94/95 Mars Note (5)

The Reccession of the North Polar Cap observed by Tohoru IWASAKI (Ls 010-080 degrees)

From -- 1994/1995 Mars Note (5) -- "C M O" #170 ( 25 Dec. 1995 )
* Mostly in the last 1994/95 aphelic apparition of Mars, the north polar cap (npc) appeared very evident and looked to be regularly thawing. The apparent disk diameter (delta) went up to 10 arcsecs when the Martian season was at 033 deg. Ls (around on 19 December 1994), and delta again decreased to 10 arcsecs at 081 deg. Ls (around on 5 April 1995): Hence the last apparition was quite relevant to the observation of the recession of the npc from about 030 to 080 deg. Ls.

* Our member Tohru IWASAKI (Iw) was asked by us in advance to pay a special attention to the observation of the regressing size of the npc, and he did his best to meet our expectation and observed continuously from from 032 to 079 deg. Ls. Iw used a 400-power Newtonian having a 21cm speculum. It is remarkable that he started from the circle of 3cm in diameter at 010 deg. Ls (when delta = 6.9 arcsecs yet) and gradually enlarged the circle bit by bit upto 5cm as the real apparent diameter grew up, and then also gradually made the circle smaller after opposition to avoid the ambiguity due to the mismatching of the real diameter and the drawing disk: he employed this method because anyone has a tendency to draw any marking (bright or dark) smaller if the drawing disk is unfitly large, and contrarily draw anything larger if the disk is too small. He also tried to average the size of the npc even if he could not see the details any other than the npc due to poor seeing or transparency, and hence the total observations of the npc exceeded the total number of his drawings.

* Here we pick out about 250 numbers of Iw's drawings made during the period from 010 to 079 deg. Ls. This is the first case we treat of the npc regression in the 1994/95 apparition, but Iw's observation and result we shall see will turn out to be very good and reasonable at least at the period from 032 to 079 deg. Ls. (We note tentatively that Iw observed this season until 138 deg. Ls.)

* The analysis here performed is similar to the previous ones already made in the CMO; All are originally based on the method in A DOLLFUS, Icarus 18 (1973) 142. The preceding results about Iw's observations of the npc was published in CMO #138 p1300. This time also, MINAMI and NAKAJIMA selected Iw's observations and measured the sizes of the npc on his drawings, and then NISHITA processed and made Figures representing the regression curve and the spiral representations of the boundaries of the npc at several intervals of the season.

* Let (psi) denote the central angle of the npc, and let (phi) be the central latitude seen from the Earth, then we can obtain the values of psi by the use of the following formulae (cf. CMO #003 p0017)

1) when psi >/= phi, cos(psi + phi)= 1 - (d/r),
2) when psi </= phi, 2sin(phi)sin(psi)= (d/r),

where 2r is the diameter of the drawing disk and d the depth of the npc along the CM. Formula 2) holds just when the npc was totally inside the apparent disk. If psi = phi, 1) is identical to 2) as easily seen under a simple double angle formula.

Fig.1 Fig.1

* (Fig.1) thus represents the recession curve of the npc observed by Iw in 1994/95. The raw data are plotted and not averaged. Between 050 and 060 deg. Ls there are several plots deviated about ten degrees wide at a certain values of Ls. As seen in (Fig.4) this is however due to the fact that the values at different longitudes are compiled, and reflects the deformed shape of the npc from omega = 000 to 070W deg., and so the deviation must be within five degrees at each longitude. The same phenomenon occurred around at 073 to 074 deg. Ls, again near the similar longitudes (cf. Fig.6). In (Fig.1), supplemented are the Dollfus curve (solid), and the James line (dotted) and the Baum dots (small circles). As to the James linear formula, an interpretation was given in CMO #130 p1199 (originally P JAMES Icarus 52 (1982) 565).

* The so-called Baum plateau is seen at 030 to 040 deg. Ls in the Iw case during which the speed of the thawing was quite slow because of the stagnation of the air above the north polar region at the particular season. Iw's result after 040 deg. Ls is quite in accord with the Dollfus curve.

fig.2Fig.2 (028-040 Ls) fig.3Fig.3 (040-050 Ls)

fig.4Fig.4 (050-060 Ls) fig.5Fig.5 (060-070 Ls)

fig.6Fig.6 (070-079 Ls)

* Figures (Fig.2) to (Fig.6) are the Fournier-type diagrammes showing the recession of the perimeter of the npc spirally as the Martian days proceeded. As the observation-time advances in one day, the dots go round clockwisely, while if the observation-days advance the group of the dots moves counterclockwisely. In (Fig.2), the spiral is broken off once because Iw could not observe from 8 Dec. to 15 Dec. 1994, and the data at 029 to 031 deg. Ls are missing. From 032 deg. Ls on however he was able to record the size rather continuously, and as seen from the Figures his observations were quite successful during the season for delta >/=10" arc. It is notable that his method of observing every 40 minutes was also effectual in obtaining the good results about the sizes of the npc. Finally we would like to express our appreciation of his nice observations of the npc of this season.