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Images and data collected by NASA's Mars Exploration Rover Opportunity and presented yesterday at the American Geophysical Union's conference in San Francisco indicate that billions of years ago water once flowed through underground fractures in the Martian landscape.

The exposed vein--seen here in an image taken from the rover's panoramic camera--is apparently made of gypsum, deposited by water. Called "Homestake," the vein 0.4 to 0.8 inches long (about the width of a human thumb) and 16 to 20 inches long, NASA said in a press release.

Opportunity, which has been on Mars for almost eight years, has been exploring the area located along the west rim of the Endeavour Crater since August. The exposures were taken on November 7.

After identifying the visible deposit in November, Opportunity's Microscopic Imager and Alpha Particle X-ray Spectrometer on the rover's arm examined the vein and "the spectrometer identified plentiful calcium and sulfur in a ratio pointing to relatively pure calcium sulfate," NASA said.

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Cape York

Positioned at the edge of Cape York on the western rim of Mars' Endeavour Crater, the bright "Homestake" vein that scientists believe may be the calcium-sulfate mineral gypsum is visible on the right side of the image.

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Homestake close-up

A close-up view of the "Homestake" vein blends three exposures taken by the microscopic imager aboard the Opportunity rover. Scientists say the site near the edge of the "Cape York" segment of the western rim of Endeavour Crater is evidence of water on Mars.

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Shadow of Opportunity's arm

The shadow of Opportunity's arm is visible alongside the gypsum deposit known as "Homestake," which scientist believe is evidence of water on the Red Planet. The photo was taken by the rover's front hazard-avoidance camera.

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False color

This view of the deposit is presented in false color to make some differences between materials easier to see.

Exposures combined into this view were taken through Opportunity's panoramic camera (Pancam) filters admitting near infrared, green, and violet wavelengths.

Currently, the Opportunity rover is active and continues exploring Mars, but as the Martian winter approaches, it is looking to hunker down on a slope facing the sun on the northern end of Cape York. There, it can draw enough solar energy to sustain its basic operating requirements. If it can survive through the winter, Opportunity will resume exploration sometime in summer 2012, Earth time.

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