MARS:

Tentative Topics

• A physiographic regionalization of Mars and the processes behind it
  • Introduction: a foray into a whole new vocabulary for landforms
    • Some Martian feature types and conventions used for naming them (modified from USGS Astrogeology Research Program Gazetteer of Planetary Nomenclature "Categories for Naming Features on Planets and Satellites"):

      Mars Features	Conventions for Naming Features
      Albedo Features	Names from classical mythology originally assigned by Schiaparelli and Antoniadi
      Large craters (craters > ~60 km)	Dead scientists who contributed to the study of Mars; writers and others who added to the lore of Mars
      Small craters (craters < ~60 km)	Villages and towns on Earth having populations < 100,000
      Large valles	Name for "Mars" or "star" in various languages
      Small valles	Classical or modern names of rivers
      Other features	From a nearby named albedo feature on Schiaparelli or Antoniadi maps
      Deimos	Authors who wrote about Martian satellites
      Phobos	Scientists involved with the study of the Martian satellites, and characters and places from Jonathan Swift's Gulliver's Travels

    • Why familar geographical terms are too misleading to use
    • A medley of new words:

      Feature	Approximate Definition/Analogy with Earth
      Albedo feature	a geographic area distinguished by amount of reflected light
      Facula, faculæ	a bright spot
      Macula, maculæ	a dark spot or irregularity
      Regio, regiones	a broad geographic region with color or reflectivity distinctiveness
      Vastitas, vastitates	an extensive, vast plain
      Terra, terræ	an extensive land mass
      Planum, plana	a plateau or high plain
      Planitia, planitiæ	a lowland or low-lying plain
      Chaos	an area of broken or blocky terrain
      Cavus, cavi	a hollow or irregular, steep sided depression, usually in clusters
      Chasma, chasmata	a deep, elongated, and steep-sided depression
      Vallis, valles	a valley or canyon
      Fossa, fossæ	a long, narrow depression
      Linea, lineæ	a straight or curved elongated marking of contrasting color
      Virga, virgæ	a streak or stripe in a contrasting color
      Arcus	an arc-shaped feature
      Labes	a landslide
      Fluctus	a flow terrain feature
      Labyrinthus, labyrinthi	a complex of intersecting valleys or ridges
      Sulcus, sulci	Parallel or sub-parallel furrows and ridges
      Dorsum, dorsa	a ridge
      Sinus	a small plain that looks like a bay on a shore
      Crater	a circular depression or impact feature
      Catena, catenæ	a chain of craters
      Mensa, mensæ	a flat-topped prominence with steep sides like a mesa or table
      Lingula, lingulæ	an extension of plateau having rounded lobate boundaries
      Rupes	a scarp
      Scopulus, scopuli	a lobate or irregular scarp
      Colles	small hills or knobs
      Tholus, tholi	small, conical mountain or hill
      Mons, montes	a large mountain
      Patera, pateræ	an irregular volcano or crater or one with scalloped edges
      Tessera, tesseræ	tile-like or polygonal terrain (Venus)
      Undæ	dunes

  • The orders of relief: Scale of topographic variation
    • The orders of relief scheme is sometimes encountered in geography textbooks as a means of organizing the variations in Earth's topography in a scale-dependent manner. Geographers often focus, not only on spatial analysis and regional synthesis, but on the scales at which regions and processes operate and the interactions between and across scales.
      • In spatial statistics, there's the Modifiable Areal Unit Problem (MAUP)
      • In human geography, there's local cultural and political responses to global economic and political processes
      • Biogeography works with alpha, beta, and gamma measures of biodiversity
      • Geomorphologists have recently been addressing "megageomorphology" as remote sensing technology has made the simultaneous examination of form and process at large scale (small map scale) possible
    • It's usually presented as a descriptive scheme (as in the Robert W. Christopherson Geosystems many of you may have used in introductory physical geography), but sometimes these days people may try to tie it to plate tectonics (first order being the plates, second order being the features that develop along the margins of plates, third order being largely erosional and depositional features at a smaller scale -- as in Michael E. Ritter's online physical geography textbook.
    • I think it's a handy descriptive scheme for organizing the emerging geography of Mars
  • First order of relief: the great crustal dichotomy
    • If Earth's oceans evaporated, which they one day will, there would remain a crustal dichotomy
      • The former ocean basins would show as low-elevation areas of thinner crust, dominated by the heavy, dense, dark basalts, in many places covered with a veneer of continent-derived sediments
      • The former continents would show up as raised areas of thick crust, with materials derived from the lighter granites (e.g., granite, andesite, rhyolite; the alluvial and nearshore marine sediments derived from them, such as shale, sandstone, and limestone; and the metamorphosed rocks deriving from any of these, such as slate, quartzite, and marble)
      • So, too, on Mars, we see a crustal dichotomy: the northern lowlands and the southern uplands
    • The low, smooth northern third of the planet: Vastitas Borealis
      • Regional subdivisions:
        • Utopia Planitia, northwest of the Elysium volcanic rise: Huge crater
        • North Polar Basin, northeast of the Tharsis volcanic rise
        • Embayments:
          • Elysium Planitia west and northwest of the Elysium volcanic rise and south of Utopia
          • Isidis, east of Arabia Terra and between Utopia and Hellas, an impact basin
          • Acidalia Planitia, north of Arabia Terra
          • Chryse Planitia, north of the eastern outflows of the Valles Marineris system, which flow into it
          • Arcadia Planitia, north of Alba Patera/Tharsis and of Tempe Terra
          • Amazonis Planitia, west and northwest of Tharsis, the young terrain for which the youngest era of Martian history is named, the Amazonian
      • Relatively young surface: few craters
      • This is what you would expect if an ocean had existed there and received sediments from rivers, floods, and coastal processes and distributed them over the underlying rock
      • Also, the great outflow channels dump into the northern plains in a manner you would expect if there were an ocean there
        • Valles Marineris' outflow channels to its east, which drain into Chryse Planitia
        • Check out the channel system that seems to cut from the highest elevations around the south polar cap (from subcap liquid water?), drain into Argyre Planitia from the southeast, cut through the north rim, winding from crater to crater into more and more distinct channels, and then out into Chryse Planitia
        • Ares Vallis that cuts into and drains out of Aram Chaos and forms a channel that drains northwest into Chryse Planitia
        • Nanedi Vallis that starts just north of Ganges Chasma and drains north into Chryse Planitia
      • It gets better: There are suggestions of coastal-type landforms that are found all around the edge of the northern plains
        • The transition between the southern highlands and the northern lowlands is quite abrupt, several kilometers' worth!
