PHYSICAL PARAMETERS OF PLUTO AND OTHER DWARF PLANETS

In the Solar System, eight classical planets, several recently discovered dwarf planets, and hundreds of thousands of small planets or asteroids orbit the Sun; many of these have their own satellites; and there are also comets, meteoroids, and countless small meteorite particles and dust. Since 2006, dwarf planets have included until recently such a “large” planet as Pluto, as well as the previously largest asteroid of the Main Asteroid Belt, Ceres, and the largest bodies in the Kuiper Belt and, probably, the largest bodies in the Oort Cloud; some of the dwarf planets have a multiple structure. It is believed that, like many Kuiper Belt bodies, Pluto consists of ice mixed with rocky rocks. In 1978, D. Christie discovered an asymmetric appearance in the image of Pluto, which gave reason to talk about the presence of a satellite, which was soon found. It was called Charon. It has a diameter of 1186 km, a mass of ~1/30 of Pluto's mass, a distance from the center of Pluto of ~20,000 km and a synchronous rotation period of 6.39 Earth days. Pluto and Charon are considered a double dwarf planet. Somewhat later, two more satellites of Pluto were found in images from the Hubble Space Telescope, which were given the names Nix and Hydra. On 06/28/2011, using the same images, another satellite of Pluto with a diameter of 13-34 km was discovered. It was called Kerberos. It rotates between the satellites Nix and Hydra in 32.167 Earth days. On 06/11/2012, the fifth satellite of Pluto was discovered. It turned out to be the smallest of Pluto's five known moons at the time. Its dimensions were refined to 16×9×8 km in a single image from the New Horizons space probe in July 2015. It orbits between the moons Charon and Nix with a period of about 20.16 Earth days at a distance of about 42,656 km from the barycenter of the Pluto-Charon system. It was named Styx. Pluto is the best-known dwarf planet in the Solar System and the largest trans-Neptunian object to date. It is currently the ninth largest and tenth most massive celestial body orbiting the Sun (excluding several planetary satellites). Pluto's surface is very heterogeneous. Studies have shown that this body is the second most contrasting body in the Solar System after Iapetus. The albedo of individual areas on its surface varies from 10% to almost 70%. Such heterogeneity leads to periodic changes in Pluto's brightness, sometimes reaching 0.35m, as it rotates. Also, its spectrum changes significantly as Pluto rotates. Spectral observational data have shown that water ice is also present on Pluto's surface. Although most of the surface is covered with other more volatile ices; mainly - more than 95% - it is nitrogen ice. In addition, spectral observations have indicated the presence of frozen methane, carbon monoxide, and - in small quantities - some other compounds, which may well be formed from methane and nitrogen under the influence of hard radiation. These may include more complex hydrocarbons and nitrites, as well as the so-called tholins, which give the surfaces of Pluto and some other bodies located far from the Sun a brown color. The most noticeable geological detail on the visible part of Pluto's surface is the Sputnik Plain. This depression with dimensions of more than 1000 km occupies about 5% of its entire surface. Pluto's atmosphere was discovered only in 1985 during observations of its occultation of the star. When Pluto is at aphelion, most of the atmosphere should freeze and fall to the surface. Currently, Pluto's atmosphere is a relatively thin gas shell. It consists mainly of nitrogen, as well as methane and carbon monoxide. The presence of such an atmosphere is maintained due to constant evaporation from the surface ice layer. The interaction of the surface layers with the atmosphere also affects the temperature of Pluto's surface. Calculations show that the atmosphere, despite the very low pressure near the surface, is able to significantly smooth out daily temperature fluctuations. However, there are still temperature variations of about 20 degrees. This may be because the areas of the surface from which nitrogen ice evaporates are significantly cooled. Due to the elongated orbit, Pluto receives 2.8 times more heat at perihelion than at aphelion. This causes strong changes in its atmosphere.