WASHINGTON —
The initial data
from a NASA experiment studying mysterious clouds near the edge of space indicate there are processes in Earth’s upper atmosphere that are more complex than previously thought. Most importantly, researchers believe those clouds could reveal
important clues about the nature of global climate change.
NASA’s Aeronomy of Ice in the Mesosphere (AIM) mission was developed as a way
to study noctilucent clouds, fleeting wisps of crystallized water vapor that exist in the mesosphere at an altitude of about
80 kilometers
and are seen only
in the higher latitudes.
The mission completed its first season observing the northern hemisphere last summer, and AIM researchers revealed preliminary findings at a December meeting of the American Geophysical Union in San Francisco.
Noctilucent
clouds were first observed in 1885 by an amateur astronomer. They can be seen with the naked eye from the ground in the summer months at latitudes above 50 degrees. But starting in the late 1990s, reports started coming in that the clouds were being spotted in places like New Jersey, Colorado and Utah and at altitudes as low as 40 degrees latitude. They also seemed to be getting brighter.
“There’s something these clouds are trying to tell us,” said Scott M. Bailey, the mission’s deputy principal investigator. “They’re definitely changing, so the atmosphere must be changing as well.”
Prior to the satellite’s launch in April 2007, scientists knew very little about these clouds beyond where and when they form and what they look like from the ground.
AIM is orbiting the Earth at an altitude of about
600 kilometers, sweeping over the North Pole every 96 minutes. Three of its onboard instruments
are measuring data more precisely than any previous satellites ever recorded. The Cloud Imaging and Particle Size instrument has four cameras that allow scientists to take a two-dimensional look at the clouds as the satellite passes by them. The Solar Occultation for Ice Experiment measures cloud particles, temperature and gaseous composition. And the Cosmic Dust Experiment records the amount of space dust that enters the atmosphere to determine if these particles are involved in the clouds’ formation.
One surprising result the researchers have found is the structural complexity of the clouds. Clouds in the lower atmosphere have features that vary so widely that one must only gaze upward to see clouds that resemble
all sorts of animals or other objects. This is because these clouds are affected by all the variation of the Earth’s surface. Clouds in the mesosphere were thought to be simpler, as was the mesosphere itself.
“For a long time it looked like these clouds were one giant mass like the sky looks like on an overcast day,” Bailey said. “But now we see they are very structured and vary a lot, like cumulous clouds in the troposphere. If the clouds are that variable, then the mesosphere itself must be that variable.”
The experiment also confirmed the existence of a suspected population of very small ice particles responsible for strong radar echoes coming from the mesosphere in the summer. These particles of unknown origin may be the genesis of the main layer of these clouds.
Conclusive results that say why these clouds are getting brighter and appearing in lower latitudes will not be known until the remaining observational seasons are complete. But James M. Russell, the AIM mission’s principal investigator, has suspicions about
what is causing the changes.
“Humans are doing something that makes this occur,” Russell said. “For the kinds of changes we’re seeing, that’s the only plausible thing I and my colleagues can think of. But we can’t conclude that yet. That’s one of the things AIM is all about.”
The most likely possibility, Russell said, is that increasing levels of methane, carbon dioxide and water vapor in the upper atmosphere are responsible. Increased methane in the atmosphere is a natural result of Earth’s growing population. Increased carbon dioxide is a result of fossil fuels being burned.
Increased methane creates more water vapor in the atmosphere when it reacts with oxygen. In the lower atmosphere, increased carbon dioxide has a warming effect, as it is a greenhouse gas. In the upper atmosphere, it has a cooling effect. A cooler upper atmosphere with more water vapor is a perfect recipe for more clouds, Russell said.
The satellite now is observing the summer in the southern polar region. The team knows the altitude of noctilucent clouds in the southern hemisphere is typically about a kilometer higher than in the northern hemisphere. Their first observations show those clouds are even higher than previously thought.
The AIM mission currently is funded to study this polar season and two more, but the team is working on a proposal to extend it through 2012. The satellite cannot run out of fuel, as it is entirely solar powered. The orbit should last another 10 to 12 years.
Noctilucent clouds are affected by solar cycles, appearing more frequently during periods of high solar activity. But the mechanism is still not understood, as the clouds’ frequency appears to peak about a year after the peak in solar activity.
“If we can gather data for 10 years, going from solar minimums to solar maximums, that will be a big help,” Bailey said. “That much data would be a great benefit to understanding these clouds.”