The Tharsis ridge region and Olympus Mons when they moves to the afternoon
hours were well observed by Mariner 9 in the northern summer and the cloud
distribution to the western flanks of Montes was well observed and
identified with the old observation of the so-called "W-cloud". Viking
Orbiters then observed several new aspects of the Montes region in the
morning, and for example in mid-June 1976 at 083degs Ls they detected the
craters of the Tharsis region stood out as dark spots beneath which low
lying fogs spread out. Especially from Ascraeus Mons a peculiar plume-like
cloud flowed out far to the west for a few hundred km. Viking also recorded
the area at 110degs Ls, 122degs Ls and so on.
Note however the phenomenon seen at the morning hours must be very different than the famous case of dark craters detected on the afternoon side by E E BARNARD in September 1894 by use of a 90cm refractor at Lick to the extent that the season was very different.
Since the morning-hour observation by the Viking mission of the Tharsis Ridge and Olympus Mons in the summer of the northern hemisphere (NH) was not global but just treated many much smaller partial regions, and hence it was dubious whether we will be able to observe by smaller telescopes the dark small craters. However an excellent whole-disk image taken by HST this season on 30 Mar 1997 at 097degs Ls proved that it might be possible to detect the craters uncovered by the morning cloud if the seeing condition is good. The images are the ones cited in CMO #191 p2102, and as commented there, we came to know that Hiroshi ISHADOH (Id) already detected some of them visually on 27 Mar (096degs Ls) at LCM=071degs W (Id-100D) and on 30 Mar (097degs Ls) at LCM=069degs W (Id-101D) (see Figs 1 & 2 cited here).
Fig 1: ISHADOH's drawing (raw) on 27 Mar 1997 (096degs Ls) at LCM=071degs W (left)
Fig 2: ISHADOH's drawing (raw) on 30 Mar 1997 (097degs Ls) at LCM=069degs W (right)
Strange enough the present writer (Mn) didn't recollect at that time that he himself
had also observed the dark crater of Ascraeus Mons near the dawn terminator
on 27 Mar (096degs Ls) at LCM=054degs W (Mn-441D). But later he found that the
observing note said "A dark spot is visible on the morning side near the
limb to be checked later" (see Fig 3).
This was later noted in CMO #192 p2107-2108. The night he observed from LCM=044degs W to LCM=138degs W, but
seeing at LCM=064degs W and so on did not much improve and he forgot the
preceding observation henceforward.
The remark in #192 was made on the opportunity to find the dark Tharsis ridge on the Yasunobu HIGA (Hg)'s image taken on 5 May (114degs Ls) at around LCM=089degs W (reproduced here in Fig 4).
Gianni QUARRA (GQr) and his colleagues (SGPG) took nice images on 8 Apr (101degs Ls) in which we can also see the dark segment of the Tharsis ridge (Fig 5 at LCM=097degs W, and cf also #189 p2069).
Fig 3: From a page of MINAMI's sketch book
(on 27 Mar 1997 (096degs Ls) at LCM=054degs W)
Fig 4: Yasunobu HIGA's images on 5 May (114degs Ls) at LCM=089degs W (left)
Fig 5:The CCD Images by QUARRA and his colleagues on 8 Apr (101 degs Ls): See the blue image at 21:30GMT at LCM=097 degs W (right)
We also learned much later
that Paolo TANGA (PTg) had detected visually the three spots more clearly at
Torino by use of a 42cm refractor (cited here the drawing at LCM=063degs W in
It should however be noted that more conspicuous than the shadowy segment was the white large patch in the vicinity of Ascraeus Mons in the CCD images of GQr as well as the Video images of Hg. The patch, as is easily verified from the HST image (at p1202 in #191), is the one stretching at the western flanks of Ascraeus Mons. We called this matter Ascraeus Mons for example in #188 p 2050, and the hazed (whitish) patch in the figure at the page corresponds to the cloud.
Fig 6: Paolo TANGA's drawing at Torino on 13 Apr (104degs Ls) at LCM=063degs W
Another characteristic seen in the HST images cited in #192 p2102 is an indefinite shadowy area at Arsia Mons which is densely reddish in colour while irregularly dark in blue (see also MGS's images cited in CMO #196 p2177 / 78: Note however the Tharsis region is still in the morning). This aspect was also caught by GQr (SGPG). Visually Id detected it clearly on 27 Mar (Id- 100D). The present writer (Mn) also saw the indefinite segment on 27 Mar at LCM=093degs W. From the time of Viking, it has been known that the cloud does scarcely develop at the area around the morning Arsia Mons (see below).
In 1982, we also observed the dense cloud at Ascraeus Mons. The example by Mn is here at 114degs Ls (Fig 7).
On the other hand, the Tharsis ridge and Olympus Mons region, as they move to the afternoon side, become thickly covered by the lee clouds in northern summer. As reported in #185 and #188, the cloud featured aspect of the region was well observed in February 1997 before opposition: Especially Olympus Mons was brought into relief as if it was covered by a very white "cotton-ball" (eg Fig 8). At the time of opposition (on 17 May at 092degs Ls), Olympus Mons was seen from Japan to pass the meridian (eg Fig 9).
