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NASA Science News for February 18, 2003

Strange Clouds
==============

Astronauts onboard the International Space Station have been observing 
electric blue "noctilucent" clouds from Earth-orbit. 

February 18, 2003: They hover on the edge of space. Thin, wispy
clouds, glowing electric blue. Some scientists think they're seeded by 
space dust. Others suspect they're a telltale sign of global warming.

They're called noctilucent or "night-shining" clouds (NLCs for short). 
And whatever causes them, they're lovely. 

"Over the past few weeks we've been enjoying outstanding views of
these clouds above the southern hemisphere," said space station
astronaut Don Pettit during a NASA TV broadcast last month. "We
routinely see them when we're flying over Australia and the tip of
South America." 

Sky watchers on Earth have seen them, too, glowing in the night sky
after sunset, although the view from Earth-orbit is better. Pettit
estimated the height of the noctilucent clouds he saw at 80 to 100 km 
... "literally on the fringes of space." 

"Noctilucent clouds are a relatively new phenomenon," says Gary
Thomas, a professor at the University of Colorado who studies NLCs.
"They were first seen in 1885" about two years after the powerful
eruption of Krakatoa in Indonesia, which hurled plumes of ash as high
as 80 km into Earth's atmosphere. 

Ash from the volcano caused such splendid sunsets that evening sky
watching became a popular worldwide pastime. One sky watcher in
particular, a Briton named T. W. Backhouse, noticed something odd. He
stayed outside after the sun had set and, on some nights, saw wispy
filaments glowing electric blue against the black sky. Noctilucent
clouds. Scientists of the day figured the clouds were some curious
manifestation of volcanic ash. 

Eventually the ash settled and the vivid sunsets of Krakatoa faded.
Yet the noctilucent clouds remained. "It's puzzling," says Thomas.
"Noctilucent clouds have not only persisted, but also spread." A
century ago the clouds were confined to latitudes above 50o; you had
to go to places like Scandinavia, Russia and Britain to see them. In
recent years they have been sighted as far south as Utah and Colorado.

Astronaut Don Pettit is a long-time noctilucent cloud-watcher. As a
staff scientist at the Los Alamos National Laboratory between 1984 and 
1996, he studied noctilucent clouds seeded by high-flying sounding 
rockets. "Seeing these kinds of clouds [from space] ... is certainly a 
joy for us on the ISS," he said on NASA TV. 

"Although NLCs look like they're in space," continues Thomas,
"they're 
really inside Earth's atmosphere, in a layer called the mesosphere 
ranging from 50 to 85 km high." The mesosphere is not only very cold 
(-125 C), but also very dry--"one hundred million times dryer than air 
from the Sahara desert." Nevertheless, NLCs are made of water. The 
clouds consist of tiny ice crystals about the size of particles in 
cigarette smoke. Sunlight scattered by these crystals gives the clouds 
their characteristic blue color. 

How ice crystals form in the arid mesosphere is the essential mystery
of noctilucent clouds. 

Ice crystals in clouds need two things to grow: water molecules and
something for those molecules to stick to--dust, for example. Water
gathering on dust to form droplets or ice crystals is a process called 
nucleation. It happens all the time in ordinary clouds. 

Ordinary clouds, which are relatively close to Earth, get their dust
from sources like desert wind storms. It's hard to waft wind-blown
dust all the way up to the mesosphere, however. "Krakatoa may have
seeded the mesosphere with dust in 1883, but that doesn't explain the
clouds we see now," notes Thomas. "Perhaps," he speculates, "the
source is space itself." Every day Earth sweeps up tons of
meteoroids--tiny bits of debris from comets and asteroids. Most are
just the right size to seed noctilucent clouds. 

The source of water vapor is less controversial. "Upwelling winds in
the summertime carry water vapor from the moist lower atmosphere
toward the mesosphere," says Thomas. This is why NLCs appear during
summer, not winter. 

One reason for the recent spread of noctilucent clouds might be global 
warming. "Extreme cold is required to form ice in a dry environment 
like the mesosphere," says Thomas. Ironically, global warming helps. 
While greenhouse gases warm Earth's surface, they actually lower 
temperatures in the high atmosphere. Thomas notes that noctilucent 
clouds were first spotted during the Industrial Revolution--a time of 
rising greenhouse gas production. 

Are NLCs a thermometer for climate change? A unusual sign of
meteoroids? Or both? "So much about these clouds is speculative," says 
Thomas. 

A NASA spacecraft scheduled for launch in 2006 should provide some
answers. The Aeronomy of Ice in the Mesosphere satellite, or AIM for
short, will orbit Earth at an altitude of 550 km. Although it's a
small satellite, says Thomas, there are many sensors on board. AIM
will take wide angle photos of NLCs, measure their temperatures and
chemical abundances, monitor dusty aerosols, and count meteoroids
raining down on Earth. "For the first time we'll be able to monitor
all the crucial factors at once." 

Meanwhile, all we can do is wait ... and watch. There's never been a
better time to see noctilucent clouds. "During the summer months, look 
west perhaps 30 minutes to an hour after sunset when the Sun has
dipped 6o to 16o below the horizon," advises Thomas. If you see
luminous blue-white tendrils spreading across the sky, you've probably 
spotted an NLC. Observing sites north of 40o latitude are favored.

One more thing: don't forget your camera. According to astronaut Don
Pettit, "you can never have too many pictures of noctilucent clouds." 
                                                                     
Editor's note: Astronaut Don Pettit's remarks and his pictures of NLCs 
that appear in this story were first broadcast on NASA TV in January 
2003. 

Credits & Contacts
Author: Dr. Tony Phillips
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

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