Using Cassini VIMS Stellar Occultations to Investigate
Geostrophic Winds in Saturn’s StratosphereNicholas I. Merritt 1 2,
Dr. Philip D. Nicholson1
Citations and Acknowledgments
Results and Future WorkProbing Saturn's Atmosphere With
Starlight
Cassini's Visual and Infrared Mapping Spectrometer
(VIMS) was designed to gather information about
the temperature and composition of Saturn and its
moons. VIMS was a 64 x 64
pixel imaging spectrometer, each pixel
having 256 spectral channels between 1-5µm. VIMS
Stellar occultations were performed by using a
single pixel at high temporal resolution to track a
star as it set behind Saturn’s atmosphere. This
allowed VIMS to capture the changes in the stellar
spectra as the starlight is absorbed and refracted by
the atmosphere. The time series of
spectra hold information about
the composition, temperature, and structure of
the atmosphere. During Saturn occultations, the
primary sources of stellar signal extinction
are refraction and molecular absorption
by hydrocarbons.
Alpha Centauri setting over Saturn as seen
by Cassini in viewing style of an occultation.
[1] R. French, E. Marouf, N. Rappaport,
and C. McGhee. 2010 Occultation
Observation of Saturn’s B Ring and
Cassini Division. The Astronomical
Journal, 139:1649–1667.
[2] Beatty, J. Kelly, et al., editors. The New Solar System.
Cambridge University Press, 1999.
[1] Cornell University, [2] Colorado College
Thanks to the Cornell Center for Astrophysics and Planetary
Science, The 2018 Cornell REU program, P.I. Philip Nicholson, Todd
Ansty, Matthew Hedman,
the Cassini Team, and to the NSF for funding the
REU program via NST/AST-1659264.Additional thanks to Cornell
faculty Peter Gierasch,
Don Banfield, and graduate students Andrew Foster,
Paul Corlies, Cody Lamarche, and Matt Hankins.
Data Analysis
A time series of spectra
from an occultation
event. The horizontal
dark spikes are molecular
absorption features, and
the vertical dark and
light lines are
scintillations caused by
atmospheric lensing of
the starlight.
With the goal of understanding Saturn’s geostrophic winds, we
measured the shape of the
planet in order to identify distortion in the radius caused by
the winds. Using the occultation
data, we assessed specific wavelength ranges to isolate the
light attenuation curves caused
by refraction and molecular absorption. In the VIMS λ range of
1-5µm, there are multiple
hydrocarbon absorption features (methane bands) as well λ ranges
where signal attenuation
is dominated by refraction (continuum bands). By finding the
altitudes at which stellar flux is
attenuated by 50% (half-light altitude) within different
wavelength bands at different
latitudes via 35 different occultation measurements, we created
multiple shape profiles of
the planet.
[email protected]
Plots courtesy of Dr. Phil Nicholson, Cornell
University: The six shape curves calculated with
respect to the standard geoid. All plots show a
prominent equatorial bulge, a strong indication that
cloud-level zonal winds extend up into the
stratosphere.
You can see the
view of Saturn
from Cassini at
any time during
the mission via
the URL of this
QR Code.
From spacecraft orbit geometry, half-light
times were converted to radii. When
plotted vs latitude of the occultation
event, these radii create a shape of Saturn.
In order to isolate possible effects of zonal
winds, a reference ellipsoid shape of
Saturn, referred to as the “Geoid”, was
subtracted from the data. The half-light
radii vs latitude profiles all show a
prominent equatorial bulge of 100-150
km, strongly suggesting that the zonal
winds seen at the cloud-top level extend
largely undiminished up into the
stratosphere. Future work will include
correcting radii for the effects of refraction
and varying occultation geometry.
