Geography of Mars

• Mars' atmosphere differs sharply from Earth's in its gaseous composition.
  • The most common gas in the martian atmosphere is carbon dioxide.
    • On Mars, it makes up 95.32% of clean air by volume.
    • On Earth, CO₂ is only about 0.037%.
    • On Earth that small bit of gas, which is rising as a result of human activities, is implicated in global warming.
    • On Mars, even 95% of air being CO₂ isn't going to create runaway global warming because, remember, the atmosphere is so insubstantial and doesn't have that much heat density.
  • The second most common gas on Mars is our own most prevalent gas: nitrogen, as N₂, but it's only a piddling 2.7% of martian air, versus 78% on Earth.
  • The third most common gas is argon.
    • This is a gas, which rarely reacts with any other element
      • It's a "noble gas," the column on the far right of a periodic table (group 18).
      • This means that the outer electron shells of these elements are already saturated with electrons, so they aren't cruising around looking to swipe electrons.
    • It makes up 1.6% of the martian atmosphere but even less on Earth: only 0.934%.
    • Argon's percentage actually has been found to vary a little (more on that later, as it was a surprising finding from the Mars Exploration Rovers).
  • Trace gasses include:
    • Oxygen, the second most common gas on Earth, is only 0.13% of Mars' atmosphere.
      • Oxygen is a chalcogen (member of Group 16 in the periodic table). It is, thus, highly, crazy-reactive, being two electrons short of a full outer orbital and hungry to acquire/share them.
        • Because of this "electronegativity," free oxygen tends to attach itself to various metals and other substances ("oxidation"), as do the other members of its group.
        • It is, therefore, really difficult for oxygen to build up in an atmosphere.
        • It does build up in Earth's atmosphere, because plant photosynthesis and microbial chemosynthesis generate such copious amounts of oxygen that it outstrips the ability of rock materials to oxidize it all, so it builds up in the atmosphere.
        • If we ever spot another planet with a strong molecular oxygen (O₂ spectral signature, we will then be almost certain that we have found life there.
        • Mars, then, does not show evidence of photosynthesis going on because of the paucity of oxygen.
      • Because there is so little free oxygen, there is virtually nil ozone.
        • Ozone is produced by the photo-dissociation of oxygen in the presence of ultraviolet radiation and its reconfiguration into O₃.
        • Because of the lack of ozone, there is no real stratosphere on Mars
        • There is, therefore, no protection from an ozone layer for surface life forms: Mars has high ultraviolet radiation at its surface.
        • UV shielding will be one of the major problems faced by astronauts sent to Mars: Settlements may well be underground for this reason.
      • Carbon monoxide comes in at 0.07%, which is a lot more than on Earth (~150 parts per billion, 1.5 x 10^-7), despite all the faulty combustion going on here.
      • Water vapor makes up only 0.03% of Mars' atmosphere; on Earth, it is a highly variable gas, making up as much as 4% in certain situations.
      • There are some other gasses present on Mars, but at incredibly dinky doses, 0.0015 (hydrogen's abundance) or less.
• Mars has a dusty atmosphere, the amount varying with meteorological events, from nearly clear to almost opaque with dust.
  • There is usually so much that it gives Mars' sky an orange, ochre, or pink cast, rarely opening up to a bluer sky.
  • There appear to be particles of two different size ranges:
    • A coarser group in suspension, containing what may be water ice grains averaging about 1.2 μ in radius, and larger dust grains, averaging about 0.7 μ. Their density is quite sparse, perhaps about 2/cm³. These would be likelier to settle down out of the air onto the ground because of their size.
    • A much finer group, a kind of ærosol, with radii between 0.04 μ to 0.07 μ. These are found in greater density, averaging about 3,000/cm³. These are the fine particulates that can serve as condensation/freezing nuclei for water vapor or carbon dioxide ice to form and create clouds.
    • Though wind and storms have mixed and homogenized particulates in the martian atmosphere, leading to a very uniform composition of surfaces in high albedo areas on Mars, the actual mineral composition of the airborne material is still poorly known.
      • It is very challenging to read spectra from the atmosphere, what with the complications of spectral influences from the surface and from water and carbon dioxide ices in the atmosphere.
      • Various attempts to sort out the mineralogical composition of the dust mention both unaltered and weathered materials, including feldspars, zeolites (hydrated feldspars), silica, possibly gypsum (hydrated calcium sulfate), and calcite (calcium carbonate), as well as the expected ferric oxides (Fe₂O₃.

