SAN FRANCISCO — Scientists at NASA’s Goddard Space Flight Center have produced the world’s first maps of global plant health based on measurements of fluorescence, a miniscule amount of light emitted by plants at near-infrared wavelengths during photosynthesis.
By measuring fluorescence and combining that data with available space-based observations, scientists hope to develop a way to gauge plant health far more quickly and accurately than is possible with widely used space-based tools.
“Fluorescence gives you a more instantaneous view of what the plant is doing,” said Joanna Joiner, NASA Earth Observing System deputy project scientist and leader of the team that created the fluorescence maps.
Much of the satellite sensor data used to evaluate the health of vegetation relies on measurements of the light reflected by plant leaves. Healthy plants absorb most of the sunlight in the visual part of the spectrum and reflect light in the near-infrared portion of the spectrum. That measurement, while useful, does not change immediately when plants lack water or sunlight. During a drought, for example, plants may remain green for days or weeks before turning brown and dying. However, the fluorescence signal may change much more rapidly, Joiner said.
Chlorophyll inside plants absorbs specific wavelengths of light and uses that light to provide the power for photosynthesis. The magnitude of the fluorescence varies based on the amount of stress plants experience, Joiner said.
Still, it will be years before fluorescence becomes a standard component of plant health indices. The fluorescence maps created by NASA Goddard scientists are not designed to monitor plant health continually but rather are proof that space-based sensors can be an effective means for measuring plant fluorescence. “This is still a research area. We are still figuring out the best way to measure fluorescence, but there is wide agreement that fluorescence gives you a good indication of the actual photosynthetic function of plants,” said Goddard biologist Elizabeth Middleton, a member of the team that created the fluorescence maps.
In spring 2010, the NASA Goddard team began analyzing data gathered in 2009 by the spectrometer onboard the Japan Aerospace Exploration Agency’s Greenhouse Gases Observing Satellite (GOSAT) to determine whether it would be possible to retrieve information on fluorescence from space. “We didn’t know whether we would be able to find it or not,” Joiner said. “We thought we had a pretty good chance of seeing it, and sure enough, we did.”
Once they realized it was possible to detect plant fluorescence in satellite sensor data, the scientists created global maps of plant fluorescence for July and December 2009. Those maps were published in March. “The next step is to look into how to apply information on fluorescence to global models of plant productivity,” Joiner said.
In addition, the NASA Goddard team is trying to gain a better understanding of the calibration of the instrument being used to measure fluorescence, GOSAT’s Fourier Transform Spectrometer, which was designed to detect carbon dioxide and methane in the atmosphere. That understanding will help the team improve the accuracy of global fluorescence data, Joiner said.
Further progress in measuring plant fluorescence is likely to come from future satellite missions. NASA’s Orbiting Carbon Observatory (OCO)-2, a mission scheduled for launch in 2013 to measure atmospheric carbon dioxide, will produce data on fluorescence, according to Christian Frankenberg, a scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who recently published a research paper on space-based fluorescence measurements.
“We are working to make [fluorescence] an official data-product of OCO-2,” Frankenberg said in an email. OCO-2, a replacement for the first Orbiting Carbon Observatory, which was lost during a February 2009 launch failure, will provide approximately 50 times as much data on fluorescence as GOSAT, Frankenberg said. The data drawn from OCO-2 sensors also is likely to be more reliable because the OCO-2 instrument is designed to minimize retrieval noise, one of the dominant sources of errors in fluorescence data, Frankenberg said.
Even more accurate measurements may be produced by a proposed European Space Agency (ESA) mission focused on gathering fluorescence data. The Fluorescence Explorer mission, known as FLEX, is a candidate for an ESA Earth Explorer-class mission scheduled for launch in 2019.
In November 2010, FLEX and CarbonSat, a mission designed to measure atmospheric carbon and methane, were selected by ESA scientists to be finalists for the agency’s Earth Explorer-8 mission. The two project teams are scheduled to begin 20-month feasibility and definition studies later this year. Once those studies are completed, ESA scientists will meet to assess the results and recommend one mission to continue on to the development phase, ESA spokesman Robert Meisner said in an email.
In May 2014, ESA’s Earth Observation Program Board is scheduled to review that recommendation, provide final approval for the mission selected and invite the winning team to begin development work. ESA’s Earth Explorer missions are designed to address specific scientific goals, Meisner said. The Earth Explorer-8 mission will be selected by the European scientific community based on its research priorities, he added.
