98/99 Mars CMO Note (9)

1998/99 Mars CMO Note
- 09 -

from CMO #231

The spc

  @ Mars in 1999 was at opposition when the season was at 129°Ls on 24 Apr 1999, and hence it was too early to watch the south polar cap (spc) when the apparent diameter was well large. Furthermore the central latitude stayed long at the northern hemisphere; even when the vernal equinox visited the southern hemisphere on 1 August 1999, the central latitude read 19°N, and hence it was apparently impossible to watch the neighbourhood of the south pole. As inspected from the figure of the grid surface diagramme presented by NISHITA at p2494 in CMO #216, the latitude 50°S was quite near the southern limb on 1 Aug 1999.

  @ However we are soon to confront the 2001 case where the southern hemisphere will be expected to face to us: We are then to watch the south polar region from around 130°Ls to observe the growth of the spc as the south polar hood (sph) dispersing, and thus we consider the 1999 season was a preliminary skirmish.

  @ It is appropriate to observe the sph in the later stage and the spc in the early stage from the side where Hellas is implicated near the CM, but when the central latitude is deep to the north, it is advisable to observe the evening side when Hellas is quite near the p limb. In this sense, the very scene faced us in Japan from around 6 July (166°Ls) this season. For instance, the observations on 7 July (166°Ls) at LCM=335°W and 345°W(Mn-725D & 726D respectively) were important since a less light or shadowy slit appeared clearly between the sph and Hellas at the p limb: The sph thus proved to have been localised. MURAKAMI (Mk) observed a similar scene on 8 July (167°Ls) at LCM=345°W (Mk-188D). MINAMI(Mn) also detected the shadowy bridge on 8 July (167°Ls) at LCM=343°W (Mn-732D) and on 9 July (168°Ls) at LCM=326°W. ISHADOH (Id) happened to watch on 12 July (169°Ls) at LCM=332°W.

  @ In mid-July, Hellas largely came near the CM, and so it was rather difficult to judge. On 21 July (174°Ls), we judged however the sph still much prevailed. The cases came soon where the spc was caught a bit: Possibly the observations by Id on 23 July (175°Ls) at LCM=224°W (Id-138D) and by Mn on 24 July (176°Ls) at LCM=173°W (Mn- 760D) first caught a glimpse of the cap at the southern limb, and certainly it looked well bright on 25 July (176°Ls) at LCM=173°W (Mn-767D). Different longitudes still showed tails of the sph however. The scene seen on 31 July (180°Ls) at LCM=113°W (Mn-773D) also proved the spc when the apparent diameter=9.4" and the central latitude=19°N (cf CMO #222).
  @ The seeing conditions in mid-Aug were poor. Mk's observation on 8 Aug (184°Ls) at LCM=041°W suggests the perimeter of the bright spc.

  @ From an early stage, Don PARKER (DPk) produced good colour images of the thick sph: the Red images were however suggestive about the s limb spc, though the production in late-June and in mid July was not so dense. The Red images on 14 June (154°Ls) LCM=076°W~081°W (whose integrated image was as cited in the preceding issue) showed clearly the light Argyre, but not so evidently the spc. The images on 7 July (166°Ls) at LCM=212°W proves the sph down to 45°S, and the Red image may show a bit of a bright s-limb. The Red images on 28 July (178°Ls) at LCM=007°W~011°W depict a bright s-limb, maybe the spc. The image on 7 Aug (184°Ls) at LCM=264°W also possibly shows the spc, but already the apparent diameter shrunk to 9.0".

  @ We should finally note that the MGS shot partly an early aspect of the spc (released as MGS MOC Release No. MOC2-165): The date is not quite disclosed but said to have taken one week before the vernal equinox of the southern hemisphere at 180°Ls (on 1 Aug 1999) and hence the season must have been around 176°Ls. The image cited here is such that the south is up: the crater at the lower right is Arsia Mons. We can judge the snow line was at about 55°S~53°S since the crater arrowed here is Porter (114°W, 50°S) which was named after Russell Williams PORTER, 1871~1949.

  @ We should also recall that Viking VO2 already produced the image of the spc at 176°Ls in 1977. The snow line was just outside the 55°S latitude (G BRIGGS et al, JGR 84 (1979) 2889), and so not much difference. This value was employed by P B JAMES and K LUMME in considering the recession of the spc boundary (Icarus 50 (1982) 368). The angle shot by VO2 was further eastern than that of the MOC, but both show an occurrence of dust near the perimeter near Argyre. It should be remembered that the snow perimeter from 330°W to 030°W was clear without obscurity.

  @ The central latitude passed the Martian equator around on 12 Oct 1999(223°Ls). From that time, the roundish spc was quite apparent to the eyes of any observers (cf #224). And also from this time, Depressiones Hellesponticae turned to be dark enough, and a shadowy rift inside the spc was witnessed (#225). The apparent diameter went however down to the apparent diameter=6.2" at the end of October and hence it was impossible to observe any detail about the spc. The southern hemisphere with a smaller roundish spc itself inside the disk was observed up until quite late in the season to be shared with the 2001 apparition.