Observing Mars with a Telescope

Photograph of Mars taken by the Hubble Space Telescope in 2016. Click to enlarge and view the visible features which have been labeled.

Mars as it appears through a small telescope at 150x magnification.

Mars constitutes the only planet in the solar system with surface features easily distinguishable from Earth—of the inner planets, Mercury’s small size and proximity to the sun make viewing exceptionally difficult, while Venus’ surface hides behind a thick layer of clouds.

As such, Martian surface features have long captured the imagination of astronomers, even helping lead to the longstanding popular belief of its inhabitation by extraterrestrial civilization. In 1877, Italian astronomer Giovanni Schiaparelli began mapping areas on Mars, including long, dark lines he named “channels”, or “canali." Mistranslations of this word to English “canals” along with his own observations encouraged American Percival Lowell to theorize that these dark lines were the work of intelligent beings attempting to irrigate their dying planet.

Although these lines were soon after revealed to be an optical illusion resulting from the mind’s attempts to connect dark areas at the extent of its vision, even amateur astronomers can make out a variety of other surface features with the aid of a telescope.

When can Mars be seen?

Chart depicting the progression of Mars through the zodiac constellations at three-month intervals over a period of 10 years. Gaps in the sequence indicate when Mars is too close to the Sun to be seen.

Diagram illustrating the relative positions of Earth and Mars at opposition and conjunction. The two planets reach opposition roughly once every 26 months.

Timing plays an essential role in viewing Mars from Earth. The differing orbits between the two planets mean that Mars enters opposition—that is, the point where it and Earth are on the same side of the Sun and therefore at their closest approach—once every 26 months. For the majority of the year, Mars appears fairly small in the sky, although not so small that it is invisible to the naked eye. During a window of several months before and after opposition, however, it appears brighter and large enough to make out surface features with the help of a telescope.

The ease with which these surface features can be observed varies from one opposition to another. The orbit of Mars around the Sun is elliptical in nature, and the timing of Earth’s closest approach does not always occur at the same point along this ellipse. If opposition happens to coincide with Mars’ closest approach to the Sun (perihelion), substantially less distance separates the two planets than if they near one another when Mars is farthest from the Sun (aphelion). Depending on the timing of opposition, then, this distance can range between 34 and 71 million miles (55-100 million kilometers).

Naturally, such a large variation in distance has a profound effect on the visibility of Mars from our planet. This visibility can be described both in terms of apparent size—measured in degrees, arcminutes, and arcseconds—and brightness, measured by an inverse logarithmic scale known as “apparent magnitude.” The brighter an object appears in the sky, the lower its magnitude, with 1 point of magnitude representing a difference in brightness of 2.5 times. The brightest celestial objects have negative magnitudes, and the unaided human eye can perceive magnitudes up to about 6.0.

The relative apparent size of Mars during opposition in 2003 was the largest in 60,000 years.

The apparent size of Mars can be as small as 3.5 arcseconds when farthest away (as a point of comparison, the Sun and Moon both have an angular diameter of around 30 arcminutes—1,800 arcseconds—in diameter, while Jupiter can reach 50 arcseconds at closest approach), but at its closest to Earth this number grows to anywhere between 15 and 25 arcseconds, depending on when opposition occurs. Similarly, the apparent magnitude of Mars can reach +1.6 at its greatest separation from Earth, but at opposition the brightness increases to somewhere between -1.8 and -3.0.

The range in values of apparent size and magnitude demonstrate the impact of the distance between Earth and Mars at opposition. In general, this cycle between opposition occurring at perihelion and aphelion takes 15 to 17 years. Mars will appear smaller at its next opposition of October 13, 2020 than it did on its previous opposition of July 27, 2018. This latter opposition was itself the closest approach of Mars since August 2003, when it and Earth were the closest they had been in 60,000 years. The next closest opposition will not occur until 2035.

What features can be observed?

As occurs with all superior planets—that is, those planets whose orbits around the Sun are more distant than Earth’s—Mars’ path across the sky describes a retrograde loop. That is, it typically moves eastward against the stars, but as opposition occurs and the Earth catches up to it, Mars appears to slow down and reverse direction. Mars’ retrograde loop is the largest one observable in our solar system.

Even without the help of any equipment, the reddish tint of Mars (right) can be discerned in the night sky.

Even to the naked eye, the planet has a distinctive appearance. Its bright, red-orange color, which originally caused the planet’s association with the Roman god of war, results from the iron-oxide sands covering its surface.

With a relatively small telescope, the planet’s surface features begin to appear.Three main regions can be seen: the polar caps, lighter areas of rust-colored dust, and darker areas of exposed volcanic rock.

The appearance of Mars as viewed through a 4", 8", and 10" telescope.

The polar caps are perhaps the easiest feature to identify. Using a telescope of at least 50x magnification during opposition, one can make out the carbon dioxide and water ice frozen at the poles. Because of the planet’s axial tilt, only one of these poles will typically be visible at a time.

Early observers theorized that the lighter areas on Mars were continents, and as such they were given names associated with land or plains: Elysium Planitia, Arabia Terra, Hellas Planitia, and Amazonis Planitia.

Similarly, the darker regions, or albedo markings, were thought to be seas or large patches of vegetation. These markings can appear to morph in size and shape due to changes in atmospheric dust, a phenomenon which was interpreted by early astronomers as resulting from Martian rainfall cycles. Somewhere between four and ten of these dark patches can be easily observed with the help of a telescope, while additional experience may allow the viewer to distinguish up to twenty. Portions of Valles Marineris and Olympus Mons are included within this category.

Global dust storms such as that of July 2018 can obscure even the most readily apparent features. South is up in this image.

All three of these regions can be obscured by dust storms which can grow to cover the entire planet. These typically occur during the northern hemisphere’s summer, when the planet is closest to the Sun and additional heat increases wind activity. This, combined with Mars’ low gravity and atmospheric pressure contribute to the ease witch which dust is kicked up into planet-encompassing storms.