Researchers find ingenious solution to map ocean plastics from orbit

This makes them especially hard to track, even as the potential long-term health concerns for marine and animal life that ingest them, including humans, especially pressing. But how do you map the location of a patch of 5mm plastic particles in the ocean when 70% of the planet is covered in water?

That is the challenge that University of Michigan (UM) marine engineer Yulin Pan and his colleagues set out to solve, and with the help of NASA's Cyclone Global Navigation Satellite System (CYGNSS), it looks like they've found a real, effective mapping solution that can significantly help in ocean remediation and cleanup efforts.

“In general, there are insufficient data about microplastics concentration in the ocean,” Pan said, according to Scientific American. “That is one of the reasons that we really want a remote sensing technique, to have a general understanding.”

This is especially needed since the current method, trawling the ocean with nets to detect microplastic levels, is tragicomically insufficient to the scope of the problem. The solution that the CYGNSS researchers developed, described in a new paper in Scientific Reports, takes a much wider view than an ocean-going trawler, instead taking a 300-mile satellite's-eye view of the problem.

Originally designed to track wind over the oceans to help scientists better understand how hurricanes form and predict their development, CYGNSS data provided a key insight for the researchers in 2021, who saw that data from the system showed an unexpected smoothness in the waves of an area of the Pacific Ocean they were not expecting.

This area of smoothness was later found to line up with the location of the Great Pacific Garbage Patch, and they were able to find other plastic concentrations in a similar fashion but were unable to figure out what was creating the smoothness they were seeing.

“The original idea for this detection technique really arose from a related technique that’s used with satellites to track oil spills,” UM Climate and Space Professor Christopher Ruf said last year. “They do that by looking at how the presence of the oil suppresses the roughness of the ocean, so we just expanded on that idea.”

“The wind over the water causes these little ripples and you can easily see them with radars and satellites,” Ruf added. “We looked at how windy it was using other satellite measurements and predicted from that how rough the surface should be, and then we made direct measurements of how rough it actually was with those satellites.

"The difference between those is caused by something suppressing the roughening of the ocean. We hypothesized that was that presence of microplastics, because that’s a similar type of physical mechanism as happens with oil spills.”

Isolating the cause to identify a microplastic marker

Correlation doesn't equal causation, obviously, so the researchers needed to confirm that the plastics in the ocean were creating a smoother ocean surface and not some other cause like marine life or other chemicals was responsible.

Using a 750,000-gallon water tank to simulate the kind of real-world ocean currents that carry microplastics, the researchers found that microplastics alone did not create the smoothing that they were seeing, but after they added surfactants—which are chemical byproducts of the production of the plastics themselves and are released as plastics break down—the surface smoothing appeared.

Surfactants decrease the surface tension of water, and surface tension is how waves are formed. So, the surfactants themselves become a marker for the presence of microplastics, since plastics breaking down in the ocean will continue to release surfactants that will then get carried along on the same current as the plastics.

That's the idea anyway. Final confirmation will come over the next few months when researchers will use the data to predict the concentration of microplastics at select spots in the Great Pacific Garbage Patch and compare it with ocean samples collected by NOAA research vessels. The comparison between satellite data and actual ocean samples should help cement the method as an effective map for cleanup efforts going forward.