On the Stability of Super-Earth Atmospheres

We investigate the stability of super Earth atmospheres around M stars using a 7-parameter, analytical framework. We construct stability diagrams in the parameter space of exoplanetary radius versus semi-major axis and elucidate the regions in which the atmospheres are stable against the condensation of their major constituents, out of the gas phase, on their permanent nightside hemispheres. We find that super Earth atmospheres which are nitrogen-dominated ("Earth-like") occupy a smaller region of allowed parameter space, compared to hydrogen-dominated atmospheres, because of the dual effects of diminished advection and enhanced radiative cooling. Furthermore, some super Earths which reside within the habitable zones of M stars may not possess stable atmospheres, depending on the mean molecular weight and infrared photospheric pressure of their atmospheres. We apply our stability diagrams to GJ 436b and GJ 1214b, and demonstrate that atmospheric compositions with high mean molecular weights are disfavoured if these exoplanets possess solid surfaces and shallow atmospheres. Finally, we construct stability diagrams tailored to the Kepler dataset, for G and K stars, and predict that about half of the exoplanet candidates are expected to habour stable atmospheres if Earth-like conditions are assumed. We include 55 Cancri e and CoRoT-7b in our stability diagram for G stars.Comment: Accepted by ApJ. 10 pages, 6 figures. No changes from previous version, except for added hypen in titl

Exoplanetary Atmospheric Characterization Using Polarimetry and Other Radiative Transfer Modeling Problems

This thesis deals with a pair of current problems with the remote sensing of planetary atmospheres. First is the modeling of polarization of scattered light from the atmospheres of exoplanets. With the first such observations becoming possible in the last year, there is a need to understand what these measurements actually mean. To that end, we developed families of radiative transfer models that simulate polarized phase curves for different atmospheric scenarios on hot Jupiters. These models were then used in the interpretation of scattered light from HD 189733b and WASP 12b, two hot Jupiter exoplanets, to determine their albedos and gauge what type of scattering particles might be present in their atmospheres. The last part of this half deals with observing oceans on distant Earth-like exoplanets using polarization from glint off the water surface. Though this measurement is not possible with current telescopes, but it may become accessible in the next decade with a slew of high powered ground and space telescopes in the pipeline. The second half of the thesis is devoted to the development of a fast radiative transfer model. The goal of this model is to be able to process the massive amounts of data coming in from Earth observing satellites such as GOSAT and OCO-2 in a timely and accurate manner. We refined the principal component analysis based fast radiative transfer model to be accurate enough to retrieve carbon dioxide concentrations to the part per million accuracy that is necessary to track spatial and temporal changes in this important greenhouse gas.</p

Aggregate Particles in the Plumes of Enceladus

Estimates of the total particulate mass of the plumes of Enceladus are important to constrain theories of particle formation and transport at the surface and interior of the satellite. We revisit the calculations of Ingersoll & Ewald (2011), who estimated the particulate mass of the Enceladus plumes from strongly forward scattered light in Cassini ISS images. We model the plume as a combination of spherical particles and irregular aggregates resulting from the coagulation of spherical monomers, the latter of which allows for plumes of lower particulate mass. Though a continuum of solutions are permitted by the model, the best fits to the ISS data consist either of low mass plumes composed entirely of small aggregates or high mass plumes composed of mostly spheres. The high particulate mass plumes have total particulate masses of (166 $\pm$ 42) $\times$ 10$^3$ kg, consistent with the results of Ingersoll & Ewald (2011). The low particulate mass plumes have masses of (25 $\pm$ 4) $\times$ 10$^3$ kg, leading to a solid to vapor mass ratio of 0.07 $\pm$ 0.01 for the plume. If indeed the plumes are made of such aggregates, then a vapor-based origin for the plume particles cannot be ruled out. Finally, we show that the residence time of the monomers inside the plume vents is sufficiently long for Brownian coagulation to form the aggregates before they are ejected to space.Comment: 44 pages, 8 figures, 2 tables, Published in Icaru

Principal Components of Short-term Variability in Venus' UV Albedo

We explore the dominant modes of variability in the observed albedo at the cloud tops of Venus using the Akatsuki UVI 283-nm and 365-nm observations, which are sensitive to SO2 and unknown UV absorber distributions respectively, over the period Dec 2016 to May 2018. The observations consist of images of the dayside of Venus, most often observed at intervals of 2 hours, but interspersed with longer gaps. The orbit of the spacecraft does not allow for continuous observation of the full dayside, and the unobserved regions cause significant gaps in the datasets. Each dataset is subdivided into three subsets for three observing periods, the unobserved data are interpolated and each subset is then subjected to a principal component analysis (PCA) to find six oscillating patterns in the albedo. Principal components in all three periods show similar morphologies at 283-nm but are much more variable at 365-nm. Some spatial patterns and the time scales of these modes correspond to well known physical processes in the atmosphere of Venus such as the ~4 day Kelvin wave, 5 day Rossby waves and the overturning circulation, while others defy a simple explanation. We also a find a hemispheric mode that is not well understood and discuss its implications.Comment: 9 pages, 6 figures, accepted in A&

