The nearby Red Planet displays remarkable changes every apparition — including dust storms!
Ever since mid-2017, Earth has been steadily closing in on Mars. The approach culminates July 27th 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 differences.
The Shrinking South 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 southern hemisphere to Earth, giving us a nightly view of the rapidly receding Sorth Polar Cap (SPC). At opposition in July, the planet will be approaching its long summer season, so the cap should still be large enough to be visible from Earth. That said, the SPC virtually disappears in the summer months.
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 SPC 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 when you are seeing the SPC as opposed to the cloudy shroud of the South Polar Hood, which should dissipate sometime around opposition.
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. Indeed, they can be the only albedo features visible during a global dust event, appearing as four dark spots reaching above the homogeneous tan dust clouds.
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.
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, bright, yellow clouds. They are most prominent when hovering over dark albedo features. In fact, a massive storm broke out in the northern hemisphere at the end of May, and by mid-June has spread to cover close to half of the red planet!
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 effect of making parts of gigantic Valles Marineris visible for a few weeks from Earth in amateur telescopes. This phenomena is occurring right now, so be sure to catch a glimps while it’s still visible.
Although these 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°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°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 current storm on the Red Planet doesn’t rage on for several months, a dynamic, changing world beckons observers as it draws relatively close for a few brief months. Let’s see what other surprises it has in store for us this year.
An earlier version of this text appeared in Sky & Telescope's March 2014 issue. It has been updated to reflect relevant changes.