The US space agency National Aeronautics and Space Administration (NASA) said on Tuesday that it has stopped using one of the mineral-identifying tools on the Mars rover Opportunity, while experts are identifying what caused the instrument to produce incomplete data.

Engineers have turned off the instrument, a miniature thermal emission spectrometer, to protect it while they plan diagnostic tests, according to Jim Erickson, project manager for the Mars Exploration Rover Project at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.

This spectrometer, from high on the rover's mast, observes rocks and other targets from afar. It measures the infrared radiation they emit in 167 different wavelengths, providing information about the targets' composition.

On March 3 and 4, Opportunity transmitted data sets for 17 successful readings by this spectrometer but also reported that eight other attempted readings yielded incomplete data sets.

Researchers are considering several possible root causes for the problem, said Phil Christensen of Arizona State University, Tempe, who leads the team for the miniature thermal emission spectrometers.

Scientists took a calculated risk by disabling this instrument on Opportunity 10 months ago, though the current problem appears unrelated to potential damage anticipated then.

At that time, rover operators began using a "deep sleep" technique to conserve energy on Opportunity during reduced-sunshine months of Mars' winter. Turning off power to overnight heaters let the instrument get cold which could possibly damage its beam-splitter. However, the spectrometer kept working through the coldest months.

"What we're seeing now does not appear to be any problem with the beam-splitter," Christensen said.

Christensen added that if the root cause cannot be remedied, scientists could still get useful data from the instrument in its currently impaired condition, and even a total loss of the miniature thermal emission spectrometer would not end the rover's usefulness.

Opportunity's other instruments are healthy and providing excellent science, and its entire instrument suite is working well and being kept busy by the science team, Erickson said.

Both Opportunity and its twin rover Spirit have been examining Mars since January 2004, more than four times as long as their successful three-month primary missions. Images and other geological data from Spirit and Opportunity are successfully providing unprecedented evidence about wet environmental conditions in Mars' past, the JPL said.