Haze production rates in super-Earth and mini-Neptune atmosphere experiments

Abstract

Numerous Solar System atmospheres possess photochemically generated hazes, including the characteristic organic hazes of Titan and Pluto. Haze particles substantially impact atmospheric temperature structures and may provide organic material to the surface of a world, potentially affecting its habitability. Observations of exoplanet atmospheres suggest the presence of aerosols, especially in cooler (<800 K), smaller (<0.3× Jupiter’s mass) exoplanets. It remains unclear whether the aerosols muting the spectroscopic features of exoplanet atmospheres are condensate clouds or photochemical hazes1,2,3, which is difficult to predict from theory alone4. Here, we present laboratory haze simulation experiments that probe a broad range of atmospheric parameters relevant to super-Earth- and mini-Neptune-type planets5, the most frequently occurring type of planet in our galaxy6. It is expected that photochemical haze will play a much greater role in the atmospheres of planets with average temperatures below 1,000 K (ref. 7), especially those planets that may have enhanced atmospheric metallicity and/or enhanced C/O ratios, such as super-Earths and Neptune-mass planets8,9,10,11,12. We explored temperatures from 300 to 600 K and a range of atmospheric metallicities (100×, 1,000× and 10,000× solar). All simulated atmospheres produced particles, and the cooler (300 and 400 K) 1,000× solar metallicity (‘H2O-dominated’ and CH4-rich) experiments exhibited haze production rates higher than our standard Titan simulation (~10 mg h–1 versus 7.4 mg h–1 for Titan13). However, the particle production rates varied greatly, with measured rates as low as 0.04 mg h–1 (for the case with 100× solar metallicity at 600 K). Here, we show that we should expect great diversity in haze production rates, as some—but not all—super-Earth and mini-Neptune atmospheres will possess photochemically generated haze.

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Fig. 1: Composition of the initial gas mixtures used for our experiments.
Fig. 2: Schematic of the PHAZER Chamber at Johns Hopkins University.
Fig. 3: Production rate values.

Change history

  • 16 March 2018

    In the version of this Letter originally published Table 2, which tabulates the production rates shown in Fig. 3, was mistakenly omitted. It has now been included in all versions of the Letter.

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Acknowledgements

This work was supported by National Aeronautics and Space Administration Exoplanets Research Program grant NNX16AB45G. C.H. was supported by the Morton K. and Jane Blaustein Foundation.

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S.M.H., N.K.L., C.H., M.S.M. and J.I.M. conceived the study. J.I.M. calculated the starting gas mixtures. C.H. performed the experiments and measurements. S.M.H. prepared the manuscript. All authors participated in discussions regarding interpretation of the results and edited the manuscript.

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Correspondence to Sarah M. Hörst.

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Hörst, S.M., He, C., Lewis, N.K. et al. Haze production rates in super-Earth and mini-Neptune atmosphere experiments. Nat Astron 2, 303–306 (2018). https://doi.org/10.1038/s41550-018-0397-0

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