2001 Mars CMO Note

- 07 -

from CMO #261

Dissipation of the 2001 Dust Cloud

Compared with Previous Cases





he Global Dust Storm that started from 24 June 2001 was unprecedented in various respects. To measure its scale, it may be a way to try to tag the 2001 dust storm with the (possibly looking longer) decay phase or dissipation phase to compare with the known phases of the preceding cases of major dust storms. It is however not easy to pin down the decay phase since, in contrast to the rise of the dust cloud, the decay rate is quite gradual. If there still exist turbulences inside the dust cloud as well as dust convection inside the troposphere, the state of the storm is not stable, but as it becomes stable, it implies tautologically no drastic sign might be found. Furthermore, we are now using different methods and apparatus than before, and even the celestial motions and seasons of Mars as well as the seeing conditions on our side are never repeatable, and so it is quite difficult to compare.


We however dare to start by comparing with the case in 1971 because fortunately the 1971b dust cloud was witnessed by Mariner 9 and the Earth-based observations were thickly given (mostly due to Shotaro MIYAMOTO).


 1971b Case: The global dust cloud first observed on 21 September 1971(260°Ls) has ever been known as having caused one of the most furious and global dust storms, designated 1971b. The cloud was onset after the southern hemisphere was well warmed, and so the primary state must have been essentially different from the 2001 case, while we can expect once the stability of the troposphere was attained some similarity can be suggested concerning the dissipation phase.


 In 1971, Mars was at opposition on 10 August and most approached on 12 August upto 24.9" angular diameter. The dust cloud occurred and was observed after 19.3", and so the observations from the Earth were preferable to check. It was furthermore very fortunate that the Mariner 9 spacecraft launched on 30 May managed to reach within 800,000km from the Martian surface on 10 November and its TV camera was switched on, and thus Mariner 9 knows well the decay phase of the 1977b dust.


 One of the earliest reports of the Mariner 9 observations was written by C B LEOVY and others in The Martian Atmosphere: Mariner 9 Television Experiment Progress Report, Icarus 17 (1972) 373: This describes that the Martian atmosphere began to be clear as early as the first week of December and the surface markings became drastically visible within a fortnight interval during the last half of December. By mid-January 1972 most of the surface features were identified. Their analysis is deeper and for example minute variations of dust inside the lower Hellas were traced until the end of February. As far as the TV showed, in a global sense the up-down convection or stirring ceased to make a stability of the air-borne dusty atmosphere in mid December.


W K HARTMAN & M J PRICE in Mars: Clearing of the 1971 Dust Storm, Icarus 21 (1974) 28, examined a total of 1000 Mariner 9 images corrected, and by using an eleven-point scale similar to the optical depth from 0.0 (clear, no obscuration evident) to 1.0 (obscured, no detail visible) determined that a fortnight before 22 December 1971 (315°Ls) the air mass of 0.9 was down near to the surface and the upper side of atmosphere was near 0.3, while the decay became halt around 22 December perhaps because of an occurrence of stirring. However after 22 December the clarity took place rapidly. So the end of December should be a standard point of clearing.


 If we regard the dissipation period of the 1971b to be around 31 December (320°Ls), the days of cloudy phase continued 100 days from 21 September.


 The analysis of B J CONRATH in Thermal Structure of the Martian Atmosphere during the Dissipation of the Dust Storm of 1971, Icarus 24 (1975) 36 traced the variation of the temperatures at 0.3 hPa and 2 hPa levels. At 2 hPa, the temperature 220K in mid-December decreased gradually to 190K much later around June 1972, and so the storm had a long tail. However this article assumed a model whose exponential decrease of the optical depth gave a half-life period of 60 days. R W ZUREK in Martian Great Dust Storm: An Update; Icarus 50 (1982) 288 employs this 60 sol as the decay phase of the 1971b. Day 60 falls in a day in late-November, but not far from the starting of the Mariner 9 activity, and certainly it implies they detected a decaying from its early stage. From the view-point of the visual observations, this must be a false clearing.


