Mars will approach closer to the Earth during the 2003 apparition than at any other time in over 59,000 years! Always an intriguing world, Mars offers both casual and serious observers many challenges and delights. It also provides astronomers a laboratory to study the atmosphere and surface of another planet, including the behavior of condensates and their effects on its atmosphere and surface. Mars is similar to Earth in that it has four seasons, exhibits global climates, changing weather patterns, annual thawing and growing of polar caps, storm clouds of water ice, howling dusty winds, and a variety of surface features that predictably change in color and size and appear to shift position over extended periods of time.

Mars appears more Earth-like to us than most of the other planets because we can observe its surface, atmospheric clouds and hazes, and its brilliant white polar caps. The latter are composed of frozen CO₂ and underlying water ice, and wax and wane during the Martian year. These aspects, along with the changing seasons and the possibility of life, have made Mars one of the most studied planets in our solar system.

Mars offers both casual and serious observers many challenges and delights, as well as providing astronomers a laboratory to study another planet's atmosphere and surface. Some Martian features even appear to shift position around the surface over extended periods of time.

There are several cooperating international Mars observing programs under way to assist both professional and amateur astronomers. These include the International Mars Patrol (I.M.P.) coordinated by the Mars Section of the Association of Lunar and Planetary Observers (A.L.P.O), the International MarsWatch, the Terrestrial Planets Section of the British Astronomical Association (B.A.A.), and the Mars Section of the Oriental Astronomical Association (O.A.A.).

As a general rule, an "apparition" begins when a planet emerges from the glare of the Sun shortly after conjunction. Early in an apparition, a superior planet such as Mars rises in the east or morning sky and sets with the rotation of the Earth in the western or evening sky. Practically speaking, however, quality telescopic observations are only possible once the apparent diameter of the Martian disk exceeds 6 arc-seconds. However, for those eager to begin observing at the earliest opportunity, it is probably safe to do so when Mars is at least 12 degrees east or west of the Sun. As a side note, we urge observers to use caution when observing an object too close to the Sun without proper filters. Accidentally catching the intense light from Sun in an unfiltered telescope WILL result in severe eye damage and possible blindness.

There is a rule of thumb to determine when Mars is at certain cardinal point in its orbit relative to the orbit of Earth. Mars is in conjunction when the Sun is between it and the Earth, and the planet will not appear in our morning sky until approximately 54 days later. Nearly 300 days after that, Mars begins retrogression, or retrograde motion against the background stars, when it appears to move backwards toward the west for a brief period during the apparition. In 2003 Mars will be in retrograde motion from July 31 to September 30.

Opposition occurs approximately 390 days after conjunction, when Mars is on the opposite side of the Earth from the Sun. At that time, the two planets will lie nearly in a straight line with respect to the Sun, and about 33 days after that retrogression ends. It should also be noted that closest approach between Earth and Mars is not necessarily coincident with the time of opposition but varies by as much as two weeks.

Mars will remain visible for another 300 days (approximate)after opposition and then become lost in the glare of the Sun again as it approaches the next conjunction. The cycle is complete in 780 Earth days.

Another general rule for predicting oppositions of Mars is the following: the planet has an approximately 15.8-year periodic opposition cycle, which consists of three or four Aphelic oppositions and three consecutive Perihelicoppositions. Perihelic oppositions are also called "favorable" because the Earth and Mars come closest to each other on those occasions. We sometimes refer to this as the seven Martian synodic periods. This cycle is repeated every 79 years (+/- 4 to 5 days), and if one were to live long enough, one would see this cycle nearly replicated in 284 years.

For nearly five weeks during 2003, from August 10th until September 14th, the Red Planet's apparent diameter will be greater than 23.8 arc-seconds, larger than it has been anytime in the past 15 years. Mars will be at a distance of 0.37271 astronomical units (AU), or 34,649,589 miles (55,756,622 km) from Earth at closest approach. [NOTE: one (1) A.U. equals 92,955,621 miles or 149,597,870 km].

The 2003 Mars apparition is considered Perihelic because the orbital longitude at opposition will be at the perihelion longitude 250° Ls (Ls will be defined later.) Closest approach occurs on August 27, 2003 (248°.9 Ls) with an apparent planetary disk diameter of 25.11''. Opposition occurs the following day on August 28 (249°.5 Ls) with no appreciable difference in the apparent diameter. The observable disk diameter of Mars will be greater than 6 arc seconds starting Feb 25, 2003 and will not fall below this value until February 18, 2004. The geometry of the heliocentric aspects of Mars relative to the Earth is shown in Figure 1.
 
