Peter J. Gierasch
Emeritus Professor of Astronomy
Dynamics of Local Regions on Jupiter, Outer Planets Atmospheric Structure, Stability of Jets on Jupiter and Saturn., Cassini Imaging Science Subsystem
Peter Gierasch's research is on the dynamics and thermal structure of planetary atmospheres. The general objective is to advance understanding in atmospheric science by studying the range of behaviors exhibited on the different planets. Topics of particular interest to him are the zonal jets of Jupiter and Saturn, the cloud structure and energy balance of Jupiter, Saturn, Uranus, and Neptune, the general circulation and strong super rotation of the atmosphere of Venus, and Martian atmospheric dust and its influence on climate.
This work depends on data from spacecraft. Gierasch has been involved in the NASA Viking orbiter-lander mission to Mars, the Pioneer Venus mission, and the Voyager flybys of the outer planets. He was an interdisciplinary scientist on the Galileo orbiter-probe mission to Jupiter and is a member of the infrared spectroscopy experiment on the Cassini Saturn orbiter and Titan probe mission. Gierasch has participated in planning of observations, in data analysis, and in interpretation, including modeling.
Temperatures on Saturn's largest satellite, Titan, can be mapped with data from the NASA Cassini spacecraft infrared spectrometer, and these maps have proved useful tools to diagnose atmospheric dynamics. Visible light images show featureless clouds. Titan, Saturn's largest satellite, possesses a thick atmosphere of nitrogen, argon and methane. Previous data of several kinds had suggested that this atmosphere rotates much more rapidly than the solid satellite, at least at high altitudes. Data from Cassini confirm the strange behavior, showing that the upper atmosphere rotates about 16 times more rapidly than the solid surface. Thus Titan is the second case in our solar system, along with Venus, of a super-rotating atmosphere. These flows represent major puzzles in atmospheric science.
In collaboration with Research Associate Anthony Toigo, Gierasch is studying dynamics in the atmosphere of Pluto. There is evidence from stellar occultations of wave-like disturbances in this atmosphere, and it has been a mystery how dynamical activity could be driven with the weak solar heating available. Toigo and Gierasch hypothesize that sublimation from surface N2 frost cause diurnal "breathing" in and out of the surface, driving atmospheric tides.
Gierasch has taken the lead in using observations of stellar occultations by Saturn made by the Cassini VIMS (Visual and Infrared Mapping Spectrometer) experiment team, an effort led by prof. Philip Nicholson, to study the atmosphere of Saturn. We do not know the helium mixing ratio on Saturn to better than 5%. Gierasch hopes the new data will allow a determination to within 1%, which is needed to discriminate between alternative theories of planetary formation.
In addition to this research, Gierasch is also working, with Senior Research Associate Don Banfield, in developing laboratory exercises involving rotating fluids with extensive web-based visual resources for remote use. Rotating fluids have Coriolis accelerations. The associated constraints on motion produce striking and non-intuitive effects. These can be studied mathematically, but visual exposure to rotating tanks gives deeper understanding of phenomena and brings the mathematics to life.
- Roman, M.T., R.A. West, D.J. Banfield, P.J. Gierasch, R.K. Achterberg, C.A. Nixon, P.C. Thomas, 2009. Determining a tilt in Titan’s north-south albedo asymmetry from Cassini images. Icarus, 203, 242-249.
- Read, P. L., B. J. Conrath, L. N. Fletcher, P. J. Gierasch, A. A. Simon-Miller, L. C. Zuchowski, 2009. Mapping potential vorticity dynamics on Saturn: Zonal mean circulation from Cassini and Voyager data. Planetary and Space Science, 57, 1682-1608.
- Simon-Miller, A. A. and P. J. Gierasch, 2010. On the Long-Term Variability of Jupiter’s Winds and Brightness from Hubble. Icarus, 210, 258-269.
- Toigo, A., P. J. Gierasch, B. Sicardy and E. Lellouch, 2010. Thermal tides on Pluto. Icarus, 208, 402-411.
- Li, L., B. J. Conrath, P. J. Gierasch, R. K. Achterberg, C. A. Nixon, A. A. Simon-Miller, F. M. Flasar, D. Banfield, K. H. Baines, R. A. West, A. P. Ingersoll, C. C. Porco, A. R. Vasavada, A. D. Del Genio, A. A. Mamoutkine, M. E. Segura, G. L. Bjoraker, G. S. Orton, L. N. Fletcher, P. G. J. Irwin, P. L. Read and T. Fouchet, 2010. Saturn’s emitted power. J. Geophys. Res, submitted.
- Achterberg, R. K., P. J. Gierasch,B. J. Conrath, F. M. Flasar, C. A. Nixon, 2011,. Temporal variations of Titan’s middle-atmospheric temperatures from 2004 to 2009 observed by Cassini/CIRS. Icarus, 211, 686-698.
- Simon-Miller, A.A., J.H. Rogers, P.J. Gierasch, D. Choi, M.D. Allison, A. Adamoli, and H.-J. Mettig, 2012: Longitudinal variation and waves in Jupiter's south equatorial wind jet. Icarus, 218, 817-830, doi:10.1016/j.icarus.2012.01.022.