ABSTRACT: The fourth is a series of A.L.P.O. Mars reports presenting results of a computerized statistical analysis of the meteorology of Mars. Using computer programs developed for analysis of the Martian environmental and climatic surveys, an extensive statistical evaluation of fourteen apparitions is presented. A statistical analysis for past meteorological activities for clouds, bright surface frosts, and dust clouds are shown by general table of percentage of occurrence sub-divided in seasonal intervals and averaged over fourteen apparitions.

Starting with the 1964-65 Apparition of Mars, the International Mars Patrol (I.M.P.) began a systematic observing program designed to record all meteorological activity on Mars using pre-selected colored filters and observing techniques developed by the well known Mars authority Charles F. (Chick) Capen, Senior A.L.P.O. Mars Recorder. A continuation of the systematic ground-based support for Mars studies represents fifteen apparitions of study. The I.M.P. coordinated the efforts of 1,074 astronomers located the United States and 44 foreign countries interested in detailed study of the planet Mars organized for a 24-hour surveillance program of the planet during each apparition. The I.M.P. is the primary observing program for the Mars Section of the Association of Lunar and Planetary Observers (A.L.P.O.).

The I.M.P. archives contain 26,161 observations of Mars to date. A catalog of 24,130 observations of Mars has been used for this survey. This paper presents statistical analyses from the wealth of data obtained during the 31-year period from 1964 through 1995 for investigating seasonal and long-term patterns in the Martian meteorology and climate.

Part-I of the Meteorology of Mars report series briefly described methods used in the analysis and presented detailed results of the 1981-82 Martian Environmental and Climatic Survey, [Beish et al, 1986]. Information pertaining to part one was obtained from observations of the Institute for Planetary Research Observatories (I.P.R.O.). Further analysis using data from the Association of Lunar and Planetary Observers (A.L.P.O.) Mars Section observation records were presented in Meteorology of Mars Parts-II, [Beish et al, 1987], and Part-III, [Beish et al, 1987].

A complete systematic survey of I.M.P. observations of Mars resulted in publication of a more detailed trend analysis of bright aerosols and condensates reported by A.L.P.O./I.M.P. observers during 1968 throughout 1985 and was presented to the American Geophysical Union [Beish and Parker, 1990].

Clouds, hazes, and white surface areas are observed on Mars during every Martian season. Observational records indicate that these bright features exhibit certain characteristics similar to the familiar terrestrial clouds, fog’s and hazes. They are especially bright in blue light and are sometimes observed to brighten in all colors. From these observations and from the data gathered by the Mariner spacecraft and the Viking Landers/Orbiters, we now know that H₂O ice clouds and CO₂ hazes do exist on Mars. We should feel comfortable with the idea that what we observe as bright patches from Earth are clouds and hazes on Mars. Our Earth-based telescopic observations are more significant with this new knowledge [Capen, 1982]

Martian clouds, fogs, frost, and dust clouds come in various shapes and sizes and are sometimes observed to move around on the planet blocking out portions of the surface. We were particularly interested in their locations and movements, and their seasonal counts. Various methods have been employed to enhance these bright areas, one of which is the use of colored transmission filters. These filters are regularly used by I.M.P. observers for visual and photographic observations of Mars and other planets [Capen, 1982].  Table 1 and Figure 1 are general histograms of the observational coverage during each Mars apparition used in this study. Figure 2 plots the number of visual, photographic, micrometric, and CCD images observations contributed during each opposition year.
 

Table I. History of ALPO/IMP observations from 1965 through 1993. Given are the dates from the first to last observations, the opposition date and Planetocentric of the Sun (Ls), Ls range, total span of Ls from first top last observation, actual number of Ls observed, number of observers (OBS), and total observations (Visual, Photographic, Micrometer, and CCD).

 

Rarely observed are the Planetary System Cloud Banding or Equatorial Cloud Band (ECB) ECBs appear as broad and diffuse hazy streaks usually observed crossing within ±20 degrees of the Martian equatorial zone. Cloud bands are detected visually using a deep blue (W47B) or violet (W47) filter or photographed in ultraviolet or violet light. Cloud bands are probably composed of thin CO2 ice crystals carried aloft by high altitude winds.

