The past 20 years have revealed an unexpected diversity of exoplanets in our galactic neighborhood. From afar, these planets are only pale dots of various colors, but future technologies will enable high-resolution spectroscopy. The scientific returns from these observations will depend on our ability to translate these spectroscopic observations into physical (dynamics, pressure and temperature) and chemical (molecular abundances) conditions, and vice versa. 

Earth, the only world known, so far, to harbor Life, would currently look like a “pale blue dot” from a distant vantage point [1]. However, Earth may not have been always so blue. Instead, it is becoming increasingly more acknowledged that early Earth had a lower atmospheric pressure [2,3], and a periodically haze-rich atmosphere [4-8] which would have substantially modified the observable features of our home planet in its past. Thus, making it closer to a “pale orange dot” [9]. Additionally, such an organic haze would have produced dramatic effects on the early climate, on the primitive atmospheric chemistry, and on the biochemistry of the very first living organisms.

1D and, more recently, 3D photochemical-climate models have been widely used to assess exoplanetary physical and chemical conditions and/or to define mission/instrument priorities through simulated spectral observations. Such tools self-consistently treat the atmospheres, the surface environments and the spectral features of these planets with a wide variety of parameters. The predicted results are therefore highly dependent on the model input parameters. Except for the very hottest exoplanets, chemical equilibrium does not hold. As a consequence, the atmospheric compositions are controlled by hundreds to thousands of individual chemical reactions. Chemical reactions are based on empirical parameters that must be known at temperatures ranging from 100 K to above 2500 K and at pressures from millibars to hundreds of bars. Derived or - more often - extrapolated from experiments, calculations and educated-guessed estimations, these parameters are always evaluated with substantial uncertainties.

Although practical, few models of planetary atmospheres have considered these underlying chemical uncertainties and their observational consequences. Recent progress has been made that now enables us (1) to evaluate the accuracy and precision of 1D models of planetary atmospheres and quantify uncertainties on their predictions for the atmospheric composition and associated spectral features; (2) to identify the ‘key parameters' that contribute the most to the modeled predictions and should therefore require further experimental or theoretical analysis, (3) to reduce and optimize complex chemical networks for their inclusion in multidimensional atmospheric models.

Here, we consider these chemical uncertainties in modeling a haze-rich Archean Earth. We rely on recent 1D photochemical-climate simulations of Archean Earth with fractal hydrocarbon hazes. These hazes are consistent with geochemical data and account for the lower end of paleopressure estimates (0.5 bar) [9]. This detailed “case study” of Archean Earth provides us with an example of a habitable planet for which the propagation of chemical uncertainties may have major climatic and observational effects, due to a chemically-produced global haze that may have been present at the time.

References: [1] Sagan (1994) Pale Blue Dot: A Vision of the Human Future in Space. [2] Som et al. (2012) Nature, 484, 359-62. [3] Marty et al. (2013) Science, 342, 101-104. [4] Pavlov et al. (2001) Geology 29, 1003-1006. [5] Haqq-Misra et al. (2008) Astrobiology, 8, 1227-37. [6] Domagal-Goldman et al. (2008) Earth Plane Sc. Lett., 269, 29-40. [7] Wolf and Toon (2010) Science, 328, 1266-8. [8] Zerkle et al. (2012) Nature Geoscience, 5, 359-63. [9] Arney et al. (2015) Astrobiology Science Conference 2015, 7264.