Encyclopedia of the Solar System 2nd ed

432 Encyclopedia of the Solar System

FIGURE 1 Global view of Europa’s trailing hemisphere,
acquired early in theGalileospacecraft’s tour of the jovian
system. The colors in this image have been enhanced to show
detail. This image shows the extent of the reddish-brown
material that mottles Europa’s surface, along with numerous
linear features, many of them over 1000 km in length. Two large
impact craters are also visible: Pwyll, surrounded by bright rays,
is thought to be the youngest crater on Europa, and Callanish is
the large circular feature toward the west. (NASA/JPL.)

Such intriguing findings meant there was much anticipa-
tion for theGalileomission, which launched from the Space
ShuttleAtlantisin 1989 and entered orbit around Jupiter
in 1995. The primary mission included dropping a probe
into Jupiter’s atmosphere, as well as observations of all the

satellites and Jupiter’s atmosphere and local environment.

Despite severe data rate limitations during the mission (be-

cause the spacecraft’s main antenna did not open), the data

fromGalileowere so intriguing that the mission was ex-

tended in 1997 for a further 3 years, in order to make 8

further close flybys of Europa, and also to study its volcani-

cally active neighbor, Io. Data from the extendedGalileo

Europa Mission afforded many more high-resolution im-

ages of Europa, as well as magnetic data that strongly imply

the presence of a briny layer beneath the surface today.

The physical and orbital properties of Europa are sum-

marized in Table 1.

2. Formational and Compositional Models

During the formation of our solar system, the growing gas

giant planet Jupiter pulled material from the solar nebula. It

is now understood that, in contrast to early models of satel-

liteaccretion, the solids of the Joviansubnebulawere

probably grabbed from the solar nebula in nearly primor-

dial form, and that the subnebula may have been gas-poor.

Thus, the material incorporated into the Galilean satellites

was probably similar in composition to the asteroids of the

outer asteroid belt, containing ice,anhydrous silicates,

carbonaceousmaterial, and nickel–iron metal alloy.

The Galilean satellites formed by aggregation of these

solids, with the proportion of ice varying with distance from

the warm protoplanet Jupiter. Io formed relatively close to

Jupiter, so it was not able to accrete and retain significant

amounts of water ice. As the next moon outward, Europa

formed as a mostly rocky satellite (density=3.0), able to

accrete sufficient volatiles to form a∼100 km thick outer

layer of H 2 O. In the colder reaches of the jovian subnebula,

Ganymede and Callisto formed with near-equal amounts

TABLE 1 Properties of Europa

Discovered 1610

Discoverers Galileo Galilei, Simon Marius

Mean distance from Jupiter 671,100 km

Radius 1560. 8 ± 0 .5km

Mass (4. 8017 ± 0 .000014)× 1022 kg

Density 3. 014 ± 0 .005 g/cm^3

Orbital period 85 hours (3.551 Earth days)

Rotational period 85 hours (3.551 Earth days)

Orbital eccentricity 0.0094

Orbital inclination 0.469°

Visual geometric albedo 0.68

Escape velocity 2.026 km/s

Spacecraft visitors Voyager 1(March 1979)

Voyager 2(July 1979)

Galileo(July 1994 to 2000)

Predicted average surface Temperature ∼50K (poles) to∼110K (equator)