The Cassini Ion and Neutral Mass Spectrometer - INMS

ASTR 5835 Katharina Otto

Overview

•  Saturn and Titan •  Instrument Goals •  Functionality •  Results •  Outlook •  Summary

■ Overview □□□ Instrument Goals □□ Results □ Summary □□ Saturn and Titan □□□□□□□□□□ Functionality □ Outlook

Saturn and Titan What is interesting about Saturn’s system?

•  Magnetosphere

•  Ring system

•  Icy moons (Enceladus, Rhea, Titan, …)

•  Neutral cloud of water molecules

■ Overview □□□ Instrument Goals □□ Results □ Summary ■□ Saturn and Titan □□□□□□□□□□ Functionality □ Outlook

Saturn and Titan What is interesting about Titan?

•  Atmosphere’s composition (mostly nitrogen and methane)

•  Atmosphere’s structure

•  Hydrocarbon-nitrile lakes on Titan

•  Volcanism

•  Conditions like on early earth

■ Overview □□□ Instrument Goals □□ Results □ Summary ■■ Saturn and Titan □□□□□□□□□□ Functionality □ Outlook

Instrument Goals Who built INMS?

•  NASA’s Goddard Space Flight Center (Planetary Atmospheres Laboratory)

•  The University of Michigan (Space Physics Research Laboratory)

■ Overview ■□□ Instrument Goals □□ Results □ Summary ■■ Saturn and Titan □□□□□□□□□□ Functionality □ Outlook

Instrument Goals What is INMS capable of measuring?

•  Neutral gas mass composition

•  Ion gas mass composition

•  Gas number density

•  Mass range from 1u to 99u

•  Neutral species and low energy ions (< 100eV)

•  Mass resolution M/ΔM = 100 at 10% of mass peak height

Mass H = 1u [detectable] Mass CH4 = 16u [detectable] Mass N2 = 28u [detectable] Mass PAH > 99u [not detectable]

■ Overview ■■□ Instrument Goals □□ Results □ Summary ■■ Saturn and Titan □□□□□□□□□□ Functionality □ Outlook

Instrument Goals Why was INMS built?

•  Composition and structure of Titan’s atmosphere (900km to 1000km)

•  Titan’s atmosphere interaction with Saturn’s magnetosphere plasma

•  Neutral and plasma environment of Saturn’s ring system and icy moons

•  Information about formation of neutral cloud of water molecules (at ~12 planetary radii)

■ Overview ■■■ Instrument Goals □□ Results □ Summary ■■ Saturn and Titan □□□□□□□□□□ Functionality □ Outlook

Functionality What are the INMS components?

■ Overview ■■■ Instrument Goals □□ Results □ Summary ■■ Saturn and Titan ■□□□□□□□□□ Functionality □ Outlook

Functionality What are the operating modes?

•  Closed source neutral mode for non-reactive neutrals (N2, CH4)

•  Open source neutral mode for reactive neutrals (N)

•  Open source ion mode for positive ions with energy < 100eV

INMS sensor and electronics

■ Overview ■■■ Instrument Goals □□ Results □ Summary ■■ Saturn and Titan ■■□□□□□□□□ Functionality □ Outlook

Functionality Closed Source

•  ~120 collision with chamber walls depending on ram angle

•  Enhanced gas density

•  Gas ionized by two electron guns with different electron energies

■ Overview ■■■ Instrument Goals □□ Results □ Summary ■■ Saturn and Titan ■■■□□□□□□□ Functionality □ Outlook

Functionality Closed Source

•  ~120 collision with chamber walls depending on ram angle

•  Enhanced gas density

•  Gas ionized by two electron guns with different electron energies

■ Overview ■■■ Instrument Goals □□ Results □ Summary ■■ Saturn and Titan ■■■■□□□□□□ Functionality □ Outlook

Functionality Open Source

•  Antechamber contains four electrodes for trapping or focusing ions

•  Top plate lens (TPL)

•  Open source Ion lenses (OL)

•  Gas ionized by two guns with different electron energies (e.g. 25eV and 70eV)

■ Overview ■■■ Instrument Goals □□ Results □ Summary ■■ Saturn and Titan ■■■■■□□□□□ Functionality □ Outlook

