Coming Mars 2001 (06)

COMING 2001 MARS (06)


The spc and the northern summer

Masatsugu MINAMI


      The forthcoming opposition occurs near at the autumnal equinox (180°Ls) of the northern hemisphere, and hence the first half of the apparition we shall watch the summer phenomena expected to occur on the northern hemisphere.
  It is customary to regard the phenomena this season as caused by the rapid thawing of the north polar cap (npc), and we usually disregard the trend of the opposite south-polar region (spr) as unrelated with the northern phenomena.
  It is true we cannot watch the spr governed by the southern winter from the Earth, but it is unreasonable to consider that the growing south polar cap (spc) does not affect the phenomena on the northern hemisphere that faces to the Earth.

    At those times when the polar cap was supposed to consist of the water snow or ice, it was considered that the spc and the npc did not exist at the same time because of a scarcity of the water budget on the bone-dry Mars as stressed especially by W H PICKERING.
  On the other hand if the polar cap mainly consists of dry ice, the opposite cap needs not refrain from beginning to form as soon as the Sun at meridian goes down after the autumnal equinox (000°Ls in the southern case) since the atmosphere is abundant in the material CO2.
  According to the famous observations from the Viking L-1 and L-2 sites, the daily averaged pressures at 22°N and 48°N respectively showed similarly a long term decrease from the southern winter solstice (maybe from more before) and both attained a minimum near at 149°Ls ± 5°Ls (for instance see Fig 16 in S L HESS et al, JGR 82 (1977) 4559 or Fig 1 in J E TILLMAN et al, JGR 84 (1979)2947).
  The surface pressure then went upward up until the southern summer solstice and it is also well known that both Landers met small disturbances from dust storms twice. It is quite natural to consider that the decrease in pressure is caused by a gradual loss of CO2 from the atmosphere to give rise to the polar cap, and the bottom implies a completion of the spc (if not otherwise another CO2 adsorption occurs to a unknown megaregolith).
  One can also find an evidence in the Viking Orbiter images showing an early rise of the spc: An area at 68°S was frost free at 023°Ls, while the point was already inside the frost cap at 039°Ls (on 27 Jan 1978) (for example see P Thomas et al, Frost Streaks in the South Polar Cap of Mars, JGR84 (1979) 4621). The half-breadth of 20 degrees is comparably large and it corresponds to the thawing spc at around 230°Ls.
  Theoretically or numerically the growth and decay of the polar cap under the pure CO2 atmosphere has long been known at the latest from the time of Y H MINTZ who was proud of his model describing a maximal snow line to which his edge curve just fell down. To give another example, the work by H TANAKA and Y ABE introduced in CMO #142 p1354 also shows a rise of the spc after the autumnal equinox and attains its maximum at 090°Ls and keep its size deep in winter. Accumulation of CO2 frost seems to continue thick until 150°Ls though the size does not develop any more.

    We should thus consider that the alternation theory of the polar caps does not hold if the caps are mainly made of CO2, but the water component of the caps must alternate. If the water is confined through the Martian year under 150K, the water does not work in the air.
  VO-2 measured the brightness and the amount of water vapour every 30°~50° in longitudes from 185°Ls to 194°Ls and yielded a result for example that water vapour did scarcely exist inside the spc, while it rapidly increased from the rim-side of the spc toward the equator (for example from 70°S to the north)(D W DAVIES and L A WAINIO, Measurement of Water Vapor in Mars' Antarctic, Icarus 45 (1981) 216).
  Since it is known that the remnant spc is made of dry ice, it is natural to see that the inside was free from water. At any rate if it does not work in the atmosphere we may disregard it. We should however remember that later at 205°Ls in 1976, a dust cloud brought much of a water ingredient into the inside area of the spc.

    The abundance in water vapour near the spc's periphery implies the brighter part seen visually after 175°Ls at the boundary of the spc is made from water ice or snow (note it is quite possible to watch the aspect in 2001). This brighter periphery ironically rejects the absorption of the sunbeams and does not warm up the polar surface.
  The southern hemisphere passes the perihelion in its summer, and so the water vapour will be richer this season than the opposite case, but as a whole the annual mean amount of the insolation received by the southern hemisphere is less than that received by the opposite hemisphere. The spr in summer is thus colder than the npr in summer, and accordingly the migration of water cannot be symmetrical.

    The migration of water from the spr seems thus to start around from 160°Ls, earlier than the period supposed so in PICKERING's time, and the vapour may be maximal from southern summer to autumn (270°Ls to 360°Ls).
  The npc might have started already to form after northern autumnal equinox, but this may be mainly of dry ice, but will acquire water from winter to the spring equinox 360°Ls during which the global pressure may go down. The npc must be at maximum at that season and the temperature at the circumpolar region must be low enough to allow still the CO2 condensation trapping water. The npr will then gradually release the water as the npc begins to rapidly thaw after the spring equinox (or more around from 060°Ls), and the northern atmosphere phenomena will be often seen because of the opposite migration. Difference from the opposite case is that the npr is not so low in temperature in summer (near 205K, 50K above the CO2 sublimation temperature), and that the water vapour still haunts the npr.
  We are not sure whether we can observe any phenomena related with the fall down of global pressure, but it is highly possible for such phenomena as the dust clouds in the southern hemisphere to occur because of the asymmetry of the water distribution as well as the pressure differences. The 2001 apparition is a favourable opportunity to watch the polar regions equally and it may be recommended to pay much attention to the behaviours of the peripheral side of the big spr white matter as well as the residual element at the npr.

  Finally we call the reader's attention to the fact that the polar cap is never any smooth field covered by snow or ice to allow a classical image of fissures or cracks or rifts. The spr is highly cratered, and the frosted craters are naked even at its height, and just the sublimation near the vernal equinox brings a shadowy area inside the spc.
  The snow layers in the npr are also fixed by an eternal topography. The both polar regions should be said so interesting not because of their topographies but only by the atmosphere phenomena that are to occur near there.
  As to the bird-eye view of the npr at 042°Ls (belonging to period 22 of the Viking mission, when the water vapour is uniform in the northern hemisphere, but the circumpolar area is near the dry ice temperature), see a recent image provided by the MGS. It is notable that the water ice core known as the residual cap already shows up, and it is hard to imagine other topographical aspects except for such dust disturbances or other atmospheric phenomena over there.


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