Massive Antarctic ozone hole over past four years: What is to blame?

Despite public perception, the Antarctic ozone hole has been remarkably massive and long-lived over the past four years, University of Otago researchers believe chlorofluorocarbons(CFCs) aren't the only things to blame.

In a study, just published in Nature Communications, the group analysed the monthly and daily ozone changes, at different altitudes and latitudes within the Antarctic ozone hole, from 2004 to 2022.

Lead author Hannah Kessenich, PhD candidate in the Department of Physics, says they found there is much less ozone in the centre of the ozone hole compared to 19 years ago.

"This means that the hole is not only larger in area, but also deeper throughout most of spring.

"We made connections between this drop in ozone and changes in the air that is arriving into the polar vortex above Antarctica. This reveals the recent, large ozone holes may not be caused just by CFCs," she says.

While the Montreal Protocol on Substances that Deplete the Ozone Layer, which has been in place since 1987, regulates the production and consumption of human-made chemicals known to deplete the ozone, the researchers believe other complex factors are also contributing to the ozone hole.

"Most major communications about the ozone layer over the last few years have given the public the impression that the 'ozone issue' has been solved.

"While the Montreal Protocol has vastly improved our situation with CFCs destroying ozone, the hole has been amongst the largest on record over the past three years, and in two of the five years prior to that.

"Our analysis ended with data from 2022, but as of today the 2023 ozone hole has already surpassed the size of the three years prior -- late last month it was over 26 million km2, nearly twice the area of Antarctica."

Ms Kessenich believes understanding ozone variability is important because of the major role it plays in the Southern Hemisphere's climate.

"We all know about the recent wildfires and cyclones in Australia and New Zealand and the Antarctic ozone hole is part of this picture.

"While separate from the impact of greenhouse gases on climate, the ozone hole interacts with the delicate balance in the atmosphere. Because ozone usually absorbs UV light, a hole in the ozone layer can not only cause extreme UV levels on the surface of Antarctica, but it can also drastically impact where heat is stored in the atmosphere.

"Downstream effects include changes to the Southern Hemisphere's wind patterns and surface climate, which can impact us locally."

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Massive 2022 eruption reduced ozone layer levels

When the Hunga Tonga-Hunga Ha'apai volcano erupted on January 15, 2022 in the South Pacific, it produced a shock wave felt around the world and triggered tsunamis in Tonga, Fiji, New Zealand, Japan, Chile, Peru and the United States. It also changed the chemistry and dynamics of the stratosphere in the year following the eruption, leading to unprecedented losses in the ozone layer of up to 7% over large areas of the Southern Hemisphere, according to a recent study published in the Proceedings of the National Academy of Sciences (PNAS) from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the University of Maryland.

Driving those atmospheric changes, according to the research, was the sheer amount of water vapor injected into the stratosphere by the undersea volcano.

The location of the stratosphere is approximately 8 -- 30 miles above Earth's surface and is where the protective ozone layer resides.

"The Hunga Tonga-Hunga Ha'apai eruption was truly extraordinary in that it injected about 300 billion pounds of water into the normally dry stratosphere, which is just an absolutely incredible amount of water from a single event," said David Wilmouth, a project scientist at SEAS and first author of the paper.

"This eruption put us in uncharted territory," said Ross Salawitch, professor at the University of Maryland's Earth System Science Interdisciplinary Center and co-author of the study.

"We've never seen, in the history of satellite records, this much water vapor injected into the atmosphere and our paper is the first that looks at the downstream consequences over broad regions of both hemispheres in the months following the eruption using satellite data and a global model."

The Hunga Tonga-Hunga Ha'apai eruption was the largest explosion ever recorded in the atmosphere.

The eruption hurled aerosols and gases deep into the stratosphere.

Some material reached the lower mesosphere, more than 30 miles above the Earth's surface, altitudes never recorded from a volcanic eruption.

Previous studies found that the eruption increased water vapor in the stratosphere by 10% worldwide, with even higher concentrations in some areas of the Southern Hemisphere.

