Forthcoming 2005 Mars
(7)
Start of Parva and Magna Depressiones
Masatsugu MINAMI
Japanese here
his is just a remark to invite the readers to an
urgent observation of the spc in the coming month as was already somewhat
cautioned in #301 (25 Jan 2005) at page Ser2-0029.
As remarked there,
the Lowell team
in 1894 started on 22 May at 16:25 Mountain Standard Time (or 23 May at 11:25
GMT). In fact on the day W H PICKERING detected a dark spot inside the big spc;
this being important because it was a starting of LOWELL's idea of the polar sea. Percival LOWELL stated in his
first book on Mars (1895) as "On the first night that Professor W. H.
Pickering observed it, on May 22, with the six-inch telescope, he suspected a
rift crossing the cap from longitude 330°W to 170°W. This rift grew more and more evident, until, in
the early part of June, it was unmistakable." (Mars, p85). Since PICKERING's observing note on 22 May just goes as follows: "I
cannot be sure whether there is really a mark in the snow or whether the effect
is due to shading as one approaches the terminator."
(Ann. Lowell Obs.
vol 1, 1898), it is not certain it was really a
complete rift from the outset, but presumably A E DOUGLASS might have minutely
checked it later: If we compare the polar map of LOWELL
with the one made by E M ANTONIADI, the rift LOWELL assumed must have been partially a
straightened Rima Australis.
Near it there are two dark Depressiones called Magna Depressio and Parva
Depressio (both named by ANTONIADI).
The date 23 May 1894
implies the Martian season λ=210°Ls, and the very
season will come this year on 12 May, and so it may be urgent and interesting
to try to search it. In 1894 the apparent diameter was slightly over 8",
but this year it is only 7.5", and so less convenient, but considering the
telescope PICKERING used was a 15 cm refractor we should say it is well
possible that the present day ccd image will well produce the dark spot inside
the spc.
We here recall that
the apparition in 1894 looks quite the similar to the one in 1973. Shotaro MIYAMOTO at the Kwasan
Observatory, Kyoto,
really detected dark spots inside the spc on 14 May 1973 (λ=206°Ls): He noted: “Dark markings inside the cap". The apparent
diameter was 7.3" on the day and the observation was made at ω=213°W. The drawing printed in his article (Contri. Kwasan and Hida Obs, Kyoto Univ No 217, 1974) is not
so clear, but two dark spots are seen inside the spc. If we compare with the Antoniadi polar map, they look to correspond to Magna
Depressio and Parva Depressio. According to ANTONIADI, both appear quite at the
same season, and his 1924 observations showed they were first observed at η=292°~294° (p214; here η
being the heliocentric longitude) and so it was really at about λ=207°Ls~209°Ls.
The observations in
2003 also revealed a dark spot and the associated rift as reported in CMO
Mars Report #11 in CMO #276 where drawings and ccd images are compared 
with each other, each
being caught from ω=123°W~129°W during the period of season from λ=207°Ls to 223°Ls. The angle ω=123°W~129°W implies the dark
spot must be Parva Depressio or its low angle superposition with Magna
Depressio. Here is shown an excellent image by VALIMBERTI (MVl) made on 24 June 2003 (λ=209°Ls) at ω=127°W. Note however that the apparent diameter is very
different this year.
So we recommend the readers in alert to watch
the inside of the spc around from 6 May 2005 (λ=206°Ls) in the range of ω=100°W~300°W. On 7 May, the
angles may be watched from 0 hrs to 14 hrs GMT and on 14 May from 5 hrs to 18
hrs GMT. Thus the chance does not come to Japan
unfortunately, but it is possible to face the angle from Europe and the US. Since on 17
May (λ=212°Ls) at 5 hrs JST the
angle ω=303°W comes and so we may
begin to catch it from Asia.
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