The international planetary patrol program: An assessment of the first three years

Planet.

Space Sci. 1973, Vol. 21, pp. 1511 to 1519.

THE INTERNATIONAL AN ASSESSMENT

Ptman~on

Press.

Printedin Northern

InSand

PLANETARY PATROL OF THE FIRST THREE

W. A. BAUM Planetary Research Center, Lowell Observatory,

PROGRAM: YEARS

Flagstaff, Arizona, U.S.A.

Abstract-Day-to-day and hour-to-hour changes in the large-scale atmospheric and surface features of the planets can now be studied more effectively than previously possible. Since 1969 a network of observatories has obtained almost uninterrupted photographic coverage during ail apparitions of Jupiter and Mars, plus some of Venus. Patrol films and catalogues are available to the scientific county. Recent or current analyses include the distribution and motion of clouds on Mars, the development and de-cay of Martian dust storms, the seasonal, diurnal and random fluctuations in contrast between adjacent light and dark regions on Mars, the detection of vertical shear in the Jovian atmosphere, the longitudinal oscillation of the Red Spot, the dependence of rotation period on xenographic latitude and on time, the eruption and spread of SEB disturbances, and the retrograde circulation of the Venus cloud deck.

The International Planetary Patrol is an intensive Earth-based photographic observing program that has been in operation since 1969 (Baum et al., 1970). Eight observatories, distributed around the Earth, have thus far participated, The products of the patrol are available to the scientific community, and some significant research findings have begun to emerge. The purpose of the patrol is to obtain uninterrupted imaging so that we can investigate the day-to-day and hour-to-hour changes in the large-scale atmospheric and surface features of the planets. We can do this much better from the Earth than from spacecraft, We can see afl sides of a planet every day with only very gradual changes in lighting and viewing angles, and we can detect very subtle regional changes of albedo or coloration that are completely missed by spacecraft. Thus, for large-scale atmospheric effects, an Earthbased patrol is surprisingly effective. Earlier studies of short-term changes in the atmospheric and surface features of the planets have been hindered by the discontinuous nature of the imaging available. The problem is particularly severe in the case of Mars, because any one observatory sees approximately the same face of Mars for several nights in succession. This difficulty can be overcome only by having a network of observatories spaced in longitude around the Earth. Cooperative ventures like this have been attempted before, but they have met with only limited success because they were much too loosely organized and because of the inhomogeneity of the instr~entation used. In order to avoid the defects of earlier patrols, we have made contractual agreements with the participating observatories, have provided them with identical equipment, and have covered the actual costs. The result is more than ten times the annual output of any previous effort. Figure 1 shows the distribution of the principal observatories that have participated in the International Planetary Patrol Program. Starting from the left, these include the Mauna Kea Observatory on the island of Hawaii, the Lowell Observatory in Arizona, the Cerro To1010 Inter-American Observatory in northern Chile, the Republic Observatory in South Africa, the Kavalur Station of the Indian Institute of Astrophysics in southern India, the Perth Observatory in Western Australia, and the Mount Stromlo Observatory in eastern Australia. The program is coordinated by the Planetary Research Center at Lowell Observatory. New Mexico State University also participated during the first year. Operation at the Mount Stromlo Observatory has recently been discontinued. 1511

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W. A. BAUM 1969

Mars

Jupiter

Venus

._.-.-._ 1c -_._.-.

,_.-_._._

_._._._-_._

I

I

1

FIG. 2. FAVORABLE

I

I

I

TIMES FOR PATROL OBSERVATIONS.