 

Although the Mars' large apparent disk diameter permits unusually favorable observing in 2003, the planet will appear low in the sky for observers in the middle northern latitudes, being below the celestial equator for a good portion of the apparition. Good news for those observing from the Southern Hemisphere though -- Mars will be seen high in their sky. The apparent declination of Mars begins at -17° in January 2003, and then continues southward to -23°.6 by mid-March, before climbing northward again. By mid-July, the declination of Mars will be -13°.0 with a slow decrease to -15°.7 by the opposition date (August 28, 2003). Mars will continue to decrease to -16°.5 (September 14, 2003) and then slowly climb northward until it reaches the celestial equator (0°) by mid-December, when the planet will still be a respectable 9.5 arc-seconds in apparent diameter.

The Ephemeris of Mars that lists the Sub-Earth Point or "De" is tabulated in the Mars Section of the A.L.P.O.'s Internet Web Page located at

and published in the A.L.P.O. Mars Section newsletter, The Martian Chronicle. Look under the heading, "Ephemeris for Physical Observations - 2003," in the "Mars Observing Ephemeris for 2003." The sub-earth (De) and sub-solar (Ds) points are graphically represented in Figure 2.
 

Figure 2. As it approaches Earth, it will swell from a small apparent disk of 6" in April 2005 to a maximum diameter on October 30, 2005, and then shrink as it moves away.
 

Mars will exhibit a disk diameter greater than 10 arc seconds for a period of more than 7 months, from May 8 through December 12. During this time, useful film-based photography will be possible. Useful imaging by CCD and digital cameras, however, can begin earlier in April, when the disk diameter is 8 arc seconds or less. The geometry of the heliocentric aspects of Mars relative to Earth is shown in Figure 3 through Figure 4. Figure 5 provides information about several close apparitions, 1988, 2001 and 2003, for comparison.

Figure 3. Graphic plot of Mars during the 2003 apparition from January 1, 2003 through December 31, 2003.  Plot illustrates the Declination (solid line),   the latitude of the sub-earth point (De) or the apparent tilt (dashed line) in areocentric degrees, and the latitude of the sub-solar point (dotted line) in areocentric degrees.  The areocentric longitude (Ls) of the Sun, shown along the bottom edge of the graph defines the Martian seasonal date.  The value of Ls is 0°at the vernal equinox of the northern hemisphere, 70°when Mars is at aphelion, and 90°at the summer solstice of the northern hemisphere 250°when Mars is at perihelion, and 180° is northern autumn.
 
 

Figure 4.  Graphic plot of Mars during the 2003 apparition from January 1, 2003 through December 31, 2003.  Plot illustrates the apparent diameter of Mars in seconds of arc.  The areocentric longitude (Ls) of the Sun, shown along the bottom  edge of the graph defines the Martian seasonal date.
 
 

Figure 5.  Graphic plot comparing the declination of Mars during the 1988, 2001, and 2003 apparitions. Dates from January through December of each year.  The Areocentric longitude (Ls) of the Sun, shown along the bottom  edge of the graph, defines the Martian seasonal date. Oppositions years are shown at top of vertical lines. Opposition date 1988 (solid line) Ls = 280°, for 2001 (dashed line) Ls =177°, and 2003 (dotted line) Ls = 250°.
 

The Martian solar day, or "sol", is about 40 minutes longer than a day on Earth. Consequently Mars only rotates through 350° of longitude in 24 hours. As a result, astronomers on Earth observing a particular Martian surface feature one night, will see that same feature positioned 10° further west on Mars (or closer to its morning limb) the next night at the same civil time.

Mars and Earth have four comparable seasons because their axes of rotation are both tilted at about the same angle to their respective orbital planes, 25°.2 for Mars and 23°.5 for Earth. In describing Martian seasons, scientists use the term "Ls" which stands for the Areocentric longitude of the Sun along Mars' ecliptic. The zero point, 0° Ls, is set at the Martian vernal equinox ¾ when the Sun, moving northward, appears to cross the celestial equator in Mars' sky. Thus, 90° Ls is the northern hemisphere summer solstice, 180° the autumnal equinox, etc. The seasons are, of course, reversed for the southern hemisphere.

