General

During December 1–31, the disk diameter increased from 6”.5 to 8”.0. Shirreff (UK), Tanga (Italy) and Cave (USA) joined the contributors. Bad weather in Europe generally (with snow in the UK in the last week) has much hindered observations. At the Director’s suggestion Shirreff contacted the Mir cosmonauts by radio to invite their observational help: they replied that they had powerful binoculars but no telescopes. OAA observations frol CMO No. 182 are referred to below.

North Polar Region

During December, the polar collar was distinct. R. Schmude (51-cm refl., Villa Rica, GA, USA) on December 20, CML = 228 deg., found the cap had an orange–white tint instead of the usual white. No-one else has reported this colour change; the observation was made in a similar longitude to that of the September dust storm. The HST October 15 image showed the dust dispersing over the cap in a spiral formation. Has this spiral pattern disappeared? Careful observations of the cap colour will be of value.

Surface features

There is nothing of significance to add to the previous notes, IRTF images by Jim Bell et al., (Mauna Kea, Dec 8, his first of the apparition) showed Idaeus–Achillis Fons on Nilokeras still large and dark.

Dust storms (yellow clouds)

As reported in the Second Report, on 1996 Nov 22 Gray found two clouds over Ophir and Candor respectively, the former the larger, impinging upon Aurorae Sinus. Both clouds were marginally luminous in yellow (W15) and orange (W22) filters, but rivalled the NPC in brightness in red light (W25). Dust? Gray had the same impression next day in poorer conditions. In the absence of further data at the time of writing, a recent letter from Gray adds that on Nov 27 at 0700 UT (CML = 11 deg.) he found a pale yellow-coloured ‘tongue’ of brightness running from the morning limb into much of Chryse. [Here I refer to classical Chryse (IAU/Ebisawa lap), and not to Chryse Planitia (classical northern Xanthe!)] An observation by Warell on Nov 28 (CML = 303 deg.), received since the last Report, shows prominent morning limb haze including the latter region. Dan Troiani emailed me to say that he had contacted the HST people, and that the latter would try to image the area. Ophir/Candor was examined by OAA observers on Dec 7, when it appeared normal, just brightening a little near the evening terminator. The aforementioned IRTF images (Dec 8) showed albedo features in the area normal; under CML = 353 deg., however, there was a trace of brightness over Candor on the a.m. side. The OAA also watched Chryse during Dec 8–14: it became most opaque and whitish under CML = 65 deg.: apparently normal diurnal cloud. Gray’s dust storm is thus neither confirmed nor contradicted, but if objective it cannot have been more than a local event. The OAA noted: ‘In 1993 at Ls = 39 deg., Morita discovered a burst of cloud over Elysium, and the same season came around in this period, On 25 Nov [Ls = 43 deg.] the area of Elysium was visible near the CM but looked very faint… Just on 3 Dec [CML = 155 deg.], Elysium was seen light at the morning limb.’ Thus there was no ‘seasonal repeat’ of the 1993 Elysium–Cebrenia dust storm [described in the 1993 BAA Apparition Report].

White clouds

There is little of special interest to report here; the December data will therefore be given in a later summary, See the comment about Elysium under ‘Dust Storms’ above.
 

Richard McKim, Director

1997 January 14

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