Mars Sketch (4)
from CMO #202 (25 April 1998)
-- The Rift-Aspects inside the
North Polar Cap
between the Period 080°Ls ~ 140°Ls --
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 (082°Ls) 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 050°Ls in
December 1996 when the apparent diameter was yet 7 arcsecs,
and after a while when it became large enough, Rima
On the other hand, Chasma Boreale was detected visually by Hiroshi ISHADOH (Id), as reported in #186 p2018, on 25 Feb 1997 (083°Ls) when the apparent diameter was 13 arcsecs (Id-054D).
Fig 1: ISHADOH's drawing of the np
region on 6 Mar 1997 (087°Ls) at ω=300°W
Fig 2: MINAMI's rough sketch of the np region on 20 Mar 1997 (093°Ls) at ω=137°W
Figure 1 reproduces here his drawing made ten days later on 6 Mar (087°Ls) at ω=300°W 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 (093°Ls) at ω=137°W where the eastern end of Olympia was caught and the much morning area was slightly hazed.
Similarly Toshiaki HIKI (Hk) observed
Many Japanese observers detected
Figure 3 so shows the HST image of
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 (076°Ls):
Fig 3: HST image around at ω=200°W on 30 Mar (097°Ls) 1997
The HST image on 10 Mar (089°Ls) 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 011°Ls 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 119°Ls from ω=285°W which is comparable with the HST one at 8 Apr 1995 (082°Ls) at ω=282°W 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=270°W. 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=285°W on 17 May (119°Ls) 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=275°W 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=000°W (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=000°W 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 180°Ls does not show well the npc (cf #200 p2236).
Fig 6: The 1997 npc compared with Viking's image