Within the past fifteen years, the surface of Venus has gone from
being the least well understood of all the terrestrial planets to
the most thoroughly mapped surface of any terrestrial planet,
including the Earth. This is primarily due to the Magellan
mission, which has collected a variety of data on the surface
morphology, physical properties, and interior density structure of
Venus amounting to more than 1 Terabit (
bits) of data.
Synthetic aperture radar images have been obtained for over 95%
of the surface; their high resolution reveals most surface
features larger than 100--200 meters across. Using its radar
altimeter, Magellan has collected data on surface elevations,
sub-meter scale roughness, and radar reflectivity at a resolution
of approximately 10 km. Further information on the physical
properties of the surface was gathered by measuring the passive
microwave emissivity of the surface [ Pettengill et
al., 1992]. Two-way Doppler tracking of the spacecraft has
yielded line-of-sight (LOS) gravity data and a spherical harmonic
model of gravity and geoid out to degree 75. Collection of
high-resolution gravity data has been aided by an innovative
aerobraking maneuver, which used Venus' atmosphere to brake the
spacecraft and lower it from a highly elliptical orbit to a
near-circular orbit.
This flood of new information is profoundly altering our basic perceptions of Venus. Before the Magellan mission, the most widely accepted (if mutable) view was that Venus was characterized by a relatively thin and easily deformed mechanical lithosphere and a relatively thin (<30 km thick) crustal layer. The surface was relatively young, and internally-driven geologic processes were thought to have continued to reshape the surface up to (and most likely through) the present epoch. The mode of global tectonics remained controversial, with some [ Head and Crumpler, 1990] suggesting plate-tectonic-like scenarios despite strong criticisms [ Kaula and Phillips, 1981; Grimm and Solomon, 1989], while others examined models in which lithospheric deformation was directly linked to convective flow in the interior [ Phillips, 1990; Bindschadler and Parmentier, 1990].
This paper attempts to review some of the more substantive discoveries in Venus geology and geophysics made over the past four years. Readers seeking a more complete perspective on the results of the Magellan mission are referred to the first report of results in the 12 April 1991 issue of Science, to the volumes of the Journal of Geophysical Research/Planets for August and October of 1992 (comprising 48 articles and 1065 pages) and an upcoming issue of Icarus containing the initial results of Magellan gravity data analysis. This review focuses on issues related to the hypothesis that Venus underwent near-complete global resurfacing approximately 300--500 m.y. (million years) ago and that subsequent tectonic and volcanic activity has been extremely limited [ Schaber et al., 1992]. In turn, it reviews the distribution of impact craters, properties of volcanic features that may indicate high effusion rate eruptions, evidence for a relatively strong mechanical lithosphere (and possibly, a thicker crust), and current ideas about global tectonics. In addition to being fundamental to our understanding of Venus' geologic evolution, these issues both highlight fundamental geophysical processes and sharpen the contrast between Venus and her sister planet, Earth.