However, it was time enough for their cameras to discover active volcanoes on Io, smooth ice plains on Europa, grooved terrain on Ganymede, and the crater-pocked surface of Callisto. The incredible complexity and rich diversity of their surfaces, which rival those of the terrestrial planets, are only visible by close-up scrutiny from nearby spacecraft. Ground-based telescopes provide only a blurred view of the tiny, distant moons. They all have a large metallic core, a rocky silicate mantle, and an outer layer of either water ice, for Europa and Ganymede, or rock, for Io.
In contrast, Callisto is a relatively uniform mixture of ice and rock. The innermost Galilean satellite, Io, has a radius and density that are nearly identical to those of our Moon, but contrary to expectation, there are no impact craters on Io.
The dramatic landscape is instead richly colored by hot flowing lava and littered with the deposits of volcanic eruptions. The active volcanoes emit a steady flow of lava that fills in and erases impact craters so fast that not a single one is left. Its red and yellow hues are attributed to different forms of sulfur, probably formed at different temperatures.
Volcanic plumes of sulfur dioxide gas fall and freeze onto the surface, forming white deposits that were first detected by ground-based infrared spectroscopy in the s. Whereas our Moon has been geologically inactive for eons, Io is the most volcanically active body in the solar system. Scientists estimate that Io has about active volcanoes, and the hotspots of at least of them have been observed. The cameras aboard the Voyager 1 spacecraft discovered nine active volcanoes during its flyby in , and the most active volcanoes, such as Prometheus, Loki and Peli, were observed from the Galileo spacecraft two decades later.
And Pele, the first volcano to be seen in eruption on Io, has repeated the performance for Galileo and the Hubble Space Telescope. Like other volcanic centers on Io, these active volcanoes have been named for gods of fire, the Sun, thunder and lightning. They spread out in graceful, fountain-like trajectories, depositing circular rings of material about a million meters in diameter.
Instruments aboard Galileo have practically smelled the hot, sulfurous breath of the eruptions, monitoring the sulfur dioxide gas as it rises, cools and falls. Diatomic sulfur, consisting of two sulfur atoms joined in pairs, has also been detected gushing out of the active volcanoes by instruments on the Hubble Space Telescope. What is keeping Io hot inside, warming up its interior, melting its rocks, and energizing its volcanoes?
But this tidal distortion does not melt the rocks by itself. If Io remained in a circular orbit, one side of the moon would always face Jupiter, its tidal bulges would not change in height, and no heat would be generated. The three Galilean satellites Io, Europa and Ganymede resonate with each other in a unique orbital dance, known as the Laplace resonance, in which Io moves four times around Jupiter for each time Europa completes two circuits and Ganymede one.
This congruence allows small forces to accumulate into larger ones. The resultant gravitational tug-of-war between Jupiter and the satellites distorts the circular orbits of all three moons into more oblong elliptical ones. The effect is greatest for Io, which revolves nearest to Jupiter, but there is a noticeable consequence for Europa and perhaps even Ganymede.
During each lap around its slightly eccentric orbit, Io moves closer to Jupiter and further away, wobbling back and forth slightly as seen from Jupiter. The interior of Io is composed of an iron or iron sulfide core and a brown silicate outer layer, giving the planet a splotchy orange, yellow, black, red, and white appearance. Based on data from scientific computer models, Io formed in a region around Jupiter where water ice was plentiful. Io's heat, combined with the possibility that there was water on Io shortly after it was formed, could have made life possible, although Jupiter's radiation would have removed the water from the surface.
The moon's most distinctive features are its volcanoes. Aside from Earth, Io is the only known body in the solar system to have observed active volcanoes. While Galileo had made some cryptic notes inferring possible volcanic activity, NASA's Voyager spacecraft discovered Io's volcanoes in The volcanic activity is a result of Io being stretched and squeezed as it orbits Jupiter. Io's rock surface bulges up and down by as much as meters feet during the process.
This affects Io's volcanic activity in a similar way to which Earth's oceans react to the moon. Io's irregularly elliptical orbit also heightens the tidal activity. Researchers are interested in watching the long-term activity of Io, which used to be difficult because telescopes on Earth lacked the resolution to look at hotspots.
However, using the adaptive optics systems of two of the world's largest telescopes, astronomers were able to cancel out 6 the wavy effects of our atmosphere, allowing more close looks at the moon's active volcanoes. They tracked 48 hotspots from through Earth-based observations have also looked shown waves in lava lakes s, providing more detail about how magma cycles on Io's surface. Because of the volcanic activity, Io's atmosphere contains mostly sulphur dioxide.
Io's orbit cuts across Jupiter's powerful magnetic lines of force, turning Io into an electric generator. As Jupiter rotates, the magnetic forces strip away about a ton of Io's material every second. The material becomes ionized and forms a doughnut-shaped cloud of radiation called a plasma torus. Some of the ions are pulled into Jupiter's upper atmosphere and create auroras.
An example of this activity was spotted by the Hubble Space Telescope , which revealed the influences of Io and another Jovian moon, Ganymede, in Jupiter's auroras in The sulfur dioxide envelope of gas freezes up while Io is in the shadow of Jupiter every day. Since they periodically line up in this fashion, the gravitational tugs the moons exert on each other stretch their orbits into elliptical shapes.
Europa's surface and crust are made almost entirely of water ice, and its bizarre, fractured appearance is proof enough that tidal heating has acted there. The icy surface is nearly devoid of impact craters and may be only a few million years old. Observations made by the Galileo spacecraft show that Europa has a metallic core and a rocky mantle.
Surrounding the rocky interior appears to be an icy layer kilometers thick, the top few kilometers of which seem to be frozen solid.
The stretching and squeezing of tidal friction should provide enough heat to melt some of this into liquid water beneath a thin ice shell. If it does, then Europa may have an ocean with more than twice as much liquid water as all of Earth's oceans combined. Close-up photos of the surface of Europa support the idea of a liquid ocean beneath the surface. These photos, taken by the Galileo spacecraft, show what appears to be icebergs stuck in a layer of ice.
Other evidence comes from double-ridged cracks on the surface. Tidal flexing that allows water to well up and build ridges may create these cracks. The surface of Ganymede shares many similarities with Europa. Ganymede's surface is also made of water ice, but unlike Europa's surface, it shows signs of varying age.
The darker regions are heavily cratered, suggesting they are billions of years old. The lighter regions show no signs of craters and it is thought that eruptions of water covered the surface before freezing over. These areas are geologically younger than the darker regions. If liquid water occasionally makes its way to the surface to fill in craters, could that suggest a liquid ocean similar to the one that might exist on Europa?
Not necessarily. The case for Europa's subsurface ocean comes from the strong probability of tidal heating, melting the ice under the surface. Ganymede has a much weaker tidal force, and thus weaker tidal heating than Europa and Io.
0コメント