Figure 2 shows the general observing plan for the first 5 yr of the patrol, commencing in 1969. Mars, Jupiter and Venus are the principal targets, because those are the three planets about which we can learn the most by reading a large number of images obtained on a patrol schedule. The stations have been operated continuously throughout all apparitions of Mars and Jupiter, but not all those of Venus. Observing schedules are printed in advance by a computer, and the patrol observers are expected to account for their results hourly. Quite a high degree of uninterrupted coverage is actually achieved for several months surrounding a planet’s opposition. Suppose, for example, that you want to see what a particular Martian feature looked like on a particular date in a particular color. The chance of finding a suitable set of images in the patrol collection turns out to be better than 80 per cent. Patrol performance statistics provide an objective comparison of observatory sites with one another. Mauna Kea is clearly the best, and Cerro To1010 is second. The French made use of our data in their decision to put their new 3.6-m telescope on Mauna Kea. In order that the photographs from the various patrol stations have the desired degree of homogeneity, the stations were provided with nearly identical equipment. Telescope focal lengths have all been made equal within about 1 per cent. All stations were provided with identical 35-mm camera systems controlled by electronic timing units that provide for the automatic exposure of a counted sequence of frames in rapid succession. The cameras record automatically the place, the date, the UT and the color filter on the edge of each film frame. There is also a built-in device for projecting sensitometric calibration strips onto the film. Exposure sequences are normally made in four colors: red, green, blue and U.V. A standard sequence in each color consists of fourteen frames taken in rapid succession, from which several images can typically be selected for analysis or composite reproduction. Rolls of exposed film from all stations are returned to the Planetary Research Center in Flagstaff for calibration and for processing under controlled conditions. After development, the rolls are edited and the individual image sequences are graded for quality. Sequences of poor quality are removed. Good sequences are copied in roll form, and the film is then cut into fourteen-frame strips that are mounted in aperture cards like the one shown in Fig. 3. These mounted filmstrips are all catalogued on IBM cards, and printouts of the catalogue, or various portions of it, can be produced according to need. The catalogue entry

Fre.1. THE INTERNATIONAL

PLAN~TARYPATROLNE~ORK.

FIG. 3. PATROL FILMSTRIPS ARE MOUNTED IN 30-cm APERTURE CARDS ~0 FACILITATE FILING AND HANDLING. THIS PARTICULAR CARD 1s SHOWN RESTING IN A STEEL FRAME THAT HOLDS IT ACCURATELY PLAT WHEN KMAGES ARE BEING PROJECTED FOR THE MEASUREMENT OF FEATURES.

FIG. 4. SKETCHILLUSTRATINGHOWTHERE~~NGOFDATAFROMPLANETARYIMAGESIS BY SUPE~~POSI~G

THEM

ON ORTKOGR~HIC

SPEEDED GRIDS IN SPECIAL OPTICAL PROJECTORS.

BLUE Bt/Bz

1.18

1.09

1.03

FIG. 6.

SAMPLE IMAGES ILLUSTRATING VARIATIONS IN THE CONTRAST BETWEEN

SYRTISMAJOR

AND ITS LIGHTER SURROUNDINGS.

For blue light, the true brightness ratios (light area divided by dark area) are labeled at the right.

Beginning of Storm

Before Storm

Sept. 21 0.48

Sept. 22 0.48

Surface

Surface

Partly

Completely Obscured

Obscured

Oct. 0.54

3

Oct. 21 0.64

Surface Partly Cleared

Dec. 20 1.09

AM.

from Earth

FIG. 8. VARIOUS STAGES OF THE 1971 YELLOW DUST STORM ON MARS.

June 23 hG.

14,

RAPID

SPREAD

OF THE

SEB

DISTURl)rdNCE

LIGHT

ON

20

June

22

June

24

T&MT’ WAS &NE

1971.

