1996/97 Mars Sketch (4)
from CMO #202 (25 April 1998)


-- The Rift-Aspects inside the North Polar Cap
between the Period 080Ls ~ 140Ls --


Japanese here


I

n the preceding 1994/95 apparition, it was difficult to watch the latter period of the north polar cap (npc) when some detailed rifts inside the npc were seen until we did not refer to such an HST image taken on 8 April 1995 (082Ls) as cited in #177 p1871.We there compared it with the bird-eye view of the npc made from the images taken by Mariner 9 and Viking Orbiters to conclude that no serious difference was there. That is, such a fissure called Rima Tenuis was absent.

  This apparition in 1997 we were able to watch the latter period of the npc even by the naked eyes because the season shifted, and further results were brought about by HST. The season reached 050Ls in December 1996 when the apparent diameter was yet 7 arcsecs, and after a while when it became large enough, Rima Borealis and Olympia were already apparent.

  On the other hand, Chasma Boreale was detected visually by Hiroshi ISHADOH (Id), as reported in #186 p2018, on 25 Feb 1997 (083Ls) when the apparent diameter was 13 arcsecs (Id-054D).

Fig 1: ISHADOH's drawing of the np region on 6 Mar 1997 (087Ls) at ω=300W
Fig 2: MINAMI's rough sketch of the np region on 20 Mar 1997 (093Ls) at ω=137W

 

  Figure 1 reproduces here his drawing made ten days later on 6 Mar (087Ls) at ω=300W where Chasma Boreale was apparent again as well as the western end of Olympia in the evening.

  Figure 2 is a rough and partial sketch of the north polar region made by the present writer (Mn) in the Observing Note of a drawing (Mn-404D) on 20 Mar (093Ls) at ω=137W where the eastern end of Olympia was caught and the much morning area was slightly hazed.

  Similarly Toshiaki HIKI (Hk) observed the morning Olympia on the same day at ω=149W, and Masami MURAKAMI (Mk) at ω=152W. Among several overseas observers, SamWHITBY (SWb) detected Olympia clearly in the outskirts of the npc at ω=161W on 8 April (101Ls) by use ofW21.We hear from Mk that Don PARKER (DPk) shot the same aspect on 7 April, but his observation is not filed by us yet.

  Many Japanese observers detected Olympia when the planet was near at opposition as a detachment to the very south of the npc at around ω=200W. On 17 Mar, it was observed by Tomio AKUTSU (Ak) at ω=197W by CCD (R), on 18 Mar visually by Mk at ω=198W, on 20 Mar by Mn at ω=183W etc, by Takashi NAKAJIMA (Nj) at ω=188W etc and by Kunihiko OKANO (Ok) at ω=196W (CCD in R) and on 22 Mar by Yukio MORITA (Mo) at ω=191W (CCD in R) and so on.

 

  Figure 3 so shows the HST image of Olympia seen from around ω=200W on 30 Mar 1997 (097Ls): The white spot near the CM is the Korolev crater located at (196W, 73 N).

 The image in Fig 3 proves if seen in colour that one third of the npc at the morning side is more faded in a tint of ocher. This was anticipated in CMO #185 p2003 where it was stated that one fourth of the npc looked faded already on 10 Feb (076Ls):

Fig 3: HST image around at ω=200W on 30 Mar (097Ls) 1997

 The HST image on 10 Mar (089Ls) shows this more clearly, while there must be a problem of unstable calibration to the extent that since HST found out a dust streak inside the npc at 011Ls on 18 Sept 1996 they tended to use less Blue light to make the details clear inside the npc until the HST image on 30 Mar 1997 proved the abundance of water vapour on the summer hemisphere (cited in #191 p2102).

  We next show as Fig 4 the HST image of the npc taken on 17 May 1997 at 119Ls from ω=285W which is comparable with the HST one at 8 Apr 1995 (082Ls) at ω=282W cited in #177. Figure 4 is from the B image cited in #200 p2234. To further compare, we cite as Fig 5 a bird-eye view of the npc made from the images taken by Mariner 9 and Viking Orbiters whose top is directed to ω=270W. This is the same map that we cited in #177 p1872.We easily see that no rift to be called Rima Tenuis is visible.

 

Fig 4: HST image around at ω=285W on 17 May (119Ls) 1997
Fig 5: The npc by Mariner 9 and Viking Orbiters

 

  It should be then remarked that the image in Fig 4 is taken in the same season as Audouin DOLLFUS obtained the detailed drawings of the 1982 npc cited in CMO #183 p1985. The one at ω=275W really proves that the HST reproduces the main features observed by DOLLFUS (by the use of a 200cm telescope at Pic) and eventually we see that no sign of Rima Tenuis was there in 1982. A small difference is the direction of the inlet of Chasma Boreale. This occurs also in Id's drawing in Fig 1 proving that the visual detection of the markings very near the northern limb of the disk is no easy task.

  Finally we show a sequence of the HST npc in 1997 seen near from ω=000W (Fig 6, prepared by Mk). The upper left image is the same with Fig 5 but rotated by 90 and changed aspect ratio so that the top is directed to ω=000W from which side it is difficult to see Olympia. The B light was fully used in May and the npc area looks as if more snowed, but note that the HST's B image taken on 12 Sept 1997 at 180Ls does not show well the npc (cf #200 p2236).

Fig 6: The 1997 npc compared with Viking's image    


(Mn)


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