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…showed last 75 words of 312 total…helium, hydrogen, and oxygen. Mariner 10 also detected a magnetic field 1 percent that of the earth. Unlike that of the earth's moon, the surface of Mercury is criss-crossed by long escarpments, dating perhaps from the period of contraction the planet experienced as it cooled some time early in its history. In 1991 powerful radio telescopes on earth revealed unmistakable signals of vast sheets of ice in Mercury's polar regions, areas that had not been covered by Mariner 10.