Since the Martian year is about 687 Earth days long -- nearly twice as long as ours, the Martian seasons are similarly extended. While the Earth's seasons are nearly equal in duration, the Martian seasons can vary by as much as 52 days from each other due to that planet's greater orbital eccentricity (see Figure 1)

The axis of Mars does not point toward Polaris, our North Star, but is displaced about 40° towards Alpha Cygni. Because of this celestial displacement, the Martian seasons are 85° out of phase with respect to terrestrial seasons, or about one season earlier than ours. Consequently, when you observe Mars next spring and summer, it will be winter and spring, respectively, in the Martian southern hemisphere.

It is very important that each visual or photographic observation (including CCD images) be accompanied by a written data record made at the time of observation and/or imaging and not left to memory the following day. Whether or not the observations include visual drawings, it is recommended that the following data be recorded:

• Universal Date and time is desirable
• Telescope and ocular power (magnification) or Barlow lens employed
• The astronomical seeing and sky transparency conditions
• Which filters -- if any -- were used
• A description of the Martian disk as observed through different color filters (again, if any were used)
• Orientation, usually with south at the top and Mars' preceding (evening) limb marked "P." This is most important when few surface landmarks are visible, as occurs with dust storms or observations made in violet light. It is also important for observers employing star diagonals, where the east-west image is reversed.

Anyone who observes Mars will find it rewarding to make a sketch of whatever is seen, both to create a permanent record and to help train the eye in detecting elusive detail. Start with a circle 1.75 in. (42 mm) in diameter. Draw the phase defect, if any, and the bright polar caps or cloud hoods. Next, shade in the largest dark markings, being careful to place them as accurately on the disk as possible. At this stage, record the time to the nearest minute. Now add the finer details, viewing through various color filters, starting at the planet's sunset limb. Finally, note the date, observer's name, the instrument(s) used, and any other relevant information.

Modern technology, like CCD and digital cameras, has greatly increased the efficiency even of small telescopes that in the past were considered less than optimal for serious planetary observing. In addition, image processing can often compensate for such factors as low contrast, poor color balance and even sharpness. Another plus is that because CCD and digital cameras can capture images much faster than conventional photography, atmospheric turbulence is less likely to spoil the results. It is important that serious imagers of Mars carefully calibrate their images by doing bias, flat fields, and dark frames. This way the images become quantitatively accurate for analysis and they will be much easier to process.

Recently many amateurs have been using webcams for imaging the planets. These inexpensive little devices do require a computer but are relatively easy to use and, with inexpensive (or even free!) software they can produce striking images of Mars. It is suggested that amateurs wishing to image Mars for the first time try using a web cam.

It is highly recommended that all astronomers, whether photographers, CCD or digital camera imagers, or visual observers, use at least a basic set of tricolor filters according to the following guide:

• Red or orange (W-25 or W-23A)
• Green (W58)
• Blue-green (W-64)
• Blue (W-38A or W-80A)
• Violet (W-47)

Those who use CCD cameras often employ filters designed for the spectral response of their cameras. If this is the case, it is necessary to provide information about what filters were used, so that those who receive the images will know the wavelengths involved. It is also suggested that, when using infrared or ultraviolet filters, the spectral range or the "bandwidth half-maximum" (BWHM) be provided. This information is usually readily available from the filter's manufacturer.

The Mars observer will make his observations and studies more profitable if he familiarizes himself with a few other physical parameters:

De. The axial tilt of Mars relative to Earth is defined by the declination of the planet Earth (De) as seen from Mars. De is also equal to the areographic latitude of the center of the Martian disk, which is known as the sub-earth point. ("Areo-" is a prefix often employed when referring to Mars or "Ares.") The latitude is (+) if the north pole is tilted toward Earth and (-) if the south pole is tilted toward Earth. This quantity is an important factor when drawing Mars or when trying to identify certain features. The aspects and range of the axial tilt of the globe of Mars make it possible to observe the south polarregion of Mars during the 2003 apparition.

The Martian Central Meridian (CM), an imaginary line passing through the planetary poles of rotation and bisecting the planetary disk, is used to define the areographic longitudes on the disk during an observing session. It is independent of any phase that may be present; if Mars presents a gibbous phase, then the CM will appear to be off center. The CM is the areographic longitude in degrees, as seen from Earth at a given Universal Time (U.T.). It can be calculated by adding 0.24°/min., or 14.6°/hr., to the daily CM value for 0h U.T. as listed in The Astronomical Almanac).

