|Note:This archived article pertains to the visibility of Mars in 2003.|
It's not enough to describe the 2003 apparition of Mars as unique. In late August, as if beckoning us to touch its enchanting, exotic shores, the red planet will reach magnitude –2.9 and will dominate the southern sky with its fiery coloration. Finally, on the night of August 26–27, Mars will be closer to Earth — if by only a little — than at any time in some 60,000 years (see "A Mars Record For The Ages").
When the broadcast entertainment industry awakens to this remarkable fact, the airwaves will be filled with replays of classic movies like The War of the Worlds and the 1938 radio hoax staged by Orson Welles. A rush not seen since the 1986 visit of Halley's Comet could overwhelm the telescope market. The event is almost a certainty to fire the public's imagination as few other astronomical events can.
But amateur astronomers already know that Mars is always a telescopic challenge. Despite its remarkable proximity this time, Mars's features will be more elusive than its next-door-neighbor status would suggest. Faced with a public that's clamoring for views at summer star parties, inexperienced observers will have a hard time impressing their audience. For Mars, an angular extent of 25.1" is as good as it gets, but that's barely more than half the apparent diameter of Jupiter.
Mars becomes almost this large every 15 or 17 years — whenever it passes closest to Earth (near opposition) within a few weeks of the date it is also nearest the Sun (perihelion). For example, in August 1971 the disk became as large as 24.9", and in September 1988 it reached 23.8". Less-ideal views come at intervals of about 2 years 2 months, as in May 1999 when it reached 16.2", and in June 2001 when it attained 20.8". Putting August 2003 in perspective, this is one of five chances (at most) in your entire lifetime that you'll see Mars so clearly.
How to Observe Mars
During most of 2003, as the diagram above indicates, the southern hemisphere of Mars will be tipped toward Earth. For this reason the south polar cap will be quite large and prominent during May and June. But because the season is spring in that Martian hemisphere, observers can expect to see this cap shrink dramatically as it begins to thaw during July and August.
Opinions vary as to how large Mars's disk must appear to be studied effectively. For film photographers 10" has been a good rule, but visual observers can glimpse interesting details whenever the disk exceeds 6". CCD detectors are even better at this than the human eye.
But there is another consideration besides the planet’s size for observers living in the Northern Hemisphere. Mars begins May well south of the celestial equator at declination –20°, so it fails to rise more than 30° above the southern horizon (as seen from latitude 40° north). Seeing will suffer, and any crisp markings are likely to be smeared and softened by low-sky haze and turbulence in Earth’s atmosphere. By the end of August it will be getting a little higher each night, but not until mid-December does the planet cross the celestial equator heading north.
Good news for Southern Hemisphere readers: When Mars is nearest Earth in late August, it will pass almost directly overhead in your night sky!
Filters, which are very useful for visual observations of Mars, are now considered almost imperative for digital imaging. Whether you are working visually, with film, or with a CCD, a basic tricolor set of filters is highly recommended. Commonly identified by their Wratten (W) designations, they can be purchased from many telescope dealers and camera stores.
Red or orange filters (W25 or W23A) penetrate Mars’s atmosphere rather handily, exposing such features as the polar caps. They increase the contrast of dark surface markings, and they are best for spotting dust storms. If a patch is bright in red and dim in blue, it's dust.
Green (W58) and blue-green (W64) filters bring out surface fogs, frost patches, and polar-cap extensions.
Blue (W38A or W80A) and violet (W47) filters, because of the Martian atmosphere’s opacity to short wavelengths, are best at highlighting water-vapor clouds and polar hoods. Only in the early stages of the 2003 apparition is there expected to be much cloud activity, however.
Use the filter that provides the highest contrast for the type of feature you are trying to study. Observers with small (3- to 6-inch) telescopes will find that a yellow filter (W15) provides a brighter image and may perform better than a deep-red one. Those doing CCD imaging are forewarned to employ filters that reject infrared light. (The filters currently being sold for this purpose usually do; older photographic filters may not.)
Remember that Mars can always throw in something unexpected. The great dust storm that appeared right around the opposition date in 2001 had not been predicted to happen at all!
The dark spots and streaks on Mars are vast expanses of rock covered by fine layers of surface dust. Many are subject to seasonal windstorms that move and redistribute the dust, sometimes changing the shapes and boundaries we see. These "albedo features" were sketched and named by astronomers long ago; only with the advent of spacecraft visits to the planet have we begun to understand how they are related to the planet’s extinct volcanoes, valleys, and craters.
Seasonal variations in the markings are usually predictable, for they tend to recur every 22 or 23 months, when Mars returns to the same place along its orbit. But our best views of the planet come at a slightly longer interval, 25 or 26 months, so our picture of seasonal changes must be pieced together from what can be seen for only a few months at a time at different apparitions. For example, in 2003 we’ll be able to study what takes place during spring and summer in the Martian southern hemisphere.
