[Paper Review] Ammonia, Water Clouds and Methane Abundances of Giant Exoplanets and Opportunities for Super-Earth Exoplanets

Renyu Hu|arXiv (Cornell University)|Dec 24, 2014

Stellar, planetary, and galactic studies1 references5 citations

TL;DR

This study demonstrates that low-resolution (R=70) reflection spectroscopy (600–1000 nm) with moderate signal-to-noise (S/N=20) can retrieve both cloud deck pressure (0.6–1.5 bars) and methane mixing ratio in cold giant exoplanets, provided the cloud deck is not too shallow; for super-Earths with H2O-dominated atmospheres, high-altitude water clouds mute most gas features, confining them to a distinct region in color-color diagrams.

ABSTRACT

Future direct-imaging exoplanet missions such as WFIRST/AFTA, Exo-C, and Exo-S will measure the reflectivity of exoplanets at visible wavelengths. The exoplanets to be observed will be located further away from their parent stars than is Earth from the Sun. These "cold" exoplanets have atmospheric environments conducive for the formation of water and/or ammonia clouds, like Jupiter in the Solar System. We study the science return from direct-imaging exoplanet missions, focusing on the exoplanet atmospheric compositions. First, the study shows that a low-resolution (R=70) reflection spectrum of a giant exoplanet at 600 - 1000 nm, for a moderate signal-to-noise ratio of 20, will allow measurements of both the pressure of the uppermost cloud deck and the mixing ratio of methane, if the uppermost cloud deck is located at the pressure level of 0.6 - 1.5 bars. Further increasing the signal-to-noise ratio can improve the measurement range of the cloud deck pressure to 0.2 - 4 bars. The strong and the weak absorption bands of methane allow the simultaneous measurements of cloud and gas; when the uppermost cloud deck is located shallower than the pressure level of 0.2, the weak bands are muted, and the cloud deck pressure and the mixing ratio of methane are not distinguishable from a single reflection spectrum. Second, future direct-imaging exoplanet missions may detect the broadband reflectivity of a few super-Earth exoplanets. If having H2O-dominated atmospheres, directly imaged super Earths are likely to have water clouds located shallower than 1E-3 bars. The very high clouds on these planets would mute most gas absorption features except for H2O, and these planets would occupy a confined phase space in the color-color diagrams.

Motivation & Objective

• To assess the scientific return of future direct-imaging exoplanet missions such as WFIRST/AFTA, Exo-C, and Exo-S.
• To determine the detectability of atmospheric composition—specifically methane and cloud deck pressure—via low-resolution reflection spectra in the visible range.
• To evaluate the potential for detecting super-Earths with H2O-dominated atmospheres and high-altitude water clouds.
• To identify the conditions under which gas absorption features remain observable despite cloud opacity.

Proposed method

• Modeling the visible reflection spectra (600–1000 nm) of cold giant exoplanets with varying methane mixing ratios and cloud deck pressures.
• Using a low-resolution spectral dispersion (R=70) to simulate instrument performance for upcoming direct-imaging missions.
• Applying signal-to-noise ratios of 20 to assess retrieval feasibility under realistic observational conditions.
• Analyzing the impact of cloud deck depth on the detectability of methane's strong and weak absorption bands.
• Simulating the broadband reflectivity of super-Earths with H2O-dominated atmospheres and high-altitude water clouds.
• Projecting the resulting color-color diagram behavior to identify observable constraints on atmospheric properties.

Experimental results

Research questions

• RQ1 Can low-resolution visible reflection spectroscopy retrieve both cloud deck pressure and methane mixing ratio in cold giant exoplanets?
• RQ2 How does the depth of the uppermost cloud deck affect the detectability of methane absorption features?
• RQ3 What spectral conditions allow for unambiguous retrieval of cloud and gas properties from a single reflection spectrum?
• RQ4 What observational signatures would super-Earths with H2O-dominated atmospheres and high-altitude water clouds exhibit in color-color diagrams?
• RQ5 To what extent do high-altitude clouds mute gas absorption features, and how does this constrain atmospheric composition?

Key findings

• With a signal-to-noise ratio of 20 and R=70, cloud deck pressure and methane mixing ratio can be retrieved when the cloud deck is located at 0.6–1.5 bars.
• Increasing the signal-to-noise ratio extends the measurable cloud deck pressure range to 0.2–4 bars.
• When the cloud deck is shallower than 0.2 bars, the weak methane absorption bands are muted, making cloud and gas parameters indistinguishable from a single reflection spectrum.
• For super-Earths with H2O-dominated atmospheres, water clouds are expected to form at pressures below 10^-3 bars, resulting in very high-altitude cloud decks.
• These high-altitude clouds mute most gas absorption features except for water vapor, leading to a confined region in color-color diagrams.
• The distinct color-color signature of such planets enables their identification and atmospheric constraint via broadband photometry.