The Fluorescence Explorer (FLEX) mission is ESA’s eighth Earth Explorer. FLEX will provide global maps of vegetation fluorescence, which can be converted into an indicator of photosynthetic activity. This new information will improve our understanding of how much carbon is stored in plants and the role of plants in the carbon and water cycles. FLEX was selected in November 2015 and is expected to be launched by 2022. (Image courtesy of ESA.)
Global agricultural efforts will soon have one more tool to help manage food production and environmental concerns.
A novel satellite mission aims to monitor and assess the health of the world’s vegetation. It will do this by detecting and measuring the faint photosynthetic fluorescence emitted by plants as they convert sunlight and carbon dioxide into energy.
FLEX Satellite and FLORIS Spectrometer
The FLEX satellite will carry a spectrometer called FLORIS (Fluorescence Imaging Spectrometer) that will measure the photosynthetic fluorescence emitted by vegetation.
This will allow researchers to analyze the state of natural vegetation and agricultural land to identify whether a region is healthy and productive or under stress.
The FLORIS instrument is an imaging grating spectrometer. It images at resolution and throughput covering a spectral range of 500nm to 780nm.
FLORIS is comprised of two complementary spectrometers:
- FLORIS-LR, a wide-band spectrometer with low spectral resolution covering 500nm-780nm.
- FLORIS-HR, a narrow-band spectrometer covering the O2-A and O2-B bands at a high spectral resolution.
The instrument’s telescope lens images the ground onto a slit. The spectrometer then images the slit onto a focal plane and disperses the light in Along Track (ALT) direction.
Spatial resolution on the ground is 300m2 x 300m2 and a swath width of 150km. This will facilitate the capture of detail on a scale of individual forestry and agricultural land units.
FLEX will orbit in tandem with one of the Copernicus Sentinel-3 satellites, taking advantage of its optical and thermal sensors to provide an integrated package of measurements.
In an experiment to support the development of the FLEX candidate Earth Explorer mission, the HyPlant instrument was used on an aircraft to detect vegetation under stress.
The experiment involved rolling out two fields of turf and applying one with a common herbicide and leaving the other untreated. As the image shows, the treated field on the left glows red, emitting more fluorescence compared to the control field on the right and in fact, more than the surrounding vegetation. Fluorescence is generally an indicator of photosynthetic activity.
In this case, the herbicide interrupted the plant power systems so that absorbed solar energy could not be used for photosynthesis. To get rid of this energy the plants emitted more fluorescence. By detecting these abnormal peaks of florescence from space, FLEX could offer early warning of stress in plants that may appear healthy to the eye. (Image courtesy of ESA.)
Photosynthesis and Fluorescence
The light reaction of photosynthesis converts solar energy into biochemically-useable energy. This process is a complex cascade from light absorption to electron transfer to biosynthesis.
Electrons are energized to an excited state when chlorophyll molecules in a leaf absorb photons. The fate of these “excitons” depends on the physical state of the plant.
Optimal plant and environmental conditions result in approximately 82 percent of absorbed light being used for carbon assimilation. The remaining part is lost as heat and dissipates as chlorophyll fluorescence.
This means that the fluorescence measured is the most direct indicator of photosynthetic activity. When analyzed, it will present a distinct picture of the state, health and stress level of a vegetation region.
The light referred to as chlorophyll fluorescence is emitted from two photosystems working in sequence.
Fluorescence from the initial reaction in one photosystem occurs at wavelengths between 650-780nm with a peak at approximately 740nm.
The second photosystem’s fluorescence occurs in the far-red/near-infrared spectrum at greater than 700nm with a peak at approximately 740nm.
The full chlorophyll emission spectrum covers a wavelength range across the visible and near-infrared spectrum from approximately 640nm up to 800nm.
According to the FLEX mission overview, it will focus on primary measurements of this vegetation fluorescence and relevant characteristics of the fluorescence emission spectra including:
- Fluorescence emitted in the range of the oxygen absorption bands O2-A (687nm) and O2-B (760nm), with an accuracy of 0.2 mW m-2 sr-1 nm-1.
- Maximum fluorescence emission of the two peaks including the wavelength position of the peaks, with an accuracy of 0.2 mW m-2 sr-1 nm-1 and 5nm respectively.
- Total fluorescence emission integrated over the full emission spectrum. The full two-peak emission spectrum of fluorescence will be retrieved with an accuracy of 10 percent for normal reference conditions.
Fluorescence products from FLEX. (1) Fluorescence emission is retrieved in the range of the two oxygen absorption lines, i.e. at 687nm and at 760nm (F687 and F760). (2) The maxima of fluorescence emission (maxF<685> and maxF<740>) and the position of the peaks’ maxima around 685 and 740 nm are also derived. (3) From these anchor points the full fluorescence emission spectrum (Ftot) is reconstructed (area under the curve). Finally, (4) the emissions originating from photosystems II and I are calculated. (Image courtesy ESA/Forschungszentrum Jülich.)
Agriculture Under Stress
The primary objective of the FLEX mission is to provide data on the state and health of vegetation on a global scale. The ESA team envisions the FLEX data being used to monitor global food production capabilities in order to improve agricultural practices and production levels.
In situations such as the current California drought, for example, this program can become a valuable tool to identify which regions of the state are experiencing the greatest effects on agriculture.
This can make it easier to direct water rations appropriately to ensure crops are supported.
The FLEX (Fluorescence Explorer) satellite and its FLORIS spectrometer are the next entry into the ESA’s family of Earth Explorer missions. It is set to launch in 2022.
The Earth Explorer missions are designed to discover and exploit new methods of observing Earth from space to improve human understanding of how the planet works as a system.
For more information on the FLEX mission and ESA’s Earth Explorers program, visit the European Space Agency’s website.