An Observer’s Guide to Mars

Mars in 1999
One of a series of views of Mars during its 1999 opposition, imaged by António Cidadão of Oeiras, Portugal, with an SBIG ST-5C CCD camera on a 10-inch Meade LX-200 telescope. CM is the longitude of the planet's central meridian. One side of the planet shows many more dark markings than the other. Celestial south is up; the planet's North Polar Cap is at lower right. Click on the image to see all 5 views.
Not often do we Earthbound observers get a good look at Mars. It's a small planet to begin with, and it spends most of its time far away. Usually it's just a tiny, fuzzy orange blob in the eyepiece. The only time we get a good look at its surface markings, clouds, dust storms, and changing polar caps is during the months around its oppositions, which come a little more than two years apart. And not all Mars oppositions are created equal. The best ones come in bunches of two or three that repeat in a cycle 16 years long.

Now, after a decade-long dearth of close Mars apparitions, one of those bunches is well underway. Mars is heading toward an August 2003 opposition in Aquarius, the best of this 16-year cycle, when the planet will attain an apparent diameter of 25.1".

An intriguing world now more than ever, Mars near opposition offers both casual and serious observers many challenges and delights. In a high-quality 4- or 6-inch telescope on a night of excellent seeing, you should be able to make out one or both of the planet's tiny, white polar caps or at least the polar cloud hoods, many dark surface markings (depending on which side of Mars is facing Earth at the time), limb hazes, occasional white clouds, and possibly signs of a moving dust storm.

During most of 2002 Mars is poorly placed for viewing, is in conjunction with the Sun on August 10th, and remains less than 6" in diameter. But as the red planet begins its journey toward opposition in 2003, it will certainly take center stage for most planetary observers. The following synopsis of the article "A Grand Return of Mars" (Sky & Telescope: May 2001, page 102) describes many of the Martian features to watch for when the planet next puts on a show.

Using the Map

Map of Mars
Surface features on Mars, to compare with what you see. South is up; longitude is labeled along the top. Most telescopes will show only the largest dark regions. The features here, recorded during the 1999 apparition, were assembled by Daniel Troiani from CCD images, videos, photographs, and drawings made by International Mars Patrol observers worldwide. The disks along the bottom show the same map projected onto a globe, rotated at 45° intervals (corresponding to the central-meridian longitudes directly above them on the big map). Click on the image to see the complete map.
Courtesy Daniel Troiani.
It's one thing to detect a tiny, vague smudge on a small, shimmering disk. But the smudge becomes much more exciting if you can identify it and name it. This Mars map will allow you to identify what you've seen by referring to the globes shown below it. In addition, the globes with grids on the facing page show how the map's latitude and longitude lines project onto the planet's apparent disk as it changes its tip and orientation throughout this observing season.

To know which side of Mars faces Earth at the time you observe, you need to know the central longitude of Mar's central meridian. The central meridian is the imaginary line crossing Mars's disk from pole to pole. Once you determine its longitude, locate this on the scale along the top of the map. Features near this part of the map are the ones that will be facing you on the planet's disk.

Visual Observing

The ancient arts of visual observing and scientific sketching are still useful for Mars, and these have long been the forte of the amateur astronomer. Sketching focuses your attention, so you actually see more. Start with a circle on white paper. A diameter of 42 millimeters is the standard long used by the Association of Lunar and Planetary Observers (ALPO).

Next, draw in the planet's expected phase. Use a soft, dull pencil to lightly rough in the largest markings, being very careful to place them correctly. You can smooth the pencil shading by rubbing with a fingertip. Use the edge of an eraser to lighten an area. To indicate a particularly bright spot, outline it with a dashed line. Write down the Universal Time (UT) to the minute as soon as the largest areas are roughed in, then add details at your leisure. Finally, note the date (also in UT!), the telescope, magnifications, and color filters used, the quality of the seeing, your name, and all other relevant information.

How to Observe

Mars through RGB filters
The color filter you use determines how far down through Mars's atmosphere you see. Red light reveals the surface best; blue light usually shows only the clouds and hazes. Even though Mars was just 6' across, Donald C. Parker of Coral Gables, Florida, captured the most prominent surface markings as well as equatorial cloud bands in these amazing CCD images with his 16-inch f/6 Newtonian reflector in December, 1998. Celestial south is up.
Mars is never easy to study. The best telescope for planetary observing is generally a large, high-quality refractor or a large Newtonian reflector with first-rate mirrors in perfect collimation. But fine planetary views have also been obtained with Schmidt-Cassegrains and Maksutovs; there are no hard and fast rules in this game. In the end, the limiting factor is usually the atmosphere. Studying the planets means spending a lot of time watching and waiting for brief glimpses of detail when the seeing steadies. Just as important, the longer you watch, the better trained your eye becomes. Plan to spend lots of time at the eyepiece.

Color filters are an important aid for planetary observing. They improve the delicate contrast of Martian features, sometimes help in diagnosing their nature, and can even steady the seeing slightly (a red or orange filter especially). The improvements are subtle, but with experience they become important.

