Deviation of the spc from the pole
Masatsugu MINAMI
from CMO #240 (25 February 2001)
T |
he present apparition
gives an good opportunity to observe the south polar region (spr) at around 180°Ls. As was elucidated in the 3rd of this
series in #237, the apparition in 1986 was similar to the present one, while
the season and the apparent diameter as well as the sub-Earth point at
opposition were slightly different. The season in 1986 when the planet was
closest to the Earth was around 205°Ls, and so we should say we could have
acquired a most plausible data concerning the spr
much after 180°Ls. So this must be a good season to accumulate the data of the
season preceding the 1986 season.
To show how we did observe the
south polar cap (spc) in 1986 in the appropriate season, we here cite a page
from CMO #029 (25 March1987 issue) at p0225 where the locations of the spc's edge were plotted on a polar coordinate
based on the observations of the present writer (Mn) during the season from
198°Ls (1 July 1986) to 216°Ls (31 July 1986). The period included the
opposition on 10 July and the day 16 July when Mars most approached. The Figure
was obtained by Le-Hsia CHANG at Taipei who measured
the spc's depth d on Mn's drawings 140 times during
the time Mn stayed at Taipei and calculated the latitudes of the edges based on
the procedure introduced in CMO #003 (25 February 1986); one of the main
formulae was given
psi
= - |phi| + cos-1(1- (d/r)),
where psi is
the half-width of the spc, phi the central latitude, and r
the radius. CMO #003 also shows a Mariner 6 photograph of the spc on 30 July
1969 at 199°Ls (the season in 1969 when the planet was closest was 170°Ls
however). The spiral does not show the contour of the spc at
a fixed Ls, but the observations by Mn were performed every 40 minutes
and hence locally the data were dense.
The result proved
that the spc was normal or was normally thawing in 1986 at the period; this was
known if compared with the averaged results compiled by G E FISCHBACHER
et al (Lowell Observatory, 1969) which were read from the Lowell Archives from
1905 to 1956. The second map in Fig 2 (which is cited from a page in P B JAMES
et al, JGR 84 (1979) 2889) is the very map and shows the contours at
200°Ls and 210°Ls to be compared. The first map shows the data acquired by the
Vikings.
Later Kyosuke IWASAKI et al published their data on the 1986 spc
boundary observations at Bosscha (K IWASAKI, Y SAITO,
Y NAKAI, T AKABANE, E PANJAITAN, I RADIMAN & S WIRAMIHARDJA, Behavior of
the Martian South Polar Cap 1986, Publ. Astron. Soc.
Japan 41 (1989) 1083) and we cite here one of their results during the
period from 205°Ls to 214°Ls. The circle is claimed as the best-fit circle
and its center is located at (88.4°S, 10.8°W). No other earlier data in 1986 is
there.
We thus recommend the readers
to observe the spr intensively this season also, if
possible every forty minutes, to make the spiral polar coordinate map of the spc's edges and to compare the results with the data of the
spc perimeters in Fig 2. The Viking observation began from 176°Ls, and hence no
data before the period, but the data from the
Another but
interesting observations may be made when the centre of the spc deviates
from the south pole. As to the phenomenon, we touched in CMO #008 p0057:
Referring to the observations also by A DOLLFUS in 1956, we called the readers'
attention that they should concentrate on the spr
from 230°Ls to 240°Ls. This year the season of 230°Ls will
reach on 9 September 2001 and 250°Ls on 10 October 2001. The deviation
was well observed in 1986 (cf CMO #018 p0145 (10 Oct
1986)), but more minutely in 1988:
The last but not the least we
make mention of the detachment phenomenon of Novus Mons (the Mountains of
Mitchell): According to E C SLIPHER, Novus Mons begins to detach from the spc
between 250°Ls and 252°Ls, but the Viking image shows that it was still a part
of the spc as a peninsula even at 255°Ls. The peninsula is however to be
checked around from 230°Ls, and hence the attention should be called from the
beginning of September this year. Note also the Viking already found the shape
of the brighter Novus Mons well inside the spc even at 183°Ls. All these are
stated in CMO #007 p 0047 (25 April 1986).
Found is a report on the
appearance of the early stage of the spc in 1986 and 1988 in CMO #115 p1004,
and a report about the detachment of Novus Mons in the case of 1988 was given
in CMO #111 p0963. The head page of CMO #116 shows a TP photo of MURAKAMI (Mk)
by the use of a 10cm refractor on 29 Aug 1988 (at 262°Ls) at LCM=000°W where
Novus Mons is clearly shot.
We add that to compare the spc
shown here in Fig 1 with the case this year, we should acquire the data of the
spc perimeter at the beginning of August by taking account of the fact that the
tilt of the central latitude is different. The following spiral data in 1986
was for example cited in CMO #040 p0335 where the season from 216°Ls to 235°Ls
(during August 1986) was treated. There was observed no abnormality in the rate
of thawing in 1986.
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