Io

Jovian System

 Of the 16 known satellites orbiting Jupiter,
all but four are smaller than 250 km diameter, and many are 80
km diameter or less and very irregular in shape. Most of these
smaller bodies are believed to be captured asteroids. The four
largest satellites, however, collectively known as the Galilean
satellites, are almost a planetary system of their own, with two
of them being comparable in size to Mercury. Like the Solar System,
this satellite system exhibits a distinct change in composition
from one end to the other: ice content increases, and density,
reflectivity, and signs of geologic activity decrease with distance
from Jupiter.

  Io is the densest of the Galilean satellites
at 3.5 times that of water, and so must consist mostly of rock.
As first discovered by the Voyager missions, it is the most active
object in the Solar System, with 5 to 10 major eruptions ongoing
at any given time. Individual eruptions can last several years,
reach 300 km high, and over the years have obliterated all traces
of impact craters. The energy driving this over-active volcanic
landscape is heat created by constant flexing of Io's interior
from tides caused by Jupiter and Europa. The rest of Io's surface
is dominated by solidified sulfur flows and sulfur dioxide frosts,
and dark liquid sulfur, which give Io its multi-hued surface.
Silicate lava's also occur, appearing gray or black, but are denser
and so usually remain below the surface. The sulfur dioxide tends
to concentrate in topographic basins. Io is thought to have a
dense iron-nickel-sulfur core, similar to Earth's. Clearly, various
volatiles and liquids are rapidly, and sometimes violently, cycling
through Io's crust. Heat from these gigantic eruptions are detectable
from sensitive telescopic instruments on Earth, allowing continued
monitoring of their activity.