Red and orange filters show Mars's surface markings best. As you move away from the red end of the spectrum toward the blue, you see less of the surface and more of the Martian atmosphere and clouds. In fact, with a violet filter such as a Wratten 47, surface markings are usually invisible.

A good basic filter set includes red or orange (Wratten 25 or 23A), green (W58), blue-green (W64), blue (W38A or W80A), and violet (W47). The W25 and W47 filters are very dark and good only for large apertures or photography. Users of telescopes in the 4- to 6-inch range may find that even the W23A is too dark; in this case a yellow W15 may do better.

CCD enthusiasts can try a variety of filters in quick succession. Photographers are encouraged to take unfiltered color slides, which capture three color images in different layers of the film with one shot. These three images can be separated later with color filters for analysis.

Things to Look For

Mars before and during a dust storm
In 2001 the Hubble Space Telescope captured a dramatic change in Mars's weather. When HST imaged Mars in June (left), the seeds of the storm were caught brewing in the giant Hellas Basin (oval at 4 o'clock position on disk) and in another storm at the northern polar cap. When Hubble photographed Mars in early September (right), the storm had already been raging across the planet for nearly two months obscuring all surface features.
Courtesy NASA, James Bell (Cornell University), Michael Wolff (STScI), and the Hubble Heritage Team (STScI/AURA).
Albedo features. The Martian surface markings, the "maria" and "terrae" named by the great early Mars observers such as Giovanni Schiaparelli and E. M. Antoniadi, are merely differences in the reflectivity, or albedo, of vast expanses of rock and dust. Windstorms sometimes move the dust, resulting in seasonal changes.

Among the areas where yearly variations have been recorded are Syrtis Major, Solis Lacus, Elysium, and the distinctive forked streak of Sinus Sabaeus-Meridiani. Syrtis Major, the planet's most prominent dark marking, has undergone dramatic long-term changes since the 1950s. Classical observations revealed seasonal variations in its breadth; it was widest in the northern hemisphere's midsummer and narrowest during early northern winter. However, recent observations by ALPO astronomers and the Hubble Space Telescope suggest that no such variations have occurred since 1990.

Solis Lacus, the "Eye of Mars," is notorious for undergoing major changes. Another feature of great interest to professional Mars researchers is the dark Trivium-Cerberus on the southern rim of the Elysium shield. Classically some 1,300 by 400 km in size, it had all but disappeared in the 1990s.

Polar regions. In 2001 Mars presents its equator toward us, so we don't get a very good look at either pole. However, both polar regions may show whiteness — either the sharp, bright polar caps themselves or hazier polar cloud hoods. It's summer in Mars's northern hemisphere until June 17th and fall thereafter, so the north polar cap should be small and the south cap large during the apparition's best couple of months.

Dust storms. Recent studies, including our own Martian meteorology study, have shown that dust storms can occur during virtually any season. Their peak, however, comes during Mars's southern summer, and a secondary peak has been observed in early northern summer. Neither of these times comes during this year's months of good observing. Identifying the places where dust storms begin, and following their subsequent spread, is most important to future Mars exploration missions. The critical diagnostic of a dust storm is movement of a relatively bright patch that obscures previously well defined dark features.

Clouds and Hazes

Clouds over Mars
Orographic (mountain-generated) clouds in Elysium mark the center of this CCD image taken by António Cidadão on April 18, 1999.
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. The amount of cloudiness seems related to the seasonal sublimation of polar-cap material into the atmosphere, especially from the north cap. Clouds and surface fogs are more abundant during the spring and summer of the Martian northern hemisphere than in the same seasons for the southern hemisphere. So we may see a lot of cloud activity in the next couple of months as Mars approaches opposition, and less thereafter. If you want your observations to be included in this ongoing cloud study, it is essential to use a blue filter.

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 an intense blue. The Blue Cloud is best seen when Syrtis Major is near the planet's limb. Viewing this cloud through a yellow filter causes Syrtis Major to appear a vivid green.

Orographic clouds recur 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. The W clouds may be active during northern fall (after opposition this year). Other orographic clouds are observed in Elysium.

Limb brightenings ("limb arcs") are caused by scattered light from dust and dry-ice particles high in the Martian atmosphere. They should be present often on both the east and west limbs.

Morning clouds are bright, isolated patches of surface fog or frosty ground 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 occur on the preceding limb and are usually larger and more numerous.

Limb clouds tend to diminish rapidly after early northern summer, so they may be sparse during this apparition. The more diffuse limb hazes, on the other hand, become more frequent and conspicuous during the northern summer.

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. In fact many limb clouds may simply be equatorial cloud bands seen edge on. CCD cameras and modern image processing may bring them into better amateur reach.

The blue clearing. Normally the surface features of Mars appear vague through a light blue filter, such as the Wratten 80A. With a dark blue or violet 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 several days. The clearing can be limited to only one hemisphere and can vary in intensity. A Wratten 47 filter is the standard for studying the blue clearing. There is renewed professional interest in this phenomenon, and we encourage observers to watch for it.