1998-1999 Apparition

Linking Amateur and Professional Mars Observing Communities.

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The International MarsWatch Electronic Newsletter

Volume 3; Issue 6
October 26, 1998
Circulation: 1479

Mars Symbol Mars Symbol

Dear Marswatch participant,

Enclosed is the third installment of the series of Marswatch emails dedicated to preparations for the 1998-1999 observing season. The series was written by Jeff Beish and other experienced Mars-observing colleagues who are involved with the Association of Lunar and Planetary Observers (A.L.P.O.). A calendar of important Mars observing dates is also enclosed. Now is the time to start preparing to observe the Red Planet through your telescope...

We are still trying to figure out how and where Marswatch images from the 1998-1999 apparition can be uploaded to and downloaded from via a web and/or ftp site. More on this soon, hopefully...

--Jim Bell
Cornell University

Mars Symbol Mars Symbol

1998-1999 Aphelic Apparition of Mars: Part 3

By: Jeffrey D. Beish
With Donald C. Parker, M.D., Daniel Troiani, and Daniel Joyce

Surface Features of Mars

The dark surface markings were once thought by some astronomers to be great lakes, oceans, or vegetation, but space probes in the 1970's revealed the markings to be vast expanses of rock and dust. Windstorms sometimes move the dust, resulting in both seasonal and long-term changes.

Among the areas where yearly variations have been recorded are Trivium-Elysium, Solis Lacus, Syrtis Major, and Sabaeus Meridiani. The Syrtis Major is the planet's most prominent dark area. Classical observations have revealed seasonal variations in the breadth of this feature: maximum width occurring in northern mid summer (145° Ls), and minimum during early northern winter, just after perihelion (290° Ls) [Antoniadi, 1930, Capen, 1976]. However, recent observations by ALPO astronomers and by the Hubble Space Telescope (HST) suggest that no such variations have occurred since 1990 [Lee, et al., 1995. Troiani, et al., 1997].

Solis Lacus, the "Eye of Mars", is notorious for undergoing major changes. In 1977 amateur observers discovered a new dark feature in the Aetheria desert at longitude 240° west, 25° north, between Nubis Lacus and Elysium. 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 the Solar System.

Another feature that is of great interest to professional Mars researchers is the Trivium Cerberus, on the southern rim of the Elysium shield. A classically dark feature 1300 x 400 km in size, it has all but disappeared during the 1990's [Moersch et al., 1997. Troiani et al., 1997].

Martian Meteorology

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. An intensive study of Martian meteorology has been conducted by the ALPO Mars Section using visual data and photographs from professionals and amateurs around the world. 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 between 1965 and 1993. Statistical analysis indicates that discrete water ice crystal cloud activity and near-surface fogs occurrence is significantly higher in the spring and summer of the Martian Northern Hemisphere than the same seasons for the Southern Hemisphere. For inclusion in this unique study, it is essential that ALPO astronomers employ blue filters when making visual, photographic, or CCD observations.

Discrete clouds -- have been observed on Mars for over a century. In 1954, a remarkable W-cloud formation was found to be recurring each late-spring afternoon in the Tharsis-Amazonis region. A decade later, C.F. Capen proposed that the W-clouds are orographic (mountain-generated), caused by wind passing over high peeks. Indeed, in 1971 the Mariner 9 spacecraft probe showed them to be water clouds near the large volcanoes Olympus Mons (longitude 133° west, latitude 18 north), 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 1999 apparition at least until opposition (129° Ls) and, perhaps, late in the apparition, during southern spring. Although often observed without filters, they are best seen in blue or violet light when they are high in altitude and in yellow or green light at very low altitudes. Other orographic clouds are observed over the Elysium Shield.

In addition to the dramatic orographic clouds, Mars exhibits many localized discrete clouds. These rotate with the planet and are most often found in northern spring-summer in Libya, Chryse, and Hellas. One remarkable example of 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 has been 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, or "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 frosty ground near the morning limb (Mars' western edge as seen on Earth's sky). 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 viewed best with a blue-green, blue, or violet filter. Occasionally, very low morning clouds can be seen in green or yellow light.

Evening clouds -- give the same appearance as morning clouds but are usually larger and more numerous than morning clouds. 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 rapidly decrease in frequency after early summer, while limb hazes become more numerous and conspicuous throughout the northern summer.

