Surface Markings

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.