The Solar System

508 PART 4^ |^ THE SOLAR SYSTEM

Plume from

volcano

Prometheus

Shadow

of plume

Five months after the

previous image, a new

volcano has emerged.

Volcano

Pele

Volcano

Pillan Patera

Volcanic caldera

Plume from

volcano

Prometheus

Shadow

of plume

Plume from volcano

Pillan Patera rises

140 km.

Hot lava at front of

advancing lava flows

Lava curtain erupting

through a fault

Caldera Culann Patera

has produced multiple

lava flows.

Debris ejected

from Pele

Visual + infrared images

moon Amalthea may be caused by sulfur pollution escaping from
Io. Th e main problem for you to consider before walking across
the surface of Io would be radiation. Io is deep inside Jupiter’s
magnetosphere and radiation belts. Unless your spacesuit had
impressive shielding, the radiation would be lethal. Io, like
Venus, may be a place that humans will not visit easily.
You can use basic observations to deduce the nature of Io’s
interior. From its density, 3.5 g/cm^3 , you can conclude that it is
rocky. Spectra reveal no trace of water at all, so there is no ice on
Io. In fact, it is the driest world in our solar system. Th e oblate-
ness of Io caused by its rotation and by the slight distortion
produced by Jupiter’s gravity gives astronomers more clues to its
interior. Model calculations suggest it contains a modest core of
iron or iron mixed with sulfur, a deep rocky mantle that is par-
tially molten, and a thin, rocky crust.
Th e colors of Io have been compared to those of a badly
made pizza. Th e reds, oranges, and browns of Io are caused by
sulfur and sulfur compounds, and an early hypothesis proposed
that the crust is mostly sulfur. New evidence says otherwise.
Infrared measurements show that volcanoes on Io erupt lava with
a temperature over 1500°C (2700°F), about 300°C hotter than
lavas on Earth. Sulfur on Io would boil at only 550°C, so the
volcanoes must be erupting molten rock and not just liquid
sulfur. Also, a few isolated mountains exist that are as high as
18 km, twice the height of Mount Everest. Sulfur is not strong

■ Figure 23-12

These enhanced-color images of volcanic fea-

tures on Io were produced by combining visual

and near-infrared images and digitally enhanc-

ing the color. To human eyes, most of Io would

look pale yellow and light orange. (NASA)

enough to support such high mountains. Th ese are all indica-

tions that the crust of Io is probably silicate rock.

Volcanism is continuous on Io. Plumes come and go over

periods of months, but some volcanic vents, such as Pele, have

been active since the Voyager spacecraft fi rst visited Io in 1979

(Figure 23-12). Earth’s explosive volcanoes eject lava and ash

because of water dissolved in the lava. As rising lava reaches

Earth’s surface, the sudden decrease in pressure allows the water

to come out of solution in the lava, like popping the cork on a

bottle of champagne: Th e water fl ashes into vapor and blasts

material out of the volcano, the process that was responsible for

the Mount St. Helens explosion in 1980. But Io is dry. Instead,

its volcanoes appear to be powered by sulfur dioxide dissolved in

the magma. When the pressure on the magma is released, the

sulfur dioxide boils out of solution and blasts gas and ash high

above the surface in plumes up to 500 km high. Ash falling back

to the surface produces debris layers around the volcanoes, such

as that around Pele in Figure 23-12. Whitish areas on the surface

are frosts of sulfur dioxide.