THE ATMOSPHERE OF NEPTUNE'S MOON TRITON AND ITS SEASONAL CHANGES.

Abstract

The largest natural satellite of the planet Neptune, Triton, was discovered by William Lassell on October 10, 1846, just seventeen days after the discovery of Neptune itself. Triton's main dynamic anomaly is its highly inclined retrograde orbit. It is the only large satellite in the Solar System with this type of motion. Tidal dissipation of energy during the transformation of its initially elongated orbit into a nearly circular one led to the release of sufficient heat to melt the inner layers of the satellite and to its complete differentiation into a massive silicate-metal core, an icy mantle, and a global liquid water-ammonia ocean. Triton has a thin, global, and dynamic atmosphere. Along with Saturn's moon Titan, Triton is the only satellite i n the Solar System with a confirmed global gas envelope. Triton's atmosphere is composed mainly of molecular nitrogen (N 2) with minor admixtures of methane (CH4) and carbon monoxide (CO), in thermodynamic equilibrium with surface ices. The weak surface gravity (g = 0.779 m/s²) and low second cosmic velocity (1.455 km/s) make its atmosphere gravitationally weakly bound; as a result, it extends to an altitude of more than 800–870 km, where it passes into the exosphere and directly interacts wi th Neptune's magnetosphere. The average surface temperature of Triton during the Voyager 2 flyby in 1989 was 38 ± 1 K. Under these conditions, most chemical compounds are in a solid state. The surface of the satellite is covered with a complex layer of condensed ices. About 55% of the area is covered by nitrogen ice, 10 –20% by carbon dioxide (CO2) ice, and 15 –35% by the solid phase of water ice (H2O). Carbon monoxide (CO) and methane (CH4) ices are also present as microscopic impurities. The phase state of nitrogen ice is of particular importance for the thermodynamics of Triton. At a temperature of 35.61 K, nitrogen undergoes a first-order phase transition from a low-temperature cubic crystal structure (α-nitrogen) to a high-temperature hexagonal modification (β-nitrogen). Spectroscopic studies confirm that the nitrogen ice on Triton is in the hexagonal β -phase, which imposes a strict lower limit on the temperature of the surface deposits: that is, it cannot be lower than 35.6 K. Since the satellite's atmosphere is in thermodynamic equilibrium with the surface nitrogen ice, the saturated vapor pressure directly controls the total atmospheric pressure. Using a radio obscuration system and an ultraviolet spectrometer, the first accurate profiles of Triton's atmosphere were obtained. Voyager 2 determined that the surface pressure of the nitrogen atmosphere is 1.4 Pa, and the temperature at the tropopause height drops to a minimum of about 36 K. The spacecraft also recorded the presence of thin clouds of nitrogen ice at altitudes of 1–3 km and layers of organic haze extending to a height of 30 km above the surface. The vertical profile of Triton's atmosphere is clearly structured and directly influences the behavior of cryovolcanic emissions. The atmosphere can be divided into several key zones based on temperature gradient: the troposphere (0–8 km), thermosphere (8–850 km), exosphere (> 850 km), and ionosphere.