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North polar hood observations during Martian dust storms
ICARUS 26, 341--352
(1975)
North Polar Hood Observations during Martian Dust Storms LEONARD
J. MARTIN
Planetary Research Center, Lowell Observatory, Flagstaff, Arizona 86001 Received May 1, 1975 The 1956, 1971, and 1973 major dust storms on Mars affected the apparent contrast and extent of the northern polar hood. A survey of photography from seven apparitions indicates that this seasonal feature is consistently prominent and identifiable in the absence of major storms throughout half of the Martian year. During the 1956 dust storm, the hood was not seen for a period of over one month on Lowell photographs. The effects of a storm are also seen on 1971 International Planetary Patrol photographs; the hood quickly became faint and tenuous even in ultraviolet light, which normally shows it as very bright. Patrol photographs of 1973 cover the complete progression from a prominent and extensive hood before the storm, to an intermittent disappearance at the height of the storm, to the subsequent return of a normal hood as the storm died out. Hourly and daily mapping from these photographs indicates that the hood developed a southward protrusion during the first few days of the storm as it began its apparent recession. The hood was seen on at least one side of the planet on every day throughout the duration of the storm, although its normal extent and contrast were greatly reduced. images t a k e n during t h e height of t h e 1973 m a j o r d u s t s t o r m (see Fig. 1 ; n o r t h is up). P o l a r hoods o f light-colored clouds norF u r t h e r investigation showed a progressive m a l l y b l a n k e t t h e higher l a t i t u d e s o f t h e a p p a r e n t recession of t h e hood, beginning a u t u m n - w i n t e r h e m i s p h e r e o f Mars. These during t h e second d a y o f t h e s t o r m ( L 8 hoods are well k n o w n a n d quickly recog301°). L a t e r , as t h e d u s t s t o r m waned, t h e nizable on u l t r a v i o l e t - a n d blue-filter h o o d a g a i n b e c a m e larger, e v e n t u a l l y rep h o t o g r a p h s . H o o d clouds are t h e m o s t t u r n i n g to its n o r m a l expanse. r a p i d l y changing of all M a r t i a n p h e n o m e n a Since t h e N o r t h Pole o f Mars was tilted t h a t c a n be easily i n v e s t i g a t e d f r o m E a r t h a w a y f r o m the E a r t h b e t w e e n 15 ° a n d 21 ° b a s e d observations, a l t h o u g h few studies during this d u s t storm, t h e b o u n d a r y o f t h e o f t h e i r r a p i d l y changing a p p e a r a n c e h a v e a p p a r e n t hood would be visible unless it b e e n published. T h e m a j o r d u s t s t o r m s of were n o r t h of t h e 70 ° parallel; these p o l a r 1956, 1971, a n d 1973 all occurred during h o o d clouds are n o r m a l l y seen e x t e n d i n g periods in which t h e higher n o r t h e r n latid o w n to b e t w e e n 40 ° a n d 50 ° n o r t h l a t i t u d e t u d e s were in t h e middle of t h e i r polar hood (Fig. 4). T h e d u s t s t o r m r e a c h e d its m a x i seasons. This p a p e r e x a m i n e s t h e possible m u m e x t e n t a t a n age of a b o u t 2weeks, a n d correlation b e t w e e n d u s t s t o r m a c t i v i t y b y t h a t t i m e t h e hood h a d d i s a p p e a r e d comand changes in t h e a p p e a r a n c e o f t h e n o r t h p l e t e l y a t some longitudes, a l t h o u g h n o t a t p o l a r hood, b a s e d on I n t e r n a t i o n a l Planeothers. These clouds were n o t a b l y smaller tary Patrol photographs. a n d less b r i g h t where traces o f t h e m remained. T h e h o o d was identifiable on a t least one face o f t h e p l a n e t t h r o u g h e v e r y I~rr~AL STUDY OF 1973 HOOD AND d a y of t h e storm. STORM T h e b r i g h t e n i n g seen on t h e D a y 14 N o sign o f a n o r t h p o l a r hood could be i m a g e in F i g . 1 is a m o r n i n g l i m b cloud a t 75°W, 10°S, n o t a h o o d on a m i s o r i e n t e d seen on some excellent ultraviolet-filter AI~.~.~: ¢~ 1975 by Academic Pre~, Inc. 341 of reproduction in any form reserved. INTRODUCTION
Great Britain
42
LEONARD J. MARTIN
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POTJAR HOOD I)U.RING ~ k N _ ~
image. This cloud appears to be unobscured b y dust, although it is just north of the region of the most intensive storm activity seen on red-filter photographs. SURVEY OF SEVEN APPARITIONS
