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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
    • Previously, we discussed the great impact craters of Mars.
    • The next second order features are endogenic in character, originating from tectonic processes (if not plate tectonics), that is, processes internal to the planet, which tend to increase topographic contrast:
      • Volcanic processes: The other great volcanic rise (Elysium)
      • Rifting: Valles Marineris
    • Here, we'll go over the two features first seen on the martian surface: the Blue Scorpion and the polar ice caps
    • See Viewgraphs: "2nd order: Syrtis 'Blue Scorpion, Polar Ice Caps"

    • Syrtis Major "Blue Scorpion" and æolian processes
      • This feature was the first martian landform recorded in a sketch map drawn by Christiaan Huygens in 1659 (and, debatably, as early as 1636 by Francisco Fontana)
      • It is that large, triangular low albedo object that dominates the area west of Isidis Planitia and north of Hellas Planitia, connected loosely to a band of low albedo surfaces in the Southern Highlands.
      • The feature is persistent, though the edges shift around through time.
      • Its dark color and stability invited early speculations about an ocean or vegetation-dominated area, seeming greenish or blueish from Earth in contrast to the bright orange/ocher light albedo areas surrounding it.
      • Orbiter imagery has revealed it as a volcanic province (lavas from Nili Patera and Meroë Patera in Syrtis Major Planum, which has been swept clean of dust by a prevailing northeast wind (winds are named for the direction from which they blow).
      • One of the striking demonstrations of this prevailing wind pattern is imagery of craters on the lava, which feature bright tails of dust deposited in the lee of the crater rims: Winds deflecting around an obstacle rejoin leeward of it, creating cross-interference, which reduces the resultant velocity of the wind, and this reduces its carrying capacity for supporting dust, which then deposits in the low-energy zone leeward of the obstacle.
      • This persistent prevailing wind seems related to the global circulation of Mars as distorted by topographic effects (deflection of the global circulation's wind systems by the Tharsis Rise).