Wilmouth, Salawitch and the rest of the research team used data from the Microwave Limb Sounder (MLS) aboard the NASA Aura satellite, to track not only how that water vapor moved across the globe but also monitor temperature and levels of chlorine monoxide (ClO), ozone (O3), nitric acid (HNO3), and hydrogen chloride (HCl) in the stratosphere for the year following the eruption.

They then compared those measurements to data collected by MLS from 2005 to 2021 prior to the eruption.

The team found that the injection of water vapor and sulfur dioxide (SO2) changed both the chemistry and the dynamics of the stratosphere.

In terms of chemistry, the SO2 led to an increase of sulfate aerosols, which provided new surfaces for chemical reactions to occur.

"Certain reactions that might not happen at all or only happen slowly can happen faster if there are aerosols available on which those reactions can take place," said Wilmouth.

"The injection of SO2 from the volcano allowed sulfate aerosols to form and the presence of water vapor led to the additional production of sulfate aerosols."

The increased sulfate aerosols and water vapor kicked off a chain of events in the complex atmospheric chemistry that led to widespread changes in the concentrations of a number of compounds, including ozone.

The extra water vapor also had a cooling effect in the stratosphere, leading to a change in circulation, which drove decreases in ozone in the southern hemisphere and an increase of ozone over the tropics.

The researchers found that the peak decrease in ozone occurred in October, nine months after the eruption.

"We had this enormous increase in water vapor in the stratosphere with modest increases in sulfate that set off a series of events that led to significant changes in temperature and circulation, ClO, HNO3, HCl, O3, and other gases," Wilmouth said.

Next, the researchers hope to continue the study by following the impact of the volcano into 2023 and beyond as the water vapor moves from the tropics and midlatitudes to the Southern Hemisphere pole, where it has the potential to amplify ozone losses in the Antarctic.

The water vapor is expected to stay elevated in the stratosphere for a period of several years.

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Smoke particles from wildfires can erode the ozone layer

A wildfire can pump smoke up into the stratosphere, where the particles drift for over a year. A new MIT study has found that while suspended there these particles can trigger chemical reactions that erode the protective ozone layer shielding the Earth from the sun's damaging ultraviolet radiation.

The study, which will appear in Nature, focuses on the smoke from the "Black Summer" megafire in eastern Australia, which burned from December 2019 into January 2020. The fires -- the country's most devastating on record -- scorched tens of millions of acres and pumped more than 1 million tons of smoke into the atmosphere.

The MIT team identified a new chemical reaction by which smoke particles from the Australian wildfires made ozone depletion worse. By triggering this reaction, the fires likely contributed to a 3-5 percent depletion of total ozone at mid-latitudes in the southern hemisphere, in regions overlying Australia, New Zealand, and parts of Africa and South America.

The researchers' model also indicates the fires had an effect in the polar regions, eating away at the edges of the ozone hole over Antarctica. By late 2020, smoke particles from the Australian wildfires widened the Antarctic ozone hole by 2.5 million square kilometers -- 10 percent of its area compared to the previous year.

It's unclear what long-term effect wildfires will have on ozone recovery. The United Nations recently reported that the ozone hole, and ozone depletion around the world, is on a recovery track, thanks to a sustained international effort to phase out ozone-depleting chemicals. But the MIT study suggests that as long as these chemicals persist in the atmosphere, large fires could spark a reaction that temporarily depletes ozone.

"The Australian fires of 2020 were really a wake-up call for the science community," says Susan Solomon, the Lee and Geraldine Martin Professor of Environmental Studies at MIT and a leading climate scientist who first identified the chemicals responsible for the Antarctic ozone hole. "The effect of wildfires was not previously accounted for in [projections of] ozone recovery. And I think that effect may depend on whether fires become more frequent and intense as the planet warms."

The study is led by Solomon and MIT graduate student Peidong Wang, along with collaborators from the Institute for Environmental and Climate Research in Guangzhou, China, the National Oceanic and Atmospheric Administration, the National Center for Atmospheric Research, and Colorado State University.

Chlorine cascade

The new study expands on a 2022 discovery by Solomon and her colleagues, in which they first identified a chemical link between wildfires and ozone depletion. The researchers found that chlorine-containing compounds, originally emitted by factories in the form of chlorofluorocarbons (CFCs), could react with the surface of fire aerosols. This interaction, they found, set off a chemical cascade that produced chlorine monoxide -- the ultimate ozone-depleting molecule. Their results showed that the Australian wildfires likely depleted ozone through this newly identified chemical reaction.

