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 τ [unknown] 0.01. Preliminary representative results include the following: 1.0243 x 10²⁶ and 2.141 x 10²² 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 J₂ = 3411 ± 10(x 10^-6) and J₄ = -26⁺¹²_-20(x10^-6) for Neptune's gravity field (J>₂ and J₄ 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 10⁵-pascal (1 bar) pressure level. Neptune's atmosphere was probed to a pressure level of about 5 x 10⁵ 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 10⁵ 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 10⁹ per cubic meter at an altitude of 340 kilometers measured during occultation egress. Its topside plasma temperature is about 80 ± 16 K if N₂⁺ 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.