 On the Earth, Shotaro MIYAMOTO was on the watch for the dissipation period of the 1971b dust storm from the Kwasan Observatory, Kyoto University by the use of a 45cm heterotyped Zeiss refractor up until 24 May 1972 (033°Ls) when the apparent diameter was down to 4.0" and produced a total of 571 drawings during the 1971 season (S MIYAMOTO, Meteorological Observations of Mars during the 1971 Opposition, Contr. Kwasan & Hida Observatories, Kyoto University, No.206 (1972)). According to MIYAMOTO, the Martian surface became semi-transparent at the end of December, and main dark markings generally turned visible. On 18 Jan 1972 (330°Ls) at LCM=074°W (No.514) he noted "dark surface clearly visible for the first time," and so this is a good point to pin down. The seeing on the day was better. We should however note that even after the day MIYAMOTO seemed to detect locally stirrings of dusts near Solis L as similarly as we met in 2001. MIYAMOTO's drawings showed no clear surface until the end of his observations in May with his note that he did never witness the total clearing. If we pick out the day 18 Jan (330°Ls, App.Diam.=7.0") as the X day, the storm lasted 120 days from the outset. It is so reasonable to regard the decay phase as 100 days to 120 days.


 We can also turn our attention to the revival of the white cloud activity: MIYAMOTO noted for instance, on 11 November (291°Ls, Day 52) at LCM=095°W that the area of Solis L was featureless but white. On 28 December (318°Ls, Day 99), the northern part of Hellas was whitish, and on 6 January (323°Ls, Day 108) the area of M Sirenum near the evening limb was white. This must be the phenomena that re-started associated with the dissipation of the dust storm. The observations of the present writer (Mn) in 1971 were never dense, but on 8 November (289°Ls) at LCM=073°W we noted a considerable white haze near the southern evening limb (phase angle =43°, Day 49) and also noted the decay started. Mn however stopped his observation on 25 November 1971 (299°Ls, App/Diam.=10.5"), and even on the day the surface was cloudy and we have never dreamt the stormy aspect could easily be erased.


 Observations of polarisation associated with the dust clouds in 1971 were reported in A DOLLFUS, S EBISAWA & E BOWELL, Polarimetric Analysis of the Martian Dust Storms and Clouds in 1971, Astron. Astrophys. 131 (1984) 123. The visual observation on 2 December (302°Ls) by DOLLFUS proved that the features had considerably recovered. At 379 nm, the polarisation caused by dust decreased in a dust free state in mid-December, while the longer waves proved slower recovering and at 590 nm, the dusty state continued until the end of February 1972. This is concerned with the quite lower and creeping dust, and the polarisation observation is not contradictory to the above assertions.


The Dust Storm in 2001: As repeatedly stressed, the 2001 dust cloud was singular, started quite earlier in season and was not so furious but very global. The yellow cloud however showed a rapid expansion through high-level altitude, and proved very global on 2 July 2001 (Day 9) as shown by the warming up of the whole Martian globe as detected by the Thermal Emission Spectrometer (TES). The dust pall stayed quite long, longer than expected. The yellow cloud appeared thus never inferior to the 1971b storm. Once the yellow cloud turned globally stable, the seasonal elements must have diminished along the middle-latitude and the equatorial band, and the conditions must have proved similar to the 1971b.


 As was stated in CMO Reports #17 (resp #18) in #253 (resp #254), a day around 13 October (252°Ls, Day 112) when Syrtis Mj was detected by naked eyes was a key-day. S Sabæus and S Meridiani were however not complete, and if we wait for their clearer appearance, after a merry-go-round, the days around 10 November (269°Ls, Day 140) proved final keys. Already on 24 October (259°Ls, Day 123) Don PARKER (DPk) produced interesting images at LCM=023°W~040°W where fundamental markings show up and it is already difficult to tell the dust pall from the dust fallout. On 31 October (263°Ls, Day 130) Teru KUMAMORI (Km) obtained good images at LCM=074°W & 083°W where the dark markings considerably recovered. The images of DPk on 8/9 November (269°Ls) at LCM=213°W - 222°W also show the dark band clearly. Tomi AKUTSU (Ak) on 11 November (270°Ls, Day 141) shot the surfaces in the daytime much before the Sunset, and produced easily the IR images of Syrtis Mj and others. We thus judge the final day of October, that is 31 October (Day 130) is an X-day. The visual observations of the present writer on the day showed still a dust stirring at the area between S Auroræ and the Solis L area, but these markings were quite dark: Such a dust stirring was also present on MIYAMOTO's drawings in the late-January 1972, and we consider such a disturbance to be quite lower and quiet.