FIRST

DETECTED

IN ULTRAVIOLET

THE INT~~A~ON~

PLANETARY

PATROL

PROGRAM

1513

for each f&n&rip includes about a dozen useful ephemeris values, such as the local central meridian, the declination of the sub-Earth point, the meridian of the sub-solar point, the latitude of the sub-solar point, the angular diameter of the planet, the position angle of the polar axis, etc. These entries also indicate the observatory of origin and the quality of the images. For completeness, our catalogue includes observations obtained at other observatories not participating in the patrol schedule, such as the Pit-du-Midi Observatory in France, the New Mexico State University Observatory and the Catalina Observatory of the Lunar and Planetary Laboratory at the University of Arizona. It therefore is a relatively complete catalogue of Earth-based planetary imaging. At the moment, the complete catalogue totals approximately 1500 computer printout pages and is issued in nine separate volumes. These catalogue volumes and copies of the patrol films themselves can be made available to anyone having a legitimate need for them. Special purpose catalogues can also be printed by the computer for selected values of the ephemeris parameters. TABLE 1.IAU COLLFXXION OF PLANETPHOTOGRAPHS AT THE PLANETARYRESEARCHCENTER,LOWELL OBSERVATORY, FLAGSTAFF, ARIZONA,U.S.A. Source International Planetary Patrol (1969-1971) Lowell plate collection (1903-1968) Lick Observatory New Mexico State University Observatory IAU Data Center, Observatoire de Meudon Table Mountain Observatory Miscellaneous sources

Number of image sequences 56,000 10,086 1940 2052 1507 970 800 Total = 73,355

Just to put the scope of the patrol in context, Table 1 lists all the Earth-based photography on file at our Center at the end of last year. In the past 3 yr the patrol has produced more than three times as much as all other sources combined during the previous 60 yr. The total of 73,000 image sequences represents more than a million individual images. Figure 4 illustrates how positional information can be read rapidly and accurately from a large number of images. We have built special projection readers to superimpose planetary images onto orthographic grids. The tilt of the grid and the adjustment of the image size to the grid are determined by the ephemeris-not by the visual judgement of the film analyst. Figure 4 indicates how the boundary of a Martian polar cap would then be read at a number of selected meridians (Fischbacher et al., 1969). Another example of projection image reading is the mapping of Martian ‘clouds.’ Martin and Smith (1971) produced daily Martian ‘weather maps’ for 69 consecutive dates centered on the 1969 opposition. These are mercator projections on which the outlines of clouds or transient brightenings were plotted in accordance with their detection on red, green, blue and U.V.photographs of all sides of the planet. Some of these brightenings were seen most distinctly on blue images, while others showed up only on green and red. When all of these daily weather maps of 1969 are combined, they yield the distribution map shown in Fig. 5. The upper half of this figure can be taken to represent the distribution of the blue-white clouds, while the lower half shows the dist~bution of yellow clouds {more precisely, transient yellow b~ghtenings). The darkest cross hatching identifies those regions rl

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W. A. BAUM

that were observed to be abnormally bright more than half the time during this particular season (Martian September). A few places of high white cloud frequency are obviously correlated with prominent topographical features such as the western slopes of Nix Olympica and South Spot. But the overall correlation with regional elevation is weak. Some of the regions having very high white cloud frequencies are at relatively low elevation. The most obvious characteristic of the distribution of the transient yellow brightenings in the lower half of Fig. 5 is their almost total avoidance of dark areas. They occur only as transient enhancements of light areas. This very interesting finding is almost certainly related to the contrast variations that are illustrated in Fig. 6. The contrast between light and dark markings on Mars varies considerably with time. In the top pair of images Syrtis Major is seen with unusually high contrast, while in the bottom pair the contrast is unusually low. Historically this phenomenon has been confusingly associated with terms like ‘blue clearing’ or ‘violet haze.’ From both photographic and photoelectric data we now believe that the light areas (not the dark) are responsible for much of the observed variation, that their rise and fall in brightness is relatively yellowish, and that the variations of one region are not synchronous with those of other regions of Mars. What these patrol results clearly imply is chronic dust activity in the light areas. Using a step scale of reference images for each of four Martian regions, Thompson has made an extensive statistical study of the fluctuations observed in 1969 and 1971. His results for 1969 (Boyce and Thompson, 1972) are illustrated in Fig. 7. The top two regions

Mare Clmmerium

-10 -10 Phase angle.

deg

Nilokeros 1969

Phase FIG.

7.