The next step was to determine how unusual this apparent recession of the hood is. Did the same thing occur during other major dust storms? Also, how consistent is the hood's presence when no dust storm is in progress? The literature confirms the regular presence of these hoods during the autumn and winter seasons (Michaux and Newburn, 1972), but we could find no statement that hoods can be expected to be seen continuously during these seasons, and it is known that their boundaries fluctuate daily (Capen and Capen, 1972; Fischbacher a/., 1969) and even hourly (Martin and McKinney, 1974). The results of studies of the Lowell Observatory photographic collection for the presence of north hoods during seven recent apparitions that cover this season are shown in Fig. 2. The three more recent apparitions are covered b y the International Planetary Patrol films, while the four earlier apparitions were studied from the Lowell Observatory historical collection of plates. Although photographic coverage NORTHERN
Ls
, ~ao°
AUTUMN
I
,
=~o°
during these earlier years was much less comprehensive than that of the Planetary Patrol, no significant interruptions in the visibility of the hood during these seasons were found, except during global dust storms. Other short=term hood recessions could have taken place on some sides of the planet during these less-photographed apparitions, but certainly none so extensive as during the 1956 storm, when no hood was apparent on photographs for over a month (L s 267°-289°). A search of the literature reveals no mention of this extended disappearance of the hood, although the disappearance of the south polar cap during the same dust storm was discussed at length b y a number of observers. A marked recession of the visible hood did occur during all three of the recent planetwide storms, as shown b y the dashed lines in Fig. 2. These are periods when these hood clouds either were not identifiable or were seen to be much thinner and weaker than normal. The solid lines, which cover periods of positive identification of a normal hood, confirm the persistence of these clouds during storm-free conditions. Phase and apparent disk diameter limit the length of time this feature can be observed during any one apparition, b u t the seven shown here provide enough overlapping coverage to provide an adequate sampling. NORTHERN
I
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343
~UST STORMS
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WINTER
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NORTH POLAR H O O D OBSERVATIONS
'~
1939 1941 STORM ¥
1956
1958
1969 1971
STORM
¥
STORM
1973
FIG. 2. A survey of seven Mars apparitions which fell within the season of north polar hoods showed t h a t this feature is persistently prominent except during dust storms. The scale across the top is t h e planetocentric longitude of the Sun (L s), spanning nearly half of the Martian year. Solid lines identify intervals of positive hood identification, while dashed lines indicate intervals in which the hood was thin, weak, or not identifiable on available photographs. See text.
344
LEONARD
J. MARTIN
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POLAR HOOD DURING MARTIAN DUST STORMS
BRIE~ EVENT IN 1939 An interruption of the 1939 hood was noted b y Slipher (1940) of Lowell Observatory, b u t he m a y not have been aware of how unusual this was. His observational notes (Slipher, 1939) from those nights emphasize the new polar brightening at 55°N, which he saw on J u l y 23 and 27. Some of his excellent photographs from Bloemfonrein, South Africa (Lowell Observatory historical plate collection), showing the progression of events during that week, are reproduced in Fig. 3 (north is up). The green-filter photographs taken on J u l y 22, 23, and 27 all show clouds in the north that are brighter than normal polar hood clouds would be at that wavelength. Slipher's blue-filter photographs taken during this time interval also show the same new brightening and the intermittent absence of hood clouds seen on the green-filter images, b u t none of them is very suitable for reproduction. Slipher described this northern brightening as being as bright as the south polar cap and suggested it could be a deposit of new snow. Neither the season nor the latitude seems consistent with that interpretation, however, so we propose that it m a y have been a dust cloud, although measurements do not indicate any notable hourly motion. The Martian season was midautumn, which is not known for its dust clouds or new deposits of polar surface brightening material (CO 2 or H 2 0 ). Also, this new brightening was further south than the polar surface cap is believed to reach at its greatest extent (Baum and Martin, 1973). The bright spot seen just to the right of center of the north hood on J u l y 22 could well be the beginning of this cloud. Both the hood and the new brightening disappeared for two days (L~ 217°), followed b y the reappearance of the brightening on J u l y 27. Its western boundary was displaced to the west, and its brightness was diminished since its previous appearance. B y J u l y 28 the hood reappeared, and the other new brightenings were not seen again.
