1998/99 Mars CMO Note
- 14 -
from CMO #236 (25 October 2000)
Two faint light patches in Tharsis
CASE 1
: (095°W, 05°N)
|
A |
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.
(Fig 3)
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.
(Fig 4)
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)
|
W |
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.
(Fig 5)
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 &
172°W.
Figure 4 contains the area inside. Its LMT is unknown, and looks to be
free from any atmospheric matter.
(Fig 6)
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.
(Mn)
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