        • There are what appears to be three terraces on the north slopes of Alba Patera, which look like the wave-cut benches and wave-built terraces you see on Earth coasts, which have been proposed as possible locations for standstills during the evaporation of Mars' putative ancient oceans
        • JPL's Tim Parker began arguing for Martian oceans back in the late 1980s and this idea eventually became his Ph.D. dissertation in 1994.
          • He thought he could see two separate sets of ancient shorelines around the northern lowlands, which left traces a lot like the Utah hillsides that show traces of the Pleistocene Lake Bonneville.
          • This was at a time when the Mars community figured that Mars was a bone-dry planet that had, maybe, traces of water here and there way back when to account for valley networks and outflow channels.
          • Where could the water come from?
            • Those outflow channels and valley drainage networks? Could there have been a dense enough atmosphere for precipitation and overland flow to channel into stream networks?
            • Parker and a colleague, Stephen Clifford, proposed that the Martian water was primordial, or collected from the outgassing of water in the very beginning of the planet, when the atmosphere was thicker and capable of holding onto water
            • They calculated that there was enough primordial water to make a global geoid-covering ocean that would be anywhere from 550 to 1,400 m deep: In the real world of Martian uneven topography, that would mean much deeper oceans in lowland areas, such as the northern plains and Hellas Planitia.
        • Parker was supported by Victor Baker, who proposed that watery (well, watery-icy, like our Arctic Ocean) conditions on Mars not only once existed but existed repeatedly, clear up to the Amazonian era, perhaps the result of long-term climate changes on Mars having to do with its orbital behavior.
        • Parker's ideas remained pretty controversial among most in the Mars community and even he despaired of their validity when he got a look at the MGS images coming back in 2001: Finer scale imagery actually made the putative shorelines disappear (as it would if you trained too sharp a camera from too close up on the old Lake Bonneville traces).
        • The MOLA data, however, provided an alternate source of information that actually validated at least some of Parker's ideas: He was able to show that the features he had seen in the coarser Viking data actually were at the same elevation, about 3,700 m below the geoid, running for hundreds of kilometers all around the basin. The water elevation should be roughly level (except for areas of gravitational anomaly, as around Tharsis, which would be able to pull sea level up about a mile!)
        • One of Parker's oceans would have averaged about 570 m deep but get as deep as 3 km in the Utopia Planitia region. If we imagine Mars smoothed out to its gravitational geoid, that ocean would average about 100 m.
        • Things are looking pretty good for a global ocean, eh? Mars, however, is the "yes, but ..." planet.
      • That "ocean" floor is dominated by andesitic rock, not sediments with minerologies consistent with precipitation out of water. No carbonates!
        • This striking divergence of exposed Martian rocks gives rise to the ST1 and ST2 dichotomy: "Surface Type 1" (basalts dominating the southern highlands) and "Surface Type 2" (andesites and andesitic basalts dominating the northern lowlands)
        • Andesite is an extrusive igneous rock enriched in silica compared with basalt
        • This can be the result of vulcanism: fractionation of basaltic magmas with olivine (Mg₂SiO₄ or Fe₂SiO₄) crystallizing out at higher temperatures, thereby enriching the still liquid magma in SiO₂ which reacts with the olivine to form pyroxene (MgSiO₄ or FeSiO₄ + SiO₂ ==> 2MgSiO₃ or 2FeSiO₃, which crystallizes out at a cooler temperature. Andesitic magma is more silicious or sialic and usually contains pyroxene, quartz, feldspar, hornblende, and biotite. It is generally extruded onto the surface by volcanic eruptions, which can be explosive sometimes, since its higher viscosity traps gasses in the flowing magma and these expand violently as the magma comes out from under the overburden toward the surface.
        • Andesite dominance can result from sustained plate tectonics, because the more silicic rock materials are more buoyant than the more mafic, and resist subduction back into the mantle.
          • Crustal materials become both thicker and more silicic on Earth's continental plates
          • When magma is produced under or on the edges of continents, it incorporates the more silicic materials in the country rock, leading to a more andesitic or even rhyolitic magma.
          • But the Earth analogy puts the formation of andesitic magmas under and on the edges of continental plates and, on Mars, the andesitic material is precisely where we would expect more basaltic rocks corresponding to Earth's oceanic crust. Mars, once more, is the "yes, but..." planet.
          • And Mars shows little evidence of plate tectonics, certainly not sustained, mantle-recycling plate tectonics, complete with subduction zones and divergence zones
        • The dominance of andesite can also result from alteration of basalt through interactions with ice or water. So, water is back in the picture.