• Mars' average pressure at the elevation of the "areoid" is roughly 0.67% that of Earth at sea level, or 6.75 hPa versus 1,013 hPa for Earth
• It varies far more than Earth's:
  • It ranges from ~6 to ~10 hPa
  • Extremes on Earth range from Typhoon Tip's 870 hPa back in 1979 to 1,084 hPa in Agata, Siberia, in 1968
  • So, Mars surface air pressure varies something like 60% of its mean pressure, versus 20% for Earth
• As on Earth, there is an inverse association between pressure and altitude, an exponential falloff in barometric pressure with a gain in altitude. For the morbidly curious, that would be:
  • P = 0.699 * e^{-0.00009 * A}, where P = pressure and A = altitude in meters
  • This is an inverse exponential curve (Y = a e ^{-b X} )
  • Multiply Altitude by -0.00009, then raise e (2.71818) by that answer, and, lastly, multiply THAT answer by 0.699. That gets you the predicted air Pressure (in hPa) for that altitude.
• Temperature, too, drops with altitude, but in a linear fashion:
  • This is a simple linear regression of the Y = a +bX form, except the curve is different below and above ~7,000 m
  • Below 7,000 m, it's T = -31 - 0.000998 * A
  • Above 7,000 m, it's T = -23.4 - 0.00222 * A
• Not too surprisingly, the drop in pressure causes a drop in density with altitude, modified by temperature:
  • D = P / [0.1921 * (T + 273.1)]
  • Where D = density in kg/m³; P = pressure in kP (kilopascals, 1/100th or percentage of Earth sea level average barometric pressure); and T is temperature in ° C.
  • To convert density into hectopascals/millibars, multiply by 10
• Vertical temperature structure
  • Like Earth, Mars' lower atmosphere usually displays an inverse relationship between altitude and temperature: Temperature cools as you rise.
    • Sometimes this lower atmosphere is called the troposphere on Mars
    • This band goes up to about 45 km above the ground, where on Earth, it extends up to only about 17 km or so (variable in thickness. thicker at the equator, ~20 km, and thinner over the poles, ~7 km).
    • The martian troposphere, like Earth's, is the zone of active radiative interaction between the surface of the planet, which absorbs solar radiation and then re-emits it at longer wavelengths as thermal radiation and the atmosphere.
      • The lower atmosphere is warmed by this re-radiation and its effectiveness drops with the square of the distance to the ground.
      • Dustiness in the lower atmosphere plays an intriguing rôle:
        • It can absorb a lot of insolation and also outbound surface thermal emissions, and this leads to elevated temperatures above the surface whenever there's a lot of dust present there. Dust absorbs and re-radiates, both insolation and planetary thermal re-emissions.
        • Dustiness, however, can also shade the surface, reducing the surface absorption/emission of solar radiation and the warming from below.
        • So, the presence of dust can produce elevated temperatures higher in the troposphere and reduced temperatures at the surface, leading to stable air.
      • As on Earth, there is also heat transfer through adiabatic processes, as air masses move up or down, especially when rising air forces the freezing of water vapor or carbon dioxide.
        • As air masses move up, they expand in volume as they move into zones with less air pressure from above.
        • This expansion dilutes the energy density of the air mass, thus cooling it.
        • Depending on the amount of water vapor or carbon dioxide in the air, this adiabatic cooling may reduce the temperature below the saturation condensation level, which on Mars will be below the triple point of either gas. The result is freezing and possible precipitation as snow.
        • If that happens, the adiabatic lapse rate reduces because the phase change from vapor to ice liberates some latent heat as thermal heat, partly offsetting the dry adiabatic process.
        • Going in the opposite direction, sinking air compresses, thus concentrating its energy density, which warms it (and precludes condensation and precipitation).
        • This adiabatic effect is above and beyond any heat transfer due to conduction or radiation.
      • At night, convectional uplift of the air closest to the ground falters and then ceases or even reverses.
        • Meanwhile, the rapid chilling of the ground begins to reverse the flux of heat, drawing it out of the atmosphere into the ground, creating a chilled layer close to the ground.
        • Above it, the collapse of the daytime convection column causes adiabatic warming and the construction of a warm layer on top of the chilled surface layer.
        • So, at nighttime, you get temperature inversions, much as we see here on Earth at night.
    • The martian troposphere, like Earth's, is the location of weather: convection cells, clouds, dust devils, wind and dust storms
  • Unlike Earth, there is no real stratosphere
    • There is no ozone level, because its source, oxygen, makes up so very little of the martian atmosphere. Therefore, there is no band in the middle atmosphere in which molecular oxygen is absorbing ultraviolet radation during photodissociation into atomic oxygen and recombination into O₃. In other words, there isn't a zone of a direct relationship between altitude and temperature comparable to Earth's stratosphere.
