Mars Sketch (5)
from CMO #203 (25 May 1998)
-- White Hellas,
from Its Brightest through its Decline,
Observed in 1997 --
White Hellas (086°Ls ~ 140°Ls)
S to the trend of Hellas in the previous 1994/95 apparition, we tried to
review our observations in CMO #174 p1923 (Mars Note 10): In that apparition
the first half of the period when Hellas was whitish bright was well observed,
and the so-called "Very-Active" state was judged to have reached
Hellas at 085°Ls to 090°Ls. On the evening limb Hellas
at 075°Ls was already, however, as bright as the north polar cap (npc) .
In the present apparition in 1996/97, our observations
of the primary state of the white Hellas was
reported in #189 p2065/2066. The Japanese observations were made by Nr, Nk, Oh, Id, Hk, Mk, Nj, Mn and others, while the overseas observers such as GTc, RSc, DTr,
NFl, ANl and others
produced somewhat earlier observations of the bright Hellas from around 092°Ls.
The report in #189 so employed the threshold where Hellas
turned Very Active to be around 090°Ls in mid-March 1997. We did not refer at
that time to the observations by the present writer (Mn) just before the
threshold but they were as follows: On 4 Mar (086°Ls) 1997, Hellas
was already well whitish. On 5 Mar (087°Ls) when seeing was better, Hellas was blue-white but not brilliant at ω=263°W,
while it could not be said to be brilliant even at ω=272°W (phase angle=10deg ). Hellas was still
weaker than the npc at ω=314°W to ω=333°W
though some partial areas looked fairly bright.
As is widely known, the HST took the pictures of Hellas
on 10 Mar (089°Ls) and on 30 Mar (097°Ls). See #200 p2233 for the latter. The
Blue images vividly show Hellas, but the
density is not so high though the blue ingredient looks reduced to show the
minor details of the npc. Considering the whole
apparition, there were few observations of the brilliant Hellas.
If any, they must be done overseas at around 120°Ls.
Hellas looked detailed on 11 Apr 1997 (103°Ls) at ω=309°W at Fukui (MINAMI drawing)
The observations in April show Hellas
as follows: Mn observed on 9 Apr that it was light and whitish in the evening
after ω=327°W, while on 10 Apr (102°Ls) it was not so light in the
morning. Also, Hellas showed internal
structure on 11 Apr at ω=309°W (Fig 1) and similarly on 12
Apr and 13 Apr. Later in April, the following observers detected Hellas to be
very bright white: GRAHAM (DGh) on 20 Apr (107°Ls) at
ω=323°W; GROSS (HGr) on 21 Apr (107°Ls) at ω=302°Wet
al (Fig 2), STARZINSKI (HSt) on 25 Apr
(109°Ls) at ω=282°W etc (cf #195), and WHITBY (SWb) on 30 Apr (111°Ls) at ω=321°W. LEHMAN (DLm) observed that Hellas
was very bright on 5 May (114°Ls).
Bright Hellas by Horst GROSS (Germany)
on 23 Apr 1997 (108°Ls) at ω=279°W
Fig 3: Creamy Hellas by Johan WARELL (Sweden) on 31 May 1997 (126°Ls) at ω=327°W
In May, IWASAKI (Iw) observed the whitish
bright Hellas on 17 May (119°Ls) at ω=306°W,
and similarly MURAKAMI (Mk) on 18 May (120°Ls) at ω=297°W. On the same day
AKUTSU (Ak) took the bright Hellas
in G and B on the CCD images at ω=294°W. Mn saw that the interside was not uniform at ω=302°W. Again in Europe,
on 30 May (125°Ls) HGs at ω=306°W and WARELL (JWr) on 31 May at ω=327°W observed that Hellas was very bright in cream colour
(Fig3). JWr also obtained a similar
result on 5 June (129°Ls).
From 23 June (137°Ls), the evening Hellas
faced us again, and Iw felt that the evening part was
whiter than the npr at ω=324°W. On 24 June (138°Ls)
at ω=307°W, Mn observed that Hellas was
as light as the npc, which by then had decreased
considerably in size. At ω=317°W Hellas
was milky white. On 29 June (140°Ls) at ω=283°W, the morning Hellas was detected but was very weak.
the White Hellas (140°Ls ~ 197°Ls)
From the end of July to August, we have for
example the following observations: HIKI (Hk) made on
30 July (156°Ls) at ω=318°W that Hellas
was whitish but not bright. On 1 Aug (157°Ls) Mn observed at ω=286°W, 296°W
etc that Hellas was slightly light but never
brilliant and looked milky white. On 3 Aug Mk, using a 20cm refractor, recorded
that Hellas was whitish but not so bright.
Similar observations were obtained at Fukui up
until 4 August and indicated that the npc looked
sharp but Hellas was dull. As reported in CMO
#195, the observations of Hellas near the CM
were few during the period from 16 Aug (165°Ls) to 15 Sept (181°Ls) and we
regrettably cannot say much . The next apparition will
cover that period, and we encourage our readers to make repeated observations
of Hellas during the summer in 1999.