          • Michael B. Wyatt et al. (2004) point out that MGS TES data show basalts dominated by plagioclase feldspars and pyroxenes and, locally, olivines, which normally alter rapidly into other minerals, such as hæmatites, iddingsite, goethite, serpentine, chlorite, smectite, and maghemite in the presence of water. None of these secondary alteration prodeucts have been found on Mars, except for a small amount of hæmatite.
          • Neither have other minerals indicative of persistent water been found, such as the carbonates that should be abundant.
          • From this, Mars looks as though it's been very dry and probably very cold for all or most of its existence.
          • Michael B. Wyatt et al. argue that the andesite was produced by another process: alteration of basaltic magma through interactions with ice near the surface and ice mantling the surface, which, during periods of high orbital obliquity, could well have formed as far from the north polar cap as 40°N.
          • They conclude that the partially altered basalts formed in the Dry Valleys of Antarctica and the summit of Mauna Kea are better Earth analogues for the andesites and basaltic-andesites of Mars
        • Patterned ground that resembles polygonal structures formed in Earth soils over permafrost in the active layer as water expands upon freezing (water ice is alone among minerals in expanding at this phase change)
          • On Earth, the size of such polygonal patterns is correlated with how far down the permafrost is and how thick it is
          • The first images of these polygons on Mars at first seemed too large (up to 30 km across!) to have anything to do with subsurface ice, but that might simply have reflected the resolution of the spacecraft of the 1970s (e.g., Viking orbiters)
          • The new crop of orbiters are showing these patterns at the 10 m to 2 km range, which are plausibly related to ice freeze-thaw stresses.
          • Estimates based on these newer images are that the ice deposits begin no more than 200 m below the northern lowland surface, where the putative possible ocean lay
        • Rampart craters are pretty unique on Mars: They are surrounded by clearly fluidized ejecta blankets, kind of a wet splat effect, scientifically speaking. They are believed to represent an impactor whacking an icy soil, a soil or regolith with ice filling its interstitial spaces. The soil ice liquefies in thermal and compressional shock, and the resulting glop flies and flows out in a kind of impact lahar, forming an ejecta blanket of a very distinctive sort, which flows over and around topographic obstacles, complete with flow striations. Again, water seems to be back in the picture.
        • Neutron spectroscopy on Mars Odyssey suggests hydrogen in soil, which you would expect over subsurface ice or water bodies. Again, water gets a brownie point.
        • Plot complication: It appears that there are as many craters under that smooth northern plains surface as there are in the ancient southern highlands, buried by those smooth deposits that look so, well, oceanic. These are becoming evident through radar imagery (e.g., Mars Express MARSIS radar altimeter).
          • So, there is this ancient, pockmarked surface under there.
          • There might be younger pelagic sediments on top of those (and an ocean would explain the smoothing of those old craters so their walls aren't poking out of the newer materials)
          • And then even younger (Amazonian?) basaltic magma spewed over all this after having been altered in the andesitic direction by interaction with buried ice deposits or ice mantling the surface during high obliquity eras in Mars' orbit? Could such a magma flood have buried every single last trace of the pelagic sediments?
          • Isn't Mars maddening?
    • The old, rough, cratered, high-elevation, and dusty southern two thirds of the planet
      • The southern highlands are generally about 1-5 km above the mean Martian geoid (versus 0-3 km below for the northern lowlands), with a sharp ~1 km scarp dividing the two
      • The southern highlands are also topographically the most diverse terrain on Mars, ranging from 21 km above the "areoid" in the case of Olympus Mons down to the floor of Hellas Planitiae, which is 4 km below the geoid (or ~8 km below the regional high country)
      • The total elevational contrast in the southern highlands, then, is ~25 km!
      • Highest crater densities lie around Arabia Terra, probably the oldest part of the Martian surface
        • Noachian era, the time of the great bombardment from the time the planets emerged up until about 3.8 billion years ago
        • So many craters, we can't even begin to use superposition to figure out the oldest ones
        • Arabia contains some of the few places on Mars where pretty much nothing happened after that, so we can still see the ancient havoc
        • It also contains some areas that have these weird layers that have been exposed by what looks like wind erosion -- sediments? volcanic ash? lava floods?
      • Noachis Terra is another ancient, pummelled landscape
        • It does show more signs of erosion and deposition, possibly by water
          • Softened crater rims
          • Channels
          • Alluvial fans?
