The Sand Ships of Mars By: Jeffrey D. Beish (Rev. 12-19-2018) INTRODUCTION It was Ray Bradbury who wrote of the "Sand Ships of Mars" in his science fiction novel, The Martian Chronicles (Bradbury, 1950). One can imagine the fictional inhabitants of Mars trekking across its deserts in their floating machines stirring up huge dust devils. This great science fiction story set the stage for this author’s interest in Mars and who now writes about similar accounts of the real dusty whirlwinds on the Red Planet. [NOTE: see Mars Chart in Reference section for location names]. One of the most spectacular events to watch in our Solar System is the development of a major Martian dust storm. From Earth a Martian dust cloud may seem to be just a nuisance to any would-be telescopic explorer on that planet. However, if you consider the actual size of these clouds then you suddenly realize that many of them would cover entire western United States! Imagine if a Mars-sized planet-encircling dust storm would occur on Earth; it would blank out most of an entire Hemisphere! Some would-be Mars experts claim that after dust storms the dust settles quickly to the surface and reduces contrast of albedo features. This is contrary to the known facts; dust remains in the atmosphere of Mars for weeks -- at times months -- before settling to the surface [Pollack, 1989], [Kahn, 1992]. This dust is very fine particles of volcanic ash and when raised aloft during these storms they reach heights of several kilometers. Because the gravity of Mars is relatively weak, compared to the Earth, the high altitude planetary winds carry the dust around the upper atmosphere causing it to settle very slowly. Dust provides the nuclei for CO2 ad H2O ice to produce cloud formations and it stays in the upper wind system for months [Lee, 1985]. A dusty atmosphere is the real cause of the loss in contrast of surface features, not because it settles onto those features [Kahn, 1992]. As local dust traps sunlight, it heats the atmosphere further [Kahn, 1992]. The warmer air flows to cooler regions where it helps to generate local winds and to raise more dust. Regional clouds spread continuously [Houben, 1981], [Zurek, 1989]. Major dust storms often have more than one "core" and development is more rapid. This appears to represent a classic example of a non-linear response - - true catastrophic phenomena, where only a small change in circumstances, such as an increase in the solar radiation from the Sun, can produce a huge shift [Sheehan, 2003]. THE DUST STORMS OF MARS During the favorable Perihelic opposition of Mars in September 1877 (opposition occurred on September 5, 1877 at 260° Ls); Giovanni Schiaparelli noticed a large, bright cloud east of Solis Lacus. Soon albedo features began to vanish from the region and we now speculate that Schiparelli had witnessed the beginning of a dust storm that, by December 1877, had covered much of the planet with dust. A few "local" and "regional" dust storms have temporarily altered familiar dark markings by hiding sections of them from view; however, a more violent and electrifying event has been seen that can have catastrophic effects on our Red Planet. These rare events we call a "planet-encircling" dust storm that engulfs an entire hemisphere of Mars, at times invading much of the north and south of Mars. These great dust storms have obscured all but the tallest peaks of the huge volcanoes and mountain ranges of Mars from view. What is interesting about these events is that great dust storms on Mars seem to come in pairs, with the first storm being a major (though not global) event and the second storm being a planet-encircling one (see Figure 1). Figure 1. As the dust began to clear from the Martian sky during the Great Dust Storm of 2001 the high volcanoes of the Tharsis region on Mars began to pop put from the tops of the dusty haze. The left image taken by Ed Grafton (C14) shows four Tharsis volcanoes poking up through the dust veil. Center image by Maurizio Di Sciullo (10"Newt) shows same features and right image by Don Parker (16" Newt) demonstrates that some of the volcanoes on Mars can be seen from Earth at least in certain circumstances. Abbreviations: Ar = Arsia Mons, P = Pavonis Mons, As = Ascraeus Mons and O = Olympus Mons. Dr. Richard McKim, Director of the Mars Section of the British Astronomical Association (BAA), performed an exhaustive historical study of Martian dust storms and has concluded that there have been only ten planet-encircling events reported since 1873. These took place in 1909, 1924, 1956, 1971, 1973, 1975, 1977 (2 storms), 1982, and 2001. Of these, only the 1971 storm was considered truly "global" [McKim, 