CONTOUR Neutral Gas and Ion Mass Spectrometer (NGIMS)
 
The Neutral Gas and Ion Mass Spectrometer will measure the abundance and
isotope ratios for many neutral and ion species in the coma of each comet
during the flyby. These measure- ments together with data from the dust
experiment will contribute to the understanding of the chemi- cal composition
of the nucleus itself and allow differences between the comets to be studied.
NIGMS is a quadrupole mass spectrometer that employs two ion sources each
optimized for a specific set of measurements. Using these two sources, NGIMS
will rapidly switch between measurements of the cometary neutral gas and
ambient ions from the coma as the CONTOUR spacecraft flys by the nucleus.
Understanding the chemical state of comets is expected to provide the best
possible record of conditions present in the outer solar nebula early in its
history, since the comet nuclei of comets likely contains primitive materials
unaltered by the chemical and physical processes that have trans- formed
other solar system objects.
 
 
Science Objectives of NGIMS
 
The chemical composition measurements of NGIMS are expected to provide an
understanding of the relative abundances of simple species such as H 2 O, CH
4 , CO 2 , NH 3 , and H 2 S and considerably more complex molecules in the
early stages of the protosolar nebula. Since comets have been in cold storage
since their formation, this record is ex- pected to be largely preserved.
Cometary infall contributes volatile species to the planets including the
atmosphere and oceans of the Earth and the chemi- cal complexity of this
material from space can be understood through the NGIMS data. Another
scientific goal is to clarify the relationship of the solar nebula to the
parent inter- stellar cloud. A key to each of these scientific objectives is
the measurement of isotope ratios such as D/H and the abundance of
'thermometer' gases in comets such as argon and other simple species. The
history of the chemical and physical processing of the nucleus material can
only be understood though such a detailed study.
 
 
 
NGIMS Functions
 
Direct Gas Sampling in the Open Ion Source:
The open source of the NGIMS minimizes gas/surface interaction effects by
directly sampling gaseous species that are formed into a beam before the
ioniza- tion region by collimating apertures. The open source measures
ambient particle density directly for all neutral species.
 
High Sensitivity Sampling in the Closed Ion Source:
The closed ion source will use RAM density enhancement to provide
measurements of higher accuracy and sensitivity for the more inert atomic and
molecular species than provided by the open source. The sensitivity
enhancement is achieved by sam- pling the ambient gas through an orifice into
an enclosed antechamber. The density enhancement in the source compared to
the ambient density can be as high as several hundred depending on the flyby
velocity, the molecular weight of the sampled species, and the source
temperature. The closed source works best with species that do not chemically
transform on the source walls.
 
Open Source Sampling of Thermal and Medium Energy Ions:
Ion species present in the coma within the mass range of the NGIMS will be
sampled through the open source. These ions are created by photoionization,
electron ionization, and other processes acting on gases in the coma.
 
Ion Source Selection in the Quadrupole Deflector:
Ions are sequentially directed to the mass analyzer from the selected ion
source by changing the potentials on the quadrupole deflector. This
electrostatic device allows either source of ions to be focusd into a common
exit lens system.
 
The Quadrupole Mass Analyzer:
The quadrupole mass analyzer consists of four pre- cision ground hyperbolic
rods mounted in a rigid me- chanical assembly. The transmitted mass to charge
value and the resolution are controlled by variations in RF and DC electric
fields between adjacent rod pairs while opposite rod pairs are kept at the
same potential. Three fixed frequencies (in the 1 to 5 MHz range) are
selected to cover the mass range (1 to 300 amu). The Dual Detector System:
The detector system counts ions transmitted through the quadrupole analyzer
to pro- duce a mass spectrum. The system is redundant and con- figured for a
wide dynamic range.
 
 
 
NGIMS Management
 
 
NGIMS Co-Investigators: Hasso Niemann (NASA/GSFC) Paul Mahaffy (NASA/GSFC)
NGIMS Instrument Manager: Jack Richards (NASA/GSFC)
 
 
From http://www.contour2002.org/instruments.html
 
Copyright 2002 NASA Discovery Program
Used by Permission