    • On Earth, the stratosphere, containing the ozone layer, extends from roughly 25 km up to about 50 km.
    • On Mars, the middle atmosphere is an isothermal belt above the troposphere, which extends from roughly 45 km up to about 110 km.
      • This is sometimes called the martian mesosphere, though I've seen some authors even call it the "stratosphere."
      • On Earth, the mesosphere is a zone, in which temperatures resume an inverse relationship with altitude (temperatures go down as you go up), and it extends from about 50ish km up to about 80 km. Given this inverse temperature and altitude relationship on Earth as the very definition of the mesosphere, use of the same term to describe something quite different on Mars is a little misleading.
      • On Mars, temperatures remain essentially static for kilometers, neither warming nor cooling consistently with a gain in altitude: isothermic
      • This roughly isothermic middle atmosphere is a sort of transition zone between the inverse temperature-altitude relationship of the lower atmosphere and the direct relationship seen in the thermosphere.
      • It is actually probably best analogous to the tropopause, stratopause, and mesopause in Earth's atmosphere, these isothermic bands that separate the different trends in the altitude and temperature relationship.
  • Above the mesosphere, as on Earth, there is a wide band in which temperatures rise with altitude: The upper atmosphere, sometimes called the thermosphere, the same name used for the same phenomenon on Earth.
    • This band extends from roughly 110 km up to the top of the martian atmosphere about 200 km out.
    • On Earth, by comparison, the thermosphere extends from about 90 or so km up to where Earth's denser atmosphere gives way to interplanetary space around 10,000 km up.
    • Temperatures really get up there (on Earth, it can hit 1,225° C), but that degree of molecular motion isn't all that impressive, really, when you think how very few molecules are going nuts up there.
    • The thermosphere on Mars, as on Earth, can be further subdivided on the basis of and gravitational effects on composition.
      • The inner thermosphere contains the ionosphere.
        • On Mars as on Earth, this is made up of ions, or atoms and molecules stripped of electrons by the intensity of ultraviolet (short-wave, high energy) light.
        • The electrons, with their negative charge, and the remaining atoms/molecules and even isolated protons or alpha-particles (two protons and two neutrons, with no electrons), with their positive charge, thus, are segregated, and the resulting ionized gas affects radio wave propagation.
        • The ionosphere also glows as UV light alters the energy state within an atom or molecule.
          • Electrons are blasted out of lower orbital shells (a process similar to the X-ray electron stripping in the APXS spectrometer on the various rovers).
          • Some of them are cut loose, part of the segregation of electrons and ions.
          • This sudden relocation is unstable, so some of the excited electrons, if not booted out entirely, will crash back down to the lower orbitals, back closer to the positive charge in the nucleus. Other electrons will quickly move down to lower orbitals vacated by an evicted electron.
          • When electrons move to the "desirable" lower energy state, they emit a photon or light particle as they move, to "pay" for the trip "downstairs," creating a glow.
        • On Earth, the ionosphere is markedly shaped by our still-strong planetary magnetosphere.
          • Ions align with the magnetic lines of force due to their electrical charge.
          • Where the field lines are concentrated, as at the magnetic poles, the airglow of the ionized gas will be concentrated to the point of visibility: aurora displays.
          • On Mars, there is no planetary magnetic field, but there are local magnetic fields, and these have produced little, local auroræ through their effects on the martian ionosphere.
          • We talked about that in the third order of relief, in the discussion of Terra Sirenum and Terra Cimmeria.
      • The lower thermosphere (~110 km to ~125 km) is an extension of the homosphere on Mars, as on Earth. The homosphere is that portion of a planet's atmosphere that is maintained in a relatively homogeneous blend of the various gasses due to mechanical and thermal mixing: winds, convection cells.
      • Above ~125 km, the gasses in the atmosphere are pretty much beyond the reach of these mixing forces, so that allows gravitation to exert a density layering effect, kind of like letting a shaken oil and vinegar bottle settle down and form layers.
        • The area subject to this kind of quiet layering is called the heterosphere on Earth.
        • Heavier molecules, such as nitrogen, carbon dioxide, and oxygen, settle out first, lowest down, letting lighter gasses, such as hydrogen, be pushed out farthest.
        • This effect is also seen with different isotopes of a given gas: For example, deuterium or "heavy" hydrogen (with one proton plus one neutron in its nucleus) will be lower than regular hydrogen (with only one proton serving as a nucleus).
        • The heterosphere layer defined chemically and mechanically is roughly equivalent to the exosphere, which is defined by the motion of those few stray atoms and molecules out there.