Equatorial Cloud Bands (ECB's)-- These features appear as broad, diffuse hazy bands along Mars' equatorial zone and are difficult to observe with ground-based telescopes. HST has revealed that these clouds may be more common than we have 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 ECB's. ALPO astronomers are encouraged to watch for these elusive features during the 1999 apparition. Are they really more common, or are our improved technologies merely allowing us to detect them more easily? ECB's are best detected 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 given rise to a virtual explosion in advanced techniques of studying our Solar System. Never before have we been able to readily detect the delicate wispy Martian Equatorial Cloud Bands so well as we do 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 Mars' southern summer, just after southern summer solstice, but a secondary peak has been observed in early northern summer, around 105° Ls. Classically, the storms occurring during southern summer are larger and more dramatic: they can even grow rapidly to enshroud the whole planet. It should be remembered, however, that these global dust storms are quite rare--only five have been reported since 1873, and these have all occurred since 1956. Much more common is the "localized" dust event, 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:

  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 must be bright 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 in fact 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.

  3. There are numerous reports of anomalous transient albedo features appearing near dust clouds, especially when the solar phase angle was reasonably large. When these clouds reach heights of several kilometers, they may cast shadows that are observable from Earth. Dr. Richard McKim (BAA) has written an excellent review of Martian dust storms [McKim, 1996].

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 regions. When a little-understood phenomenon known as the "blue clearing" occurs, however, 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 as well as in white light). The Wratten 47 filter or equivalent is the standard for analyzing blue clearing. Recently there has been renewed professional interest in blue clearing. We encourage ALPO Mars observers to watch for this phenomenon during the 1999 apparition.

Calendar of Events -- Mars 1998-1999


1998 Dec 16 Ls 70° Aphelion. Mars at 5.7" apparent diameter Views of surface details well defined. NPC Rima Tenuis may appear. Northern hemisphere mid-spring. NPC beginning rapid retreat? Are limb arcs present? NPR clouds increasing in frequency, intensity. Use filters! Antarctic hazes, hood? Cloud activity increases. Watch for "Aphelic Chill" in NPR (usually between 60° and 70° Ls).

1998 Dec 25 Ls 75° Mars at 6" apparent diameter. Apparition begins for observers using 4-inch to 8-inch apertures telescopes and up. Begin low resolution CCD imaging. Blue Syrtis Cloud? Is Hellas brightening?

1999 Jan 29 Ls 90°. Northern Summer/Southern Winter Solstice. Mars at 7.8" apparent diameter; Orographics over the Tharsis volcanoes -- W-Cloud? Look for orographic clouds (violet filter and blue-green filter).

1999 Feb 02 Ls 96° Mars at 8" apparent diameter. Quality micrometer measurements of NPC possible.

1999 Feb 26 Ls 102° Mars at 10" apparent diameter. Continue NPC measurements. Is North Cap fairly static or entering rapid retreat phase? South polar regions becoming difficult to observe. Any signs of SPH? Some photography now possible. Begin high resolution CCD imaging.

1999 Mar 16 Ls 110° Mars at 12" apparent diameter. Begin high resolution. Visual observations and high quality photography begins. Hellas bright? Watch for limb clouds. Blue Syrtis Cloud.

1999 Apr 24 Ls 128.7° Opposition. Mars at 16.02" apparent diameter. Dec -11.5°. Distance 0.58435 AU (54,313,968 miles)

1999 May 01 Ls 132° Closest approach, Mars at 16.18" apparent diameter., Dec. -11.0°, Distance 0.57846 AU (53,771,107 miles). Polar clouds. NPC remnant. Is Syrtis Major broad?

1999 Jun 25 Ls 160° Mars drops below 12" apparent diameter. Is north polar hood forming? Look for NPC remnant in red light.

1999 Jul 20 Ls 173° Mars drops below 10" apparent diameter. Are both polar hood visible? Decrease in discrete clouds.

1999 Jul 31 Ls 180° Northern Autumn/Southern Spring. Watch for increase in dust clouds. Is south polar cap visible free of its hood?

1999 Aug 28 Ls 195° Mars drops below 8" apparent diameter. Early southern spring. Possible W-clouds reforming. Is Syrtis Major narrowing?

1999 Nov 08 Ls 240° Mars drops below 6" apparent diameter. Mars near perihelion. Watch for dust. De = -9° Late spring south polar cap visible.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.

Mars Symbol Mars Symbol

Jim Bell will continue to maintain the email distribution list as well as the various Cornell and JPL Marswatch-related WWW archives. If you are receiving duplicate copies of the International MarsWatch Electronic Newsletter, or you want your name added to or removed from the distribution list, please send him an email at

Jim Bell
Cornell University
Department of Astronomy
Center for Radiophysics and Space Research
424 Space Sciences Building
Ithaca, NY 14853-6801
Phone: 607-255-5911; fax: 607-255-9002

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This page is maintained by David Knighton for the International MarsWatch. The 1998-1999 MarsWatch site it hosted by the Astronomical League as a service to the astronomical community. Comments, corrections, and suggestions can be addressed to This page last updated January 17, 1999.