1969 HOOD STUDY The International Planetary Patrol Program began in 1969 (Baum, 1973), provi-
34~
ding us with enough continuity of photographic coverage to make a detailed study: of the north hood's changing boundaries during its early phases (Martin and McKinney, 1974). Summary maps f r o m this study are reproduced in Fig. 4. They show how extensive the hood becomes, even during the time of its formation. Although all three of these maps cover time intervals long before :the season known for its large dust storms, the lower two (L S 171°-196 °) show boundaries which are representative of those seen during the entire hood season. Thus we can expect to see the north hood on blue-filter photographs, even though the North Pole of Mars m a y be tilted away from us b y as much as 24 ° during this season. 1971 MAJOR STORM The 1971 Patrol photographs (oriented north up) in Fig. 5 show the effects of the largest well-observed dust storm (Martin, 1974a). Starting with a normal appearance (L s 261°), the north hood rapidly becomes difficult to identify as the storm progresses. The original ultraviolet-filter negatives do show a slight suggestion of polar and connected limb brightening throughout the storm. Although the hood does seem to revive as the storm wanes, the increasing phase and distance of Mars prevented us from getting photographs which could be reproduced well enough to show this revival. Mariner 9 photographs were used b y Briggs and Leovy (1974) to establish the hood boundaries as again reaching to between 40 ° and 50 ° north latitude. These were obtained during February and March 1972, near the end of the northern hemisphere winter on Mars (L s 343°-353°), when good Earth-based photographs could no longer be obtained. As in 1956 and 1973, the apparent recession was temporary and was limited to the approximate duration of the dust storm. MAPPING THE 1973 HOOD
Changes in the north hood during the developing stages of the 1973 dust storm are shown on the nine daily maps in Figs.
346
LEONARD-J. MARTIN
FxG. 4. Summary maps from a study of hood boundaries using 1969 International Planetary Patrol photographs. The southern limits shown on the bottom two maps are typical of those seen throughout n o r t h e r n a u t u m n a n d w i n t e r o n b l u e - f i l t e r p h o t o g r a p h s . T h e n u m b e r s o n t h e l i n e s refer t o t h e n u m b e r
of days (out of 23 ) that hood clouds appeared on blue-filter images within the areas either enclosed by, or north of the lines. 6, 7, a n d 8. Since this p h e n o m e n o n occurred h o u r l y b o u n d a r i e s o f t h e h o o d were p l o t t e d n e a r t h e 1973 opposition a n d during a f r o m ultraviolet-filter P a t r o l p h o t o g r a p h s period w h e n several P a t r o l s t a t i o n s h a d using o u r p l a n e t i m a g e projector. T h e b a s e excellent seeing, t h e p h o t o g r a p h i c record m a p t h a t w a s used is f r o m t h e Lowell series o f these changes in t h e h o o d ' s a p p e a r a n c e (Inge, 1974) a n d shows t h e p l a n e t as seen during t h e s t o r m is u n u s u a l l y good. T h e " on I n t e r n a t i o n a l P l a n e t a r y P a t r o l red-
347
POLAR H O O D D U R I N G MARTJAN D U S T STORMS
o o
o
o
FIG. 6. Hourly hood boundaries plotted from ultraviolet- or blue-filter Patrol photographs during the first days of the 1973 dust storm. Sample ultraviolet photographs for each d ay are exhibited together with red-filter International Planetary Patrol photographs taken during the same hour. The broader boundary lines are those plotted from these sample photographs. Brightenings seen on redfilter images are outlined on the maps with dashes. The central meridians of the sample photographs are also denoted on the maps. Note the recession of the hood on D a y 2, followed by the protrusion on
FIo. 7. A continuation of Fig. 6 showing 1973 storm Days 4, 5, 6. Each day is a Mars rotation, beginning with the first day of the dust storm. I n the sample ultraviolet photograph of D ay 4, note the dark cloud over Sinus Meridiani, which became bright in red light on the following day. Th e dashed lines at 50°N indicate gaps between available photographs, as explained in the text. 349
FIo. 8. A continuation of Fig. 6 and 7 showing Days 7, 8, 9 of the 1973 storm. Some boundary lines are labeled with tiny triangles to indicate which side of the boundary is bright. Broken boundary lines denote plots from blue-filter images, which were used when no suitable ultraviolet images were available. The photographs for D a y 9 had diagonal scratches on the original film. 350
POT,A~ HOOD D v R r ~ o : ~ a ~ S T