          • In most of the atmosphere, molecules and atoms are so tightly packed that any one of them ricocheting after a collision with another one isn't going to get too far before being blocked in its travels by yet another one.
          • Out here in the exosphere, however, a quick moving molecule or atom may rarely bump into another, maybe going 10 km in a straight line before hitting something.
          • Well, eventually, with few obstacles to tackle them, heated (and sped up) by the intense radiation out there, and less and less subject to the gravitation of the planet, they might bounce and just keep on going in a straight line right out into Space: The Final Frontier! This is called "sputtering."
          • Atmospheres tend to ablate in this way, losing disproportionately the lightest of all elements and isotopes: hydrogen more than deuterium, for example.
          • Thus would have been lost much of any ocean on Mars once the atmosphere dropped in density back in the Noachian.
            • Water ice, sublimated directly into vapor, then, is often photodissociated by solar radiation into hydrogen and oxygen.
            • The oxygen, being a "promiscuous" chalcogen atom, will quickly recombine with something, perhaps an iron-bearing mineral on the surface.
            • The hydrogen, meanwhile, may drift up into the exosphere, given its lightness, and be lost to Mars through random ricochets at the top of the exosphere.

• As on Earth, there is relatively lower pressure in the equatorial regions and higher pressure over the polar caps.
  • This reflects the concentration of solar energy at the equator and convective uplift there, which lowers surface pressure.
  • The extreme cold at the poles creates subsiding air there, and this raises barometric pressure.
• As on Earth, there is a Hadley circulation:
  • There are the familiar temperature and pressure variations just mentioned:
    • Air warms in the tropics and rises there, spreading poleward in the upper atmosphere.
    • Air chills in the polar regions, sinks, and spreads equatorward along the surface.
    • Since the planet is rotating, horizontal air flows are distorted by the Coriolis Effect.
      • In the northern hemisphere, horizontal motion is deflected to the right with respect to the planet's surface.
      • In the southern hemisphere, deflection is to the left.
      • There is no Coriolis deflection along the equator, and the deflection becomes progressively stronger farther and farther from the equator.
  • These generate surface air flows:
    • Air rising in the equatorial regions pulls surface air into a westward moving (easterly) system of winds, rather like our easterly Trade Winds.
    • Air sinking over the polar caps is squeezed out katabatically in a clockwise surface circulation that corresponds to our Polar Easterlies.
  • As on Earth, Coriolis Effect distorts the circulation by breaking it into smaller eddies.
  • As on Earth, too, the Hadley circulation tends to move first north and then south with the seasons as the obliquity of Mars' axis points first the northern hemisphere and then the southern hemisphere toward the sun.
  • Unlike Earth, however, there is a marked difference in the strength of the Hadley circulation and the pattern of flow over the course of the year, due to the much greater eccentricity of the martian orbit compared with Earth's.
    • The Hadley cells develop best and in a most earthlike way during the martian spring and fall, when the sun's direct rays strike the equatorial regions. This is the familiar pair of Hadley cells we see on Earth.
    • During the summer and winter, however, when the greater axial tilt of Mars leads to a more pronounced shift of solar energy into or away from the polar regions, there tends to develop a single big Hadley engine in the warmer hemisphere, which extends into the other hemisphere.
      • The companion cell in the cold hemisphere is lost in the strong polar vortex circulation that develops at the surface around the dark pole.
      • This is due to the interaction between descending and adiabatically warming air over the pole and the really, really cold and denser winter air mass that develops in the darkened "Arctic" or "Antarctic" regions, which is enough to disrupt the Hadley cell in that hemisphere.
  • Due to the planet's high eccentricity, too, the Hadley circulation is best developed during northern winter/southern summer rather than during southern winter/northern summer: Remember, perihelion hits during southern summer on Mars as on Earth, but there really is a marked difference in energy receipt between perihelion and aphelion.
  • Interestingly, the Hadley cells are associated with aurora-like faint glows in the ultraviolet and the infrared.
    • As we saw in the discussion of the thermosphere, carbon dioxide and molecular nitrogen are photodissociated there to create atomic nitrogen and oxygen, as well as carbon monoxide.
    • These get caught up in the descending branch of the Hadley circulation and sink there on the night and winter sides.
    • This brings them into greater density with the adiabatic concentration there, so a lot of these loose-cannon atoms recombine into molecular oxygen (accounting for the tiny trace amounts of oxygen in the martian atmosphere) and nitrogen (N₂ and nitric oxide (NO).
      • The oxygen emits a concentration of near-infrared energy at 1.27 microns, which was picked up by the OMEGA spectrometer on Mars Express.
