an observer's guide to mars

Basic Information

Mars is the fourth planet from the Sun - the next planet out from the Earth. It has an orbit that is more eccentric than average (only Mercury and Pluto have more eccentricity). So while it averages 229 million km (142 million miles) from the Sun, it gets as close as 208 million km (129 million miles) and as far as 258 million km (160 million miles). Mars' distance from Earth varies from 58 million km (36 million miles) to over 403 million km (250 million miles). Proportionately, this six-fold distance variation is the greatest of any planet.

Mars is much smaller than the Earth -- at a diameter of 6794 km (4222 miles), it could fit inside the Earth 6½ times over. However, the lack of oceans on Mars means that its surface area is about the same as the land area of the Earth. Another similarity, the day on Mars is almost the same as on Earth: just over 24 hours 43 minutes. So if you observe Mars at the same time each night, it would take 33 nights to see the entire planet.

The Martian atmosphere is composed mostly of carbon dioxide (95.3%) with small amounts of nitrogen (2.7%), argon (1.6%), oxygen (0.15%), and water (0.03%). There is not that much atmosphere to speak of though. The average surface pressure is only 1% of that on Earth. It is, however, thick enough to support strong winds and large dust storms.

Mars has some of the most varied and spectacular scenery in the solar system including:

• Olympus Mons - an extinct volcano rising 24,000 m (78,000 feet) above the surrounding plain - 2.7 times the size of Mount Everest. The ancient caldera on top is 80 km (50 miles) wide.
• Valles Marineris - a canyon system 4000 km (2480 miles) long and 7 km (4.3 miles) deep.
• Hellas Planitia - an ancient impact crater in the southern hemisphere over 6 km (3.7 miles) deep and 2000 km (1240 miles) in diameter.

Oppositions

Mars completes one orbit of the Sun in about 1 year 10½ months (686.98 days to be exact). So the Earth, completing an orbit once a year, catches up to and passes Mars roughly every 26 months.

Opposition occurs when the Earth catches up to and is at its closest point to Mars. It is called opposition because Mars then lies opposite the Sun in our sky.

The 26 month period between oppositions (779.94 days to be exact) is rounded off to exactly two years in the graphic at right -- in actuality, the point where Earth and Mars are closest would shift about 1/6 of the way around the orbit at each successive opposition.

Because we are so much closer to Mars at opposition, that is the best time to observe the red planet. In fact, from about 3 months before opposition until about 3 months after is the only time that observing Mars is worthwhile. Outside of these "observing windows" Mars is too small and distant to be very interesting, even with a fairly large telescope.

Recent and upcoming oppositions are as follows:

Neither Earth nor Mars has a perfectly circular orbit -- both experience perihelion and aphelion, especially Mars, since it has a more eccentric orbit. As a result, the date of closest approach does not always coincide exactly with the opposition date. Closest approach is usually a few days before or after opposition.

When an opposition occurs near Martian perihelion, like in 2003, it is called a perihelic opposition. Since Mars is closer to the Sun, it is also closer to the Earth. More distant oppositions, like in 2012, are called aphelic oppositions.

Retrograde Motion

click for animation

As the Earth nears opposition with Mars, and as it passes, Mars will appear to stop its normal eastward motion in the sky. It will then appear to move westward against the background stars for a couple of months. Finally, Mars will stop again and then resume its normal eastward motion. This "reversing of course" is called retrograde motion and it is observed to some extent with all of the planets further from the Sun than Earth. Retrograde motion is caused by the Earth moving faster in its orbit than the other planet. As we pass an outer planet the effect is the same as passing a slower car on the highway -- you know the other car is moving forward almost as fast as you are, but relative to your own car, it will appear to be moving backwards.

Albedo Features

The brightness of a reflective body is called albedo and bright and dark areas are referred to as albedo features. Many albedo features do not represent topographic surface structures on Mars, they are just large areas of differently colored material which covers the planet's surface.

Now that Mars has been visited by many different spacecraft, "real" maps have emerged which depict actual structures and not just light or dark areas. This has lead to a dichotomy of Martian maps -- some depict what we see from Earth, some show what's really there.

In addition, many features have two different naming conventions depending on whether they are named as an albedo feature or a real feature. For instance, the albedo feature Mare Sirenum ("Sea of Sirens") changes to the actual area Terra Sirenum ("Land of Sirens"). Many observers continue using the old terminology and names to describe what they see because the albedo maps were maps of what is visible to observers, while new maps are about what is there and yet mostly invisible to Earth-based observers.

The map below is adapted from an older "albedo map," but most of the nomenclature has been updated to modern standards. The globes below it are photo mosaics from the Mars Global Surveyor spacecraft that can be used to compare actual features to albedo features. Click on one of the global images for a larger version.

Polar Caps

Mars has both a northern and southern polar cap that change with the seasons.

The southern polar cap is usually the larger and more prominent of the two, but it undergoes significant changes and can split in two or even disappear completely during the Martian spring. Small fissures can sometimes be seen on the edges of the cap during spring and summer melting. Perihelic oppositions are especially favorable for observing the southern polar cap since the south pole of Mars is tilted towards the Earth at that time.

The northern polar cap does not vary as greatly with the seasons as the southern cap and it never completely disappears. However, changes in the northern polar cap are far less predictable and so more interesting to observe. At times, especially in Martian autumn, the northern cap produces a haze that partially obscures the cap and appears to stop or even reverse its melting. Expansion of the cap and the presence of the northern polar haze are almost always linked.

Clouds

Clouds on Mars may be composed of dust, fog, frost, or other phenomena and come in a variety of colors and shapes.

White Clouds - The majority of white clouds are seen in the temperate and equatorial regions of Mars, especially between Martian spring and autumn. White clouds are often daily phenomena, appearing near the morning terminator and dissipating as the temperature climbs towards mid-day. They are often more numerous in the hemisphere where the polar cap is currently melting.

Blue-White Clouds - These sometimes appear with white clouds and are often indistinguishable. It is believed that blue-white clouds are a lower atmosphere phenomena and may in fact be fog or surface frost. They often occur in sheltered and/or depressed areas.

Yellow Clouds - Yellow clouds are most present near the time of perihelion when the Sun heats the surface and gives rise to high winds that kick up the Martian dust. While they can develop into planet-wide dust storms, most yellow clouds are small and short-lived. The regions of Hellas Planitia and Serpentis Mare are frequent origins of yellow clouds. Other areas to look for yellow clouds are Chryse, Solis Lacus, Isidis Regio and Aeria.

W-Shaped Clouds - Usually associated with large volcanoes (Olympus Mons, Pavonis Mons, Ascraeus Mons and Arsia Mons), W-shaped clouds are usually large and fast moving.

Dust Storms

Dust storms can occur on Mars anytime, but are much more common during perihelion when the Sun's energy contributes to high surface winds. They may be regional and short-lived or can completely cover the planet, obliterating all features from view for several weeks or even months.

Filters

Color filters aid greatly in seeing various details on Mars. What follows is a list of Kodak Wratten filter numbers, general colors, and features best observed with each.