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Neutral Mass Spectrometer (LNMS)

NSSDCA ID: 1978-078D-06

Mission Name: Pioneer Venus Large Probe
Principal Investigator:Dr. John H. Hoffman


The objective of this investigation was to measure the composition of the lower atmosphere of Venus. This investigation used a ceramic micro-leak gas inlet and a double-focusing magnetic deflection mass spectrometer. About 50 analyses of the Venusian atmosphere were planned during the probe descent. A separate sample of the atmosphere was analyzed for rare gasses. The analyzer had a mass range of 1 to 208 u and a dynamic range of 1.E+7. The instrument was based on a design flown previously.

The Large Probe Neutral Mass Spectrometer (LNMS) has a mass of 10.9 kg, a volume of 10650 cubic cm, and uses 14 W power. The instrument comprises a miniature magnetic sector-field mass analyzer, a gas inlet and pumping system, and an electronics (microprocessor) package all mounted on a 31 x 36 cm baseplate. The inlet is mounted so it protruded through the wall of the shell just below the equator. The gases entered the apparatus through two microleaks, passed through the spectrometer and then to one zirconium-aluminum-alloy and two zirconium-graphite chemical getters. An ion getter pumping system was used to transfer gases. The instrument also had a noble gas enrichment cell, the Isotope Ratio Measurement Cell (IRMC), which collected an atmospheric sample at the beginning of instrument operation. This sample was purged of carbon dioxide using sorption and getter pumping during the descent to enrich the inert gases in the cell, and then introduced the into the mass spectrometer just before parachute jettison.

The inlet system consisted of a primary microleak and a secondary microleak. The primary miroleak had a conductance of 0.0001 cubic mm per second. The secondary microleak was larger (0.001 cubic mm per second) and was only used in the upper atmosphere where the pressure was lower. The secondary was open for approximately 12 minutes and was valved off shortly before parachute jettison, which took place at about 47 km.

The single focusing magnetic sector-field mass spectrometer consisted of the electron bombardment ion source, with selectable electron energies of 70 eV, 30 eV, or 22 eV, a magnetic momentum analyzer and a detector system. The detector had two electron multiplier-counter channels, one covering 1 - 16 amu and the other covering 15 - 208 amu. The instrument was designed to measure a preselected set of 232 mass peaks. A complete spectrum of the 232 mass peaks was scanned in 58 seconds at the rate of 4 peaks per second. 6 seconds were required for background readings and returning the peak stepping program. Thus each scan covered 64 seconds. It also had two calibration gases containing 136Xe+, 136Xe++, and CH3+ producing peaks at 135.907, 67.954, and 15.035 amu.

The instrument was fully activated at 64 km altitude (after heat shield ejection at 67 km) by exposing the inlets to the atmosphere through use of a breakseal cap. Prior to this the instrument was operated for 4 scans to establish background peak amplitudes. The end of the 4th scan and the entire 5th scan have no data, this is presumably related to the instrument turn-on procedure. The IRMC gases were collected at approximately 62 km altitude. The first 17 minutes of data collection occurred while the probe was descending under the parachute. Just before parachute jettison the IRMC cell gases were introduced into the instrument. After the parachute was jettisoned at 47 km altitude, the probe continued to descend and collect data for another 37 minutes until loss of signal at the surface (at 19:39:53 UT). Each 64 second scan took place over an average altitude change of roughly 1.2 km. 51 scans were taken over the course of the descent.

The normal electron energy was 70 eV, but three times during the descent (beginning at altitudes 54, 30, and 9 km) the ionization energy was cycled on successive scans of the mass spectrum to 30 and 22 eV. (All doubly charged ion species are removed from the spectrum when low energy electrons are used.) Apparently from approximately 50 to 28 km altitude the microleaks were blocked by an overcoating of cloud particle materials, mainly background residual gases and gases from the blocking material were detected. Below about 30-28 km the primary leak unblocked and reopened and flow returned to normal. The final scan began to an altitude of 0.2 km and ended before it was completed when the probe hit the ground and the signal ceased.

Alternate Names

  • LNMS
  • PioneerVenusLargeProbe/LNMS

Facts in Brief

Mass: 10.9 kg
Power (avg): 14 W

Funding Agency

  • NASA-Office of Space Science (United States)


  • Planetary Science: Atmospheres

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Dr. Michael B. McElroyOther InvestigatorHarvard
Prof. Thomas M. DonahueOther InvestigatorUniversity of
Dr. Richard R. Hodges, Jr.Other InvestigatorUniversity of Texas, Dallas
Dr. M. KolpinOther InvestigatorTRW Systems Group
Dr. John H. HoffmanPrincipal InvestigatorUniversity of Texas, Dallas

Selected References

  • Hoffman, J. H., et al., Pioneer Venus sounder probe neutral gas mass spectrometer, IEEE Trans. Geosci. Rem. Sens., GE-18, No. 1, 80-84, Jan 1980.
  • Hoffman, J. H., et al., Composition of the Venus lower atmosphere from the Pioneer Venus mass spectrometer, J. Geophys. Res., 85, No. A13, 7882-7890, Dec. 1980.
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