100-C 25-C 50-C 75-C C+M 50-C+M C+Y 50-C+Y M+Y 50-M+Y 100-M 25-M 50-M 75-M 100-Y 25-Y 50-Y 75-Y 100-K 25-K 25-19-19 50-K50-40-40 75-K 75-64-64CHAPTER 23
Europa
Louise M. Prockter
Johns Hopkins University
Applied Physics Laboratory
Laurel, MarylandRobert T. Pappalardo
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California- Introduction and Exploration History 6. Surface Physical Processes
- Formational and Compositional Models 7. Surface Age and Evolution
- Stress Mechanisms and Global Tectonic Patterns 8. Astrobiological Potential
- Landforms on Europa 9. Future Exploration
- Surface Composition and Thermal State Bibliography
1. Introduction and Exploration History
Europa and her sibling satellites were famously discovered
by Galileo in 1610, and less famously by Simon Marius at
essentially the same time, but it took almost 4 centuries be-
fore any detailed views of their surfaces were seen and the
grandeur of theGalilean satelliteswas revealed. In the
1960s, ground-based telescopic observations determined
that Europa’s surface composition is dominated by water
ice, as are most other solid bodies in the far reaches of the
solar system.
ThePioneer 10and 11 spacecraft flew by Jupiter in
the 1970s, but the first spacecraft to image the surfaces of
Jupiter’s moons in detail were theVoyagertwins.Voyager
1 ’s closest approach to Jupiter occurred in March 1979, and
Voyager 2’s, in July of the same year. BothVoyagers passed
farther from Europa than from any of the other Galilean
satellites, with the best imaging resolution limited to 2 km
perpixel. These images revealed a surface brighter than
that of the Earth’s moon, crossed with numerous bands,
ridges, cracks, and a surprising lack of large impact craters
or high-standing topography (Fig. 1). Despite the distance
from which the images were acquired, they were of suffi-
ciently high resolution that researchers noted some of the
dark bands had opposite sides that matched each other ex-
tremely well, like pieces of a jigsaw puzzle. These cracks
had separated, andductiledark icy material appeared to
have flowed into the opened gaps. This suggested that the
surface could have once been mobile. The relative youth of
Europa’s surface is demonstrated by a lack of large impact
craters—Voyagerimages showed only a handful—which
are expected to build up over time as a planetary surface is
constantly bombarded by meteorites over billions of years,
until the surface is covered in craters (such as on Mercury).
A lack of craters implies that something has erased them—
such as volcanic (or in Europa’s case,cryovolcanic) flows
orviscous relaxationof the icy crust. Researchers study-
ing theVoyagerdata also noted that the patterns of some
of the longest linear features on the surface did not fit with
predicted simple models of global stresses that might arise
from tidal interactions with Jupiter. However, if the shell
was rotated back a few tens of degrees, the patterns fit ex-
ceptionally well to a model ofnonsynchronous rotation,
by which the icy surface had slowly migrated with respect to
the satellite’s tidal axes. This mechanism probably requires
a ductile layer between the surface ice and the deeper inte-
rior. Combined with the observations of dark bands, there
were tantalizing hints that perhaps Europa had a warm in-
terior at some time in the past, and perhaps still today. In-
creasingly sophisticated theoretical models oftidal heat-
ingof Europa, discussed later in more detail, suggested that
a global subsurface ocean might exist within Europa today.