Devoir de Philosophie

Io (astronomy) - astronomy.

Publié le 11/05/2013

Extrait du document

Io (astronomy) - astronomy. I INTRODUCTION Io (astronomy), large satellite of the planet Jupiter and the most volcanically active body in the solar system. Io circles Jupiter in a slightly elliptical (oval-shaped) orbit over Jupiter's equator at an average distance of 422,000 km (262,200 mi). It completes an orbit once every 1.769 Earth days and rotates once in the same period. Io is slightly larger than Earth's moon, with a radius of about 1,820 km (about 1,130 mi). Like Earth's moon, it ranks among the ten largest moons in the solar system. Io has a large, dense iron core at its center surrounded by a mantle that contains molten compounds of silicon and oxygen. Its crust, the moon's outer layer, is made up mostly of sulfur and sulfur compounds, which color the moon's surface with areas of yellow, orange, red, white, blue, brown, and black. II VOLCANOES Io has hundreds of volcanoes dotting its surface. Some shoot plumes of molten sulfur and sulfur dioxide gas up to 300 km (186 mi) high. These volcanic vents also release lava hotter than any other planetary surface temperature ever recorded in the solar system--as hot as 1727°C (3140°F). Astronomers have identified magnesium-rich silicates, a type of lava that only melts at very high temperatures, around these vents. Most of Io's volcanism is driven by gravitational forces that act on the planet. The gravitational force created by Jupiter's mass would make Io travel in a smooth elliptical path, if the gravitational forces of Ganymede and Europa, two other large satellites of Jupiter, did not also affect Io's orbit. Io is squeezed and stretched by the conflicting attractive forces of Jupiter, Ganymede, and Europa, causing rocks inside the moon to rub together and produce heat. In addition, Io's orbit takes the moon through Jupiter's powerful magnetic field, generating about 1 trillion watts of electricity--and yet more heat--within the satellite. Io's largest volcanic feature is Ra Patera, a volcano surrounded by lava flows that stretch up to 300 km (186 mi) from the volcano to cover an area about the size of the state of New Jersey. The Haemus Mountains at Io's south pole measure up to 10 km (6 mi) above the surrounding area, making them higher than Mount Everest, the highest mountain on Earth. III ATMOSPHERE Io has a thin atmosphere made mainly of sulfur dioxide gas. The gas has accumulated over millions of years from volcanic eruptions on Io. The atmosphere is much denser on the sunlit dayside of Io, where the temperature is about -133°C (-207°F), than on the nightside, where the temperature drops to -188°C (-306°F). This difference occurs because most of the gas freezes out as frost on the surface when a face of Io is darkened by night or by an eclipse by giant Jupiter. The frozen sulfur dioxide turns to gas again when the same area passes back into sunlight. Only about 1 to 3 percent of the gas in Io's dayside atmosphere comes from volcanoes that are now active. These active volcanoes account for nearly all the atmosphere on the nightside. The volcanoes on Io emit other gases in addition to sulfur dioxide. However, these gases quickly turn to frost in the cold temperatures present even on the dayside. Atoms of sulfur, oxygen, sodium, potassium, and chlorine escape into space from the surface and from the atmosphere. As a result, Io is surrounded by a cloud of neutral sodium atoms and moves through a torus of charged oxygen and sulfur particles that lies along the moon's orbital path. The charged particles are held in place by Jupiter's magnetic field. Auroras are visible in Io's atmosphere when the moon is in darkness. Unlike auroras that occur at the poles of planets, Io's auroras are brightest over the equator and in regions with active volcanic plumes. The electrically charged particles that cause the auroras come from interactions with Jupiter's magnetosphere. IV DISCOVERY AND EXPLORATION Io was discovered independently in 1610 by Italian astronomer Galileo and German astronomer Simon Marius. Io and the three other largest moons of Jupiter, all found in 1610, are called the Galilean moons. Marius named Io and the other Galilean moons for mythical lovers of the Greek god Zeus, whom the Romans renamed Jupiter. Zeus transformed Io into a cow to protect her from his jealous wife, Hera. Io's volcanoes are named for gods of fire, the sun, and thunder from mythologies from all over the world. The United States spacecraft Voyager 1 and 2, which passed Jupiter in 1979, and the Galileo spacecraft, which went into orbit around Jupiter in 1995, have provided much of the information astronomers know about Io. In 1999, for example, Galileo took detailed photographs of Io's volcanic eruptions; scientists hope to use the observations to learn more about similar volcanic activity that occurred on Earth eons ago. The Hubble Space Telescope has also made important observations of Io's volcanic activity. The New Horizons spacecraft studied Io when the probe passed by Jupiter in 2007. The probe was able to observe Io's atmosphere, auroras, and volcanoes, including an eruption by the volcano Tvashtar that sent a plume 330 km (200 mi) high. New Horizons used Jupiter's gravitation to boost its velocity for a planned flyby of distant Pluto in 2015. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

Liens utiles