The terminator (phase defect) is the line where daylight ends and night begins The phase , or defect of illumination, is given in seconds of subtended arc on the apparent disk, or in degrees (i), or the ratio (k), to define how much of the Earth-turned Martian disk is in darkness. The sunset terminator appears on the east side, or evening limb, before opposition; after opposition, the terminator becomes the sunrise line on the west side, or morning limb. At opposition, there is no perceptible phase defect (See Figure 6).
 

Figure 6. The Martian Disk and Useful terms. The orientation and nomenclature of the Martian globe as seen from Earth through an astronomical telescope. The figure indicates a simple inverted view of the disk of Mars, where south is at the top, bottom is north, the right side is terrestrial east or the Martian west (morning limb), and the left side is terrestrial west or Martian east (evening limb). Mars appears to rotate from Martian west to east, or right to left. Most classical charts of Mars show this same orientation.
 

The dark Martian surface markings, called "maria" or "albedo features," were once thought by some astronomers to be great lakes, oceans, or vegetation, but space probes in the 1970's revealed them to be vast expanses of rock and dust. Windstorms sometimes move the dust, resulting in both seasonal and long-term changes in these markings. These features seem to darken during early Martian spring in such a manner that a "wave of darkening" appears to sweep from the thawing polar cap towards the equator. This event, which occurs during each hemisphere's spring season, lent credence to the theory that the maria were composed of vegetation, which was replenished when water flowed from the melting polar cap towards the equator.

Now we know that this concept is false. In fact, C.F. Capen showed that the wave of darkening is in actuality a "wave of brightening" [Michaux, 1972, Capen, 1976, Dobbins, 1988]. The albedo features only appear to darken because the adjacent ochre desert areas have brightened during early spring. This has been confirmed by Viking Lander photos, which reveal a fresh, bright layer of dust appearing on the ground during early spring.

Light and dark surface features tend to change in albedo and color contrast diurnally and more slowly as the seasons change. Seasonal variations are usually predictable, but secular or long-period changes are unpredictable.

Seasonal Changes. Several regions that display seasonal changes are:

• Syrtis Major (300° W, 10° N)
• Pandorae Fretum (345° W, 25° S)
• Nilokeras-Lunae L. (60° W, 25° N)
• Candor-Tharsis (90° W, 10°N)
• Elysium-Trivium Charontis (210° W, 22° N)
• Mare Australe (90° W, 75° S)
• Aonius Sinus (105° W, 47° S).

• Nodus Laocoontis-Amenthes (245° W, 10° N)
• Nepenthes-Thoth (268° W, 08° N)
• Thoana Palus (256° W, 35° N)
• Moeris L. (270° W, 08° N)
• Antigones Fons-Astaboras complex (298° W, 22° N)
• Margaritifer S.-Hydaspis S. (30° W, 02° S)
• Solis Lacus (85° W, 26° S)
• Nilokeras-Lunae L. (60° W, 25° N)
• Acidalium Fons-Tempe C. (60° W, 58° N).

Syrtis Major is the planet's most prominent dark area. Classical observations indicated seasonal variations in the breadth of this feature: maximum width occurring in northern mid summer (145° Ls), when its eastern edge expands eastward to about 275° W. longitude [Dollfus, 1961]. Minimum width classically occurs during early northern winter, just after perihelion (290° Ls) [Antoniadi, 1930, Capen, 1976]. However, recent observations by A.L.P.O. astronomers and the Hubble Space Telescope (HST) suggest that no such variations have occurred since 1990 [Lee, et al., 1995.].

The Syrtis Major area has also undergone some rather dramatic long-term, or "secular," changes over the years. During recent apparitions it has become narrower and more blunted in appearance compared to the 1950's. After the 2001 dust storm this feature appeared thinner and more tapered to the north than it was before the storm [McKim, 2002]. Osiridis Promontorium became very dark in 1984, appearing as a dark bar jutting out into Libya from the northeast border of Syrtis Major. This feature was conspicuous in 1879, 1909, and during the 1940's and 1950's. The broad "canal," Nilosyrtis that curves northeast from the northern tip of Syrtis Major, was inconspicuous in 1984 [Parker et al., 1999].

