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Geography of Mars

Lecture Notes

Christine M. Rodrigue, Ph.D.

Department of Geography
California State University
Long Beach, CA 90840-1101
1 (562) 985-4895
rodrigue@csulb.edu
https://home.csulb.edu/~rodrigue/

Lecture Notes for the Final

  • Noachian surfaces: The oldest
    • See Viewgraphs:
      • "3rd order: Noachian regions, Part A"
      • "3rd order: Noachian regions, Part B"
    • The Noachian covers the period from the earliest formation of the planet during the process of the gravitational accretion, collision, and consolidation of planetesimals, asteroids, comets, meteoroids, and dust, ending when the flux of large bolides eventually fell off.
      • Some people have begun to divide the traditionally understood Noachian into the "pre-Noachian" and the Noachian proper, with the pre-Noachian reserved for the time of planetary accretion, differentiation, and development of the planetary magnetic field.
        • In this view, the pre-Noachian ended at the point where crater saturation doesn't allow you to discern really old surfaces.
          • This would be the bold straight line on the Neumann-Hartmann isochron chart.
          • That line represents a crater density so intense that new craters obliterate old craters, so that you can no longer say one surface is older than another.
        • When the pre-Noachian term is used, it continues up to the time by which the dynamo had clearly shut down (as evidenced by the lack of remanent magnetization in Hellas and other huge impacts).
      • Most commonly, though, the whole period from the time of the planet's origins to the end of the Late Heavy Bombardment is referred to as the Noachian. So, the traditional Noachian includes:
        • Primordial accretion
        • The kinetic, compressional, and radioactive heating of the accreted materials
        • Differentiation begins with melting of these materials and the "iron event," when iron, melting first, began to drift in blobs toward the center of the planet, pulling some siderophiles with it (particularly nickel).
        • Formation of the mantle magma ocean.
        • Formation of a crust on top of the magma ocean, in Mars' case, apparently quite a thick one, for reasons unknown.
        • Mantle overturn because of the gravitational instability created when magnesium-rich olivine cumulates that crystallized out first at the hottest temperatures were overlain by denser iron-rich olivine cumulates that crystallized out later at a somewhat cooler temperature.
        • 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 sys tem was cleaned up of most of the stray smaller objects by the gravitational fields of the early planets.
        • Some differentiated bodies were themselves smashed into asteroids and D meteoroids, giving rise to the many different types of meteorites: chondrites and carbonaceous chondrites from primordial material and achondrites, irons, ? and stony irons from previously differentiated bodies.
        • There was a drastic drop-off in the rate of bombardment around 3.7 or 3.8 Ga, but it may have crescendoed toward the end before tapering off: The Late Heavy Bombardment (peaking between 3.92 and 3.85 Ga).
        • While widely accepted, the LHB does have its critics, who feel that too much was made of the concentration of lunar impact melts in the Apollo rocks, which dated to a narrow interval around 3.9 Ga. Though rock samples were taken from a variety of sites on the moon by Apollo (and the robotic Soviet Luna sample-return program), there is a possibility that these may over-sample ejecta from a single massive impact, the one creating the Imbrium Basin.
        • The LHB, assuming the majority position, may have had a specific cause in the movement of the giant planets Jupiter, Saturn, Uranus, and Neptune from their areas of formation. This argument is called the Nice model ("neese," after l'Observatoire de la Côte d'Azur in Nice, France).
          • Jupiter formed farther out than we see it now, while the other three were formed closer in.
          • Neptune was formed between Saturn and Uranus and migrated outward.
          • The giant planets, in cleaning out their neighborhood of smaller objects, exchanged angular momentum with them, causing shifts in their orbits
          • Jupiter and Saturn eventually attained a 1:2 resonance (Jupiter orbits the sun two times for every orbit of Saturn), and this created great concentrations of their gravitational influences on Neptune and Uranus.
          • Neptune was flung out past Uranus, right into the Kuiper Belt, and it destabilized many of these objects, disrupting their orbits.
