Titan-like exoplanets: Variations in geometric albedo and effective transit height with haze production rate

• The slope of the geometric albedo profile changes greatly in the UV–visible range when varying the haze production rate.
• This spectral region is the most effective way to determine the presence of a thick haze from remote observations.
• The same shape of the effective transit height persists when varying the haze production rate.
• When increasing the haze production rate from 0x to 0.01x, a large slope onset was noticeable for the effective transit height.
• The lowest altitude probed by the effective transit height spectra is 34 km, making it insensitive to changes in surface properties such as the surface albedo.
• Geometric albedo spectra provide the most sensitive indicator of the haze production rate and the background Rayleigh gas.

Extensive studies characterizing Titan present an opportunity to study the atmospheric properties of Titan-like exoplanets. Using an existing model of Titan's atmospheric haze, we computed geometric albedo spectra and effective transit height spectra for six values of the haze production rate (zero haze to twice present) over a wide range of wavelengths (0.2–2 µm). In the geometric albedo spectra, the slope in the UV–visible changes from blue to red when varying the haze production rate values from zero to twice the current Titan value. This spectral feature is the most effective way to characterize the haze production rates. Methane absorption bands in the visible-NIR compete with the absorbing haze, being more prominent for smaller haze production rates. The effective transit heights probe a region of the atmosphere where the haze and gas are optically thin and that is thus not effectively probed by the geometric albedo. The effective transit height decreases smoothly with increasing wavelength, from 376 km to 123 km at 0.2 and 2 µm, respectively. When decreasing the haze production rate, the methane absorption bands become more prominent, and the effective transit height decreases with a steeper slope with increasing wavelength. The slope of the geometric albedo in the UV–visible increases smoothly with increasing haze production rate, while the slope of the effective transit height spectra is not sensitive to the haze production rate other than showing a sharp rise when the haze production rate increases from zero. We conclude that geometric albedo spectra provide the most sensitive indicator of the haze production rate and the background Rayleigh gas. Our results suggest that important and complementary information can be obtained from the geometric albedo and motivates improvements in the technology for direct imaging of nearby exoplanets.