MAS January Monthly Meeting

Topic: Killing the hurricane at the south pole of Saturn

Description: Cassini observations show that Saturn’s polar regions contain giant cyclones and unusual cloud configurations, including dark eyes that bring to mind the eyes of earthly hurricanes. Both on Earth and on Saturn, these eyes are regions of reduced cloud cover resulting from descending motions. Cassini images of the south polar regions also showed that circular cloud bands at the edge of the eye cast very long shadows. This led to the conclusion that there were towering convective storms reaching into the stratosphere, producing eyewalls that were casting shadows, much like hurricanes on earth, but on a vastly larger scale. I will show that this conclusion is completely wrong, that it is inconsistent with other observations, and that the shadows themselves can be explained by a very different phenomenon.

The trail begins with spectral observations by the Visual and Infrared Mapping Spectrometer (VIMS). Fitting these observations with model clouds failed to confirm the presence of optically thick eyewalls. But how can shadows be produced without an eyewall? The answer is surprisingly simple, as I will show qualitatively by a simple physical model, and quantitatively with Monte Carlo calculations that were needed to deal with light scattering at discontinuous boundaries. The physical model and Monte Carlo calculations also show that something called antishadows should also be produced on Saturn, and they are in fact observed, further confirming the explanation.

Dr. Larry Sromovsky is a Senior Scientist at the UW Madison’s Space Science and Engineering Center (SSEC). He was a co-investigator for Voyager Imaging Team support at SSEC through the Neptune encounter. He later served as principle investigator of the Galileo Net Flux Radiometer, which successfully probed Jupiter’s atmosphere in 1995. Since 1993 his research has focused primarily on dynamics and cloud structures of outer planet atmospheres, making use of spectral and imaging observations by Cassini and New Horizons space missions, the Hubble Space Telescope, and NASA’s IRTF, Gemini, and the Keck ground-based observatories. Dr. Sromovsky received his Ph.D. in physics from the UW Madison in 1970.

Space Place is located in the basement of the Villager Mall in Madison, at 2300 South Park Street, on the west side of the street, between Hughes Place and Ridgewood Way, about three blocks north of the Beltline (Google Map).

Park in front near the center of the Mall, and enter at the Atrium Entrance. Once inside, walk down the hallway to the stairs and go downstairs. Space Place occupies the left-hand part of the basement just beyond the doors at the bottom of the stairs. There is also an elevator available.