Mars fascinates us because it is more Earth-like than any other planet. We see its surface, clouds, hazes, and white polar caps. These brilliant white caps, composed of CO₂ and underlying water ice (in the north at least), wax and wane during the Martian year.

Today's amateurs contribute much of our knowledge about the weather and surface conditions on Mars. With a set of color filters and a quality telescope of 4” to 16” in aperture, the amateur is well equipped to conduct professional-quality research. CCD cameras that are marketed to amateurs produce results that rival those of any professional observatory. 

The Orbit of Mars

Like the orbit of Earth, Mars’ orbit is elliptical. In fact, Mars’ orbit is considerably more eccentric than that of the Earth. Due to this difference in eccentricities, the closest distance between the Earth and Mars at any given opposition can range from 34,670,000 miles during the most favorable oppositions to about 61,000,000 at less favorable approaches.

The orbital position of Mars at any given time defines the Martian seasonal date. It is expressed in degrees, measured along the ecliptic of the Mars’ celestial sphere. Similar to terrestrial seasons, this system is reckoned directly from the Sun's ascending node or the Martian northern spring equinox, where the (Martian) longitude of the Sun is 0°.

Mars and Earth have four comparable seasons because their axes of rotation are each tilted at about the same angle to their respective orbital planes. Mars' axial tilt is 25.2° as compared to 23.5° for that of the Earth. The Martian year is 687 Earth days, nearly twice as long as ours, so that the Martian seasons are similarly longer. While Earth's seasons are nearly equal in duration, because of the greater eccentricity of Mars’ orbit, the length of a Martian season can vary (from ¼ of its year) by as much as 52 days.

The axis of Mars does not aim at 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 the terrestrial seasons, or about one season in advance of ours. Consequently, when you observe Mars in the summer and autumn you will be seeing spring and summer respectively, in the Martian Southern Hemisphere.

Mars is favorably placed for observation when it is in opposition to the Sun.  This happens at intervals of about 26 months. Because Mars’ orbit is more eccentric than the Earth’s the planet is closer to us at some oppositions than at others. The closest oppositions, called perihelic oppositions, occur about every 15 years. The next perihelic opposition will occur in 2001. During a perihelic opposition the apparent size of the planet can exceed 25” of arc for a few weeks.

 The term "apparition" refers to the time span centered on the date of opposition to the Sun during which observation of a planet can be carried out. An apparition begins when the planet first emerges from the glare of the Sun in the pre-dawn morning sky and ends when it disappears in the sunset shortly before its next conjunction.

The Planet

The diameter of Mars is only about 53% that of Earth, and its mass is about 10% of the Earth's. But surprisingly, its charted land area is about equal to that of our own planet. This is because Mars has no oceans. Mars has a polar diameter of 4,194 miles and an equatorial diameter of 4,217 miles. Mars's surface gravity is 38% that of Earth's.

The Martian Day

The rotation period of Mars, called a "sol" by space scientists, is about 40 minutes longer than an Earth day. Thus Mars rotates through only 350° of longitude in 24 hours. Consequently, a particular surface marking appears to back up about 10° from night to night, causing an illusory retrograde rotation in about 36 days. Any given Martian region can be observed for about 10 consecutive terrestrial days at intervals of about 36 days.

 The Terminator and Central Meridian

Because it is so close to us, Mars can sometimes show a slight phase, similar to the gibbous phase of the Moon. The terminator is the line where daylight ends and night begins. The phase, or defect of illumination, is expressed in seconds of arc, or in degrees, of Martian longitude. It defines how much of the geometrical Martian disk is in darkness. The sunset terminator appears on the eastern side, or evening limb, before opposition. After opposition, it becomes the sunrise line on the morning limb on the western side. At opposition there is no perceptible phase defect.

The Martian Central Meridian (CM) is an imaginary line passing through the planet’s poles of rotation and bisecting the disk. It is used to define which Martian longitudes are present on the disk during an observing session. It is independent of any phase that may be present — if Mars presents a gibbous phase the CM will appear to be off center. The value of the CM is the Martian longitude, in degrees, which is on the central meridian of the disk as seen from Earth at a given time. 

