Galileo Project: Galilean Moons

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Europa is a puzzle. The sixth largest moon in our Solar System, Europa confounds and intrigues scientists. Few bodies in the Solar System have attracted as much scientific attention as this moon of Jupiter because of its possible subsurface ocean of water. The more we learn about this icy moon, the more questions we have. Because the nature of science is to ask questions, we cannot resist the mystery of Europa and its potential for possessing an ocean.

Early Imaging
The United States sent two spacecraft, Pioneer 10 and 11, to Jupiter in the early 1970's. No one knew if a spacecraft could survive a flyby of Jupiter, but the Pioneers did survive, and they sent back valuable information for the next space mission. However, the Pioneer photographs of Jupiter's largest moons were fuzzy and dim. The twin Voyager spacecraft flew by Jupiter and its moons in 1979, giving us our first close-up view of Europa. Voyager pictures show pale-yellow icy plains with red and brown mottled regions. Long cracks run for thousands of kilometers over the surface. On Earth, these cracks would indicate such features as tall mountains and deep canyons. But none of these features are higher than a few kilometers on Europa, making it one of the smoothest objects in our Solar System.

Images from Galileo
If we look at the surface more closely, as we have with the instruments on the Galileo spacecraft, we see some fascinating features. Europa looks like broken glass that is repaired by an icy glue oozing up from below. Low ridges, straight and curved, crisscross the surface. Flows and fractures, pits and frozen "puddles" - all hint at a unique geologic history. Large circular features could be the sites of impacts or the result of upwelling of material from beneath the surface. Making sense of this chaotic landscape is a challenge to planetary scientists.

Europa's Puzzling Surface
Despite the chaos of its surface, Europa is probably the kind of puzzle that science can solve. Some of our questions are: "How old is the surface? How were the cracks and other features made? What is under the ice?" To answer them, we collect data and make careful obervations, applying what we know about geology, physics, and chemistry. Geologists figure out the age of a surface by counting the impact craters formed where comets, meteorites, and other debris hit the surface.

Comparison to Our Moon
Earth's Moon has young and old craters literally everywhere, which tells us that it has been geologically inactive for more than a billion years. Earth has been impacted at least as many times as the Moon, but Earth's surface has been smoothed by active geological processes such as plate tectonics and volcanic flows, and by constant weathering. Like our Moon, Jupiter's satellites Ganymede and Callisto are heavily cratered Ð evidence of very old and inactive surfaces. On Europa, however, only a few large craters have been identified. Unless Europa has somehow avoided these impacts, which is unlikely, relatively recent events must have smoothed over the craters.

Geologic Action on Europa
Looking at the pictures from Galileo, we see evidence of geologic action on Europa. Small blocks of crust float like icebergs over an invisible sea. Some blocks are tilted, others rotated out of place. Dark bands of ice and rock spread outward from a central ridge. What is the cause for this activity? In a gravitational tug of war of incredible dimensions, Europa is pulled in different directions by Jupiter and by the planet's other moons in a process called tidal flexing. Over one Europan day, it stretches and compresses up to several tens of meters. The outer surface of Europa is a rigid sphere. Imagine Earth covered by a blanket of ice that traps the oceans below. In the course of a day, these oceans rise and fall. This is what happens to Europa. The flexing of Europa's surface continues until the brittle crust cracks. We don't know what happens when the crust fractures. The process may be slow and steady, advancing only centimeters at a time - or, it may cause ice volcanoes or geysers to erupt violently, showering the surface with material from below.

Is There Life in the Oceans of Europa?
Another interesting possibility arises from this tidal flexing of Europa. Heat generated by the expansion and contraction may be enough to melt part of the crust underneath the surface, creating lakes or oceans below. The possibility of liquid water just below Europa's surface naturally leads to the question of whether life could have evolved there. Scientists have discovered marine life on Earth that thrives in the deep ocean near hydrothermal vents. This discovery provides us with a model for how similar organisms might survive on Europa. However, liquid water is just one of life's key ingredients. Many other factors, including organic material and a continuous energy source, must be present. Even if there is no ocean currently on Europa, one may have existed in the past, perhaps leaving fossilized remains to be found by a future mission.

Science Findings

General / Overview