|
+ Play
Audio
|
+ Download Audio | +
Email to a friend | +
Join mailing list
August
25, 2008: When in space, keep an eye on the window.
You never know what you might see.
Last
month, astronauts on board the International Space Station
(ISS) witnessed a beautiful display of noctilucent or "night-shining"
clouds. The station was located about 340 km over western
Mongolia on July 22nd when the crew snapped this picture:
Above:
Noctilucent clouds photographed by the crew of the ISS: more.
Atmospheric
scientist Gary Thomas of the University of Colorado has seen
thousands of noctilucent cloud (NLC) photos, and he ranks
this one among the best. "It's lovely," he says.
"And it shows just how high these clouds really are--at
the very edge of space."
He
estimates the electric-blue band was 83 km above Earth's surface,
higher than 99.999% of our planet's atmosphere. The sky at
that altitude is space-black. It is the realm of meteors,
high-energy auroras and decaying satellites.
What
are clouds doing up there? "That's what we're trying to
find out," says Thomas. People
first noticed NLCs at the end of the 19th century after the
1883 eruption of Krakatoa. The Indonesian supervolcano hurled
plumes of ash more than 50 km high in Earth's atmosphere.
This produced spectacular sunsets and, for a while, turned
twilight sky watching into a worldwide pastime. One evening
in July 1885, Robert Leslie of Southampton, England, saw wispy
blue filaments in the darkening sky. He published his observations
in the journal Nature and is now credited with the
discovery of noctilucent clouds.
Scientists
of the 19th century figured the clouds were some curious manifestation
of volcanic ash. Yet long after Krakatoa's ash settled, NLCs
remained.
"It's
a puzzle," says Thomas. "Noctilucent clouds have
not only persisted, but also spread." In the beginning,
the clouds were confined to latitudes above 50o;
you had to go to places like Scandinavia, Siberia and Scotland
to see them. In recent years, however, they have been sighted
from mid-latitudes such as Washington, Oregon, Turkey and
Iran:
Above:
Noctilucent clouds over Mt. Sabalan, a 15,784 ft extinct volcano
in northwestern Iran. Photo credit: Siamak Sabet. [more]
"This
year's apparition over Iran (pictured above) was splendid,"
says Thomas. The Persian clouds appeared on July 19th, just
a few days before the ISS display, and were photographed from
latitude 38o N. "That's pretty far south,"
he says.
The
genesis and spread of these clouds is an ongoing mystery.
Could they be signs of climate change? "The first sightings
do coincide with the Industrial Revolution," notes Thomas.
"But the connection is controversial."
NASA
is investigating. The AIM satellite, launched in April 2007,
is now in polar orbit where it can monitor the size, shape
and icy make-up of NLCs. The mission is still in its early
stages, but already some things have been learned. Thomas,
an AIM co-Investigator, offers these highlights:
1.
Noctilucent clouds appear throughout the polar summer, are
widespread, and are highly variable on hourly to daily time
scales. A movie made from daily AIM snapshots shows the
2007 NLC season unfolding over the north pole: watch
it.
 Right:
A daily snapshot of noctilucent cloud activity over the
North Pole in 2007. Click on the image to set the scene
in motion. Credit: AIM/Goddard Space Flight Center Scientific
Visualization Studio.
2.
There is a substantial population of invisible
noctilucent clouds. Thomas explains: "NLCs are made
of tiny ice crystals 40 to 100 nanometers wide—just the
right size to scatter blue wavelengths of sunlight. This
was known before AIM. The spacecraft has detected another
population of much smaller ice crystals (< 30 nm) that
don't scatter much sunlight." Clouds made of these
smaller crystals are stealthy and hard to see, but a key
part of the overall picture.
3.
Some of the shapes in noctilucent clouds, resolved for the
first time by AIM's cameras, resemble shapes in tropospheric
clouds near Earth's surface. AIM science team members have
described the similarities as "startling." The
dynamics of weather at the edge of space may not be as unEarthly
as previously supposed.
These
findings are new and important, but they don't yet unravel
the central mysteries:
Why
did NLCs first appear in the 19th century?
Why
are they spreading?
What
is ice doing in a rarefied layer of Earth's upper atmosphere
that is one hundred million times dryer than air from the
Sahara desert?
AIM
has just received a 3-year extension (from 2009 to 2012) to
continue its studies. "We believe that more time in orbit
and more data are going to help us answer these questions,"
says Thomas.
Meanwhile,
it's a beautiful mystery. Just ask anyone at the edge of space.
SEND
THIS STORY TO A FRIEND
Author: Dr.
Tony Phillips | Credit: Science@NASA
| more
information: |
| AIM
-- Aeronomy of Ice
in the Mesosphere, mission home page
at Hampton University
The
Discovery of NLCs: A Brief History
Krakatoa
erupted in 1883, yet NLCs did not appear until 1885,
a time lag of 2 years. "It took the atmospheric
circulation that long to transport Krakatoa's ejecta
(water vapor and/or ash) up to the 83 km region,"
explains Gary Thomas. "It is certain that NLCs
were not observed in the summer of 1884."
A
German named T.W. Backhouse is often cited as the discoverer
of noctilucent clouds, but scientific precedence should
go to a different observer. Thomas explains: "We
now give the credit for the first published account
of noctilucent clouds to Robert Leslie of Southampton,
England, whose letter appeared in Nature in
July 1885, a month before Backhouse's letter (who acknowledged
Leslie's earlier publication). Backhouse claimed that
he saw the unusual clouds for the first time on June
8, 1885, nearly a month before Leslie's date of July
6, 1885, but his letter was not published in Nature
until Sept 10, 1885. Backhouse has historically received
credit for the first sighting, but Leslie is now cited
as being the first because he published first."
"Otto
Jesse of Berlin and many others reported sightings that
same summer. Jesse pointed out in a letter to Nature
in 1890, that he had always paid great attention to
clouds (in 25 years of observing), and 'on this account
[on June 23, 1885] these bright clouds appeared to me
the more surprising and puzzling.'"
NASA's
Future: US
Space Exploration Policy
|
|
