The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth

Abstract

Recognizing whether a planet can support life is a primary goal of future exoplanet spectral characterization missions, but past research on habitability assessment has largely ignored the vastly different conditions that have existed in our planet's long habitable history. This study presents simulations of a habitable yet dramatically different phase of Earth's history, when the atmosphere contained a Titan-like, organic-rich haze. Prior work has claimed a haze-rich Archean Earth (3.8-2.5 billion years ago) would be frozen due to the haze's cooling effects. However, no previous studies have self-consistently taken into account climate, photochemistry, and fractal hazes. Here, we demonstrate using coupled climate-photochemical-microphysical simulations that hazes can cool the planet's surface by about 20 K, but habitable conditions with liquid surface water could be maintained with a relatively thick haze layer (τ ∼ 5 at 200 nm) even with the fainter young Sun. We find that optically thicker hazes are self-limiting due to their self-shielding properties, preventing catastrophic cooling of the planet. Hazes may even enhance planetary habitability through UV shielding, reducing surface UV flux by about 97% compared to a haze-free planet and potentially allowing survival of land-based organisms 2.7-2.6 billion years ago. The broad UV absorption signature produced by this haze may be visible across interstellar distances, allowing characterization of similar hazy exoplanets. The haze in Archean Earth's atmosphere was strongly dependent on biologically produced methane, and we propose that hydrocarbon haze may be a novel type of spectral biosignature on planets with substantial levels of CO₂. Hazy Archean Earth is the most alien world for which we have geochemical constraints on environmental conditions, providing a useful analogue for similar habitable, anoxic exoplanets. Key Words: Haze-Archean Earth-Exoplanets-Spectra-Biosignatures-Planetary habitability. Astrobiology 16, 873-899.

FIG. 1.

Shown is an example of the Atmos model convergence process. This atmosphere, which has CH₄/CO₂ = 0.17 and pCO₂ = 0.02 (total pressure 1 bar) goes through five coupling iterations. The initial temperature profile it uses was stored from a previous similar atmosphere. Here we show the temperature, water, haze number density, haze particle radii, C₂H₆ profile, and CH₄ profile for each iteration of the coupled model.

FIG. 2.

The top panels present the extinction efficiency (Qext) and single-scattering albedo ( = Qscat/Qext) of four sizes of fractal hydrocarbon particles used in this study and in Wolf and Toon (2010). The spherical monomers comprising these particles are 0.05 μm in radius. The radii on the plot correspond to the radii of equivalent-mass spherical particles, and the fractal dimensions of these particles, from smallest to largest, are 3 (spherical), 1.51, 2.28, and 2.40. The number of monomers in these particles are 1, 8, 1000, and 8000. These particles tend to scatter and absorb light more efficiently at shorter wavelengths, and larger particles have flatter wavelength dependence for the scattering efficiency. Refractive indices, shown in the bottom panels, are presented from information in Khare et al. (1984a).

FIG. 3.

The gas profiles for H₂O, CH₄, CO, CO₂, and C₂H₆ for planets with pCO₂ = 0.01 bar for CH₄/CO₂ = 0.1 (on the left) and CH₄/CO₂ = 0.2 (on the right). Also shown are the profiles for the haze particle number density (in pale orange). The CH₄/CO₂ = 0.1 haze profile is divided by 1000, and the CH₄/CO₂ = 0.2 haze profile is divided by 1 × 10⁵ in order to plot it on the same axis as the gases. The profiles in the right panel show larger amounts of CH₄, H₂O, and C₂H₆ above 60 km in altitude and illustrate how haze-induced shielding can prevent photolysis of these gases. The sharp decrease in haze particle number density between 60 and 70 km in the right panel shows where fractal coagulation occurs. The atmosphere above the fractal coagulation region is populated by spherical submonomer particles.

FIG. 4.

Mean surface temperatures as a function of CH₄ for Archean Cases A–D. The dashed blue line shows the freezing point of water (273 K), and the dashed orange line marks our lower threshold of habitability (248 K) for an equatorial ocean belt (Charnay et al., 2013). The X in each panel indicates the initiation of haze-induced cooling.

FIG. 5.

The left panel presents the temperature profiles of three CH₄/CO₂ ratios for the Case B planet. Note the strengthening temperature inversion as the CH₄ content of the atmosphere increases. The right panel shows the size of haze particles produced in these three atmospheres, showing the dependence of haze particle size on temperature. From least to most CH₄ (and thinnest to thickest haze), the particles reach a maximum radius of 0.067, 0.28, and 0.57 μm. Note that the temperature profiles become isothermal at the top of the climate model grid when transferred to the larger photochemical model grid.

