NASA's Mars Reconnaissance Orbiter has officially completed it two-year primary mission of examining Mars in unprecedented detail.

The orbiter, set to continue observing the red planet for the next two years, has returned 73 terabits of science data, more than all earlier Mars missions combined. That data has revealed signs of a complex Martian history of climate change that produced a diversity of past watery environments.

Among the major findings during MRO's primary science phase was the revelation that the action of water on and near the surface of Mars occurred for hundreds of millions of years.

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

Since moving into position 186 miles (300 km) above Mars' surface in October 2006, the orbiter has imaged nearly 40 percent of the planet at a resolution that can reveal house-sized objects in detail, and 1 percent in enough detail to see desk-sized features.

The orbiter also assembled nearly 700 daily global weather maps, dozens of atmospheric temperature profiles, and hundreds of radar profiles of the subsurface and the interior of the polar caps.

"These observations are now at the level of detail necessary to test hypotheses about when and where water has changed Mars and where future missions will be most productive as they search for habitable regions on Mars," said Richard Zurek, Mars Reconnaissance Orbiter project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

The Mars Reconnaissance Orbiter has found repetitive layering in Mars' permanent polar ice caps, which may record possible effects of cyclical changes in Mars' tilt and orbit on global sunlight patterns.

Recent climate cycles are indicated by radar detection of subsurface icy deposits outside the polar regions, where near-surface ice is not permanently stable. Other results reveal details of ancient streambeds, atmospheric hazes and motions of water, along with the ever-changing weather on Mars.

MRO also imaged its surface-bound teammates, including the Mars rover Opportunity poised on the rim of Victoria Crater and NASA's Phoenix Mars Lander during its descent earlier this year to the surface.

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