Improved simulation of extreme precipitation in a high-resolution atmosphere model

Climate models often underestimate the magnitude of extreme precipitation. We compare the performance of a high-resolution (∼0.25°) time-slice atmospheric simulation (1979–2005) of the Community Earth System Model 1.0 in representing daily extreme precipitation events against those of the same model at lower resolutions (∼1° and 2°). We find significant increases in the simulated levels of daily extreme precipitation over Europe, the United States, and Australia. In many cases the increase in high percentiles (>95th) of daily precipitation leads to better agreement with observational data sets. For lower percentiles, we find that increasing resolution does not significantly increase values of simulated precipitation. We argue that the reduced biases mainly result from the higher resolution models resolving more key physical processes controlling heavy precipitation. We conclude that while high resolution is vital for accurately simulating extreme precipitation, considerable biases remain at the highest available model resolutions

A Multiple Scattering Polarized Radiative Transfer Model: Application to HD 189733b

We present a multiple scattering vector radiative transfer model which produces disk integrated, full phase polarized light curves for reflected light from an exoplanetary atmosphere. We validate our model against results from published analytical and computational models and discuss a small number of cases relevant to the existing and possible near-future observations of the exoplanet HD 189733b. HD 189733b is arguably the most well observed exoplanet to date and the only exoplanet to be observed in polarized light, yet it is debated if the planet's atmosphere is cloudy or clear. We model reflected light from clear atmospheres with Rayleigh scattering, and cloudy or hazy atmospheres with Mie and fractal aggregate particles. We show that clear and cloudy atmospheres have large differences in polarized light as compared to simple flux measurements, though existing observations are insufficient to make this distinction. Futhermore, we show that atmospheres that are spatially inhomogeneous, such as being partially covered by clouds or hazes, exhibit larger contrasts in polarized light when compared to clear atmospheres. This effect can potentially be used to identify patchy clouds in exoplanets. Given a set of full phase polarimetric measurements, this model can constrain the geometric albedo, properties of scattering particles in the atmosphere and the longitude of the ascending node of the orbit. The model is used to interpret new polarimetric observations of HD 189733b in a companion paper.Comment: 13 pages, 13 figures. Accepted for publication in Ap

Quantifying the impact of aerosol scattering on the retrieval of methane from airborne remote sensing measurements

As a greenhouse gas with strong global warming potential, atmospheric methane (CH₄) emissions have attracted a great deal of attention. Although remote sensing measurements can provide information about CH₄ sources and emissions, accurate retrieval is challenging due to the influence of atmospheric aerosol scattering. In this study, imaging spectroscopic measurements from the Airborne Visible/Infrared Imaging Spectrometer – Next Generation (AVIRIS-NG) in the shortwave infrared are used to compare two retrieval techniques – the traditional matched filter (MF) method and the optimal estimation (OE) method, which is a popular approach for trace gas retrievals. Using a numerically efficient radiative transfer model with an exact single-scattering component and a two-stream multiple-scattering component, we also simulate AVIRIS-NG measurements for different scenarios and quantify the impact of aerosol scattering in the two retrieval schemes by including aerosols in the simulations but not in the retrievals. The presence of aerosols causes an underestimation of CH₄ in both the MF and OE retrievals; the biases increase with increasing surface albedo and aerosol optical depth (AOD). Aerosol types with high single-scattering albedo and low asymmetry parameter (such as water-soluble aerosols) induce large biases in the retrieval. When scattering effects are neglected, the MF method exhibits lower fractional retrieval bias compared to the OE method at high CH₄ concentrations (2–5 times typical background values) and is suitable for detecting strong CH₄ emissions. For an AOD value of 0.3, the fractional biases of the MF retrievals are between 1.3 % and 4.5 %, while the corresponding values for OE retrievals are in the 2.8 %–5.6 % range. On the other hand, the OE method is an optimal technique for diffuse sources (<1.5 times typical background values), showing up to 5 times smaller fractional retrieval bias (8.6 %) than the MF method (42.6 %) for the same AOD scenario. However, when aerosol scattering is significant, the OE method is superior since it provides a means to reduce biases by simultaneously retrieving AOD, surface albedo, and CH₄. The results indicate that, while the MF method is good for plume detection, the OE method should be employed to quantify CH₄ concentrations, especially in the presence of aerosol scattering

