Programme

This is the latest version to appear in the BAA Observing Guide (ed. R.J. McKim), 2002 edition.

Instruments and observing techniques

Mars is not particularly accommodating to the observer, for its oppositions occur at intervals of over two years. In contrast to the other outer planets, the appreciable ellipticity of Mars’ orbit dictates that not all oppositions are equally favourable as regards the apparent angular disk size. Also, whilst Jupiter and Saturn can be satisfactorily observed for a considerable part of each year, Mars presents an acceptably large disk for only a few weeks about opposition, at least to small-telescope users. Perihelic oppositions occur in August and September, at which time Mars may have an apparent diameter of nearly 26 arcsec. Unfortunately for observers in the UK, the planet’s considerable southern declination at such times gives it an undesirably low altitude. Because the summer solstice of the southern hemisphere of Mars occurs soon after perihelion, this hemisphere is tilted towards us at these ‘favourable’ oppositions. Aphelic oppositions, when Mars may subtend only about 14 arcsec, occur in February and March, but its northern declination compensates for the reduced image size. At conjunction, the disk may shrink to only 3.5 arcsec diameter.

Useful observations of Mars are certainly possible when the disk exceeds a diameter of 6 arcsec, but those observers with larger apertures have found it possible to draw or image details on a disk as small as 4 arcsec, greatly extending the period over which the planet can be watched. Observers are encouraged to follow the planet for many months before opposition and to continue for as long as possible after opposition, as a martian apparition can last for much more than a terrestrial year if one is prepared to develop the necessary skill in observing the small disk far from opposition. Successive oppositions occur during different portions of the martian year so that a full picture of the seasonal changes can only be built up by observing the planet over seven or eight apparitions. This corresponds to a period of 15 or 17 terrestrial years. The terms areocentric longitude, Martian Date and heliocentric longitude are all means of describing the position of Mars in its orbit. A martian year consists of 687 Earth days or 669 Sols (martian days).

The close similarity in rotation periods of the Earth and Mars has a restrictive effect upon the range of martian longitudes that may be sampled by a fixed observer on any given night. The range of longitudes will be different for observers at different terrestrial longitudes, so the need for international cooperation is immediately obvious. Since Mars rotates in 24h 37m, any given martian CM longitude recurs on successive nights 37m later, giving rise to an illusory ‘reverse rotation’ effect with a period of 5.5 weeks, when the planet is observed at the same hour of the night.

The objectives of the Mars Section are: 1) to detect secular and seasonal changes in the shapes and intensities of the dark albedo markings, and the presence of new features; 2) to map the extent and shape of the polar caps, to note their brightness and definition, and to observe any interior details or peripheral detached portions; 3) to chart the positions and movements of martian yellow and white clouds both on the visible disk and at the limb or terminator of the planet, and also to record the incidence of the ‘blue clearings’.

Effective participation in the programme requires instruments of the order of 200 mm for reflectors and 15 cm for refractors. Apertures of 30 cm and above are particularly desirable. Magnification depends on personal choice, but generally speaking, at least 200x is desirable, and will be essential at any aphelic opposition. With his 41-cm Cassegrain, the Director finds that with a power of more than 400x he can still make useful observations with the disk diameter below 6 arcsec.

The Mars Section report form uses drawing disks 50 mm in diameter. Data from the BAA Handbook allows a blank disk to be prepared with the correct phase and orientation before observing. Some observers prefer to be ignorant of the CM longitude when commencing work, but this is an old-fashioned habit if the observer is to be on the lookout for specific features. Nevertheless, even when the region is well-known to the observer, he or she must not forget that the outlines and intensities of the dark markings are subject to change in a manner that cannot be predicted easily. At the telescope, the observer might first sketch in the polar cap or hood visible at each pole, then the larger dark makings, and finally any clouds or other brighter patches. The time of completion of this outline must be noted, after which fine details can be filled in. Is there a dark fringe to the polar cap? Is its outline sharp? Are there any irregularities at the terminator due to projecting cloud?

Photographic work, especially around opposition, will impartially record the relative intensities and shapes of the dark markings. This is very useful in supplementing visual data. Image projection by a good achromatic eyepiece or Barlow lens will be needed, and a fine-grained film. By making multiple exposures on one frame, film can be saved. A driven telescope on a steady mounting is a necessity. Filter photographs should be attempted if possible, using the Wratten filters recommended below for visual work. CCD detectors, generally being rather red-sensitive, are excellent for recording the surface features and dust storms when used in conjunction with a red filter such as a Wratten 25. They can also work well in the blue end of the spectrum (as, for example, with the blue–violet W47), to record white clouds, but they then need longer exposure times. Colour composite CCD images are welcome. Lightweight modern camcorders are also useful in making a video record.

