Is there life on Europa?
Graeme Ing, HET602A
Introduction
Europa is the fourth largest moon of Jupiter, comparable in size to Earth’s moon. Its surface is a thick layer of ice covered in cracks and lines, below which we may find an ocean harboring life. This paper intends to discuss the possibility of life on Europa, what form that life will take and how it will overcome its harsh environment. We will also consider likely methods of finding this life.
Can Europa support life?
The three basic requirements for life as we know it are liquid water, organic compounds and a source of heat or other energy. We can argue that Europa possesses all of these:
When the Galileo probe encountered Europa in 1996, its Near
Infrared Mapping Spectrometer confirmed Kuiper’s 40-year-old theory that Europa
was covered in water ice (PlanetScapesWeb). The surface spectrum was almost
identical to that of water ice on Earth (Freedman & Kaufmann 2001).
Photographs show an incredible surface covered in cracks and ridges, remarkably
similar to those found in Earth’s
Further evidence of liquid water beneath the ice comes from analyzing the tidal force effects of Jupiter. The enormous gravitational pull of Jupiter and the other Galilean moons exerts enormous pressure on Europa, trying to pull it out of shape. These internal stresses are likely to cause considerable friction in the rocky core, providing enough continual heat to create and maintain a huge liquid ocean under the ice.
Our final piece of evidence comes from Galileo’s magnetometer. Europa does not appear to generate its own magnetic field and yet Galileo observed regular fluctuations in Europa’s field. Kivelson at NASA (JPLWeb2) speculates that as Europa moves through Jupiter’s magnetosphere, the changing polarity of the field during each orbit induces currents within an expanse of charge-carrying liquid beneath the surface. Could this be an ocean of water with conductive minerals in suspension?
The most likely origin of organic compounds on Europa is through meteoritic bombardment. Comets are known to contain up to 100 organic compounds, most of which are stable in ice form at the orbit of Jupiter (Hey 2002). Many of these, such as ammonia and methane, when combined with liquid water and a source of energy will produce amino acids, the basic building block of proteins and hence life (NewScientistWeb1). Pierazzo & Chyba (NewScientistWeb3) speculate that a lifetime of comet impacts could have deposited billions of tonnes of carbon, another important building block of life, on Europa’s surface. Extensive cratering throughout the Solar System confirms a history of such bombardment from space. Pappalardo at Brown (ScienceNasaWeb) speculates that Europa’s surface may repeatedly thaw and freeze. During such a thaw, the organic compounds spreading across the surface could mix with the slush ice and be dispersed beneath the surface. Even without a thaw, the force and heat of a meteoritic impact near a young crack could allow a similar penetration of material from the object deep into the ice.
Europa is too far from the Sun to be warmed by its weak rays. Unlike Io, Europa has very little evidence of volcanism, but earlier we presented evidence of plentiful heat due to the stress of Jupiter’s gravity. This might be released as superheated gas by hydrothermal vents similar to those found on Earth’s ocean floors, or more directly by lava welling up on the ocean bottom. If there is adequate heat to sustain life, where did Europa obtain the energy to form life? Miller’s experiments (NewScientistWeb1) created basic amino acids by firing “bullets” at a mixture of water, ammonia and methane, frozen to the surface temperature of Europa. The bullets simulated a kilometer-sized asteroid hitting the surface. Energy released from the impact of a large object could have sent electrical currents through any organic compounds carried on the object or frozen in Europa’s ice, creating amino acids.
What life forms do we expect to find on Europa?
If the development of life on Europa mirrors that of Earth, we are likely to find microorganisms thriving around undersea hydrothermal vents. Such vents spew hot gas and chemicals that the microorganisms might find nutritious (AstroBioWeb2). There is no possibility of photosynthesis in the darkness of Europa’s ocean. After hundreds of millions of years, such organisms might have developed the ability to swim and explore beyond their home vents. We might even find larger forms of life such as tubeworms (UcarWeb), or even simple predatory swimmers. If we could reach these deep seabed vents, we might discover a microcosm of life.
Another potential habitat for microorganisms is the numerous ice cracks. If the ice is thin enough (perhaps one to ten kilometers thick), it is possible that larger cracks could provide a conduit between the surface and an ocean below. Ocean tides might wash warm water up and down the cracks often enough to keep them in a slush-like, navigable state. Organisms might cling to the walls of the cracks, using photosynthesis to feed on the feeble light filtering from the surface (NewScientistWeb2), or might move up and down with the tides, collecting both light and warmth from the deeper water. Pappalardo and Barr (2003) propose evidence that the brown stains around these cracks are chloride salts and sulfuric acid deposits. Such substances lower the melting point of ice, supporting the theory of liquidity in the cracks. Organic compounds from comet impacts might wash into the cracks to further enrich the habitable cracks.
Can life survive the harsh environment?
