Research
Titan is a strange place, but in strange ways it is not unlike Earth. The surface temperatures on Titan would be considered cryogenic on Earth. But the main component of its thick atmosphere is molecular Nitrogen, just like Earth. Water is abundant on Titan, but of course it is all frozen in the crust at such low temperatures. But another substance, methane, is thermodynamically active in the lower atmosphere; much like water vapor on Earth, methane on Titan forms clouds, precipitates, and is resupplied from surface sources. The runoff then weathers the cold surface of Titan, creating what appears to be river patterns. But one of the more interesting comparisons between Titan and Earth is a bit more subtle. It is thought that Earth, shortly after it formed an atmosphere, had large amounts of methane and very little oxygen. Since methane is very active radiatively, it provided an important greenhouse warming that probably prevented the Earth from staying perpetually in a completely frozen state that otherwise would have resulted from the weaker sunlight from the very young Sun. So by studying Titan's modern climate, we may be understanding new things about the way the early Earth's climate was.
To that end, we have developed a simplified atmospheric model to study the climate of Titan. Our model shows some interesting things about the cloud patterns on Titan (see image above). First, we find that we must include the thermodynamic effects of methane convection in order to produce the observed sparse clouds. Second, we find the seasonal response of the large-scale circulations of the atmosphere can explain the location of the observed clouds during the current Southern Summer Solstice, but again, only with the effect of methane thermodynamics included. This result is somewhat contradictory to others since methane thermodynamics was until now thought to have little effect on the overall state of the atmosphere. We feel it is important to use a simplified model like ours to better understand the mechanisms controlling aspects of the climate, and in part to inform more realistic GCM simulations that are otherwise too complicated to understand in detail. Were we to include the full effect of radiation, for example, the computational cost of doing parameter studies with a GCM would be prohibitive. See our PNAS article for a full report of our findings.
The climate dynamics of Titan
Image credits: NASA/JPL/Univ. of Arizona
Geophysical Fluid Dynamics
In order to best understand planetary climate, it is necessary to develop theoretical tools that fill the spectrum of complexity from simple “paper and pencil” theories to full General Circulation Models. At the simpler end, attempts have been made to distill the circulation into fundamental physical laws like energy and momentum conservation. A canonical theory on tropical dynamics due to Held and Hou (1980) posits that the overturning of the atmosphere, or Hadley cell, conserves angular momentum aloft and does work on the background stratification in order to transport heat out of the tropics. The results of this theory makes predictions for the width and strength of the Hadley cell on fundamental, non-dimensional parameters, including the Rossby number. The original theory had simplifications including a Boussinesq approximation and assumed diabatic heating (moist convection and radiation) can be represented by Newtonian cooling. It has been our goal to extend this theory in two ways: firstly, we use a fully compressible fluid, and secondly, we use gray radiation and a “hard adjustment” for convection. We find that in the Boussines limit, we can reproduce the Held and Hou result for the width of the Hadley cell exactly. The theory compares almost perfectly with numerical integrations of an axisymmetric GCM.
The major advantage of such a simple theory is the predictive power it enables for certain planetary climates. We think this will become an important tool in the coming decades as terrestrial extrasolar planets will be discovered, and the issues of the physics of habitability come to the forefront of the search for life. In the meantime, it offers a simple framework in which to classify planets and moons in our Solar System with thick atmospheres.
We are currently summarizing the results in a manuscript that we will soon submit for review.
Earth’s Paleoclimate
There is substantial geological evidence that the Earth has gone through up to two epochs when the global oceans were completely iced-over, or a “hard snowball”. Obviously, the Earth was able to get out of these snowball states, if they did occur. But due to the feedback of the high albedo of snow, it would have been very difficult for it to. So the question becomes, how did it melt? The strongest theory for deglaciating a hard snowball is that globally ice-covered oceans prohibit the uptake of carbon into the ocean and subsequent burial; assuming volcanoes continue to outgas, carbon dioxide would build in the atmosphere to very high levels until the greenhouse warming it provides is enough to trigger deglaciation. Our models show this mechanism to be weaker than previous studies have predicted. The culprit is a reduced lapse rate in a snowball climate, especially in the winter hemisphere, which weakens the greenhouse warming from elevated concentrations of carbon dioxide.
We are currently writing this work up, and will soon submit it for review.
Former research:
Gravitational Lensing
Published in ApJ 20 March 2005, v622. PDF
Statement of Research Interests PDF
The focus of my current research is primarily Titan, which has led to some important discoveries that are explained in the attached manuscripts. First, I identified methane thermodynamics as an important mechanism controlling the latitudinal positions and seasonality of convective methane clouds in Titan’s lower atmosphere; previous work had largely dismissed this effect. Second, I identified a seasonally oscillating overturning circulation, or Hadley cell, as the mechanism that dries the low-latitude surface and supports equatorial desert-like dunes. Third by comparing simulations with available data, I estimated that the current methane reservoir is largely contained in the atmosphere, which was born out by Cassini observations. And most recently, I suggested the temporal phase lag seen in recent observations of Titan’s spin rate is the result of methane thermodynamics in the atmosphere. Some of my current work is aimed at linking fluid instability to the periodicity of Titan’s mid-latitude clouds. I plan to address more of Titan’s mysteries in future work including the persistence of Titan’s methane reservoir over long timescales, the distribution of atmospheric angular momentum, and the role of spin-orbit coupling in the origin of Titan’s large eccentricity.
In the near term, I am extending my work on planetary atmospheres to two extrasolar contexts. The first application is to close-in extrasolar giant planets (CEGPs) for which there are now data from the Spitzer Space Telescope. Current interpretation of observed phase curves of CEGPs is limited by our understanding of heat redistribution by atmospheres. I am currently developing a simplified general circulation model (GCM) to identify mechanisms leading to observed phenomena. There may also be use for certain analytic, steady circulation theories developed in the context monsoons on Earth due to the permanent day-night contrast on CEGPs.
My second interest in extrasolar planets has been described as the “M-dwarf opportunity”; current transit searches for extrasolar planets are targeting M-dwarfs because the transit probability is high. It is likely the first terrestrial planet of several Earth masses will be discovered around an M-dwarf in the next few years. Measurements of phase curves will become possible for those discovered closest to their host stars by the James Webb Space Telescope, as is currently being done for CEGPs using Spitzer. The discovery and characterization of “super-Earths” around M-dwarfs will require new theories for planetary formation, dynamics, atmospheres, and climate.
As an astronomer with the training of a geoscientist, my interests are at the intersection of astrophysical environments and planetary phenomena. The scope of discoveries in the era of extrasolar planets will continue to open exciting new opportunities in the area of my interests.