US-Sized Dust Storms Seen on Mars

Last week, scientists were surprised to see a second regional dust storm on Mars blooming only two weeks after another one in the same storm track.

NASA’s Mars Reconnaissance Orbiter (MRO) showed both storms generated in the Acidalia area of northern Mars, then moving to the southern hemisphere and expanding to sizes bigger than the United States. While the path is normal, the frequency of the storms is unexpected.

“What we’re trying to understand is the weather of Mars,” said Richard Zurek, chief scientist for the Mars program at NASA’s Jet Propulsion Laboratory and the project scientist for MRO.

One mystery is what determines the scale of a dust storm. There are many local storms, a few that become more regional, and then even fewer where enough dust is lofted into the atmosphere to become global, Zurek said.

So far, scientists see that global dust storms tend to happen during the spring and summer in the southern hemisphere, when Mars is closest to the sun and heating is at a maximum to generate winds. The orbit tends to change every 100,000 years. So in older times, when Mars’ elliptical orbit exposed other parts of the planet to maximum heating, dust generation may have happened differently — but scientists don’t know that for sure yet, Zurek pointed out.

Only the smallest dust particles are lifted high in the atmosphere; sometimes, larger bits of dust hop along the surface and dislodge finer materials that float up. Global dust storms have happened a few times since NASA started observing Mars. One famous example was a 1971 dust storm that raged as Mariner 9 orbited the planet. Scientists saw the peaks of volcanoes peeking above the clouds, but not much else. The last global dust storm was 2007.

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