What Venus has taught us about protecting the ozone layer

Understanding Venus’ atmosphere helps us understand Earth’s past, present, and a potential future. Keith Mosley

SAVING THE OZONE: Part six in our series exploring the Montreal Protocol on Substances that Deplete the Ozone Layer – dubbed “the world’s most successful environmental agreement” – looks at the atmosphere of Venus and what it means for us on earth.

In June 2012, Venus passed in front of the Sun. This won’t happen again until 2117. The transit of Venus played a well-recognised, significant role in British exploration of Australia. But Venus has also played less-well-known roles in understanding the chemistry in the Earth’s atmosphere. Venus helped us recognise that chlorofluorocarbons (CFCs) can decompose stratospheric ozone (O₃).

Mario Molina and Sherry Rowland from the University of California, Irvine, shared the 1995 Nobel Prize for Chemistry “for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone”. Their seminal contribution was captured in an article in Nature in 1974, which identified the threat that CFCs posed to Earth’s ozone layer.

The Swedish Academy noted that Molina and Rowland’s work built directly on two important contributions by other researchers.

One was James Lovelock, who during 1971-73 showed that CFC gases had spread throughout the global atmosphere. And from 1972-74, Richard Stolarski and Ralph Cicerone from the University of Michigan examined the impact of the space shuttle on the stratosphere. They showed that free chlorine atoms (Cl) produced from the breakdown of hydrogen chloride (HCl) emitted from the shuttle’s solid rocket boosters could catalytically convert O₃ to molecular oxygen (O₂).

In parallel, Michael McElroy and Steven Wofsy from Harvard University were studying catalytic destruction of O₃ by chlorine and nitrogen gases on Earth, and McElroy’s group and Ronald Prinn at MIT were studying the broader impact of chlorine on another planet’s atmospheric chemistry – Venus.

By 1970, scientists knew there were five gases in the atmosphere of Venus – carbon dioxide (CO₂) (the vast majority of the atmosphere), water vapour (H₂O), carbon monoxide (CO), hydrogen chloride (HCl), and hydrogen fluoride (HF).

Carbon dioxide, like O₃, is not stable against the intense ultraviolet (UV) light present in the uppermost parts of a planetary atmosphere. With the vertical mixing that occurs in an atmosphere, an initially pure CO₂ atmosphere can be expected to decompose. This will produce a few percent each of carbon monoxide (CO) and molecular oxygen (O₂) in less than a thousand years. So why does CO₂ remain stable on Venus?

Understanding the stability of CO₂ on Venus helps us understand the early Earth’s atmosphere. It also tells us about planets similar to Earth orbiting other stars. More generally, studying chemistry on Venus can help identify chemistry we did not know was occurring on Earth, as happened for CFCs, chlorine chemistry, and ozone.

Scientists thought CO₂ might be stabilised on Venus by chemical reactions involving hydrogen and chlorine gases. Hydrogen chemistry is critical for understanding the atmosphere on Mars. Trace amounts of water on Mars are split apart by UV light to produce reactive hydrogen gases. But hydrogen chemistry alone couldn’t explain the stability of CO₂ in Venus’ denser atmosphere.

Hydrogen chloride (HCl) can be split apart more readily than water by UV light in a dense CO₂ atmosphere. Hydrogen chloride is decomposed by UV light at longer wavelengths which are not absorbed by CO₂. So the atmospheric chemistry models developed for Venus in 1971-73 by Ronald Prinn, Michael McElroy, and McElroy’s students explored how HCl chemistry could make CO₂ stable.

McElroy’s group focused on HCl as a source for reactive hydrogen species. Prinn examined the potential for Cl, ClO, and ClO₂ to react with CO and, thus, produce CO₂ via catalytic cycles similar to those which destroy O₃ in Earth’s stratosphere. The chlorine catalytic cycles now believed to dominate production of CO₂ on Venus were not proposed until the early 1980s and experimentally verified in the early 2000s.

O₃ chemistry was not a primary focus for this early work on Venus as there were no observations at the time of O₃ on Venus. However, the potential for catalytic conversion of odd oxygen (O and O₃) to O₂ by chlorine gases was noted before its importance for the Earth’s stratosphere was fully recognised.

Similar parallels can be drawn between other studies of Venus and Earth. For example, production of sulphuric acid (H₂SO₄) from carbonyl sulfide (OCS) was examined in Venus atmospheric chemistry models several years before the same reactions were found to be important in Earth’s stratosphere. Also, the first versions of key instruments still used by Earth-orbiting meteorological satellites were designed for NASA’s Mariner 2 spacecraft, which flew past Venus in 1962.

Ozone and chlorine monoxide (ClO) have only recently (2011 and 2012, respectively) been measured on Venus. These observations - and the chlorine catalytic chemistry that has been studied to understand the Earth’s ozone layer - may finally help resolve what catalytic chemistry keeps CO₂ stable on Venus. Despite several decades of research, this question still hasn’t been answered.

References

Anderson and Sarma, Protecting the Ozone Layer: The United Nations History, Earthscan Publications Ltd, 2002, pp 6-8

Kowalok, Environment 35, 12-20+35-38, 1993

Stolarski and Cicerone, Canadian Journal of Chemistry 52, 1610, 1974

Wofsy and McElroy, Canadian Journal of Chemistry 52, 1582, 1974

Read more on the Montreal Protocol’s 25th anniversary.

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