          • Some were thrown into very inclined orbits (such as Pluto), some entered into resonances with Neptune (such as Pluto, with a 2:3 resonance with Neptune), others were scattered to the farthest reaches of the solar system.
          • And a whole bunch of them were flung into the inner solar system, where they hammered Mars, the Earth and Moon, Mercury, and, presumably, Venus.
          • If you'd like to learn more about this:
            • Here is an animation of the Nice model, with plenetesimals shown in green, Jupiter's orbit in red, Saturn's in orange, Uranus' in purple, and Neptune's in blue: http://www.psrd.hawaii.edu/WebImg/LHB-sim-small.gif
            • Here is one of the original presentations of the Nice model:
              Gomes, R.; Levison, H.F.; Tsiganis, K.; and Morbidelli, A. 2005. Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets. Nature 435, 7041 (26 May): 466-469. doi: 10.1038/nature03676.
          • This hellacious era went on until about 3.9 to 3.7 billion years ago in most accounts and as "recently" as 3.5 billion BP in others: There are still a lot of controversies about when, exactly, the Noachian drew to a close. The most commonly cited time in recent writings is 3.8 or 3.7 Ga.
      • The Noachian roughly corresponds with the Hadean time on Earth (4.6 to 4.0 Ga) and the early Eoarchean era (4.0 to 3.6 Ga), but, unlike on Mars, we have very few rocks on Earth that date from this time because of the intense geological activity here.
        • Well, let me qualify that: There are a few grains of zircons that old on Earth, small crystals that were once part of igneous and metamorphic rocks.
        • Zircon contains some uranium, thorium, and lead, the ratios among which has allowed them to be radiometrically dated to as old as 4.4 billion years on Earth, in the case of the Jack Hills zircons from Australia!).
        • There's been a controversy more recently about the age of actual metamorphosed mafic/ultramafic rocks in Canada that might be nearly as old as these zircons: the Nuvvuagittuq greenstone belt just east of Hudson Bay in northern Province Québec. These have been dated to 4.3 Gya but the results are contested with claims that they're no older than a "mere" 3.8 billion years old.
        • So, where on Earth Hadean eon materials consist of a very few zircon grains and a controversial claim for Canadian greenstones, on Mars, roughly 40% of the planetary surface dates back to the comparable Noachian (Barlow 2010). Here is the Barlow map of martian surface ages: http://bulletin.geoscienceworld.org/content/122/5-6/644/F8.large.jpg.
      • So, while Mars is geologically active, it's nowhere near the level of activity seen on Earth with its plate tectonism, and that has allowed the preservation of ancient surfaces on Mars and their obliteration on Earth (except for those zircons and maybe the Canadian greenstones)
      • The constraint on the Noanchian timeframe is based on analysis and dating of Moon rocks from similarly cratered surfaces brought back to Earth by Apollo.
        • This is a fairly elaborate reasoning process. Rocks were taken back to Earth from the Moon by the Apollo astronauts from regions that had been previously relative-dated by crater-counting techniques. The returned rocks, then, allowed for an absolute date to be assigned to surfaces of previously described as of particular relative dates.
        • Then, the size-frequency curve for the Moon had to be calibrated for use on martian surfaces, factoring in Mars' atmosphere (which would both destroy more of the smaller objects and slightly reduce their incoming velocity), Mars' location closer to the putative source of orbiting debris in the solar system (closer to the asteroid belt and to Jupiter, the gravity of which dislodges objects and puts them on new orbits, including orbits that intersect the inner solar system bodies).
        • You can get an overview of the Moon to Mars isochron correction system (optional link for the curious: http://www.psi.edu/epo/isochrons/chron04b.html).
      • Characteristics of Noachian surfaces
        • 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 a lot of geomorphic reworking:
          • Very distinctive softening of the rims, as though they'd sagged and spread out.
