As 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 085deg Ls to 090deg Ls. On the evening limb Hellas at 075deg 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 092deg Ls. The report in #189 so employed the threshold where Hellas turned Very Active to be around 090deg 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 (086deg Ls) 1997, Hellas was already well whitish. On 5 Mar (087deg Ls) when seeing was better, Hellas was blue-white but not brilliant at LCM=263deg W, while it could not be said to be brilliant even at LCM=272deg W (phase angle=10deg ). Hellas was still weaker than the npc at LCM=314deg W to LCM=333deg W though some partial areas looked fairly bright.
As is widely known, the HST took the pictures of Hellas on 10 Mar (089deg Ls) and on 30 Mar (097deg 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 120deg Ls.
The observations in April show Hellas as follows: Mn observed on 9 Apr that it was light and whitish in the evening after LCM=327deg W, while on 10 Apr (102deg Ls) it was not so light in the morning. Also, Hellas showed internal structure on 11 Apr at LCM=309deg 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 (107deg Ls) at LCM=323deg W; GROSS (HGr) on 21 Apr (107deg Ls) at LCM=302deg Wet al (Fig 2), STARZINSKI (HSt) on 25 Apr (109deg Ls) at LCM=282deg W etc (cf #195), and WHITBY (SWb) on 30 Apr (111deg Ls) at LCM=321deg W. LEHMAN (DLm) observed that Hellas was very bright on 5 May (114deg Ls).
In May, IWASAKI (Iw) observed the whitish bright Hellas on 17 May (119deg Ls) at LCM=306deg W, and similarly MURAKAMI (Mk) on 18 May (120deg Ls) at LCM=297deg W. On the same day AKUTSU (Ak) took the bright Hellas in G and B on the CCD images at LCM=294deg W. Mn saw that the interside was not uniform at LCM=302deg W. Again in Europe, on 30 May (125deg Ls) HGs at LCM=306deg W and WARELL (JWr) on 31 May at LCM=327deg W observed that Hellas was very bright in cream colour (Fig3). JWr also obtained a similar result on 5 June (129deg Ls).
From 23 June (137deg Ls), the evening Hellas faced us again, and Iw felt that the evening part was whiter than the npr at LCM=324deg W. On 24 June (138deg Ls) at LCM=307deg W, Mn observed that Hellas was as light as the npc, which by then had decreased considerably in size. At LCM=317deg W Hellas was milky white. On 29 June (140deg Ls) at LCM=283deg W, the morning Hellas was detected but was very weak.
Decay of the White Hellas (140deg Ls ~ 197deg Ls)
From the end of July to August, we have for example the following observations: HIKI (Hk) made on 30 July (156deg Ls) at LCM=318deg W that Hellas was whitish but not bright. On 1 Aug (157deg Ls) Mn observed at LCM=286deg W, 296deg 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 (165deg Ls) to 15 Sept (181deg 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 (167deg 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 (174deg Ls) at LCM=332deg 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 (177deg Ls) ISHADOH (Id) watched the morning Hellas at LCM=293deg W and described that it as yellowish . On 8 Sept at LCM=285deg W, Iw saw Hellas as than the npc, while Id observed a dirty Hellas on 9 Sept (178deg Ls) at LCM=276deg W. On 10 Oct (196deg Ls), Mk observed at LCM=299deg W, and detected the southern limb to be light, but it must have not been Hellas. The observations by NAKAJIMA (Nj) at LCM=299deg W and by Mn at LCM=303deg W show Hellas as misty but not bright. At LCM=313deg W, Hellas was weaker even compared with its northern limb. The apparent diameter was 5.1 arcsecs, and De=08deg N. On 12 Oct (197deg Ls) Id observed at LCM=301deg 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 085deg Ls~090deg Ls and maintained its white brightness up until 140deg Ls~150deg Ls. We should however remark that even at the heights at 090deg Ls or at 120deg 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 150deg Ls and further after 180deg 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 150deg 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, 090deg 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 224deg Ls~227deg 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 232deg 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: LCM=328deg 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 reddish Hellas.
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 246deg Ls, and the first small dust disturbance in 1971 happened at 213deg Ls.
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 080deg Ls at the beginning of the year but will attain 180deg 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.
(Mn : Masatsugu MINAMI)