          • Flattened floors
      • Syrtis Major
        • Dark wedge north of Hellas, west of Isidis, east of Arabia
        • Basalt terrain clean of dust
        • Hesperian in age, going back to the times just after the great bombardment ended
        • A volcanic terrain with basalts washing over it to a depth of ~0.5 - 1.0 km, covering up the ancient craters, but old enough to have been pockmarked by newer collisions
      • Terra Tyrrhena
        • Just south of Isidis and northeast of Hellas
        • Basalt-dominated surfaces, covered with dust
        • Classic old, battered terrain, consisting of crater floors, crater rims, crater ejecta blankets, and intercrater lands
        • There may be old craters buried under newer surfaces, themselves smacked with craters
        • These surfaces have been modified in places by seeming fluvial processes e.g., Vichada Valles network, Libya Montes) especially and by æolian processes
      • Promethei Terra
        • Just east of Hellas Planitia
        • Another basaltic old cratered highland
        • Evidence of landslides
        • Some spectacular images of dust devil tracks taken by ESA Mars Express HRSC
      • Terra Cimmeria
        • East of Hellas Planitia, southwest of Tharsis, south of Elysium Rise
        • Another basaltic old cratered highland
        • Ma'adim Vallis, fluvial channels
      • Terra Meridiani
        • East of Valles Marineris outflow, northwest of Hellas, south of Acidalia
        • Younger but badly cratered countryside
        • Home of the hæmatite concentration, which might indicate water
        • Opportunity's stomping grounds
      • Margaritifer Terra
        • Where Valles Marineris outflows swing northward, west of Meridiani
        • Younger terrain filled with outflow channels (Ares Vallis) and chaos terrain (Aram)
      • Terra Sirenum -- transition between Tharsis and Cimmeria
      • Xanthe Terra -- directly north of eastern Valles Marineris, south of Chryse
      • Tempe Terra -- directly north of Nanedi, east of Alba
  • Second order of relief
    • The great volcanic "plana"
      • Tharsis
        • Huge volcanic rise along the equator at roughly 250° (~110° W), about 8,000 km across
        • Nearly 10 km thick, not counting the volcanoes on and near it
        • Would cover most of the United States and portions of Canada and Mexico
        • Supports the Tharsis Montes (Arsia, Pavonis, Ascraeus) running along its central spine, with Olympus Mons and Alba Patera just off the main rise
        • This feature is such a gravitational anomaly that it would affect Mars' rotation, perhaps determining the axis of rotation itself through the centrifugal force that creates planets' oblate ellipsoid shape
        • Arabia Terra is a smaller bulging area that is opposite Tharsis, perhaps a kind of compensating counterweight
        • There is a kind of topographic depression that surrounds Tharsis, again possibly a geoid compensation for the vast weight of the Tharsis "lump," like a dip in the bed when you lie on it
        • This depression affected the path of surface water/fluid
        • The Tharsis bulge is comprised of basalts from eruptions
        • Valles Marineris' floor is covered with sheets of lava from the Tharsis activities
        • The Tharsis magmas are so massive that Roger Phillips estimated that the amount of carbon dioxide and water that would outgas during Tharsis' early eruption history back in Noachian times would have been enough to increase the density of the Martian atmosphere to 1.5 bars and create a global ocean averaging 120 m above the geoid!
        • Signs of stresses and deformations from whatever caused the uplift of Tharsis:
        • Radial fractures and grabens (including in a sense Valles Marineris)
        • Compressional ridges ringing it
        • Some explanations for the Tharsis rise:
          • Dynamic support by an underlying great plume of mantle material, which becomes upon depressurization as it approaches the surface, but is such a plume stable for over 4 billions years?
          • Maybe plate tectonics, e.g., a subducting plate originating in the northern hemisphere, which can produce uplift, extension, and faulting in the plate above (much as the Farallon Plate uplifted the western United States) - but 4 billion years is a long time for such a process to be persistent and stable
          • Some kind of mantle anomaly in terms of temperatures or chemical composition, but, again, 4+ billion years is a long time for that not to have reached equilibrium
        • But what could feed this kind of persistent vulcanism?
          • On Earth, plate tectonics generates magma through heating and decompression, but Mars has had little to no plate tectonics
          • But, then, again, the three Tharsis Montes are aligned along a single 1,500 km long line, and Arsia looks older and more eroded than Pavonis, which looks older than Ascraeus -- perhaps, like Hawai'i, the bulge is moving over a stationary hotspot?
          • Maybe, though plate tectonics never really advanced, a volcanic hot spot did develop as one of perhaps two major ventings of magma created in the mantle to channel lava onto the surface, and it just kept on doing so, building this huge mound of lava, so heavy it depressed the surrounding countryside
          • C. Reese and V. Solomatov are not convinced by the core/mantle magma plume and hotspot theory and think that maybe Tharsis is the result of locally focussed heat energy from comet/asteroid impact
          • Their work is echoed in Linda Elkins-Tanton's work on massive impacts on Earth excavating/vaporizing chunks of crust enough to cause the surrounding lithosphere to rebound through isostacy, creating a bulge and magma as mantle materials experience a reduction in pressure - the magma may be so copious as to produce flood basalts
          • Jonathan Hagstrum criticizes the narrow plume model of hot spot vulcanism by pointing to antipodal pairs of hotspots on Earth and the possibility that large impacts' seismic energy may be focussed in the antipodal aesthenosphere, resulting in heating, melting, rifting, flood basalts, and persistent hot spot vulcanism
          • There has been some speculation that Hellas is the guilty party, nearly antipodal to Tharsis and huge
      • Elysium
        • Another huge rise, dwarfed only by the sheer scale of Tharsis
        • "Only" 2,000 km
        • "Only" 6 km thick
        • Also houses multiple volcanoes:
          • Elysium Mons on the west central side of the rise (12.5 km high)
          • Albor Tholus to the southeast (4.5 km high, with a 3 km deep caldera!)
          • Hecates Tholus to the northeast
        • Hecates may have been active at least as recently as 350 million BP and this looks like an explosive event creating a flank caldera on the northwest side of the volcano
          • An article by a team led by Ernst Hauber, based on Mars Express HRSC data, discusses an longated depression running NE to SW at the bottom of the northwest slop of the volcano (~45 km by 20 km)
          • It contains some 50 m wide ridges that look like terminal moraines on Earth
          • Another, shorter depression is completely full of striated materials running downslope and have some cracks perpendicular to them that look like stuff that would be deposited in crevasses and then exposed as the glacier melted or sublimed back
          • There are steep sided valleys pouring out onto the top of the bigger depression: Could these be hanging valleys carrying materials onto the top of the "valley glacier"?
          • These features have few craters on them, implying an age of something like 100-ish million BP
          • Similar features have been reported on the northwest flanks of Olympus, Arsia, Pavonis, and Ascraeus, too
          • Ice age?
        • Elysium may have erupted 20 million BP, meaning it could well be an active volcano (error bars X 4 - 80 million BP to 5 million BP)
        • The "recent" vulcanism has put dust in the eye of the traditional theory that Mars, being a dead planet with a cooled core, stopped being volcanically active two billion years ago!