          • Hardened ejecta blankets with that "wet splat" look, sometimes with two or more layers of flowing ejecta, sometimes with the kinds of striations produced by very rapid and fluidized movement, often ending in a rampart edge.
          • Some of these craters were clearly buried by wind or water deposits, and then subsequently re-exposed by erosion as pedestal craters perched like crater-dented mesas high above the remaining landscape level.
          • floors flattened by the deposition of alluvial, lacustrine, marine, or æolian materials in them: You do not see that on the Moon, which lacks such familiar geological activities as wind and water erosion, transport, and deposition.
        • There was quite a bit of this geological work back in Noachian times:
          • Valley networks are found almost exclusively on Noachian surfaces, showing fluvial action by what is more and more accepted as water, even precipitation-fed channelization.
          • There was some early and distinctive vulcanism in the highlands, featuring plains formed from very low viscosity lavas (flood basalts, possibly emanating from long rupes or fossæ), small cones, and very shallow-sided vent-volcano edifices (pateræ).
          • Later in the Noachian, volcanic activity became increasingly concentrated in the two great volcanic rises, Tharsis and Elysium, which built up at this time. The viscosity of lavas associated with the later volcanism allowed the construction of very tall shield edifices and, in some cases, ashy eruptions were part of the mix, which allowed the construction of steep sided tholi.
          • The Late Noachian saw such extensive and massive volcanism that global geochemistry was drastically changed.
            • Early Noachian geochemistry was dominated by phyllosilicate chemistry (alteration of basalts in water to liberate silicas, including the kind of one silicon/four oxygen tetrahedrons that produce micas, talcs, and clays).
            • Late Noachian and Early Hesperian geochemistry shows a strong sulfate signal, as volcanoes spewed out massive amounts of sulfuric acid, carbon dioxide, and water and created a strongly acidic aqueous chemistry.
            • This would explain the near lack of calcium carbonate on Mars: The presence of sulfate (SO2-4) and sulfur dioxide (SO2) prevents the formation of calcium carbonate and favors the formation of hydrated calcium sulfite (CaSO3 - H2O) instead, which can oxidize to create sulfates, iron oxides, and more acidity.
            • Most of the arguments about possible oceans on Mars place it in the Noachian time frame, and, given the previous argument about sulfate chemistry, if those oceans were strongly acidified, the lack of calcium carbonate on the putative ocean floors becomes more comprehensible.

      • Tour of Noachian regions
        • I'll use a "walkabout" style of presentation, starting with the type province (Noachis Terra) just west of Hellas Planitia and then go generally west through Aonia Terra, Terra Sirenum, Terra Cimmeria, to Promethei Terra, which takes us back to Hellas Planitia. From there, we'll swing up north to Terra Tyrrhena and then go west and northwest into Terra Sabæa, Arabia Terra, Margaritifer Terra, Xanthe Terra, and then Tempe Terra, leaving us northwest of Alba Patera.
        • Noachis Terra, the prototype, is a large region to the west of Hellas and east and north of Argyre.
          • This is a contender for the greatest crater density on Mars prize.
          • Mariner 4 got images of Noachis Terra during its flyby, which created the (then rather shocking) image of Mars as a dead, dry planet much like the Moon.
          • Subsequent closer looks showed it to be a lot more interesting:
            • The craters themselves turned out to be pretty strange
                They often have softened rims and flattened floors, including some "ghost craters" that are so softened and infilled that they have practically vanished.
              • Softened craters turned out not to be the result of standard-issue erosion and deposition mechanisms: It's as though entire landscapes of old craters sagged, spread out, and flattened, but new craters haven't.
              • This suggests that there was a lot of soil moisture and ice back then, which could flow, deform, and relax, softening the look of the ancient craters.
              • Pedestal and rampart craters were found here, too.
            • These look like impacts into surfaces loaded with ice, which vaporized and liquefied on impact, creating that "wet splat" look.
            • The ejecta blankets appear to have solidified as a particularly resistant material, which functioned kind of like a cap rock of resistant material.