The Nepenthes-Thoth (268° W, 08° N) feature, lying to the west of the Elysium shield, so prominent in the 1940's, and 1950's, decreased in size in 1960 and began fading in 1971. It was virtually undetectable in 1984. Nodus Laocoontis (246°W, 25°N), first described by S. Kibe in 1935, had faded during the 1970's and was not seen during the 1983-1985 apparition.

Hellas. One of the most active areas on Mars is the Hellas Basin (292° W, 50° S), not only because of its dynamic meteorology but also for its never-ceasing albedo changes. Surface structure becomes apparent in this area when its darker center (Zea Lacus) seems to extend its arms or canals (Alpheus) to the north, and connect Mare Hadriacum (265° W, 40° S) and Yaonis Fretum (318° W, 43° S) eastward to the western edge of Peneus. As the Martian southern summer solstice approaches, the basin often becomes flooded with dust if a violent storm begins. Hellas was the initial site of the great planet-encircling dust storm of 2001 and is a region that bears careful scrutiny during the 2003 apparition, since Mars will then be in its "dusty season."

Hellas was also was involved in both the December 11, 1983, the January 5, 1984, dust storms [Beish et al., 1984]. As these apparitions progressed and southern hemisphere winter got underway, Hellas and the high basin Argyre (30°W, 50°S) appeared brilliant white on the southern limb. Both of these great basins are the water-ice reservoirs of the southern hemisphere and are often covered with frost or with low clouds. These features were often confused with the South Polar Cap (SPC) or its winter hood, owing to their foreshortened appearance due to the planet's axial tilt.

Solis Lacus is called the "Eye of Mars" because, with the surrounding light area called Thaumasia, it resembles the pupil of an eye. Centered at 90° W, 30° S, Solis Lacus is notorious for its variability. Small and relatively inconspicuous in 1971, it underwent a major dark secular change in 1973, perhaps as a result of the major dust storms occurring during those years [Dobbins et al., 1988]. During the ensuing two decades it remained large dark oval with a north-south orientation [McKim, 1992]. During the 1992-1993 apparition Solis Lacus presented as a small, dark oval, but it enlarged and elongated in 1975 and has remained a large dark oval feature oriented slightly east-west until late 2001. At that time, after the massive dust storm had subsided, it appeared smaller than it had before the storm and the "canal" Nectar had all but disappeared [McKim, 2002].

Just west of Solis Lacus other areas have undergone changes are Daedalia-Claritas and Mare Sirenum. In 1973, the normally light region located between Sirenum M. and Solis Lacus, Daedalia-Claritas, underwent a dramatic darkening, which persisted through 1980. In 1984, this region had returned to its normal light intensity. However, during March and April 1984, A.L.P.O. observers reported that northeastern M. Sirenum had weakened considerably, possibly as a result of dust deposition from the storms sighted earlier in that region [Capen, 1986].

Early in 2001, after the dust had cleared from the 2001 storm, IMP observers reported a significant darkening in Daedalia-Claritas that extended eastward into Thaumasia near the site of the Phasis "canal." Both this region and nearby Solis Lacus bear careful watching in 2003.

Trivium-Cerberus (210° W, 22° N), lying on the southern rim of the Elysium shield, is another feature of great interest to professional Mars researchers. During the 1950s it was a classically dark feature 808 x 249 miles (1,300 x 400 km) in size, but it weakened somewhat in the 1960s. During the 1970s it varied in size and intensity from prominent to near invisibility. This area appears to have been covered over with dust during February and March of 1982 [Parker et al., 1990]. A generally "washed out" appearance was reported during the remainder of that apparition and very low contrast has been observed ever since. Dust storms during 1983 and 1984 appeared to further lower the contrast of the Elysium and Trivium Charontis region [Parker et al., 1999]. On May 14, 1984,A.L.P.O. observers reported that the Trivium Charontis--Cerberus was very difficult to see or missing from the face of Mars [Beish, 1984 and Troiani, 1996]. Except for a brief darkening in 1995 it has remained nearly invisible, appearing as two or three dots on a half-tone background [Moersch et al., 1998. Troiani et al., 1998].