FLUCTUATIONS

angle,

dsg

IN THE BLUE-LIGHT MSIBIL~ FEATURES.

0

‘Phase

10 10 30 angle,

,o

deg

Mare SirenumAonius Sinus 1969

I

Phase

angle,

OF SOME PROMINENT

dw MARTIAN

ALBWO

Each step of the ordinate scale corresponds approximately to a 3 per cent increment in the blue brightness ratio (Boyce and Thompson, 1972).

of the diagram differed in behavior from the lower two, which were farther from the subsolar latitude. This finding is compatible with the concept that the stirring of dust will be related to the amount of solar heating. The differences of observed behavior emerging from the analysis of 1971 images seem qualitatively to confirm this inte~retation. An even more int~guing feature of Thompson’s analysis is a diurnal effect, The regional contrasts were found to be systemati~lly a few per cent greater in the Mayan afternoon than in the morning, even though these data were taken near opposition when the lighting and being were both sy~et~c. Using Mead’s (1970) analysis for judging the predominant particle size, the writer (Baum, 1972)has interpreted this diurnal brigh~ning of the light areas in terms of a ground haze that is rejuvenated each day by noon-time heating, and has c~culat~ the bori~ontal visibility range of a lander eamera to be less than 5 km. The quality and abundant of the patrol images have made it possible to produce much improved albedo maps of Mars (Inge et cal.,1971). The 1969patrol map represents Martian ‘September,” while the 1971 map represents Martian ‘November.’ Our method of optical projection of images directly auto coordinate grids makes it possible to achieve good positional accuracy, even for subtly shaded markings. The enhancement of contrast is equivalent to a gamma of about 3, relative to actual red-light albedo ratios on Mars. The difference between the 1969 map and the 1971 map is consistent with the seasonal ‘wave of b~ghtening’ of light areas (centered near the sub-solar lati~de~ that one might expect from the other manifestation of dust described above, We do not favor the traditional view that it is a ‘wave of darke~ing~ of the darker areas. Another Mars study that has utilized projection image reading is the detailed analysis of the 1971 global dust storm (Capen and Martin, 1971). As can be seen in Fig, 8, the storm eommen~~ with the sudden brightening of the Noa~his-Se~en~s region as it came around the moving limb on 22 September. By the end of September, the dust storm had enveloped most of the southern hemisphere, incl~d~g the south pofar cap. In October it moved into the northern hemisphere, until the entire planet became almost featureless. Familiar albedo features did not begin to reappear strongly until about mid~~~ember. Capen and Martin’s map of the developing stages is shown in Fig. 9. Contours represent the boundaries of anomalously brightened areas detected on green and red patrol photographs. The numbers on the contours represent successive days, the first 7’ days being plotted in the upper half of the slide, while day #&Ithrough day #17 are plotted in the lower half. As the storm developed, it arose in areas that were not all co~t~gnous with the initial cloud nor with one another. In a vague sort of way, the profession seemed to be more westward than eastward, a fact that probably has no f~d~ental si~i~~an~. There was a lot of similarity between the 1971 storm and that of 1956. Although the 1956storm was the best d~umented earlier event of this kind, there is only a tenth as much material as now available for 197X. Both storms commenced with a sudden brightening of the Noac~~~e~entis region, and some of the other regions invaded by both during their first 2 weeks were similar. Afthongh there is good evidence that a yellow storm has occurred most Martian years during the season just following perihelion, the storm of 1971is the only one for which one can follow in detail what happened on all sides of the planet from beginning to end. We are very much looking forward to 1973, when the patrol network will again have the oppo~unity of fo~owin~ a Martian dust storm in detail. Not only should we get a good look at the onset of the 1973 storm in July or August, but the planet will be more favorably situated for documenting the waning phase of the storm than it was in 1971.

W. A. BALJM

1.516

P s

250 240 230

1

210 I 6 January

IS.

