruptions may occasionally occur, judging from the very young lava flows found on Tharsis and Elysium, but this is nowhere as pervasive as the scale of late Noachian and Hesperian volcanism. The possibility of magmatic heating of soil and ground ice, though, means that massive outflows might still occur.

Cycles in axial tilt (obliquity) and the eccentricity of the planet's orbit over the course of the Amazonian can be expected to create changes in the amount of glaciation and its spatial distibution.

Even though Mars' atmosphere is so thin, wind is the dominant geomorphic agent in Amazonian Mars. Dust devils are very common, especially in the Southern Hemisphere spring and summer, and major regional and even planet-covering dust storms are occasionally witnessed. With 3 billion years to act, wind has created impressive æolian landforms: dunes, yardangs, fluting and faceting of rock surfaces.

Alternative Geochemical Periodization

As if to stir the pot, a group of geochemists has proposed an alterative periodization for Mars, this based on the prevailing geochemistries:

• Phyllocian Period
  • Dominated by neutral or alkaline hydrological conditions.
  • Phyllosilicate clays formed in surface waters and in groundwater
  • Early and middle Noachian
• Theiikian Period
  • Intense volcanism of late Noachian and early to mid Hesperian acidifies surface water
  • Sulfate hydrochemistry
  • Hydrated sulfates (such as gypsum, magnesium sulfate) and evaporites
• Siderikian Period
  • Transition to dry modern Mars
  • Dominated by oxidation geochemistry and the formation of anhydrous iron oxides
  • Late Hesperian and all of the Amazonian

Geography of Deep Time on Mars

Noachian landscapes are spatially the most extensive, dominating most of the Southern Highlands and also Tempe Terra in northeastern Tharsis. Noachian regions include Noachis Terra west of Hellas Planitia, Aonia Terra south of Tharsis, Terra Sirenum southwest of Tharsis, Terra Cimmeria west of Sirenum and south of Elysium Rise, Tyrrhena Terra northeast of Hellas Planitia, Promethei Terra southeast of Hellas Planitia, and Terra Sabæa west of Syrtis Major and northwest of Hellas, Arabia Terra west of Terra Sabæa, Margaritifer Terra east of Valles Marineris, and Xanthe Terra north of eastern Valles Marineris and west of Margaritifer Terra.

Hesperian surfaces are found in a few places in the Southern Highlands, northeast of Hellas Planitia (Hesperia Planum), south of Hellas (Malea Planum) and also to the west of Isidis Planitia (Syrtis Major Planum). The Thaumasia block is predominantly Hesperian, as is Vastitas Borealis, Utopia Planitia, and Chryse Planitia in the Northern Lowlands.

Amazonian landscapes are spatially even more constricted than Hesperian ones: Acidalia Planitia northwest of Arabia Terra and northeast of Tempe Terra, Arcadia Planitia north west of Tharis, and Amazonis Planitia west of Tharsis, all three in the Northern Lowlands. As well, the bulk of the lava surfaces on Tharsis Rise are also Amazonian, including Lunæ Planum north of Valles Marineis.

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