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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

  • Second order of relief: gigantic features and the dominant processes shaping the martian surface
    • Mars' surface physiography shows conspicuous evidence of several geomorphic processes: impacts and cratering, volcanism, rifting, glaciation, hydraulics, and æolian processes.
      • These processes have created enormous landscape features, some visible with telescopes from Earth, which constitute the second order of martian relief.
      • These features are generally between 1,000 km to 8,000 km in diameter or length: four enormous impact basins, the other great volcanic rise, the Valles Marineris rift system, the possible mega-slide of Thaumasia, the polar ice caps, the Chryse Trough drainage system, and the Syrtis Major wind-cleared basalt region.
      • Together with the first order great crustal dichotomy and Tharsis Rise, these second order features provide a framework on which to hang an increasingly detailed mental map of Mars.

    • The great impact basins
      • See Viewgraphs:
        • "2nd order: Great craters (Hellas, Argyre)"
        • "2nd order: Great craters (Isidis, Utopia)"
      • Much of Mars is cratered, but there are four impact craters that stand out by their tremendous size, ranging from 1,500 km to 3,300 km in diameter. They also feature positive gravitational anomalies (mass concentrations, or mascons, coïnciding with topographic lows), which seem counterintuitive, given the tremendous excavation of mass from them.
      • Hellas Planitia
        • This crater spans about 50° of longitude and 30° of latitude, centered about -42° at 70° E.
        • It is 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
        • Indeed, the material blasted out of Hellas accounts for a large share of the higher elevation of the Southern Highlands over the Northern Lowlands, according to Arden Albee (2000, Annual Reviews of Earth and Planetary Science). It amounts to several hundred kilometers in width by some 2 km in depth.
        • This argument is what got me to thinking that, if Hellas could disgorge this much ejecta, wouldn't the Northern Lowlands impactor have deposited vastly more ejecta, perhaps accounting for a very significant share of the raised elevation of the Southern Highlands?
        • The Hellas event 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 such remanent magnetization, so it formed after the collapse of the Martian magnetic field
        • The crust is very thin here, < 10 km thick, perhaps as little as 7 km, related both to the explosive excavation and to the rebounding of the mantle afterwards.
        • Hellas went through extensive reworking after its excavation:
          • It may have contained a great inland sea, with a volume about two thirds that of the proposed Northern Lowlands ocean.
          • The floor deposits are largely Hesperian in age (younger than the Noachian times of its formation, but older than the Amazonian age of the Northern Lowlands surface)
        • Hellas shows all sorts of interesting erosional and depositional landforms expressing this complex geological history:
          • Depositional:
            • Volcanic wrinkle ridges and pyroclastic flows
            • Mass wasting/landsliding
            • Fluvial alluvial fans
            • Lacustrine/marine layered deposits
            • Æolian dunes
            • Ground ice or glaciers
          • Erosional:
            • Fluvial outflow channels
            • Lacustrine/marine shorelines
            • Æolian yardangs
      • Argyre Planitia
        • Another girnormous crater centered around -49° lat. and 318° E. lon.
        • It's not as large as Hellas, with a diameter about 1,800 km and a depth of 5 km
        • It is visually distinctive due to the rugged mountain massifs that form ring and radial fretting patterns around the floor of the crater.
        • The radial pattern is enhanced by five major channels flowing into and out of the basin: four entering from the south and one flowing out of the north rim.
        • Muddying my tidy nested regionalization scheme, apparently, Argyre, a second order feature, is involved in another second order feature I'll discuss later, a tremendous seemingly 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.
        • Many of the same erosional and depositional features seen in Hellas Planitia can be found in Argyre Planitia
          • Dunes trapped on crater floors: Easily transported into craters but nearly impossible to be lifted out
          • Dust devils do try, though, and there are dust devil tracks in Argyre
          • Complex layering, possibly of lacustrine deposits, lavas, dust, which then get differentially eroded, nowadays mostly by wind
          • Gullying is seen in Argyre: water? brines? carbon dioxide dry ice?
        • Of the four great impact basins, the floor of Argyre is the oldest, judging from the superposed crater density, probably late Noachian in age.
      • Isidis Planitia
        • Isidis is the third of the great impact basins found in the Southern Highlands, but, unlike the previous two, it is found right on the first order crustal dichotomy border, again kind of messing up my tidy "orders of relief" scheme.
        • It's centered roughly at 15° at 90° E.
        • Very distinctively, Isidis has almost no remnant of its northern and northeastern rim structure: The crater opens out onto the Northern Lowlands over a gradual rise of only about 500-600 m from the lowest point of the crater floor.
        • It also has the thinnest crust of the four great craters, ~6 km.
