CALLISTO IS THE THIRD LARGEST NATURAL SATELLITE IN THE SOLAR SYSTEM AND YET APPEARS UNUSUALLY INACTIVE FOR A BODY OF ITS SIZE. GALILEO IMAGERY SHOWS A BODY COVERED WITH IMPACT CRATERS AND VERY FEW OTHER LARGE-SCALE GEOLOGIC FEATURES. IN CONTRAST ITS ICY NEIGHBORS GANYMEDE AND EUROPA SHOW DIVERSE GEOLOGY AND SURFACE AGE UNITS INDICATIVE OF INTERNAL ACTIVITY. IF OUR CURRENT UNDERSTANDING IS CORRECT CALLISTO REPRESENTS A PUZZLING END MEMBER FOR LARGE ICY WORLDS. INVESTIGATION OF CALLISTO S CURRENT AND HISTORIC HEAT FLOW AND INTERNAL STRUCTURE CAN THEREFORE INFORM OUR UNDERSTANDING OF DIFFERENTIATION AND PLANETARY EVOLUTION. IMPACTS CAN BE USED AS PROBES OF PLANETARY SUB-SURFACES AS CRATER MORPHOLOGY IS AFFECTED BY TARGET PROPERTIES. THE PROPOSAL TEAM HAS ALREADY STUDIED THE NEAR-SURFACE INTERIORS (DEPTH&LT;200 KM) OF GANYMEDE AND EUROPA USING ANALYSIS OF IMPACT CRATERS (BRAY 2009 BRAY ET AL. 2012 BRAY ET AL. 2014). THESE STUDIES PRODUCED ESTIMATES OF HISTORIC HEAT FLOW AND GLOBALLY AVERAGED CRUSTAL THICKNESS FOR GANYMEDE AND EUROPA. THE TRIED AND TESTED METHODS EMPLOYED IN THESE ANALYSES WILL BE APPLIED TO CALLISTO AS PART OF THE PROPOSED STUDY AND USED TO ESTIMATE HEAT FLOW AND NEAR-SURFACE CRUSTAL STRUCTURE.