Four hundred years ago, the astronomer Galileo's discovery of Jupiter's four large moons forever changed humanity's view of the universe, helping to bring about the understanding that Earth was not the center of all motion. Today one of these Galilean moons could again revolutionize science and our sense of place, for hidden beneath Europa's icy surface is perhaps the most promising place to look for present-day environments that are suitable for life.
This new appreciation began to unfold in 1995, when a spacecraft named in Galileo's honor arrived in the Jupiter system to follow up on earlier discoveries by the Voyager mission. The Galileo spacecraft sent tantalizing samplings of data that provided strong evidence for a deep global ocean beneath Europa's icy crust, leading to speculation on the potential for life within icy moons.
Meanwhile, over the last quarter century we have learned that Jupiter-like planets are common around other stars, and that many could have icy moons like Europa. This realization means that studying Europa will help us understand the habitability of icy worlds throughout the cosmos.
What Makes Europa Special
As Europa orbits Jupiter it experiences strong tidal forces - somewhat like the tides in Earth's oceans caused by our Moon. The tidal forces cause Europa to flex and stretch because its orbit is an ellipse, rather than a circle, and the tide is much higher when the moon is close to Jupiter than when it is farther away. This continuous flexing creates heat, which makes Europa's interior warmer than it would be from the Sun's heat alone. In addition, the flexing could produce volcanic activity from the rocky interior, as on the neighboring moon Io. The tidal forces also cause Europa's icy outer shell to flex, likely causing the long, linear cracks seen in images of its surface.
Thanks in large part to measurements made by visiting spacecraft, scientists think it is probable that Europa has a saltwater ocean beneath a relatively thin and geologically active icy shell. Although evidence exists for oceans within several other large icy satellites in the outer solar system, Europa is unique because its ocean is believed to be in direct contact with its rocky interior, where conditions could be similar to those on Earth's biologically rich sea floor. (In contrast, Jupiter's other large, icy moons, Ganymede and Callisto, are thought to contain "ocean sandwiches," where a liquid ocean exists between two layers of ice.) Our planet has geologically active places on its sea floor, called hydrothermal zones, where water and rock interact at high temperatures. These zones are known to be rich with life, powered by energy and nutrients that result from reactions between the seawater and the warm, rocky ocean floor.
The Stuff of Life
Life as we know it depends upon three key "ingredients":
- Liquid water, to create an environment that facilitates chemical reactions
Europa appears to meet these minimum requirements for life. It is special among the bodies of our solar system in having a potentially enormous volume of liquid water, along with geological activity that could promote the exchange of useful chemicals from the surface with the watery environment beneath the ice. However, our current understanding of how material moves within Europa's icy crust is not well-developed. Even the existence of a subsurface ocean, while strongly suspected, is not yet proven.
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