Future NASA satellites will extend the reach of global ocean color observation and research.
The physical, chemical, and biological processes of Earth’s ocean, atmosphere, land surface, and interior are naturally and intricately linked and constantly in flux. Monitoring the many ways these systems change and interact is an important part of NASA’s mission. NASA satellites utilize the comprehensive viewpoint of space to collect information on things such as air quality and composition, sea ice, soil moisture, solar radiation, natural disasters, and land and ocean ecosystems. This information is used by scientists and other individuals all over the world and is relevant to almost every aspect of life.
Ocean color is the tool by which scientists, with the aid of specialized satellite technology, can infer the biological and biogeochemical constituents of the ocean.
According to the National Oceanic and Atmospheric Administration (NOAA), “Ocean color is the water hue due to the presence of tiny plants containing the pigment chlorophyll, sediments, and colored dissolved organic material.”
Examining and measuring this characteristic of the ocean is especially important because these tiny plants, called phytoplankton, are the base of the marine food-web, are a crucial element of global climate, and are the source of around half of the oxygen we breathe.
NASA has used satellites to study ocean color for more than 35 years. The Coastal Zone Color Scanner (CZCS), the proof-of-concept ocean color mission, was launched in 1978 on NIMBUS-7. After CZCS, there was a gap from 1986 until 1997, when the next generation ocean color instrument, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), a collaboration between NASA and Orbital Sciences, began scientific operations.
Since that time, with the addition of newer sensors like the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua and Terra and the Visible Infrared Imager Radiometer Suite (VIIRS) onboard Suomi NPP, along with several international projects, there’s been an almost 18 year continuous time-series of ocean color data.
Jeremy Werdell, a research oceanographer at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, an expert on ocean color satellites, explains how the technology operates.
“The way an ocean color instrument works is it measures the intensity of different wavelengths of light, predominantly ultraviolet to near-infrared wavelengths,” Werdell says. “The intensity of one color relative to the other colors provides information on what’s in the upper water column, where sunlight penetrates all the way in and all the way out.”
He notes that this is because most constituents in the water, including seawater itself, absorb and scatter light differently.
“When you think about why leaves are green, it’s because they absorb blue light, they absorb red light, and green light becomes dominant and is what’s ultimately left to be seen. The contents of the ocean operate exactly the same way.”
According to a 2011 National Research Council (NRC) report, “Ocean color measurements from space have revolutionized our understanding of the ocean on every scale, from local to global and from days to decades.”
The applications and benefits of ocean color remote sensing are extensive. They include the mapping of chlorophyll concentrations, fisheries management, monitoring water quality, and the detection of harmful algal blooms (like the severe bloom that’s occurring now along the U.S. West Coast), as well as advancing our knowledge of the carbon cycle and climate change.
To extend and enhance the collection of ocean color data into the future, NASA has started working on the next generation of space-based ocean color instruments to begin operations in the coming decade. The Pre-Aerosol, Clouds, and ocean Ecosystem (PACE) and the Geostationary Coastal and Air Pollution Events (GEO-CAPE) are two projected missions that will improve upon existing capabilities and applications of ocean color research.
The PACE mission, scheduled for launch in 2022, will be managed by NASA’s Goddard Space Flight Center.
Originally conceived as a precursor to the larger, more advanced Aerosol, Clouds, and ocean Ecosystems (ACE) mission that was suggested in the 2007 NRC Decadal Survey, PACE will provide a combination of new observations on ocean ecology and biogeochemistry, as well as clouds and aerosols (minute particles suspended in the atmosphere).
One improvement that PACE will have relative to prior and current ocean color sensors is that it will be able to essentially “see more light.” PACE will employ a hyperspectral radiometer to make its ocean color measurements, as opposed to the multispectral imagers used by previous satellites. This will supply higher spectral resolution, and thus more accurate and useful data sets.
“With existing instruments there are wavelength gaps,” says Werdell, also the PACE project scientist. “Making near-continuous hyperspectral measurements will fill in those gaps.”
Werdell says that while multispectral sensors like SeaWiFS look at only six to nine different wavelengths on the electromagnetic spectrum, “The idea behind PACE is to look at every five nanometers, from ultraviolet to near infrared.” This means instead of six to nine wavelengths, PACE will capture more than 80.
One of the advantages of this increased spectral capability, Werdell explains, is that scientists will be better able to differentiate between the types of phytoplankton in the ocean.
“With the current instruments, we’re very good at estimating the total abundance of all phytoplankton in the upper water column, but the wavelengths are spaced in a way that you don’t see the full phytoplankton absorption spectra,” he says.
