1995 Mars

Mere Hellas, But No Mean Hellas

(When Did Hellas Start to Be Bright in 1995?)

from CMO #174 ( 25 April 1996 )
-- 1994/1995 Mars Note (10) --

ellas is a basin which in a season becomes very whitish bright: The last apparition in 1994/95 and the next apparition was and is the rare occasions for us to be able to observe how it changes itself from a state having a dull aspect to a conspicuous state with a whitish covering. When it is brilliant on its CM line it looks like a polar cap. If you find however a statement that there is the south polar cap in addition to the north polar cap on the same disk, the statement is very false caused by the presence of the brilliant Hellas. On the other hand if some may consider that Hellas is always whitish bright, this is also false: For instance refer to the D PARKER'S CCD image at 055°Ls printed on the top cover of the CMO memorial issue #171 (25 Jan 1996), and Hellas is apparently dull though partly whitish.

• Later in the last apparition, however, Hellas was very whitish bright: For example, from our Oriental side, the following declare the brightening of Hellas:

Iw-284D 13 Jly 1995 (125°Ls) ω=326°W φ=26°N

Id-073D 13 Jly 1995 (125°Ls) ω=329°W φ=26°N

Tohru IWASAKI (Iw) observed that Hellas was clearly white and considerably bright and Hiroshi ISHADOH (Id) that the evening Hellas is extraordinarily brilliant enough to be mistaken with the south polar cap. The observation

Iw-286D 15 Jly 1995 (128°Ls) ω=277°W φ=26°N

also showed that even the morning Hellas was bright: Note that the apparent diameter at that time was only δ = 05.3", which shows that Hellas at that time was distinctly a bright marking to be seen even for such a small diameter.

･ It is rather well known that the season around 120°Ls well brings the whitish bright aspect to Hellas, as was analysed by SMITH and SMITH (Icarus 16 (1972) 509) as once paraphrased in CMO # 134 pl251. According to them, Hellas gradually becomes weakly active from the vernal equinox and very active (VA) from around 090°Ls. Their discussion was based on the world-wide observations and also on the older documents and hence was comprehensive though the data used were rather a few. However if we base our discussion of Hellas only on our own observations, we are not always be able to reach an inclusive conclusion.

• For example the present writer (Mn) observed in 1980 on 15 Apr at ω=292°W that Hellas was very whitish bright; the season being 093°Ls, but we could not compare the result with the data at the other regions. In 1982, Hellas was blue-whitish bright at the morning side and whitish brilliant near the CM on 27 Feb (091°Ls), and successively around on 29 Mar (105°Ls), and around on 5 May (122°Ls) we also observed the bright aspect of Hellas, but these also did not produce any organic results because our knowledge was necessarily quite limited. The polar caps are always visible from any region and hence anyone would like to make a recession curve irrespective of the surface difference. Hellas however only appears every forty days, and if any world-wide communication is not established, the continuous trace of the brightness change cannot be obtained.

• Before mid-July, we could catch Hellas at the beginning of June for example as follows:

Iw-262D 5 Jne 1995 (108°Ls) ω=235°W φ=24°N

Iw-265D 6 Jne 1995 (108°Ls) ω= 256°W φ=24°N

Mn-715D 7 Jne 1995 (109°Ls) ω=299°W φ=24°N

Iw there emphasised a strong brightness of Hellas. Masatsugu MINAMI (Mn) also, catching Hellas near the CM, observed that it was large whitish like a polar cap. Of course the season was inside the VA one as called by SMITH and SMITH.

One month before, the season was critical however: For example. Masami MURAKAMI (Mk)'s observation:

Mk-248D 2 May 1995 (093°Ls) ω= 295°W φ=21°N

showed that Hellas was blue-whitish and

Nj-191D 5 May 1995 (094°Ls) ω= 279°W φ=21°N

Mn-659D 5 May 1995 (094°Ls) ω= 284°W φ=21°N

(where Nj=Takashi NAKAJIMA) showed that Hellas was roundish white, not so brilliant but it gave an impression of a deposit.

