MAGELLAN PROJECT OFFICE

MIT ID: GEDRP.1;2
JPL ID: P-40347
Date:   6/15/92

These  images  display  the  distribution  of   radio-thermal   emission
efficiency  (emissivity)  over  the  Venus  surface,  observed using the
Magellan SAR  radar  receiver.  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  that  part  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.  Easterly
longitudes run across the Mercator map from left to  right,  and  around
the  periphery of the polar stereographic projections. The emissivity of
the surface is determined primarily by its  bulk  electrical  properties
and  on the angle at which the emitted radiation is viewed. The decrease
in average emissivity at low latitudes  results  from  the  higher  view
angle  there. The horizontal resolution also 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,  but  at
high latitudes it degrades to as much as 100 kilometers.

 There is a tendency for elevated regions, e.g. the Maxwell Montes (left
of  the  data  gap  at  top  left center) and Aphrodite Terra (along the
equator at right center), to show lower values of  emissivity  than  are
typical  of  lower-lying areas. These low values confirm the high values
of radar reflectivity seen in  corresponding  regions  by  the  Magellan
altimeter,  but  do not offer significant additional information to help
solve the considerable  puzzle  of  what  mechanism  may  underlie  this
unexpected  behavior. On a cooler planet such as Earth or Mars, water or
ice might explain the anomalous observations,  but  at  the  470-degree-
Celsius  temperature  of the Venus surface, neither can be present. Some
theories require the presence of minerals such as iron  pyrites;  others
suggest  a  material,  as  yet  unidentified,  that  has  extremely  low
electrical loss. The data shown here were analyzed and projected at  the
Center for Space Research, Massachusetts Institute of Technology.