National Aeronautics and Space Administration scientists were awaiting arrival of the first images taken by the robot's microscopic imager, a camera coupled to a microscope and mounted on Spirit's robotic arm.

Atop the arm, another camera has already transmitted back to Earth spectacular high-resolution panoramic images of the Martian landscape.

The explorer, controlled by NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, lowered its robotic arm Friday and "we took the first microscopic images of the surface of another planet," said mission controller Mark Alden.

The soil analysis is scheduled to continue for two more days, during which time Spirit will venture a few meters (yards) from the landing package that allowed it to survive a blistering descent through the Martian atmosphere on January 3 and bounce to a soft stop encased in huge airbags, now deflated.

On Saturday, the robot was for the first time to use its Mossbauer Spectrometer, a delicate and sophisticated instrument developed at Johannes Gutenberg University in Mainz, Germany, to identify ferrous minerals.

"It will be our first contact with the soil," said Jessica Collisson, another mission official.

The spectrometer, placed against a rock or the martial soil, can identify any mineral containing iron, which is expected to help researchers determine the role of water in the formation of these minerals and provide clues to Mars' past climate.

Once it is oriented and ready to role, the robot will check out one of two rocks situated just a few meters from its landing site, then head out to inspect a cratered region known as "Sleepy Hollow."

Spirit will then be put on hold for a few days to allow the JPL team to concentrate on the arrival of its twin robot, Opportunity, due to touch down on the opposite side of Mars during the night of January 24-25.

Martian weather conditions for the touchdown date are forecast as favorable and "things are looking very good for Opportunity's landing," said project scientist Joy Crisp.

Once the second robot is on the Martian surface, Spirit will resume its exploration, moving at the rate of some 15 meters a day towards the Gusev Crater, situated some 250 meters away.

NASA scientists hope the crater will reveal the past presence of water and other conditions that could indicate life once existed on the red planet, which is the ultimate aim of the mission.

Researchers chose the Gusev Crater for exploration because of its resemblance to a dry lake bed.

The Mars Exploration Rover mission, at a cost of 820 million dollars, is to last three months for each of the two robots, the size of a golf carts and weighing 280 pounds (180 kg) apiece.

So far, the project is being viewed as a success, with the picture-perfect landing of Spirit and the transmission to Earth of some 4,000 images taken by the robot's nine cameras showing the rocky martian surface in mesmerizing detail and clarity.

And, so far, all of the first robot's spectrometers and other scientific instruments are working well.

SPACE.WIRE