Mars 2000

Twenty years after the Viking Mission, Mars is again being scrutinized in t he Light of a flood of information from spacecraft missions to Mars, the Hu bble Space Telescope, and the SNC meteorites. This review provides an overv iew of the current understanding of Mars, especially in Light of the data b eing returned from the Mars Global Surveyor Mission. Mars does not now have a global magnetic field, but the presence of crustal anomalies indicates t hat a global field existed early in Martian history. The topography, geodet ic figure, and gravitational field are known to high precision. The norther n hemisphere is lower and has a thinner and stronger crust than the souther n hemisphere. The global weather and the thermal structure of the atmosphere have been mo nitored for more than a year. Surface-atmosphere interaction has been inves tigated by observations of surface features, polar caps, atmospheric dust, and condensate clouds. The surface has been imaged at very high resolution and spectral measures have been obtained to quantify surface characteristic s and geologic processes. Many questions remain unanswered, especially abou t the earliest period of Mars' history.