 The TES also tells us that the globular temperature on 30 November (Day 160) looks lower than that on AKUTSU's day of 30 June. Temperature went down from the beginning of October, and in mid-October the middle latitude bands as well as the equatorial band were well cooled, and we may say 31 October (Day 130) was a good bound. The cooling along the equatorial band suggests a slightly earlier clearing, and so it will be appropriate to consider that the period from 110 days to 130 days constituted the dissipation phase of the 2001 dust storm.


 The above consideration is also confirmed by the revival of the water vapour activity. Details are in #253: From late October, the white mist as well as white cloud re-appeared and from this view-point, the activity of water vapour should be said to have returned from around Day 120. 


The 1973 Dust Cloud: The 1973 Mars appeared strange. It was dusty from the outset from the beginning of the season, and an area of Dædalia-Claritas was already singularly dark, while Claritas implied originally 'brilliance'. Late in season at 300°Ls, another great dust cloud (maybe denoted 1973b) occurred near Solis L. The day was 13 October 1973 (300°Ls), and the planet was closest to the Earth on 17 October (maximal diam=21.5") and hence the dust cloud was well pursued as well as the north polar hood. However no Mariner was working without any comparable data available.


 We so first refer to the result of the polarisation observation obtained by A DOLLFUS, E BOWELL & S EBISAWA in The Martian Dust Storms of 1973; A Polarimetric Analysis, Astro. & Astrophys. 134 (1984) 343: The data near 352 nm show that the yellow dust storm caused by the October cloud increased its transparency by the end of December 1973, and was very cleared in mid-January 1974. At the end of December, 80 days passed since the onset on 13 October, and 15 January implies Day 95.


 MIYAMOTO's observations were complete again in the later part of 1973 (Meteorological Observations of Mars during the 1973 Opposition, Contr. Kwasan & Hida Obs, Kyoto Univ. No 217, 1974): He secured a total of 679 drawings from the beginning of the season up until 16 June 1974 (063°Ls) when App.Diam.=4.0". The October dust cloud was major and global, but rather rapidly decayed. He noted on 29 November (327°Ls, Day 48) that there was no dust cloud except the global haze, and on 4 December (330°Ls, Day 53) at LCM=297°W he described the global haze also diminished. A drawing on 18 December (337°Ls, Day 67) shows Cerberus. On 10 January 1974 (349°Ls, Day 90) S Sabæus and S Meridiani were apparent to his eye. He described that the disk was cloud free on 17 January (353°Ls) at ω=250°W.


 In 1973, Mn observed until 17 January 1974 while Syrtis Mj and its environments were apparent during the period from 7 December (331°Ls, Day 56) to 14 December, though the southern continents looked dusty for a while.


 We thus conclude that the 1973 dust cloud (or 1973b) was alive for 60 days to 80 days up until the end of the 1973 year.


The Case of the 1956 Dust Cloud: As to this first attention-getting great dust storm, Tsuneo SAHEKI gave a standard summary in his book (Mars and Its Observations, Third Edition in 1968): The dust cloud appeared brilliantly at Noachis on 20 August 1956 (246°Ls) and encircled the southern hemisphere within 10 days. Some dark markings remained however unaffected. Around 10 September (Day 22) the storm began to decay to make the cloud split and dispersed. In late-September yellow haze still haunted near Noachis and Deucalionis R, and it aroused a very bright dust devil in a narrow area of Deucalionis R on 3 October (274°Ls, Day 43). The bright part was however short-lived, and soon dispersed. SAHEKI notes the area appeared already at the morning side associated with a thick whitish water mist. The last observations of the area were made by MIYAMOTO and S FUKUI on 6 October, and when it became into sight again on 26 October, no dust cloud was detected. So we can consider the storm ended in mid- October around on Day 55.