            • Erosive agents attacked the surrounding landscape, but the area under the ejecta blankets was protected from whatever the regionally dominant erosive agent was, leaving the crater and its ramparts of ejecta perched high above the worn-down landscape, kind of like mesas with holes punched in the top.
          • Drainage networks that looked like fluvial systems on Earth showed that water or some other similar fluid ran over martian landscapes and eroded them.
            • Long networks featuring several tributaries, most of them fairly short with few of their own tributaries, such as Nirgal Vallis
            • Several smaller drainage basins with relatively long tributaries and drainage densities larger than the Nirgal Vallis system's but smaller than typical for Earth catchments and with nowhere near the degree of interfluve dissection common on Earth
            • The origins of such valley networks have long been contentious.
              • Some authors argue for a precipitation-fed runoff history and the evidence their existence gives to arguments that Noachian Mars had higher atmospheric density and warmer temperatures, allowing at least for snow to fall and liquid water to exist long enough to flow overland into drainage channels (e.g., Gulick and Baker 1990; Ansan and Mangold 2006). This would pertain to the dendritic drainages.
              • Others have pointed out that most such networks have fewer, shorter tributaries than most Earth valley networks and that many of the short tributaries originate in alcoves or theater-shaped headwalls most akin to the slope morphologies of groundwater sapping-fed networks in arid Earth environments (e.g., Laity and Malin 1985; Malin and Edgett 2000)
              • Such morphologies can also be produced by meltwater from under snow or ice cover even in very cold, arid conditions, as seen at a small scale in and around Haughton Crater on Devon Island in northern Canada (Lee et al. 1999).
        • Aonia Terra southwest of Noachis Terra and Argyre Planitia
          • Its central areas are classic Noachian landscapes, highland cratered units with many small dendritic valley networks.
          • There is much evidence of contemporary æolian processes, including large dune deposits at the base of some crater rims, with some evidence of dunes overtaking older dunes trapped against a topographic barrier.
          • Much of the Aonian cratered landscape shows signs of being subdued in contrasts, very akin to the crater softening and flattening seen in the discussion of Noachis Terra.
          • There is a heavy profusion of larger craters in Aonia Terra, many showing the pedestal structure seen in Noachis Terra. The pedestals preserve craters on a surface once higher than today's, about 500 m higher (Head et al. 2003), which was eroded away around the craters and their resistant ejecta blankets by, presumably, meltwater from once larger polar ice deposits.
          • Aonia Terra has extensive development of Hesperian aged flat and rather featureless plains, particularly in the northern part of the region just south of the Tharsis mountains. These have been interpreted as being comprised of thick beds of alternating lava flows and æolian deposits that have buried underlying terrain (Scott and Tanaka 1986).
        • Terra Sirenum west of Aonia and south of Tharsis
          • Terra Sirenum is a profusely cratered basaltic terrain of the Southern Highlands, located to the southwest of the Tharsis rise.
          • It shows a diversity of surface ages, though the preponderant surface exposure is Noachian
          • There are several large craters with diameters exceeding 100 km and some exceeding 300 km.
          • Again, we have the valley networks of apparent fluvial origin
          • A particularly striking feature of Terra Sirenum and its neighbor, Terra Cimmeria, was revealed by the Mars Global Surveyor magnetometer: marked linear bands of alternating remanent magnetization, trending east-west across these two adjacent regions.
            • Linear magnetic bands like Earth's spreading zones that record polarity changes in our planetary magnetic field?
            • Accretion of terranes through plate tectonics, each with a different magnetic signal from the long-vanished martian magnetic field?
            • Intrusion of magnetite/ilmenite dikes associated either with rift zone spreading or some other magmatic source?
        • Terra Cimmeria northwest of Sirenum
          • In many ways, Terra Cimmeria is essentially the westward extension of Terra Sirenum into the eastern hemisphere, out to ~ 120° E: It shares the same common range of elevations, the same general distribution by size class of ancient craters, and, with Terra Sirenum, houses the same east-west bands of remanent magnetization, and it is rarely discussed without its neighbor.
