1996/97
Mars Sketch (3)
from CMO #201 (25 March 1998)
-- Clouds at the Tharsis Ridge and Olympus
Introduction
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y
the terminology Olympus Mons, for example, we do not microscopically imply the
crater of Olympus Mons: That is to say, when we just say that we observed
Olympus Mons, we do not necessarily imply that we detected the very crater of
Olympus Mons. Instead we mostly mean by Olympus Mons a macroscopic aspect of
the area, sometimes covered by a white large cloud. The cloud covers sometimes
the volcano asymmetrically around flanks. Therefore for example the
terrestrial-based observational results of its position must be sometimes
different to the true Martian-based value of the position of the volcano itself.
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 083°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 110°Ls, 122°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.
Morning Clouds
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ince 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 097°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 (096°Ls) at ω=071°W (Id-100D) and on 30
Mar (097°Ls) at ω=069°W (Id-101D) (see Figs 1 & 2 cited here).
Fig 1: ISHADOH's drawing
(raw) on 27 Mar 1997 (096°Ls) at ω=071°W (left)
Fig 2: ISHADOH's drawing (raw) on 30 Mar 1997 (097°Ls)
at ω=069°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 (096°Ls) at ω=054°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 ω=044°W
to ω=138°W, but seeing at ω=064°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 (114°Ls) at around ω=089°W (reproduced
here in Fig 4).
Gianni QUARRA (GQr) and his colleagues (SGPG)
took nice images on 8 Apr (101°Ls) in which we can also see the dark segment of
the Tharsis ridge (Fig 5 at ω=097°W, and cf also #189 p2069).
Fig 3: From a page of MINAMI's
sketch book
(on 27 Mar 1997 (096°Ls) at ω=054°W)
Fig 4: Yasunobu HIGA's images on 5 May (114°Ls) at ω=089°W (left)
Fig 5:The CCD Images by QUARRA and his colleagues on 8 Apr (101 °Ls): See the
blue image at 21:30GMT at ω=097 °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 ω=063°W in Fig 6).
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
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 ω=093°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 114°Ls (Fig 7).
Fig 7: 1982 Mars by Mn (on 18 Apr 1982, 114°Ls) at ω=092°W
by use of 450x 15cm Refr (App.Diam.=14.3")
Evening Clouds
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n 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 092°Ls), Olympus Mons was seen from
Fig 8: Cotton-ball like Olympus
on 10 Feb 1997 (076°Ls, App Diam=11.5", Ph.
Angle=25degs) at ω=165°W (left)
Fig 9: Y HIGA's images
on 20 Mar 1997 (093°Ls) at ω=142°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 ω=135°W. The map of the US Geological Survey
pins down the position of the crater at ω=133°W, and hence the cloud on
the day must have been deviated to the western flanks of Olympus Mons.
HST image of Olympus
Fig 10: on 10 Mar (089°Ls)10 Mar (left) ,
Fig 11: on 30 Mar (097°Ls) at 
ω=195°W (right)
The HST image taken on 10 Mar (089°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 (097°Ls) at ω=195°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).
Fig 12: Olympus
Diurnal Change of Clouds
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t 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 050°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 volcano.
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
observable.
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 ω=195°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.
(Mn)