        • It also features a higher level of post-impact fill, nearly 3 km deep, giving it the flattest floor of the three, with a slope about 0.015°, tilting down toward the southwest and then reversing to form a smooth but steeper slope rising to the southwest into Syrtis Major.
        • There has been energetic debate about what the nature of that flat fill is.
          • One group argues that this is a basalt flow from the Nili and Meroë Patera volcanoes in the Syrtis Major region to the west the likeliest sources
          • Others point out that most of the basin tilts downward toward the southwest, so that would be weird if this were lava from those volcanoes.
          • Another argument against the Nili and Meroë Patera volcanoes is that their lavas have much greater surface roughness than the Isidis fill.
          • There's been speculation that the fill might be catastrophic debris flows triggered by Syrtis Major volcanic dikes interacting with ice-rich soils, particularly ices rich in carbon dioxide. That interaction would trigger an explosive outflow, perhaps destroying the crater's northeastern rim.
          • Going against that idea, though, is the lack of chaos terrain and channeled outflows of the sort we see farther west in the borderlands of Tharsis.
          • Still others think that, however that northeast rim was broken, its failure allowed a marine intrusion from the posited Northern Lowlands ocean, kind of a big lagoon, complete with smooth marine deposits.
          • The various positions on this debate are summarized in Hiesinger and Head, 2004, Lunar and Planetary Science Conference.
        • Like the previously discussed craters, Isidis has a very complex geological history: volcanic, marine, permafrost, mass wasting, and æolian features
        • Given the possibility of ocean incursion from the Northern Lowlands through the blown-out norther rim of Isidis, some of the coastline suggestions (the Deuteronilus Contact) have been traced into Isidis.
        • Among these are a lot of dunes forming fields with ripple structures
        • It has a high density of smaller craters. It is probably younger than Hellas, though, basically puncturing its annular ring.
          • However, many of these craters are eroded mounds with pits at the top
          • Their appearance suggests that there was once some kind of sediment or other filling in Isidis even higher than it is now, which was then smacked by craters, which consolidated the areas around the impacts under the ejecta blankets. If the area contained permafrost, the result was that "wet-splat" rampart crater.
          • Later, erosion (wind?) removed whatever these beds were, leaving the consolidated crater rims to stick out more and more prominently above the lowering floor: pedestal craters.
        • This intriguing crater is where Beagle 2 was to land on 25 December 2003
      • Utopia Planitia
        • A lava plain in the northern lowlands, located roughly at the antipode from Argyre, about 46° lat. and 119° E lon.
        • This is where Viking 2 landed in 1976.
        • This is 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
        • This is also the landing site of the Tianwen-1 lander and Zhurong rover, which are working to the southwest of Viking 2 and west of Elysium Rise, about halfway from Viking 2 to Isidis Crater. A map of all landers and rovers can be accessed here.
        • Zhurong has identified hydrated minerals there in sediments that date to the Amazonian (relatively recent), probably materials altered by groundwater rather than surface water.
        • The consensus now is that Utopia Planitia is a humongous crater buried in whatever it is that resurfaced the Northern Lowlands. This was first proposed in 1989, when G.E. McGill published an article in JGR arguing from geomorphic evidence that there was some kind of circular structure buried in the Northern Lowlands. His argument has basically received increasing support with every new data source collected on it, though there are still some holdouts saying that not all alternative explanations have been systematically ruled out.
        • There are odd circular grabens on that Northern Lowlands surface material.
          • 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?
        • If this is, indeed, a crater, it is the largest of the four discussed here as second order features at 3,300 km across (conservative estimate) to 4,700 km across (more inclusive definition).
        • It is, moreover, covered by the buried "quasi circular depressions" that MOLA and Mars Express have found all over the Northern Lowlands, revealing an ancient surface under that smooth resurfacing. Since, the resurfacing is newer, Amazonian material and the QCD are necessarily much older (probably Noachian like much of the Southern Highlands), then something buried under them is older still.
        • Utopia is far from the equator and gives a lot of evidence of ice-related features and processes:
          • Viking 2 documented the first evidence for the frequent formation and sublimation of frost and ice fogs.
          • There's patterned ground, the polygons often seen on Earth over permafrost.
          • There's evidence of sublimation of subsurface ice in the form of scalloped pits and thermokarst.
          • There are lobate debris aprons of the sort you see in solifluction affected Arctic terrain.
          • Pedestal craters are found here and in other high latitude locations: An impact crater sits at the top of a mesa several times wider than it is, surrounded by a steep scarp that perches the whole landform dozens of meters above the surrounding plains. These have been interpreted as impact-hardened ground and ejecta blankets set in a soil substrate susceptible to æolian erosion.
          • There is also evidence that groundwater has operated in Utopia Planitia, altering and hydrating minerals there all the way into Amazonian (recent) times (Kornei 2022).
    • [ 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: 10/26/22