2001 Mars CMO Note
- 07 -
from
CMO #261
Dissipation of the 2001 Dust Cloud
Compared with Previous Cases
|
T |
he Global
Dust Storm that started from
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
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
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
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
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
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
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.
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
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
In 1973, Mn observed until
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
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
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
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
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
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
Masatsugu MINAMI
Back to CMO #261 Home Page / Back to Façade / Back to the CMO Home Page