Observing Oceans in Tightly Packed Planetary Systems: Perspectives from Polarization Modeling of the TRAPPIST-1 System

The recently discovered TRAPPIST-1 system is exciting due to the possibility of several rocky, Earth-sized planets harboring liquid water on their surface. To assess the detectability of oceans on these planets, we model the disk-integrated phase curves and polarization signals for planets in this system for reflected starlight. We examine four cases: (1) dry planet, (2) cloud-covered planet, (3) planet with regional-scale oceans, and (4) planet with global oceans. Polarization signals are strongest for optically thin (≾ 0.1) atmospheres over widespread oceans, with the degree of polarization being up to 90% for a single planet or on the order of 100 parts per billion for the star–planet system. In cases where reflected light from different planets in a tightly packed system cannot be separated, observing in polarized light allows for up to a tenfold increase in star–planet contrast compared to photometric observations alone. However, polarization from other sources, such as atmospheric scattering and cloud variability, will pose major challenges to the detection of glint (specularly reflected starlight) polarization signals. Planned telescopes like LUVOIR may be capable of observing glint from Earth-like planets around Sun-like stars, and if equipped with a polarimeter can significantly improve our ability to detect and study oceans on rocky exoplanets

A Ground-Based Albedo Upper Limit for HD 189733b from Polarimetry

We present 50 nights of polarimetric observations of HD 189733 in $B$ band using the POLISH2 aperture-integrated polarimeter at the Lick Observatory Shane 3-m telescope. This instrument, commissioned in 2011, is designed to search for Rayleigh scattering from short-period exoplanets due to the polarized nature of scattered light. Since these planets are spatially unresolvable from their host stars, the relative contribution of the planet-to-total system polarization is expected to vary with an amplitude of order 10 parts per million (ppm) over the course of the orbit. Non-zero and also variable at the 10 ppm level, the inherent polarization of the Lick 3-m telescope limits the accuracy of our measurements and currently inhibits conclusive detection of scattered light from this exoplanet. However, the amplitude of observed variability conservatively sets a $3 \sigma$ upper limit to the planet-induced polarization of the system of 58 ppm in $B$ band, which is consistent with a previous upper limit from the POLISH instrument at the Palomar Observatory 5-m telescope (Wiktorowicz 2009). A physically-motivated Rayleigh scattering model, which includes the depolarizing effects of multiple scattering, is used to conservatively set a $3 \sigma$ upper limit to the geometric albedo of HD 189733b of $A_g < 0.37$. This value is consistent with the value $A_g = 0.226 \pm 0.091$ derived from occultation observations with HST STIS (Evans et al. 2013), but it is inconsistent with the large $A_g = 0.61 \pm 0.12$ albedo reported by (Berdyugina et al. 2011).Comment: 10 pages, 9 figures, submitted to Ap

Reflected Light Curves, Spherical and Bond Albedos of Jupiter- and Saturn-like Exoplanets

Reflected light curves observed for exoplanets indicate that a few of them host bright clouds. We estimate how the light curve and total stellar heating of a planet depends on forward and backward scattering in the clouds based on Pioneer and Cassini spacecraft images of Jupiter and Saturn. We fit analytical functions to the local reflected brightnesses of Jupiter and Saturn depending on the planet's phase. These observations cover broadbands at 0.59–0.72 and 0.39–0.5 μm, and narrowbands at 0.938 (atmospheric window), 0.889 (CH4 absorption band), and 0.24–0.28 μm. We simulate the images of the planets with a ray-tracing model, and disk-integrate them to produce the full-orbit light curves. For Jupiter, we also fit the modeled light curves to the observed full-disk brightness. We derive spherical albedos for Jupiter and Saturn, and for planets with Lambertian and Rayleigh-scattering atmospheres. Jupiter-like atmospheres can produce light curves that are a factor of two fainter at half-phase than the Lambertian planet, given the same geometric albedo at transit. The spherical albedo is typically lower than for a Lambertian planet by up to a factor of ~1.5. The Lambertian assumption will underestimate the absorption of the stellar light and the equilibrium temperature of the planetary atmosphere. We also compare our light curves with the light curves of solid bodies: the moons Enceladus and Callisto. Their strong backscattering peak within a few degrees of opposition (secondary eclipse) can lead to an even stronger underestimate of the stellar heating