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.
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.
A source of energy that could be utilized by living things 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.
Water is essential to life, serving as a perfect liquid medium for dissolving nutrients for ingestion or wastes for excretion, and for transporting chemicals living things can use. Several lines of evidence strongly suggest that the planet-sized moon contains an ocean of liquid water many tens of miles deep. If it does exist, the ocean lies beneath an ice shell that is at least a few miles thick, and perhaps tens of miles thick. At the ocean bottom lies a rocky seafloor in direct contact with the water, possibly supplying chemical nutrients into the ocean by hydrothermal activity.
Important clues to the presence of an ocean within Europa: Observations by NASA's Galileo spacecraft confirmed that Europa's surface is sparsely cratered and therefore young. (Heavily cratered surfaces are older.)Models for the formation of the many linear ridges and fractures on Europa's surface suggest that the moon's icy shell is relatively thin and flexes in response to tidal forces as the moon orbits Jupiter.
Flexing of the icy crust above an ocean could create pockets of salty impurities and partially melted areas leading to features seen in spacecraft images.
Favorable environments for the chemistry of life (or even life itself, in microbial form) could exist in areas within Europa's ice shell that contain salty fluids or around possible hydrothermal systems driven by tidal heating. An ocean rich with chemistry conducive to life could be maintained by a cycle that moves water through the moon's ice shell, ocean and rocky interior.
Studying Europa's chemistry - on the surface and within the suspected ocean - is important for understanding its habitability because living things extract energy from their environments via chemical reactions. Interactions between materials from Europa's surface and those in an ocean environment beneath the ice could produce elements essential for life such as carbon, hydrogen, nitrogen, oxygen, phosphorous and sulfur.
Europa's surface is mostly water ice (H2O), but the surface is bombarded by intense radiation from Jupiter, which can alter the chemistry of the ice. Through this process, the hydrogen and oxygen from water ice can combine with other materials on the surface to create a host of molecules like free oxygen (O2), hydrogen peroxide (H2O2), carbon dioxide (CO2) and sulfur dioxide (SO2).
If these compounds are finding their way into an ocean as part of an ongoing cycle, they could be used to power the reactions living things depend upon. Meanwhile, cycling of ocean water through minerals in the seafloor could replenish the water with other chemicals that are crucial for life.
How material cycles between the ice, the ocean and the rocky interior is the greatest uncertainty about energy as it relates to Europa's habitability. Image credit: NASA/JPL-Caltech
Image credits Flexing of Europa's icy crust could create partially melted pockets, or even lakes, scattered throughout the moon's outer shell. Image credit: Britney Schmidt/Dead Pixel VFX/Univ. of Texas at Austin.
Life extracts energy from its environment in order to carry out biological processes like maintaining cellular structures, growing and reproducing. Most living things on Earth's surface depend (directly or indirectly) on energy supplied by the sun, but there are many organisms that extract their energy from chemical sources like those produced by hydrothermal activity.
Europa's constant tidal flexing provides heat energy to drive chemical reactions in the rocky interior, recycling the elements and making them available for potential use by living things. If Europa's seafloor has volcanoes (as its sibling moon Io does) or hydrothermal vents, they may drive the chemistry of the ocean and play an important role in cycling nutrient-rich water between the ocean and the rocky interior. Tidal flexing of the ice shell could create slightly warmer pockets of ice that rise slowly upward to the surface, carrying material from the ocean below. Jupiter's intense radiation also provides a source of energy by ripping apart chemicals on the surface, where they can recombine to form new compounds.
The greatest uncertainty about energy as it relates to Europa's habitability is in how material cycles between the ice, the ocean and the rocky mantle on the ocean bottom. There are, potentially, sources of chemical energy for life being created on the surface and in the rocky interior, but their availability for use by living organisms depends on how well Europa's different layers are able to exchange material. In essence, the more energetic Europa is, the more energy would be available for life. Determining the balance of all these forces - Europa's energy balance - is a major hurdle toward understanding the icy moon's habitability.
Artist's concept of Europa's surface. Image credit: NASA/JPL-Caltech
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