Ordinary Meeting and Exhibition Meeting, 2002 September 21
Mars in 2001
Dr McKim started by reminding observers that the Martian opposition of 2003 would be one of the best opportunities to view the planet for many years, possibly many tens of thousands of years. This talk would first summarise the observations of the 2001 opposition, before moving onto prospects for 2003.
In 2001, Mars had been at declination 26° south, and hence only 10° above the horizon in Cambridge. This low altitude had hindered high resolution work. Imaging techniques had progressed tremendously in the past half-century, however, and the speaker summarised these by showing an image of the 1954 opposition by the great E.C. Slipher of the Lowell Observatory. This was an opposition which had been at similarly low altitude. The speaker went on to show an amateur image from 2001 with a 20cm instrument, which did not compare too unfavourably with an image by the HST. The improvement between the early professional image and modern amateur work was stunning.
In June 2001, the Martian year was at Ls=180, or the beginning of southern spring. (Ls, the areocentric longitude, is a measure of the Martian season, and is explained in the Association's Observing Guide.1 ) At this time the southern polar cap begins to become visible. Dust storms are frequently seen in the warmer southern spring and summer seasons of the Martian year, and this was the case in 2001. A storm blew up in the Hellas basin, and on June 26 spread beyond this locality. A second storm grew from a separate source in Claritas-Daedalia, on the opposite side of the planet, and the two rapidly merged to form the first planet-encircling storm for decades. The best estimate for the duration of the storm was 185 days, although such durations are difficult to measure accurately due to the subjective nature of when a storm is deemed to have cleared. The return of all surface features, or the restoration of the degree of polarisation to normal, were two possible criteria. The latter is a useful indicator of the structure of the Martian atmosphere.
The speaker speculated that the Martian climate might be undergoing short-term change, since a number of planet-encircling dust storms were observed in the 1970s: those in 1971, 1973, 1975, and two in 1977. Yet no such great events had been observed during the 1990s. Dr McKim speculated that 2001 might mark the start of another 'dusty' epoch. It could be argued that the decade in which Mariner and Viking probed the red planet was by no means typical.
Moving onto amateur observations, the speaker first referred to the cover of the 2002 June Journal, which featured several images by Ed Grafton from Houston. In this sequence, surface features are visible before the storm sets in, but are soon masked. In the centre-right image of July 31, the summit of Olympus Mons can be seen protruding through the dust. Damian Peach had taken images when Mars had subtended only a 4-5" diameter, and although the surface detail at first sight appeared modest, they represented a much greater resolution than Slipher had ever achieved when the small angular size was considered.
Measurement of the southern polar cap is an area where amateurs can contribute. The southern cap is highly asymmetric, and professional images from the HST are lacking in quantity, and often only available from a single CM longitude. Precise measurement was hindered in 2001 by haze surrounding the cap, as is often the case. Measurements shortly after Ls=180 had revealed close correspondence with data from 1988. After Ls=200, however, the cap retreated more slowly than in 1988, suggesting that the dust storm had preserved it. This is an indication that dust storms can affect the size of the polar caps.
As normality returned in October, it was observed that Syrtis Major was narrower than before. Also a new dark region had appeared close to Solis Lacus, which was itself reduced and reorientated. It is not uncommon for the Martian bedrock to become exposed during storms, or for dust to pile up against ridges.
In 2003, Dr McKim recommended all observers to take a look at Mars. Even those without telescopes could contribute useful observations of the naked eye colour of the planet. In 2001, the usual red colour had turned to a dull yellow as the storm progressed. Mars would be at declination 15° south next year, giving an altitude of 20-22° from the UK. At opposition, the Martian year would be at Ls=249.5, and it would be the closest opposition for many thousands of years. If there is another dust storm, it may be under way as early as 2003 May, so early observation was urged. It would be of interest to see whether the dusty conditions were returning.
The Association's programme for the opposition would include watching for dust, which is bright in red light, and can be distinguished from the surface, which is bright with a blue filter. The southern polar cap could be measured and sketched, most preferably with a red filter. Such drawings should be submitted at regular intervals. Observers should also keep an eye out for small changes in surface features such as Solis Lacus. The speaker urged observers to consider photographic observation as well as CCD imaging and sketches, both for their aesthetic value and the easy comparison they give with earlier observations.
In response to a question, Dr McKim noted that there was probably no need to look at variations in the Sun's output in explaining the variability in dust storms from year to year.
After the applause for Dr McKim's thorough account, the President invited Bob Marriot to play an amusing video clip featuring Tom Boles at an Association event three years earlier. The President then reiterated his thanks to all who had contributed to the organisation of the Exhibition Meeting, and reminded members that feedback would be particularly welcome. The meeting was then adjourned until the Annual General Meeting, to be held at Savile Row on Wednesday October 30.
----
Dominic Ford