Fig 8: Cotton-ball like Olympus Mons by MINAMI
on 10 Feb 1997 (076degs Ls, App Diam=11.5", Ph. Angle=25degs) at LCM=165degs W (left)
Fig 9: Y HIGA's images
on 20 Mar 1997 (093degs Ls) at LCM=142degs W (right)
At noon it
was not yet so bright (seen so by use of a Green filter) but light enough to
make CMT timing, and as stated in #188 p2048, on 18 Mar, Takashi NAKAJIMA
(Nj) and the present writer (Mn) alternatively watched the possible transit
of Olympus Mons and determined the centre of the cloud was at LCM=135degs W.
The map of the US Geological Survey pins down the position of the crater at
LCM=133degs W, and hence the cloud on the day must have been deviated to the
western flanks of Olympus Mons.
HST image of Olympus Mons ;
Fig 10: on 10 Mar (089degs Ls)10 Mar (left) ,
Fig 11: on 30 Mar (097degs Ls) at LCM=195degs W (right)
The HST image taken on 10 Mar (089degs Ls)
clearly proves the ring of the crater of Olympus Mons as well as the
following cloud stretching to the west (or north-west) (see Fig 10).
The HST image on 30 Mar (097degs Ls) at LCM=195degs W shows much thicker clouds which develop upto the top of the mountain (Fig 11).
We should make a supplementary note on a dark band which lies between Olympus Mons and the Tharsis ridge region seen in the evening. This is not so evident in colour on the HST images but appears in blue, and to the naked eyes, the band is rather evident. The present writer noticed it vividly for example in 1982 (Fig 12). Compare the aspect with this season's. Nelson FARSARELLA (NFl) also detected this band this apparition (cf CMO #186 p2021, p2023).
Diurnal Change of Clouds
It was the summer meteorology of the NH that we could observe during the 1996/97 apparition as stated in the preceding issue (1996/97 Mars sketch(2)). Roughly, the characteristics in the season were:
i) the north polar region was warmer
than the equatorial region, and so an upper migration of air occurred from
the Arctic region towards beyond the equatorial band,
ii) in the midlatitudes of the NH the strong easterly winds blew,
iii) noon insolation was so prevailed that westerly winds blew in the morning areas while the evening areas were affected by easterlies both at lower and higher altitudes, and
iv) the migration was accompanied by an amount of water vapour originated from the thawing of the north polar cap.
The Alba phenomenon which was observed in January 1995 at 050degs Ls (CMO #
179 p1895) was the first example where the moist air mass starting from the
Arctic met the up-windy hilly area at Alba (40degs N) and as a result the
white-cloud burst occurred. As an extension of the elevated area, Olympus
Mons is located at 18degs N, and so the next example may be this largest
As the area of Olympus Mons goes to the afternoon side, the easterly winds from the evening terminator and the easterlies at the upper atmosphere combine to become upslope winds at the eastern flanks of the volcano just like the up- draughts. The orography is however different from the terrestrial case in the sense that the mountain is extraordinarily high ( 27km above the mean reference level ) and so the summit works well as a heat source since the top of the volcano faces longer to the sunward to receive much more insolation. The upslope air is therefore not easily cooled, and so it must pass high up far the summit to become condensed on the lee side and may be in the very high altitudes. It may be followed by the mountain waves which may be similarly high up over the western flank of the volcano. (Fig 10) We don't know at present whether this phenomenon was confirmed by the Viking Missions or not, but the presently working MGS will give some answer to the mountain waves.
The Tharsis ridge region must also receive the similar updraughts in the afternoon side, though the lee clouds may be lower because the ridges, working as weaker heat sources, are lower. Note that Arsia Mons is situated on the opposite hemisphere, while Ascraeus Mons is at 10degs N, and hence the retardation must occur concerning the water-vapour migration at Arsia and the cloud formation there must be weaker even in the evening side.
The morning Tharsis is also affected by the easterlies while at the lower
layer the situation differs. The summit does not work well as a heat source
and the lee clouds must be much lower. At the lower places near the dawn
terminator where westerly winds prevails to the day side, the low lying fogs
like stratus are given rise to by the nocturnally cooled air and cooled
ground. The lee cloud of Ascraeus Mons must be resonanced with the fogs from
the morning terminator and must become a large white patch. On the other
hand Arsia Mons at 10degs S is free from the easterlies, and the updraughts
at the eastern flanks are weaker so that the indefinite shadowy marking is
The floating fogs or mists will vanish as the area approaches the noon line. The orographic cloud may remain, but it must be much weaker than the cloud which will be developed at the evening side. As stated above, the HST image on 30 March 1997 at LCM=195degs W shows a thick cloud patch which covers almost all the summit to the lee side (Fig 11). The local time was about 4 o'clock pm.