filter photographs taken n e a r t h e time of the 1973 opposition. When no suitable ultraviolet photograph was available for a particular hour, a bhie-filter photograph was substituted, and the hood was then plotted as a broken line. Gaps between the central meridians of available ultraviolet or blue photographs exceeding 60 ° have been indicated b y a dashed line at 50°N on the map. Boundaries of the hood and other brightenings seen on the particular ultraviolet photographs exhibited along the left margins of Figs. 6, 7, and 8 are identified b y lines slightly heavier than the rest. Dust storm brightenings seen simultaneously on the exhibited red-filter photographs are outlined on the maps with broad dashes. The central meridians for the exhibited photographs are also shown. The numbering of "days" is based upon rotations of Mars, with each beginning at the time that the subsolar longitude on Mars is 180 ° and starting with D a y 1 as the first day of the dust storm (October 13). The apparent disturbance of the hood b y the 1973 storm began on the second day as a northward recession of the hood at the same longitude which showed maximum storm activity. On the following day the hood protruded somewhat southward at a more easterly longitude, while continuing to recede northward in the region directly north of the storm. B y the fourth day the hood was no longer visible at some longitudes and was generally farther north than it was the previous day. Ultraviolet photographs also show the dust storm area as unusually bright in that color compared with the 1971 storm. They also reveal a remarkably dark cloud over Sinus Meridiani, which developed into a brightening in red light on the fifth day. Although photographic coverage was incomplete on the sixth day, there was a longitudinal expanse of 150 ° for which we had photographs that showed no hood. The apparent hood boundary continued to fluctuate (primarily north of 50 °) during the seventh, eighth, and ninth days of the storm. As the storm continued to expand, we have increasing evidence of the hood's
STORMS
351
disappearingfrom view in part
of the eastern hemisphere, as well as the western. I~ote also that the areas of greatest dust storm activity (according to red-light photographs) are no longer particularly bright on these ultraviolet photographs. CONCLUSIONS
Do these polar clouds actually recede, or are they merely obscured b y hazes associated with the dust storm ? Although a dust storm of these proportions (Martin, 1974b) must disturb the planet's heat balance (Gierasch and Goody, 1972) and therefore could affect these seasonally dependent cloud formations, the observational evidence is not conclusive. The southward protrusion of the 1973 hood boundary at one longitude, simultaneous with a northward retreat at another longitude (Fig. 6), is possible evidence for actual changes in the hood. Another clue is the reappearance of the hood long before the contrast of albedo features had returned to normal in red light (Fig. 1), which suggests that dust had not yet cleared from the atmosphere. Also, the reappearance of bright limb clouds in ultraviolet (Fig. 1) brings up the question as to w h y they also were not obscured, despite their being closer to the most intense storm activity. Apparent interactions between red clouds and ultraviolet clouds during the 1956 (Sharnov, 1959), 1971 (Martin, 1974a), and 1973 (Fig. 7) storms lend further support to the hypothesis that we m a y be dealing with more than simple obscuration b y high dust. The rapid hourly motions of these polar clouds during nonstorm periods (Martin and McKinney, 1974) suggests that they are highly unstable to begin with and could easily be affected b y massive dust storms. ACKNOWLEDGMENTS W. A. Baum and C. F. Capen have both provided generous assitance in the preparation of this paper. This research is supported by NASA grant NGR-03-0O3-001. REFERENCES
B A ~ , W. A. (1973). The International Pl~etary Patrol Program. An assessment of the first three years. Planet. Space Sci. 21, 1511.
352
LEONARD J. MARTIN
BAUM, W. A., ~ MARTn~, L. J. (1973). Behavior of the Martian polar caps since 1905. Bull. A m . As~ron. Soc. 5, 296. BRIGGS, G. A., AND LEOVY, C. B. (1974). Mariner 9 observations of the Mars north polar hood. Bull. A m . Meteorol. Soc. 55, 278-296. CAPEN, C. F., A~-DC~'EN, V. W. (1972). Meteorological phenomena. I n Physical Observations of Mars 1966-1967-1968. Jet Propulsion Lab., Pasadena, CaliL FISCHBA.CHER, G. E., MARTIN, L. J., AND BAUM, W. A. (1969). Mars polar cap boundaries. Planetary Research Center, Lowell Obs., Flagstaff, Ariz., J P L Contract 951547, May, GIERASCH, P. J., AND GOODY, R. M. (1972). The effect of dust on the temperature of the Martian atmosphere. J. Atmos. Sci. 29, 400-402. INGE, J. L. (1974). Mars--1973. Albedo map, 1 : 25,000,000. Lowell Obs. Map Series. MARTIN, L. J. (1974a). The major Martian yellow storm of 1971. Icarus 22, 175-188. MARTIN, L. J. (1974b). The major Martian dust
storms of 1971 and 1973. Icarus 23, 108115. M ~ T I N , L. J., AND McKtNN-~Y, W. M. (1974). North polar hood of Mars in 1969 (May 18-July 25). I. Blue light. Icarus 23, 380-387. MICHAUX,C. M., ANDNEWBURN,R. L., JR. ( 1972 ). Mars Hciontific Model, Section 4.2. J P L Doc. No. 606-1. SHARONOV, V. V. (1959). The surface and atmosphere of Mars according to photographic, photometric, and colorimetric observations conducted in 1956 at Tashkent. I n Result~ of Observations of Mars in the UHHR During the Great Opposition of 1956 (The Astronomic Council, Committee of Planetary Physics), p. 122. USSR Academy of Sciences Publishing House, Moscow, Technical Translation F-3, NASA, 1962. SIGHER, E. C. (1939). Visual observation notes, Lowell Obs. (unpublished). SUPHEtt, E. C. (1940). Martian phenomena observed from our southern hemisphere. Telescope 7, 102-109.