On 20 Aug (167°Ls) the MGS took the Red picture of Hellas
on the way to the planet, as cited in #195 p2165. Hellas
is clear but its southern part seems to be covered by mist.
On 2 Sept (174°Ls) at ω=332°W, Mn saw the Hellas on the evening
side of the CM, but it was not lighter than the similarly placed Libya.
On 7 Sept (177°Ls) ISHADOH (Id) watched the morning Hellas
at ω=293°W and described that it as yellowish. On 8 Sept at ω=285°W,
Iw saw Hellas as than the npc, while Id observed a
dirty Hellas on 9 Sept (178°Ls) at ω=276°W.
On 10 Oct (196°Ls), Mk observed at ω=299°W, and detected the southern limb
to be light, but it must have not been Hellas.
The observations by NAKAJIMA (Nj)
at ω=299°W and by Mn at ω=303°W show Hellas
as misty but not bright. At ω=313° W, Hellas
was weaker even compared with its northern limb. The apparent diameter was 5.1 arcsecs, and De=08deg N. On 12 Oct (197°Ls) Id observed at ω=301°W
and described Hellas to be slightly yellowish.
This was the last observation this apparition from our side.
Trend of Hellas in the Present Period
We have thus described how Hellas
changed during the period. The observation of the intensity of the bright
region is not uniform from observer to observer, but if roughly summarised, Hellas became quite whitish bright at 085°Ls~090°Ls
and maintained its white brightness up until 140°Ls~150°Ls. We should however
remark that even at the heights at 090°Ls or at 120°Ls we could detect a light
and shade inside the bright Hellas, and as a whole we have never witnessed the
very brilliant Hellas (at least from Japan). It was furthermore observed that
the brightness of Hellas did not remain
constant even in the very active period.
Hellas reduced to the "not so
bright" state after 150°Ls and further after 180°Ls it began to show a
yellowish tint. At this period the state of Hellas
must have been very different.
The period when Hellas is white and
bright is coincident with the period when the Tharsis
ridges and Olympus Mons are very active with white clouds. The mechanism of Hellas is however different from that of the phenomenon
in Tharsis. Hellas
remains bright from the morning to evening while the mountainous region in Tharsis become more
active in the evening. Furthermore we should recall that Hellas
is a basin.
The whiteness of Hellas was not due to
the cloud cover but must have been the frost or ice deposit on the ground of
the basin. After 150°Ls the thawing of the frost or ice brought a change of the
colour inside, which might have been caused by a
superficial dust disturbance.
At the southern hemisphere, 090°Ls implies the winter solstice, and in
such a basin as Hellas the ground tends to be
covered by the H2O or CO2 ice. Another point is that clouds caused by the
ascending air seldom appear at the basin during this period, and the area will
be duller without clouds. As spring approaches that
hemisphere the situation changes: the equatorial band becomes warmer than the
both hemispheres and Hellas then becomes
affected by the warmer condition.
Aftermath and Outlook
If the vernal equinox of the southern hemisphere passes, the sub-Solar
point moves to the southern hemisphere and that hemisphere becomes warmed up.
At the moment the south polar cap (spc) must have begun to thaw already. By
then Hellas is free from ice, and the dust
shows a tendency to rise due to the ascending current.
This apparition we could not observe that very moment from the Earth,
but the Mars Global Surveyor detected the dust disturbance at Noachis in late-November 1997 at 224°Ls~227°Ls as reported
from the IMW Electronic Newsletter on 2 Dec 1997 cited in CMO #198 p2209. we should say Hellas must
have been under a similar meteorological condition.
This suggests we should remember this transitional period in the next
apparition. At this point we should also recall the observation by Toshihiko
OSAWA (Os) who observed a "yellow cloud" in 1954 at 232°Ls. This fact
is not well known, and hence in the original edition of CMO #203 we recited his
drawing and a caption from the Tenmon-Guide 1971 July
extra issue (originally at page 103). His observation was carried out on 11
Sept 1954 at 10:05 GMT. Fortunately Mn was observing at the same time at Fukui
by use of a 15cm refractor and described that Hellas was bright in a reddish
(yellowish?) tint at 09:46GMT, and so we also cited in the issue the drawing
(No 116) as well as a part of the observing note (written in Japanese). The
data was as follows: ω=328°W, De=02deg S and the apparent
diameter=13.8". According to the note, Noachis
was also reddish though Hellespontus was apparent. We
just remarked these observations to show how and when Hellas is well called the
We should further comment, for the sake of the coming 1999, 2001 and
2003 apparitions that the great Noachis dust cloud in
1956 occurred at 246°Ls, and the first small dust disturbance in 1971 happened
Hellas, as the vernal equinox approaches, thus decreases its
whiteness, and another two months will bring the reddish Hellas
with the possible dust disturbance. In the 1999 apparition, the season will be
near 080°Ls at the beginning of the year but will attain 180°Ls on the first
day of August when the apparent diameter will be still 9 arcsecs;
and hence we must be able to watch the change of Hellas
as just described.
* * *
The writer would like to thank Sam WHITBY for his kind and fruitful help
in preparing the present article.
(Mn : Masatsugu MINAMI)
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