Exploring Mars With a Telescope

© 2003 by Robert C. Moler

This is an updated article which originally ran in 1999.

On August 27th Mars will come closer to earth than it has in thousands of years. One day later it will reach opposition from the sun. At its closest Mars will be 34.63 million miles away. This planet, half the size of earth, will appear only 25 seconds of arc in diameter in telescopes. That's less 5 seconds or arc smaller than Jupiter ever appears in telescopes at its smallest.

The resolution of features on Mars through a small telescope will be little more than resolution of the moon by the unaided eye. So yes, Mars is going to be a challenge. But unlike Jupiter which is bigger or Saturn which is prettier, Mars is a planet my grandchildren could be the first to explore on foot, and their grandchildren could grow up on it and be the first true Martians.

It was Mars that led Percival Lowell to build his observatory in Flagstaff Arizona. It was Mars from whom a fictitious invasion produced a Halloween scare in pre World War II America. No other planet has done that for or to us, for Mars was named for the god of war. Jupiter is, well, jovial; and Saturn is an old guy.

Mars requires the observer's best observing skills, and equipment. As a tiny planet Mars requires high magnification. The rule of thumb is that the highest useful magnification a telescope can give is about 50 to 60 power per inch. My highest magnification for a 6 inch telescope would be 300 to 360 power. The reason is due to the wave nature of light itself. The larger the aperture the sharper the image. There is just so much detail available. Higher power just gives fuzzier views. Personally, I'd rather view a small sharp image rather than a bigger but fuzzier one. Subtle details are lost at higher powers.

To view the planet at its sharpest it is best to let it rise well above the horizon before trying to seriously view the planet. The earth's atmosphere is thickest near the horizon, magnifying image destroying turbulence. I know it's hard to wait for the planet to rise high enough, especially early in the Mars viewing season when it rises long after sunset but patience will be rewarded. However clear nights just after a cold front comes through tend to have poor seeing, that is the atmosphere is very turbulent, so Mars may be a lost cause, even after it rises higher.

So what is there to see on Mars? The first thing one notices is that Mars isn't really red. It appears a yellow-orange. The first detail on the surface of Mars easily discernible is one of its polar ice caps whose appearance changes with the Martian Seasons. Currently the northern hemisphere is in autumn, so the north polar cap is tilted away from the sun, while the southern hemisphere is enjoying spring and its polar cap is tilting more and more towards the sun. Mars' southern hemisphere spring started May 6th. Summer will begin September 29th. Mars' seasons last about twice as long as our seasons.

Also visible on Mars are dark markings called maria or seas like the dark lunar features. Of course there are no water filled seas on Mars. The coloration appears to be in the soil itself, and seems to change in intensity with the seasons. The easiest of these features to spot is a large roughly triangular area called Syrtis Major at about 270 degrees Martian longitude. Above is a reproduction of a drawing I made on July 29th, 2003 showing the southern polar cap to the upper right. The srraight feature on theleft heading to the lower right is Sabaeus Sinus ending with Meridiani Sinus. The straight feature above is Mare Serpentis. The feature on the upper left edge outlined by dashed lines is the Hellas Basin. When this drawing was made I was just geeting reaquanted with the planet.

Dust storms can obscure features on the face of Mars. They are apt to happen near the start of the southern hemisphere summer. I saw one during the 1973 observing season. They appear light in color, but I first noticed one because the expected Martian features I was trying to see were missing.

The famous Martian Canals are illusions. I noticed in my observation that I tended to see and draw circular, geometric or linear forms, and can be seen with in the drawing above. Much higher resolution Hubble photographs tend to show the dark marking as being more splotchy than linear or circular. The one big linear feature, that does exist on Mars, Mariner Valley, some 3,000 miles long was missed as such. The feature visible from earth was called Coprates.

There are other cloud formations visible besides clouds of dust. These are icy clouds near the summit of Olympus Mons called Nix Olympus or Snows of Olympus before the Mariner 9 spacecraft discovered a real mountain there. There are also W shaped clouds that appear in the Martian afternoon. These last clouds I have personally never seen.

One of the best ways to help bring out the subtle Martian features is with the use of filters. Filters increase contrast. Red filters are used to make the dark Martian marking stand out. Wratten numbers 21, 23a and 25 in increasing density can be used. Dust clouds stand out with amber filters number 8, 12 and 15 (again in increasing density). Other clouds can be enhanced with filters 38a and 64. Frosts and hazes can be seen with filters 38a, 47 and 58. Filter recommendations are from Observing and Photographing the Solar System by Dobbins, Parker and Capen, published by Willmann-Bell. I've found that a moon filter also helps, because Mars is very bright.

Questions? Comments? Send Email to me at bob@bjmoler.org

Updated 08/03/03