2001 Mars CMO Note
- 06 -
from
CMO #260
The North Polar Hood during the Dust Clouded Period.
I. Introduction
Introduction: It has been widely recognised that the great dust storms on Mars occur
near the summer solstice in the southern hemisphere (or more loosely at
250ｱ50ｰLs): This implies the northern hemisphere is governed by the season
before or near the winter solstice, and hence the northern polar region (npr)
should usually be covered by a big northern polar hood (nph) preceding the
formation of the north polar cap. Once a major dust cloud occurs in the
southern hemisphere however it is believed the nph shrinks and sometimes
disappears. As to this hood-shrinking phenomenon, the article entitled North
Polar Hood Observations during Martian Dust Storms (Icarus 26 (1975) 341) written by Leonard J MARTIN, Lowell
Observatory, is a good summary: It was made based on the results of the
International Planetary Patrol (IPP).
The 2001 global dust cloud however
proved several consequences very deviated from these assertions, and this is a
theme of the present series of Notes. Prior to the analysis of the 2001 case,
we first summarise the foregoing results.
Leonard MARTIN: One of the most typical examples of the shrinkage phenomenon is the one
associated with the 1973 great dust storm that started on 13 October 1973 at
300ｰLs (observed at
L MARTIN also reports that in the
case of the 1956 great dust storm (occurred at 250ｰLs) the nph cannot be
clearly detected on the images made during a month period from 267ｰLs to
289ｰLs. The central latitudeφwas around 24ｰS, and so the
deep polar region was not visible. The difference of 17ｰby Ls implies
about 35 terrestrial days.
The 1939 year was the first year
when E C SLIPHER made the expedition to
In 1971 the great dust
cloud (1971b) started at 260ｰLs on 21 September 1971. The planet was closet to
the Earth already on 12 August at 233ｰLs with Apparent Diameter＝24.9". And thus when the dust disturbance was onset, the central
latitude was up 16ｰS or more and the angular diameter decreased under 19.3" with increasing phase angle and so the
images secured don't seem to give details. The images at the Meudon Observatory
made on 17 September show a thick nph, but the nph on 25 September looks weaker
(A DOLLFUS, S EBISAWA & E BOWELL, Astronomy
& Astrophys 131 (1984) 131). L MARTIN's presentation based on
the IPP indicates that the Day 11 image seems to cease to show the nph, and
more completely at Day 18.
In this respect the case in 1973
was very preferable since the southern disturbance occurred near at opposition.
L MARTIN plots the perimeter of the nph near the area of M Acidalium from Day 1
to Day 9. On Day 2, a part of border slightly receded, while it protruded much
on Day 3. We may say the boundary began to recede gradually around from Day 8.
On Day 18, the central diameter =19ｰS. L MARTIN considers that the northward
recession on Day 2 was because of the arrival of the higher dust stream. On Day
4 the nph was not apparent for an interval of some longitudes.
L MARTIN is of the opinion that the
shrinkage of the nph is not simply due to the coving by the high altitude dust,
but more than that, namely due to a serious interaction of the dust stream and
the nph.
Appendix: The NPC in 1969: Because of the tilt to our side of the north pole,
the nph in 1954, 1969 or 1986 (as well as in 2001) was well observable as often
noted. As to the 1969 nph, the paper by L J MARTIN & W M
McKINNEY North Polar Hood of Mars in 1969 (May 18 - July 25). I.
Blue Light (Icarus 23 (1974)
380) shows systematically the observations of the nph during the period 158ｰLs
- 196ｰLs. We say thus it is possible to compare the results with those in 2001
up until the occurrence of the global dust cloud. In 1969, some dust disturbances
were observed for example by C F CAPEN at Hellas on 28 May (163ｰLs), and
another was checked that divided M Sirenum into two on 24 June (178ｰLs) (though
S MIYAMOTO regarded it as caused by a white cloud, while T SAHEKI whitish
yellow haze), but as a general consequence no great dust storm was reported on
the occasion. The nph was active southward up to 50ｰN to 35ｰN. Around from 9
June (171ｰLs) some activity was seen at the border of the nph which protruded
to southward.
