This is the first-ever map of temperatures over large areas of Io's night-side, obtained with the photopolarimeter-radiometer instrument onboard NASA's Galileo spacecraft during flybys of Io in November 1999 and February 2000. Temperature contours are in degrees Kelvin (K): 90 K is -297F, and 105 K is -270 F. The view is dominated by numerous volcanic hotspots. The brightest are Loki (L); Amaterasu (A); Daedalus (D); Pillan (P); Pele (Pe); Marduk (M); Babbar (B); and a huge, old lava flow called Lei-Kung. Lei-Kung erupted sometime before the 1979 flybys by NASA's Voyager spacecraft, but it is apparently still warm. Nighttime maps of this type allow estimates of the total amount of heat coming out of Io's interior. High temperatures seen along the north and west (left) margins of the map may not be real.
Additional information about the Hubble Space Telescope is available at http://www.stsci.edu/hst/. Additional information about the Galileo mission is available at http://galileo.jpl.nasa.gov.