      • The NO emits a similar spike in the ultraviolet, which was picked up by the SPICAM spectrometer on Mars Express.
      • These constitute a faint nighttime emissions glow in the descending limb of the Hadley cells during winter. They would not be visible to us, though, because they peak well outside the 0.4-0.7 micron wavelengths of visible light.

Temporal patterns in these spatial pressure variations

• Strong temperature differences develop across the differing surfaces of Mars, especially in the mid-latitudes.
  • As on Earth, temperature differences create pressure differences, with warmer areas producing rising air and low pressure (cyclones) and vice-versa (anti-cyclones).
    • These cyclone/anti-cyclone patterns create wind flows between them.
    • These systems move eastbound around the planet, as on Earth.
    • Unlike on Earth, though, they tend to be more regular and predictable, with about a 3 or 4 day cycle.
  • They mix the atmosphere, moving surface and equatorial heat upward and toward the poles, a function that varies with the clarity of the atmosphere:
    • They are stronger during times when the atmosphere is clearer, perhaps because the clear sky allows more radiation to hit the surface, inducing those surfaces with a relatively low specific heat to heat up more rapidly and produce pressure variations.
    • They are weaker during dusty episodes, which might shade the surface and reduce the contrasts in surface temperatures.
• There's an interesting diurnal thermal tide that develops between the day side of Mars and the drastically colder night side of the planet. The difference in temperatures can be as much as 50° C or 90° F, worse than even a bone-dry desert on Earth experiences between night and day (though NOAA reports two incidents in Montana, one in 1916 and another in 1972, where the diurnal temperature difference hit 102°F or nearly 57° C!).
  • This sets up atmospheric tides of strong local winds that move around Mars with the sun.
  • They are much stronger when the air is clear and more subdued when the atmosphere is in one of its dustier "moods," shading surfaces in the daytime and keeping the atmosphere warmer in the nighttime.
• At a smaller scale, the area around the northern ice cap gets some interesting weather in the northern hemisphere summer.
  • The ice cap shrinks due to heating and sublimation of carbon dioxide and water, which provides water vapor for clouds and storms.
  • You might remember those odd comma-like storms I showed you earlier in the semester, rather like the subpolar lows that get going on Earth
  • On Earth, this is more of a winter phenomenon, the source of the mid-latitude wave cyclones that give California its winter precipitation.
  • At the most local scale, there are small wind systems, kind of like the upslope-downslope breezes, land-and-sea breezes, and dust devils that develop in certain Earth locations.
    • On Mars, they are generated by localized, differential heating of air due to:
      • Differences in surface albedo
      • Differences in aspect: Adret (sun-facing) and ubac (shady) slopes
      • Differences in soil thermal inertia and specific heat:
        • Thermal inertia is a function of a substance's:
          • heat capacity or specific heat (how quickly it changes in temperature in response to inputs of heat energy)
          • density
          • thermal conductivity (how quickly it transfers heat to something else or within itself)
        • These are affected by state of matter (liquid vs. solid), grain size, interstitial spaces among grains.
        • On Earth, water has high thermal inertia/high specific heat, and land has low thermal inertia/low specific heat, so land heats up and cools down quickly compared to nearby water, which then creates relative temperature differences, pressure differences, and compensating breezes.
        • Mars doesn't have bodies of water, but the surfaces of Mars do vary in thermal inertia, specific heat, and albedo (solid lava flows, impact-gardened regolith, high albedo dust).
          • Solid rocks, including the dark, low albedo basalts so common on Mars, have high thermal inertia as rock molecules and mineral grains readily conduct heat to their neighbors and away from the area receiving the energy: It takes more heat input to produce warming of their surfaces.
          • Regolith and dust, with their abundant interstitial spaces between rock grains, transfer heat from one to the next only at the small points and edges where they touch one another. This is low thermal inertia. It takes relatively little energy input to produce warming of such a surface because the surface can't transfer the heat away as readily.
        • These differences in thermal inertia and specific heat may be significant on Mars, creating analogues of diurnally reversing land-and-sea breezes there or even seasonally reversing monsoons, an effect significant enough to affect global circulation models of Mars' climate!
    • These local wind systems are strongest near perihelion, especially in areas with topographic extremes (e.g., Tharsis' gigantic volcanoes or the steep slopes leading down to Hellas Planitia). They are common around the "Blue Scorpion" of Syrtis Major, which plays a "sea" to the surrounding dusty areas as "continents." In a manner of speaking, the Syrtis Major "Blue Scorpion" is, indeed, the "Hourglass Sea" it was often called in early Mars telescopic observation, at least in terms of air flow!