The ocean is full of tiny plastic particles – we found a way to track them with satellites

Plastic fragments washed onto Schiavonea beach in Calabria, Italy, in a 2019 storm.
Alfonso Di Vincenzo/KONTROLAB /LightRocket via Getty Images

Christopher Ruf, University of MichiganPlastic is the most common type of debris floating in the world’s oceans. Waves and sunlight break much of it down into smaller particles called microplastics – fragments less than 5 millimeters across, roughly the size of a sesame seed.

To understand how microplastic pollution is affecting the ocean, scientists need to know how much is there and where it is accumulating. Most data on microplastic concentrations comes from commercial and research ships that tow plankton nets – long, cone-shaped nets with very fine mesh designed for collecting marine microorganisms.

But net trawling can sample only small areas and may be underestimating true plastic concentrations. Except in the North Atlantic and North Pacific gyres – large zones where ocean currents rotate, collecting floating debris – scientists have done very little sampling for microplastics. And there is scant information about how these particles’ concentrations vary over time.

Two people lower conical nets off a research ship into the water.
Researchers deploy plankton sampling nets in Lake Michigan.

To address these questions, University of Michigan research assistant Madeline Evans and I developed a new way to detect microplastic concentrations from space using NASA’s Cyclone Global Navigation Satellite System. CYGNSS is a network of eight microsatellites that was launched in 2016 to help scientists predict hurricanes by analyzing tropical wind speeds. They measure how wind roughens the ocean’s surface – an indicator that we realized could also be used to detect and track large quantities of microplastics.

This story is part of Oceans 21

Our series on the global ocean opened with five in depth profiles. Look out for new articles on the state of our oceans in the lead up to the UN’s next climate conference, COP26. The series is brought to you by The Conversation’s international network.

Looking for smooth zones

Annual global production of plastic has increased every year since the 1950s, reaching 359 million metric tons in 2018. Much of it ends up in open, uncontrolled landfills, where it can wash into river drainage zones and ultimately into the world’s oceans.

Researchers first documented plastic debris in the oceans in the 1970s. Today, it accounts for an estimated 80% to 85% of marine litter.

The radars on CYGNSS satellites are designed to measure winds over the ocean indirectly by measuring how they roughen the water’s surface. We knew that when there is a lot of material floating in the water, winds don’t roughen it as much. So we tried computing how much smoother measurements indicated the surface was than it should have been if winds of the same speed were blowing across clear water.

This anomaly – the “missing roughness” – turns out to be highly correlated with the concentration of microplastics near the ocean surface. Put another way, areas where surface waters appear to be unusually smooth frequently contain high concentrations of microplastics. The smoothness could be caused by the microplastics themselves, or possibly by something else that’s associated with them.

By combining all the measurements made by CYGNSS satellites as they orbit around the world, we can create global time-lapse images of ocean microplastic concentrations. Our images readily identify the Great Pacific Garbage Patch and secondary regions of high microplastic concentration in the North Atlantic and the southern oceans.

Tracking microplastic flows over time

Since CYGNSS tracks wind speeds constantly, it lets us see how microplastic concentrations change over time. By animating a year’s worth of images, we revealed seasonal variations that were not previously known.

This animation shows how satellite data can be used to track where microplastics enter the water, how they move and where they tend to collect.

We found that global microplastic concentrations tend to peak in the North Atlantic and Pacific during the Northern Hemisphere’s summer months. June and July, for example, are the peak months for the Great Pacific Garbage Patch.

Concentrations in the Southern Hemisphere peak during its summer months of January and February. Lower concentrations during the winter in both hemispheres are likely due to a combination of stronger currents that break up microplastic plumes and increased vertical mixing – the exchange between surface and deeper water – that transports some of the microplastic down below the surface.

This approach can also target smaller regions over shorter periods of time. For example, we examined episodic outflow events from the mouths of the China’s Yangtze and Qiantang rivers where they empty into the East China Sea. These events may have been associated with increases in industrial production activity, or with increases in the rate at which managers allowed the rivers to flow through dams.

