98/99 Mars CMO Note (1)

1998/99 Mars CMO Note
- 01 -
from CMO #225

-- The morning bluish Syrtis Mj beneath the white mist --

  It is well known that Syrtis Mj shows a bluish tint, and sometimes appears conspicuously blue near the evening or morning limb beneath the vapour mist.
  The present writer (Mn) remembers well a colour drawing made by Tsuneo SAHEKI in the 1950's in which a blue morning Syrtis Mj was clearly shown. He also left a drawing on 24 Apr (120ºLs, Phase Angle=7º) at LCM=242ºW whose observing note describes Syrtis Mj to be sky-blue.
  Several drawings of his in 1963 also were accompanied by the comments of grayish blue or bluish green for the morning Syrtis Mj (on 9 Feb 1963 (049ºLs) at LCM=240ºW, 15 Feb 1963 (051ºLs) at LCM=241ºW and so on).
  The present writer also gained experience even by the use of the old 15cm refractor at Fukui in seeing impressively the blue morning Syrtis Mj, and recent years he has been much paying his attention to it. At the same time he has recommended our CMO members to do the observation.
  The blue Syrtis Mj has a long history ever since the father SECCHI in 1838, but there has been a confusion about the interpretation. Note that no blue cloud is concerned.

  Note first that the clear light-blue tint is caused by a presence of the early morning (or late evening) white mist.
  Furthermore it was not mainly because of the direct penetration of the light through the mist, but the colour is rather caused by the dispersion. The longer wave easily goes through usual water vapour obstructs, while the shorter wave part receives easily scattering or dispersion.

  Figure 1 shows schematically how the light is bent by the mist over the surface as the marking S goes from S1 to S2, and to S3. Apparently the angle alpha 1 is sharper than alpha 2, and alpha 2 than alpha 3, and hence the reflection must be more bluish at S1. Furthermore we may say the light from S3 comes more directly toward us because the mist must be thinner at the place more inside.

In fig 1, 2 and 3, the planet is observed from the bottom and the mist surface is dotted.

fig 1 is for the case of opposition,
and the case in fig 2 is before opposition and the Sun light comes from the lhs,
while the case in fig 3 is after opposition and the Sun light is from the rhs.

  Similarly we can compare the cases before opposition and after opposition at the same local Martian time. Figure 2 is for the former, and Figure 3 is for the latter. The reflection angle is duller after opposition, and hence we may recognise that the light before opposition is more bluish.

  Observations went as follows (we just pick out this time Mn's observations because the comparison cannot be complexed).
  Before opposition we had a chance to watch the morning Syrtis Mj from the end of March to the beginning of April when the apparent diameter was 13.8 arcsecs. On 31 Mar (Phase Angle=19º) at as early as LCM=215ºW, Syrtis Mj was witnessed on the morning limb. At LCM=224ºW, Syrtis Mj proved quite light-blue. Same at LCM=234ºW. It remained still bluish at LCM=244ºW. It became dark blue at LCM=253ºW. We obtained the same results also on 1 Apr, 3 Apr and 4 Apr. Already on 27 Mar at LCM=250ºW, Syrtis Mj was well blue-greenish.
  Let Phase Angle=20º, and choose LCM=235ºW, and then the local time of Syrtis Mj was 9:40 am. On the other hand, let us choose the case Phase Angle=10º after opposition, and then the local time 9:40 am implies LCM=265ºW. On 6 May (at Phase Angle=10º), after opposition, Mn observed at LCM=265ºW in which Syrtis Mj was already fairly dark. Before that, it was blue-greenish dark at LCM=255ºW, and densely blue at LCM=245ºW. And furthermore it was faint and whitish blue at LCM=235ºW. (Just found at LCM=216ºW on the terminator). Same results were obtained on 5 May, 7 May, 8 May and 9 May.
  The above comparison also shows that at LCM=235ºW, Syrtis Mj was rather white-blue after opposition while it had been light-blue before opposition. This may be because the mist is thicker at the more early morning localtime after opposition. On 1 May, we observed that Syrtis Mj at LCM=239ºW was already darker than expected.
  In June, Syrtis Mj appeared again on the morning side after opposition around on 12 June (at Phase Angle=34º), but we could not see the clear-blue Syrtis Mj any longer. At LCM=251ºW on the day, Syrtis Mj was quite dark. On 15 July ( Phase Angle=42º) at LCM=257ºW, Syrtis Mj was just after dawn, but it was already dark. These were also observed similarly by ISHADOH.

  We thus described that Syrtis Mj varied its blue colour because of the presence of white mist and it was mainly caused by the dispersion. The marking is situated more morning side after opposition, but the clear light-blue colour is more easily seen before opposition or at opposition.