Communications companies like Starlink plan to launch tens of thousands of satellites into Earth orbit over the next decade or so. The growing swarm is already causing problems for astronomers, but recent research has raised another question: what happens when they start to descend?
When these satellites reach the end of their useful lives, they will fall into the Earth’s atmosphere and burn up. Along the way, they will leave a trail of small metal particles.
According to a study published last week by a team of American researchers, this satellite rain can throw 360 tons of tiny aluminum oxide particles into the atmosphere every year.
The aluminum will mostly be injected at altitudes between 50 and 85 kilometers, but will then descend into the stratosphere – home of Earth’s protective ozone layer.
What will she say? According to the study, satellite tracks can facilitate chemical reactions that destroy ozone. This is not wrong, but as we shall see, the story is far from simple.
How is ozone destroyed?
Ozone loss in the stratosphere is caused by “free radicals” – atoms or molecules with a free electron. When radicals are produced, they start cycles that destroy many ozone molecules. (These cycles have names that Dr. Seuss would admire: NOx, HOx, ClOx, and BrOx, since they all involve oxygen, as well as nitrogen, hydrogen, chlorine, and bromine, respectively.)
These radicals are created when stable gases are broken down by ultraviolet light, which is abundant in the stratosphere.
Nitrogen oxides (NOx) start with nitrogen oxide. This is a greenhouse gas produced naturally by microbes, but human manure production and agriculture have increased the amount in the air.
The HOx cycle involves hydrogen radicals from water vapor. Not much water vapor enters the stratosphere, although events such as the 2022 Hunga Tonga–Hunga Ha’apai submarine volcanic eruption can sometimes inject large amounts.
Water in the stratosphere creates many tiny aerosol particles, which create a large surface area for chemical reactions and also scatter more light to make beautiful sunsets. (I will return to both of these points later.)
How CFCs made the ‘ozone hole’
ClOx and BrOx are the cycles responsible for the most famous damage to the ozone layer: the “ozone hole” caused by chlorofluorocarbons (CFCs) and halons. These chemicals, now banned, were commonly used in refrigerators and fire extinguishers and introduced chlorine and bromine into the stratosphere.
CFCs rapidly release chlorine radicals into the stratosphere. However, this reactive chlorine is quickly neutralized and locked into molecules with nitrogen and water radicals.
What happens next depends on the aerosols in the stratosphere, and near the poles it also depends on the clouds.
Aerosols accelerate chemical reactions by providing a surface on which they can occur. As a result, aerosols in the stratosphere release reactive chlorine (and bromine). Stratospheric polar clouds also remove water and nitrogen oxides from the air.
So in general, when there are more stratospheric aerosols around, we are likely to see more ozone loss.
An increasingly metallic stratosphere
The details of the specific injection of aluminum oxides from falling satellites would be quite complex. This is not the first study to highlight the increase in stratospheric pollution from space debris reentry.
In 2023, researchers studying aerosol particles in the stratosphere detected traces of metals from the spacecraft’s reentry. They found that 10 percent of stratospheric aerosols already contain aluminum and predicted that this will increase to 50 percent over the next 10-30 years. (About 50 percent of stratospheric aerosol particles already contain metals from meteorites.)
We don’t know what effect this will have. A possible result would be that aluminum particles seed the growth of ice-containing particles. This means there would be more smaller, cooler, reflective particles with more surface area on which chemistry can occur.
We also don’t know how the aluminum particles will interact with the sulfuric acid, nitric acid and water found in the stratosphere. As a result, we cannot really say what the consequences will be for ozone loss.
Learning from volcanoes
To really understand what these aluminum oxides mean for ozone loss, we need laboratory studies, to model the chemistry in more detail, and also to see how the particles would move in the atmosphere.
For example, after the Hunga Tonga–Hunga Ha’apai eruption, water vapor in the stratosphere was quickly mixed around the Southern Hemisphere and then moved poleward. At first, this extra water caused intense sunsets, but a year later, these water aerosols have diluted well across the southern hemisphere and we no longer see them.
A global current called the Brewer-Dobson circulation moves air up into the stratosphere near the equator and back down again at the poles. As a result, aerosols and gases can stay in the stratosphere for no more than six years. (Climate change is speeding up this circulation, meaning the time that aerosols and gases are in the stratosphere is shorter.)
The famous eruption of Mount Pinatubo in 1991 also created beautiful sunsets. It injected more than 15 million tons of sulfur dioxide into the stratosphere, which cooled the Earth’s surface by just over half a degree Celsius over about three years. This event is the inspiration for geoengineering proposals to slow climate change by deliberately placing sulfate aerosols in the stratosphere.
Many questions remain
Compared to Pinatubo’s 15 million tons, 360 tons of aluminum oxide look like small potatoes.
However, we do not know how aluminum oxides will behave physically under stratospheric conditions. Will it make aerosols that are smaller and more reflective – thus cooling the surface, much like geoengineering scenarios of stratospheric aerosol injection?
We also don’t know how aluminum will behave chemically. Will it nucleate ice? How will it react with nitric and sulfuric acid? Will it release trapped chlorine more effectively than current stratospheric aerosols, easing ozone depletion?
And of course, aluminum aerosols won’t stay in the stratosphere forever. When they eventually fall to earth, what will this metal contamination do to our polar regions?
All these questions need to be addressed. By some estimates, more than 50,000 satellites could be launched between now and 2030, so we’d better address them quickly.
Robyn Schofield, Associate Professor and Associate Dean (Environment and Sustainability), University of Melbourne
This article is republished from The Conversation under a Creative Commons license. Read the original article.