 The 1956 year was the very year when MIYAMOTO started by the use of a 30 cm Cooke (later by a 45 cm Zeiss) his routine observations of Mars fascinated by the occurrence of the encircling dust storm (MIYAMOTO was senior by four years to SAHEKI, and MIYAMOTO was a graduate student at the Kwasan Observatory but was away because of military service when SAHEKI observed Mars in 1937 by the use of the 30 cm Cooke at Kwasan). MIYAMOTO describes the behaviour of the dust cloud in Contr. Kwasan & Hida Obs. Kyoto Univ. No.71(1957). He reports dusty cloud mass began to subside on 28 September (271°Ls), and S Sabæus recovered. He describes also the bright condensation at Deucalionis R on 3 October. He judged the storm around Noachis totally ceased in mid-October. The conclusion about the decay phase is thus similar to SAHEKI's.


1977a, 1977b: The dust clouds in 1977 were the ones observed by the Viking Orbiters and Landers, and no other systematic observations from the Earth were known concerning the 1977 clouds. The 1977b occurred when the apparent diameter was recovering to 4.9", and so ground-based observations were not impossible and some were known but not systematic: MIYAMOTO already retired (on 31 March 1976 from Kyoto University). The planet was closest to the Earth on 19 January 1978 with maximal diameter=14.3". The dust clouds were pinned down to have occurred at 205°Ls and 274°Ls, and their characteristic profiles of the optical depths were made by J B POLLACK and others in Properties and Effects of Dust Particles Suspended in the Martian Atmosphere, JGR 84 (1979) 2929, and by T E THORPE in Mars Atmospheric Opacity Effects Observed in the Northern Hemisphere by Viking Orbiter Imaging, JGR 86 (1981) 11419. The former reports the opacity variation of the sky seen from the Viking Lander sites, and the latter records the observations from the Orbiters through Red and Clear filters. The trend of the optical depth in both cases looks quite similar. Just before the occurrences, the optical depth was near 1, while it rapidly grows higher than 2, and if passing the height the depth falls rather rapidly down.


 According to POLLACK and others (op cit), the optical depth of 1977a was above 1 from 16 February (205°Ls) to 19 April (244°Ls) and 1977b was so from 6 June (274°Ls) to 4 September (327°Ls) (App.Diam.=6.2" on 4 September). If we employ these period the durations of 1977a and 1977b were 63 days and 91 days respectively.


 It appears that the information before and after the occurrence day of the 1977b was not thickly delivered (on the level of the visual observations). The well-known report by G A BRIGGS, W A BAUM & J BARNES in JGR 84 (1979) 2795 shows a mosaic image at 274°Ls in which a furious dust rises at an area west of Argyre, while the very place is vacant on the preceding image taken at 268°Ls (on 26 May). Since the dust cloud expands but does not easily move, we should say the dust cloud started on a day between 26 May and 6 June. If we take this value POLLACK and others' life period of the 1977b should be about 95 days.


On the other hand, the observation at T15 shows a steep rise of temperature only during the period from 272.4°Ls to 273°Ls (M B JAKOSKY & Terry Z MARTIN, Icarus 72 (1987) 528), and so the day around 1 June (271°Ls) is a good candidate. Terry MARTIN, in Icarus 45 (1981) 427, employs a characteristic curve of the optical depth secured from the temperature observations (his Fig 2) in which the depth is under 0.3 from 256°Ls to 273°Ls while it rises abruptly after 273°Ls. The data about 1977a rather ends at 250°Ls when the depth is about 0.4 (higher). So if we choose 255°Ls (7 May) as the end mark of 1977a, we must add about 27 days to POLLACK and others' data on 1977a. Furthermore, if we expect the depth should be as low as down to the value at 273°Ls, it corresponds to 348°Ls (12 October) if extrapolated, and so we should further add about 38 days to the half-life period of POLLACK and others'. Thus we obtain the days of the decay phase 90 days and 130 days for 1977a and 1977b respectively.