Functionality Electrostatic Quadrupole Deflector and Lenses

•  Four hyperbolic shaped electrodes

•  Focuses ions from closed source

•  Deflects ions from open source

•  Energy selection possible in open source mode

■ Overview ■■■ Instrument Goals □□ Results □ Summary ■■ Saturn and Titan ■■■■■■□□□□ Functionality □ Outlook

Functionality What is the radio frequency quadrupole mass

analyzer? •  Opposing poles

electrically connected

•  Radio frequency applied to rods

•  Direct current voltage superimposed

•  Oscillating ions depending on m/q ratio

•  Filtering of certain m/q ions

www.analyticalspectroscopy.net

■ Overview ■■■ Instrument Goals □□ Results □ Summary ■■ Saturn and Titan ■■■■■■■□□□ Functionality □ Outlook

Functionality Dual Electron Multiplier Detectors

•  Multipliers electronically biased for different ions energies

•  Electron multiplier converts ion hit into charge signal

•  Pulse rate of 10MHz

■ Overview ■■■ Instrument Goals □□ Results □ Summary ■■ Saturn and Titan ■■■■■■■■□□ Functionality □ Outlook

Functionality INMS Properties

•  Height 20.3 cm [8.0 in] •  Length 42.2 cm [16.6 in] •  Width 36.5 cm [14.4 in] •  Weight 10.29 kg •  1.4 kg tantalum radiation shield

0.23 cm [0.090 in]

■ Overview ■■■ Instrument Goals □□ Results □ Summary ■■ Saturn and Titan ■■■■■■■■■□ Functionality □ Outlook

Functionality Difficulties and Limitations

•  Gas composition can be misinterpreted (fractioning, multiple ionization)

•  Mass range too small for complex molecules

•  Density threshold of 104 – 105 cm-3

•  Gases adsorb to instrument surfaces

■ Overview ■■■ Instrument Goals □□ Results □ Summary ■■ Saturn and Titan ■■■■■■■■■■ Functionality □ Outlook

Results What did we learn about Titan’s atmosphere?

•  Horizontal and vertical density distributions of various species determined

•  Atmospheric composition investigated

•  Major species: N2, CH4, H2

■ Overview ■■■ Instrument Goals ■□ Results □ Summary ■■ Saturn and Titan ■■■■■■■■■■ Functionality □ Outlook

Results What else did we learn Enceladus?

•  Plumes at Enceladus’ south pole

•  Grains ejected in jets

•  90% water vapor

•  Related to the neutral cloud of water molecules

•  INMS supports other Cassini instruments

■ Overview ■■■ Instrument Goals ■■ Results □ Summary ■■ Saturn and Titan ■■■■■■■■■■ Functionality □ Outlook

Outlook What is still and will be analyzed?

•  Cassini mission extended till 2017

•  Further density measurements in Titan’s atmosphere

•  54 additional Titan flybys

•  11 additional Enceladus flybys

■ Overview ■■■ Instrument Goals ■■ Results □ Summary ■■ Saturn and Titan ■■■■■■■■■■ Functionality ■ Outlook

Summary What to remember?

•  INMS measures densities and compositions of neutral and ionized gases

•  Operates in open or closed mode

•  Uses a quadrupole mass analyzer to determine m/q ratio

•  Supports interpretation of other Cassini instruments’ data

■ Overview ■■■ Instrument Goals ■■ Results ■ Summary ■■ Saturn and Titan ■■■■■■■■■■ Functionality ■ Outlook

References

•  Waite, J. et al.: The Cassini Ion and Neutral Mass Spectrometer (INMS) investigation, Space Science Reviews 114, 2004

•  http://www.quantum.physik.uni-mainz.de/lectures/2007/ws0708_massenspektrometrie/Kapitel7.pdf

•  Waite, J. et al.: Ion Neutral Mass Spectrometer results from the first flyby of Titan, Science 308, 2005

•  Cui, Cravens, T. et al.: Composition of Titan’s Ionosphere, Geophysical Research Letters 33, 2006

•  J. et al.: Analysis of Titan’s neutral upper atmosphere from Cassini Ion Neutral Mass Spectrometer measurements, Icarus 200, 2009

•  Teolis, B. et al.: Detection and measurement of ice grains and gas distribution in the Enceladus plume by Cassini’s Ion Neutral Mass Spectrometer, Journal of Geophysical Research 115, 2010