Forthcoming 2007/2008 Mars
(14)

Coming NPC in 2008

CMO #338 (25 November 2007)

Masatsugu MINAMI


 

Japanese here


F

rom this apparition, we are to observe the planet Mars of the northern season in a favourable condition. First of all we are concerned with the north polar cap (npc). On 9 December 2007, the spring equinox of the northern hemisphere arrives with the tilt φ=4N so that it is preferable to watch the north polar region (npr). Recent technique by IR ccd makes us allow digging out the ground configurations including the existence of the outer frozen ring of the npc in comparison with the RGB images, but since the usual naked eye (should-be also by the usual RGB images) shows more or less the north polar hood (nph), it is not easy to grasp the largest npc before the spring equinox. It is usual just after λ=000Ls for us to be able to probe the final struggle of the npc and the nph, the latter behaving spontaneously in its own way. After the spring equinox we encounter the period of the recession of the npc.

In any macroscopic world it is quite rare to have a linear variation, while at the regression of the npc, there is one example which claims a linear recession curve such that Θ=57.7+ 0.216λ, where Θ is the latitude of the snow line: This was put forward by P B JAMES in Icarus 52 (1982) 565 and based on the Viking data in 1979/1980. At λ=000Ls, Θ=57.7N, and λ=010Ls, Θ= 59.9N. At λ=355Ls, just before equinox, there seem to exist good red and blue images of the npc which told Θ= 55N; this result however is not necessarily on the line.

Figure 1 was made by M MURAKAMI (Mk), and the line on the Fig is the James straight line. The dotted line has nothing to do with the npc recession, but is just shown to assert how this apparition is favourable (not best favourable) to watch the rudimental state of the npc.

P B JAMES had already published the recession curve of the 1977/1978 npc based on the Viking early data in JGR 84 (1978) 8332 in which the npc did not thaw linearly: As mentioned in CMO#130 (25 Feb 1993), the npc looked to come to halt around from λ=010Ls to 030Ls and it belongs to the Baum plateau (as we called so there). We here cite his Fig. 1 as Fig. 2 here. Apparently JAMES knows well about the Baum standstill (originally in FISCHBACHER et al 1969). Here IWASAKI implies the work by Kyosuke IWASAKI and others at the Kwasan Observatory, Kyoto University.

What's the Baum plateau? W A BAUM and L J MARTIN published an exquisite work on the polar caps with G E FISCHBACHER based on the Lowell plate collection covering more than 60 years, and as a by-product they put forward a figure which showed a recession of the npc. In 1973, BAUM and MARTIN revised the plots adding the results obtained by the IPP (Bull. Am. Astron. Soc. 5 (1973) 296). Here we cite their work as Fig 3 cited from W BAUM, in Exploration of the Planetary System 241-251 (1974) (ed. Woszczyk & Iwaniszewska), "Results of Current Mars Studies at the IAU Planetary Research Center." Just we here beg his pardon if we reverse his figure in our way because he uses a conversion where the north is upward. This shows apparently the npc comes to a standstill from λ=010Ls to λ=060Ls. The essence of this plateau was first shown us from Audouin DOLLFUS since his work on the npc recessions in 1946, 1948 and 1950 was compared with the work of BAUM and others. The Dollfus curve was cited in CMO #109 (15 Sept 1991) p0944 (by the courtesy of Prof DOLLFUS) in which Baum plateau was shown as halting dots on λ=010Ls, 020Ls, 030Ls, 040Ls. These were used in Fig 1 here. Note that it stopped at λ=040Ls in early work, but in Fig 3 it was extended to λ=060Ls because a new data was added (what year was it when the new dot was obtained?). Incidentally the Dollfus curve (very well-known) does not show any data at around λ=000Ls, and those at λ= 020Ls and λ=030Ls look to suggest a halt. At λ= 040Ls the diagram implies that the snow line was lower than the Baum's dot.

It should be remarked that the Baum-Martin diagram is made from the selected excellent data in an averaged form ranging over 60 years. Therefore it shows no more than a general tendency and does not imply the trend is all the same every Martian year. So it is not impossible if such a linear case as in 1979/80 is found. It was suggested the case was the one when no dust storm preceded the npc season, while it was known in 1977 preceding the 1977/79 case that two great dust storms appeared. So does the Baum-Martin diagram imply that the dust storm occur every Martian year in an averaged sense?

Since at the spring equinox the pole-to-pole grand circulation changes its direction via the equator-to-pole circulation, it is plausible the changing at the pole is accomplished slowly in a macroscopic world so that the wind-blow dies down for a while, though an instantaneous return is possible in a mathematical chart (no dust case or more in a vacuum atmosphere?). If the wind remains to die for a while, the recession of the npc also must come to a standstill. The retardation may be affected by several macroscopic elements, and the distribution of dust may be an ingredient. If a delay proves enough for the npr to become warmer than the equator band, the npr will became well ventilated. So we should say that the halt or retardation may be different from year to year depending upon some hidden conditions; that is, the delay may not occur, or a shorter delay may occur or a longer delay may appear, different continuously. Since we are not necessarily allowed to chase the npr every Martian year, we need to file up the data when it is possible for a longer period.

Previously we had chances to chase the recession of the npc in 1990 and 1992. However as shown in a chart in CMO #327 (25 Jan 2007 issue), in 1990 the tilt φ of the north pole was away from us, so that it was inconvenient to observe the npr. Contrarily in 1992/1993, φ13N when λ=000Ls on 21 Nov 1992 (φ=4N at λ=040Ls), and so it was quite favourable (more than this apparition). As to the npc regression we picked out the data of several observers in the CMO. Here we cite from CMO #139 (25 Nov 1993) p1314 a diagram of the present writer (Mn) plotted out by A NISHITA (Ns). We should say in 1992/93 the Baum plateau continued up to λ=035Ls. From Japan we encountered M Acidalium at around λ=027Ls. The data was processed by Ns based on the Dollfus formula. Solid curve shows DOLLFUS' data, and the dotted line JAMES' 1979/80 line. The open circles denote the BAUM-MARTIN dots.

 


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