Satellite images, color-coded to show density of microplastic particles in the water.
These images show microplastic concentrations (number of particles per square kilometer) at the mouths of the Yangtze and Qiantang rivers where they empty in to the East China Sea. (A) Average density year-round; (B) short-lived burst of particles from the Qiantang River; (C and D) short-lived bursts from the Yangtze River.
Evans and Ruf, 2021., CC BY

Better targeting for cleanups

Our research has several potential uses. Private organizations, such as The Ocean Cleanup, a nonprofit in The Netherlands, and Clewat, a Finnish company specializing in clean technology, use specially outfitted ships to collect, recycle and dispose of marine litter and debris. We have begun conversations with both groups and hope eventually to help them deploy their fleets more effectively.

Our spaceborne imagery may also be used to validate and improve numerical prediction models that attempt to track how microplastics move through the oceans using ocean circulation patterns. Scholars are developing several such models.

Large barge with conveyor belt pulling plastic debris out of river.
A solar-powered barge that filters plastic out of water, designed by Dutch NGO The Ocean Cleanup, deployed in the Rio Ozama, Dominican Republic, in 2020.
The Ocean Cleanup, CC BY

While the ocean roughness anomalies that we observed correlate strongly with microplastic concentrations, our estimates of concentration are based on the correlations that we observed, not on a known physical relationship between floating microplastics and ocean roughness. It could be that the roughness anomalies are caused by something else that is also correlated with the presence of microplastics.

One possibility is surfactants on the ocean surface. These liquid chemical compounds, which are widely used in detergents and other products, move through the oceans in ways similar to microplastics, and they also have a damping effect on wind-driven ocean roughening.

Further study is needed to identify how the smooth areas that we identified occur, and if they are caused indirectly by surfactants, to better understand exactly how their transport mechanisms are related to those of microplastics. But I hope this research can be part of a fundamental change in tracking and managing microplastic pollution.

[The Conversation’s science, health and technology editors pick their favorite stories. Weekly on Wednesdays.]The Conversation

Christopher Ruf, Professor of Climate and Space Sciences and Engineering, University of Michigan

This article is republished from The Conversation under a Creative Commons license. Read the original article.


Lights in the sky from Elon Musk’s new satellite network have stargazers worried

The panel of 60 Starlink satellites just before they were released to go into orbit around Earth.
Official SpaceX Photos

Michael J. I. Brown, Monash University

UFOs over Cairns. Lights over Leiden. Glints above Seattle. What’s going on?

Starlink satellites travel silently across the skies of Leiden.

The launch of 60 Starlink satellites by Elon Musk’s SpaceX has grabbed the attention of people around the globe. The satellites are part of a fleet that is intended to provide fast internet across the world.

Improved internet services sound great, and Musk is reported to be planning for up to 12,000 satellites in low Earth orbit. But this fleet of satellites could forever change our view of the heavens.

Will we lose the night sky to city lights and satellites?
Jeff Sullivan, CC BY-NC-ND

Starlink’s ambitious mission

Starlink is an ambitious plan to use satellites in low Earth orbit (about 500km up) to provide global internet services.

Read more:
What caused the fireballs that lit up the sky over Australia?

This is different from the approach previously used for most communication satellites, in which larger individual satellites were placed in high geosynchronous orbits – that stay in an apparently fixed position above the Equator (about 36,000km up).

Communications with satellites in geosynchronous orbits often require satellite dishes, which you can see on the sides of residential apartment buildings. Communication with satellites in low Earth orbit, which are much closer, won’t require such bulky equipment.

But the catch with satellites in low Earth orbit, which move quickly around the world, is they can only look down on a small fraction of the globe, so to get global coverage you need many satellites. The Iridium satellite network used this approach in the 1990s, using dozens of satellites to provide global phone and data services.

Starlink is far more ambitious, with 1,600 satellites in the first phase, increasing to 12,000 satellites during the mid-2020s. For comparison, there are roughly 18,000 objects in Earth orbit that are tracked, including about 2,000 functioning satellites.

Lights in the sky

It’s not unusual to see satellites travelling across the twilight sky. Indeed, there’s a certain thrill to seeing the International Space Station pass overhead, and to know there are people living on board that distant light. But Starlink is something else.

The first 60 satellites, launched by SpaceX last week, were seen travelling in procession across the night sky. Some people knew what they were seeing, but the silent procession of light also generated UFO reports. If you’re lucky, you may see them pass across your skies tonight.

