Special Features for 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 the best 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. A smaller fragment, Ierne, is harder to see. Likewise the SPC breaks into various portions with the onset of evaporation: the Mountains of Mitchell (Novus Mons) can sometimes flash out as a bright spot if sunlight catches an icy surface 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. Flashes were seen over Edom in 2001, when the sub-Earth and subsolar points very nearly coincided, repeating a classic but previously unconfirmed observation under similar circumstances made by Tsuneo Saheki from Japan in 1954.
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. The last such event occurred in 2007, so statistically speaking we are due for another. Recognition of the onset of a dust storm requires a good level of 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 (Volume 44, in 1999). 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 may have heralded the return to more dusty conditions upon Mars. This latter event began unusually early in the martian year, and observers should be on the alert for dust events of all sorts. In recent years improved levels of resolution have enabled amateurs to record small dust storms across or adjacent to the north polar cap. In 2012 and 2014 for example, the separation of Olympia from the NPC was accompanied by several episodes of local dust activity, while in 2010 a much larger dust cloud crossed the cap from one side to the other. All these events are described in the Section reports that are published in the BAA Journal and then uploaded to the website.
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 in the martian afternoon and evening can sometimes 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. The ECB becomes visible in part around Ls 0, in the longitudes of Chryse and Xanthe but is only complete around the planet from Ls = 50° to 145°. Its brighter extremities at the E and W limbs give rise to the well-known 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, though it has been remarkably stable since the early 1980s. In recent years the regions of Aetheria, Aethiopis, Cerberus, Solis Lacus, Mare Hadriacum, Phasis, Claritas-Daedalia, NW Mare Sirenum and other regions have all been affected. Cerberus today is nearly invisible, but appears as a dark linear feature on the older telescopic maps, whereas Solis Lacus for some years has been considerably larger and darker than on many occasions in the 20th Century.
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