"But that didn't explain all the changes that were observed in the stratosphere," Solomon says. "There was a whole bunch of chlorine-related chemistry that was totally out of whack."

In the new study, the team took a closer look at the composition of molecules in the stratosphere following the Australian wildfires. They combed through three independent sets of satellite data and observed that in the months following the fires, concentrations of hydrochloric acid dropped significantly at mid-latitudes, while chlorine monoxide spiked.

Hydrochloric acid (HCl) is present in the stratosphere as CFCs break down naturally over time. As long as chlorine is bound in the form of HCl, it doesn't have a chance to destroy ozone. But if HCl breaks apart, chlorine can react with oxygen to form ozone-depleting chlorine monoxide.

In the polar regions, HCl can break apart when it interacts with the surface of cloud particles at frigid temperatures of about 155 Kelvin. However, this reaction was not expected to occur at mid-latitudes, where temperatures are much warmer.

"The fact that HCl at mid-latitudes dropped by this unprecedented amount was to me kind of a danger signal," Solomon says.

She wondered: What if HCl could also interact with smoke particles, at warmer temperatures and in a way that released chlorine to destroy ozone? If such a reaction was possible, it would explain the imbalance of molecules and much of the ozone depletion observed following the Australian wildfires.

Smoky drift

Solomon and her colleagues dug through the chemical literature to see what sort of organic molecules could react with HCl at warmer temperatures to break it apart.

"Lo and behold, I learned that HCl is extremely soluble in a whole broad range of organic species," Solomon says. "It likes to glom on to lots of compounds."

The question then, was whether the Australian wildfires released any of those compounds that could have triggered HCl's breakup and any subsequent depletion of ozone. When the team looked at the composition of smoke particles in the first days after the fires, the picture was anything but clear.

"I looked at that stuff and threw up my hands and thought, there's so much stuff in there, how am I ever going to figure this out?" Solomon recalls. "But then I realized it had actually taken some weeks before you saw the HCl drop, so you really need to look at the data on aged wildfire particles."

When the team expanded their search, they found that smoke particles persisted over months, circulating in the stratosphere at mid-latitudes, in the same regions and times when concentrations of HCl dropped.

"It's the aged smoke particles that really take up a lot of the HCl," Solomon says. "And then you get, amazingly, the same reactions that you get in the ozone hole, but over mid-latitudes, at much warmer temperatures."

When the team incorporated this new chemical reaction into a model of atmospheric chemistry, and simulated the conditions of the Australian wildfires, they observed a 5 percent depletion of ozone throughout the stratosphere at mid-latitudes, and a 10 percent widening of the ozone hole over Antarctica.

The reaction with HCl is likely the main pathway by which wildfires can deplete ozone. But Solomon guesses there may be other chlorine-containing compounds drifting in the stratosphere, that wildfires could unlock.

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Underlying assumptions of air quality need to be redefined

The 40-meter-high monitoring tower of the Innsbruck Atmospheric Observatory near the city center of Innsbruck in Austria, Europe, continuously provides data on the composition of the atmosphere near the surface. Every hour, 36,000 data points are recorded. Using a special measuring method -- the so-called eddy covariance method -- the concentration of air components can be continuously monitored. An international team led by Thomas Karl from the Department of Atmospheric and Cryospheric Sciences at the University of Innsbruck has now used these data to study the chemistry of ozone, nitrogen monoxide and nitrogen dioxide in urban areas in detail. The high proportion of diesel vehicles in European cities leads to strong concentrations of nitrogen monoxide. This reacts with ozone to produce nitrogen dioxide. In the atmosphere, nitrogen dioxide decomposes again to nitrogen monoxide and atomic oxygen, which immediately combines with atmospheric oxygen to form ozone.

