Earth Explorer future mission summaries

ACECHEM (Atmospheric Composition Explorer for CHEMistry and climate interaction) We know that emissions from human activities are changing the chemical composition of the atmosphere. What we don’t know is how these changes could alter our climate. The Antarctic ozone hole - the end result of a long chain of complicated chemical reactions - was not anticipated before it was observed. What other complex and unanticipated processes might man-made increases in trace gases be causing? ACECHEM proposes to investigate this gap in knowledge, concentrating on the elusive-to-study region where the dynamic troposphere exchanges trace gases with the more tranquil stratosphere, between seven and 26 miles up.

A platform with dedicated instruments will observe the Earth’s limb, the smear of atmosphere between the curve of the surface and the blackness of space. Infrared and millimetre-wave spectrometers will chart the chemical composition of the upper troposphere and lower stratosphere while a cloud imager determines cloud and aerosol populations for even greater accuracy.

EarthCARE

(Earth Clouds, Aerosol and Radiation Explorer) Clouds represent a major area of uncertainty in our current climate models. What is sure is that they play a key role in feedback effects associated with global warming. At low levels, clouds cool the Earth down by reflecting solar heat straight back to space. At high levels, clouds warm the Earth up by blocking the loss of infrared heat off the surface. Different types of aerosols (suspended particles) can alter cloud formation, lifetimes and reflectivity. So depending on their position and composition, clouds can either inhibit climate change or amplify it. EarthCARE has been designed to peer down through the entire body of a cloud or aerosol layer, and provide a vertical profile charting what the clouds and aerosols are made up of. It will do this with a suite of sensors including a Lidar, Doppler radar, imager, radiometer and spectrometer.

SPECTRA

(Surface Processes and Ecosystem Changes Through Response Analysis) In seeking to understand global climate, it has become ever more obvious that the planet’s distinctive ecological habitats or biomes play a major role. The great forest and tundra biomes function as carbon stores, absorbing vast amount carbon dioxide out of the atmosphere where it would otherwise add to greenhouse warming. The exact level absorbed by terrestrial vegetation is in flux, depending on the amount of vegetation, its health and the season of the year - it increases in summer and decreases in the winter die-off. The polar-orbiting SPECTRA intends to thoroughly study the importance of biomes in the global carbon cycle. A range of the world’s key habitats will be selected before the mission to be studied with a high-resolution spectrometer and a thermal imager. Remote sensing offers numerous advantages over solely ground-level investigations, not least of which is the ability to document these complex environments at many different scales at once, and over time, gather a data set that extends into four dimensions.

WALES

(Water vapour and Lidar Experiment in Space) This single satellite is based on a scientific instrument known as a Lidar (Light detection and ranging), which works on the same principle as a radar but substitutes laser pulses for radio pulses. WALES' Lidar will fire down two slightly different laser pulses, one emitted at a wavelength where it will be absorbed by water vapour, the other at a wavelength where absorption is negligible. As the pulses travel through the atmosphere they are backscattered by contact with aerosols, and this backscatter is picked up by WALES’ receiver. The degree of backscattering for the two pulses should be identical except for water vapour absorption, so the amount of water vapour can be deduced.

Increased accuracy in global water vapour profiling will help improve our climate models. At the moment we use instrument-laden giant balloons for the same task, launching hundreds every day, but coverage remains patchy, especially over the poles and the oceans.

WATS

(Water vapour and temperature in the Troposphere and Stratosphere) This mission employs a scientific technique called radio occultation, first used back in the Sixties and Seventies to investigate the Martian and Venusian atmospheres. When radio transmissions from interplanetary probes passed through them, the signals received back on Earth were found to be distorted. Scientists realised these changes in frequency and amplitude were actually yielding useful data about the temperature and structure of these alien atmospheres. The WATS mission basically seeks to do the same, except the radio signals are a little closer to home. Each of the 12 WATS satellites will track navigation satellites such as GPS, which transmit continuously as they orbit Earth. The WATS satellites also track each other. Radio signals between the satellites pass through the atmosphere, causing refraction and attenuation. This happens hundreds of times every 24 hours, giving near-global coverage. The radio occultation provides highly accurate vertical measurements of water vapour and temperature in the atmosphere.

For more information see the future missions page of ESA's Living Planet site: http://www.esa.int/export/esaLP/YJEVCKSC_futuremissions_0.html