Mars 2003: Night of the Red Planet
Seen through a telescope, the disk of Mars now looks about three times larger than it did in January. Even modest equipment reveals the icy gleam of the south polar region, and amateur astronomers using larger telescopes have captured digital images -- and even movies -- that reveal bright and dark shading across the disk.
Mars still has some growing to do. Over the next two months, as Earth swings toward its exceptional close approach, Mars will enlarge by another two-thirds and reach its maximum apparent diameter of 25 arcseconds. That's big for Mars, but it's only about half the apparent diameter of Jupiter's disk. For this reason, Mars remains an observing challenge even at its best.
The image above comes from a movie showing the orientation of martian
surface features at every hour, Universal Time, on August 27. The left panel
contains a map of Mars showing the features turned toward the Earth. The right
panel lists the martian longitude at the center of the disk, prominent
visual features, and displays a map showing the Earth hemisphere turned toward Mars.
On the night of August 27, when Earth finally laps Mars and the two planets reach their minimum separation, sky watchers around the world will be looking toward the Red Planet. What will they see?
The most obvious martian feature will be the south polar region. This includes the ice cap itself, as well as bright white seasonal frosts that form at its periphery. Both consist of frozen carbon dioxide -- "dry ice" -- as well as water. Despite their alien composition, the ice caps are the Red Planet's most evocative telescopic features, advancing and retreating in seasonal rhythm just like their terrestrial counterparts. Nothing like them exists anywhere else in the solar system, except on Earth.
Astronomer William Herschel gave the south "polar spot" considerable attention, observing its changing orientation as Mars rotated. He was able to detemine that the tilt of the Red Planet's spin axis was similar to that of Earth and thus realized that Mars must experience seasons akin to our own. He compared the known qualities of the two planets in 1781:
The analogy between Mars and the earth is, perhaps, by far the greatest in the whole solar system. The diurnal motion is nearly the same; the obliquity . . . on which the seasons depend, not very different; of all the superior planets the distance of Mars from the sun is by far the nearest alike to that of the earth: nor will the length of the martial year appear very different from that which we enjoy . . . . If, then, we find that the globe we inhabit has its polar regions frozen and covered with mountains of ice and snow, that only partly melt when alternately exposed to the sun, I may well be permitted to surmise that the same causes may probably have the same effect on the globe of Mars; that the bright polar spots are owing to the vivid reflection of light from frozen regions; and that the reduction of those spots is to be ascribed to their being exposed to the sun.
The rest of the martian surface exhibits continent-sized variations in brightness and color. Darker regions make up about one-third of the planet's surface. Although some of these features had been observed for centuries, no one attempted to combine observations to produce a global map of Mars until 1840. In the following decades, astronomers observed Mars with telescopes of greater power and the number of identifiable martian features grew. But astronomers had yet to devise a naming system everyone agreed on. For instance, "Kaiser Sea" and "Mer du Sablier" (Hourglass Sea) were both common terms for the prominent dark region we know today as Syrtis Major.
"Continents" and "Oceans"
In September 1877, the Italian astronomer Giovanni Schiaparelli began a series of observations that culminated in the most detailed Mars map of the time, with more than sixty geographic features identified. One consequence of Schiaparelli's map was a solution to the nomenclature problem. Following the pattern already in place on the moon, he named bright regions after terrestrial land areas and dark patches after bodies of water, using terms derived from history, mythology and literature. Many of these names remain in use today.
The most prominent dark areas visible during this opposition are: Syrtis Major, named after the Libyan Gulf of Sidra; Mare Sirenum, "Sea of the Sirens"; Sinus Sabaeus, from an older name for the Red Sea; and Mare Tyrrhenum, for the Tyrrhenian Sea between Italy and Sicily.
Brighter regions apparent on Mars may include the smooth oval patches of Argyre ("Silver") and Hellas ("Greece"). Both of these broad, flat depressions were created in ancient asteroid collisions. Sometimes clouds collect over the depressions, enhancing their visibility.
The Right Word
Schiaparelli's Mars map had another consequence, too. It included some unusual linear features he referred to as canali. The word can mean either "channels" or "canals" in Italian, but in English the term "channel" connotes a natural waterway while "canal" indicates an artificial construction. Since Schiaparelli often used the word fiume (river) as a synonym for canali, the preferred translation would have been "channel." Instead, the dark linear features became known as canals -- and a modern mythology was born.
Today we know that the canal network was a product of the limited resolution of the telescopes at the time, the smearing-effect of the atmosphere, and wishful thinking on the part of observers. The canals do not, in fact, correspond to any natural features on the planet.
All of the features mentioned -- except the canals -- can be found in the movie above, which shows the face of Mars for each hour Universal Time on August 27 (8:00 p.m. EDT on August 26 to 8:00 p.m. EDT, August 27). Think of it as a "sneak preview." The movie identifies prominent features along the center of the planet's disk and also shows the Earth's Mars-facing hemisphere.
The Mars views, which are based on imagery from spacecraft, reveal much more detail than one could see through a telescope. For comparison, this map shows how planetary scientists saw Mars before 1970, when detailed spacecraft reconnaissance was about to begin.
If you don't own a telescope, check with your local planetarium, science center, or amateur astronomy group for upcoming public viewing opportunities. They will be happy to help you encounter the Red Planet.