94/95 Mars Note (12)

1994/1995 Mars Note

From -- 1994/1995 Mars Note (12) -- "C M O" #177 ( 25 July 1996 )
On the North Polar Cap by the HST on 8 Apr 1995 at 082degs.Ls
npc by the HST
* James BELL kindly sent us an interesting HST image of the north polar region taken on 8 April 1995 at 082degs.Ls which was seen from LCM=282degs.W when the central latitude was De=18degs.N (See Fig 1).The image shows several rifts inside the polar region to be identified. It is however difficult for us to project the region stereographically onto a polar coordinate while the work may be inevitable to say something about the minute structure of the region.
Fig 1: The npc on 8 Apr 1995 at LCM=282degs.W taken by the HST (courtesy of Jim BELL)

* We so choose a different way to approach the problem: We instead first try to employ an established polar map which was made at the US Geological Survey (mainly by J L INGE) based on the results from Mariner 9 and Viking Orbiter mosaics in the 1970's. Figure 2 is copied from a colour version of the airbrush portrayal of the north polar region, and Fig 3 is a similar one overlaid on a coordinate grid. Note that we set the longitude 270degs.W upward (usually 180degs.W being topwards). These maps are originally obtained from the mosaics taken on 12 Oct 1972 at 094degs.Ls by Mariner 9. It is however known that the similar configuration was seen during the Viking period up until 150degs.Ls, though the brightness was much weakened with several spiral-type fissions which were more apparent inside the npc.

Fig 2: The npc by Mariner9 and Viking Orbiters
Fig 3: The Mariner 9-Viking npc overlaid

* Figure 2 clearly shows the presence of Olympia made of several white fragments, and Rima borealis is seen broadly separating Olympia and the main part of the npc. It should be remarked that another rift cuts deep into the cap on the right-hand side around from 050degs.W. This is the fissure called Chasma boreale (1973, IAU). Note that at least during the Mariner and Viking period the so-called Rima tenuis was not visible. This is often reasoned that in the 1960s and 1970s the climate was cold enough to hide Rima tenuis. On the other hand there can also be another opinion that such a fine line is similar to a lot of fine canals observed by Giovanni V SCHIAPARELLI, and since the canals have been denied these days, such a fine line inside of the npc must also be truly absent. Incidentally Rima tenuis, the name of which was given by E M ANTONIADI in 1929, was claimed found by SCHIAPARELLI in 1888.
* We next prepare a grid like Fig 4 for the north polar region with the tilt of the north pole towards to the Earth to De=18degs.N, and try to map the npc in Figs 2 and 3 into Fig 4 with LCM=280degs.W to make Fig 5.
* The mapping here may however be somewhat sloppy, but such rifts as Chasma boreale were carefully checked.

Fig 4: A grid of the np region at De=18degs.N: the smallest circle is at 85degs.N, the next one at 80degs.N.
Fig 5: The npc in Figs 2&3 is mapped on Fig 4 with LCM=280degs.W.
Fig 6: The same as Fig 1: HST image at LCM=282degs.W compared with Fig 5.

* Our plan is finally to compare the configu- ration of the npc in Fig 5 (at LCM=280degs.W) with the npc in the HST image of Fig 1(at LCM=282degs.W) which is reproduced again here as Fig 6.

* The comparison between Fig 5 and Fig 6 readily implies that the configuration of the so-called perennial npc taken by the HST in 1995 is not so different to the one supposed from the data of the Mariner 9 and Viking Orbiters: The big gap of Rima borealis is very evident in both cases with the remnant fragments of Olympia, and Chasma boreale really appears in the HST image as a deep fissure. It is however difficult to find the so-called Rima tenuis in the HST image as another rift which otherwise might have run through near the pole.
* We incidentally note that the npc which appeared in the well-known HST images taken on 24 Feb 1995 at 064degs.Ls (cf. CMO #159) also suggests several rifts inside which may be identified with those seen in the famous image taken by Mariner 9 on 7 Aug 1972 at 066degs.Ls: Chasma boreale is just appearing to the north of M Acidalium, and Rima borealis broadly complex. A fine fissure is seen to the east of the main part of Rima borealis.
* The image of Mariner 9 at 066degs.Ls shows that the snow line is just to the north of the crater called Korolev at 73degs.N, and hence the size of the npc in 1972 was not so large but very normal (Phil JAMES recorded the values in a Figure showing the regression in his paper in Icarus 52(1982)565 -- the very Figure in the article was once cited in CMO #130 p1199 where the solid boxes inside the Fig represent the values indicated by the observation of Mariner 9.) This shows that the npc was not large enough to hide the supposed rift Rima tenuis at least in 1972.