These maps of Venus show how well various regions of the surface
radiate heat compared to a perfect radiator.  They display in color a
quantity called emissivity, observed using a Magellan Synthetic
Aperture Radar (SAR) receiver during its 24 months of systematic
mapping in Venus orbit.  Color is used to code the emission efficiency
(see color bar).  Red corresponds to the highest, blue to the lowest
values of emissivity.  The upper image shows the portion of the planet
between 69 degrees north and 69 degrees south latitude in Mercator
projection; beneath it are the two polar regions covering latitudes
above 44 degrees in stereographic projection.  The horizontal
resolution varies with latitude, being determined solely by the
"footprint" of Magellan's high-gain SAR antenna beam.  Near the equator
the surface resolution is about 20 kilometers (12.4 miles) but at high
latitudes it degrades to as much as 100 kilometers (62 miles).
Emissivity of the surface is a measure of how well the surface radiates
heat.  Higher regions, such as the Maxwell Montes (at top left center)
and Aphrodite Terra (along the equator at right center), usually show
lower values of emissivity than are typical of lower-lying areas.  On a
cooler planet, such as Earth or Mars, water or ice might explain the
puzzling observations but at the surface temperature of Venus -- 470
degrees C (878 F) -- neither can be present.  Some theories call for
the presence of an electrically-conducting mineral such as pyrite (the
minerals have an electrical field when illuminated by radar); others
suggest a material as yet unidentified that has an extremely low
electrical loss.  The data shown here were compiled and analyzed at the
Center for Space Research, Massachusetts Institute of Technology.