(1975)
North Polar Hood Observations during Martian Dust Storms LEONARD
J. MARTIN
Planetary Research Center, Lowell Observatory, Flagstaff, Arizona 86001 Received May 1, 1975 The 1956, 1971, and 1973 major dust storms on Mars affected the apparent contrast and extent of the northern polar hood. A survey of photography from seven apparitions indicates that this seasonal feature is consistently prominent and identifiable in the absence of major storms throughout half of the Martian year. During the 1956 dust storm, the hood was not seen for a period of over one month on Lowell photographs. The effects of a storm are also seen on 1971 International Planetary Patrol photographs; the hood quickly became faint and tenuous even in ultraviolet light, which normally shows it as very bright. Patrol photographs of 1973 cover the complete progression from a prominent and extensive hood before the storm, to an intermittent disappearance at the height of the storm, to the subsequent return of a normal hood as the storm died out. Hourly and daily mapping from these photographs indicates that the hood developed a southward protrusion during the first few days of the storm as it began its apparent recession. The hood was seen on at least one side of the planet on every day throughout the duration of the storm, although its normal extent and contrast were greatly reduced. images t a k e n during t h e height of t h e 1973 m a j o r d u s t s t o r m (see Fig. 1 ; n o r t h is up). P o l a r hoods o f light-colored clouds norF u r t h e r investigation showed a progressive m a l l y b l a n k e t t h e higher l a t i t u d e s o f t h e a p p a r e n t recession of t h e hood, beginning a u t u m n - w i n t e r h e m i s p h e r e o f Mars. These during t h e second d a y o f t h e s t o r m ( L 8 hoods are well k n o w n a n d quickly recog301°). L a t e r , as t h e d u s t s t o r m waned, t h e nizable on u l t r a v i o l e t - a n d blue-filter h o o d a g a i n b e c a m e larger, e v e n t u a l l y rep h o t o g r a p h s . H o o d clouds are t h e m o s t t u r n i n g to its n o r m a l expanse. r a p i d l y changing of all M a r t i a n p h e n o m e n a Since t h e N o r t h Pole o f Mars was tilted t h a t c a n be easily i n v e s t i g a t e d f r o m E a r t h a w a y f r o m the E a r t h b e t w e e n 15 ° a n d 21 ° b a s e d observations, a l t h o u g h few studies during this d u s t storm, t h e b o u n d a r y o f t h e o f t h e i r r a p i d l y changing a p p e a r a n c e h a v e a p p a r e n t hood would be visible unless it b e e n published. T h e m a j o r d u s t s t o r m s of were n o r t h of t h e 70 ° parallel; these p o l a r 1956, 1971, a n d 1973 all occurred during h o o d clouds are n o r m a l l y seen e x t e n d i n g periods in which t h e higher n o r t h e r n latid o w n to b e t w e e n 40 ° a n d 50 ° n o r t h l a t i t u d e t u d e s were in t h e middle of t h e i r polar hood (Fig. 4). T h e d u s t s t o r m r e a c h e d its m a x i seasons. This p a p e r e x a m i n e s t h e possible m u m e x t e n t a t a n age of a b o u t 2weeks, a n d correlation b e t w e e n d u s t s t o r m a c t i v i t y b y t h a t t i m e t h e hood h a d d i s a p p e a r e d comand changes in t h e a p p e a r a n c e o f t h e n o r t h p l e t e l y a t some longitudes, a l t h o u g h n o t a t p o l a r hood, b a s e d on I n t e r n a t i o n a l Planeothers. These clouds were n o t a b l y smaller tary Patrol photographs. a n d less b r i g h t where traces o f t h e m remained. T h e h o o d was identifiable on a t least one face o f t h e p l a n e t t h r o u g h e v e r y I~rr~AL STUDY OF 1973 HOOD AND d a y of t h e storm. STORM T h e b r i g h t e n i n g seen on t h e D a y 14 N o sign o f a n o r t h p o l a r hood could be i m a g e in F i g . 1 is a m o r n i n g l i m b cloud a t 75°W, 10°S, n o t a h o o d on a m i s o r i e n t e d seen on some excellent ultraviolet-filter AI~.~.~: ¢~ 1975 by Academic Pre~, Inc. 341 of reproduction in any form reserved. INTRODUCTION
Great Britain
42
LEONARD J. MARTIN
~o
i f-~
Q O
POTJAR HOOD I)U.RING ~ k N _ ~
image. This cloud appears to be unobscured b y dust, although it is just north of the region of the most intensive storm activity seen on red-filter photographs. SURVEY OF SEVEN APPARITIONS
