BioEd Online

NASA is about to launch a spacecraft to study mysterious invaders lurking above Earth's poles - not UFOs, but the shimmering, high-altitude apparitions known as noctilucent clouds.

The Aeronomy of Ice in the Mesosphere (AIM) mission, slated for launch on 25 April, will be the first mission dedicated to these clouds. Scientists are looking to gather the most basic information about how they form, including any hints as to whether their appearance could be linked to climate change.

First spotted two years after the 1883 eruption of Indonesia's Krakatoa volcano, noctilucent, or 'night-shining', clouds have been showing up more and more often over the years. They are now spotted more frequently, for longer periods of time, and at lower latitudes than ever before, says Jim Russell, an atmospheric physicist at Hampton University in Virginia.

Just why this is remains a mystery. Over the past three decades, several satellites have looked at noctilucent clouds, but never before has there been a mission dedicated to studying them. Of the earlier work, Russell says: "All it has done is pricked our imagination and interest, and left us wanting."

At dusk and dawn

AIM is scheduled to start answering those questions soon after its launch from Vandenberg Air Force Base in California. A jet aeroplane will carry it over the Pacific Ocean to an altitude of 12 kilometres, where it will release the craft. AIM will then burn its engines and shoot upwards, eventually entering a polar orbit about 600 kilometres above Earth.

From there, it will spend the next two years studying noctilucent clouds above both the Arctic and Antarctic. There are important differences between the two hemispheres, says Russell. The clouds appear brighter in the north than in the south, and occur at lower altitudes. But in both places, only three things are needed to form them: cold temperatures, water vapour and dust particles that allow the water to freeze as tiny ice crystals.

The three scientific instruments aboard AIM are designed to scrutinize the clouds from several angles. The ice particles that make up the clouds are typically 50 nanometres across — just the width of a human hair. Sunlight reflects off the particles, making them visible only around dawn and dusk when the rest of the sky is dim. The effect, says Russell, is similar to seeing a high-flying aeroplane reflecting sunlight after the Sun has set locally.

The clouds also ripple and shimmer. "They're very bright, silvery-blue and iridescent," Russell says. "They capture the imagination." Sky-watchers in high latitudes track the clouds throughout the peak season — which runs from 15 May to 20 August in the Northern Hemisphere, with the most activity in early July.

Cooling effect

Results from AIM could eventually pin down the exact conditions under which noctilucent clouds form, and specifically whether increased greenhouse-gas levels will promote more frequent cloud formation. Rising amounts of greenhouse gases such as carbon dioxide warm the atmosphere at lower elevations but actually cool things down at the higher elevations that the clouds form at, says Cora Randall, an AIM project co-investigator at the University of Colorado, Boulder. In addition, higher levels of the greenhouse gas methane actually leads to the formation of more water vapour at higher elevations.

So colder temperatures and the presence of more water vapour could lead to more of the noctilucent clouds forming more often, Randall explains. "The question is, what combination of those might be leading to more frequent and brighter clouds?" she says.