Authors: D.R. Anderson et al.
First Author’s Institution: Keele University, UK.
Pollution and Temperature Inversions: The Making of Smog
My grandfather likes to tell the story of how he, an unknowing asthmatic, moved across the country in 1952 and nearly moved back again due to the Los Angeles smog. Although Los Angeles is iconic for its brown haze, it is not the only smoggy city in the modern world: Beijing, New Delhi, and Santiago ranked 1,2, and 3 in a recent air pollution survey. I could discuss the negative consequences of smog and the actions we could take to clean Earth’s air, but I won’t. Instead, I will talk about what makes smog and about how we can find the necessary conditions (or lack thereof) for creating smog on planets around other stars.
Smog can only occur in an atmosphere with a temperature inversion. Usually, a planet smoothly radiates heat away from its hot surface through its atmosphere. Thus, the base of the atmosphere is warmest, and the temperature of the atmosphere gradually decreases with altitude. However, some atmospheres have temperature inversions, or layers of hot air higher up in the atmosphere that trap colder layers underneath. Rising air laced with pollutants becomes trapped in the lower layer, producing smog.
What causes temperature inversions? On Earth, local temperature inversions are often caused by pressure systems (i.e. weather). High-pressure cells of lower-altitude air can produce local temperature inversions, such as the one over Los Angeles that my grandfather despises. Although we are beginning to learn about the weather patterns (such as “circulation,” which describes how global winds redistribute the heat from the dayside to the nightside of the planet) on other planets, we are not nearly so well versed in their effects as we are in Earth’s climatology, and even that is not particularly well known. However, we do have the ability to measure global temperature inversions on Earth and other planets. We have a global temperature inversion in Earth’s atmosphere that we call the stratosphere. Some exoplanets have similar temperature inversions. The authors of this paper determine that Wasp-19b, a hot Jupiter orbiting an active star, does not display a strong temperature inversion. In fact, it appears to display no temperature inversion at all. Why do some hot Jupiters have strong temperature inversions, while others do not?
Recent astrobites by Sukrit Ranjan and Nathan Sanders describe how the technique of transmission spectroscopy can determine the composition of a planet’s atmosphere at different heights. The similar technique of emission spectroscopy involves taking spectra of the planet before and while it passes behind the star in what is called a secondary eclipse. These spectra show thermal emission (which includes both blackbody radiation from the warm planet and chemical signatures) rather than absorption features, but can reveal molecules at specific layers of the planet’s atmosphere nonetheless. The authors used the InfraRed Array Camera (IRAC) aboard the warm Spitzer Space Telescope to observe the secondary eclipse of Wasp-19b at 3.6, 4.5, 5.8 and 8.0 microns. After a careful data reduction that attempted to remove the systematic effects of warm Spitzer, they did a best-fit analysis to the data to measure the secondary eclipse depth at each wavelength. (See figure 1.)
In addition to the secondary eclipse depths measured from Spitzer, the authors included eclipse depths measured from Hawk-I at 1.6 and 2.09 microns. They used an atmospheric modeling code to find model atmospheres that best described the data. The best models all lacked temperature inversions, regardless of the ratio of carbon to oxygen in the planet’s atmosphere. (See figure 2.)
What’s the upshot of all this? Well, we can’t determine the composition of Wasp-19b, since either a carbon- or oxygen-rich spectrum produces the observed lack of temperature inversion. However, the absence of a temperature inversion is itself interesting. Why do some Jupiters have temperature inversions, while others do not? The authors refer to a hypothesis that active stars prevent the formation of temperature inversions in hot Jupiters (Knutson, Howard & Isaacson, 2010). Wasp-19 is indeed an active star. However, we will need to wait and see whether this relation between temperature inversion and stellar activity holds for a significant number of un-inverted hot Jupiters and their active hosts (right now there are five such examples).
Whatever the reason, Wasp-19b has no temperature inversion, which means that it can’t hold a global smog. So the next time my grandfather feels the need to move cross-country (or cross-galaxy), I have at least one suggestion of a place with clean air.