Common assumptionneeds to be refined

This chemical cycle was described mathematically over 60 years ago in the first air pollution textbook by Philip Leighton. The relationship between the two processes has since been referred to as the Leighton ratio. Computer models of atmospheric chemistry use the Leighton ratio to minimize complexity by deriving the concentration of ozone, nitric oxide, and nitrogen dioxide from the concentration of each of the other two. In practice, this has been used, for example, to derive ozone concentrations in areas polluted by nitrogen oxides. The Innsbruck atmospheric researchers' data now show that in the presence of high nitrogen monoxide emissions, computational simplifications made by Leighton lead to incorrect results. Thomas Karl points out that "in cities with high nitrogen monoxide emissions, this ratio can be overestimated by up to 50 percent, which can lead to model calculations overestimating ground-level ozone concentrations in urban areas." The effect of chemistry -- turbulence interactions plays a significant role in the lowest layer of the atmosphere, up to 200 meters above the ground.

Responsible for the effect studied in Innsbruck is the combination of strong turbulence in urban areas in the presence of high nitrogen monoxide emissions. . The mixing of the gases combined with the relatively rapid chemical processes lead to more ozone being converted into nitrogen dioxide. The researchers' data also show that direct emissions of nitrogen dioxide from urban traffic are largely negligible in comparison to secondary formation. "It remains important to note that environmental regulations do not rely on model calculations but come into effect depending on actual measured pollutant concentrations," Thomas Karl emphasized.

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Ozone hole continues shrinking in 2022, NASA and NOAA scientists say

The annual Antarctic ozone hole reached an average area of 8.9 million square miles (23.2 million square kilometers) between Sept. 7 and Oct. 13, 2022. This depleted area of the ozone layer over the South Pole was slightly smaller than last year and generally continued the overall shrinking trend of recent years.

"Over time, steady progress is being made, and the hole is getting smaller," said Paul Newman, chief scientist for Earth sciences at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We see some wavering as weather changes and other factors make the numbers wiggle slightly from day to day and week to week. But overall, we see it decreasing through the past two decades. The elimination of ozone-depleting substances through the Montreal Protocol is shrinking the hole."

The ozone layer -- the portion of the stratosphere that protects our planet from the Sun's ultraviolet rays -- thins to form an "ozone hole" above the South Pole every September. Chemically active forms of chlorine and bromine in the atmosphere, derived from human-produced compounds, attach to high-altitude polar clouds each southern winter. The reactive chlorine and bromine then initiate ozone-destroying reactions as the Sun rises at the end of Antarctica's winter.

Researchers at NASA and NOAA detect and measure the growth and breakup of the ozone hole with instruments aboard the Aura, Suomi NPP, and NOAA-20 satellites. On Oct. 5, 2022, those satellites observed a single-day maximum ozone hole of 10.2 million square miles (26.4 million square kilometers), slightly larger than last year.

When the polar sun rises, NOAA scientists also make measurements with a Dobson Spectrophotometer, an optical instrument that records the total amount of ozone between the surface and the edge of space -- known as the total column ozone value. Globally, the total column average is about 300 Dobson Units. On Oct. 3, 2022, scientists recorded a lowest total-column ozone value of 101 Dobson Units over the South Pole. At that time, ozone was almost completely absent at altitudes between 8 and 13 miles (14 and 21 kilometers) -- a pattern very similar to last year.

Some scientists were concerned about potential stratospheric impacts from the January 2022 eruption of the Hunga Tonga-Hunga Ha'apai volcano. The 1991 Mount Pinatubo eruption released substantial amounts of sulfur dioxide that amplified ozone layer depletion. However, no direct impacts from Hunga Tonga have been detected in the Antarctic stratospheric data.

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People generate their own oxidation field and change the indoor air chemistry around them

People typically spend 90 percent of their lives inside, at home, at work, or in transport. Within these enclosed spaces, occupants are exposed to a multitude of chemicals from various sources, including outdoor pollutants penetrating indoors, gaseous emissions from building materials and furnishings, and products of our own activities such as cooking and cleaning. In addition, we are ourselves potent mobile emission sources of chemicals that enter the indoor air from our breath and skin.

But how do the chemicals disappear again? In the atmosphere outdoors, this happens to a certain extent naturally by itself, when it rains and through chemical oxidation. Hydroxyl (OH) radicals are largely responsible for this chemical cleaning. These very reactive molecules are also called the detergents of the atmosphere and they are primarily formed when UV light from the sun interacts with ozone and water vapor.

