Looking Deeper At Phytoplankton from Space

NASA is currently in the middle of a joint airborne and sea campaign to study the ocean and atmosphere in preparation for developing instruments for future spaceborne missions. The Ship-Aircraft Bio-Optical Research (SABOR) campaign has brought together experts from a variety of disciples to focus on the issue of the polarization of light in the ocean; it runs from 17th July to 7th August and will co-ordinate ocean measurements with overflights.

One of the instruments on SABOR is an airborne Lidar-Polarimeter aimed at overcoming the limitation of vertically integrated surface measurements as captured by many existing Earth Observation satellites. These traditional satellites measure the water-leaving radiance, which is the signal returned from an area of water; the problem is that the signal is returned from a variety of different depths and it’s then aggregated to provide a single vertically integrated measurement for that area.

Diffuse attenuation depth at 490 nm, Kd(490), created from the SeaWiFS mission climatological data; data products retrieved from http://oceancolor.gsfc.nasa.gov/

Diffuse attenuation depth at 490 nm, Kd(490), created from the SeaWiFS mission climatological data; data products retrieved from http://oceancolor.gsfc.nasa.gov/

In effect, this means that a phytoplankon bloom at the surface will show up as a strong concentration on an image, however the same bloom at a deeper depth will show as having lower concentrations. The figure on the right shows the diffuse attenuation depth at 490 nm, blue light, created from the SeaWiFS mission climatological data collected between 1997 and 2010; the higher the value the shallower the depth of maximum passive light penetration. So, in summary, the light penetrates further within the open ocean than in many coastal waters that are more turbid.

The SABOR Lidar is based on lasers and will provide depth-resolved profiles, so instead of having a single value for an area of water, the measurements will be separable for different depths; expected to penetrate to around 50m. This will enable a much more detailed analysis of what’s happening within the water column. Satellite Lidar measurements have already been used to provide initial insights into the scattering of light resulting from phytoplankton through the CALIPSO satellite, an atmospheric focused Lidar mission launched in 2006.

In addition, the polarimeter element of SABOR will improve the quantification of the in-water constituents, such as the concentration of Chlorophyll-a (the primary pigment in most phytoplankton as well as land based plants) plus an understanding of the marine aerosols and clouds. Polarimeters have been launched before with the POLDER/PARASOL missions being examples.

The SABOR campaign will provide valuable information to support a proposal to have an Ocean Profiling Atmospheric Lidar (OPAL) deployed from the International Space Station (ISS) in 2015. If successful, it will join the existing Earth Observation mission on the ISS, called the Hyperspectral Imager for the Coastal Ocean (HICO), which I discussed in an earlier blog.

The potential offered by depth profiled oceanic measurements is exciting and will offer much more granularity beyond the ocean’s surface. I’m looking forward to the campaign’s results.