ScienceWise - May/Jun 2009

The Evil Twin

Article Illustration
Dr frank Mills
Article Illustration
The Anglo Australian Telescope

How Modelling the Atmosphere of Venus Helps Us Understand the Earth

One of the difficulties in modelling the climate of the Earth under various greenhouse scenarios is that scientists only have one set of parameters against which to compare such models, that is, the   climate history of our own planet. When making future predictions it’s hard to be 100% sure that you haven’t simply tweaked your model to fit the past without the underlying science being good enough to give a degree of certainty to the future scenarios it predicts. One way this can be overcome is by looking at the atmospheres of other planets such as Mars and Venus where the parameters are different but the science is exactly the same. Venus is an especially interesting and alarming case as its dense CO2 atmosphere has led to a runaway greenhouse effect heating the surface to temperatures that would easily melt lead.

Dr Frank Mills is an atmospheric and climate scientist with a joint appointment in The Fenner School of Environment and Society and The Research School of Physics and Engineering at the ANU. A primary interest is numerical modelling of the chemical processes within the atmosphere of Venus. “Many of us are strongly interested in creating better models of the Earth’s atmosphere and because the physics and chemistry are the same on Venus but the density and composition of the atmosphere are vastly different, it gives us an opportunity to test our models in two very different cases. If a model of planetary atmospheres holds good to observations when you plug in the pressures, densities and chemical compositions of two planets, then it gives you confidence that you’ve got the underlying science right.” Dr Mills explains.

Of course the problem with Venus is that it’s not nearly so easy to gather atmospheric and climate data as it is on the Earth. The data that Dr Mills feeds into his models come from two sources. The first is from spacecraft such as the European Space Agency’s Venus Express for which Dr Mills is a member of the scientific team.

The other way to gather Venus data is using large Earth based telescopes such as the 4m Anglo Australian Telescope in NSW.  An important portion of the Earth based data for this research program has come from collaborations with Dr Jeremy Bailey of the University of New South Wales. Although it may seem strange to observe a bright planet with a 4 metre diameter telescope designed for very faint objects, the AAT has an advanced suite of spectroscopic instrumentation that make it one of the best telescopes in the world for gathering such data.

One of the difficulties with making measurements of molecular absorption lines in the atmosphere of Venus using terrestrial telescopes is that the Earth’s atmosphere contains nearly all the same molecules as those on Venus so the two spectra overlap. To get around this scientists make atmospheric measurements on Venus when the relative velocity between the two planets is large enough to Doppler shift the spectral lines from Venus away from the terrestrial lines. This makes it possible to distinguish the Venus spectrum from that of the Earth’s own atmosphere.

Within the atmospheres of planets, chemistry, thermodynamics and convection combine with solar radiation and surface interactions to create a highly complex and dynamic system. The high densities and temperatures on Venus coupled with the intense solar radiation make it a particularly interesting system to study. One intriguing aspect of the photochemistry of Venus is the creation and destruction of free oxygen.

In Venus’ upper atmosphere ultraviolet light from the sun splits carbon dioxide molecules into carbon monoxide and atomic oxygen (O). The direct recombination reaction to produce carbon dioxide from carbon monoxide and atomic oxygen is very slow so the majority of this atomic oxygen forms O2. The 2O to O2 reaction emits a characteristic light which forms an airglow that is visible on the dark side of Venus. By measuring the intensity of this glow scientists can estimate the rate of molecular oxygen production. “We know the rate of oxygen generation on Venus is quite high from the airglow data,” Dr Mills explains, “but when we look at the bright side of Venus using absorption spectroscopy we see very little molecular oxygen. So some process has to be consuming it at a very fast rate. Our current models can’t really plausibly explain this so it’s an area of great interest to planetary scientists.”

Venus is sometimes termed the Earth’s twin due to its almost identical size, its similar proximity to the sun, and the likely (but not yet proven) similarities in its initial composition. The Earth’s first stable atmosphere was dominated by carbon dioxide, much like Venus today except not nearly so dense. Over the course of billions of years, the evolution of life on Earth transformed the atmosphere by releasing free oxygen and binding the CO2 in biomass and sedimentary rocks. “In a sense you can think of the early Earth as being quite Venus-like.” Says Dr Mills, “It would be unfortunate if the Earth turned back to that state!”

Understanding how to prevent that happening has been a primary objective of climate scientists. A very important step in doing so has been to develop models of atmospheric processes so thorough and reliable that no one can question the need to take heed of their future predictions.

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How Modelling the Atmosphere of Venus Helps Us Understand the Earth
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