1996/97 Mars Sketch

1996/97 Mars Sketch (10)
from CMO #207 (25 September 1998)

-- A Shadowy Band between Olympus Mons and Tharsis Montes --

FALSARELLA's drawing on 27 Feb 1997 (084°Ls)
at ω=160°W by 260x 20cm spec
showing a dark band preceding Olympus Mons

NFL 27 Feb

Nelson FALSARELLA (NFl) in Brasil communicated to us on 27 Feb 1997 and on 1 Mar 1997 his interesting observations of a dark band which was seen running at the channel between Olympus Mons and Tharsis Montes when they moved to the evening side (see CMO #186 p2023 for more details).

His observations of the dark band at the time were repeatedly made as follows:

26 Feb (083°Ls) at ω=155°W - 170°W

27 Feb (084°Ls) at ω=154°W - 169°W

28 Feb (084°Ls) at ω=148°W

01 Mar (085°Ls) at ω=131°W - 151°W

(while he could not detect the segment at ω=185°W -192°W on 26 Feb). We here reproduce his drawing made on 27 Feb as Fig 1. The apparent diameter was 13.2 arcsecs.

In April NFl also saw the same band on

06 Apr (101°Ls) at ω=133°W - 141°W

The dark band was considered to be similar to the one once alluded to at p1610 of #158, and observed well in 1982 from Japan. It was seen since 7 Mar 1982 (095°Ls) and the drawing cited in #201 p2247 as Fig 10 is an example which shows the aspect at that time. The apparent diameter was 12.9 arcsecs before opposition. The band was also seen in April 1982.
In 1997, we observed the area from 9 Feb to 14 Feb from Japan when the disk diameter was rising, and the drawing in Fig 8 shown at the same page (#201 p2247) is an example on 10 Feb (076°Ls) when the apparent diameter =11.5". One round after, we could observe the area from 17 Mar (092°Ls) through 25 Mar (095°Ls) when the planet was closest to the Earth.

Hk-065 : HIKI's drawing on 20 Mar 1997 (093°Ls) at ω=161°W by 340x 16cm spec
Mn-406 : MINAMI's drawing on 20 Mar 1997 (093°Ls) at ω=154°W 630x 20cm Refra

Unfortunately however the observation of the angle was scarcely made in Japan except at Fukui. An exceptional case was the one in Fig 2 made by Toshiaki HIKI (Hk) at Nagano by the use of 340x16cm spec on 20 Mar at 14:30 GMT (ω=161°W, Hk-065). This is compared here with the drawing (Mn-406) made thirty minutes earlier at ω=154°W by Mn shown in Fig 3. NAKAJIMA (Nj) took drawings of the dark band on the day at ω=159°W & 169°W.

As pointed by NFl, the shadowy band is scarcely seen on the images by HST. This must just be because it is different from the so-called the dark marking, but it must be the surface ground itself which appears when it is free from any atmospheric water vapour. Thus the band must be seen if taken through blue light. The naked eyes can catch the wider range of colour waves under good seeing and so the shadowy band is more apparent when the Montes appear to be very whitish bright.

As to this point, the HST images taken on 30 Mar as cited in #191 p2102 are suggestive. On the blue-light disk, we can easily see that the areas that are free from the white mist are not distinguishable from the so-called dark markings. The area in question (the channel between Olympus Mons and the Tharsis ridges) is white at this moment because the area is covered by the morning mist. The observations by NFl and others so imply that the area will be free from the white mist as it moves to the late afternoon side, while the summits of Montes become thickly covered by the white clouds.

We are similarly aware that the canal called Deuteronilus is not exactly the dark marking but mostly the orange-coloured channel which is casually free from the atmosphere. This however appears frequently as a dark band to the naked eyes.

We finally call your attention to the fact that the next opposition will be the last chance to watch the detail of the dark band as well as the white cloud over the evening Olympus Mons.

(Mn : Masatsugu MINAMI)

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