The next step was to determine how unusual this apparent recession of the hood is. Did the same thing occur during other major dust storms? Also, how consistent is the hood's presence when no dust storm is in progress? The literature confirms the regular presence of these hoods during the autumn and winter seasons (Michaux and Newburn, 1972), but we could find no statement that hoods can be expected to be seen continuously during these seasons, and it is known that their boundaries fluctuate daily (Capen and Capen, 1972; Fischbacher a/., 1969) and even hourly (Martin and McKinney, 1974). The results of studies of the Lowell Observatory photographic collection for the presence of north hoods during seven recent apparitions that cover this season are shown in Fig. 2. The three more recent apparitions are covered b y the International Planetary Patrol films, while the four earlier apparitions were studied from the Lowell Observatory historical collection of plates. Although photographic coverage NORTHERN
Ls
, ~ao°
AUTUMN
I
,
=~o°
during these earlier years was much less comprehensive than that of the Planetary Patrol, no significant interruptions in the visibility of the hood during these seasons were found, except during global dust storms. Other short=term hood recessions could have taken place on some sides of the planet during these less-photographed apparitions, but certainly none so extensive as during the 1956 storm, when no hood was apparent on photographs for over a month (L s 267°-289°). A search of the literature reveals no mention of this extended disappearance of the hood, although the disappearance of the south polar cap during the same dust storm was discussed at length b y a number of observers. A marked recession of the visible hood did occur during all three of the recent planetwide storms, as shown b y the dashed lines in Fig. 2. These are periods when these hood clouds either were not identifiable or were seen to be much thinner and weaker than normal. The solid lines, which cover periods of positive identification of a normal hood, confirm the persistence of these clouds during storm-free conditions. Phase and apparent disk diameter limit the length of time this feature can be observed during any one apparition, b u t the seven shown here provide enough overlapping coverage to provide an adequate sampling. NORTHERN
I
I
~
,
24o"
343
~UST STORMS
i =7~
I
i
~"
o
I ]
WINTER
i
I =3~
,
i
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NORTH POLAR H O O D OBSERVATIONS
'~
1939 1941 STORM ¥
1956
1958
1969 1971
STORM
¥
STORM
1973
FIG. 2. A survey of seven Mars apparitions which fell within the season of north polar hoods showed t h a t this feature is persistently prominent except during dust storms. The scale across the top is t h e planetocentric longitude of the Sun (L s), spanning nearly half of the Martian year. Solid lines identify intervals of positive hood identification, while dashed lines indicate intervals in which the hood was thin, weak, or not identifiable on available photographs. See text.
344
LEONARD
J. MARTIN
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POLAR HOOD DURING MARTIAN DUST STORMS
BRIE~ EVENT IN 1939 An interruption of the 1939 hood was noted b y Slipher (1940) of Lowell Observatory, b u t he m a y not have been aware of how unusual this was. His observational notes (Slipher, 1939) from those nights emphasize the new polar brightening at 55°N, which he saw on J u l y 23 and 27. Some of his excellent photographs from Bloemfonrein, South Africa (Lowell Observatory historical plate collection), showing the progression of events during that week, are reproduced in Fig. 3 (north is up). The green-filter photographs taken on J u l y 22, 23, and 27 all show clouds in the north that are brighter than normal polar hood clouds would be at that wavelength. Slipher's blue-filter photographs taken during this time interval also show the same new brightening and the intermittent absence of hood clouds seen on the green-filter images, b u t none of them is very suitable for reproduction. Slipher described this northern brightening as being as bright as the south polar cap and suggested it could be a deposit of new snow. Neither the season nor the latitude seems consistent with that interpretation, however, so we propose that it m a y have been a dust cloud, although measurements do not indicate any notable hourly motion. The Martian season was midautumn, which is not known for its dust clouds or new deposits of polar surface brightening material (CO 2 or H 2 0 ). Also, this new brightening was further south than the polar surface cap is believed to reach at its greatest extent (Baum and Martin, 1973). The bright spot seen just to the right of center of the north hood on J u l y 22 could well be the beginning of this cloud. Both the hood and the new brightening disappeared for two days (L~ 217°), followed b y the reappearance of the brightening on J u l y 27. Its western boundary was displaced to the west, and its brightness was diminished since its previous appearance. B y J u l y 28 the hood reappeared, and the other new brightenings were not seen again.
