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 Asia). The planet was at opposition on 25 October. Leonard MARTIN describes that the nph was normally bright visible on Day 10, but on Day 14 it became quite faint and around Day 18 it looked completely vanished. On Day 24 the nph began to recover and on Day 34 it returned very normal.

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 Bloemfontein, South Africa, where he produced an excellent series of Mars photos. The planet was closest to the Earth on 28 July with the maximum apparent diameter of 24.1". L MARTIN points out that the nph disappeared for about two days around 25 July (217ーLs). He thinks of this as caused by a bright dust cloud at 55ーN found by SLIPHER on 23 July. This year the central latitudeφwas not far from the equator so that the npr was easier to be watched; but no great dust storm was reported.

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 Diameter24.9". 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-m Band Measurements (JGR, 84 (1979) 2843) treats the IRTM results of the brightness temperature measured at 15m 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 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)


Back to CMO #260 Home Page / Back to Fa軋de