from CMO #236 (25 October 2000)
Two faint light patches in Tharsis
CASE 1 : (095°W, 05°N)
s the summer solstice visits on the northern hemisphere, the region of Tharsis becomes gradually conspicuous covered thickly by the evening white cloud together with the cotton-ball like Olympus Mons in the afternoon. It is however not easy to discern the inside of the evening Tharsis cloudy region because the cloud near at the limb or the terminator looks heavily condensed. See for example the drawing in Fig 10 at p2247 in CMO #201. Since the topography of the region is particular and if the meteorological condition remains constant for a period, the aspect of the thickly covered region will repeat every Martian day. Some images by the HST show some complicated details of the region near the limb (see eg Fig 11 on p2248 in #201). We became however to be aware that the drawing of Fig 7 made on 18 April 1982 at 114°Ls proved suggestive since the pattern of the cloud structure seemd to be apparent already when Tharsis was more inside.
(Fig 1) : By
HST on 30 March 1997 (097°Ls)
It is advisable to consult the HST image taken on 30 March 1997 at 097°Ls (shown on p2102 of CMO #191) where, preceding to the Ascraeus cloud, shown is a fainter cloud patch to the north of Tithonius L. See Fig 1 here. This is suspected to become thicker near the evening limb. The position of the centre of the cloud is about at (095°W, 05°N), located around Fortuna Fossae: According to the map of the US Geological Survey, the area is quite a slope ascending from east to west. The plateau is as levelled as Alba Patera, and looks to yield a weaker orographic cloud in the evening. This area seems also to be haunted by a low mist in the morning just like the Ascraeus mist/cloud. (As to this quasi-plateau we touched at p2719 in CMO #230.)
(Fig 2): By Don
PARKER on 3 April 1999 (119°Ls)
To compare, we cite here as Fig 2 a piece from an excellent image taken by Donald PARKER (DPk) on 3 April 1999 (119°Ls, Phase Angle = 17°) at ω = 071°W. This also clearly shows a presence of the cloud patch at the site of Fortuna Fossae. Succeeding observations by the present writer (Mn) on 16 April 1999 (125°Ls, Phase Angle = 07°) show the cloud patch from ω = 068°W and so on. Yasunobu HIGA (Hg)'s Tingara-Video also shows the area on 15 April (125°Ls) at ω = 076°W, and on 16 April (125°Ls) at ω = 084°W et al.
The HST produced the area in the evening on 28 April 1999 (131°Ls). A cloud patch is also shown at the area. See Fig 3. The image was taken at a time between 00:22GMT~01:17GMT, and the ω is possibly at around 115°W, and so it lies about at 13:20 LMT.
Finally we cite an MGS image made in April 1999 as Fig 4 whose original image is Release No. MOC2-143 released on 19 July 1999.
We have thus reviewed a faint cloud patch seen at (095°W, 05°N) in April 1999 (so from 119°Ls to 131°Ls). It may be possible to give a review similar to the above on the patch for May 1999. The area was also checked in June as seen on the CCD images cited in CMO #230 p2717. DPk's morning image on 14 June 1999 at 154°Ls shows the area thinly covered or nakedly surface lit. We also note that DPk also produced a good image on 10 June (152°Ls) at ω = 107°W.
CASE 2 : (150°W, 00°N)
ith Tharsis sinking, Olympus Mons increases its brightness in northern summer evening. We then sometimes witness a following faint light patch to the south-west of Olympus Mons. For example the drawing by the present writer made on 8 May 1999 (136°Ls) at ω = 186°W as cited in #232 p2751.
The spot is apparent to the
naked eyes, but it is not easy to tell whether it is atmospheric or barely surface
lit. Fortunately Tomio AKUTSU (Ak) produced a good
series of RBG images on 12 May 1999 (137°Ls) at ω = 175°W which proves it
is not so atmospheric because the spot is just less evident in B than in G and
R. The topography is complexed at the area, but has a
plateau near (150°W, 00°N) and it must be mostly surface lit but the slope
seems to yield a faint misty patch. Y HIGA (Hg)'s Tingara-Video
shows the light spot clearly on 13 May (138°Ls) at ω = 156°W & 165°W.
His images on 7 Apr (121°Ls) also show the patch at ω = 152°W, 162°W &
Figure 4 contains the area inside. Its LMT is unknown, and looks to be free from any atmospheric matter.
We supplement here with the images made by DPk on 29 April 1999 (131°Ls) at ω = 170°W. Fig 6. The spot is also faintly seen in B.