Indoors, on the other hand, the air is of course far less affected by direct sunlight and rain. Since UV rays are largely filtered out by glass windows it has been generally assumed that the concentration of OH radicals is substantially lower indoors than outdoors and that ozone, leaking in from outdoors, is the major oxidant of indoor airborne chemical pollutants.

OH radicals are formed from ozone and skin oils

However, now it has been discovered that high levels of OH radicals can be generated indoors, simply due to the presence of people and ozone. This has been shown by a team led by the Max Planck Institute for Chemistry in cooperation with researchers from the USA and Denmark.

"The discovery that we humans are not only a source of reactive chemicals, but we are also able to transform these chemicals ourselves was very surprising to us," says Nora Zannoni, first author of the study published in the research magazine Science, and now at the Institute of Atmospheric Sciences and Climate in Bologna, Italy. "The strength and shape of the oxidation field are determined by how much ozone is present, where it infiltrates, and how the ventilation of the indoor space is configured," adds the scientist from Jonathan Williams' team. The levels the scientists found were even comparable to outside daytime OH concentrations levels.

The oxidation field is generated by the reaction of ozone with oils and fats on our skin, especially the unsaturated triterpene squalene, which constitutes about 10 percent of the skin lipids that protect our skin and keep it supple. The reaction releases a host of gas phase chemicals containing double bonds that react further in the air with ozone to generate substantial levels of OH radicals. These squalene degradation products were characterized and quantified individually using Proton Transfer reaction Mass Spectrometry and fast gas chromatograph-mass spectrometry systems. In addition, the total OH reactivity was determined in parallel enabling the OH levels to be quantified empirically.

The experiments were conducted at the Technical University of Denmark (DTU) in Copenhagen. Four test subjects stayed in a special climate-controlled chamber under standardized conditions. Ozone was added to the chamber air inflow in a quantity that was not harmful to humans but representative of higher indoor levels. The team determined the OH values before and during the volunteers' stay both with and without ozone present.

In order to understand how the human-generated OH field looked like in space and time during the experiments, results from a detailed multiphase chemical kinetic model from the University of California, Irvine were combined with a computational fluid dynamics model from Pennsylvania State University, both based in the USA. After validating the models against the experimental results, the modeling team examined how the human-generated OH field varied under different conditions of ventilation and ozone, beyond those tested in the laboratory. From the results, it was clear that the OH radicals were present, abundant, and forming strong spatial gradients.

"Our modeling team is the first and currently the only group that can integrate chemical processes between the skin and indoor air, from molecular scales to room scales," said Manabu Shiraiwa, a professor at UC Irvine who led the modeling part of the new work. "The model makes sense of the measurements -- why OH is generated from the reaction with the skin."

Shiraiwa added that there remain unanswered questions, like the way humidity levels impact the reactions the team traced. "I think this study opens up a new avenue for indoor air research," he said.

Adapt test methods for furniture and building materials

"We need to rethink indoor chemistry in occupied spaces because the oxidation field we create will transform many of the chemicals in our immediate vicinity. OH can oxidize many more species than ozone, creating a multitude of products directly in our breathing zone with as yet unknown health impacts." This oxidation field will also impact the chemical signals we emit and receive," says project leader Jonathan Williams, "and possibly help explain the recent finding that our sense of smell is generally more sensitive to molecules that react faster with OH."

The new finding also has implications for our health: Currently, chemical emissions of many materials and furnishings are being tested in isolation before they are approved for sale. However, it would be advisable to also conduct tests in the presence of people and ozone, says atmospheric chemist Williams. This is because oxidation processes can lead to the generation of respiratory irritants such as 4-oxopentanal (4-OPA) and other OH radical-generated oxygenated species, and small particles in the immediate vicinity of the respiratory tract. These can have adverse effects, especially in children and the infirm.

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Discovery reveals large, year-round ozone hole over tropics

An ozone hole, seven times larger than the Antarctic ozone hole, is currently sitting over tropical regions and has been since the 1980s, according to a Canadian researcher.