• We at the Oriental side could not observe by the geographical reason the area of Hellas in April, and the information preceding was those obtained in March: The observations

Iw-242D 26 Mar 1995 (076°Ls) ω=275°W φ=17°N

Nj-158D 26 Mar 1995 (076°Ls) ω=302°W φ=17°N

Iw-243D 26 Mar 1995 (076°Ls) ω=304°W φ=17°N

show that the morning Hellas was becoming clearer, and it became well whitish bright near the CM. On the other hand, Mn saw some details inside the area:

Mn-585D 26 Mar 1995 (076°Ls) ω=287°W φ=17°N

Mn-586D 26 Mar 1995 (076°Ls) ω=297°W φ=17°N

In the former, Hellas rather dull bright, but there was another brighter patch existed between Hellas and Ausonia Australis (at the southern limb), and in the latter the brighter patch looked located at the southern extreme part of Hellas. (The former aspect is reminiscence of the area on the HST image on 25 Feb). Id also observed a cloud patch inside Hellas in

Id-060D 27 Mar 1995 (077°Ls) ω=312°W φ=17°N

while Mk observed a bluish-white bright part at the south-eastern edge of the disk in

Mk-238D 27 Mar 1995 (077°Ls) ω=325°W φ= 17°N

Several observations at Fukui on 28 Mar also did not yet disclose the large bright area covering Hellas near the CM (eg as shown in the following):

Nj-162D 28 Mar 1995 (077°Ls) ω=272°W φ=17°N

Mn-593D 28 Mar 1995 (077°Ls) ω=286°W φ=17°N

• We however note that even that time Hellas was already very whitish bright similar to the npc when it moved to the evening limb side as was observed in

Mn-553D 20 Mar 1995 (074°Ls) ω=320°W φ=17°N

Nj-129D 20 Mar 1995 (074°Ls) ω=325°W φ=17°N

(where δ = 11.5")(cf CMO #160).

• The observations above listed are not intended to be complete, but above all, we from the Oriental side cannot collect denser accumulation by the physical reason: Hellas thus faced toward us around only when the season was 063°Ls, 076°Ls, 093°Ls, 108°Ls and 125°Ls. We may safely say from the above argument that Hellas became rapidly active from 074°Ls to 093°Ls, while this period, mainly in Apr 1995, we were unfortunately unable to observe the area of Hellas. Thus we expect that the overseas observations will fill the gap.

he planet was going away at that time, but happily there were obtained several nice observations in April abroad which caught the area of Hellas. We are sorry we cannot well afford to be exhaustive in listing them up all, but we think the following represent the observations of the Hellas area watched from necessary world-wide regions.

The observer's name codes we use are as follows:

NFl: Nelson FALSARELLA (Brasil)

DGh: David GRAHAM: (England)

KGm: Katja GRÜTZMACHER† (Germany)

AHt: Alan HEATH (England)

DLm: David LEHMAN (USA)

FMl: Frank MELILLO (USA)

DPk: Donald PARKER (USA)

PRp: Patrick RAPHAEL (Germany)

RSc: Richard SCHMUDE Jr (USA)

GTc: Gérard TEICHERT (France)

JWr: Johan WARELL (Sweden)

SWb: Samuel WHITBY (USA)

• As April came in, Hellas faced toward Europe and for example the following observations all describe that Hellas was particularly bright (first four in #160, and DGh's in #163):

PRp 04 Apr 1995 (080°Ls) ω= 311°W φ=18°N

KGm 05 Apr 1995 (081°Ls) ω=308°W φ=18°N

GTc 05 Apr 1995 (081°Ls) ω= 308°W φ=18°N

JWr 06 Apr 1995 (081°Ls) ω=298°W φ=18°N

DGh 08 Apr 1995 (082°Ls) ω= 302°W φ=18°N

Especially Katja Gm observed that Hellas was noticeably bright like the npc. Johan Wr declared that Hellas was brighter than the npc, and David Gh commented that it was as bright as the npc if not more brilliant (δ= 09.7”). GTc used the symbol =, but it was difficult for him to see Hellas through denser filters. Succeedingly, in England Alan Ht

AHt 10 Apr 1995 (083°Ls) ω=295°W φ=18°N

observed that Hellas was white and bright at intensity 0.5. On the same day, JWr observed at ω=260°W that Hellas was moderately light. On 13 Apr, DGh caught Hellas at the morning side at intensity 2.

