HOUSTON — NASA’s $2.5 billion Mars Curiosity rover has experienced a computer issue that has placed the mechanized geologist in a “safe mode,” temporarily curtailing science investigations.

The “validation” error, which occurred late March 17, is hindering full recovery from the loss earlier this month of the “A side” of the mission’s redundant flight computer, which may have been caused by radiation, according to Curiosity Project Scientist John Grotzinger.

“This is not something which is rare or extraordinary. It does mean that science will have to stand down for a couple more days here,” Grotzinger said at the 44th annual Lunar and Planetary Science Conference here.

Curiosity engineers are currently configuring the B-side flight computer to manage the mission, while the A side is restored to a backup role, Grotzinger said March 18. The swap includes a series of software patches to prevent a repeat of the initial difficulties with the A side.

The root cause of the initial A-side difficulties remains under investigation, though the source of the original corruption has been isolated, according to Grotzinger.

Curiosity landed in Gale Crater on Aug. 6, initiating a two-year mission to determine whether the landing site was once habitable for microbial life. Initially, investigations quickly determined that water had flowed through the region in the past. The latest chemical and mineral studies suggest the water was of a neutral quality that would have accommodated a wide range of microbes, though direct evidence of bacterial activity has not been established.

The rover also has found evidence of at least two ancient episodes in which water altered the mineral formation of basaltic rocks at the mission’s Gale Crater landing site.

The timescales for the two episodes await further investigation, though they likely occurred no earlier than 2.5 to 3 billion years ago, said Melissa Rice, a science team collaborator from the California Institute of Technology, speaking at the same conference. Rice works with the Mastcam instrument on the rover that has imaged evidence of calcium sulfate and carbonate — water-bearing minerals — in the bright veins of rocks strewn around the “Yellowknife Bay” area of the site.

The new findings join those announced on March 12, in which Curiosity’s science team announced the discovery of sulfur, nitrogen, hydrogen, oxygen, phosphorus and carbon — some of the key chemical ingredients for life — in the powder that Curiosity drilled out of a sedimentary rock called John Klein in February.

Two findings

The two findings suggest the rocks either rolled from the walls of Gale Crater or Mount Sharp, which rises from the crater floor, for the first round of exposure. Over time, as the rocks cracked, they were exposed for a second time to surface or subsurface water that changed the mineral composition.

“With Mastcam, we see elevated hydration signals in the narrow veins that cut many of the rocks in this area,” Rice said.

The hydrated minerals are forms of water or hydroxide left over from watery exposures, said Jim Bell of Arizona State University, who serves as the Mastcam co-investigator.

A second instrument on Curiosity, the Russian-made Dynamic Albedo of Neutrons (DAN), has also found evidence of previous concentrations of water beneath the rover’s easterly traverse from its landing site, according to Maxim Litvak, the DAN deputy principal investigator from the Space Research Institute of Moscow.

“More water is detected at Yellowknife Bay than earlier on the route,” Litvak reported. “Even within Yellowknife Bay, we see significant variation.”