SCIENTIFIC OBJECTIVES: MARS 2020 CARRIES BOTH THE FIRST MARTIAN GROUND PENETRATING RADAR RIMFAX AND AN INFRARED RADIOMETER MEDA. RIMFAX IS DESIGNED TO MEASURE LAYERING AND STRUCTURE WITHIN THE MARTIAN SUBSURFACE BY SENDING ACTIVE 150 MHZ-1200 MHZ SIGNALS INTO THE SUBSURFACE AND TIME RETURNED PULSES TO PROVIDE A PICTURE OF SUBSURFACE LAYERING THAT IS DEPENDENT ON THE DIELECTRIC PROPERTIES OF THE SUBSURFACE. IN ADDITION TO THE ACTIVE MODE THE RADAR ALSO HAS TWO PASSIVE MODES DESIGNED TO DETECT NOISE ON THE CALIBRATION CABLE (TO MEASURE THE INTERNAL NOISE) OR ON THE ANTENNA (TO MEASURE EXTERNAL NOISE) (HAMRAN ET AL. 2016 3RD IWIPM). HERE WE ARE INTERESTED IN USING THIS EXTERNAL NOISE AS A MEASUREMENT TURNING RIMFAX INTO A PASSIVE RADIOMETER TO MEASURE SUBSURFACE TEMPERATURES LINKED WITH MEDA S SURFACE CHARACTERIZATION. PASSIVE RADIO SIGNALS CAN COME FROM SPACECRAFT AND NATURAL SOURCES BUT THESE ARE SIGNALS ARE TIME AND FREQUENCY DEPENDENT. HERE WE ARE INTERESTED AT THE MICROWAVE EMISSIONS COMING FROM THE TEMPERATURES OF THE GROUND ITSELF. ANY WARM BODY EMITS MICROWAVES FROM A DEPTH DETERMINED BY THE: WAVELENGTH OF THE EMITTED MICROWAVE RADIATION PHYSICAL TEMPERATURE AND THE DIELECTRIC PROPERTIES OF THE OVERLYING MATERIAL. USING INFORMATION AT MULTIPLE FREQUENCIES THAT RIMFAX CAN SUPPLY WE CAN RECONSTRUCT SUBSURFACE TEMPERATURES AND DIELECTRIC PROPERTIES. AT LOW FREQUENCIES THIS SHOULD ALLOW FOR A CONSTRAINT OF THE LOCAL GEOTHERMAL GRADIENT DIRECTLY ENHANCING THE SCIENCE RETURN OF BOTH THE MARS 2020 AND INSIGHT MISSIONS. METHODOLOGY: MICROWAVE RADIATION FROM THE SUBSURFACE DEPENDS PRIMARILY ON PHYSICAL TEMPERATURE AND THE DIELECTRIC PROPERTIES OF THE MATERIAL THROUGH WHICH THE RADIATION TRAVELS. USING MEDA TIR SURFACE TEMPERATURE MEASUREMENTS TO CONSTRAIN LOCAL THERMAL INERTIA AND THERMAL MODELS DEVELOPED FOR THE INSIGHT MISSION (SIEGLER ET AL 2017). PHYSICAL (T) AND MICROWAVE BRIGHTNESS (TB) TEMPERATURES ARE RELATED AS TB(Z)/T(Z)W(Z) WHERE W IS THE MICROWAVE WEIGHTING FUNCTION WHICH DEPEND DIRECTLY ON THE DIELECTRIC PROPERTIES. THEREFORE OUR PHYSICAL TEMPERATURE MODELS ALLOW US TO TURN PASSIVE MEASUREMENTS OF THE MICROWAVE BRIGHTNESS TEMPERATURE AT HIGH FREQUENCIES INTO A DIRECT CONSTRAINT OF DIELECTRIC PROPERTIES (FENG ET AL. 2019 SIEGLER ET AL. 2020). DIELECTRIC PROPERTIES CONTROL THE PROPAGATION OF ELECTROMAGNETIC ENERGY THROUGH THE REGOLITH SO SOME LEVEL OF ASSUMPTION OF DIELECTRIC PROPERTIES TO TURN THE TIME OF THE RADIO PULSE RETURNS INTO DEPTHS. OUR SEPARATE MEASUREMENT OF DIELECTRIC PROPERTIES WILL DIRECTLY ALLOW RIMFAX TIME INFORMATION TO BE MORE ACCURATELY CONVERTED INTO DEPTH. THIS INFORMATION WILL ALSO ALLOW FOR MAPPING OF LAYERING RICH IN DIELECTRICALLY LOSSY MAGNETITE OR ILMENITE (AS HAS BEEN SHOWN ON THE MOON FENG ET AL. 2019). LONGER WAVELENGTH MEASUREMENTS WILL SOUND BELOW THE DEPTH OF DIURNALLY VARYING TEMPERATURES. AT THESE WAVELENGTHS RIMFAX SHOULD BE CAPABLE OF CONSTRAINING BURIED ROCK AND THE GEOTHERMAL GRADIENT AND WITH IT THE LOCAL GEOTHERMAL HEAT PRODUCTION ON THE MARS 2020 LANDING SITE. THIS WOULD BE A MAJOR BREAKTHROUGH IN MARS INTERIOR SCIENCE CHARACTERIZE THE POTENTIAL FOR PAST GEOTHERMAL-DRIVEN HABITABILITY AT THE LANDING SITE AND ENHANCE THE SCIENTIFIC RETURN OF THE MARS INSIGHT MISSION. RELEVANCE TO THE NASA MARS 2020 PSP: THIS PROPOSED WORK DIRECTLY ADDRESSES MANY ASPECTS OF THE MARS 2020 PS CALL. NAMELY IT WILL AID IN CHARTER(IZING) THE PROCESSES THAT FORMED AND MODIFIED THE GEOLOGIC RECORD WITHIN A FIELD EXPLORATION AREA AND PERFORM THE FOLLOWING ASTROBIOLOGICALLY RELEVANT INVESTIGATIONS ON THE GEOLOGIC MATERIALS AT THE LANDING SITE. THIS PROJECT WILL ENHANCE THE SCIENTIFIC RETURN FROM THE MISSION BY BROADENING PARTICIPATION IN THE MISSION AND AUGMENTING THE EXISTING MARS 2020 SCIENCE TEAM TO INCLUDE NEW INVESTIGATIONS THAT BROADEN AND/OR COMPLEMENT EXISTING RIMFAX SCIENCE.