PRIMER ON SULFUR FOR THE PLANETARY GEOLOGIST.

SULFUR HAS BEEN PROPOSED AS THE DOMINANT COMPOSITION FOR THE VOLCANIC MATERIAL ON IO. SULFUR IS A COMPLEX ELEMENT WHICHFORMS MANY INTRAMOLECULAR AND INTERMOLECULAR ALLOTROPES EXHIBITING A VARIETY OF PHYSICAL PROPERTIES. CYCLO-S//8 SULFUR IS THE MOST ABUNDANT AND STABLE MOLECULAR FORM. THE IMPORTANT MOLECULAR SPECIES WITHIN LIQUID SULFUR CHANGE IN CONCENTRATION WITH TEMPERATURE AND CONSIST OF CYCLO-S//8, POLYMERIC CHAINS, AND SMALL MOLECULES SUCH AS S//3, S//4, AND S//5. CONCENTRATIONS OF THE ALLOTROPES CONTROL THE PHYSICAL PROPERTIES OF THE MELT. DISCONTINUITIES IN DENSITY, VISCOSITY, AND THERMAL PROPERTIES REFLECT THE POLYMERIZATION PROCESS WITHIN LIQUID SULFUR. INCREASING CONCENTRATIONS OF S//3, S//4, AND S//5 WITH TEMPERATURE CAUSE THE COLOR TO CHANGE FROM YELLOW TO RED. VARIATIONS IN THE MELTING POINT ARE RELATED TO AUTODISSOCIATION OF THE LIQUID. MANY SOLID FORMS OF SULFUR HAVE BEEN IDENTIFIED, BUT ONLY ORTHORHOMBIC ″ALPHA″ AND MONOCLINIC ″BETA″ SULFUR, BOTH COMPOSED OF CYCLO-S//8 SULFUR, ARE STABLE UNDER TERRESTRIAL CONDITIONS. OTHER SOLID ALLOTROPES ARE COMPOSED OF VARIOUS MOLECULAR SPECIES AND MAY BE FORMED THROUGH REACTIONS OF SULFUR COMPOUNDS OR BY QUENCHING THE MELT. PHYSICAL PROPERTIES OF SOLID SULFUR ARE DEPENDENT ON THE ALLOTROPE AND, IN SOME CASES, THE THERMAL HISTORY. ALTHOUGH ONLYTWO SOLID ALLOTROPES ARE CONSIDERED STABLE, OTHERS MAY BE IMPORTANT ON IO.