1969 HOOD STUDY The International Planetary Patrol Program began in 1969 (Baum, 1973), provi-
34~
ding us with enough continuity of photographic coverage to make a detailed study: of the north hood's changing boundaries during its early phases (Martin and McKinney, 1974). Summary maps f r o m this study are reproduced in Fig. 4. They show how extensive the hood becomes, even during the time of its formation. Although all three of these maps cover time intervals long before :the season known for its large dust storms, the lower two (L S 171°-196 °) show boundaries which are representative of those seen during the entire hood season. Thus we can expect to see the north hood on blue-filter photographs, even though the North Pole of Mars m a y be tilted away from us b y as much as 24 ° during this season. 1971 MAJOR STORM The 1971 Patrol photographs (oriented north up) in Fig. 5 show the effects of the largest well-observed dust storm (Martin, 1974a). Starting with a normal appearance (L s 261°), the north hood rapidly becomes difficult to identify as the storm progresses. The original ultraviolet-filter negatives do show a slight suggestion of polar and connected limb brightening throughout the storm. Although the hood does seem to revive as the storm wanes, the increasing phase and distance of Mars prevented us from getting photographs which could be reproduced well enough to show this revival. Mariner 9 photographs were used b y Briggs and Leovy (1974) to establish the hood boundaries as again reaching to between 40 ° and 50 ° north latitude. These were obtained during February and March 1972, near the end of the northern hemisphere winter on Mars (L s 343°-353°), when good Earth-based photographs could no longer be obtained. As in 1956 and 1973, the apparent recession was temporary and was limited to the approximate duration of the dust storm. MAPPING THE 1973 HOOD
Changes in the north hood during the developing stages of the 1973 dust storm are shown on the nine daily maps in Figs.
346
LEONARD-J. MARTIN
FxG. 4. Summary maps from a study of hood boundaries using 1969 International Planetary Patrol photographs. The southern limits shown on the bottom two maps are typical of those seen throughout n o r t h e r n a u t u m n a n d w i n t e r o n b l u e - f i l t e r p h o t o g r a p h s . T h e n u m b e r s o n t h e l i n e s refer t o t h e n u m b e r
of days (out of 23 ) that hood clouds appeared on blue-filter images within the areas either enclosed by, or north of the lines. 6, 7, a n d 8. Since this p h e n o m e n o n occurred h o u r l y b o u n d a r i e s o f t h e h o o d were p l o t t e d n e a r t h e 1973 opposition a n d during a f r o m ultraviolet-filter P a t r o l p h o t o g r a p h s period w h e n several P a t r o l s t a t i o n s h a d using o u r p l a n e t i m a g e projector. T h e b a s e excellent seeing, t h e p h o t o g r a p h i c record m a p t h a t w a s used is f r o m t h e Lowell series o f these changes in t h e h o o d ' s a p p e a r a n c e (Inge, 1974) a n d shows t h e p l a n e t as seen during t h e s t o r m is u n u s u a l l y good. T h e " on I n t e r n a t i o n a l P l a n e t a r y P a t r o l red-
347
POLAR H O O D D U R I N G MARTJAN D U S T STORMS
o o
o
o
FIG. 6. Hourly hood boundaries plotted from ultraviolet- or blue-filter Patrol photographs during the first days of the 1973 dust storm. Sample ultraviolet photographs for each d ay are exhibited together with red-filter International Planetary Patrol photographs taken during the same hour. The broader boundary lines are those plotted from these sample photographs. Brightenings seen on redfilter images are outlined on the maps with dashes. The central meridians of the sample photographs are also denoted on the maps. Note the recession of the hood on D a y 2, followed by the protrusion on
FIo. 7. A continuation of Fig. 6 showing 1973 storm Days 4, 5, 6. Each day is a Mars rotation, beginning with the first day of the dust storm. I n the sample ultraviolet photograph of D ay 4, note the dark cloud over Sinus Meridiani, which became bright in red light on the following day. Th e dashed lines at 50°N indicate gaps between available photographs, as explained in the text. 349
FIo. 8. A continuation of Fig. 6 and 7 showing Days 7, 8, 9 of the 1973 storm. Some boundary lines are labeled with tiny triangles to indicate which side of the boundary is bright. Broken boundary lines denote plots from blue-filter images, which were used when no suitable ultraviolet images were available. The photographs for D a y 9 had diagonal scratches on the original film. 350
POT,A~ HOOD D v R r ~ o : ~ a ~ S T