With apertures of 150 mm or more, examine Mars with colour filters of known transmission characteristics. Filters reveal details of the structure of the Martian atmosphere, unobtainable by observation in integrated (white) light alone. Some of these filters are fairly narrow-cut and dense and so have low transmittance. The blue–violet W47 requires at least 20 cm aperture. When employing filters, always avoid ‘threshold’ observations with a dim image, for these will only prove to be misleading and useless. The W25 red and W15 yellow (and any orange filter) will be of help in intensifying the dark markings and in enhancing the visibility of faint desert details. Furthermore, they assist the observer in recognizing discrete yellow dust clouds as these appear brighter through the filters: strictly speaking, only the W25 can be used diagnostically here, for some white patches also appear bright in yellow light. Dust clouds are, however, very difficult to detect when they lie wholly over desert regions. The W44A blue, 58 green and 47 blue–violet filters enhance the limb brightening and the outlines of limb and terminator clouds and bright patches. The dark markings will be visible, though subdued, with the W44A filter. Surface details are rarely seen at all with the W47, except when a so-called ‘Blue Clearing’ renders them more or less visible. The scale of assessment runs as follows: 0: No observable surface features; 1: Some features visible with difficulty; 2: Features easily seen and identifiable; 3: Features are about as well defined as in white light. Category 3 is exceptional; negative observations are just as useful as positive ones. Although the phenomenon is considered to be an ‘opposition effect’ due to the scattering properties of the Martian atmosphere, it is still worth recording, and in any case the white clouds are better seen in blue–violet light. The cones of the human retina are not very sensitive to this waveband, and the W47 is much better when used photographically, or with a CCD. Because the filter transmits some infrared, to which the CCD is very sensitive, the CCD should be used with an infrared-rejection filter when working in this waveband.

Atmospheric hazes or clouds also occur over the polar caps and may be rendered visible (if present) with the W47 or W58 filters. Some authorities consider that if a white disk cloud appears larger or brighter with the W58 (compared with W47) it lies nearer to the Martian surface and may be a frost patch. However, it is best not to be too dogmatic because some dust storms have appeared white rather than yellow in the past. In the latter case, the expansion and motion of a storm over successive days, and its obscuration of surface details, are the real tests.

The assignment of numerical intensity estimates to features is another line of observation. The black night-sky background is rated at intensity 10 and the brightest scale point at 0, representing the normal brightness of the polar caps. As an intermediate guide, most of the desert regions would normally be 2. Estimates to the nearest half or quarter point will be adequate and can be recorded on an outline blank. Estimates should be made in white light only. Careful series of observations will reveal slow changes in intensity of the surface features, and yield figures enabling successive apparitions to be compared.

Maps

All intending observers should obtain a copy of the IAU Mars map of 1957. This map, compiled by G.de Mottoni shows the average appearance of the albedo features. Although certain features have undergone significant and apparently permanent changes since its compilation, it has been adoped by the Section for general reference and nomenclature. Dr Shiro Ebisawa’s more detailed general map is useful for small details, and the writer’s paper in the Bibliography includes reproductions of both the IAU map and the Ebisawa map. These maps, and Mario Frassati’s map (2001), are available on this web site (see Maps.)

Special points about the various areas of observation

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Polar regions Regular seasonal events should be carefully looked for, during the seasonal decline of the two caps. The Director measures the size of the cap from good drawings, photos or CCD images and constructs a regression curve to compare with historical data. In this way any differences between successive Martian years can be detected. Sometimes the cap may evaporate faster; in other years the evaporation may be delayed. In the North Polar Cap (NPC) there is a seasonal detachment of a large bright portion known as Olympia, separated from the main mass by the dark rift, Rima Borealis. Likewise the SPC breaks into various portions with the onset of evaporation: the Mountains of Mitchell can sometimes flash out as a bright spot if sunlight catches the ground at the right angle, and there are various dark rifts. It is interesting to try to follow the cap remnant to determine whether or not it evaporates completely in the corresponding summer season. Dust storms Planetologists are particularly interested in the yellow dust clouds for they may have an appreciable effect on the Martian climate. Whilst discrete yellow clouds have been recorded at all Martian seasons, the obliterating planet-wide storms occur only at or near perihelion, starting in the Hellas, Noachis or Solis Lacus regions. An event in 2001 June began at an usually early date in the Martian year, however, and observers should always be alert for short-lived events. Recognition of the early stages requires familiarity with the surface features: here the amateur has an important role to play in alerting professional colleagues. The writer has compiled a narrative and catalogue of historical dust storms, and this appeared as a recent BAA Memoir. Other atmospheric phenomena White clouds, frequently seen at the limb and terminator, can occur in conjunction with specific topographic features, such as the great volcanoes like Olympus Mons, those in the Tharsis region, or Elysium. Such topographic or orographic clouds are carried round with the planet’s rotation, and may be enhanced with the recommended blue and green filters. The clouds over the Tharsis volcanoes can sometimes coalesce on the evening side and form a ‘W’-shaped cloud (or ‘M’ (for ‘Mars’!) if viewed with north up). Some of the basin areas act as cold-traps for volatiles; Argyre and Hellas can show frost patches at the appropriate season. Dark markings Seasonal changes are well-known, and arise from the interaction of the Martian winds with the light dusty surface deposits. Thus darker, underlying features can be uncovered and others covered up. A good example is the darkening of Pandorae Fretum in the southern hemisphere, a semi-regular phenomenon related to dust activity in nearby Hellas–Noachis. Syrtis Major can show irregular variations in width, and in recent years the regions of Aetheria, Aethiopis, southern Utopia, Nodus Alcyonius, Nepenthes, Solis Lacus, Phasis, Claritas–Daedalia, NW Mare Sirenum and other smaller regions have been affected. If today’s Mars observer works systematically there is a great deal that can still be contributed to this branch of astronomy.