Terry MARTIN: Here we move on to refer to the results derived from the Viking
observations in 1977. As noted in Notes (1)
& (2),
the article by Terry Z MARTIN & H H KIEFFER, Thermal Infrared Properties
of the Martian Atmosphere. 2. The 15-ｵm Band Measurements (JGR, 84 (1979) 2843)
treats the IRTM results of the brightness temperature measured at 15ｵm band T15 and
alludes to the north polar warming due to the dust arrival: In 1977, as the
Vikings observed, just after the occurrence of the second dust storm (1977b) at
274ｰLs, the npr at an altitude of about 25km (0.5 hectopascal atmosphere)
became quite warmer. It rose by 10 to 20K compared with the temperature at
264ｰLs. It is supposed that the temperature at this season should be low to
form the npc, and so it was appropriate to consider the warming must have been
caused by the arrival of the higher airborne dust.
As to the effect of the first dust
storm (1977a) arisen at 205ｰLs, the data was few and just reported a rise in
temperature at 55ｰS by 222ｰLs, but later B M JAKOSKY & T Z MARTIN Mars:
North-Polar Atmosphere Warming during Dust Storms (Icarus 72 (1987) 528) reported that no warming had been
observed at the npr during the 1977a.
A detail of the warming seen after
274ｰLs (on 6 June 1977) is as follows: Before the onset, the 15 ｵm temperature went down
northward to 140K, but once the southern dust storm was entrained, the northern
temperature at 60ｰ- 80ｰN went up by 0.7K/h, and at 281ｰLs the side of the north
polar night was also warmed, and at 285ｰLs it attained a maximum at 60ｰN. The
temperature rose at the pole by 50K, and at 72ｰN by 80K (north of 65ｰN was night
whole day). The maximum was attained on Day 19. The maximum value at the pole
was 195K at 287ｰLs. The temperature began to fall from 295ｰLs, and turned
normal at 309ｰLs (on 3 August) being Day 59, though the cooling was slower at
the outskirts apart from the pole. The CO2 condensation temperature is around 130K and hence the warming must have
been effective to the formation of the npc in the year.
The paper by JACOSKY & Terry
MARTIN also shows the distribution of temperature by T9 and T20. T9 is a channel to indicate
the direct influence of the dust cloud, and this proves the outer area at 65ｰ -
70ｰN was warmer than the polar area. T20 is originally concerned with the surface temperature (if not dusty) and
it is notable this does not show much variation or influence of the dust
(however Terry Z MARTIN, Icarus 45
(1981) 427, cited in Note (2),
implies that the T20 measurements were scarce at the npr
compared with those obtained at the spr).
VL-1 & 2: It is interesting to know the temperature variation when the dusty air
reached the Viking Landers (VL-1 at 22.5ｰN, and VL-2 at 48ｰN). According to J A
RYAN & R M HENRY Mars Atmospheric Phenomena during Major Dust Storms as Measured
at Surface (JGR 86 (1979)
2821), the maximum, mean, and minimal temperatures (at 1.6m above the ground)
at VL-1 remained rather constant before the arrival of the 1977a dust, while
the maximum rather went down (minimum went up) when the front of the dust storm
1977a arrived. The propagation of the 1977b also caused similar consequences to
the effect that the diurnal difference of the temperature diminished. At the
more northerly located VL-2, the temperature was going to fall, and accelerated
because of the first arrival. After the second the temperature stayed low. At
VL-1, the second arrival was more gradual than the first, and at VL-2 the
arrival of both dust streams was not easy to tell; especially the 1997b arrival
was not pinned down. This is very in contrast with the behaviour at the npr of the higher dust stream of the 1997b.
We add here a confusing
story asserting that at 205ｰLs the nph reached VL-2 (at 48ｰN, 230ｰW) from the
north (confusing because the day is nominated as the day 1977a was onset). The
camera at Vl-2 observed a serious obscuration between 11:00 LM and 13:10 LM.
There must have been an argument whether this is caused by the dust or the ice
cloud (J E TILLMAN, R M HENRY & S L HESS, JGR 84 (1979) 2949). We don't repeat it here, but the nph
floating around the northern belt is also interesting. As MARTIN & McKINNEY discuss the haunting phenomena until 196ｰLs, we may
suppose several arrivals of the nph were there at the belt of 48ｰN by the time.
Possible Interpretation: We are now in a position to give a possible interpretation to the
relationship of the nph shrinkage in 1973 with the npr
warming associated with the 1977b storm. It may be not easy to give a
meteorological theory to interpret the rapid and enormous rise of temperature
at the npr, but here it may be appropriate to consider
that the airborne dust at a higher altitude caused by the southern storm
rapidly propagated northward and brought the heat to the upper npr. Any arisen
dust should not necessarily be a thermal medium as seen from the case of the
1977a storm.
