Mars Sketch (16)

1996/97 Mars Sketch (16)

from CMO #215

Ascræus Cloud and Ascræus Lacus

  As was stated several times, by the term Olympus Mons, we don't necessarily imply the crater itself of Olympus Mons. Often the mountain shows up itself when its summit or its frank is covered by a thick patch of white cloud, and then we sometimes just say Olympus Mons is visible. Classically this is quite identical with Nix Olympica, but unfortunately Olympus Mons and other Tharsis Montes are located differently from the areas in classical Martian Maps (see the Figure at p1386 in #144 for more details) and furthermore we don't have such a nomenclature as Nix something to Ascræus Mons and others. So we usually make use of the new terminology of Olympus Mons et al even if any is not exactly a summit or a crater of the mountains.

  We have however encountered recently with a troublesome problem: As analysed in CMO #201 and #209, the very craters proved to be visible as densely reddish spots to us without presence of the orographic clouds in the early morning. We are therefore bound to discriminate between the clouded mountains and the summit without cloud.

  By the same token, we should be cautious when we use the terminology Mons or Montes if we find some white spots or patches. In this sense we were misled in #188 p2049 where we regarded a white patch found on 31 Mar 1997 at LCM=058°W as Ascræus Mons in the morning. Fortunately we were given in May 1997 a detailed HST Blue image taken on 30 Mar 1997 in which the white patch was located between Ascræus Mons and Olympus Mons (perhaps a veiling still near the ground). The HST image was cited in #191 (Mar 1997) and again in #211.

  The present writer observed on 31 Mar 1997 (098°Ls, apparent diameter=14.0", central latitude= 24°N, phase angle=11°) as follows:

                  LCM                Seeing
    Mn-464D    LCM=000°W    v. poor->clouded
    Mn-465D    LCM=019°W    moderate
    Mn-466D    LCM=029°W    very poor
    Mn-467D    LCM=048°W    very poor
    Mn-468D    LCM=058°W    moderate
    Mn-469D    LCM=068°W    very poor
    Mn-470D    LCM=078°W    moderate->cloud coming
    Mn-471D    LCM=092°W    moderate
    Mn-472D    LCM=102°W    moderate
    Mn-473D    LCM=112°W    good

  We here cite the drawings Mn-468D, Mn-470D, Mn-471D and Mn-473D which we believe show how the white patch developed when going into inside.

The Ascræus Cloud observed on 31 March 1997 by MINAMI by the use of a 20cm refractor

LCM=058 LCM=058 LCM=058 LCM=058

 from left to right :
  Mn468D(LCM=058°W),   Mn470D(LCM=078°W),   Mn471D(LCM=092°W),   Mn473D(LCM=112°W)

  The description in #188 implied the coming white spot to be Ascræus Mons, but it should be called the "Ascræus Cloud" or "Ascræus Mist" following Ascræus Mons in the morning. From the view-point of LCM, Mn-471D is comparable with the HST image. We further note that the drawing Mn-441D cited in #201 was made at LCM=054°W (on 27 Mar 1997), and hence we could have detected the Ascræus crater on 31 Mar also if we were not much impressed with the compact white patch.

    We did know in advance that the HST became in conjunction with us on the 30th day of March. We observed so the planet on the day also, but the seeing was poor though at LCM=091°W we saw the white patch without much impression. (The patch was also seen at LCM=064°W on 27 Mar also).

    The local time of Ascræus Mons on the HST image on 30 Mar 1997 is about 10:30am as easily verified if we take into account LCM=094°W, phase angle=11° (after opposition) and the Mons position LCM=104°W (since (104-94+11)/15=1.4). Recent images of Don PARKER (on 30 Mar 1999), as shown in another column this issue, prove that Olympus Mons is already covered by the summit cloud at 10:40am, and so the time is possibly a very divide. The season is however different: it was 097°Ls on 30 Mar 1997 while 117°Ls on 30 Mar 1999. It is currently an interesting problem to know the time when each summit becomes orographically covered by cloud.

    We furthermore have another problem of nomenclature: We thought the name Ascræus Lacus was renewed to Ascræus Mons. And so we often used instead the terminology Mareotis Lacus to denote the dark or densely reddish area preceding Olympus Mons. It was included in the schematic map in #188 p2050/51 where the "densely reddish" area was also defined. We therefore used Mareotis L for example in #190 at p2082. We are however now led to consider that the terminology Ascræus Lacus is still convenient and available to denote the densely reddish area around (110°W, 25°N).

(Mn : Masatsugu MINAMI)