With Mars just having passed opposition on October 13, I thought it might be fun to challenge you to see a specific surface feature on the Red Planet before it slips too far away.

First, most readers probably realize that some Martian oppositions are better than others. An aphelic opposition occurs at or near Martian aphelion, when the Red Planet comes no closer than 50 to 61 million miles (81 to 98 million km) to Earth. During these comparatively poor viewing periods, Mars, which measures 4,219 miles (6,794 km) in diameter, will measure no more than 14" across. In more favorable years, when Mars reaches opposition at or near perihelion, the planet will be less than 35 million miles (56 million km) from Earth and will appear about 25" diameter. These are called perihelic oppositions. This year's Martian opposition was perihelic, as was the previous opposition in 2018.  Both afforded observers some prime Mars-watching. The next periherlic oppositions won't be until June 27, 2033, and September 15, 2035. In between now and then, the oppositions in 2022, 2025, 2027, 2029 and 2031 will be aphelic.

No other planet in our solar system appears so enticing, yet proves so frustrating, through backyard telescopes as does the Red Planet, Mars. On one hand, the planet's thin carbon dioxide atmosphere affords us a nearly cloud-free, round-the-clock view of its sun-drenched surface. On the other hand, however, the planet's small size coupled with its distance away conspire together to shrink the planet's disk to no more than 25" across at its best. Usually, Mars appears far smaller than that. As a result, whatever surface details are visible through our telescopes prove small, vague, and tenuous, at best.

This contradictory set of conditions undoubtedly led to some of the controversial surface features that early Mars observers claimed to see. Without a doubt, the best-known case of Martian illusions has to be the widespread misconception that the planet is covered in a web of thread-thin canals. Many references attribute the "discovery" of Martian canals to the Italian astronomer Giovanni Schiaparelli. Viewing Mars in 1877, Schiaparelli saw what he interpreted as dark, thin lines stretching across the lighter areas of the planet's surface and connecting the darker regions. He described these vague markings as "canali," which in Italian, means channels or grooves. Once his observations, published in 1878, reached the ears of English-speaking astronomers, canali was mistranslated to mean "canals," which of course, are artificial waterways constructed by intelligent beings. Suddenly, the hunt for the Martians was on!

Actually, Schiaparelli was not the first person to see "canali." At least half a dozen observers recorded linear features on Mars as far back as 1840. In 1867, Richard A. Proctor published a map of Mars based largely on observations and drawings by William Dawes (of "Dawes Limit" fame). Proctor presumed that the darker parts of the planet were seas and the reddish tracts continents, and proceeded to name several features after English astronomers, such as Dawes Ocean, Herschel Continent, and Terby Sea.

Schiaparelli's 1878 report also included a map of Mars, showing far more detail than Proctor's, which contained several fanciful errors. To correct these errors, Schiaparelli decided to abandon any names previously assigned and instead create his own references based on biblical and mythological entities. Terby Sea, for instance, became Solis Lacus. For the most part, the names we still use when discussing features on Mars are those assigned by Schiaparelli. That is, minus the canals, of course.

While we may chuckle today at the thought of canals crisscrossing the planet, many of the surface features that perplexed generations of astronomers continue to intrigue observers today. Even with robotic spacecraft scurrying about the surface of the Red Planet or in orbit high above, Mars still beckons backyard planet watchers. There are many striking features across the Martian surface, from the fork-shaped Sinus Meridiani (or what Proctor had christened Dawes's Forked Bay) to the dark wedge of Syrtis Major (formerly Kaiser Sea).

Since it was first detected in the 19th century, the region Solis Lacus, located at Martian longitude 85° west and Martian latitude 26° south, has puzzled observers. Nicknamed the Eye of Mars, or Oculus, for its cyclopic appearance, this feature has been observed to undergo dramatic changes in size and appearance. Normally, Solis Lacus appears as a dark, elliptical feature measuring some 500 miles east-to-west by 300 miles north-to-south, surrounded by a brighter region known as Thaumasia. Together, they resemble a human eye, almost as if Mars is looking back at us.

Since Schiaparelli first drew a detailed view of Solis Lacus in 1877, observers have watched it go through a variety of changes, as the figure above demonstrates. Schiaparelli's original drawing recorded a dark, segmented viaduct across Thaumasia, connecting the "eye" to Mare Erythraeum to the south. Within 30 years, others recorded not one, but several thin straits radiating outward from Solis Lacus, bridging the gap between it and the mare, as if the Eye was bloodshot. As the early 20th century wore on, Solis Lacus continued to morph from oval to circular, blending in part into Mare Erythraeum before separating again. By the 1971 opposition of Mars, it had shrunken in size and faded in darkness, only to experience resurgence two years later. As the 21st century opened, the Eye was dark again, although not as large as it had appeared in the past.

The cause of these variations is likely due to dust storms that rage across the Red Planet. The powdery Martian soil can be picked up by high winds and swept across plains and down into basins. As this material is blown about, darker, subsurface regions are alternately exposed and covered up, accounting for what was once interpreted as growth of seasonal vegetation.

In general, dedicated planet watchers prefer refractors and long-focus reflectors because they usually produce the highest image contrast. Short-focus Newtonians and most catadioptric telescopes yield lower image contrast owing to their large central obstructions. And since magnifications over 200x are usually needed to see fine details, be sure to use a high-quality eyepiece. Popular super-wide-field eyepieces are wonderful for panoramic views of star fields and broad nebulae, but they are often surpassed by simpler conventional eyepieces, such as orthoscopics and Plössls, for planetary observing. Finally, many observers report great success using color filters to enhance various features on Mars. For Solis Lacus, try an orange (Wratten #21) or red (#23A or #25) filter to increase the contrast of the dark eye against the surrounding bright region. I am sure that many readers have their own opinions on this, so I welcome you to post them in this column’s discussion forum below. 

Have a favorite challenge object of your own? I'd love to hear about it, as well as how you did with this month's test. Contact me through my web site or post to this month's discussion forum.

Until next month, remember that half of the fun is the thrill of the chase. Game on!