The 1973 dust was triggered at
300ｰLs, while the 1977b one was at 274ｰLs (both at near the Solis L area) and
so the season was different, but the fact that the nph completely disappeared
in the case of 1977 around on Day 18 is well coincident with the fact that the
higher-level temperature of the npr in 1977 received a sudden rise and proved
maximal by Day 19 (counted from the 1977b onset); implying the northward
recession of the nph is possibly due to the arrival of the thermal medium of
airborne dust. The hood diminishing is not because the hood is concealed by the
dust covering, but because the heat transmission or the downward radiation of
the heated dust should weaken the activity of the ice cloud.
On the contrary, in the case of
1977a at 205ｰLs, it is thought that since no warming was there at the npr, the nph must have been well present, and nph haunted up
until the area of VL-2. It is possible to postulate that the 1977a dust must
have been weaker and stayed lower, and the dust did not reach far (though it is
certain it reached 48ｰN after 5 or 6 days with a speed of 40km/h) to the deep
north.
Another difference of 1977a and
1977b cases is there: The latter dust was stronger but fell out more rapidly than
the 1977a (James B POLLACK et al, Properties and Effects of Dust Particles
Suspended in the Martian Atmosphere, JGR
84 (1979) 2929). This is perhaps because the medium of the 1977b dust
must have been courser: The airborne dust was not only more subject to the
gravitational sedimentation, but also more interactive with the ice cloud to be
absorbed to fall out.
The 2001 Case: Based on the above
general trend, we next discuss the case of the nph associated with the 2001
global dust storm that started from 184ｰLs (on 24 June). To compare with the
case of 1973 storm, we first pick out the days around Day 20: They correspond
to the days in 2001 around on 12, - 14 July, but as we observed (observed every
day under good conditions) these days the nph proved very large and very
active, and never weakened. Furthermore as we will elucidate in a following
Note, the nph showed a very complicated active behaviour. We just add that the
season was quite different since 13 July (Day 20) 2001 was at 195ｰLs.
A similarity exists however between
the two or between 2001 and 1977b in the sense the dust propagation to the
northern hemisphere was rapid. The 2001 explicit dust devils showed a
characteristic to send readily the airborne dust at higher level and made it
expanded to eastward and northward rapidly. The implication of the difference
of the season may be summarised: The sub-Solar point Ds at 274ｰLs in 1977 was
24.7ｰS, while in 2001 on 24 June (at 184ｰLs) was merely 1.5ｰS. Even 20 days
after, the former was deep at 23.7ｰS, while shallow at 6ｰS in the latter case.
We thus consider the medium of the northward stream was much weaker as a
thermal source in 2001 than in the 1977b case. In this sense, the 2001 case
must be more akin to the 1977a case, though the scale was much larger in 2001.
At 205ｰLs in 1977, Ds was 10.2ｰS.
To compare with the 1956 case, if
we pick out Day 35, the nph was still furious in the 2001 case around the end
of July. The 2001 storm was still active and very different than the case of
1956 when the global dust storm waned. We note the corresponding season 267ｰLs
was far beyond, around on 7 November with central latitude =17ｰS and angular
diameter=8.4", and so not comparable.
POLLACK et al seem to postulate
that the sedimentation of the dust might mainly occur at the npr
(dust layers among the snow are well known), and hence, if so, the 2001 dust
remained longer in the atmosphere than usual because its interaction with the
nph remained milder.
To sum up, the 2001 outbreak of the
dust cloud brought the medium to higher altitude and send it rapidly northward
(as in 1973 and 1977), but it was much less heat-laden in the 2001 case.
Conversely speaking, the globalism of the yellow cloud does not necessarily
need the thermal abundance. The assertion that the appearance of the global
dust storm causes a recession of the nph is thus a mere legend, and can only
applicable to the case of higher-level dust disturbance onset near the summer
solstice.
Outlook: The dust observation in 2001 was incomparably rare in the sense that we
were able to watch the behaviour in the northern hemisphere because the central
latitude was declined northward. Several activities seen near the rim of the
nph were interesting, as well as the cool colour of the fringe that was different
from the dark areas beneath the dust covering. We may say we watched in 2001
the interaction or competition of the dust as a thermal source being to break
the season with the nph which tended to maintain the usual pace of season. We
also accumulated the data of the ice cloud around the npr
before and just after the occurrence of the yellow cloud. These are to be dealt
with in the coming issues.
(Masatsugu MINAMI)
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