Until recently, cloud bands were most often observed during the Martian northern summer, however, systematic tricolor CCD imaging has uncovered evidence these wisps of cloud bands may be more frequent and may occur in all Martian seasons. Using a special Infrared blocking filter in conjunction with high quality glass Wratten red, green, and blue filters these ECBs are readily detected and may be unseen by visual observers.

The I.M.P. has initiated an observing program for intensive investigation into these phenomena and will appeal to all planetary observers using CCD technology to assist us in this important study.

Figure 1. Graph of observational coverage during Mars apparitions as indicated by the corresponding opposition year. Graph includes number of degrees Ls span form first to last observation and number of degrees Ls covered by actual observational reports.

Each of the 24,130 Mars drawings, photographs, CCD images in the ALPO Mars Section Observational Report Library was carefully evaluated for quality and accuracy, with special attention given to proper use of color filters. When multiple observations of a particular phenomena was evident, its precise size, shape, and location was computed using the least squares method. To reduce systematic errors, the "personal equation" for each participating observer was derived from this computation and used to quantify their experience.

Systematic errors are also found in our data as a result of the nature of the reporting of Mars observations. We might think of these observations as discrete samples of time or "snap shots" of the conditions on Mars. This is due to the fact we cannot possibly record every moment of Mars’ history, even with the excellent longitudinal coverage provided to us by our world wide network of observers. Large gaps in areographic longitudes go unseen. To identify simultaneous observations of clouds or white areas seen by several astronomers is much less difficult than to separate limb phenomena. This is because limb hazes and clouds appear to stick close to the limbs and other phenomena rotate with the planet, as if these clouds are continuously being created and destroyed.
 

Figure 2. Bar graph showing number of observations by type, i.e., visual drawing reports, photographs, micrometer measurements, and CCD images.

In performing this study we have made every effort to reduce systematic errors. The least square method was employed to construct each individual observer’s personal error equation. Each observer’s experience and reliability, type and size telescope used, reported atmospheric "seeing" conditions, and general location was considered carefully when selecting observations for the survey.

Whenever available, photographs were utilized to cross check and confirm phenomena reported visually. Nevertheless, owing to the orbital geometry of Earth and Mars and Mars’ axial tilt, considerable bias is unavoidable. For example, the sub-earth point (De) can be situated more than 25° from Mars’ equator for much of an apparition. This prohibits observations of those regions near the hidden pole. Areas from latitudes of 50° and more are sometimes hidden as are the back side Mars and terminator areas. All these lost observations go into the "bucket of the unknown." Other sources of bias include the greatly varying distance of Mars from the Earth (changing Mars’ apparent size by some 4-5 times), the period near conjunction with the Sun, when Mars cannot be observed at all or only briefly each night, and the changing value and position of the phase angle that complicates observations of hazes and clouds near Mars’ poles, limbs, and terminator. These problems are real but uncontrollable, since we cannot yet change our vantage point and must remain earthbound.

Statistical analysis was carried out using a 486DX2-50 Express Business Computer and Gateway-2000 4DX2-66 Business Computer. Graphics plots presented here has been the product of Borland’s Quattro Pro version 5.0.

The tables in this report are simple percentages of the frequency with which we observe the various types of meteorological phenomena on Mars. Owing to a small difference in axial tilt, Mars’ seasonal periods are similar to those of Earth. When observing Mars from Earth, we see both the planet’s northern and southern hemispheres, so we must specify that hemisphere’s season and is indicated on each graph and table.

Due to the longer year and higher eccentricity of the Martian orbit, the seasons on Mars are not as symmetrical as Earth’s. The Martian northern spring and summer are longer than autumn and winter, (reversed for the Southern Hemisphere).

