Some time ago I heard a lecture at Stanford University by the famous astronomer (and SETI pioneer) Frank Drake.  I was especially intrigued by two things that he said.  He mentioned that the famous 19th century mathematician Carl Friedrich Gauss,  some of whose theorems I have often lectured about in my courses,  had proposed a way to send a message to inhabitants of the Moon or Mars in order to let them know that intelligent beings lived on Earth.  The proposal was to make a design in the forests of Siberia consisting of an enormous field of wheat forming a right triangle with squares on each of the three sides formed by groves of pine trees,  showing that we on Earth had discovered the Pythagorean Theorem.  Later in his lecture he surprised me by showing some interesting photographic evidence that the Galilean satellite Callisto might have an ocean under its solid crust.  During the 1970s, the theoretical models for the Galilean satellites suggested the possibility of an ocean on Europa, Ganymede, and Callisto.  In some of the books about astronomy from that period written for general audiences,  it seemed that Ganymede and Callisto were the candidates most often mentioned.  I think that this was because of their lower density and the fact that the theoretical models predicted much deeper oceans on those moons.  But the validity of these models came into question early in 1979, and the article Is There Liquid Water on Europa? (written by Cassen, Peale, and Reynolds later that year) then made Europa the most likely candidate to have an ocean.
           In the October 22, 1998 issue of the magazine Nature , there is an article entitled Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto (written by Krishan K.Khurana, Margaret G.Kivelson, and five other scientists from UCLA, Caltech, and JPL). That article provides perhaps the strongest evidence to date for the existence of an ocean under the ice of Europa, and also under the rocky, icy crust of Callisto.  So perhaps it is reasonable after all to think that at least two of the Galilean moons might have oceans of liquid water.   The authors suggest that Ganymede too might have an ocean.  Ganymede is probably much more differentiated than Callisto. But it has a much stronger magnetic field of its own, and that makes the measurement of any induced magnetic field more difficult. In that same issue of Nature, there is a nice introductory article by F. Neubauer.  Here are some links about this new discovery.  There is also an interesting lecture given by Margaret Kivelson almost one year earlier (and before the above discovery) entitled From outer space to the ocean floor: Magnetic fields in the solar system. In his 1971 paper published in Icarus, John S. Lewis first proposed the possibility of a subsurface, liquid water ocean on ice-covered bodies in the outer Solar System, including Europa, Ganymede, and Callisto. It was quite interesting for me to find at the end of that paper the suggestion that searching for a magnetic field generated by electrical conduction in such a subsurface ocean might be a way to detect its presence.  Khurana, Kivelson, et al  propose that, as Jupiter rotates, its strong magnetic field might induce an electric current in salty, subsurface oceans on Europa and Callisto, and possibly on Ganymede, and that this electric current in turn could generate an "induced" magnetic field surrounding those bodies. The magnetometer measurements obtained from Galileo flybys during 1996-97 seem to provide strong evidence that this is what is actually happening.
           The images of the surface of Europa obtained by the Galileo flybys offers compelling evidence that a liquid water ocean under Europa's icy crust has at least existed at some time in the past.  Here is a collection of articles..discussing this photographic and other evidence. Plans to determine whether Europa does indeed have an ocean and, if so, to explore its interior are carefully being studied. In 1999, the Committee on Planetary and Lunar Exploration prepared a detailed document for the National Research Council entitled A Science Strategy for the Exploration of Europa.
           The Galilean moons of Jupiter have already once proved that human beings are not the center of the Universe. The discovery that these bodies were revolving around the planet Jupiter, and not around the Earth, convinced Galileo of the validity of the Copernican theory.  Perhaps these moons will soon show us that our Earth is also not the only abode of life in our Universe, an idea which many people strongly believe, but which still has not been  proven.  If we do find life on Europa, just think of what we can learn by studying its nature. Is the underlying biochemistry totally different, or are there extensive similarities? The answer would give clues to the origin of life on Earth.  If complex creatures have evolved, it would be fascinating to see the paths that evolution, totally independent of that on Earth, has taken.  And suppose that Callisto or Ganymede has underground seas and that life has evolved there also, again totally independently of that on Earth or Europa. What would these life forms be like?  And then there are the moons of other the outer Solar System.  Perhaps, within the next twenty years or so, we will find answers to some of these questions. Arthur C. Clarke writes the following famous passage in his novel 2010: Odyssey 2: All these worlds are yours - except Europa.  Attempt no landing there. But Clarke would undoubtedly completely agree that we really have no choice but to follow our natural human curiosity.  In a letter that I received from him some time ago, he writes that  "I am very puzzled by the line at the top of Europa image PIA00588 - it is definitely not a computer artefact, and seems qualitatively different from all other markings on this extraordinary world."

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