The surface of Europa is totally different from that of Ganymede or Callisto, despite the fact that the infrared spectrum of this object indicates that it, too, is covered with ice. There are few impact craters on Europa—the number per unit area is comparable to that on the continental regions of Earth, indicating that the surface is relatively recent. Some scientists think the surface is so young that significant resurfacing is still taking place on the satellite. This resurfacing evidently consists of the outflow of water from the interior to form an instant frozen ocean.

Models for the differentiated interior suggest the presence of an iron-rich core surrounded by a silicate mantle surmounted by an icy crust some 150 km (90 miles) thick. This moon possesses both induced and intrinsic magnetic fields. Slightly mottled regions on the surface have been found to contain salt deposits, suggesting evaporation of water from a reservoir below the crust. Europa's frozen surface is crisscrossed with dark and bright stripes and curvilinear ridges and grooves. Spatter cones along some of the grooves again suggest fluid eruptions from below. The relief is extremely low, with ridge heights perhaps a few hundred metres at most. Europa thus has the smoothest surface of any solid body examined in the solar system thus far. Traces of sulfur, sulfur compounds, hydrogen peroxide, and organic compounds have been identified on the surface.

The major open question is whether there is a global ocean of liquid water beneath Europa's ice, warmed by the release of tidal energy in Europa's interior. The possibility of such an ocean arose from Voyager data, and highresolution Galileo images suggested fluid activity near the surface. In addition, explanation of Europa's induced magnetic field appears to require an interior, electrically conducting fluid medium, implying a salt-containing liquid water layer at some depth beneath the surface ice. If this ocean and its required source of heat exist, the possible presence of at least microbial life-forms must be admitted.

Seen through a telescope from Earth, Io appears reddish orange, while the other moons are neutral in tint. Io's infrared spectrum shows no evidence of the absorption characteristics of water ice. Scientists expected Io's surface to look different from those of Jupiter's other moons, but the Voyager images revealed a landscape even more unusual than anticipated.

Volcanic fissures, instead of impact craters, dot the surface of Io. Nine volcanoes were observed in eruption when the two Voyager spacecraft flew by in 1979, while the closer encounters by Galileo indicated that as many as 300 volcanic vents may be active at a given time. The silicate lava emerging from the vents is extremely hot (about 1,900 K [3,000 °F, 1,630 °C]), resembling primitive lavas on early Earth. This unprecedented level of activity makes Io the most tectonically active object in the solar system. The surface of the satellite is continually and completely replaced by this volcanism in just a few thousand years. Various forms (allotropes) of sulfur appear to be responsible for the black, orange, and red areas on the moon's surface, while solid sulfur dioxide is probably the main constituent of the white areas. Sulfur dioxide was detected as a gas near one of the active volcanic plumes by Voyager's infrared spectrometer and was identified as a solid in ultraviolet and infrared spectra obtained from Earth-orbital and ground-

based observations. These identifications provide sources for the sulfur and oxygen ions observed in the Jovian magnetosphere and prove that Io's volcanic activity is the source of its torus of particles.

The energy for this volcanic activity requires a special explanation, since radioactive heating is inadequate for a body as small as Io. The favoured explanation is based on the observation that orbital resonances with the other Galilean satellites perturb Io into a more eccentric orbit than it would assume if only Jupiter controlled its motion. The resulting tides developed by the gravitational contest over Io between the other satellites and Jupiter release enough energy to account for the observed volcanism. The interior contains a dense, iron-rich core, which probably produces a magnetic field. The interactions of Io with Jupiter's magnetosphere and ionosphere are so complex, however, that it has been difficult to distinguish the satellite's own field from the current-produced fields in its vicinity.

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