If the full constellation of satellites is launched, hundreds of Starlink satellites will be above the horizon at any given time. If they are visible to the unaided eye, as suggested by initial reports, they could outnumber the brightest natural stars visible to the unaided eye.

Astronomers’ fears were not put to rest by Musk’s tweets:

Satellites are very definitely visible at night, particularly in the hours before dawn and after sunset, as they are high enough to be illuminated by the Sun. The Space Station’s artificial lighting is effectively irrelevant to its visibility.

In areas near the poles, including Canada and northern Europe, satellites in low Earth orbit can be illuminated throughout the night during the summer months.

Hundreds of satellites being visible to the unaided eye would be a disaster. They would completely ruin our view of the night sky. They would also contaminate astronomical images, leaving long trails across otherwise unblemished images.

The US$466 million Large Synoptic Survey Telescope, based in Chile, is an 8-metre aperture telescope with a 3,200-megapixel camera. It’s designed to rapidly survey the sky during the 2020s.

With the full constellation of Starlink satellites, many images taken with this telescope will contain a Starlink satellite. Longer exposures could contain dozens of satellite streaks.

Dark skies or darkened hopes?

Is there any cause for optimism? Yes and no.

Musk has produced some amazing feats of technology, such as the SpaceX Falcon and Tesla cars, but he’s also disappointed some on other projects, such as the Hyperloop tunnel transport plan.

Read more:
A guide to ensure everyone plays by the same military rules in space: the Woomera Manual

While Starlink certainly blew up on Twitter, for now at least, Musk is 11,940 satellites short of his 12,000.

Also, initial reports may have overestimated the brightness of the Starlink satellites, with the multiple satellites closely clustered together being confused with one satellite.

While some reports have indicate binoculars are needed to see the individual satellites, they also report that Starlink satellites flare, momentarily becoming brighter than any natural star.

If the individual satellites usually are too faint to be seen with the unaided eye, that would at least preserve the natural wonder of the sky. But professional astronomers like myself may need to prepare for streaky skies ahead. I can’t say I’m looking forward to that.The Conversation

Michael J. I. Brown, Associate professor in astronomy, Monash University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Satellites reveal melting of rocks under volcanic zone, deep in Earth’s mantle

File 20170705 9733 ityqvm
Mount Ngauruhoe, in the foreground, and Mount Ruapehu are two of the active volcanoes in the Taupo volcanic zone.
Guillaume Piolle/Wikimedia Commons, CC BY-ND

Simon Lamb, Victoria University of Wellington and Timothy Stern, Victoria University of Wellington

Volcanoes erupt when magma rises through cracks in the Earth’s crust, but the exact processes that lead to the melting of rocks in the Earth’s mantle below are difficult to study.

In our paper, published today in the journal Nature, we show how it is possible to use satellite measurements of movements of the Earth’s surface to observe the melting process deep below New Zealand’s central North Island, one of the world’s most active volcanic regions.

Rifting in the Taupo volcanic zone

The solid outer layer of the Earth is known as the crust, and this overlies the Earth’s mantle. But these layers are not fixed. They are broken up into tectonic plates that slowly move relative to each other.

It is along the boundaries of the tectonic plates that most of the geological action at the Earth’s surface occurs, such as earthquakes, volcanic activity and mountain building. This makes New Zealand a particularly dynamic place, geologically speaking, because it straddles the boundary between the Australian and Pacific plates.

The central region of the North Island is known as the Taupo volcanic zone, or TVZ. It is named after Lake Taupo, the flooded crater of the region’s largest volcano, and it has been active for two million years. Several volcanoes continue to erupt regularly.

The TVZ is the southern tip of a zone of expansion, or rifting, in the Earth’s crust that extends offshore for thousands of kilometres, all the way north in the Pacific Ocean to Tonga. Offshore, this takes place through sea floor spreading in the Havre Trough, creating both new oceanic crust and a narrow sliver of a plate right along the edge of the Australian tectonic plate. Surprisingly, this spreading is going on at the same time as the adjacent Pacific tectonic plate is sliding beneath the Australian plate in a subduction zone, triggering some of the major earthquakes in the region.

Sea floor spreading results in melting of the Earth’s mantle, but it is very difficult to observe this process directly in the deep ocean. However, sea floor spreading in the Havre Trough transitions abruptly onshore into the volcanic activity in the TVZ. This provides an opportunity to observe the melting in the Earth’s mantle on land.

