Devout exobiologists fervently believe that life may exist, or could have existed, on four other worlds in our solar system. The candidates are Mars, Europa, (a moon of Jupiter), Titan, (a moon of Saturn), and now Enceladus. Like Titan, Enceladus is a small moon that orbits the planet Saturn. All four worlds to varying degrees possess the attributes that could support life as we know it: liquid water, carbon, nitrogen, and a source of energy.
Mars has failed to show any signs of even microbial life, past or present, despite its near Biblical standing as a kind of Promised Land for extraterrestrial organisms. It is also lacking in nitrogen, important in the formation of amino acids and proteins.
Enceladus is in the news this week. Analysis of the data sent back by NASA’s Cassini spacecraft suggest the presence of an underground body of water the size of Lake Superior. The small sea is perhaps 20 to 25 miles beneath the moon’s icy face. Cassini flew by Enceladus three times between 2010 and 2012, snapping pictures of the frozen surface; changes in its speed and trajectory were studied in order to develop a map of the moon’s gravitational field. Geysers of ice crystals had also been observed at the moon’s south pole.
Just over 300 miles wide, Enceladus is a shiny, smooth faced world that reflects nearly 100% of the sunlight that strikes it. It is a very cold place, being negative 330 degrees Fahrenheit on the surface. Its terrain is broken up by fissures, hilly regions, and plains, but relatively few craters, which suggests that the moon has been geologically active in the recent past. Heat energy in its core is generated by tidal forces—its orbit is affected by gravitational forces from Saturn and neighboring moons. It is thought to have a liquid interior as well as a slight atmosphere containing water vapor.
Evidence for an underground sea has not been directly observed but inferred from analysis of the satellite’s gravitational field, which is weaker at the south pole. Scientists speculate that liquid water has the requisite density to account for what should otherwise be a weaker gravitational force at the pole. Thus, there must be an enormous underground body of liquid water near the south pole. Several scientists are proposing a mission to Enceladus. If samples of ice crystals from the geyser sites can be brought back to earth—with suitable precautions—they can be examined for extraterrestrial microbes.
Exobiologists have reasoned metaphorically that because life can exist in such extreme Earth environments as deep undersea volcanic vents and the Antarctic wastes, it must also exist in similar locations on other worlds. Elaborate mathematic formulas have been used to infer the presence of optimal conditions for extraterrestrial life, when direct observation is not possible.
These speculative methods have been used to locate over a thousand “exoplanets”, circling nearly as many stars. Of these planetary systems, nearly 200 have multiple planets. Out of all these worlds, at least nine exoplanets and possibly 30 planetary moons lie in the habitable orbital zone of a star—defined as at a sufficient distance from the star to allow liquid water to exist. It seems that every month or two a few more exoplanets are discovered using this methodology; typically these are “gas giants” like our own Jupiter and Saturn.
(See also How Many Extraterrestrials Are There? )
Besides watching, we have also been listening for signs of life in the cosmos. SETI has turned its enormous ear to the heavens since 1961. Alas, no identifiable signal has come to Earth—it has been very quiet out there. My hunch is that it probably will remain so.
Strictly speaking, none of this is really science, since it involves more inference than actual evidence. The reporting about Enceladus is another example of reason in the service of a persevering faith. It verges on science fiction—entertaining, intriguing, perhaps even good public relations—but not verifiable. Based on the evidence so far, collected over decades, it would seem that the only life that exists in the universe is on Earth, where it was created. Yet the stars and planets out there were also created—and for us.
Despite my criticism of efforts to find extraterrestrial life, your humble blogger supports aggressive space exploration and colonization of the moon and nearby planets. This is not so much to aid in the search for life on other planets, but for business. Imagine the natural resource wealth available to us in the asteroid ring between Mars and Jupiter! Colonies could serve as mining and refining stations to enrich our planet’s store of valuable metals, and our environment can improve as energy production and manufacturing are moved off our home and onto nearby locations in space. Good old business will take us beyond our home world, and help bring life to an empty cosmos.