What can you see

Albedo features are the areas most subject to seasonal changes such as brightening or darkening. Through a 4-inch or larger telescope, you’ll see large albedo features — regions distinguished by the amount of light they reflect. Mars’ best include Syrtis Major (an easily seen dark, triangular feature), Chryse, Elysium, the Hellas Basin, Libya, and Solis Lacus.

Mars’ day, which astronomers call a sol, is 37.4 minutes longer than an Earth day. So, if you observe Mars at the same time each night, its markings will appear to move 9.11° per day to the west. In a little more than five weeks, the planet seems to slowly rotate backward one full spin. All of Mars’ prominent features will, at some time during this period, lie on its meridian, the line splitting the planet’s visible disk from top to bottom, as seen from Earth. All martian features look their best when on the planet’s meridian.

Even including albedo highlights, the Red Planet’s best surface features are its polar ice caps. At opposition, Mars’ south polar cap will tilt 11° in our direction.

Due to the temperature range at the martian poles, astronomers subdivide each ice cap into larger “seasonal” and smaller “residual” caps. Residual caps last through the martian year. The southern residual ice cap measures about 200 miles (320 km) across. The northern residual ice cap spans about 600 miles (960 km).

Winters are more severe in Mars’ northern hemisphere, and the seasonal ice cap there has reached a latitude of 45°. In the southern hemisphere, the seasonal ice cap never passes above a latitude of about –55°.

Don’t confuse the Hellas Basin with the polar cap. Hellas is a round, bright feature — an impact basin with lots of bright dust and sometimes fog or clouds. When seen near the limb (the planet’s edge), it can look like a polar cap.

With respect to clouds, all of them on Mars are temporary. Discrete clouds associate with a specific area, and Mars’ rotation carries them along.

Orographic clouds are a type of discrete cloud. Wind passing over high mountains and volcanoes creates these water-vapor clouds. Observe orographic clouds through a blue or violet filter to see the most contrast.

Look away from Mars’ meridian and observe morning and evening clouds. These bright, isolated patches of surface fog appear at sunrise (the western edge of Mars) and sunset. Evening clouds generally appear larger and are more numerous, and as the martian night approaches, they grow larger. These clouds respond best to blue or violet filters.

Color filters show a lot when you observe Mars, but they do take some getting used to. Be patient, and you’ll be amazed at how much more detail you’ll see.

No need to rush

Although Mars will appear biggest July 31, it will look almost as bright several weeks before and after that date. So, if you’re clouded out, or if the 31st just doesn’t work for observing, cheer up. Unlike totality during a solar eclipse, a martian opposition isn’t a short-lived event.

Just keep in mind that the best time to observe Mars is any night it’s in the sky. Once every 26 months, Mars appears bright in our sky, but some of these appearances — like this one — are better than others. Take advantage of the planet’s size and brightness, and don’t worry so much that it’s so low in the sky. Head out to a science center or observatory, contact your local astronomy club, or simply point your scope at the Red Planet. Then focus and take a good, long look.

A version of this story ran in Astronomy’s May issue.