Lake Taupo is the caldera of the region’s largest volcano.
NASA/Wikimedia Commons, CC BY-ND

In general, volcanic activity happens whenever there is molten rock at depth, and therefore the volcanism in the North Island indicates vast volumes of molten rock beneath the surface. However, it has been a tricky problem to understand exactly what is causing the melting in the first place, because the underlying rocks are buried by thick layers of volcanic material.

We have tackled this problem using data from Global Positioning System (GPS) sensors, some of which form part of New Zealand’s GeoNet network and some that have been used in measurement campaigns since 1995. The sensors measure horizontal and vertical shifts in the Earth’s surface to millimetre precision, and our research is based on data collected over the past two decades.

Bending of the earth’s surface

The GPS measurements in the Taupo volcanic zone reveal that it is widening east-west at a rate of 6-15 millimetres per year – in other words, the region, overall, is expanding, as we anticipated from our previous geological understanding. But it was surprising to discover that, at least for the past 15 years, a roughly 70-kilometre stretch is undergoing strong horizontal contraction and is also rapidly subsiding, quite the opposite of what one might anticipate.

Also unexpectedly, the contracting zone is surrounded by regions that are expanding, but also uplifting. Trying to make sense of these observations turned out to be the key to our new insight into the process of melting beneath the TVZ.

We found that the pattern of contraction and subsidence, together with expansion and uplift, in the context of the overall rifting of the TVZ, could be explained by a simple model that involves the bending and curving of an elastic upper crust, pulled downwards or pushed upwards by an underlying vertical driving force. The size of the region that is behaving like this, extending for about 100 kilometres in width and 200 kilometres in length, requires this force to originate nearly 20 kilometres underground, in the Earth’s mantle.

This diagram illustrates a patch of suction stress along the axis of the underlying upwelling mantle flow beneath the Taupo volcanic zone.
Simon Lamb, CC BY-ND

Melting the mantle

When tectonic plates drift apart on the sea floor, the underlying mantle rises up to fill the gap. This upwelling triggers melting, and the reason for this is that hot, but solid, mantle rocks undergo a reduction in pressure as they move upwards and closer to the Earth’s surface. This drop in pressure, rather than a change in temperature, begins the melting of the mantle.

But there is another property of this upwelling mantle flow, because it also creates a suction force that pulls down the overlying crust. This force comes about because as part of the flow, the rocks have to effectively “turn a corner” near the surface from a predominantly vertical flow to a predominantly horizontal one.

It turns out that the strength of this force depends on how stiff or sticky the mantle rocks are, measured in terms of viscosity (it is difficult to drive the flow of highly viscous or sticky fluids, but easy in runny ones).

Experimental studies have shown that the viscosity of rocks deep in the Earth is very sensitive to how much molten material they contain, and we propose that changes in the amount of melt provide a powerful mechanism to change the viscosity of the upwelling mantle. If mantle rocks don’t contain much melt, they will be much stickier, causing the overlying crust to be pulled down rapidly. If the rocks have just melted, then this makes the flow of the rocks runnier, allowing the overlying crust to spring back up again.

We also know that the movements that we observe at the surface with GPS must be relatively short lived, geologically speaking, lasting for no more than a few hundred or few thousand years. Otherwise they would result in profound changes to the landscape and we have no evidence for that.

Using GPS, we can not only measure the strength of the suction force, but we can “see” where, for how long, and by how much the underlying mantle is melting. This melt will eventually rise up through the crust to feed the overlying volcanoes.

This research helps us to understand how volcanic systems work on a variety of time scales, from human to geological. In fact, it may be that the GPS measurements made over just the last two decades have captured a change in the amount of mantle melt at depth, which could herald the onset of increased volcanic activity and associated risk in the future. But we don’t have measurements over a long enough time period yet to make any confident predictions.

The ConversationThe key point here is, nevertheless, that we have entered a new era whereby satellite measurements can be used to probe activity 20 kilometres beneath the Earth’s surface.

Simon Lamb, Associate Professor in Geophysics, Victoria University of Wellington and Timothy Stern, Professor of Geophysics, Victoria University of Wellington

This article was originally published on The Conversation. Read the original article.