In 1977 A.L.P.O. Mars observers reported a new dark area on the western side of the Elysium shield volcanoes [Capen and Parker, 1980] Astronomers reported that the normally insignificant "canal" Hyblaeus (240°W, 30°N) had darkened and expanded westward into Aetheria. Termed the "Hyblaeus Extension" by Capen, this change has persisted to the present. Interestingly, it was subsequently found on Viking Orbiter photographs taken in 1975, apparently undetected by Viking scientists. This is an example of the importance of ground-based observations of Solar System objects. On June 10, 1984 (162° Ls), A.L.P.O. observers photographed a further darkening in this region, located in Morpheos Lacus (228°W, 37°N). This darkening persisted into the 1980's & 1990's along with other changes near Elysium, notably the lightening of the wedge-shaped feature, Trivium Charontis. The entire region near the huge Elysium volcanoes appears to be in a state of flux and should be monitored often.

Cerberus III. A recent surface change is the appearance of a very conspicuous dark band across Hesperia. This has been named for the faint "canal" Cerberus III and was first detected in 1986 [Beish et al., 1989].

In 1990 A.L.P.O. observers reported a bright streak running east-west from 160°W to 260°W at 50-60°N. At 220°W longitude, another streak extended at right angles from it and extended southward into Elysium. These streaks, also observed in 1995 and 1997, appeared bright through all filters, and their nature is not known. This entire region bears careful scrutiny and will be well placed for observation during the Perihelic apparitions of the early 21^st Century.

Clouds and Hazes - The Martian atmosphere is ever-changing. White water ice clouds, yellowish dust clouds, bluish limb hazes, and bright surface frosts have been studied with increasing interest in the past two decades. Clouds seem to be related to the seasonal sublimation and condensation of polar-cap material. The A.L.P.O. Mars Section, using visual data and photographs from professionals and amateurs around the world, has conducted an intensive study of Martian meteorology. The first report, published in 1990, analyzed 9,650 IMP observations submitted over eight Martian apparitions between 1969 and 1984 [Beish and Parker, 1990]. This study has now been expanded to include 24,130 observations made between 1965 and 1995 [Beish, 1999]. Statistical analysis indicates that discrete, water ice crystal cloud activity and surface fog occurrences are significantly higher in the spring and summer of the Martian northern hemisphere than they are during the corresponding seasons in the southern hemisphere.

To participate in this important study, it is essential that A.L.P.O. astronomers employ blue (W-38A or W-80A) and violet (W-47) filters when making visual, photographic, CCD or digital camera observations of Martian clouds and other atmospheric phenomena.

Discrete clouds have been observed on Mars for over a century. In 1907, a remarkable, recurring W-shaped cloud formation was observed each late-spring afternoon in the Tharsis-Amazonis region [Slipher, 1962]. A decade later, C.F. Capen proposed that the W-clouds are orographic (mountain-generated), caused by the up-lifting of water vapor-laden atmosphere. [Capen, 1984 and Capen, 1986]. In 1971, the Mariner 9 spacecraft probe confirmed these observations, and showed that they were water clouds near the large volcanoes Olympus Mons (longitude 133° W, 18° N), Ascraeus Mons (104° W, 11° N), Pavonis Mons (112° W, 0° N), and Arsia Mons (120° W, 9° S). The W-clouds should be active during the 2003 apparition at least until opposition (250° Ls) and perhaps later in the apparition as well, during the southern hemisphere summer. Although often observed without filters, these clouds are best seen in blue or violet light when they are high in the Martian atmosphere, and in yellow or green light when they are at very low altitudes. Similar orographic clouds are also frequently observed over the Elysium Shield region.

In addition to such dramatic orographic clouds, Mars exhibits many localized, discrete clouds. These rotate with the planet and are most often seen in northern spring-summer over Libya, Chryse, and Hellas. One remarkable example of such a discrete topographic cloud is the "Syrtis Blue Cloud", which circulates around the Libya basin and across Syrtis Major, changing the color of this dark albedo feature to an intense blue. Originally named the "Blue Scorpion" by Fr. Angelo Secchi in 1858, this cloud usually makes its appearance during the late spring and early summer of Mars' northern hemisphere. It was prominent during the 1995 and 1997 apparitions and is best seen when the Syrtis is near the limb. Viewing this cloud through a yellow filter causes the Syrtis to appear a vivid green (yellow + blue = green).

Limb brightening ("limb arcs") are caused by scattered light from dust and dry ice particles high in the Martian atmosphere. They should be present on both limbs, often throughout the apparition, and are also best seen in blue-green, blue or violet light. When dust is present, these arcs are often conspicuous in orange light.