The environment on Europa might not be the ideal that we have proposed. Detailed analysis by Chyba and Phillips (2001) suggests that an Europan ocean might lack enough oxygen to sustain life. This might rule out respiratory life and gill breathing life, but it is known that bacteria can thrive in anoxic conditions (UniverseTodayWeb), and the environment surrounding hydrothermal vents is rich in numerous chemicals that can be absorbed by microorganisms. Something similar to Deinococcus radiodurans is the most likely bacterial candidate for life on Europa, since it is resistant to vacuum, extreme cold, high radiation and low-oxygen – typical of the environment on Europa. Another candidate is Sulfolobus shibatae (AstroBioWeb1), shown to thrive in sulfuric acid that is believed to exist widely on Europa, and might be a major component of the brown surface stains.
The major hurdle to life within the cracks is the thickness of the ice. If the ice is too thick, water below is less likely to be able to keep the cracks thawed, separating life in the crack from its source of light. It might even allow the crack to freeze, forcing the microorganisms into hibernation in the ice, to await the next thaw. Photo-analysis of surface craters allows us to predict the depth of the ice by comparing the size and distribution of ripples in the ice to the diameter and depth of the crater. Analysis by Schenk (NewScientistWeb3) suggests the ice might be as thick as 19km, probably too far for liquid water to break through to the surface.
If the cracks did remain liquid for any period, how would microorganisms fare during long periods of freezing? Frozen 44km below the Antarctic ice of Earth lies Lake Vostok, forming a model of conditions deep in Europa’s ice crust. Recent research obtained core samples from Vostok and found a multitude of frozen microorganisms and bacteria, dating back as much as 400,000 years (ScienceNasaWeb). Vostok could be proof that microorganisms on Europa could survive long periods frozen in the cracks between thaws.
How can we detect life if it is there?
Our best course of action is to put a scientific lander on Europa. Once there we have various options: A lander could extract and analyze ice samples, significantly increasing our understanding of the chemical composition of the ice. Our primary landing site ought to be the brown-stained areas surrounding a young crack. Samples could determine what organic compounds exist in the brown material, or be used to discover any microorganisms spewed from the crack itself during periods of thaw.
Whilst surface analysis might form our initial research, our real quest is an ocean beneath the ice. NASA has been experimenting with cryobot technology (ICEJPLWeb). A cryobot is a torpedo-sized device, packed with scientific instrumentation, a power source and tiny propellers. Our lander, or even an orbiter spacecraft could eject one or more cryobots to penetrate the ice, melting their way downward and reporting their findings to the lander. Recent tests in the Arctic allowed a cryobot to penetrate to a depth of 23m (NewScientistWeb4), though its engineers plan to achieve depths of several kilometers. The cryobot can serve two functions: First it allows us to analyze samples from successive depths within the Europan ice, from which we can construct a model of how the ice varies with depth, in terms of composition, liquidity and temperature. Second, we might use the cryobot to bore through the ice to an ocean below. Then it, or a daughter device, could act as a hydrobot, exploring the ocean using tiny engines for propulsion and guidance. Our biggest hurdle with these penetration robots is providing power for a mission of more than a few minutes, and retaining communication with the lander from several kilometers below the ice.
Detailed modeling of the Europan environment warns us that life may prove difficult to detect. The scarcity of oxygen and organic raw materials, along with the immense volume of a moon-wide ocean could provide a density significantly less than 1 cell per cubic centimeter (Chyba & Phillips 2001). Landers and hydrobots would have to melt and analyze enormous quantities of ice or water to have a significant chance of detecting life.
Since hydrothermal vents are likely to be too deep to reach, our best chance lies in inserting a cryobot into a large crack to search for microorganisms deposited along the crack walls in large numbers. Lake Vostok could provide a testing ground to advance our cryobot technology. In all our on-site analysis, we must be certain not to pollute Europa’s environment with microorganisms brought from Earth.
Conclusion
Europa remains one of the most likely places to find life outside of the Earth, and the evidence for liquid water under the surface ice is gathering as more scientists analyze the data and photographs supplied by the Galileo probe of 1996/1997.
If we find life on Europa, it is likely to be species of resilient bacterium and microorganisms capable of surviving the freezing temperatures, lack of oxygen, intense radiation from Jupiter and lack of sunlight. Future explorers should search for these organisms clustered around hydrothermal vents on the floor of the ocean, or in huge, slushy cracks in the ice that might provide a connection between the surface and warmer oceans below. These cracks are likely to have provided a conduit for organic compounds brought to Europa on meteoritic impacts to reach the more habitable ocean below the ice.
Our next step should be to guide a lander down beside one of these cracks, dispatching a fleet of cryobots or hydrobots to penetrate the ice, free to swim the ocean or liquid crack in search of life.
Europa is a fascinating moon and we are certain to go back there many times in our quest to discover extra-terrestrial life.
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UcarWeb: University Corporation for Atmospheric Research, http://www.windows.ucar.edu/tour/link=/life/smokers.html
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