General

This Circular summarises the period 1998 December 16–1999 January 31 (Ls = 71–91 deg., D = 5.7–7.8 arcsec., lat. of centre of disk 24–18 deg. N). I have received CCD images from Don Parker, Frank Melillo and Damian Peach; David Gray remains the most prolific visual contributor. The weather in the UK has not been very cooperative, but observers have made the most of any fine weather. From Hawaii Nicolas Biver put his 26-cm reflector to good use on a few mornings. The OAA CMO 211 described Japanese work during December 16 to January 15.

      The next Circular will discuss observations done in 1999 February, and I intend to compile it by March 7. Therefore please send any relevant work in time for it to be included. Most of the Circulars from the last apparition (and Nos. 1 and 2 from 1998–99) are to be found on the Mars Section website. There is a new MarsWatch site, now sponsored by the Astronomical League (http://www.astroleague.org/marswatch/). There is an article about how to observe the current apparition in the latest copy of the ALPO Journal (1998 October).

      I hope to say something about the current round of Mars-bound spacecraft, later, perhaps in the next Circular.

North Polar Region

The cap remains bright and conspicuous, though the latitude of the S. edge has moved much further north. On January 11 for example, Warell found it to be ‘brilliant white’. Parker’s images (e.g., January 20, under CM = 66–69 deg.) also show the dark patch Hyperboreus Lacus at the edge of the cap. Though the tiny disk militates against really detailed study, Parker’s CCD work indicated some detached outliers to the S. of the cap, and detail within the cap itself. Thus on December 25, CM = 310 deg., the cap had a well-defined brighter patch following the CM with indications of further structure around the centre of the cap. This bright spot seems to be a seasonal feature, and it was shown in Parker’s red filtered images as well as in the overall composited colour views.

      The most famous (and largest) seasonal outlier is Olympia. Antoniadi found it began to be separated from the cap near Ls = 80 and that the outlier could be followed up to Ls = 195. It is important to try to establish these seasonal dates for every favourable apparition, as some authorities have considered them variable from martian year to year. Further, being in fixed topographic positions, they provide another means of judging the seasonal progress of the polar cap. Parker’s images of January 9 (CM about 170 deg.) imperfectly resolved Olympia at Ls = 81 degrees. Further observations please!

      A new study of the NPC recession from Earth-based and HST data covering the years 1990 to 1997 has recently appeared in Icarus (136, 175–191 (1998)). Written by B.A.Cantor, M.J.Wolff, P.B.James and E.Higgs it advances evidence that the recession rate varied slightly during these four apparitions, the rate of recession being a bit slower in 1994–95. BAA NPC data from 1993 (and 1980–82) have already been published (R.J.McKim, J. Brit. Astron. Assoc., 105, 117–134 (1995)), and recent (unpublished) analyses of the Section’s work for 1995 and 1997 suggests very small interannual differences during 1993–97. It is clear that the HST images are a quantum advance upon previous Earth-based work, but being comparatively few in number it is still vital that as many accurate measures as possible are made by ground-based workers. Cantor et al. agree with the writer’s opinion (expressed in the 1993 Section Report, op. cit.) that measures of the north cap’s E–W diameter involve systematic errors (due to limb darkening or diurnal clouds) when the contour of the cap is not located entirely within the visible disk. BAA data have been analysed for the cap latitude on the CM (N–S direction): visual drawings, filar micrometer data and measures from photographs, when made by the same method have yielded comparable results. But comparisons of micrometric data measured in the E–W sense have not agreed exactly. Thus historical comparisons can present a problem if the method of measurement has not been stated in the literature. In the course of his research for the BAA Dust Storm project, among appropriately dusty archives in both the USA and Europe, the writer found a great deal of unpublished polar cap data. Another project…? Perhaps.

Atmospheric activity

No clear-cut evidence of dust storm activity has been found so far, but white cloud activity has increased, and there have been numerous records of the Equatorial Cloud Band (ECB) effect. The martian orographic clouds over Olympus Mons (Nix Olympica) and the Tharsis volcanoes have been very clearly imaged and observed visually. Rather than giving an incomplete preliminary meteorological report here, some selective notes of the more interesting observations are given below. All of Parker’s work is CCD; the others, visual.

      Parker, December 19: CM = 11–18 deg. Chryse–Xanthe is lightish, as is Tempe on the a.m. limb following M. Acidalium.

