Night-Shining Clouds May Have Metal Lining

Irene Klotz, Discovery News

Oct. 15, 2008 -- It's not a silver lining that causes night-shining clouds to bounce radar, but that's close, claims plasma physicist Paul Bellan.

 Noctilucent Clouds

He thinks the clouds, which sit on the edge of space, may be coated with a thin layer of metals.

The ability to reflect radar is one of a handful of mysteries about noctilucent clouds, so named because of their luminescence in deep twilight.

The electric-blue clouds, also known as polar mesospheric clouds, appear at the far outer fringes of the atmosphere, particularly during summer. Originally the clouds were confined to arctic latitudes, but they have appeared more recently as far south in the United States as Utah and Colorado.

Conventional theories about why the clouds bounce radar so well suggest that charged particles in the clouds may be responsible.

"You can get these reflections from charged particles," said David Rusch, with the University of Colorado's Laboratory for Atmospheric and Space Physics and a co-investigator on a NASA mission named AIM devoted to studies of noctilucent clouds.

Bellan believes otherwise. In the August issue of the Journal of Geophysical Research-Atmospheres, Bellan lays out dense mathematical calculations that correlate the radar signatures coming from the ice-laden clouds with those of a thin film of metals.

The metal film would come from sodium and iron that have been blasted off of micrometeoroids plowing into Earth's atmosphere.

"It's possible that there could be a mono-coating," Rusch told Discovery News. "I think we're a little bit skeptical at this point, but we're certainly going to look at it. There's good thinking in that research."

Past studies of sodium and iron in the atmosphere show the metal vapors are more than 80 percent depleted when noctilucent clouds are present.

"The ice is like flypaper for the sodium," Bellan said, citing other experiments showing that when sodium sticks to ice it forms a metallic film.

"That's neat. You don't usually think of metal appearing in clouds," Bellan added.

The idea that noctilucent clouds are eating out atmospheric sodium is, admittedly, a long-shot, Bellan says, but the physics adds up.

"Ripples in the cloud of metal-coated ice grains reflect in unison and reinforce each other ... like an army marching in step across a bridge causes the bridge to vibrate," he said.

"The metal is responsible for the reflectivity," Bellan said. "I did the math."