NASA's MarsReconnaissance Orbiter has officially completed it two-year primary mission ofexamining Mars in unprecedented detail.

Theorbiter, set to continue observing the red planet for the next two years, hasreturned 73 terabits of science data, more than all earlier Mars missions combined.That data has revealed signs of a complex Martian history of climate changethat produced a diversity of past wateryenvironments.

Among themajor findings during MRO's primaryscience phase was the revelation that the action of water on and near thesurface of Mars occurred for hundreds of millions of years.

Thespacecraft also observed that signatures of a variety of watery environments,some acidic, some alkaline, increase the possibility that there are places onMars that could reveal evidence of past life, if it ever existed.

Since movinginto position 186 miles (300 km) above Mars' surface in October 2006, theorbiter has imaged nearly 40 percent of the planet at a resolution that canreveal house-sized objects in detail, and 1 percent in enough detail to seedesk-sized features.

The orbiteralso assembled nearly 700 daily global weather maps, dozens of atmospherictemperature profiles, and hundreds of radar profiles of the subsurface and theinterior of the polar caps.

"Theseobservations are now at the level of detail necessary to test hypotheses aboutwhen and where water has changed Mars and where future missions will be mostproductive as they search for habitable regions on Mars," said RichardZurek, Mars Reconnaissance Orbiter project scientist at NASA's Jet PropulsionLaboratory in Pasadena, Calif.

The MarsReconnaissance Orbiter has found repetitive layering in Mars' permanent polarice caps, which may record possible effects of cyclical changes in Mars' tiltand orbit on global sunlight patterns.

Recentclimate cycles are indicated by radar detection of subsurface icy deposits outsidethe polar regions, where near-surface ice is not permanently stable. Otherresults reveal details of ancient streambeds, atmospheric hazes and motions ofwater, along with the ever-changing weather on Mars.

MRO alsoimaged its surface-bound teammates, including the Mars rover Opportunity poisedon the rim of Victoria Crater and NASA's Phoenix Mars Lander duringits descent earlier this year to the surface.