        • As with Hecates, Elysium may have been glaciated, but 5-24 million BP, judging from glacial deposit features and crater counting on the Hecates flank caldera and nearby depressions:
          • There is (and cannot be) stable ice at these low latitudes now
          • Such glaciation suggests climate change on Mars and the timing cooncides with a time of increased obliquity and seasonal extremes on Mars
          • Again, tantalizing suggestions of an ice age on Mars
    • The great impact basins
      • Hellas Planitia
        • This sucker spans about 50° of longitude and 30° of latitude, some 2,300 km across and 8 km deep relative to the surrounding countryside (about 4 km below the geoid)
        • Striking thought: If all the material excavated by the impactor that created Hellas were sifted evenly all across the contiguous continental United States and slowly built up, it would cover us up to a depth of 3.5 km or so
        • It is believed to date from the end of the Noachian era (which ran from the beginning of the planet's coalescence to maybe 3.8 billion BP).
          • When an impactor of this size hits, it vaporizes and melts solid rock.
          • Magma containing iron minerals (which the basalts of Mars' southern highlands have a lot of) aligns with the then prevailing magnetic field
          • Hellas shows no magnetization, so it formed after the collapse of the Martian magnetic field
        • The crust is very thin here
        • Isostatic rebound isn't apparent, so the impact-rebound process posited to explain Tharsis didn't do the same thing here
        • Unless seismic energy was focussed at the antipode, which is roughly where Tharsis is
        • Hellas shows all sorts of interesting erosional and depositional landforms expressing a complex geological history:
          • Depositional:
            • Volcanic wrinkle ridges and pyroclastic flows
            • Mass wasting
            • Fluvial alluvial fans
            • Lacustrine/marine layered deposits
            • Æolian dunes
            • Ground ice or glaciers
          • Erosional:
            • Fluvial outflow channels
            • Lacustrine/marine shorelines
            • Æolian yardangs
      • Argyre Planitia
        • Many of the same features as Hellas
        • Apparently involved in a tremendous fluvial system draining from beneath the south polar cap through a chain of crater lakes and river channels leading to Ares Vallis and Chryse Planitia
      • Isidis Planitia
        • Again, a very old crater
        • Again, a very complex geological history
        • Again, a lot of dunes forming fields with ripple structures
          • High crater intensity
          • However, many of these are eroded mounds with pits at the top
          • Their appearance suggests that there was once some kind of sediment or other filling in Isidis, which was then smacked by craters, which consolidated the areas around the impacts.
          • Later, erosion (wind?) removed whatever these beds were, leaving the consolidated crater rims to stick out more and more prominently above the lowering floor
        • This is where Beagle 2 was to land on 25 December 2003
      • Utopia Planitia
        • A lava plain in the northern lowlands, located at the antipode from Argyre
        • Where Viking 2 landed
        • Where Viking 2 recorded the formation of thin frost layers on rock and soil, which may form when CO₂ in the atmosphere freezes out, attaches to dust particles themselves the condensation nuclei for water, and then settle down like a kind of fog frost
        • The consensus now is that Utopia Planitia is a humongous crater buried in whatever it is resurfaced the northern lowlands
        • Odd circular grabens on that surface material and polygons
          • These look almost like the draping and sagging and fracturing of some layered material over buried crater structures
          • Mars Express has ferreted out buried craters on Chryse Planitia
          • This would be consistent with ocean sediments in an argument by Debra Buczkowski and George McGill in 2002
          • Might that consistency not preclude low viscosity lavas?
    • The great canyons
      • Valles Marineris
        • Extensional rifting
        • Pitting
        • Water or water mixtures in subsoil
        • Landslides
        • Massive outflows, like jokulhaups on Earth when vulcanism-related warming hits a glacier or ground ice or when an ice dam or moraine dam liberates a huge lake
        • Not quite a canyon in the Earth sense, since the eastern end is higher than the center
      • Subsidiary chasmata
        • Ius Chasma in the west on the south side
        • Melas Chasma in the middle on the south side
        • Coprates Chasma to the east on the south side
        • Eos Chasma, the southern fork on the east side
        • Capri Chasma, the northern fork on the east side
        • Tithonium Chasma in the west to the north of Ius
        • Candor Chasma in the center north of Melas
        • Ganges Chasma to the east north of Coprates/Eos/Capri
        • Hebes Chasma off to the northwest
        • Ophir Chasma off to the north
        • Juventae Chasma off to the northeast
    • Polar ice caps
      • Northern cap: water ice core and seasonal carbon dioxide ice extensions and layered terrain
      • Southern cap: permanent carbon dioxide ice, probably with water ice underneath, and probably seeing some basal melting in the past to form the Argyre to Ares fluvial system
  • Third order of relief: Variations in crater density: Terra and planum
    • Noachian surfaces: The oldest
      • From the earliest formation of the planet through the gravitational accretion, collision, and consolidation of planetesimals, asteroids, comets, meteoroids, and dust
        • The kinetic and radioactive heating of these
        • Melting
        • Differentiation through gravitational layering: formation of the core, mantle, and lithosphere
        • Initiation of the planetary magnetic field through motion in the outer, liquid iron-dominated core
      • The sustained bombardment of the differentiated planets as the solar system was cleaned up of most of the stray smaller objects by the gravitational fields of the early planets
      • This hellacious era went on until about 3.8 or 3.9 billion years ago in most accounts and as "recently" as 3.5 billion BP in others: There are still a lot of controversies
      • The Noachian roughly corresponds with the Hadean time on Earth (4.5 to 3.8 GYr), but, unlike on Mars, we have no rocks on Earth that date from this time because of the intense geological activity here