    • Polar ice caps
      • North Polar Ice cap:
        • The ice cap itself is about 1,000 km in diameter.
        • The North Polar Cap and the Planum Boreum plateau structure underlying it cover approximately 800,000 km2 and, with thickness ranging to nearly 3 km in places, the ice cap volume amounts to somewhere between 1.2 and 1.7 million cubic kilometers.
        • Its extent varies seasonally and also over centuries with climate change.
        • During the northern hemisphere fall and winter, the North Polar Cap is obscured by hazes and clouds and even sometimes hurricane-like storm systems that develop north of 50°, a cloud cover referred to as the polar hood.
        • Through precipitation or through frost sublimation, carbon dioxide ice on the ground expands to roughly 60° of latitude
        • This ice cap is mainly composed of water ice, which dominates the residual ice that persists through all seasons.
          • The water does sublime, whenever summer temperatures get above 205 K (-68° C or -91° F), which it sometimes does on the south-facing walls of the ice cap, which exaggerates the steepness of the south-facing slopes.
          • In the Northern Hemisphere winter, water freezes out of vapor, first at the pole and then farther and farther out, to build the seasonal water ice cover. Some of this is contributed by polar cyclones, first spotted by Viking, which can produce snow.
        • Carbon dioxide sublimes around 150 K (-123° C or -190° F), so it freezes out as frost as winter approaches, developing a seasonal carbon dioxide veneer. This is the very atmosphere itself settling out! This seasonal carbon dioxide ice extends out quite far from the polar ice cap.
        • During summer, first the carbon dioxide frost sublimates away entirely and then some of the water ice does, too, noticeably shrinking the ice cap during the Northern Hemisphere summer.
        • This adds a significant pulse of carbon dioxide to the atmosphere in the Northern Hemisphere winter, the partial pressure of which raises martian air pressures quite significantly: There's nothing like this pressure pulse on Earth.
        • The Northern Hemisphere summer is noticeably longer than the winter, so there's that much longer for air temperatures to exceed 150 K and even 205 K, so it's not surprising that the carbon dioxide veneer disappears and even some of the water ice sublimates.
        • One of the weirdest features of the Northern ice cap, which has no parallel on Earth, is the existence of deep chasmata in the ice.
          • These are very deep and curve outward in a counterclockise spiraling pattern.
          • The largest is Chasma Boreale, which opens out from the ice cap about 300-320o E, where it is about 350 km wide and cuts back some 600 km ... and spirals at an angle different from most of the others.
          • These features are etched as much as a kilometer into the cap and often their depth takes them below the elevation of the surrounding countryside.
          • Their floors have lower albedo than the surrounding polar layered deposits, suggesting that they may be traps for dust blown into them, much like crater floors often develop dune fields.
          • Very oddly, though, they trend counterclockwise outward, while katabatic winds generated by the polar high tend to spiral clockwise off the northern cap. One of those martian "yes, but ..." moments.
          • There is all kinds of speculation about what causes these weird features: Wind erosion? Jökulhlaup erosion?
        • Internal stratigraphy was revealed by the Shallow Radar (SHARAD) sensor on board the Mars Reconnaissance Orbiter (MRO):
          • Four laterally continuous concentrations of fine layers of dust
          • Three homogeneous zones of nearly pure water ice
          • A basal unit of æolian origin, comprised of dark sand-sized grains. It is believed to be of Amazonian age, meaning the ice cap is no older than the Early Amazonian.
          • This layering of pure water ice and dusty ice is a record of Amazonian climate change and coring it would be of intense interest to future human expeditions to Mars.
      • South Polar Ice Cap
        • This cap is quite different from the northern cap.
          • Much smaller, about 350-400 km in diameter, but it is somewhat thicker, getting over 3 km thick in places.
          • Like the North Polar Ice Cap, the South Polar Ice Cap features deep chasmata cutting down into the ice. These seem to spiral clockwise off the cap. That would be weird, if these are carved by katabatic winds, since those would spiral counterclockise out of the Southern Hemisphere polar regions. In this, the southern cap is just as baffling as the northern, with their counterintuitive chasmata.
          • The seasonal carbon dioxide frost extends farther out than seen in the Northern Polar Cap, though: It gets down to about -45°
          • Located on the Southern Highlands, it is about 6 km higher up than the North Polar Cap, which means that it gets colder (think of lapse rates up a mountain on Earth).
          • The Southern Hemisphere winter is also noticeably longer than the summer because of the planet's great orbital eccentricity, which means Mars is moving relatively slowly at aphelion, protracting winter in the Southern Hemisphere .
          • Aphelion is 121% as far from the Sun as perihelion, which itself means a drastically colder winter than experienced in the Northern Hemisphere.
          • Also, the Southern Hemisphere summer features more dust devils and dust storms than the Northern Hemiosphere summer, meaning the Southern Hemisphere summer is dustier and the surface is slightly shadier, also meaning the summer is cooler.
          • This means that, even in the relatively short Southern Hemisphere summer, temperatures are not going to get above 150 K for long enough to sublimate away all of the carbon dioxide ice. The permanent carbon dioxide ice remains about 8 m thick through the summer.
        • Suspicions that there was water ice below the residual carbon dioxide ice cap were affirmed by ESA's Mars Express Minerological Mapping Spectrometer or OMEGA and NASA's Mars Odyssey Thermal Emission Imaging System or THEMIS).
          • Sublimation pits have long been observed on the South Polar Cap, where carbon dioxide sublimates explosively in geysers, sometimes pulling dust up with it, forming an odd deposit on the ice, referred to as "spiders."
          • These steep-sided pits consistently show flat floors about 8 m below the surface ice.
          • Spectra from these floors evidence water ice.
          • So, the South Polar Cap has a residual carbon dioxide cover about 8 m thick on top of a permanent water ice core.
          • This water ice core probably saw some basal melting in the past, as seen in imagery of stream channels emerging from below the ice.
          • This creates at least some plausibility for the Argyre to Ares fluvial system, or Chryse Trough system proposed by Timothy Parker.
          • The South Polar Cap dominates the large air pressure swings in the atmosphere.
            • At the Viking 1 landing site in Chryse Planitia, air pressure varied annually over a range from 6.9 to 9 hectopascals or millibars, something like a 30% increase.
            • Air pressure would go up like crazy in the Viking 1 fall and winter, back down somewhat in spring, go up in late spring/early summer, and drop like a rock in late summer.
            • This coïncides with the cycle of sublimation of a lot of carbon dioxide off the South Pole Cap in its spring and summer and the migration of that CO2 to the North Polar Cap. The same thing would happen in the North Polar Cap's spring and summer, but the effect was smaller.
            • So, the southern cap has a stronger effect on the semi-annual march of air pressures on Mars, because the CO2 ice is more extensive than on the northern cap, and the winter there is longer and colder than the northern cap due to the exaggerated ellipticity of the planet's orbit interacting with the marked tilt in the axis.
        • Another weird feature of the South Polar Ice Cap is that it isn't, exactly, "polar."
          • The winter cap and seasonal hood is pretty symmetrical, extending up to ~ -45°, as noted above.
          • The residual ice cap, however, is markedly askew, developing on the western end of the actual pole and missing from the eastern end.
          • Marco Giuranna and his team in Rome used the Mars Express Planetary Fourier Spectrometer in 2008 to measure temperatures through vertical profiles above the polar region and found that there are two temperature régimes there, which they relate to the general air circulation of Mars.
          • Like Earth, Mars has a prevailing westerly air flow in the southern mid-latitudes. This is strongly affected by topographic contrasts, such that a lot of this air flow falls into Hellas Planitia (around 60° - 90° E) and then flies up the other side, creating a massive undulation, or Rossby wave, in the westerlies circulation. This airflow is deflected poleward as it first undulates up into the upper troposphere and then comes down around the eastern side of the South Pole.
          • This descending airflow, as on Earth, would be dry, which would preclude carbon dioxide or water snows (though it would not prevent surface frost developing in the extreme cold). So, the east side of the South Pole, while frost covered in winter, does not receive snow to support ice buildup.
          • Meanwhile, the air rises on the west side of the pole, creating lower pressure there and supporting, in addition to frost, a bit of snow, which can sustain glacier development on that side of the pole.

    • So concludes our tour of the "second order" features of Mars. These are large and conspicuous features that do not nest tidily within the first order features (the crustal dichotomy and the Tharsis rise) but in some ways are nearly as conspicuous, particularly on the MOLA maps. They sometimes transcend the first order, with, for example, Valles Marineris reaching beyond Tharsis to drain into the Northern Lowlands. The system of great craters, too, is found on both sides (and on top) of the crustal dichotomy. Together with the first order features, they create an easily memorized structure of reference points, lines, and polygons, with which we can fill out the details of our mental map of Mars. We can refer to a feature, for example, as "comprising a large region between Hellas Planitia and Argyre Planitia" (Noachis Terra) or "a subregion of Noachis Terra found west of the Chryse Trough and east of the Thaumasia Block" (Bosporos Planum). Interestingly, each of these features seems to be a huge example of a given geological or geomorphic process, such as cratering (the four big bruisers), rifting (Valles Marineris), jökulhlaup-type massive outflows (Kasei Valles), huge volcanic province (Elysium Rise), glaciation (polar ice caps), wind erosion ("Blue Scorpion" of Syrtis Major, hydrological drainage (Chryse Trough), and megalandslide (Thaumasia Block).

    • [ 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: 11/02/22