General

D: 13.9 to 12.9 arcsec, Ls: 98 to 105 deg. UK weather and seeing conditions remained good.

North Polar Region

Again, no N. polar haze was evident in white ight. The NPC seemed to have reached its minimum size, but measurements will be needed to confirm this. Hyperboreus Lacus was still conspicuous, and Olympia showed up on Dijon’s CCD work and to Shirreff (Apr 7) visually, on the morning side.

Surface features

Much fine structure was again seen. Observers paid attention to the fine detail around Mare Acidalium, e.g., Hendrie on Apr 12 (15-cm OG) accurately portrayed Acidalium’s dark NW tail, Tanais, together with the dusky Baltia. On Apr 15 the Director found Lunae Lacus rather large, and apparently darker than before: he felt the region to be a little different to its earlier appearance visually and upon the HST images. Observations, please! In 1982 Lunae Lacus and Uranius darkened at the time of a dust storm in Tempe–Arcadia.

Dust storms (yellow clouds)

There is nothing positive to report, but see the above note on Lunae L.

      Dr Ebisawa (49-cm refl., Tokyo, Japan) has sent details of his work thus far. Of special interest, on 1996 September 18, CML = 154 deg., Ebisawa sketched a small southward inflexion in the NPC’s contour. He notes that this corresponds to the place of the dust storm imaged by the HST (see earlier Circulars).

White clouds

Dijon’s images (reproduced in the paper edition of this Circular) were among the many received that showed the complex spotty pattern of evening white clouds over Tharsis–Amazonis–Arcadia. Nix Olympia exhibited white morning cloud, became less easy to see by local noon, then gradually brightened towards evening. Several observers including the Director drew attention to the large white morning cloud over Tharsis and E. Memnonia – E. Amazonis, visible when Mare Acidalium was on or somewhat p. the CM. S.Moore (22-cm refl.) on April 15 reported a small bright white cloud in S. Tempe (seen in blue, invisible in red light), but the Director contemporaneously saw only a general lightness in Tempe. The region showed bright a.m. and p.m. clouds but was less conspicuous about mid-disk. Elysium was bright on the morning terminator, Cebrenia being included or brightening separately. Near the CM and in the afternoon, no more than a small light cloud was seen over Elysium Mons, or a slight lightness over Elysium generally. In the evening Elysium was wholly or partly brightened by white cloud. Siegel on Apr 6, under CML = 100 deg., reported that an equatorial cloud band was faintly visible in W47 and 80A, and very faintly visible in W58 and integrated light. Other observations of the phenomenon were reported by Troiani. Some observations of the ‘Blue Clearing’ were again made.

Richard McKim, Director

1997 April 25
 

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