SwRI IRD 2009--New Experimental Set-Up: Simulation of Titan's Upper Atmosphere, 15-R9880

Principal Investigators
Edward L. Patrick
Mai-Julie Nguyen
J.Hunter Waite
Gregory P. Miller
Jacob T. Grimes
Bishun N. Khare (NASA/Ames)
Christopher P. McKay (NASA/Ames)

Background - The need for this research was based on results obtained from the Cassini Plasma Spectrometer (CAPS) and Ion and Neutral Mass Spectrometer (INMS) instruments investigating the upper atmosphere of Saturn's largest moon, Titan. Since the discovery of the main composition of Titan's atmosphere by Voyager, many laboratory experiments have been carried out to reproduce its atmospheric chemistry. However, subsequent observations made by the aforementioned instruments aboard the Cassini spacecraft reveal details of an ion-neutral chemistry that were both unanticipated and far more complex than previously understood. At these spacecraft flyby altitudes (above 1,000 km), it is believed that the ion primarily responsible for driving the ion-neutral chemistry is N₂+. Data from the INMS have revealed the presence of heavy organic molecules, while the CAPS instrument has produced evidence of positive ions up to 350 daltons and negative ions of up to 8,000 daltons. The presence of negative ions was not only a complete surprise, but also revealed a gap in our present knowledge of important organic chemical pathways in Titan's upper atmosphere.

Approach - The objective of this work is to reproduce the ion-neutral chemistry of Titan's upper atmosphere. More precisely, researchers seek to simulate the first steps of ion-neutral reactions that may occur in Titan's ionosphere by using the main neutral compounds (N₂ and CH₄) and the major ion (N₂+) present at these altitudes. This new experimental setup – dubbed the "Titan Ionosphere Experiment" (TIE) – is an attempt to produce a space environment simulation in the laboratory capable of duplicating the complex chemistry of Titan's ionosphere. TIE consists of a "cold" plasma radio-frequency (RF) source, ion "guide" optics, and a reaction chamber containing an ion "wire" guide and ion mirrors to aid in trapping ions for an extended period at pressures typical in the Titan ionosphere. From this laboratory simulation, mass spectra measurements are anticipated that will reproduce those observed at Titan and give clues to this moon's complex organic chemistry.

Accomplishments - A prototype system was constructed to demonstrate "proofs of principle" inherent in the various sub-assemblies prior to the operation of the completed system. The RF plasma ion source has been successfully operated for a variety of gases and over wider pressure ranges than previously used for the device. Fundamental beam diagnostics of the plasma source were performed, and more detailed beam diagnostics are currently under way to characterize the source ions conducted downstream using an electrostatic analyzer (ESA). Pressure in the downstream reaction chamber has been kept lower than 1x10^-6 Torr while the ion source is operating, thereby reproducing pressures observed by the Cassini spacecraft during a typical Titan flyby. The ion wire "guide" concept was also demonstrated successfully and was able to conduct ions at low eV over a long beam path to a Faraday cup detector. Early tests using a simulated Titan gas mixture (2 percent CH₄, 98 percent N₂) in the plasma source have already produced reactant products observed at Titan, including ammonia (NH₃), acetylene (C₂H₂), hydrogen cyanide (HCN), ethane (C₂H₆) and cyanogen (C₂N₂).

Figure 1. This photo captured the first introduction of a simulated Titan gas mixture (2 percent CH₄, 98 percent N₂) into the RF plasma source. This immediately produced reactant products that included ammonia (NH₃), acetylene (C₂H₂), hydrogen cyanide (HCN), ethane (C₂H₆) and cyanogen (C₂N₂) observed downstream at the mass spectrometer.

Figure 2. This photo of a later TIE system prototype was taken during tests of the ion "wire" guide, represented by the large tube on the right side of the system. On the left side of the photo is the RF plasma source, followed by the ion extraction optics. The RF power supply is the black box above the source.

Figure 3. This model represents the plan for construction of the final TIE system currently under way.