Extra Report published in CMO #213 (10 March 1999)
HELLAS, SUPPLEMENT: Hellas was observed very brilliant in mid-February from Japan. The season was 098°Ls on 15 Feb. At page 2412, we described a few observations how they observed when the bright part sank to the evening terminator: We here supplement the observations because we received further rich data made in mid-February from the Japanese observers.
To know the width of the bright part of Hellas, it will be a normal method to determine the CMT of the eastern and western border of Hellas at the time of opposition. However, when the defect of illumination is large we have another opportunity to determine the position of a bright point when it passes the terminator Sun-setting line. When the phase angle is large, the line is quite inside the disk and hence it is more advantageous than the method of making use of the very limb.
We first list up the relevant observation data as far as we received recently by emails and other means: Every observation before ω=325°W recognised the bright remnant of Hellas: On 14 Feb, HIGA's Video tape shows still Hellas at ω=322°W, and on 15 Feb IWASAKI and MURAKAMI noticed the remaining Hellas at ω=322°W and ω=324°W respectively, and it is also evident at ω=320°W on HIGA's tape. On 16 Feb, MURAKAMI observed that Hellas stayed behind at ω=322°W, and HIGA's tape records Hellas still at ω=323°W. The Video tape of HIGA on 17 Feb seems to show the remnant at ω=327°W. We also have the data that an afterimage of Hellas was recognised by HIKI on 14 Feb at ω=336°W, by IWASAKI on 15 Feb at ω=331°W, and by MURAKAMI on 16 Feb at ω=332°W.
On the other hand, the observations in which Hellas was no more evident are as follows: On 14 Feb, the present writer (MINAMI) failed to find the remnant at ω=328°W, ISHADOH at ω=324°W, and HIGA's tape also does not show the remnant at ω=329°W. On 15 Feb, NAKAJIMA missed Hellas at ω=329°W, MURAKAMI at ω=334°W, HIKI at ω=334°W. On 16 Feb, HIKI lost Hellas at ω=334°W and the situation is delicate on the HIGA's tape at ω=334°W. According to HIGA, the original tape shows clearly Hellas until around ω=332°W.
On 16 Feb (098°Ls), the present writer at Fukui watched Mars by the use of a 20cm refractor from 15:20 GMT to 22:00 GMT every forty minutes, and checked Hellas from 19:20 GMT (at ω=310°W) to 21:00 GMT (at ω=334°W) every ten minutes, and obtained the result that the bright part of Hellas was evident up to ω=325°W, while at ω=327°W it turned out to be just like a pin-point, and at ω=329°W it became quite hard to recognise the bright remnant. The disappearance is not so instantaneous as the occultation of a star, and occurred gradually so that the timing must much depend on the aperture of the telescope and the seeing as well as the apparent diameter (δ=9.1" at that time). The seeing was under moderate (good however at around 19h), but we were able to observe all of the possible features of the surface at that time.
Taking into account also the observations by NAKAJIMA, MURAKAMI, IWASAKI and HIGA, we may say that the bright remnant of Hellas, maybe having a somewhat hazed tail, sank at the terminator at an angle from ω=329°W to ω=332°W.
We next use a grid disk a la NISHITA showing the terminator on 15 Feb. On the latitude line of 40°N, the distance from the CM to the terminator is nearly 30°. Since 329-30=299 and 332-30=302, the position of the western border of the bright part must have been ω=299°W to 302°W at the 40°N latitude line. Since the phase angle was 34 degrees and (30+34)/15=4.3, the local time of the Sun-set was just before the 4:30 pm (natural near the winter solstice of the southern hemisphere).
Incidentally if we pick out the result from the CCD images by Don PARKER taken on 31 January, we may say the remnant was concealed at ω=326°W, and hence the position of the border implies ω=298°W since 326-28=298 where 28 is the degree from the CM to the terminator at 40°N on 31 Jan.
We know on the other hand that the western border of Hellas Planitia is featured by Hellespontus Montes whose position is at 315°W on the 40°N. If we take the same level of the cliff as Isidis Planitia, the border of the basin must be at 310°W.
We otherwise have several figures of the Martian surface printed from a software (Starry Night Deluxe, Sienna Software Inc) for the Mac provided kindly by HIGA (LtE below and #205 p2303): According to the model the remnant of Hellas at ω=329°W on 16 Feb is never like a point but shows quite an area. The disappearance then is very slow, and even at ω=334°W, the area is evident while it is just a little smaller than the north polar cap.
The observational ω=302°W is thus quite different to the model's 310°W: How should we then consider this situation?
It may be naive to consider that the frosted floor of Hellas was smaller than the area of the so-called Hellas Planitia: Since the centre of Hellas is nearly at 300°W, the above difference of values is to imply that the western part of the bright Hellas is too small. Hellas looks on the contrary large enough when it passed the CM. So we may consider that the decrease of the bright extension is resulted in when it approaches the evening terminator. It is however not because of the optical diffraction since the point is well inside the disk as deep as the npc is. We may so regard that the westerly border area of Hellas slopes steeply downward to the east. The angle of the slope is steep enough to be much steeper when it comes to the limb side, and the reflection rate of light over the frost will rapidly decrease as it comes to the terminator. The frosted floor including the wall of the cliff will fully shine near the CM, while we may consider that the central part on the floor will only be bright near the terminator even if the frost covers Hellas indifferent to the altitudes. Thirdly we may also take the following possibility into account: Hellas may also receive a veiling by a mist or haze as it approaches the evening terminator. A thick haze has lower intensity than the unveiled frost. The above value may suggest an optical depth of the haze near the terminator.