1999, 2008, 2017]. The 1956 and 1971 storms originated in the Hellespontus-Noachis deserts. The 1973 and 1977 storms originated in the sands of Solis Lacus. And the 2001 storm flared up in Hellas. While dust storms can form rapidly, the largest can take weeks, even months, to disperse. Since 1971, the year of the "Great Dust Storm of Mars," the ALPO Mars Recorders have suspected that these disturbances came in pairs. We have seen that in 1971 a major dust storm occurred on 213° Ls, followed by a "planet encircling" dust storm on 259° Ls. Again, in 1973 a major storm began on 244° Ls and was followed by a "planet encircling" storm on 300° Ls [Capen, 1971] [Martin, 1974]. The Viking Lander recorded two "planet encircling" storms on 204° and 268° [Tillman, 1988]. Figure 2. A sequence of photographs of the Great Dust Storm of 1971 composed by C. F. +6. The "before storm" image was taken on Sept 21 (259 Ls). Dust clouds in Hellas and westward on Sept 22 (260 Ls), spreading in both directions by Oct 3 (267 Ls), Oct 21 (278 Ls), and begins to dissipate by Dec 20 (314 Ls). Hellas is light oval area south of Syrtis Major. Photographs taken by Lowell Observatory. Figure 3. A sequence of photographs of the Great Dust Storm of 1973 composed by C. F. Capen. The "pre storm" image was taken Oct 13 (300 Ls). Bright dust cloud in Solis Lacus during "Day 1" on Oct 14 (301 Ls), increasing in size and a second dust cloud appears south of Solis Lacus in "Day 2" on Oct 15 (301 Ls), both dust clouds spread east and northeast in "Day 3" on Oct 16 (302 Ls), nearly blanks out 1/3rd of visible disk in "Day 4" on Oct 17 (302 Ls). In "Day 6" on Oct 19 (304 Ls) the dust storm is spreading north, east and west. Dust storm raging over much of the planet in "Day 8" on Oct 21 (305 Ls). "Day 34" on November xx, 1973 Mars begins to clear of planet-encircling dust storm for 1973. Hellas is light oval area south of Syrtis Major. Photographs taken by Lowell Observatory Although the two dust storms of 1988 did not reach the "planet encircling" stage, similar patterns emerged; during 206° Ls a major storm occurred, followed by another major storm on 313° Ls [Beish and Parker, 1988] and [Beish and Parker, 1989]. Perhaps the statistical significance of the three periods above appears to mislead us when attempting to predict when these disturbances should occur. However, one should be especially alert when observing Mars during these times -- even though we have not passed through the first sensitive period as of yet, be watchful in the next few weeks, alas, a major dust storm may be lurking just around the corner! HOW TO IDENTIFY DUST CLOUDS ON MARS Mars observers frequently report "albedo features" lacking in contrast, the planet is "washed out," or Mars' atmosphere is "dusty." While such descriptions may have merit, generalized yellow hazes and temporary losses in surface contrast is usually omitted in our reports. Photographic evidence for these phenomena is also weak, since the proper sensitometric calibration is usually lacking. Each apparition experienced Mars observers slowly grow familiar with the dark and light features on Mars with a regular systematic observing program. Those who regularly observe Mars during the early months of an apparition will develop the ability to recognize these features even without consulting Mars charts. An observer will then notice sudden changes in Mars’ features, such as a disappearance of a familiar feature or a bright area that was not there a few days before. You may just be seeing the beginnings of dust storm conditions. It is rare indeed when one can watch one of these great red clouds slowly move over the Martian landscape covering over features that were dark and well defined just hours before. Windstorms sometimes move the dust, resulting in both seasonal and long-term changes. In the past we have referred to dust storms as "yellow clouds" and "yellow dust storms." We feel that this is misleading. First, it is virtually impossible to see or even photograph accurate colors on Mars without employing very specialized techniques. Traditionally, observers have employed yellow filters to better reveal dust clouds. The problem is that nearly every light feature on Mars is bright through a yellow filter! When the bright planet Mars is observed against a nighttime sky, the planet's predominantly orange or golden-ochre colored surface becomes highly saturated to the eye. This leads to confusion for the observer, as some features on Mars may appear to change colors during observations. Interestingly, the darker albedo features, when observed against this saturated background are sometimes perceived as complementary hues.