FIG. 6.

The haze particle sizes for two completely isothermal atmospheres together with the coagulation and sedimentation timescales for these atmospheres.

FIG. 7.

Shown are surface UV spectra (left) and ozone column abundances (right) for Archean, Proterozoic, and modern Earth atmospheres. A modest amount of O₂ in the Proterozoic (1% PAL) produces a stronger UV shield than the Archean haze, but the haze shown here cuts out more UVA (320–400 nm) and UVB (280–320 nm) radiation than ozone in all situations. The haze can produce a stronger UV shield compared to the low O₂ atmosphere (0.1% PAL) proposed recently by Planavsky et al. (2014) for our atmospheric assumptions.

FIG. 8.

Shown here are spectra for Case B. Haze and gas absorption features are labeled with the symbols indicated. (a) At short wavelengths in direct imaging, haze absorption decreases the planet's brightness; scattering brightens the planet at longer wavelengths. (b) Thermal emission from the hot stratosphere of the thickest haze planet (CH₄/CO₂ = 0.21) fills in absorption bands near 8 and 16 μm. (c) The y axis shows the effective transit height above the planet's surface that light is able to penetrate, and absorption features are inverted compared to (a) and (b) due to an increase in the effective planet radius during transit resulting from an increase in absorption at these wavelengths. The bottom section shows the approximate color of the hazy sky and planet. Sky colors are computed using the diffuse radiation spectrum at the ground. “Effective tangent height” refers to the minimum altitude above the planet's surface that light is able to penetrate on transit transmission paths.

FIG. 9.

A reflectance spectrum for a hazy Case B planet in the visible and NIR (a) and mid-IR (b) is presented with gases and the hydrocarbon haze removed to show where each spectral component interacts with radiation. The full spectrum is shown in black. Places where the black spectrum deviates from the colored spectra indicate where each gas or haze absorbs. For example, the green line shows a spectrum where CO₂ is omitted, and a strong CO₂ feature is present near 15 μm in (B) as shown by the deviation of the green spectrum from the black spectrum. At some wavelengths, gas and haze absorptions are complex to detangle because multiple species are absorbing: in these cases, the key on Fig. 8 will indicate which gases are the dominant absorbers in a region.

FIG. 10.

A transit transmission spectrum for a hazy Case B planet in the visible and NIR (a) and mid-IR (b) is presented with gases and the hydrocarbon haze removed to show where each spectral component interacts with radiation. The full spectrum is shown in black. Places where the black spectrum deviates from the colored spectra indicate where each gas or haze absorbs. For example, the orange line in (A) indicates CH₄ absorption features near 1.15, 1.4, 1.7, 2.3, and 3.3 μm.

FIG. 11.

Example reflectance spectra, intended as analogues for exoplanets like Archean Earth, for all the types of planets investigated in this study are presented here. Fractional ice coverage is included in these spectra using a weighted average of icy and liquid water surfaces as described in the text.

FIG. 12.

Transit transmission spectra in the visible and NIR for Cases A–D are presented here. For thicker hazes, absorption features shortward of approximately 1 μm vanish. These relatively featureless spectra result because high-altitude hazes are effective at obscuring the lower atmosphere with the long path lengths taken by light in transit spectroscopy measurements.

FIG. 13.

Here we show the impact of water clouds on our Case B spectra with no haze, a thin haze, and a thick haze. The spectra with cloud and haze are shown in the pale colored lines. The dashed lines over our transit transmission spectra indicate that the spectra with and without water clouds are the same.

FIG. 14.

This shows the diversity of optical constants measured by several studies. The studies the figure key refers to are as follows: Hasenkopf et al., ; Ramirez et al., ; Sciamma-O'Brien et al., ; Khare et al., ; Imanaka et al., ; Tran et al., ; Mahjoub et al., ; Vuitton et al., . Note in particular the single point measured under Archean Earth-like laboratory conditions by Hasenkopf et al. (2010).

FIG. 15.

A comparison of reflectance spectra and surface flux spectra using Khare et al. (1984a) and Mahjoub et al. (2012) optical constants, plus a spectrum generated by shifting the Khare constants to match the Archean haze refractive indices measured by Hasenkopf et al. (2010) at 532 nm (called “Khare-Hasenkopf”).