      Parker, December 23: CM = 341–346 deg. Chryse–Xanthe is light on the morning side (especially in green and blue light).

      Gaskell, December 24: Argyre is bright and bluish on the evening side, and there is extensive morning cloud over Tharsis to Solis Lacus.

      Parker, December 25: CM = 310–318 deg. Hellas on the evening side looks only vaguely light. A thin ECB is seen across the disk.

      Parker, January 1: CM = 250–256 deg. Elysium is light on the evening side. Hellas is only very slightly white in the morning, and there is some morning cloud over Aeria following Syrtis Major. In blue light (BG12) the Syrtis is invisible and the white cloud in Aeria enlarged. In an e-mail, Parker reports having seen the ‘Syrtis Blue Cloud’ visually on this date.

      Gray, January 7: CM = 122 deg. Nix Olympica, on the CM, is not especially light, but is seen to be surrounded by a dark area, especially on the N. side. (With a phase angle of 36 deg., the local martian time would be about 1.20 pm.) Tharsis, on the evening side, contains extensive bright cloud. The S. limb was rather light, too.

      Parker, January 9: CM = 158–178 deg. Orographic clouds over the Tharsis region on the evening side, somewhat blurred by the seeing. Elysium is well onto the disk on the morning side and looks bright. (Allowing for phase it must be near local noon over Elysium.) These bright patches are imaged especially clearly in blue light.

      Gray, January 10: CM =108–113 deg. Light S. limb. Tempe lightish on the p.m. side. Alba is light, crossing the CM. Nix Olympica on the a.m. side is also rather light

      Warell, January 11: Argyre light on the evening side and extended haze over Memnonia on the morning side.

      Parker, January 15: CM = 107–128 deg. The Equatorial Cloud Band effect is in evidence both in the blue light images and in the composited colour images. A bright equatorial cloud on the evening side thins out towards the west, and runs discontinuously across the disk where it meets a large bright cloud on the morning limb. The last images show a discrete Nix Olympica very bright approaching the CM (early afternoon, local martian time). Morning cloud also completely surrounds Propontis I, a phenomenon noticed in 1997, 95 and 93 (and illustrated in the 1993 final Section Report). Cebrenia is also hazy.

      Gray, January 16: CM = 63 deg. The region just S. of Aurorae Sinus looks unusually pale as if affected by haze, but observing conditions are not perfect. On the evening side Chryse–Xanthe is a light region.

      Parker, January 18: CM = 87–101 deg. ECB and Nix Olympica again very evident (blue, green light and in composited colour CCD frames). The evening cloud is over Xanthe and Candor–Ophir.

      Parker, January 20: CM = 66–69 deg. Candor–Ophir is again light on the CM.

      Gray, January 21: CM = 355 deg. The S. limb, including Noachis, is very bright, the brightness extending to encompass the small part of Argyre that is visible on the a.m. side. On the morning side this cloud extends north over part of M. Erythraeum. Chryse–Xanthe also bright on the morning side.

      Gray, January 22: CM = 353–356 deg. Much as the 21st, but filter work shows that the a.m. limb part of the S. limb brightness is most conspicuous in blue and green light. (On both mornings this bright patch rapidly faded with time.)

      Surface features Propontis remains a conspicuous dark spot. Solis Lacus is still large and dark to Parker, Gray and Biver, as it has been since the mid-80s. It is elongated E–W. No internal details have been resolved yet. No trace has been found of the Phasis development to the W. side of Solis Lacus, which is in line with the gradual fade notice in the last few apparitions. As in 1997 the Cerberus region is not prominent, but it showed up faintly nonetheless in Parker’s CCD images, as well as in Gray’s drawings. Gaskell reports having picked up internal details in M. Acidalium.

Erratum in Circular No. 2

Apologies to David Gray for wrongly quoting the date of his Libya observation as December 23; it should, of course, have read November 23.

Reporting data to the Section

Send mail to Cherry Tree Cottage, 16 Upper Main Street, Upper Benefield, Peterborough PE8 5AN, Great Britain; home telephone 01832 205387; home e-mail Rmckim5374@aol.com. (Do not use the former e-mail address for my place of work (which was mckim@oundle.northants.sch.uk).) You can also send any really urgent fax to my place of work on 01832 274052.
 

Richard McKim, Director

1999 February 7

Return to Mars Circulars for 1998-99

Return to reports of past oppositions of Mars 

Return to the Mars Section home page