      • The constraint on this time is based on analysis and dating of Moon rocks from similarly cratered surfaces brought back to Earth by Apollo
      • Noachian surfaces on Mars are intensely cratered: craters on top of craters to the point that it becomes challenging to pick out which ones are superposed on which others
      • Noachian surfaces also show a great diversity of crater sizes, with some big craters mixed in with medium and small ones
      • Noachian craters, too, show such geomorphic reworking as softening of the rims, exposed ramparts, and floors flattened by the deposition of alluvial, marine, or æolian materials in them: You do not see that on the Moon, which lacks geological activity
      • There was some geological activity back then:
        • Valley networks may date to that time
        • There was some early and distinctive vulcanism in the highlands, featuring plains formed from very low viscosity lavas and small cones (pateræ)
        • Most of the arguments about possible oceans on Mars place it in this time frame
      • Noachian surfaces include:
        • Noachis Terra, the prototype, to the west of Hellas and east of Argyre
        • Arabia Terra northwest of Hellas Planitia and east of Chryse Planitia
        • Terra Sirenum southwest of Tharsis
    • Hesperian surfaces: Intermediate
      • Goes back from the end of the Noachian to roughly 2.9 billion BP, but there are divergent interpretations of the end of the Hesperian, ranging from 3.55 billion years ago to "only" 1.8 GYr>
      • This is believed to be when the Tharsis bulge developed
      • It was an era of intense geological activity:
        • Shield vulcanism that built up Tharsis and its humongous montes and the impressive Elysium rise and its three volcanoes
        • Folding
        • Faulting and the formation of linear grabens in areas of evident extensional tension (parts of Tharsis, for example)
        • Gigantic outwash floods far beyond the more modest valley networks seen in Noachian areas
      • This is also the era in which Martian climate began to dry out and become really dusty and water began to concentrate in permafrost, soil ice, the polar ice caps, and very tenuous cloud activity
      • Some Hesperian regions include:
        • Hesperia Planum to the northeast of Hellas
        • Terra Tyrrhena to the southeast of Isidis
        • Aonia Terra to the northwest of Argyre
        • Margaritifer Terra to the east of Valles Marineris
    • Amazonian surfaces: Youngest
      • This began with the (contested) date of the ending of the Hesperian era and continues until the present day
      • Things are much more peaceful
        • Vulcanism probably still goes on but not at the clip of Hesperian or even Noachian times
        • Water continues to be released from the subsurface somehow, creating those fresh looking gullies and possibly more dramatic outflows
        • Heck, meteoroids do still smack down from time to time
        • There may even be ice ages in the Amazonian, which coïncide with periods of exaggerated obliquity in the Martian axis of rotation
      • Examples of Amazonian terrain includes
        • Amazonis Planitia to the west of Tharsis
        • Utopia Planitia to the northwest of the Elysium rise
        • Chryse Planitia to the north of Margaritifer Terra
        • Acidalia Planitia to the north/northwest of Arabia Terra
  • Fourth order of relief
    • Fluvial valleys
      • Some valleys seem to show the kind of dendritic structure and fine dissection you'd expect from a precipitation-fed surface flow and channelization system on Earth
        • Channels in Echus Chasma, south of Kasei Valles, originating to the northwest of Valles Marineris
        • Dendritic pattern collecting flow in progressively higher order channels
        • Eventually poured over a 4,000 m cliff
      • Some seem to carry the overflow of water into or out of a crater or system of craters
        • Ma'adim Vallis flows into Gusev Crater, which is where Spirit landed
          • Ma'adim Vallis is southeast of the Elysium rise and southwest of Tharsis
          • Seems to collect fluids north from a series of possible lakes to the south to empty into Gusev
          • Lakes identified by R.P. Irwin III and G.A. Franz at the Smithsonian from what look like deltas and terraces all at the same elevation around a surface that does not show channelization itself, perhaps protected from fluvial erosion by the deposition activities of a lake bottom.
          • A channel descends from them, winding sinuously for some 900 km down into Gusev
          • Gusev Crater was chosen for Spirit's landing site partly because it looked so clear that Ma'adim Vallis was carrying water from a wide watershed into the crater.
          • The hope was the crater would yield water-altered minerals.
          • Spirit did not find water-altered minerals at first, the way Opportunity did in Meridiani, but, after a year and a half, bedrock was reached and it was dramatically altered (not coatings and veins of water-deposited minerals but pretty wholesale alteration).
          • It's bromine, sulfur and chlorine found inside the rock "Clovis."
        • Then there's that enormous chain of craters and valleys linking the melt from under the south polar cap through Argyre and Holden craters, around Aram Crater, and through Ares Vallis into Chryse Planitia, some 8,000 km long!
      • Catastrophic, jökulhlaup-like outflows
        • Kasei Vallis and Ares Valles you met earlier in the semester and last week saw how much of a flow they could have carried in a single event or series of massive events
        • Ares Vallis might have its source in collapsed terrain, such as Aram Chaos, where water or other fluids are suddenly liberated by heat or mechanical failure breaking a dam of it, which undermines the still-frozen surface terrain, producing the chaos landform
        • Last week, I mentioned Dao Vallis, Niger Vallis, and Harmakhis Vallis in eastern Hellas Planitia, with their odd "upside down" structure with wide, alcoved head"waters" and narrow V-shaped channels farther down
        • A very interesting case is Chasma Borealis, which might be a volcano-glacio-fluvial source of immense water volumes and catastrophic flooding
          • Kathryn Fishbaugh and James Head, III, have created a topographic map and profiles and used them to estimate volume of a catastrophic melt (perhaps subsurface vulcanism): 26,000 km³!