Observing Mars

Even at its best, Mars is challenging to observe. The disk is frequently tiny and its markings are blurred by the Earth's atmosphere. A telescope for planetary work should provide sharp images with the highest possible contrast. A long-focus refractor is generally considered the best, followed by a long-focus Newtonian or Cassegrain reflector. Telescopes with large central obstructions do less well.

At the extremes of an apparition, Mars is low in the sky, where the turbulence of Earth's atmosphere is most severe. Despite these problems some useful work can be done at these times. A red filter lessens the poor seeing effects and improves image contrast. While little or no fine detail can be discerned when the planet is low and small, gross features, such as Syrtis Major, are not difficult to detect. If the observer has been regularly recognizing such features and suddenly notes their disappearance, he sould suspect an obscuration, such as a dust storm.

Many of the observed surface changes and atmospheric phenomena appear to be directly coupled to the seasonal climate which causes the spring thawing of one polar cap and the autumn formation of the other.

Anyone who observers 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 1-1/4 inches in diameter (trace the bottom of an eyepiece on the paper!). 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 in exactly the right locations on the disk. At this stage, record the time to the nearest minute. Now add the finer details, perhaps viewing through various color filters, starting at the planet's sunset limb. Finally, note the date, your name, the instrument used, and any other relevant information.

The dark surface markings were once thought by some 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. These markings can be identified on maps published by the ALPO, or Sky Publishing Corporation. Syrtis Major is the planet's most prominent dark area. Its eastern side becomes streaked and shrinks during Martian spring, then widens in autumn. Watch Syrtis Major during spring and summer to see if it narrows on schedule. 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.

The Martian atmosphere is ever changing. White water clouds, yellow 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 evaporation and condensation of polar-cap material. An intensive study of Martian weather is now in progress by the ALPO Mars Section using visual data and images from professionals and amateurs around the world. Statistical analysis indicates that blue and white cloud activity and near-surface fogs tend to occur in the spring and summer of the Martian Northern Hemisphere rather than the same seasons in the Southern Hemisphere. White clouds have been observed on Mars for over a century. In 1954, a remarkable W-shaped 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 mountain-generated, caused by wind passing over high peaks. Indeed, in 1971 the Mariner 9 spacecraft probe showed them to be water clouds near the large volcanoes Olympus Mons, Ascraeus Mons, Pavonis Mons and Arisa Mons. The W-clouds should be active during future apparitions. 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.

Limb brightening is caused by scattered light from dust and dry ice particles high in the Martian atmosphere. This effect will often be present on both limbs throughout the apparition.

Morning clouds are bright, isolated patches of surface fog or frosty ground near the morning limb (Mars' western edge as seen in Earth's sky). These 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.

Dust storms usually begin in Mars' Southern Hemisphere around the time of summer solstice. Watch for a small, bright yellow cloud to appear overnight, perhaps in Serpentis-Noachis, Solis Lacus, or Chryse, sites where dust storms have been recorded before. The small cloud may soon evolve into a global obscuration that persists for months. When a dust storm reaches maturity, the planet's entire disk is a nearly uniform bright orange. This can be very frustrating to the beginning observer!

Identifying the places where dust storms begin is highly important to future Mars exploration missions. Dust clouds are best detected using a yellow, red, or magenta filter.

Much has been written about the characteristics of these dust clouds and how to detect them. They are very difficult to identify in their beginning stages and, in some cases, go undetected even after they have fully developed. On the other hand, some observers have confused bright Martian desert regions or bright fog areas with dust clouds. The bright mountain clouds in the Tharsis region of Mars may appear as dust clouds.  The yellow-white appearance of some clouds most likely indicates the presence of dust particles. However, observers should not classify all bright yellow as "dust clouds."

It is important to avoid false alarms in reporting dust storms. Our mentors in the professional community take a dim view of wasting time and money on spurious information. Therefore, after consultation with professionals, we have revised our criteria for Martian dust activity.

The reader will note that nowhere in this article will the adjective "yellow" been applied to dust clouds. In the past we have referred to dust storms as "yellow clouds" and "yellow dust storms." We now feel that this is misleading. It is virtually impossible to see or even photograph accurate colors on Mars without employing very specialized techniques. Traditionally, observers have employed yellow filters to better reveal dust clouds. The problem is that nearly every light feature on Mars is bright through a yellow filter!