• By that time, Hellas faced to the American continents: On the Nelson Fl observation:

NFl 10 Apr 1995 (083°Ls) φ= 300°W φ=18°N

Hellas was at intensity 0.0. In the US

SWb 10 Apr 1995(083°Ls) φ=341°W φ=18°N

FMI 12 Apr 1995(084°Ls) ω= 342°W φ=18°N

DPk 14 Apr 1995(085°Ls) ω=337°W φ=18°N

observed that Hellas was whitish bright at the evening side (DPk by CCD). In

RSc 15 Apr 1995 (085°Ls) ω=310°W φ=18°N

the southern part of Hellas was bright and

RSc 16 Apr 1995 (085°Ls) ω=320°W φ=18°N

described that Hellas was "distinctly" bright. And then came the record by Flank Ml:

FMI 17 Apr 1995 (086°Ls) ω=309°W φ=19°N

where Hellas was very impressive, just like the spc, and bright through every filter, but especially evident through W56 (Green), the fact bringing him to consider that Hellas was surface frosted (cf CMO #163 pl664). Finally decisive CCD photos were obtained by Don

DPk 18 Apr 1995 (086°Ls) ω=302°W φ=19°N

DPk 18 Apr 1995 (086°Ls) ω=316°W φ=19°N

where Hellas purely and largely white. This suggests that the VA state was attained even seen through the blue light. We however note that

DLm 24 Apr 1995 (089°Ls) ω= 275°W φ=19°N

observed Hellas being dull near the morning limb.

･ The Oriental observation then should follow, but unfortunately the weather in Japan was poorer, and the first naked-eye observation was not obtained until 2 May (093°Ls) by Mk (as listed above).

• We here cite the following which caught Hellas in May in the US:

SWh 22 May 1995 (101°Ls) ω=316°W φ=19°N

DPk 23 May 1995 (102°Ls) ω=318°W φ=19°N

each of which shows that Hellas was highly whitish at the evening side.

• As to the intensity, there is much room for ambiguity because of subjectivity, and hence we cannot put forward a linear comparison ranking. We will however be admitted to assume that the state of Hellas observed by us at around 075°Ls being dull light though bright at the evening limb around 20 Mar (074°Ls) was rapidly changed to be active at around 080°Ls as observed in Europe, and further at around 085°Ls, when FMl was involved; it went to another new phase of activity, and finally it reached the VA state at 085°Ls - 090°Ls. The steps look similar to those obtained by SMITH and SMITH but we have an impression that the wave went to rise up somewhat faster.

• It should be remarked that the VA state may not always be stable. The observations of the brightening Hellas in July 1995 well suggest that the veiling over Hellas was much thicker than before. The next apparition is expected to disclose much more details of the intensity in later period.

• It should also be noted that there were several observations at the beginning of March 1995 claiming of the brightness of Hellas, but other several denied the vast brightening. This was perhaps caused by the fact that some part of Hellas was locally light since January or before. We so referred to the Japanese observations at the end of February (at around 065°Ls) in the afore-mentioned Japanese context but here they are omitted. Just the photo observations by Yukio MORITA (Mo) including the following may deserve to be recorded:

Mo-147B 24 Feb 1995 (063°Ls) ω=244°W φ=18°N

where Elysium was already bright while Hellas was quite dim. This well shows that the brightening wave reached the area of Elysium much earlier than Hellas.

• One may think that observations of Hellas are very primitive to be taken up, but we here tried to treat the matter because we consider it to be highly important to observe the gross intensities of such seasonally varying markings as Hellas internationally under the world-wide communications. The coming apparition (in 1997) is more preferable to observe the status of Hellas as well as other could-be bright areas (Argyre basin in the southern hemisphere and Alba, Tharsis, Elysium et al on the other hemisphere), and we thus expect much more in the coming apparition.

(Mn)

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