Discovery of heavy negative ions in Titan's ionosphere

Abstract

Titan's ionosphere contains a rich positive ion population including organic molecules. Here, using CAPS electron spectrometer data from sixteen Titan encounters, we reveal the existence of negative ions. These ions, with densities up to ∼100 cm−3, are in mass groups of 10–30, 30–50, 50–80, 80–110, 110–200 and 200+ amu/charge. During one low encounter, negative ions with mass per charge as high as 10,000 amu/q are seen. Due to their unexpectedly high densities at ∼950 km altitude, these negative ions must play a key role in the ion chemistry and they may be important in the formation of organic‐rich aerosols (tholins) eventually falling to the surface.

Anion Photoelectron Spectroscopy of C3N− and C5N−

“…Table 1 gives peak assignments from the simulation. As inputs for the simulation, we used vibrational temperatures of 350 K for ν 4 and 100 K for ν 5 , experimental values 11 for the ν 4 and ν 5 fundamental transitions in the ground state of the neutral C 3 N, and, initially, calculated values 14 for ν 4 and ν 5 of the ground state of C 3 N -that were modified to produce a better fit. The optimized values for the ν 4 and ν 5 bending modes in C 3 N -obtained from this procedure are 538 ( 8 and 208 ( 8 cm -1 , as listed in Table 3.…”

“…They are also potentially significant constituents of Titan's ionosphere. 5 The neutral C 3 N and C 5 N radicals and their anionic counterparts were discovered in the expanding circumstellar envelope of the carbon star IRC + 10216 and in the dark cloud TMC1 with radio telescopes. The interstellar detection of C 3 N was confirmed by the observation of the radical in the low-pressure laboratory gas discharge of cyanoacetylene in nitrogen; 6 the corresponding anionic species, C 3 N -, has also been successfully made in the laboratory and has been the subject of a considerable amount of theoretical and experimental effort.…”

Interaction of Titan's ionosphere with Saturn's magnetosphere

“…The high time resolution ELS observations initially made on Ta, and subsequently on 15 other Titan encounters, were analysed by Coates et al (2007a). In addition to the electron populations discussed above, they presented evidence for negative ions in Titan's ionosphere.…”

“…As the spacecraft flies through Titan's cold ionosphere, the spacecraft velocity (approx. 6 km s K1 ) effectively provides a mass spectrometer for cold, ionospheric ions; assuming singly charged ions, the conversion is m amu Z5.32E ev (Coates et al 2007a).…”

“…The negative spacecraft potential in this region, estimated at approximately K0.6 to K1 eV (Wahlund et al 2005;Coates et al 2007a), implies that the energy of this peak would be larger by this amount in the undisturbed region a few Debye lengths away from the spacecraft. This potential would preclude the observation of any electrons from the undisturbed region that have initial energies below the spacecraft potential as they would be repelled by the spacecraft potential.…”