22

29

36 February Days

43

50 of

57

64 March

7,

76

65

92

year

FIG. 10. VERTICAL SHEARIN THE JOVJANEQUATORIALZONE.

The System I longitudes

of two red and two blue cloud features are plotted against date in 1970 (Layton, 1971).

Several investigations of Jupiter have been carried out by the projection reading of patrol images. Figure 10 shows Layton’s (1971) detection of vertical shear in the Jovian atmosphere, found by comparing the motions of features detected in blue light with those detected in red light. Both pairs of features were at +4” lat. The red features were found to move 2.7 m see-l more rapidly than the blue features, and we suppose that the red features represent a slightly greater depth in the atmosphere than the blue ones. Interpreted in terms of the thermal wind equation, this result implies that the dark north equatorial belt should be a fraction of a degree warmer than the north equatorial zone in the vicinity of the top of the visible cloud deck. Another Jupiter patrol analysis, by Millis and O’Dell (1971), is diagrammed in Fig. 11. It pertains to the longitude oscillation of the Red Spot. Reading from the bottom up, the curves here refer to the leading edge, the center, and the trailing edge of the Spot. These results confirm the go-day period of oscillation reported earlier. They also reveal that the trailing edge seems to oscillate with somewhat greater amplitude than the leading edge. Patrol coverage has provided more leverage on determining the profile of Jovian rotation as a function of latitude. Data by Inge (1972) for a 25-day interval in 1970 and another 25day interval in 1971 are compared in Fig. 12 with long-term means gathered earlier by others. The abundance and continuity of patrol coverage makes it possible to deal with relatively short time intervals and to look eventually for cyclic or secular changes. Inge’s results agree better with those of Chapman (1969) than those of Pokorny (1970). They also reveal that the center of the equatorial jet is moving slightly slower than its edges. This difference seems to be enough greater than the errors of observation to be real. Taken together with the sense of Layton’s (1971) vertical shear, this result suggests that the visible cloud top of the equatorial jet stands slightly higher in the Jovian atmosphere at its center than at its edges. A patrol network provides an improved opportunity for studying the atmospheric circulation of Venus. Images spaced a few hours apart confirm beyond any doubt the reality and approximate magnitude of this retrograde rotation. However, Caldwell’s (1972) analysis of a 40-day interval in 1970, shown in Fig. 13, yields a rotation period about 10

36

I

33..

32.. E

” 31_

P s

13_ ” 12..

I I..

IO.. L

’

L

9,

I

L

16

54

hIa,::

78

Days of

90 Apnl

102

114

126 MoY

136

150 JINX

162

year

FIG. 11. LONGITUDEOSCILLATIONOF THE JOVIAN RED SPOT IN EARLY 1970. Bottom to top, the curves represent the leading edge, the center, and the trailing edge of the Spot (Millis and O’Dell, 1971).

Sh 50m

55m

Rotation

9h 50m

55m

period

FIG. 12. ROTATIONPERIODSOF JOVIAN CLOUDSAS A FUNCTIONOF LATITUDE. Patrol film data for short time intervals in 1970 and 1971 are compared with earlier long-term means (Inge, 1972). 1517

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W. A. BAUM

FIG. 13. ROTATIONOF A RECOGNIZABLE FEATUREIN THE VENUS CLOUDDECK IN The synodic period was found to be 4.41 days retrograde (Caldwell, 1972).

1970.