          • It retains a separate name as its inheritance from the names given to albedo features seen from Earth in the nineteenth century.
          • It made news in its own right when an aurora was recorded by ESA's Mars Express SPICAM instrument (Bertaux et al. 2005) at 177deg; E at -52°.
          • It also was the destination of Mars Exploration Rover, Spirit, which landed in Gusev Crater at the end of Ma'adim Vallis in the northeastmost corner of Terra Cimmeria.
            • Ma'adim Vallis is, like Nirgal Vallis discussed under Noachis Terra, a long channel with several short tributaries suggesting some sort of sapping process more than the dissection of a fluvial network fed by precipitation and spring flow.
            • It may have had at least one jökulhlaup massive outflow episode.
            • Its morphology and the presence of delta-like deposits in southern Gusev Crater led to the selection of Gusev Crater as the landing site for the Mars Exploration Rover Spirit in the hope of finding sedimentary deposits.
            • Spirit landed, instead, on a basaltic lava flow, probably from Apollinaris Patera to the north, which was emplaced after the Ma'adim Vallis flows
            • Water-altered strata were not found for 159 sols until Spirit reached an outcrop of groundwater-altered volcanic ash exposed in the Columbia Hills.
            • This was quite a "Mars, the yes, but ..." planet scenario.
        • Promethei Terra just east of Hellas Planitia
          • West and southwest of Terra Cimmeria, Promethei Terra lies adjacent to the eastern margins of Hellas Planitia.
          • Like all the Noachian regions, it is generously covered with craters in a profusion of size ranges
          • The landscape features ancient rugged highland terrain interspersed with lower elevation basins filled with sediments eroded and transported from the highland massifs.
          • In southernmost Promethei Terra is a roughly half-circular ridge, Promethei Rupes, which is evidently the remnant of a very large impact basin now mostly covered by Planum Australe.
          • Evidence of valley networks is apparent, as well, and northernmost Promethei Terra is the source region for one of the great outflow channels debouching in eastern Hellas: Harmakhis Vallis, its tributary Reull Vallis, and the tributary of the latter, Teviot Vallis.
          • The region is quite dusty, and dust piles up in great beds dozens of meters thick in many a crater.
          • Lighter coverings of dust often show networks of ornate dark streaks and curlicues, which were shown to be dust devil tracks disturbing the dust and exposing the basalt below, a phenomenon first clearly documented in the process of formation in Promethei Terra and since found all over Mars.
          • Many of Promethei Terra's craters are dramatically softened, with eroded or sagging rims and floors flat with infill. This has long been posited as the result of a large amount of interstitial soil ice and permafrost close to the surface that has undergone viscous relaxation over time, the surface layers flowing and deforming in lineated and lobate structures, sagging and creeping into arcuate ridges in valleys and crater bottoms.
          • Interestingly enough, these thaw/melt/flow features were pole-facing at latitudes less than 45° and equator-facing at latitudes greater than 45°, reflecting a dependence on total solar radiation rather than intensity of solar radiation.
          • Total solar radiation is affected, not only by slope aspect with respect to sun angle as it varies over the course of the day and the seasons, but with changes in orbital eccentricity and obliquity.
          • Evidence of glaciation during Mars' last high obliquity phase about 5.5 Ma are abundant in Promethei Terra, including a particularly striking hourglass-shaped pair of craters with a fill showing flow lines leading from the higher to the lower, which turned up in HRSC imagery.
        • Terra Tyrrhena north of Hellas Planitia and south of Isidis Planitia
          • Like most Noachian surfaces, Terra Tyrrhena's is a crater-littered landscape, its central plateau dating back to the Late Noachian and Early Hesperian and its surrounding lower elevation plains made up of younger Hesperian materials, largely volcanic.
          • Many of the craters show substantial filling and flattening of the floors and erosion of the rims.
