................................................................. . = T H E M A R T I A N C H R O N I C L E = . . . . The electronic newsletter by the International Mars Patrol, . . an observing program of the . . Association of Lunar and Planetary Observers . . Edited by: J.D. Beish . . August 1991 No. 8 . ................................................................. METEOROLOGY OF MARS IN 1991-1992 The ALPO/IMP has over 22,000 observations of Mars and many of the late Chick Capen's scientific papers covering his meteorologi- cal studies of the Red Planet. Results of Capen's work and more recent studies by the IMP, Beish and Parker published "Meteorological Survey of Mars, 1968-1984" in the Journal of Geophysical Research, (95, B9, August 20, 1990) that can be used to predict the times when clouds, hazes, and other atmospheric phenomena are most likely to occur on Mars. As we draw nearer to closest approach (03 JAN 1993) and opposi- tion (07 JAN 1993) the orbit of Mars will pass through 20 degrees and 22 degrees Planetocentric Longitude of the Sun (Ls), respectively. These periods correspond to late Martian northern spring, therefore, observers should begin to see an increase in cloudiness on Mars. OROGRAPHIC CLOUDS: Approaching local Martian noon, discrete white orographic clouds, identified as water clouds by the Mariner 9 spacecraft, are seen in Martian spring and summer forming on the upper slopes of the large volcanoes (Olympus Mons, Ascraeus Mons, Pavonis Mons, Arsia Mons, and Elysium Mons, and between Tharsis Tholus and Valles Marineris. These seasonal clouds ("W-clouds") were well observed during northern Summer in 1984 and in 1986 after rapid thawing of the North Polar Cap (NPC) began. Look for orographic clouds during the rapid retreat of the North Polar Cap (NPC) from northern mid-Spring until Summer. Because orographic clouds are best seen through blue and violet filters, they are well-elevated and are probably generated by mechanical uplift, and growing by convection. EQUATORIAL CLOUD BAND: A faint veil of wispy white clouds with variable shapes and levels of opacity, known as the equatorial cloud band (ECB), is occasionally seen extending across Mars' disk. Because the ECB is detected best in ultraviolet and violet light, it resides at a chilly high altitude, and is probably composed of Carbon Dioxide ice crystals. Because the ECB is equatorial, it too may occur twice each Martian year, appearing with the sublimation of each cap. LIMB HAZE: Limb hazes appear as a bright misty arc of light on the sunrise or sunset limb of Mars, and is caused by the observer's oblique view through the equivalent of several Martian atmospheres of aerosols, which may consist of Carbon Dioxide ice crystals, fine dust, cirrus-type water clouds, or a mixture of these. Consequently, the observation of the global location, color, and density of limb haze is a very sensitive method of diagnosing the global system of Mar tian weather and unusual polar phenomena. Sometimes, the color and density of the hazes help to detect dust storms that have begun on the other side of the planet. Limb haze may be seasonal or nonseasonal and, since it does not rotate with the planet, disappears around local 8 or 9 A.M., Mars time. It is best seen in violet light if at high altitudes, or in blue light if at mid-altitudes. WHITE AREA: A most delicate and challenging feat of observation is the detection of volatiles at the boundary between the Mar- tian atmosphere and its surface. In this volatile regime, ice fogs and frosts, often called bright patches, can be distin- guished from elevated clouds by means of comparing their relative brightnesses and boundary definitions as seen with the aid of blue, blue green, green, and yellow filters. If the suspect bright feature appears brighter in blue light than it does in green or yellow light, it is an atmospheric cloud. If it is brighter and better defined in blue-green light than in blue or yellow light, it is probably ice-fog contiguous to the surface. If the patch appears brighter with sharp boundary in green and yellow light and is not well seen in blue light, it can be identified as surface frost. A boundary-layer volatile's diurnal behavior and location also helps to distinguish it from clouds and limb haze. Fogs and frosts form in the chill of the Martian night, rotate with the planet, dissipate in the morning sunlight, and usually disappear by local noon. Fogs normally form in valleys, in fossae (linear depressions), basins, and on upper slopes. Frosts are usually noted on cool, light albedo features, plana (plateaus), montes (mountains), and floors of large craters. Because these volatiles are