University of Maryland
Computer & Space Science Building, Room 2400
4:30 PM Monday, 27 February
Coffee, Tea & Cookies 4:00-4:30 PM
Mark Shappirio
GSFC
Ion Mass Spectrometry at Saturn: Results, discoveries, and lessons learned.
For the last year and a half, Cassini has
been orbiting Saturn, passing over, around and through the rings, and
traveling past Saturn’s many moons. The Cassini Plasma suite (CAPS) has
been sampling the magnetospheres population of ions, investigating its
interaction with Saturn, the rings and moons, and the solar wind.
Previous to the arrival of Cassini at Saturn, models existed for a
variety of sources for the different species of ions within the
magnetosphere, including the rings, the inner moons and Titan. Since
the arrival of Cassini, we have been trying to confirm which models are
the most accurate.
The Ion Mass Spectrometer (IMS) in the CAPS suite
produces a great deal of information about the magnetospheric ions, but
it is also a complicated instrument that we are still trying to
understand. IMS uses thin carbon foils to generate secondary electrons
for a start signal and the plasma ions, now neutralized, for a stop
signal, a standard time of flight technique, to obtain a high count
spectrum with a mass resolution M/ΔM ~ 8 amu. At the same time the TOF
region of the instrument has a linear electric field so that the plasma
ions which retain a positive charge when exiting the carbon foils
(typically < 10% of the incident flux) are focused onto a separate
detector which gives a low count, high mass resolution (M/ΔM ~ 50)
spectra. The TOF of these positive ions in a perfectly linear electric
field would be independent of energy. But in order to be able to
distinguish atomic species from the same species in a molecule (say
atomic H from H from a water ion) the electric field was slight
“detuned”, giving the high resolution spectra a slight, non-linear,
energy dependence. In addition, some species pick up a negative charge
from the carbon foils, namely H, C and O. These negative ions are then
accelerated and focused by the linear electric field and get
incorporated into the low mass resolution high count spectra, in some
cases interfering with neutral particle TOFs.
Despite, or in some cases
because, of the complications of the IMS instrument a .number of
discoveries have been made. IMS has found atomic and possibly molecular
nitrogen in Saturns magnetosphere and localized its source to the
e-ring/Enceladus (Young et al. 2005; Smith et al. 2005a). Water group
ions (covering O+, OH+, H2O+ and H3O+) have also been measured also
with a source in the inner region of the magnetosphere, either the
rings or the icy satellites or both. And molecular oxygen from O2+ has
been measured, with a potential source somewhere in the region of
Dione. Finally there is evidence of small amounts of methane or methane
fragments (CHx) whose source has not yet been localized.
Sponsored by: Department of Physics
and the Institute for Physical Science and Technology, University of Maryland. For information
call John Paquette at (301) 405-6208 or go to the UMD Space Physics group seminar web site.
For free parking please park in lot DD or anywhere on levels 1-2 in lot B (the big parking garage) after 4:00 pm.
Make sure that you park in a spot WITHOUT a parking meter. More parking information is at the seminar website.