Morning clouds are bright, isolated patches of surface fog or frost near the morning limb. The fogs usually dissipate by mid-morning, while the frosts may persist most of the Martian day, depending on the season. These bright features are best viewed with blue-green, blue, or violet filters. Occasionally, very low morning clouds can also be seen in green or yellow light.

Evening clouds have the same appearance as morning clouds but are usually larger and more numerous than the latter. They appear as isolated bright patches over light desert regions in the late Martian afternoon and grow in size as they rotate into the late evening. They are best seen in blue or violet light.

The size and frequency of limb clouds appear to be related to the regression of the northern, rather than the southern, polar cap. Both limb arcs and limb clouds are prominent after aphelion (70° Ls), but limb clouds tend to decrease rapidly in frequency after early summer, while limb hazes become more numerous and conspicuous throughout the northern summer.

Equatorial Cloud Bands (ECBs) appear as broad, diffuse hazy bands along the Martian equatorial zone and are difficult to observe with ground-based telescopes. CCD images and the HST have revealed that these clouds may be more common than suspected in the past. Their prevalence during the 1997 apparition led some conferees at the Mars Telescopic Observations Workshop-II (MTO-II) to postulate that many limb clouds are simply the limb portions of ECBs. A.L.P.O. astronomers are encouraged to watch for these elusive features during the 2003 apparition. Are they really more common, or could it be that our improved technologies merely allow us to detect them more easily?

ECBs are best observed visually through a deep-blue (W47 and W47B) Wratten filters and may be photographed or imaged in blue or ultraviolet light.

New technologies, such as CCD cameras, sophisticated computer hardware and software, and large-aperture planetary telescopes have resulted in a virtual explosion in advancing the study of our Solar System. Never before, for example, have we been able to readily detect the delicate wispy Equatorial Cloud Bands on Mars as well as we can now with CCD imaging.

Dust storms. Recent surveys, including our Martian meteorology study, have shown that dust events can occur during virtually any season [Martin and Zurek, 1993. Beish and Parker, 1990]. The main peak (285° Ls) occurs during the southern Martian summer, just after southern summer solstice, but a secondary peak has also been observed in early northern summer, around 105° Ls. Classically, the storms occurring during southern summer are larger and more dramatic, and can even grow rapidly to enshroud the whole planet. It should be remembered, however, that these global dust storms are quite rare - only ten have been reported since 1873, and all but two have occurred since 1956. Much more common are the "localized" dust events, often starting in desert regions near Serpentis-Noachis, Solis Lacus, Chryse, or Hellas. During the 1997 apparition, CCD and HST observations revealed localized dust clouds over the north polar cap early in northern spring.

Identifying the places where dust storms begin and following their subsequent spread is most important to future Mars exploration missions. The following criteria apply in the diagnosis of Martian dust clouds:

• Movement with obscuration of previously well-defined albedo features. Absence of this criterion disqualifies a candidate from inclusion under the dust clouds category.

• Bright appearance of these phenomena in red light. In the past, astronomers have identified Martian dust clouds and/or obscurations as "yellow clouds." It is incorrect to describe the color of Martian dust clouds as "yellow." While they may appear yellowish when observed without the aid of color filters, they are brighter in red and orange light than they are in yellow light. Dust clouds brighten faintly in yellow filters and display well-defined boundaries through orange and red filters. During the initial stages of formation, they often appear very bright in violet and ultraviolet light, suggesting the presence of ice crystals. We vigorously discourage the use of the term "yellow clouds" to describe dust. If a suspect cloud is not bright in red light, it is not to be considered a dust cloud.

Blue Clearing, a little-understood phenomena, when Martian surface features can be seen and photographed in blue and violet light for periods of several days. The clearing can be limited to only one hemisphere and can vary in intensity from 0 (no surface features detected) to 3 (surface features can be seen also in white light). The Wratten 47 filter or equivalent is the standard for analyzing blue clearing. Normally the surface (albedo) features of Mars appear vague through light blue filters, such as the Wratten 80A. With a dark blue (W47) or violet (380-420 nm) filter, the disk usually appears featureless except for clouds, hazes, and the polar caps.

Recently, there has been renewed professional interest in blue clearing. We encourage A.L.P.O. Mars observers to watch for this phenomenon during the 2003 apparition.
 

Antoniadi, E. M. (1930). The Planet Mars. Chatham: W&J Mackay, Ltd. p. 110.

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