Modelling suggests that Pluto's atmospheric temperature is regulated by haze, unlike the other planetary bodies in the Solar System. The finding has implications for our understanding of exoplanetary atmospheres. See Letter p.352
In 2015, NASA's New Horizons spacecraft captured the first close-up images of Pluto and its moons (http://pluto.jhuapl.edu). It is hard to imagine how anyone who has seen these images could not be both impressed by the achievement and excited by what the images and other data reveal. One of the big surprises1 was that Pluto's atmosphere is much colder than predicted2 at altitudes above 50 kilometres. Because temperature profiles are fundamental to understanding the physics of planetary atmospheres, scientists have been compelled to determine whether key atmospheric processes were missed in earlier studies. On page 352, Zhang et al.3 come to the remarkable conclusion that haze particles, rather than gas molecules, control Pluto's atmospheric temperature.
New Horizons revealed many layers of haze in Pluto's atmosphere1 (Fig. 1). The discrete nature of these layers and their vast horizontal extent were another big surprise. Images taken before and after the spacecraft's fly-by of Pluto show that the haze particles are highly forward-scattering — they scatter visible light without substantially changing the light's direction. This suggests that the particles have diameters near to or larger than the wavelength of visible light (400–700 nm). The particles are thought to be similar to the organic haze particles in the atmosphere of Saturn's moon Titan4.
Zhang and colleagues present a model of Pluto's atmosphere that uses the observed distributions of haze particles and gas molecules4. Their model suggests that the haze particles absorb more sunlight, and therefore generate more atmospheric heating, than the gas molecules. Furthermore, the haze particles can compensate for this heating by radiative cooling — losing heat to space in the form of infrared radiation. The results of the model are consistent with the temperature profile revealed by New Horizons, and imply that Pluto's atmospheric temperature is regulated by haze.
Why has the idea that haze can affect atmospheric temperature come so late in the game? In the case of Pluto, perhaps the main reason is that its gas molecules produce lines in its emission and absorption spectra that grab the attention of modellers. By contrast, its haze is well defined only in the images from New Horizons and only at visible and near-infrared wavelengths. Radiative cooling occurs at longer wavelengths, and there are currently no observations that are sufficiently sensitive to reveal it.
The case is not yet closed on our understanding of Pluto's atmospheric temperature. For instance, there are no observational constraints on the composition of Pluto's haze, which means that the authors' calculated heating and cooling rates are highly uncertain. Without constraints, modellers are free to consider many possible haze compositions that have a wide range of optical constants — quantities that characterize the optical properties of a material.
Only haze particles that have high infrared absorption rates can undergo radiative cooling. Zhang et al. looked through the scientific literature for suitable candidates and found that the optical constants of some organic particles are in the range needed to bring the authors' theoretical model into agreement with observations. However, without compositional constraints, there is no way to verify that the inferred optical constants are representative of Pluto's haze.
A key prediction of Zhang and colleagues' model is that the radiative cooling of Pluto's haze particles should produce detectable infrared emission. Existing infrared detectors and instruments do not have the sensitivity to detect this radiation. However, the authors show that the Mid-Infrared Instrument imager5 on the future James Webb Space Telescope should have the required sensitivity to either verify or rule out their model.
The authors' work also has implications for modelling exoplanetary atmospheres. Spectroscopic studies of transiting exoplanets (those that pass in front of their host star) show that atmospheric cloud and haze particles are important, and sometimes crucial, in the formation of the observed spectra6. In some exoplanets, cloud particles are thought to exist at high altitudes to explain the absence of gaseous absorption features in the exoplanets' spectra6. However, it is difficult to understand how these particles remain aloft in such low-pressure environments.
A possible explanation is that these particles are photochemical haze particles — for example, hydrocarbons. Such particles are the end products of a break-up of gaseous molecules by ultraviolet radiation or energetic charged particles in the upper atmosphere. This process operates at high altitudes and occurs in the giant planets of the Solar System7. Zhang and colleagues' results suggest that models of exoplanetary atmospheres that neglect these high-altitude haze particles could have unrealistic temperature profiles.
The existence of high-altitude hydrocarbon haze, at least for hot atmospheres, is questionable because carbon monoxide is the most common gaseous form of carbon in these atmospheres. Photochemical models that start with carbon monoxide do not lead to hydrocarbon-haze formation8, but work on this issue is in progress. Future studies that simulate the atmosphere of an exoplanet close to its host star — taking into account the atmosphere's high temperature and exposure to radiation and energetic particles — could lead to insights beyond what is currently possible.