In May 2007, Spirit churned up pay dirt as she dragged her broken wheel in an area around Home Plate. When Steve Ruff had the robot check it out with the mineral detecting Mini-TES and the scientists reviewed her data, it turned out to be near pure opaline silica. This unexpected discovery of concentrated silica deposits and Ruff’s subsequent research indicate that there were once hot springs or steam vents at Home Plate site, which could have provided favorable conditions for microbial life. “It was one of the most important findings by either rover,” Squyres said.
In May 2007, members of the MER Science Team were hypothesizing that Home Plate was composed of debris deposited from a hydrovolcanic explosion. That same month Spirit had uncovered a patch of light-toned, almost white soils as she was on her way to a silica rich outcrop chosen by the scientists near the base of Home Plate.

Steve Ruff, the Miniature Thermal Emission Spectrometer lead scientist, of ASU, was already on the trail of opaline silica. “I knew this opaline silica that we were beginning to recognize from Mini-TES was important, but I didn’t know at that time where this was going to lead,” he recalled. “At the time, we were struggling with dust contamination on the Mini-TES mirror that made the spectra for Mini-TES data difficult to interpret. But there was enough there that I could tell that what we were seeing and where in the landscape we were seeing more of this stuff.”

Spirit was already in place on silica-rich outcrop when Ruff and the team noticed the trench the rover had made as she dragged that right front wheel. “We measured the trench with Mini-TES, and again we saw a strong indication of opaline silica,” Ruff said.

Once work was done on the outcrop, Spirit drove over to the patch of scuffed up white stuff and checked it out up close. Turned out it was near pure silica. “It was a great moment,” said Ruff. “I can’t say it wasn’t exciting to drive up and see nearly pure opaline silica. It really made the story that much firmer and clearer for us.”

Silica at Mars has become a major highlight in Ruff’s career. Even after Spirit’s mission was over and to this day, he has been researching Earth analogues. Despite being seriously challenged by other scientists along the way, his comparative analyses and findings in Spirit’s data have successfully convinced most planetary scientists that this rover uncovered hot springs and sinter deposits. “Now he’s trying to convince the community and the 2020 project to go there and drill them,” said Arvidson, noting that Gusev is now one of the three final candidate landing sites for NASA-JPL’s 2020 rover.