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Because Mars’ orbit is far less circular than Earth’s, the Red Planet receives about 40 percent more solar energy at perihelion (closest point to the sun) than aphelion (furthest point). So while the coincidentally-similar tilts of Earth’s and Mars’s rotational axes are the primary driver of both planets’ seasons, solar distance plays a far larger role on Mars.

More solar energy translates to stronger winds, and thus a greater chance of dust storms. The fictional gale-force dust storm in “The Martian” notwithstanding, Mars’s thin atmosphere requires winds of at least 40-50 mph to keep dust grains suspended. Air that’s 100 times thinner than Earth’s is hard to stay aloft in, but Mars’s surface gravity is 60 percent less, so those two factors roughly cancel each other out.

Mars’ dust is powder fine as well, the result of billions of years of dry winds grinding away on the rocks, sans the active geologic and biologic processes that can cement grains together on Earth.

Mars appears bigger than normal in a telescope when it’s on the same side of the sun as earth (called “opposition” because Mars appears opposite the sun in the sky), which happens every 22 months on average.

Even better if this happens near the time Mars is at perihelion, which occurs at 15 or 17 year intervals. Perihelic oppositions are more likely to spawn global dust storms that can shroud the planet’s features from telescopic scrutiny during the very times that Mars is close.

Unfortunately, one of the most intense Martian dust storms ever observed sprang up in late May, and continues unabated as of this writing. Whether or not it subsides in time for Mars’s closest approach since 2003 (at the end of the month—more about that in my July 26th column) remains to be seen.

Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 208-732-6663 or canderson@csi.edu.

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