In AIP Advances, by AIP Publishing, Qing-Bin Lu, a scientist from the University of Waterloo in Ontario, Canada, reveals a large, all-season ozone hole -- defined as an area of ozone loss larger than 25% compared with the undisturbed atmosphere -- in the lower stratosphere over the tropics comparable in depth to that of the well-known springtime Antarctic hole, but its area is roughly seven times greater.

"The tropics constitute half the planet's surface area and are home to about half the world's population," said Lu. "The existence of the tropical ozone hole may cause a great global concern.

"The depletion of the ozone layer can lead to increased ground-level UV radiation, which can increase risk of skin cancer and cataracts in humans, as well as weaken human immune systems, decrease agricultural productivity, and negatively affect sensitive aquatic organisms and ecosystems."

Lu's observation of the ozone hole comes as a surprise to his peers in the scientific community, since it was not predicted by conventional photochemical models. His observed data agree well with the cosmic-ray-driven electron reaction (CRE) model and strongly indicate the identical physical mechanism working for both Antarctic and tropical ozone holes.

As with the polar ozone hole, approximately 80% of the normal ozone value is found to be depleted at the center of the tropical ozone hole. Preliminary reports show ozone depletion levels over equatorial regions are already endangering large populations and the associated UV radiation reaching these regions is far greater than expected.

In the mid-1970s, atmospheric research suggested the ozone layer, which absorbs most of the sun's ultraviolet radiation, might be depleted because of industrial chemicals, primarily chlorofluorocarbons (CFCs). The 1985 discovery of the Antarctic ozone hole confirmed CFC-caused ozone depletion. Although bans on such chemicals have helped slow ozone depletion, evidence suggests ozone depletion persisted.

Lu said the tropical and polar ozone holes play a major role in cooling and regulating stratospheric temperatures, mirroring the formation of three "temperature holes" in the global stratosphere. He said this finding may prove crucial to better understanding global climate change.

Lu's discovery builds on previous studies of the CRE-initiated ozone-depleting mechanism that he and his colleagues originally proposed about two decades ago.

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Hubble uses Earth as proxy for identifying oxygen on potentially habitable exoplanets

Taking advantage of a total lunar eclipse, astronomers using NASA's Hubble Space Telescope have detected Earth's own brand of sunscreen -- ozone -- in our atmosphere. This method simulates how astronomers and astrobiology researchers will search for evidence of life beyond Earth by observing potential "biosignatures" on exoplanets (planets around other stars).

Hubble did not look at Earth directly. Instead, the astronomers used the Moon as a mirror to reflect sunlight, which had passed through Earth's atmosphere, and then reflected back towards Hubble. Using a space telescope for eclipse observations reproduces the conditions under which future telescopes would measure atmospheres of transiting exoplanets. These atmospheres may contain chemicals of interest to astrobiology, the study of and search for life.

Though numerous ground-based observations of this kind have been done previously, this is the first time a total lunar eclipse was captured at ultraviolet wavelengths and from a space telescope. Hubble detected the strong spectral fingerprint of ozone, which absorbs some of the sunlight. Ozone is important to life because it is the source of the protective shield in Earth's atmosphere.

On Earth, photosynthesis over billions of years is responsible for our planet's high oxygen levels and thick ozone layer. That's one reason why scientists think ozone or oxygen could be a sign of life on another planet, and refer to them as biosignatures.

"Finding ozone is significant because it is a photochemical byproduct of molecular oxygen, which is itself a byproduct of life," explained Allison Youngblood of the Laboratory for Atmospheric and Space Physics in Boulder, Colorado, lead researcher of Hubble's observations.

Although ozone in Earth's atmosphere had been detected in previous ground-based observations during lunar eclipses, Hubble's study represents the strongest detection of the molecule to date because ozone -- as measured from space with no interference from other chemicals in the Earth's atmosphere -- absorbs ultraviolet light so strongly.

Hubble recorded ozone absorbing some of the Sun's ultraviolet radiation that passed through the edge of Earth's atmosphere during a lunar eclipse that occurred on January 20 to 21, 2019. Several other ground-based telescopes also made spectroscopic observations at other wavelengths during the eclipse, searching for more of Earth's atmospheric ingredients, such as oxygen and methane.