          • The region shows signs of fluvial dissection in the zones between the older highlands and the younger lower elevation surfaces, with well-developed and often well-integrated valley networks, with tributary systems attaining up to the fourth order in the Strahler system of stream ordering.
          • Unaltered olivine of the original Noachian surface rock is shown in CRISM spectroscopy, sometimes covered with somewhat altered lavas but then excavated by impacts. Olivines are very rapidly altered in the presence of water into such minerals as serpentine, goethite, iddingsite, or hæmatite.
          • The team operating the OMEGA spectrometer on the European Space Agency's Mars Express found the first clear evidence of phyllosilicates exposed in crater walls and in eroded ejecta blankets around craters in Terra Tyrrhena, notable for the dependence of phyllosilicate formation on the interaction of rock with abundant neutral to high pH water. Phyllosilicate clays are alteration products of fairly neutral water acting on basalts.
          • Subsequent work has shown that the phyllosilicates are widely distributed on Mars, but only on Noachian terrain, such as Terra Tyrrhena, and of a diverse range of specific minerals (Marble et al. 2008).
          • The presence of phyllosilicates and the neutral or somewhat alkaline aqueous chemistry they indicate goes against the impression created by all the unaltered olivine and basalt. That "yes, but ..." quality again.
        • Terra Sabæa northwest of Hellas Planitia
          • Terra Sabæa is a heavily battered low albedo landscape located northwest of the Hellas Planitia rim and wrapping around Syrtis Major to its east.
          • It shows a wide range of crater sizes, again in nearly saturated profusion, as well as a number of Late Noachian fluvial valley networks.
          • Terra Sabæa, of all the Noachian regions, seems underrepresented as a setting for particular investigations, as I found out when I did my secondary crater prospecting study there.
        • Arabia Terra northwest of Hellas Planitia, north of Noachis Terra, and east of Chryse Planitia
          • Crater density is so great here, vying with Noachis Terra for the greatest densities on the planet, that superposition breaks down as a method of picking out the oldest craters.
          • The region is bounded to the north by the transition scarp down to the Northern Lowlands but, here, it is far less distinct and more fretted and intricately graded than it is in other parts of Mars.
          • The crust is considerably thinner under Arabia than under other parts of the Southern Highlands, too, more akin to the crust under the Northern Lowlands.
          • The northern and western portions of Arabia Terra are distinctive for areas of older cratered terrain, "inliers," standing isolated as buttes and mensæ towering over the far lower terrain comprising the bulk of the landscape there.
            • These inlier features often expose marked layering, as, for example, in Cydonia in northwestern Arabia and its infamous "Face on Mars" mensa.
            • The layering suggests burial of a Noachian surface and then its exhumation from under younger materials.
          • Construction of a 1 m resolution digital terrain model from the Mars Reconnaissance Orbiter's HiRISE instrument's stereo images permitted Lewis et al. (2008) to construct detailed topographic profiles of bedding outcrops in four Arabia Terra craters and measure layer widths.
          • Beds show rhythmic variations in width, which authors attribute to extraplanetary climate drivers, such as changes in orbital eccentricity, precession, and obliquity.
          • Arabia Terra's rhythmic sedimentary layers, then, join the polar deposits as potential archives of martian climate change and calibration of the crater counting based geological record.
        • Margaritifer Terra east of Valles Marineris, west of Arabia Terra, north of Noachis Terra, and south of Chryse Planitia
          • About 60% of its area is comprised of surviving heavily cratered surfaces of Noachian age.
          • It is quite distinctive, however, for the concentration of outflow channels and chaos terrain.
          • Most of these show the reduced cratering of Hesperian age surfaces.