For statistical analyses, percentages are generally based on the number of activities of weather phenomena observed during seasonal periods versus the actual time spent observing Mars during that particular season. The Martian year of four seasons start with its vernal equinox at 0° planetocentric longitude (Ls) and moves eastward in its orbit through the seasons. Martian seasons are defined as: spring (0° - 89° Ls), summer (90° - 179° Ls), autumn (180° - 269° Ls), and winter (270° - 359° Ls). For this study, the Martian year is subdivided into 90° periods, measured in degrees of Areocentric or planetocentric longitude of the Sun (Ls); and the terms "Nsp/Sau," "Nsu/Wwi," "Nau/Wsp," and "Nwi/Ssp" corresponds respectively to "spring," "summer," "autumn," and "winter" that identifies with the Martian seasons in each of the planet’s hemispheres.

To provide a valid and systematic distribution of the observational data, I chose to (1) discard those observations made when Mars was less than 6 arcsec apparent diameter, (2) exclude seasonal periods with less than 12% observational coverage, and (3) eliminate observations by very inexperienced or novice planetary astronomers. These three criteria confine analysis to at least much of each Martian season occurring immediately before and after opposition and to observations made by experienced planetary astronomers.

Table II and III summarize the results of our statistical analysis for each phenomenon by season. Although our knowledge of "white areas" is limited, their characteristics suggest they are surface deposits of frosts. I have included white areas in this survey because these phenomena may prove to be both surface and atmospheric in nature. Also, bright areas have been observed immediately after dust storm activity further suggesting surface deposits of fresh dust.

Figure 3 represent a more detailed breakdown for each type of meteorology by season and include the north-south hemisphere occurrences as well. In each case, the percentages reflect the number of activities versus the number of degrees Ls observed for each period.
 

Table II. Martian Meteorological Survey, 1965 - 1984. Given are the apparition years, Ls (degrees) actually observed during an apparition, and seasonal percentages of meteorology observed.

Table III. Martian Meteorological Survey, 1986 - 1993. Given are the apparition years, Ls (degrees) actually observed during an apparition, and seasonal percentages of meteorology observed.

Figure 3. Graph indicating the overall simple average meteorological active degrees Ls during all apparitions from 1964 through 1993. Each type of meteorology in listed in the vertical scale and percentage of the observed Martian year is indicated in the horizontal axis. The terms Nsp/Sau = Northern spring/Southern autumn, Nsu/Swi = N. spring/S. winter, Nau/Ssp = N. autumn/S. sprint, and Nwi/Ssu = N. winter/S. summer.

One striking finding of this study is the marked proclivity for limb clouds and discrete clouds to appear during northern hemisphere spring and summer. This seasonal preference may result from different composition of the polar caps. Viking data has shown that many of the white limb and discrete clouds are composed of water-ice crystals. The Viking spacecraft have demonstrated that the primary composition of the South Polar Cap (SPC) is carbon dioxide ice (CO2 ) with perhaps a tiny core of water ice clathrate [James et al., 1979], while the North Cap consists of a layer of carbon dioxide covering a fairly large water-ice remnant [Kieffer et al., 1976]. The frequency distribution of these clouds appears to follow the regression of the north cap, increasing as the remnant cap is exposed during Martian northern summer.

The much lower incidence of white clouds during southern spring and summer agrees well with Mars’ known asymmetry in water vapor abundance [Farmer and Doms, 1979; Jakosky and Farmer, 1982]. It should be pointed out, however, that the apparitions most favorable for studying these Martian seasons were 1971 and 1973, years of global dust storm activity. During these two apparitions, dust clouds obscured large areas of the planet throughout much of the southern spring and summer, reducing the chances of observing white clouds. While this has no doubt introduced some bias, our preliminary reduction of the considerable data from the 1986 and 1988 apparitions [Parker et al., 1989] suggests that there is indeed much less discrete and limb cloud activity in southern spring and summer than there is during these seasons in the north. Even the orographic "W clouds" in 1986 around 200°-220° Ls were neither so numerous nor so long-lived that they would be able to alter the dominance of the "more usual" northern spring/summer meteorology in this survey. The 1988 apparition displayed even fewer clouds, despite a record number of experienced observers participating in the meteorological survey. As the data from 1986, 1988, 1990, and 1993 is added to this study, any bias due to global storms have been reduced considerably.