          • Picked out deposits from such an event: could fill lowest portion of north polar basin to a few 10s of m!
    • Sapping alcoves, channels, aprons
      • These structures form on Earth in layered terrain with a more resistant caprock as groundwater sapping from a subsurface layer exposed in a scarp (fault? landslide?) erodes scarp face material and undermines the caprock, which then collapses down the scarp
      • MGS' Mars Orbital Camera has recorded many smaller scale versions of this on the walls of craters and caught a few of them in the act of having recently flowed
      • Especially parallel is the imagery from Houghton Crater on Devon Island, Canada, north of Baffin Island: The only Earth impact crater on a Mars-like polar desert landscape
      • Especially interesting is the geography of such gullies: They are found on poleward-facing slopes, especially in the southern hemisphere, at latitudes with absolute values >30°
        • This is odd, given that poleward slopes are colder than equatorward slopes and Mars is such a cold place with such low atmospheric pressure!
        • Mars' air pressure is typically between 500 and 600 Pa (5-6 mb), where Earth's is about 101,320 Pa!
        • Water has a triple point pressure, or pressure at which solid, liquid, and gas phases co-exist, of 610 Pa or 6.1 mb, at 0.01° C. If you hold pressure constant, warming it turns it into a gas. If you held the temperature constant and upped the pressure, you could cause it to change to liquid. Below the triple point, increasing the pressure would only cause it to become solid ice.
        • Sooo, at this low pressure, warming ice in the soil will only make it sublimate directly into vapor. Most of the time, Mars' temperatures are below the triple point, so increasing pressure would, perversely, keep it solid!
        • And yet you see these gullies and they aren't antiques: Malin's team has found new ones popping up between MOC imaging passes!
        • Some possible explanations for this maddening Martian phenomenon:
          • Maybe it isn't water: Brine or some other such non-pure water mix might work. If you add salt to water, you depress the triple point, which allows the liquid phase to exist at Martian temperatures and surface pressures. Various salts have been detected on the Martian surface, so there might be something to this approach. Salts can be picked up from volcanic fines in the atmosphere that would interact with water or from groundwater's interactions with various rocks.
          • Maybe it's a signature from an earlier era a few thousand years ago when Mars' axis had greater obliquity. Martian obliquity swings from ~15° from the vertical of the plane of ecliptic to ~35° over a cycle of approximately 124,000 years (Earth's varies by only about 4°, with its rotation and axis stabilized by the gravitational tug of the Moon).
            • At a time of greater obliquity, Mars' polar regions would receive the concentration of solar energy, not the equatorial regions, as today and as on Earth. So, it would make sense that water might liquify on the warmer slopes, which would, paradoxically, be the poleward-facing slopes, an effect more pronounced as you approached the equator.
            • A French team led by F. Costard published a geographical analysis of 213 gullies that the MGS MOC camera had found: Almost all gullies from -28° to -40° faced south toward the pole, with 4% facing east; from -40° to -60° 55% faced south, 33% faced east or west, and 11% faced north toward the equator; in polar regions, 58 still face south, but 35% face north. This is consistent with the production of average daily temperatures above 0°C during times of high obliquity.
              • Adding more credibility to this approach, Mars at high obliquity happens to have higher atmospheric pressure as more water vapor enters the atmosphere, and the sun-facing hemisphere will have higher temperatures, a combination that allows more plain water to remain liquid on the surface.
              • Detracting from this is the obvious freshness of so many of these gullies, many caught with new activity or newly created between passes of MGS and the MOC imager.
    • Linear fossæ and catenæ
      • There are linear graben type structures in several regions of Mars:
        • Cerberus Fossæ running from the southern parts of the Elysium rise to the southern parts of the Tharsis rise
        • Claritas Fossæ, the rough terrain south of Noctis Labyrinthus, west of Solis Planum, southeast of Tharsis and clearly subject to tension from its development, and northwest of the Thaumasia mountains
        • Alba Patera Fossæ, scoring the entire old volcano and continuing on south-southwest toward the center of Tharsis: It's as though Alba Patera came up first and then the continuing uplift of Tharsis exerted tension on it, too, cutting it with these grabens
      • These often seem to track off radially from the center of Tharsis or, to a lesser extent, Elysium Planum
      • They probably reflect the extensional stress of the material coming up from the hot spots under them, leading to rock failure and faulting, and down dropping of terrain between faults, much as we see in the Basin and Range Province with the tension exerted by the insertion of the Farallon Plate under western North America
      • There are a number of oddly linear patterns of pits, too, called catenæ. The term, "catenæ," is neutral, without a cause implied.
        • So, they sometimes describe impact craters lined up in a row, which you sometimes see when there have been secondary impacts from materials ejected from the primary impact or when an impactor has already broken up before impact, creating a streak of smaller craters.
        • The term can also be applied to strings of pits that are clearly not impact craters, not having the tilted up rim structures: These are just sharp-bordered lines of circular depressions.
        • We talked about them last week, in discussing features found in the Valles Marineris system of chasmata:
          • Pits in Tithonium Chasma
          • Coprates Catenæ
        • Catenæ may be areas related to faulting and graben construction or even the collapse of lava tubes, in which a surface crust is undermined by extensional stresses and perhaps the extraction of some subsurface fluid and subsidence, leading to cave ins and pitting.
    • Folded and faulted mountains of Thaumasia
      • This is about the closest terrain to the classic folded-and-faulted montane topography so common on Earth in areas of conservative and destructive plate margins. There is nothing else like it on Mars.
      • It consists of a central lava plateau, around which are highly deformed highlands reminiscent of Earth mountain ranges.
