JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109

PHOTO CAPTION                                MAGELLAN
                                             P-37296
                                             12/14/90


Dark Halos Revealed Around Impact Craters:  New Geologic Clocks for
Venus?

     This Magellan image mosiac located in the Lavinia region shows
the crater Carson, a 38 km (23 mile) diameter crater.  The image is
centered at about 24 degrees south latitude, 344 degrees longitude. 
Impact craters in the Magellan data are frequently surrounded by
radar-dark halos.  Several of these halos were noted to have a
hyperbolic shape, extending hundreds of kilometers to the west of
the craters.   Five of these dark halos were also seen extending
out from craters in Magellan emissivity data.  Magellan emissivity
data, derived from the altimeter data, provides information about
the porosity (soil versus solid rock) and the composition of the
surface.  The darkness in the emissivity data indicates a very
smooth surface, leading to the interpretation that these halos may
be thick, smooth sediment deposits formed by the 'crushing' blow of
the atmosphere associated with the incoming projectile that formed
the impact crater.  But why are only about 5% of the craters
surrounded by these halos?  On the surface of most planets, the
population of impact craters can be used to obtain both relative
and absolute surface ages.  On Venus, as on Earth, there are
relatively few craters, making relative age dating (comparing one
region of the planet to another) statistically difficult.  The
number of craters on Venus indicates that the surface may  be only
about 400 million years old.  The dark halos may indicate the very
youngest craters on the planet-  where the harsh conditions on
Venus have not yet caused these dark halos to 'roughen up' and
disappear.  The halos could possibly be used as 'geologic clocks.' 
Volcanic flows or tectonic structures cutting one of these halos
are interpreted to have occurred even more recently than the
craters.  This image shows such a situation, where flows associated
with the crater cut across the halo, indicating a very young age. 
Scientists are continuing to identify these halos in the Magellan
data, and use them to understand the geologic evolution of the
surface.