filter photographs taken n e a r t h e time of the 1973 opposition. When no suitable ultraviolet photograph was available for a particular hour, a bhie-filter photograph was substituted, and the hood was then plotted as a broken line. Gaps between the central meridians of available ultraviolet or blue photographs exceeding 60 ° have been indicated b y a dashed line at 50°N on the map. Boundaries of the hood and other brightenings seen on the particular ultraviolet photographs exhibited along the left margins of Figs. 6, 7, and 8 are identified b y lines slightly heavier than the rest. Dust storm brightenings seen simultaneously on the exhibited red-filter photographs are outlined on the maps with broad dashes. The central meridians for the exhibited photographs are also shown. The numbering of "days" is based upon rotations of Mars, with each beginning at the time that the subsolar longitude on Mars is 180 ° and starting with D a y 1 as the first day of the dust storm (October 13). The apparent disturbance of the hood b y the 1973 storm began on the second day as a northward recession of the hood at the same longitude which showed maximum storm activity. On the following day the hood protruded somewhat southward at a more easterly longitude, while continuing to recede northward in the region directly north of the storm. B y the fourth day the hood was no longer visible at some longitudes and was generally farther north than it was the previous day. Ultraviolet photographs also show the dust storm area as unusually bright in that color compared with the 1971 storm. They also reveal a remarkably dark cloud over Sinus Meridiani, which developed into a brightening in red light on the fifth day. Although photographic coverage was incomplete on the sixth day, there was a longitudinal expanse of 150 ° for which we had photographs that showed no hood. The apparent hood boundary continued to fluctuate (primarily north of 50 °) during the seventh, eighth, and ninth days of the storm. As the storm continued to expand, we have increasing evidence of the hood's
STORMS
351
disappearingfrom view in part
of the eastern hemisphere, as well as the western. I~ote also that the areas of greatest dust storm activity (according to red-light photographs) are no longer particularly bright on these ultraviolet photographs. CONCLUSIONS
Do these polar clouds actually recede, or are they merely obscured b y hazes associated with the dust storm ? Although a dust storm of these proportions (Martin, 1974b) must disturb the planet's heat balance (Gierasch and Goody, 1972) and therefore could affect these seasonally dependent cloud formations, the observational evidence is not conclusive. The southward protrusion of the 1973 hood boundary at one longitude, simultaneous with a northward retreat at another longitude (Fig. 6), is possible evidence for actual changes in the hood. Another clue is the reappearance of the hood long before the contrast of albedo features had returned to normal in red light (Fig. 1), which suggests that dust had not yet cleared from the atmosphere. Also, the reappearance of bright limb clouds in ultraviolet (Fig. 1) brings up the question as to w h y they also were not obscured, despite their being closer to the most intense storm activity. Apparent interactions between red clouds and ultraviolet clouds during the 1956 (Sharnov, 1959), 1971 (Martin, 1974a), and 1973 (Fig. 7) storms lend further support to the hypothesis that we m a y be dealing with more than simple obscuration b y high dust. The rapid hourly motions of these polar clouds during nonstorm periods (Martin and McKinney, 1974) suggests that they are highly unstable to begin with and could easily be affected b y massive dust storms. ACKNOWLEDGMENTS W. A. Baum and C. F. Capen have both provided generous assitance in the preparation of this paper. This research is supported by NASA grant NGR-03-0O3-001. REFERENCES
B A ~ , W. A. (1973). The International Pl~etary Patrol Program. An assessment of the first three years. Planet. Space Sci. 21, 1511.
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BAUM, W. A., ~ MARTn~, L. J. (1973). Behavior of the Martian polar caps since 1905. Bull. A m . As~ron. Soc. 5, 296. BRIGGS, G. A., AND LEOVY, C. B. (1974). Mariner 9 observations of the Mars north polar hood. Bull. A m . Meteorol. Soc. 55, 278-296. CAPEN, C. F., A~-DC~'EN, V. W. (1972). Meteorological phenomena. I n Physical Observations of Mars 1966-1967-1968. Jet Propulsion Lab., Pasadena, CaliL FISCHBA.CHER, G. E., MARTIN, L. J., AND BAUM, W. A. (1969). Mars polar cap boundaries. Planetary Research Center, Lowell Obs., Flagstaff, Ariz., J P L Contract 951547, May, GIERASCH, P. J., AND GOODY, R. M. (1972). The effect of dust on the temperature of the Martian atmosphere. J. Atmos. Sci. 29, 400-402. INGE, J. L. (1974). Mars--1973. Albedo map, 1 : 25,000,000. Lowell Obs. Map Series. MARTIN, L. J. (1974a). The major Martian yellow storm of 1971. Icarus 22, 175-188. MARTIN, L. J. (1974b). The major Martian dust
storms of 1971 and 1973. Icarus 23, 108115. M ~ T I N , L. J., AND McKtNN-~Y, W. M. (1974). North polar hood of Mars in 1969 (May 18-July 25). I. Blue light. Icarus 23, 380-387. MICHAUX,C. M., ANDNEWBURN,R. L., JR. ( 1972 ). Mars Hciontific Model, Section 4.2. J P L Doc. No. 606-1. SHARONOV, V. V. (1959). The surface and atmosphere of Mars according to photographic, photometric, and colorimetric observations conducted in 1956 at Tashkent. I n Result~ of Observations of Mars in the UHHR During the Great Opposition of 1956 (The Astronomic Council, Committee of Planetary Physics), p. 122. USSR Academy of Sciences Publishing House, Moscow, Technical Translation F-3, NASA, 1962. SIGHER, E. C. (1939). Visual observation notes, Lowell Obs. (unpublished). SUPHEtt, E. C. (1940). Martian phenomena observed from our southern hemisphere. Telescope 7, 102-109.