          • Margaritifer Terra collected outflows from the following sources:
            • the eastern end of Valles Marineris (Hesperian outflows)
            • the Chryse Trough drainages (probably Noachian fluvial systems of varying connectivity and continuity)
            • sources internal to the region, in the form of the many chaos terrains that themselves would have created massive jökulhlaup-like outflows during the Hesperian:
              • Auroræ
              • Pyrrhæ
              • Asrinoses
              • Aureum
              • Margaritifer
              • Iani
              • Hydraotes
              • Hydaspis
              • Aram chaoses
          • The outflow channels cutting through Margaritifer Terra do not show the dendritic structure of precipitation-derived surface and groundwater-fed fluvial networks, such as the many small valley networks seen on Noachian surfaces and such channels as Ma'adim Vallis and Nirgal Vallis, respectively.
            • That is, they do not originate in a series of low-order streams fusing their flows into progressively higher-order, larger discharge branches and trunks per Strahler.
            • Rather, they originate in chaos terrain and emerge at full width below it, which they generally substantially preserve throughout their lengths, dwindling only far downstream.
            • They show close to U-shaped or even box-shaped cross-sections, which suggests massive, sudden, and probably short-lived flooding of the jökulhlaup character, perhaps triggered by warming of subsurface ices by magmatic intrusion, perhaps in a system of dikes.
            • Indeed, the outflow channels of Margaritifer Terra, Xanthe Terra, and Lunæ Terra have been characterized as, by far, "by orders of magnitude the most voluminous known fluid-eroded channels in the Solar System (Rodriguez et al. 2007).
            • The chaos features at the heads of these channels and the hummocky, lineated, terraced lower reaches have been characterized as thermokarstic on the basis of Earth analogues in Siberia.
          • One of the "yes, but ..." qualities of these massive outflow channels, here in Margaritifer Terra and in the other borderlands of Chryse Planitia is the question about how much atmospheric density Mars would have had to have to sustain that much liquid for the duration of the jökulhlaup-type event and for what looks like ponding or pooling of this water in the Northern Lowlands. When would Mars have been above the triple point of water? Noachian times, but the crater density pattern in the Margaritifer Terra outflow channels is much lighter and not as diverse in size as we see on Noachian surfaces: They are more in line with Hesperian times.
          • This discrepancy has fed skepticism that the fluid involved was, in fact, actual water.
            • Probably the most commonly proposed fluid is brine. The brine would be comprised of the chlorine and sodium in magma, which can combine to form salt or sodium chloride. Under impact gardening conditions, this salt would be joined by calcium and magnesium and other elements in subsurface water to form a very complex brine, and such brines have very low freezing points, in some cases as low as 225 K or -48° C.
              • Concentrated brines might thus account for the ability of subsurface fluids to sustain the kinds of flows in the outflow channels of Margaritifer Terra, as well as the small seeps and gullying witnessed even today on martian crater walls (Knauth et al. 2001; Knauth and Burt 2002).
              • The brines would be activated by subsurface warming, perhaps due to magma intrusion regionally or in the form of dikes ascending into frozen soil brine.
            • Another scenario proposed by Nick Hoffman is that the fluid involved is actually carbon dioxide: Carbon dioxide sublimes extremely violently upon depressurization and the kinds of flows seen in Margaritifer Terra and Valles Marineris could have been gas-supported flows more like pyroclastic flows in their behavior. Talk about popping the soda bottle after too long a trip in a car with bad shocks!
            • Still another proposal is that lava deposition during the formation of Tharsis could have heated thick underlying deposits of hydrous sulfate evaporites, as may be exposed, for example, in the walls of Valles Marineris. If so, the heat could have caused dehydration of the evaporites and segregation of the water, which would have increased their volume and thereby pressured the segregated water into explosive release (Montgomery and Gillespie 2005).
        • Xanthe Terra west of Margaritier Terra and northeast of Valles Marineris
          • Like any Noachian landscape, Xanthe Terra is badly battered with a wide range in sizes of craters, including many with diameters in excess of 10 km.