The large southern-hemisphere oval called Hellas is particularly prone to seasonal changes. At times it develops a structure that gives it the appearance of a bowl or basin, the darker center (named Zea Lacus) seeming to extend arms or "canals" to the north, south, east, and west. But each Martian year when the southern summer solstice approaches (as in September 2003), these details may be obliterated by a violent local dust storm.
Another region of Mars subject to dramatic seasonal change is the Solis Lacus area, often called the "Eye of Mars." At past apparitions observers have even seen it vanish completely about 2½ months before the southern hemisphere’s spring equinox. (It may have done so in mid-February 2003, but the planet’s disk was then too tiny for visual study.)
The planet’s most prominent dark area, the wedge-like Syrtis Major, actually changes shape with the seasons. It has classically been widest in southern midwinter, when its eastern edge has expanded well into the bright Libya region. The Association of Lunar and Planetary Observers (ALPO) reported no dramatic widening of Syrtis Major in 1984, however. By the onset of southern spring this boundary begins to retreat again. The feature is expected to become narrowest shortly after Mars’s perihelion, in October and November this year.
Secular changes, or long-term trends in albedo features, are also well documented on Mars. Some have been dramatic. During recent apparitions, Syrtis Major appears to have become narrower and blunted compared to its appearance in the 1950s, and the once-conspicuous region to the east, called Nepenthes Thoth, has all but disappeared.
Another area that has undergone secular changes in the past two decades is the Trivium Charontis region near Elysium. This area appears to have been covered over with dust during February and March of 1982, and a general "washed out" appearance of this feature has persisted ever since. On May 14, 1984, ALPO observers reported that Trivium Charontis and neighboring Cerberus were very difficult to see or were even entirely absent from the face of Mars.
Weather on Mars
While they usually don’t hide the dark surface markings completely, cloud formations of many types are frequent in the Martian atmosphere. Along with white water-ice clouds, bluish limb hazes and dust clouds have been studied with increasing interest in the past two decades.
Discrete clouds recur at the same Martian sites, notably Libya, Chryse, and Hellas. One remarkable example is the "Syrtis Blue Cloud," which circulates around the Libya basin and across Syrtis Major, changing its color to a distinct blue. The cloud is best seen when Syrtis Major is near the planet’s limb. Viewing this cloud through a yellow filter causes Syrtis Major to look strikingly green.
Orographic clouds appear over Martian mountains, just like orographic clouds on Earth. The most famous example is the W-shaped cloud formation sometimes caused by wind passing over the high peaks of Olympus Mons and the other volcanoes of the Tharsis plateau. Other orographic clouds are occasionally seen over Elysium.
Limb brightening ("limb arcs") is caused by scattered light from dust and dry-ice particles high in the Martian atmosphere. This effect is often present on both the east and west limbs, and it's best seen in blue-green, blue, or violet light.
Morning clouds are bright, isolated patches of surface fog or frost near the morning limb (Mars’s following or eastern edge as seen on Earth’s sky). Fogs usually dissipate by midmorning, while frosts may persist for most of the Martian day.
Evening clouds have the same appearance as morning clouds, but they occur on the preceding limb (celestial west) and are usually larger and more numerous. They look like isolated bright patches over light desert regions in the late Martian afternoon and grow in size as they rotate into late evening. They're best seen in blue or violet light.
Equatorial cloud bands appear as broad, diffuse hazy strips along Mars’s equatorial zone. These delicate, wispy streaks are difficult to observe with ground-based telescopes, but Hubble and Mars Global Surveyor have revealed them to be more common than we suspected.
Dust storms. Of all the phenomena of Martian weather, by far the most dreaded are the global dust storms that occasionally rise up (as in 1956) to hide astronomers’ views just when Mars is passing near our planet. It should be remembered, however, that these storms are quite rare — only five have been reported since 1873. Recent studies have shown that dust storms (global and local) can occur during virtually any season. Their peak, however, comes during Mars’s southern summer, so there is a definite chance that one could occur in the waning stages of the present apparition,
Blue clearings. Every once in a while a mysterious "blue clearing" will occur when surface features are even seen through a very heavy blue or violet filter. No one seems to know why this can happen. With a dark blue or violet filter the disk usually appears featureless except for clouds, hazes, and the polar regions, but a blue clearing can last for periods of several days. It can be limited to one hemisphere and can vary in intensity. A Wratten 47 filter is the standard for studying a blue clearing.
The 2003 apparition is particularly important because two NASA orbiters, Mars Global Surveyor and Odyssey, will continue routine imaging during this time. By synchronizing their close-up images with what Earth-based observers can see, astronomers are almost sure to gain insights into what has transpired on this dust-blown planet ever since the invention of the telescope.
For More Information
To learn more about how to observe the red planet, visit ALPO's Mars Section Web site. More information about making and contributing useful observations is available at the International Mars Watch Web site. This group was founded in 1996 by astronomers at Cornell University and JPL, in collaboration with ALPO, to facilitate communications between the amateur and professional Mars communities. There you can find images of Mars contributed by amateurs and professionals, along with current and past issues of the Mars Watch electronic newsletter.