High-resolution radiometry of Pluto at 4.2 cm with New Horizons

The radio thermal emission from Pluto was observed from the New Horizons spacecraft at a wavelength of 4.2 cm along two scans across the planetary disk shortly after closest approach to Pluto on 14 July 2015. The measurements were performed as part of the New Horizons Radio Science Experiment (REX) using the 2.1 m High Gain Antenna (HGA) and the spacecraft's X-Band receiver. The HGA boresight first scanned along a diametric chord across the Pluto disk and then reversed direction to traverse a chord that crossed close to Pluto's winter pole. The diametric scan reveals a “hot spot” on the Pluto nightside associated with an optically bright region centered roughly at the planetocentric coordinates 280° E, 55° S, imaged in 2002–03 with the Hubble Space Telescope. The nightside was also found to be warmer than the dayside during the polar scan. The highest emission was not observed at the maximum southern latitude, however, but rather near the outbound Pluto limb at lower latitude. The REX emission profile from the polar scan is qualitatively consistent with a bright U-shaped polar cap observed on Pluto's Charon-facing hemisphere during the recurring Pluto/Charon mutual events in the late 1980's. The REX radiometer measurements show distinct variations in microwave brightness that constrain volatile transport models and provide unique information on the thermal structure and composition on the regions in winter night during the New Horizons encounter at Pluto. •Thermal emission of Pluto was measured at λ = 4.2 cm as part of the New Horizons Radio Science Experiment REX on 14 July 2015.•Two high-resolution scans were recorded: one diametrically through the center of Pluto and a second one passing near the winter pole.•The ground track of the diametric scan was nearly identical to that of the lower resolution nightside scan performed between Pluto occultation ingress and egress.•A maximum in the nightside thermal emission was associated with an optically bright spot in the Southern Hemisphere observed from Earth in 2002-2003.•The brightness temperature increased monotonically from 20 K to 37 K during the winter pole scan.