Revised version, 2013 July 1
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 around 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. Thus 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.
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: a ‘cycle’ 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. Areocentric longitude is abbreviated to ‘Ls’ in the BAA Handbook, and the northern hemisphere seasons begin at the following values of Ls: spring, 0°; summer, 90°; autumn, 180°; winter, 270°. These naturally correspond to southern hemisphere autumn, winter, spring and summer. 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. Thus we have especially good cooperation with observers in continental Europe, Australia, Japan and the United States. 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 as follows. 1) To detect any changes in the shapes and intensities of the classical dark albedo markings, and the presence of new features. These changes are related to the occurrence of dust storms (‘yellow’ clouds). 2) To map the extent and shape of the polar caps, to note their brightness and definition, and to observe any interior details or detached portions. 3) To chart the positions and movements of martian dust storms, and to document the occurrence of seasonal or topographic white clouds.
Effective participation in the programme will require instruments of the order of 200 mm for reflectors and 150 mm for refractors. Apertures of 300 mm and above are better still. Magnification depends on personal choice, but generally speaking at least 200x is desirable. 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 visual 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?
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. Some can also work well in the blue end of the spectrum (for example, with the blue–violet W47) to record white clouds, but they then need longer exposure times. Webcams are generally best of all for the Red Planet (and other planets), for they enable many images (or a short section of video) to be combined and stacked to offset the effects of atmospheric turbulence.
With apertures of 150 mm or more, examine Mars visually 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 200 mm 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 the discrete yellow dust clouds as the latter appear brighter through these filters: strictly speaking, only the W25 can be used diagnostically here, for some white patches also appear bright in yellow light. Dust clouds are more 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, but on rare occasions they may become more or less visible. 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 (or its Schott equivalent) is much better when used with a suitable CCD or webcam (not all such devices show a very good sensitivity at this end of the spectrum). Because the filter transmits some infrared, to which imaging devices are very sensitive, it should be used in conjunction with an infrared-rejection filter (such as the Baader UV/IR filter) when working in this waveband.
Until the recent past, the assignment of numerical intensity estimates to features formed another line of observation. The black night-sky background is rated at intensity 10 and the brightest scale-point is 0, representing the normal brightness of the polar caps. As an intermediate guide, most of the desert regions would normally be 2. Estimates should be made in white light only. Their main use today is in assisting the observer to recognise veiling by dust, or other short-term changes. Unprocessed images would be even more valuable in such analyses, particularly if the observer takes a series with precisely the same equipment over a number of apparitions.
All intending observers should obtain a copy of the IAU Mars map of 1957. Although certain features have undergone significant and apparently permanent changes since its compilation, it has been adopted 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. A recent map compiled by the Planetary Section of the Unione Astrofili Italiani (UAI) is reproduced here to show what the observer might currently expect to see at the eyepiece. All these charts, together with a high-resolution map by Damian Peach (from the 2005 opposition), are available on this website (see Maps.)
Special points about the various areas of observation
Polar regions and ground ice 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 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. Surface frost shows up as brighter patches, but is hard to distinguish from white cloud unless one has a series of images available. Just occasionally there can be a momentary specular reflection from surface ice that appears as a bright ‘flash’ to the Earth-based observer. Most recently, some flashes were seen over Edom in 2001, when the sub-Earth and subsolar points very nearly coincided.
Dust storms Planetologists are particularly interested in the yellow dust clouds for they may have an appreciable effect on the martian climate. There is a classification scheme based upon the area of the planet affected. The smallest events are termed ‘local’; those which have a long axis greater than 3000 km are termed ‘regional’, and ‘encircling’ storms cover all martian longitudes, but not necessarily all latitudes. Truly global coverage, as in the storms of 1971, 2001 and 2007, is also possible. Whilst discrete ‘yellow’ clouds have been recorded at all martian seasons, the obliterating planet-wide storms occur only in southern spring or summer, and especially when Mars is at or near perihelion, starting in the Hellas, Noachis or Solis Lacus regions. Historical work suggests that the probability of having a planet-encircling storm once per year on Mars is about one in three. Recognition of the onset of a dust storm 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 has been published as a BAA Memoir. This research showed that the decade of the 1970s was an unusually dusty one, especially around the years of the Viking extended mission. The decade of the 1990s apparently produced no encircling or global telescopic dust storms. The planet-encircling event which commenced in 2001 June perhaps heralded the return to more dusty conditions on Mars. This latter event began unusually early in the martian year, and observers should be on the alert for dust events of all sorts.
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 Mons. 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 basin areas act as cold-traps for volatiles: thus Argyre and Hellas can show frost patches at the appropriate season. Another interesting phenomenon is a band of equatorial white cloud, the so-called Equatorial Cloud Band, which is best seen in blue or violet light and most apparent between about Ls = 50° to 145°, and whose brighter extremities at the E and W limbs give rise to the evening and morning clouds viewed in white light.
Dark markings Semi-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. The semi-regular darkening of Pandorae Fretum in the southern hemisphere is 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, Cerberus, Solis Lacus, Mare Hadriacum, Phasis, Claritas-Daedalia, NW Mare Sirenum and other regions have been affected.
Submission of observations
The Director prefers to receive hard copies of drawings at monthly intervals, but images, in Jpeg format, should be e-mailed to him daily. The preferred file-name format is yyyy-mm-dd-ttttUT-initials: for example, 2008-04-30-2030UT-RJM for an image taken on 2008 April 30 at 2030 UT by the Director.