topographically controlled the discovery of their locations and seasonal occurrence is most important to the study of Martian weather patterns and areography. IDENTIFYING A MARTIAN DUST STORM Although immense global dust storms are firmly entrenched in Martian lore, they may be rare. Lowell Observatory's Leonard J. Martin has written that there have been only five well-docu- mented "planet-encircling" Martian storms. These storms occurred in 1956, 1971, 1973, and two storms in 1977 (these were discov- ered by Viking Spacecraft). However, smaller dust storms or dust clouds are observed on Mars. These dust clouds are very diffi- cult to identify in their beginning stages and, in some cases, go undetected even after they have fully developed. Numerous reports of yellowish hazes have appeared in the litera- ture and in the International Mars Patrol archives. Mars observ- ers frequently report "albedo features" lacking in contrast, the planet is "washed out," or Mars' atmosphere is "dusty." These terms have been employed in the past by the A.L.P.O. Mars Recorders. While such descriptions may have merit, generalized yellow hazes and temporary losses in surface contrast is usual ly omitted in our reports. Photographic evidence for these phenomena is also weak, since the proper sensitometric calibra- tion is usually lacking. Martian dust clouds usually form rapidly when finely divided surface materials are raised by the Martian wind. These clouds may be small, localized, and short lived, or they may expand to cover most or all of the planet in a matter of days. Dust clouds brighten faintly in yellow filters and reveal sharpened boundaries through orange and red filters. During the initial stages of formation, they often appear very bright in violet and ultraviolet light (photo graphic), suggesting the presence of ice crystals. Thus dust clouds are frequently confused with bright white areas, frosts, or high localized clouds on Mars. Because these dust clouds are often confused with bright surface deposits it becomes more difficult to determine the extent of the dust cloud expansion once the observer identifies it as dust on the move. Fresh surface deposits of dust tend to brighten the area where its has fallen and appears to blend with the dust cloud on the move. While working with Leonard J. Martin (Planetary Research Center, Lowell Observatory, Flagstaff, Arizona) and Richard W. Zurek (JPL) on problems of correctly identifying and classify ing Martian dust clouds or storms, new guidelines have been established by the A.L.P.O. Mars Recorders for interpreting Martian dust clouds and dust storms, they are classified as: I. Type of Observation. 1. White cloud or bright area mistaken for dust cloud. 2. Visual observation(s) of dust clouds in a dust storm. 3. Instrumental observation(s) of dust cloud/storm (Includes photographic, polarimetric, spacecraft data, or other data obtained by instrumental means). II. Martian dust clouds. 1. Obscuration -- Not sure if surface or atmospheric. 2. Dust Haze -- Partial obscuration with displacement. 3. Bright dust cloud -- Bright obscuration with displace- ments. 4. Limb projection/terminator protrusion by dust cloud. III. Martian dust storms. 1. Local -- Dust storm with major axis not to exceed 2000 km (1,243 miles or less than 34 degrees). 2. Regional -- Dust storm with major axis that exceeds 2000 km (1,243 miles or less than 34 degrees) but not encir cling either or both hemispheres. 3. Planet Encircling -- Dust storm with major axis that completely encircles either one or both hemispheres of Mars. In recent years, the popular media has played up the romance of Martian dust storms to such an extent that virtually any form of meteorological activity has been reported as dust. There fore, in analyzing your observations, the ALPO Mars Recorders have had to exercise considerable caution in defining dust clouds. We have applied the following criteria: 1. The sine qua non of Martian dust clouds is movement with obscuration of previously well-defined albedo features. Absence of this criterion in the present study disqualified a candidate from inclusion under dust clouds. 2. They are bright in red light. 3. When these clouds reach heights of several kilometers, we have proposed that they may cast shadows -- that are observa- ble from Earth. There are numerous reports of anomalous tran- sient dark surface markings appearing near dust clouds, espe- cially when the solar phase angle was reasonably large. Confusion over the colors of Mars is nothing new. When the bright planet Mars is observed against a nighttime sky, the planet's predominantly orange colored surface becomes highly saturated to the eye.