• Geologic interpretation of images
• Observing and recording data
• Making comparisons
• Drawing conclusions

The four largest satellites of Jupiter are referred to as the Galilean satellites, since they were first discovered by Galileo in 1610. These satellites are too small and too far from the Earth to study their surfaces in detail. The surfaces of Io, Europa, Ganymede and Callisto were first imaged at high resolution by the Voyager spacecraft in 1979 and 1980, and are being observed in greater detail by the Galileo spacecraft which will be in orbit around Jupiter until December of 1997. Using the techniques discussed in the previous section, we can examine geologic processes important on these bodies, and determine the relative ages of their surfaces. This technique of comparing newly-explored worlds to those with which we are more familiar is a common one in science.

Activity:

• Look at Galileo images of Io, Europa, Ganymede and Callisto, Compare them to the terrestrial planets you looked at in the previous activity. Using the appearance of craters on the surfaces, and any other features you can identify, try to rank the Galilean satellites in terms of geologic activity.
  • Callisto:
    It's heavily cratered and looks like our Moon and Mercury; therefore,
    it appears inactive.

  • Ganymede:
    It has heavily cratered regions, regions with fewer craters which also have cracks and grooves (implying tectonic activity), and regions which appear to have experienced resurfacing (like our Moon?).
    Does this mean that some areas were recently active and some were not?

  • Europa:
    It has few craters, a smooth surface, and cracks (implying tectonics).
    Does this mean is was recently active?

  • Io:
    It has very few craters and has clear evidence of surface activity (volcanism); therefore,
    photojournal it is active.

  • Click here for all images of Io, Europa, Ganymede, and Callisto

  
  

• Did you see any examples of weathering or erosion of craters? Why or why not?
  No. Weathering and erosion require wind and water, but the Galilean satellites have no atmospheres and no liquid water on the surface.

• What are the main agents of change you see modifying craters on the Galilean satellites?
  • Volcanism:
    Io has sulfur volcanism, and Europa has possible water outflow.

  • Tectonism:
    Europa and Ganymede have cracks and grooves etc. which imply tectonism.

• Are there other differences between the craters in the Jovian system, and those in the inner solar system?
  Ganymede and Callisto have palimpsests (relaxed craters).
  These satellites are made up of a large fraction of ice. The viscosity of this ice is such that (over long periods of time) the ice flows and fills in craters instead of preserving the hole. (Relate this to the cratering exercise in section IIA with respect to the composition of the target body.)

TEACHER FEATURE

Next Page

• Introduction

• I. Location of features on the surface of a planet: latitude and longitude

• II. Crater formation, modification, and removal
  • A. How are craters made?

  • B. Where have all the craters gone?

  • C. Focus on the Galilean satellites

• III. Crater Image Interpretation
  • A. Crater size

  • B. Crater depth

  • C. Crater size distribution and surface ages

• Conclusion