Voyager Radio Science Observations of Neptune and Triton

Abstract

The Voyager 2 encounter with the Neptune system included radio science investigations of the masses and densities of Neptune and Triton, the low-order gravitational harmonics of Neptune, the vertical structures of the atmospheres and ionospheres of Neptune and Triton, the composition of the atmosphere of Neptune, and characteristics of ring material. Demanding experimental requirements were met successfully, and study of the large store of collected data has begun. The initial search of the data revealed no detectable effects of ring material with optical depth tau [unknown] 0.01. Preliminary representative results include the following: 1.0243 x 10(26) and 2.141 x 10(22) kilograms for the masses of Neptune and Triton; 1640 and 2054 kilograms per cubic meter for their respective densities; 1355 +/- 7 kilometers, provisionally, for the radius of Triton; and J(2) = 3411 +/- 10(x 10(-6)) and J(4) = -26(+12)(-20)(x10(-6)) for Neptune's gravity field (J>(2) and J(4) are harmonic coefficients of the gravity field). The equatorial and polar radii of Neptune are 24,764 +/- 20 and 24,340 +/- 30 kllometers, respectively, at the 10(5)-pascal (1 bar) pressure level. Neptune's atmosphere was probed to a pressure level of about 5 x 10(5) pascals, and effects of a methane cloud region and probable ammonia absorption below the cloud are evident in the data. Results for the mixing ratios of helium and ammonia are still being investigated; the methane abundance below the clouds is at least 1 percent by volume. Derived temperature-pressure profiles to 1.2 x 10(5) pascals and 78 kelvins (K) show a lapse rate corresponding to "frozen" equilibrium of the para- and ortho-hydrogen states. Neptune's ionosphere exhibits an extended topside at a temperature of 950 +/- 160 K if H(+) is the dominant ion, and narrow ionization layers of the type previously seen at the other three giant planets. Triton has a dense ionosphere with a peak electron concentration of 46 x 10(9) per cubic meter at an altitude of 340 kilometers measured during occultation egress. Its topside plasma temperature is about 80 +/- 16 K if N(2)(+) is the principal ion. The tenuous neutral atmosphere of Triton produced distinct signatures in the occultation data; however, the accuracy of the measurements is limited by uncertainties in the frequency of the spacecraft reference oscillator. Preliminary values for the surface pressure of 1.6 +/- 0.3 pascals and an equivalent isothermal temperature of 48 +/- 5 K are suggested, on the assumption that molecular nitrogen dominates the atmosphere. The radio data may be showing the effects of a thermal inversion near the surface; this and other evidence imply that the Triton atmosphere is controlled by vapor-pressure equilibrium with surface ices, at a temperature of 38 K and a methane mixing ratio of about 10(-4).

Upper Limits on Gaseous CO at Pluto and Triton from High-Resolution Near-IR Spectroscopy

“…The atmosphere of Triton was measured by Voyager 2 in 1989 (Broadfoot et al 1989, Tyler et al 1989, and by stellar occultations in the 1990's (Elliot et al 2000). The occultations indicate both an increase in pressure since the Voyager encounter and a change in the thermal profile (Elliot et al 2000).…”

Satellite Atmospheres and Magnetospheres

“…This technique has been used very successfully to measure the characteristics of the ionospheres and atmospheres of Venus (Kliore et al, 1967;Fjeldbo et al, 1971;Kliore and Patel 1980); Mars (Kliore et al, 1965;Kliore et al, 1972a;Lindal et al, 1979); Mercury (Howard et al, 1974); Jupiter (Fjeldbo et al, 1975;Eshleman et al, 1979;Lindal et al, 1981;Hinson et al, 1997); Saturn Tyler et al, 1981;Lindal et al, 1985); Uranus (Lindal et al, 1987); Neptune (Tyler et al, 1989;Lindal 1992); Saturn's rings (Marouf and Tyler 1985); as well as Saturn's satellite Titan (Lindal et al, 1983); Neptune's Triton (Tyler et al, 1989); and Jupiter's satellites Io (Kliore et al, 1975) and Europa (Kliore et al, 1997).…”

Planetary ionospheres – sources and dynamic drivers

“…Like Uranus, Neptune possess a magnetic field, giving rise to a magnetosphere. The ionosphere has been measured using the radio occultation experiment on Voyager 2 (Tyler et al 1989). The electron density profile between 1000 and 5000 km decreases from 2×10 3 cm −3 at 1000 km to less than 100 cm −3 at 5000 km.…”