      • These deformation structures occur at several scales and include both compressional and extensional features. They may reflect uplift associated with the construction of Tharsis, other magma-driven uplifts (e.g., Syria Planum), the opening of Valles Marineris, and the Argyre impact.
      • These processes are quite old, going back to Noachian and Hesperian times.
      • The whole thing kind of looks like the destructive margin of a plate-in-the-making, the constructive edge of which looks like Valles Marineris, and possibly Claritas Fossæ
      • Complicating the picture further, since those times, the landscape has been further altered by volcanic, æolian, and fluvial processes.
      • Probably reflecting similar processes, maybe the leading edge of a thrust that may also be expressed in Thaumasia, is the Coprates Rise
    • Lava tubes
      • These form, as on Earth, when lava flows rapidly along a surface, while the top of the flow is cooling and solidifying, forming a crust protecting the fast flow underneath. This eventually drains out, leaving a tunnel in the lava deposit, which may cave in subsequently, exposing the tube system
      • There are lava tubes all over northeasternmost California on the Modoc Plateau, where Captain Jack held off the U.S. Army for a long campaign of guerrilla raids followed by disappearance into the lava tubes that the Modocs knew all about but the army didn't
      • An example is the ESA Mars Express image of such tubes on the side of Pavonis Mons
    • Layered mesas or mensæ
      • These are all over the transition zone between the highlands of Arabia Terra and the northern lowlands
      • They may represent mesas topped by more resistant layers that form a caprock protecting the stack of sediments or lava flows or consolidated wind deposits below
      • These are attacked by erosive agents, notably wind on Mars, and sculpted into striking patterns wherever certain layers are more resistant than others
      • We see these all over the American Southwest, such as in Monument Valley, Arizona
      • Mars' answer to such structures include the Cydonia Mensæ region and its infamous Face on Mars
      • There's also now the Heart on Mars, a 255 m mesa found in the south polar region by Mars Global Surveyor
    • Yardangs
      • Erosional features created by wind, just as on Earth
      • Sandblasting of features, sometimes with intense sculpturing at base and often creating long, linear features where the wind blows in a consistent direction
      • Given the predominance of æolian processes on Mars, these structures have many similarities with mensæ
      • An example is seen in the HRSC (Mars Express) image of yardangs south of Olympus Mons at ~6° at ~220° with a comparison image in the military illustration of yardang terrain (sometimes called grooved terrain)
      • Another example is seen in the viewgraphs showing a MOC image of Æolis Mensæ (which is just south of Elysium Planum on the margins of the southern hemisphere highlands, around +1° by ~145°), again contrasted with a mystery military image of Earth yardangs
    • Dune fields
      • Depositional features created by wind, just as on Earth
      • On Earth, dominated by silica sand
      • On Mars, dominated by basalt-derived dust, so they are sometimes dark
      • Often found in craters or other depressions, where wind drops its load
      • If the wind comes from a consistent direction and the sand supply is still relatively sparse, it will group the sand into classic barchans, or crescent-shaped dunes with the horns pointing downwind: The viewgraphs show Nili Patera (Syrtis Major just west of Isidis) with its classic dark barchans
      • If the wind comes from a variety of directions, sometimes it will bunch the sand into star dunes, which may have three or sometimes four ridges going out radially from the summit, as we see in the viewgraphs contrasting Opportunity's view of a dune field in Endurance Crater with another military mystery yardang photo
    • Patterned ground
      • Polygons are often seen on Earth in periglacial environments due to the mechanical stresses caused by:
        • Freeze-thaw expansion-contraction of water
        • Expansion-constraction of other materials due to temperature changes, which can be considerable in such environments
        • Water in the active layer above permafrost is drawn by the temperature gradient toward the frozen face of ice at the surface of the polygons, the ground surface, and the permafrost itself, which desiccates the soil into blocks, and this partly explains the chunky look of these patterned grounds.
        • Finer and coarser materials respond differently to these stresses, leading to segregation by particle size, which creates the sorting you see around the edges of the polygons, with the largest clasts in the crevices between hummocks
      • Such patterned ground is found on Mars more and more frequently at higher latitudes, which is what you would expect from a planet with little permafrost or deeply buried permafrost in the tropics, a lot of permafrost closer to the surface in the mid-latitudes, and sitting atop the surface at the ice caps themselves.
    • Evidence of mass wasting
      • Landslides
        • Common on crater gully walls at a small scale
        • Very evident as a major mechanism expanding the chasmata of Valles Marineris
          • We've looked at Coprates Chasma
          • Ganges Chasma
          • Cando Chasma
          • Noctis Labyrinthus to the west of Valles Marineris and seeming at an earlier stage of development and facing different stress fields
      • Chaos
        • Collapsed, jumbled terrain
        • May be source of massive, possibly explosive outflows
        • Catastrophic melting of water or brine, possibly accelerated by carbon dioxide outgassing, totally undermining the floors of canyons and craters
        • Aram Chaos blasting a channel into the side of Aram Crater to get to Ares Vallis looks prototypical, as does Iani Chaos on the other side of Ares Vallis and Aram's crater walls
      • Softened craters
        • Very characteristic of Mars, unlike the Moon (and yet not so far gone as the craters, or astroblemes, of Earth)
        • Crater walls eroded through fluvial and æolian processes and, possibly, marine or lacustrine processes (if the state of the buried craters in the northern lowlands is an indicator)
        • Crater floors fill with æolian debris, which eventually flattens the floor
        • Some may have been filled, too, with lakes and their bed deposits or, as with the Moon, even lava deposits
        • Once filled, these flattened deposits can be eroded by wind
  • Fifth order of relief
    • Boulders and rocks
    • Frost deposits
    • Dust devil tracks
    • Sand and dust streaks
    • Light-toned new gully deposits