A short-term climatic phase of this survey is being completed. Although the seasonal coverage is incomplete for 1971 and 1973 (due to global dust storms) enough data was available to qualify the survey under the above 12% criterion. I have also included data from C.F. Capen’s observational records and photographic library for the 1964-65 and 1966-67 apparitions and further reducing the data from the 1962-63 apparition observations and photographs by C.F. Capen.  Despite the resultant gaps in the coverage, meteorological observations of the 1960s are considered credible, since C.F. Capen, who organized this survey, performed most of the work at Table Mountain Observatory.

As a result of this survey this author will not rule out the possibility of short-term and highly variable climatic changes on Mars as predicted by past I.M.P. studies and has published these suggestions before [Parker et al, 1983]. On the surface this meteorology study can be used in conjunction with past studies of the Martian polar cap behavior indicates the planet Mars was either cooler during the 1960’s and warmer in the early 1980’s or the observing techniques and equipment have so biased our results that we need to completely revolutionize the art of observing the Solar System with more objective methods.

The spacecraft missions to Mars during the 1960’s and 1970’s have resulted in a new impetuous for ground-based telescopic observing of the Red Planet. Astronomers are now armed with new knowledge about Mars made available by close-up surveillance by the Viking Orbiters and Landers. Of course, the loss of Mars Observer has ended prospects of continuing the close-up watch on Mars and all those machines are just space junk now.

The benefits of the study of Mars’ climate will help in the understanding of our own planet’s climate. The methods arrived from the meteorological survey of Mars using A.L.P.O. observations and modern techniques will increase our knowledge of the planet Mars. The amateur astronomer has earned a place in modern science.

Beish, J.D., Parker, D.C., and Capen, C.D., "Meteorology of Mars - Part I," Journal of the Association of Lunar and Planetary Observers (J.A.L.P.O.), Vol.31, Nos. 11-12, November 1986.

Beish, J.D., Parker, D.C., and Capen, C.F., "Meteorology of Mars - Part II," J.A.L.P.O., Vol.32, Nos. 1-2, March 1987.

Beish, J.D. and Parker, D.C., "Meteorology of Mars - Part III," J.A.L.P.O., Vol 32, Nos. 5-6, October 1987.

Beish, J.D., and D.C. Parker, 1990. "Meteorological Survey of Mars, 1968-1985," Journal of Geophysical Research (JGR), Vol. 95, B9, 14657-14675, August 20, 1990.

Capen, C.F. and D.C. Parker, "Observing Mars IX - The 1981-82 Aphelic Apparition," J.A.L.P.O.,, Vol. 29, Nos. 5-6, April 1982

Capen, C.F., "International Mars Observation Computer Program Studies of Seasonal Meteorological Phenomena," an invited paper based data obtained on computer programs and analysis of Martian meteorology by J.D. Beish, D.C. Parker, and C.F. Capen. Commission 16 of the 18th General Assembly of the International Astronomical Union (IAU). 19 August 1982.

James, P.B., G. Briggs, J. Barnes, and A. Spruck, "Seasonal Recession of Mars’ South Polar Cap As Seen by Viking," JGR, Vol. 84, No. B6, June 1979.

Kieffer, H.H., S.C. Chase, T.Z. Martin, E.D. Miner, and F.D. Palluconi, "Martian North Pole Summer Temperatures: Dirty Water Ice," Science, Vol. 194, 1341- 1344, 1976.

Farmer, C.B., and P.E. Doms, "Global Seasonal Variation of Water Vapor on Mars and the Implications for Permafrost," JGR, Vol. 84, 2881-2888, 1979.

Jakosky, B.M., and C.B. Farmer, "The Seasonal and Global Behavior of Water Vapor in the Mars Atmosphere: Complete Global Results of the Viking Atmospheric Water Detector Experiment," JGR, Vol. 87, 2999-3019, 1982.

Parker, D.C., J.D. Beish, and C.E. Hernandez, "Mars’ Grand Finale." Sky and Telescope Magazine, Vol. 77, No. 4. Pp. 369-372. April 1989.

Parker, D.C., C.F. Capen, and J.D. Beish, "Exploring the Martian Arctic," Sky and Telescope Magazine, Vol. 65, No. 3, March 1983.