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Mars
Section Circular No. 3 1998 December 16–1999 January 31 |
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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. |
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Richard McKim, Director 1999 February 7 |
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