"One of NASA's major goals is to identify planets that could support life," Youngblood said. "But how would we know a habitable or an uninhabited planet if we saw one? What would they look like with the techniques that astronomers have at their disposal for characterizing the atmospheres of exoplanets? That's why it's important to develop models of Earth's spectrum as a template for categorizing atmospheres on extrasolar planets."

Her paper is available online in The Astronomical Journal.

Sniffing Out Planetary Atmospheres

The atmospheres of some extrasolar planets can be probed if the alien world passes across the face of its parent star, an event called a transit. During a transit, starlight filters through the backlit exoplanet's atmosphere. (If viewed close up, the planet's silhouette would look like it had a thin, glowing "halo" around it caused by the illuminated atmosphere, just as Earth does when seen from space.)

Chemicals in the atmosphere leave their telltale signature by filtering out certain colors of starlight. Astronomers using Hubble pioneered this technique for probing exoplanets. This is particularly remarkable because extrasolar planets had not yet been discovered when Hubble was launched in 1990 and the space observatory was not initially designed for such experiments.

So far, astronomers have used Hubble to observe the atmospheres of gas giant planets and super-Earths (planets several times Earth's mass) that transit their stars. But terrestrial planets about the size of Earth are much smaller objects and their atmospheres are thinner, like the skin on an apple. Therefore, teasing out these signatures from Earth-sized exoplanets will be much harder.

That's why researchers will need space telescopes much larger than Hubble to collect the feeble starlight passing through these small planets' atmospheres during a transit. These telescopes will need to observe planets for a longer period, many dozens of hours, to build up a strong signal.

To prepare for these bigger telescopes, astronomers decided to conduct experiments on a much closer and only known inhabited terrestrial planet: Earth. Our planet's perfect alignment with the Sun and Moon during a total lunar eclipse mimics the geometry of a terrestrial planet transiting its star.

But the observations were also challenging because the Moon is very bright, and its surface is not a perfect reflector because it is mottled with bright and dark areas. The Moon is also so close to Earth that Hubble had to try and keep a steady eye on one select region, despite the Moon's motion relative to the space observatory. So, Youngblood's team had to account for the Moon's drift in their analysis.

Where There's Ozone, There's Life?

Finding ozone in the skies of a terrestrial extrasolar planet does not guarantee that life exists on the surface. "You would need other spectral signatures in addition to ozone to conclude that there was life on the planet, and these signatures cannot necessarily be seen in ultraviolet light," Youngblood said.

On Earth, ozone is formed naturally when oxygen in the Earth's atmosphere is exposed to strong concentrations of ultraviolet light. Ozone forms a blanket around Earth, protecting it from harsh ultraviolet rays.

"Photosynthesis might be the most productive metabolism that can evolve on any planet, because it is fueled by energy from starlight and uses cosmically abundant elements like water and carbon dioxide," said Giada Arney of NASA's Goddard Space Flight Center in Greenbelt, Maryland, a co-author of the science paper. "These necessary ingredients should be common on habitable planets."

Seasonal variability in the ozone signature also could indicate seasonal biological production of oxygen, just as it does with the growth seasons of plants on Earth.

But ozone can also be produced without the presence of life when nitrogen and oxygen are exposed to sunlight. To increase confidence that a given biosignature is truly produced by life, astronomers must search for combinations of biosignatures. A multiwavelength campaign is needed because each of the many biosignatures are more easily detected at wavelengths specific to those signatures.

"Astronomers will also have to take the developmental stage of the planet into account when looking at younger stars with young planets. If you wanted to detect oxygen or ozone from a planet similar to the early Earth, when there was less oxygen in our atmosphere, the spectral features in optical and infrared light aren't strong enough," Arney explained. "We think Earth had low concentrations of ozone before the mid-Proterozoic geological period (between roughly 2.0 billion to 0.7 billion years ago) when photosynthesis contributed to the build up of oxygen and ozone in the atmosphere to the levels we see today. But because the ultraviolet-light signature of ozone features is very strong, you would have a hope of detecting small amounts of ozone. The ultraviolet may therefore be the best wavelength for detecting photosynthetic life on low-oxygen exoplanets."

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