The Aeronomy of Ice in the Mesosphere (AIM) mission is the first detailed exploration of Earth's unique and elusive clouds that are literally on the "edge of space." Other space-based and ground-based measurements have probed some aspects of this unusual phenomenon in Earth's mesosphere (the region just above the stratosphere), but very little is known about how these clouds form over the poles, why they are being seen at lower latitudes than ever before, or why they have been growing brighter and more frequent. Some scientists have suggested that these polar mesospheric clouds may be the direct result of human-induced climate change.
Over the course of its two-year mission, AIM will help answer these questions by documenting for the first time the entire complex life cycle of these clouds. With this information, scientists will be able to resolve many of the mysteries about how these clouds form and be better able to predict how they will change in the future.
- Polar mesospheric clouds form over both poles only during each hemisphere's summer, which starts in mid-May in the north and November in the south. The number of clouds and their brightness is highest toward the poles. Clouds are both more frequent and brighter in the Northern Hemisphere than in the Southern Hemisphere. They are normally observed at altitudes of 83-84 kilometers (50 miles).
- Polar mesospheric clouds, as they are known to those who study them from satellite observations, are also often called "noctilucent," or night shining, clouds. This is because they are visible from the ground when illuminated by sunlight from below the horizon while lower layers of the atmosphere are in the Earth's shadow.
- The first public report of noctilucent clouds was made in 1885 by an amateur astronomer. The first observations of the clouds in the daytime were made by a satellite in 1969. Regular space-based observations began in 1982 with NASA's Solar Mesosphere Explorer. Space-based observations to date were made serendipitously by instruments designed for other purposes.
- AIM will observe two complete polar mesospheric cloud seasons over both poles
Deciphering the Recipe for Polar Mesospheric Clouds
The formation of polar mesospheric clouds at such high altitudes does not follow conventional meteorological concepts of how clouds form lower in the atmosphere. One theory is that the cloud particles grow on "seeds" of volcanic or meteoric dust. These clouds appear to be a relatively recent phenomenon, being first reported in the late 19th century shortly after the volcanic eruption at Krakatoa. The brightest clouds are now known to be primarily composed of water ice, and their seasonal lifecycle is controlled by complex interactions between temperature, water vapor, solar activity, atmospheric chemistry and small particles on which the cloud crystals form.
AIM will make simultaneous measurements of the main ingredients needed to unravel the role of natural factors such as the solar cycle and meteorology in the formation of these clouds from the possible role of anthropogenic factors such as carbon dioxide, which causes a warming in the lower atmosphere but a cooling in this region of the atmosphere. Ultimately, this research will provide the data needed to determine the role of polar mesospheric clouds as an important indicator of our planet's changing climate.
Polar mesospheric clouds occur in the region where the sun first interacts with Earth's atmosphere, causing chemical and thermal changes. Solar radiation at this altitude can break apart water vapor molecules, thus reducing the amount of water ice available to form ice crystals in the clouds. The solar ultraviolet radiation at work in this process is known to vary with the 11-year solar cycle.
Satellite observations have shown a pattern of increasing solar ultraviolet radiation followed by declining brightness and frequency of these clouds over two solar cycles. But the change in solar activity occurs nearly a year before changes are seen in the clouds, indicating that the relationship is not a simple matter of direct cause and effect. It is not known why the two are so far out of sync with each other.
Water Vapor and Temperature
Three things are needed in order for these high-altitude clouds to form: cold temperatures, water vapor and small particles that provide surfaces for water to condense. Two of the leading suspects behind the recent changes in polar mesospheric clouds are an increase of water vapor in the region and colder temperatures. Climate models predict that the result of increasing greenhouse gases in the atmosphere would be warmer temperatures in the lower atmosphere, where emitted radiation is "trapped" by the air above, but colder temperatures in the mesosphere where the radiation is lost to space. Colder temperatures would allow more icy cloud particles to form. Alternately, a buildup of water vapor in the upper atmosphere could cause the same increase in polar mesospheric clouds. By measuring water vapor, temperature and the presence of clouds at the same time, AIM will allow scientists to isolate which of these factors is the key driver of cloud formation.
Understanding the processes that control water vapor in the summer polar mesosphere will provide a basis for understanding the formation and evolution of these clouds. Water vapor is transported upward into the polar summer mesosphere from the lower atmosphere. It is also produced by the photochemical destruction of methane in the stratosphere and mesosphere.
There are few direct measurements of water abundance along with polar mesospheric cloud properties in the region where these clouds form. AIM will measure a comprehensive data set of water vapor and key chemicals that lead to water formation and that can be used as tracers of the general atmospheric motion. These combined observations will yield a detailed view of the movement and formation of water vapor in this region of the atmosphere.
One unusual source of mesospheric water vapor - exhaust from rocket engines - was recently shown to be the cause of an increase in Arctic polar mesospheric clouds (Geophysical Research Letters, M. H. Stevens et al., July 6, 2005). The exhaust plume from an August 1997 space shuttle launch moved northward to form a burst of clouds a week after launch. Water vapor from rockets, however, is not thought to be a major contributor to the long-term increase in these clouds.
Cosmic dust particles
Like clouds in other parts of the atmosphere, one element required for polar mesopheric clouds to form is tiny particles on which water vapor can accumulate and grow into ice crystals. Nearer to Earth's surface clouds form from "cloud condensation nuclei" that can be sea salt spray, desert dust, or other materials lofted from the surface. In the mesosphere it is thought that cosmic dust particles falling into the Earth's atmosphere might serve this same purpose, although this has not been confirmed. AIM will be able to identify how important the daily influx of cosmic dust particles is in the lifecycle of these clouds by measuring the amount and changes of dust in low-Earth orbit. The dust particles disintegrate as they enter the atmosphere below the spacecraft and reform into minute smoke particles that might serve as nuclei for water condensation.
Instruments on the AIM spacecraft will measure the incoming dust, and this data will be used to estimate the average number of condensation nuclei available in the mesosphere for cloud formation. Comparisons will be made between the overall influx of cosmic dust and the occurrence and brightness of clouds in the same area to see if this "cosmic seeding" is more or less important for cloud formation than temperature, water vapor or other factors.
A "Canary in the Coal Mine" for Global Change?
AIM will examine the relative contributions of solar and human-induced effects that cause change in the upper atmosphere. Scientists have pointed out a possible connection with global change because the clouds are becoming brighter and occurring more frequently with time and they are being observed at lower latitudes than ever before. One plausible explanation is that temperatures where the clouds form have become colder with time due to the build up in the lower atmosphere of greenhouse gases from human activities. High in the atmosphere, the greenhouse-gas buildup results in cooling.
AIM will test this hypothesis by providing a clearer understanding of why polar mesospheric clouds form and how they respond to short-term environmental changes. The comprehensive data from the mission will allow scientists to build computer simulations that reproduce the observed changes in these clouds. With these tools in hand, scientists will be able to predict future changes in the clouds and see to what extent they are an indicator of global climate change.
Timeline to Discovery
The three scientific instruments onboard AIM will begin to deliver images and data within a few months after launch, which is currently scheduled for April 2007. The first observations will be made of the Arctic polar mesospheric cloud season during the summer months in the Northern Hemisphere. Scientists will then begin to analyze the data with computer simulations of the mesosphere to better understand the many complex processes at work.
Gary E. Thomas, "Are noctilucent clouds harbingers of global change in the middle atmosphere?" Advances in Space Research, vol. 32, no. 9, 2003.
Matthew T. DeLand et al., "A quarter-century of satellite polar mesospheric cloud observations," Journal of Atmospheric and Solar-Terrestrial Physics, Vol. 68, 2006.