When plants take in too much energy, they don’t get fat – they lighten up. They absorb more sunlight than they need to power photosynthesis, and they get rid of the excess solar energy by emitting it as a very faint glow. The light is far too dim for us to notice under normal circumstances, but it can be measured with a spectrometer. Called solar-induced fluorescence (SIF), it’s the most accurate signal of photosynthesis that can be observed from space.
That’s important because, as Earth’s climate changes, growing seasons worldwide are also changing in both timing and length. These changes may affect world food production and the pace of greenhouse warming. It’s not possible to measure photosynthesis globally from ground level, and lab experiments can’t easily replicate all of the environmental factors affecting plant growth, such as water availability, wildfires and competition from other plants – factors that also are changing with the climate.
OCO-3 will be mounted on the International Space Station where it will measure both atmospheric carbon and plant activity from orbit. During photosynthesis, plants absorb carbon dioxide from the atmosphere and emit a small amount of light. Measuring this “solar-induced fluorescence” will help scientists better understand the role plants have in removing carbon dioxide from the atmosphere.
Parazoo and his colleagues will use previously developed algorithms to extract the SIF signal from the full set of data collected by OCO-3. The instrument consists of three spectrometers, each observing different bands of wavelengths in the electromagnetic spectrum. Every kind of gas molecule in the atmosphere – oxygen, carbon dioxide and the others – absorbs sunlight in a unique set of wavelengths. A spectrometer looking at the right wavelengths will see this absorption as a distinctive series of dark lines, like the spectral bar code of a particular gas.
OCO-3’s three spectrometers are tuned to two wavelength bands covering different parts of carbon dioxide’s bar code and one band with an oxygen bar code. As it happens, the oxygen spectrometer records not only wavelengths absorbed by oxygen, but also nearby wavelengths where SIF shines particularly strongly. “So the SIF measurement wasn’t by design but an extremely fortunate bonus,” Parazoo said.
Since NASA scientist Joanna Joiner and colleagues produced the first spaceborne SIF measurements in 2010 – before OCO-2 was launched – SIF data has been generated from earlier European and Japanese satellites. However, OCO-2 has a much finer-scale field of view, or footprint, than any preceding satellite, with each image covering an area of about a square mile (less than three square kilometers).
OCO-3 will add to that advantage something OCO-2 cannot do: As OCO-3 orbits, it will turn its sensor quickly to point at instrumented towers on the ground below the spacecraft. These towers measure SIF and photosynthesis concurrently, with similar resolution to OCO-3. Validating the data this way provides critical information on OCO-3’s performance and can increase scientific insight into the underlying SIF mechanics.
Data averaged over a large area suggest that there’s a straightforward relationship between solar energy coming in and photosynthesis taking place. With OCO-2’s fine-scale data, Parazoo said, “We’re finding that the relationship between SIF, absorbed solar energy and photosynthesis is more complicated than we thought. We’re trying to understand that.” He hopes OCO-3 will be able to shed some light on the causes of this complexity.
With so many promising avenues of study arising from SIF, OCO-3’s plant light measurements will illuminate new findings for years to come.