# Planetary Atmospheres

The Sunlight that we can observe illuminating a planet carries a tremendous about of information about the far away atmosphere. We know that a diverse range of exotic physical environments exist among the planets and their moons. I learn about these places using telescopes on Earth or instruments on spacecraft. Using the powerful technique of infrared spectroscopy, we can study the atmospheric physics of clouds, precipation, and circulation in a wide variety circumstances. A few specific examples what I've worked on are described below:

# Titan

The weather is a daily reminder of the changes in our environment, and can inspire the search for a deeper understanding of our physical world. From the local weekly weather forecast, to knowledge of regional and seasonal conditions, to predicting the global climate response of our atmosphere in the coming decades due to significant anthropogenic changes, there are intriguing challenges in understanding our atmosphere.

Since the Cassini mission to the Saturn system, it has become possible to evaluate the physics and models that we use to address these challenges on Earth by applying our understanding to exotic environments with fascinating meteorology. Saturn’s largest moon, Titan, is the one place in the Solar System beyond our planet where fluids on a rocky surface interact with dense atmosphere, forming clouds, fog and rain, in a strangely familiar hydrological cycle that operates at 290 degrees below zero. In these frigid conditions, the role of water is played by methane, the dominant component of natural gas. I use measurements from telescopes on Earth and the Cassini spacecraft to inform our knowledge of weather in the Saturn system.

###### From the literature:

Ádámkovics et al. (2010)

Mitchell et al. (2011)

# Giant Planets

Clouds are a visual diagnostic of horizontal motions on the giant planets, and spectroscopy can be used to determine the altitude structure. Measurements of both images and spectra — sometimes referred to as hyper-spectral data, integral-field spectral cubes, or spatially-resolved spectra — can provide a 3-dimensional picture of the fluid motions in the atmospheres of giant planets.

###### From the literature:

Luszcz-Cook et al. (2010)

de Pater et al. (2011)

At near-infrared wavelengths, in the cold outer solar system, the dominant processes for altering the rays of Sunlight as they travel through an atmosphere are the absorption by gas molecules and the 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 azimuthally-averaged radiative transfer equation for discrete layers, $i$, along the path $\mu$ 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 the source function is

$S(\tau',\mu) = \overbrace{ \frac{\tilde{\omega}}{4\pi} \, (\pi F_0) \, P(\mu,-\mu_0) \, e^{-\tau/\mu_0} }^{\mathrm{single \; scattering}} + \overbrace{ \int\limits\limits_{-1}^1 \frac{\tilde\omega}{2} \, I(\tau,\mu') \, P(\mu,\mu')\, d\mu' }^{\mathrm{multiple \; scattering}}$

In the top expression, the first term in the is the direct contribution of gas and particle absorption, and the second (integral) term is the diffuse contribution from scattering. Solving this problem numerically is used to calculate spectra from atmospheric models that are compared to observations.

# Protoplanetary Disk Atmospheres

The flared disk of material around young stars is observed to have a gaseous atmosphere evidenced by atomic ion and molecular emission lines. Temperatures of a few thousand Kelvin are indicated by CO, as well as temperatures of several hundred by the observations of water and small organic molecules. These gas temperatures can be much hotter than the dust continuum emission. One guess about the mechanism that is responsbile for producing such high temperatures is represented by an equation for accretion heating,

$\hspace{2cm}\Lambda_{\rm acc} = 9\alpha_{h} \rho c^2 \Omega/4.$

However, a problem exists in that $\alpha_h$ is not directly constrained by observations, nor is it unambiguously determined by theory. In fact, it's not all that clear that this equation correctly represents all of the most relevant physics, but in the absence of accretion heating it is difficult to interpret the observations of hot gas. Indeed, many of the various heating mechanisms in disk atmospheres rely on local environmental conditions that are inadequately constrained.

## Thermal-chemical models of disks

We explore the various heating (and cooling) mechanisms and resulting chemical signatures throughout disk atmospheres with chemical kinetics models and compare our numerical results to observations. Some of these heating and cooling mechanisms are illustrated below.

Many of the processes are interdependent, and sorting through the thermal-chemical kinetics can be a bit complicated.

###### From the literature:

Ádámkovics, Glassgold & Meijerink (2011)

Najita, Ádámkovics & Glassgold (2011)