per cent longer than that reported initially by Boyer and Guerin (1969). One should probably not be surprised to find variations of that magnitude in the period of a phenomenon of this kind. Additional U.V. Venus observations are being collected at some of the favorably situated patrol stations, and further analysis is planned. Patrol network coverage is particularly helpful in following the development of unusual events. We have already mentioned the 1971 and the forthcoming 1973 yellow storms on Mars. Another good example was the 1971 disturbance in the south equatorial belt of Jupiter, illustrated in Fig. 14. This disturbance first appeared as a tiny bright spot in U.V. light on 20 June (Baum, 1971a). Here we see its development each day. By 24 June, 4 days after it commenced, it was comparable in size to the Red Spot. Subsequent patrol photography showed in detail how it gradually spread out in longitude and how associated disturbances emerged at other longitudes. One of the patrol stations caught another type of Jovian event, namely, the occultation of Beta Scorpii by Jupiter on 13 May 1971. The gradual extinction of the star is a refractive effect that yields the vertical distribution of density (related to molecular weight and temperature),in the upper Jovian atmosphere. The patrol camera at Perth was operated in a tine mode with I-see exposures and an U.V.filter. These yielded a strong image of the star, while supressing the image of Jupiter. Clouds over Perth degraded the accuracy of the occultation data, but the writer (Baum, 1971b) was able to carry out some limited photometry on the patrol films and compare the initial fluctuations in the light curve with those observed during a similar event 20 yr ago (Baum and Code, 1953). The fluctuations, which result from some degree of layering in the upper Jovian atmosphere, do not match well in the light curves of the two events. Particular layers are therefore probably not long-lived and global. Acknowledgements-The planetary patrol is supported by NASA grant NGR-03-003-007 analysis has been supported by NASA grant NGR-03-003-001.

and much of the

REFERENCES BAUM, W. A. (1971a). Comments and pictures of Jovian SEB disturbance of 1971. Sky Telex. 42, 177. BAUM, W. A. (1971b). The occultation of Beta Scorpii by Jupiter; an ultraviolet light curve and its interpretation. Bull. Am. astr. Sot. 3, 374.

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PLANETARY

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BAUM, W. A. (1972). Where will the Martian dust be when Viking arrives? Bufl. Am. astr. Sue. 4, 374. J~AIJM,W. A. and CODE, A. D. (1953). A photometric observation of the occultation of Sigma Arietis by Jupiter. Asrr. J. 58, 108. BAUM, W. A., MILLIS, R. L., JONES, S. E. and MARTIN, L. J. (1970). The international program. Icarus 12,435.

planetary patrol

BOYCE, P. B. and THOMPSON,D. T. (1972). A new look at the Martian ‘violet haze’ problem-I. Syrtis Major-Arabia, 1969. Icarus 16, 29!. BOYER, C. and GUERIN, P. (1969). Etude de la rotation retrograde, en 4 jours, de la couche exterieure nuageuse de Venus. Icarus 11, 338. CALDWELL,J. J. (1972). Retrograde rotation of the upper atmosphere of Venus. Zcarus 17, 608. CAPEN, C. F. and MARTIN, L. J. (1971). The developing stages of the Martian yellow storm of 1971. Bull. Lowell Oh., No. 157,7, 211. CHAPMAN,C. R. (1969). Jupiter’s zonal winds; variation with latitude. J. armos. Sci, 26,986. FISCHBACHER,G. E., MARTIN, L. I. and BAUM,W. A. (1969). Martian polar cap boundaries. Final Report under JPL Contract 951547, Lowell Observatory, Arizona, U.S.A. INGE, J. L, (1972). Jovian rotation profiles for 1970 and 1971. Pub& asfr. Sot. Pa@ 84,641. INGE, J. L., CAPEN, C. F., MARTIN, L. J., FAURE, B. Q. and THOMPSON,D. T. (1971). Mars 1969 and Mars 1971 (albedo maps). Lowell Obs. Map Series. LAYTON,R. G. (1971). Vertical shear in the Jovian equatorial zone. Zcarm 15,480. MARTIN, L. 3. and SMITH, J. L. (1971). Mapping Martian clouds from 1969 photographs. Pubis usfr. Sot. Pact? 83, 606. MEAD, J. M. (1970). The contribution of atmospheric aerosols to the Martian opposition effect. Icarus 13, 82.

MILLIS, R. L. and O’DELL, K. D. (1971). (Red Spot oscillation). Private communication. POKORN~,Z. (1970). Changes on rotational periods of details of the Jupiter. Bull. asfr. Znsrs Cd. 21,318.