 The catastrophe 1977b must have been aroused because the energy of the preceding 1977a was not completely combustive, and this case the second one could be considered much more furious if we refer to the thick opacity. The second one however subsided rather rapidly. This must be caused because the second one is generally made of coarser grains receiving much the gravitational effect, and this case the season was late enough to make the grains be cores of the ice condensation (iced H2O or CO2 from the north polar region).

 Since the season of the 1977a occurrence (205°Ls) was akin to the case of the 2001 dust cloud (at 184°Ls) and some expected and waited another furious dust cloud to occur to name 2001b? about 50 degrees later in Ls, but we should say they failed. No symptom about the case: The June 2001 dust cloud was sufficiently great and major from the outset enough to make the catastrophic energy gush to the upper level altitudes.


Summing Up: Regardless the onset season as well as the furiousness near on the surface, if we simply compare the periods when the dust palls were alive, we obtain the following ranking of the dissipation phases: 55 days in the case of 1956 dust cloud, (60~)80 days in the 1973 case, (63~) 90 days in 1977a, (100~)120 days in 1971b, (90~)130 days in 1977b, and (110~) 130 days in 2001.


 As to the scale of the dust storms, the mean size of particles that constitutes the dust is another index: In this sense the 2001 yellow cloud was made of finer grains and so it should be inferior in violence to both of the 1971b and 1977b to the extent that the sedimental fallout was more delayed and the scavenging by the nph or its interaction was slower. We should simply say the 2001 one was less furious but more stable.


Appendix about the dust clouds in the first half of the 1973 apparition: It was certain that in 1973 before 13 October (300°Ls) the surface was often  patched by local dusty hazes but generally free from any global dust cloud. The bright October dust devils at the Solis L area just after the onset were noticed by S KURISU (25cm spec) on 14 October first in Japan, and on 15 October by several independently including observers at the Fukui City Observatory (Mn and Nj). The cloud was observed earlier in Hawaii (for example T B McCORD and others in Icarus 31 (1977) 293 reported nearly 100 images of the dust cloud taken by the use of a 224cm speculum from 8:50 GMT to 12:41 GMT on 14 October at the Mauna Kea Observatory). Opposition was on 25 October, and hence the cloud was well chased (maximum diameter was 21.5" on 17 October).


 At that time however the new dark patch at Dædalia was shown up there already: It was therefore suggested to have been caused by a preceding furious dust activity. The new patch seems to be checked by S MIYAMOTO first on 24 May 1973 (212°Ls, App. Diam.=8.3") at LCM=095°W. One round after it was vivid on 30 June (235°Ls, App.Diam.= 10.4"). Richard McKIM in Telescopic Martian Dust Storms: A Narrative and Catalogue, Memoirs. BAA No. 44 (1999) lists several minor dust disturbance since 22 June 1973, but none may be explicitly candidates. There are several possibilities about the appearance of the Dædalia dark patch: It might have been caused 1) by the great dust storm in 1971, 2) by a global dust cloud onset early 1973, or 3) by a local furious dust devil just before May or June 1973.


 In 2001, the season 212°Ls (of MIYAMOTO) corresponded to mid-August 2001 when already Dædalia showed up again, and so 2) and 3) are still possible. In 1973, MIYAMOTO started his routine observation no earlier than from 27 April 1973 (195°Ls) when App.Diam.=7.1". Toshihiko OSAWA started earlier in February checking the south polar region, but did not particularly reported about the lower dust clouds (his on 25 March at 175°Ls shows S Sabæus under a hazy aspect). Unfortunately we should say thus no systematic observation earlier to pin down the occurrence of the dust cloud was there. It is highly possible however there occurred a furious dust cloud in an earlier stage, and maybe named a phantom 1973a. The polarimetric observations at 352 nm by A DOLLFUS, E BOWELL & S EBISAWA in 1973 (op cit) suggest that the earlier dust 1973a (than the October one 1973b) might have been larger and more polarised (their observations however began from July 1973).

    Masatsugu MINAMI    


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