          • Like Margaritifer Terra next door, Xanthe Terra also has a number of outwash channels crossing it but in a more organized north/northeast direction:
            • Shalbatana Vallis to the east, originating in chaos terrain north of Ganges Chasma (site of Lab 1)
            • Maja Vallis, a large channel originating in Juventae Chasma. Maja Valles comprise Xanthe Terra's western boundary (with Lunæ Terra)
            • Nanedi Valles, a smaller channel system between Shalbatana Vallis and Maja Valles
            • There is some discrepancy in regional usage in the literature: Some authors refer to Tiu Vallis, Simud Vallis, and Ares Vallis as part of Xanthe Terra, using Ares Vallis as the eastern boundary, but the USGS, NASA, and IAU, in the planetary gazetteer prefer to confine Xanthe Terra to the area directly north of eastern Valles Marineris. I'm only mentioning it here, because you may encounter discrepant usage in older writings.
          • While many Noachian regions show modification of the basic crater-pocked surface by wind, ground ice, groundwater, or surface water, Xanthe Terra, however delimited, shows an unusual concentration of different types of modification:
            • groundwater sapping
            • weak fluvial network development
            • the massive outflows (jökulhlhaup) mentioned earlier
            • permafrost
            • chaos terrain formation (subsurface fluid withdrawal and surface collapse)
            • volcanism
            • landslides
            • æolian processes
            • diagenetic processes (alteration in situ) generating layering that resemble the stratigraphy of sediments or volcanic flows
        • Tempe Terra far northwest of Xanthe Terra and northeast of Tharsis.
          • Lying well to the northwest of Terra Xanthe and off the northeastern edge of the Tharsis Rise, Tempe Terra is the northernmost reach of the cratered Southern Highlands and a kind of isolated outlier of Noachian territory.
          • It is the lowest lying of the "highlands," as well, with perhaps half of the region lying below the geoid, though Tempe Terra surfaces may stand as high as 3-4 km above nearby Northern Lowlands elevations.
          • The transition scarp is quite steep in northern Tempe Terra.
          • As in Arabia Terra, much of the transition from the cratered highlands to the lowlands is marked by taller mensæ and knobs separated by swaths of lower and smoother terrain that descend to the lowlands in a series of steps.
          • Though much of the region lies below the geoid, the southwestern and central portions are mostly above, and there are areas in the center and scattered along the west and south that reach above 1,000 m.
          • Making Tempe Terra quite distinctive among the Noachian regions is the presence of long and sometimes sinuous fossæ, as well as catenæ (linear arrangements of subsidence pits often set off by extensional stresses), which fan out from a common center on the northern portion of the Tharsis rise, e.g.,
            • Mareotis Fossæ
            • Tempe Fossæ
            • Labeatis Fossæ
            • the largest catena is Baphynis Catena
          • This is the region of Mars showing the greatest seismic strain, according to a team headed by Matthew Golombek, who measured fault throw and distortions in the shape of craters to estimate the degree of extensional stress in Tempe Terra.
        • So, that concludes our tour of Noachian Mars, which comprises much of the planet. Each of these terræ, plana, and planitiæ share dense cratering approaching saturation levels, with a "striking" mix of different crater diameter sizes. It is in Noachian Mars we see valley networks, some of them quite dendritic and approaching fourth level stream order. There are also quite a few long trunk/short tributary systems with theatre-headed alcoves in their upper reaches, possibly groundwater-seepage systems akin to those in the American Southwest. Noachian Mars also shows a range of volcanic types, from the common flood basalts and ridged plains to shield volcanoes and some steep-sided volcanic edifices, with evidence that concentration of magma sources had taken place over the Noachian, culminating in its concentration in the two great volcanic rises, Tharsis and Elysium. Many of the ancient Noachian surfaces have been reworked, sometimes dramatically, by various geomorphic agents, such as wind, fluvial processes, glaciation, volcanism, and permafrost in more recent Hesperian and Amazonian times.

    • [ orthographic image of Mars on a black background ] [ Olympus Mons seen at oblique angle that gives a 3-d sense ] [ Mars explorer ]

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    This document is maintained by Dr. Rodrigue
    First placed online: 01/15/07
    Last updated: 04/07/16