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Changes on mars: the nearby red planet displays remarkable changes every apparition.

Ever since early 2013, Earth has been steadily closing in on Mars. The approach culminates April 8th in opposition. As the closest rocky planet to Earth whose surface is visible in modest telescopes, Mars easily garners the most attention of all the inner planets. The Red Planet often reveals subtle changes from year to year that stand out to the patient observer. But occasionally, big changes can occur from one apparition to the next.

Part of the great attraction Mars has for observers is that the planet once looked more like Earth. Even today, with white cirrus clouds, dust storms, and ice caps that grow and shrink with the seasons, Mars is the most Earthlike of any planet in our solar system. Although the planet's thin atmosphere and dry environment make it look desolate, Mars is far from an unchanging, dead world.

As the planet approaches opposition, keep an eye out for some of these dierences. A guide to observing Mars, as well as a map, can be found on page 50.

The Shrinking North Polar Cap

Among the first features even a beginning planetary observer will notice are the Red Planet's bright polar caps. This year, Mars near opposition presents its northern hemisphere to Earth, giving us a nightly view of the rapidly receding North Polar Cap (NPC). At opposition in April, the planet will be well into its long summer season, so the cap should be at its smallest as seen from Earth. That said, the NPC is substantially larger than the SPC and presents many interesting rifts and other features.

As the NPC shrinks, it will unveil what looks like a dark ring encircling the pole. In addition, shortly after New Year's a large, dark rift known as Chasma Boreale should begin to form around 300[degrees]W. And at roughly the same time on the NPC's opposite side, a dark swath known as Rima Tenuis will seem to cut off a large, crescent-shaped section of the bright ice cap from the rest of the pole. This

rift should be visible in telescopes 6 inches or larger and will look like a dark inner ring dividing the icy crescent from the main NPC.

Partly Cloudy

Clouds have been prevalent throughout the last several apparitions, and although they are most easily detected by planetary imagers using color filters, observers can enjoy them too. As the NPC recedes, its ices sublimate into the planet's tenuous atmosphere, producing thin clouds that are often visible above the planet's equatorial regions. About this time, the well-known "W" cloud formation often clings to the flanks of the large volcanoes in the Tharsis-Amazonis region. These clouds can sometimes appear so bright that you might have difficulty determining which white spot is the NPC and which is the cloud complex.

Morning clouds and frost can sometimes settle in the broad, low plains of this region, making the giant Tharsis volcanoes Arsia Mons, Pavonis Mons, and Ascraeus Mons, as well as nearby Olympus Mons, appear as dark spots above the bright plains.

In the southern hemisphere, a bright cloud often fills the great Hellas impact basin; don't mistake this for a glimpse of the South Polar Cap.

Dust Storms

In the months leading up to opposition, there's always the chance that changing weather patterns will kick up large dust storms and block parts of the surface from view.

Dust storms can spring up overnight and often appear as small, brightish, yellow clouds. They are most prominent when hovering over dark albedo features.

Historically, dust storms have often been spotted in Hellas, Elysium, Chryse, and Solis Lacus. During late October 2005, a large dust storm sprang up in Chryse and then spilled into the deep chasms within Aurorae Sinus. Because of its high albedo compared with the terrain, this storm had the rare eect of making parts of gigantic Valles Marineris visible for a few weeks from Earth in amateur telescopes.

Dust storms can arise almost anywhere on Mars and occasionally encircle the entire globe, obscuring the whole surface. Although global dust events can put a damper on observations, it's exciting to watch these massive wind-driven storms envelop the planet within a few days.

Changing Albedo Features

The dark albedo features of Mars have also experienced long-term changes over the years. Dark markings within Solis Lacus have come and gone within the last decade. Additionally, a dark feature at roughly 230[degrees]W known as Hyblaeus has also expanded and receded throughout the past quarter century.

Perhaps the largest recent albedo change occurred in the 1960s. Around that time, a large complex of albedo features connecting Syrtis Major to Utopia, located at about 270[degrees]W and known as Thoth-Nepenthes, literally disappeared. Today, only the small dark feature known as Alcyonius Nodus remains.

At about the same time as the disappearance of Thoth-Nepenthes, the northernmost tip of Syrtis Major changed from a distinctly pointed feature to the rounded end that today resembles the southern tip of Africa.

Assuming the Red Planet isn't immersed in a globe-spanning dust storm, a dynamic, changing world beckons observers as it draws relatively close for a few brief months every 26 months. Let's see what surprises it has in store for us this year.

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Title Annotation:OBSERVING Exploring the Solar System
Publication:Sky & Telescope
Date:Mar 1, 2014
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