How about the eastern border of Hellas? Since this has to be checked at present at the behaviour at the very (following) limb, any result will be of much lower accuracy.
As reported in the preceding issue, the CCD images by Don PARKER on 9 Feb showed that Hellas was not bright yet at ω=224°W, while it shined well at ω=238°W. An image of PARKER on 8 Feb shows that Hellas is inside at ω=234°W. So we may derive a result that the easterly border of Hellas lies between 224+65=289°W and 234+65=299°W, where 65 degrees is a distance from the ω to the longitude of the morning limb at the level of 40°N latitude line. The accuracy depends on the distance and 65 is much larger (and so worse) than 30 in the preceding case.
BIVER's observation (reported below) shows a white limb on 15 Jan already at ω=224°W. HIGA's tape on 26 Feb suggests that Hellas is indefinite at ω=212°W, still uncertain at ω=222°W, while it is apparent at ω=232°W. MURAKAMI also caught Hellas on 23 Feb at ω=235°W. MINAMI on 25 Feb just saw the bright Hellas at ω=243°W, and thought that it was misty at ω=233°W. These results suggest that the east border lies around 290°W by CCDs, while 300°W by naked eyes. The difference may be caused by a presence of the morning mist, and hence to obtain the real boundary, the CCD in red must be more powerful.
The eastern contour of Hellas in reality is not so steep but determined at around 285°W at 40°N, and hence we may conclude that the preceding bright end of Hellas lies also inside the bare Hellas basin. The model of HIGA on 9 Feb at ω=224°W proves that Hellas has already come out. Geometrically 285-65=220°W is another possibility. We may also say that the appearance must be quite gradual at the limb. We owe much to HIGA in the above consideration.
The above consideration is made temporarily and preliminarily, but we hope any reader to notice that a large defect of illumination has a value to give geometrically a good measure to any bright spot, white or yellow, because the spot comes at dusk quite inside the disk.
AS REPORTED in a note added in proof in the preceding report, Nicolas BIVER at Oahu, Hawaii sent us his colour drawings made during the period from 17 Dec 1998 to 15 Feb 1999. The last three made on 15 Feb (098°Ls) looked interesting to us at that time, but we failed to report.
BIVER observed Mars successively on 17, 18 and 19 Dec 1998 (072°Ls, App. Diam.=5.8") at ω=105°W, ω=096°W and ω=114°W respectively, and described several dark spots following Nilokeras. Solis L was caught. The southern limb was not whitish on 14 Jan (084°Ls) at ω=225°W, while 30 Jan 1999 (091°Ls) the south top is white at ω=049°W. On 15 Jan 1999 (098°Ls) he obtained three drawings at ω=224°W, 263°W and 285°W where Syrtis Mj, Hellas and Elysium were chased. The first drawing shows a thin Syrtis Mj near the morning limb (however the mist is not explicit) and the edge-on Hellas was there white (not known whether haze or real frost). Elysium was just identified on the first drawing, while it was quite a white area near the evening terminator on the second. The third does not show Elysium any longer, and a morning mist was first drawn. The circumpolar region was largely faded, and the npc was petite light at the bottom.
We tentatively note that the novembre-decembre 1998 issue of l'Astronomie contains two articles written by NBv: The first one, entitled Evolution de la periode de rotation de Hale-Bopp a partirde dessins, is based on his visual observations of the wavy coma of Hale-Bopp. The other is Evolution de l'activite de la comete Hale-Bopp. He more observed Hale-Bopp than Mars in 1997, using the same telescope of 25.6cm spec.
RICHARD McKIM, Director of the BAA Mars Section, sent us an alert by email on 21 February 1999 (received 22 Feb at 01:27JST), which described David GRAY's observation of a possible dust cloud over the morning Chryse-Xanthe: GRAY watched it on 21 Feb (100°Ls) from 2:00GMT to 3:20GMT (ω=010°W to 029°W); and through red light it looked invading Aurorae S and its west with a further veiling of Margaritifer S and M Erythraeum.
This alert seemed however to have been sent selected to some observers only, and since the area of GRAY was far from Japan at that time (just the area of Syrtis Mj was facing to us) and any dust cloud at this season of the Martian year must not have been global, we didn't report it in #212 though it reached us before the edition. We have not heard any case of the so-called global dust storm that occurred at 100°Ls, though this does not imply any could be a short-term dust: In fact, as was observed by Viking Orbiter 1, a sequential series of dusts lasted in 1978 locally at Echus Chasma (01°N, 082°W) from 066°Ls to 115°Ls including the present season (L J MARTIN & P B JAMES, Icarus 77 (1989) 35).
We are of the opinion that any local dust has to be observed much at the region where it was found because it is local. We thought therefore GRAY's case should and could solved in Europe and/or then in the USA. At any rate, we thought the problem was settled when we received CCD images taken by Don PARKER on 28 Feb (103°Ls) around at ω=046°W in which the normal surface looked recovered.
On 7 Mar at 04:27JST, we received the BAA Mars Section Circular 1998/99 No 4 from McKIM in which a detail of the aftermath was described. A lot of observations that were not to miss the bus are reported there, but as far as we judge, it does not look easy at present to find other definite and decisive observations near the CM than those by GRAY. The CCD images of PARKER on 28 Feb are all excellent, and they record locally a bright dusty white-cloud in the southern Chryse off Aromatum Pr as well as a shadowy extension of the Orestes protrusion. Both are to be reviewed in the forthcoming issue.