The temperature at the surface of a terrestrial planet is set by the atmosphere. The surface temperature determines if water can exist as a liquid and therefore characterizes the potential for habitability. We can use the detailed knowledge that is gained from studying the atmospheres of planets and moons within our Solar System to understand the properties of extrasolar planets, with a goal of accurately predicting the possibility of habitability.
Quantitative spectroscopy in the near-IR can be used to determine the composition and altitude of clouds, as well as to identify the conditions near the surface of terrestrial planets. We develop the tools and techniques for remote observation of planetary atmospheres, and build the radiative transfer models that are needed to interpret those images and spectra.
The passage of light through at atmosphere has the potential to deposit energy, as well as to get imprinted with the signature of the composition and structure. For example, at near-infrared wavelengths, the dominant processes for altering the rays of starlight as they travel through an atmosphere are absorption by gas molecules and scattering by particles, such as clouds (big droplets or crystals) and aerosols (small hydrocarbons, usually). The physics can be written down in a straightfoward way, and models of the atmosphere can be created. The radiative transfer equation for a set of discrete layers that are enumerated by $i$, each with optical depths $\delta\tau_i$, is
$$I(\tau_i,\mu) = I(\tau_{i+1},\mu) \, e^{-\delta\tau_i/\mu} + \int\limits\limits_{0}^{\delta\tau_i} S(\tau',\mu) e^{-\tau'/\mu} \frac{d\tau'}{\mu}$$
where $\mu = \cos \theta$ determines the slant-path through the atmosphere defined by the angle, $\theta$, between the ray path relative and the normal (i.e., $\bot$ to surface). The term that is integrated is the contribution within the layer, called the source function, $S$, which can be written as
$$S(\tau',\mu) = \overbrace{ \frac{\tilde{\omega}}{4\pi} \, (\pi F_0) \, P(\mu,-\mu_0) \, e^{-\tau/\mu_0} }^{\mathrm{single \; scattering}} + \overbrace{ \int\limits\limits_{-1}^1 \frac{\tilde\omega}{2} \, I(\tau,\mu') \, P(\mu,\mu')\, d\mu' }^{\mathrm{multiple \; scattering}}.$$
In the top expression for $I$, the first term in the is the direct contribution of gas and particle absorption, and the second (integral) term has been defined as the contribution from scattering. We show the case for an atmosphere that is cold and not self-luminous, like Titan. In the case of a Hot Jupiter, for example, the source function would include a term with Planck's expression for black body radiation. Solving this problem numerically is used to calculate spectra, since each of the terms, $\tau$, $P$, and ${\tilde\omega}$ depend on the wavelength of light being considered. Spectra from atmospheric models can then be compared to observations.
The code for our model is available on github.
If you find the code useful, please cite this 2015 paper where the code was first described, and if you encounter problems, then please get in touch.
Why do the some exoplanets essentially lack structure in the near-IR spectra that would indicate molecular absorption? Do clouds or high-altitude hazes obscure the molecular absorption? What is the source of methane on Titan, and will the currently supply dwindle under irreversible photochemical processing? What is the composition and of Jupiter's clouds? These are the types of questions that we think about and explore with numerical models and observations. Space observatories such as JWST and TESS facilitate our ability to ask new questions about the properties of exoplanets, and NASA missions within our Solar System will continue to explore in detail the atmospheres of the planets that are closer celestial neighbors.
Spectroscopic investigations of the planetary atmospheres abound in the scientific literature, and here we give a few examples of our work. We observe and interpret the images and spectra of both the Ice Giants in the Solar System: we study the aerosol structure in Neptune's atmosphere. As a comparison, we've also studied the clouds and aerosol on Uranus. Using higher spectral resolution, at slightly longer wavelengths, we can study the water cloud altitudes on Jupiter.
For terrestrial-like planets we can explore the frigid limits of a moon that has hydrological cycle of natural gas. Methane (on Titan) plays the role of water (on Earth), forming clouds, rain, and pooling in lakes. In our research we use spectroscopy to make measurements of the methane humidity on Titan. This intriguing hydrological cycle has a vast reservoir of methane in the atmosphere, where condensation and evaporation should lead to isotopic fractionation of the gas.