“With PACE … we can start looking at differences in how multiple kinds of phytoplankton absorb light across the full spectrum. With that information, we expect to be able to estimate the particular kinds of phytoplankton in the water.”
Different types of phytoplankton have their own way of responding to environmental changes and play different roles in the marine food web, so understanding their changing distributions and abundances will give scientists more pieces of information on how Earth’s living marine resources are responding to a changing climate.
PACE will also gather data on properties of the atmosphere, specifically clouds and microscopic particles like dust, smoke, and anthropogenic emissions. These measurements, made by the ocean color sensor and a likely separate instrument, a polarimeter (similar to that of the French PARASOL mission), will help scientists better understand how the myriad of substances in the atmosphere influence ocean ecosystems, and how biological processes in the ocean affect the atmosphere.
Werdell says that the quality and synthesis of ocean and atmospheric measurements that PACE will deliver will “be a substantial leap forward in what both the cloud and aerosols and the ocean color communities can do” and will more strongly encourage those communities to work collaboratively.
“The possibilities are very exciting,” he says.
Geostationary Coastal and Air Pollution Events (GEO-CAPE)
Coastal ocean environments are particularly important to monitor and understand for several reasons. Due to increasingly dense population levels and many economic resources, human impact on coastal areas is large.
Coastal regions also are exposed to a variety of hazards like storms, floods, sea level rise, and pollution, and contain higher levels of phytoplankton, sediments, and dissolved organic matter in comparison to the open ocean.
GEO-CAPE is a NASA mission designed to observe these coastal ocean dynamics, focusing on coastal ecosystem health and air quality. GEO-CAPE will be in geostationary orbit, which differs significantly from the polar orbit of PACE and other ocean color satellites. Polar orbiting satellites operate at about 700 km altitude and cover most of the Earth every two days. Geostationary satellites function at around 36,000 km, and remain stationary with respect to a desired location on the surface (see here for more information on satellite orbits).
GEO-CAPE will be NASA’s first ever ocean color mission to observe from geostationary orbit (the only other in the world is the Geostationary Ocean Color Imager launched in 2010 by the Korea Ocean Satellite Center).
Antonio Mannino, a research oceanographer in the Ocean Ecology Laboratory at NASA Goddard, says this distinction is important to the objectives of GEO-CAPE.
“Polar orbiting satellite missions, like PACE, or Terra and Aqua (which have the MODIS instrument), generally view the same location about once per day,” Mannino says.
“Having a satellite in geostationary orbit provides the capability to scan any portion of the Earth within view of the satellite multiple times during the course of a day.”
According to Mannino, this difference is critical when it comes to monitoring things like algal blooms, river discharge, or pollution events such as oil spills, which occur continuously and on relatively short time-scales.
“If you’re trying to look at a phytoplankton bloom, you might get the middle, you might get the end, you might get the beginning, but you don’t really see it forming, reaching its peak, and then dissipating from a polar orbiter,” he states. This is because of cloud cover and a limited number of opportunities to observe the Earth.
Frequent imaging will enable scientists to track the dispersion of these types of coastal phenomena, gaining a better sense of ecological and environmental impacts while serving more practical purposes like enhanced mitigation and clean up and more accurate forecasts and models.
Like PACE, GEO-CAPE will be equipped with hyperspectral imaging technology.
Two separate spectrometers will measure natural and human-produced substances in the ocean, rivers, and the atmosphere at high and low resolutions (continental and regional scales) and will serve to identify the sources and the transport of these substances. A third instrument, an infrared radiometer, will measure airborne carbon monoxide.
Mannino says that GEO-CAPE will be located near the Galapagos Islands, near the mid section of the continental U.S. so that it can view the East and West coasts of the U.S. throughout the entire day. Its range of view will cover most of North and South America, from the eastern tip of Brazil to the Hawaiian Islands.
GEO-CAPE is still in development, and is expected to launch no earlier than 2023. The Tropospheric Emissions: Monitoring of Pollution (TEMPO) mission, which is being built by the Smithsonian Astrophysical Observatory (SAO) and Ball Aerospace & Technologies Corp. (BATC) and managed by NASA’s Langley Research Center, will make all the requisite UV/visible atmospheric measurements of GEO-CAPE.
TEMPO will be the first space-based instrument to monitor air pollutants from geostationary orbit across the North American continent and will be a hosted payload on a commercial satellite, which is also the likely scenario for GEO-CAPE. TEMPO is scheduled for launch in 2019.
The continuation and enhancement of ocean color science in the future will address the expanding needs of the scientific community while creating new opportunities for observing and understanding the ocean biosphere and long-term trends in global climate. The knowledge gained from this international endeavor will be extremely valuable in learning about and managing our planet going forward.
A complete list of scheduled international ocean color sensors can be found here.