Perched on the southern wall of Marathon Valley at Endeavour Crater, Opportunity braved temperatures descending well below -100° Fahrenheit [-73.33° Celsius] in December to work the winter science campaign and home in on the remnants of ancient clays. By month’s end, the Mars Exploration Rovers (MER) mission wrapped another productive Earth year of surface exploration and just around the corner was its 12th anniversary.

While the human members of the MER ops team tried to get a little rest or catch up on work during the holidays, the veteran robot field geologist focused on the sometimes-tedious detective work needed to zero in on the rocks and soils that are harboring the remnants of clay minerals that the MER team came to uncover.

Known as smectites, they were first detected from orbit. Since they form in neutral to high pH or alkaline water, they provide strong evidence that water much like we drink on Earth once flowed through or permeated this part of Mars, at a time billions of years ago planetary scientists generally believe when Mars had water and was more like Earth.

The MER scientists are confident that the residue of the smectites are in the rocks and soils that appear deep red, as opposed to the tan red rocks that make up most Martian landscapes. But they have not confirmed that yet and the rover has just started to do the investigative work.

The Martian winter officially begins in the southern hemisphere of the planet with solstice January 3, 2016. But temperatures have been dropping at Endeavour Crater for several months.

Opportunity was in good shape producing welcomed power levels, ranging from 387 to upwards of 440 watt-hours in December. “This past winter, Opportunity's power levels were higher than the last three winters,” said John Callas, MER project manager, of the Jet Propulsion Laboratory (JPL), home to all NASA’s Mars rovers.

Even so, Mars is farther from the Sun than Earth, and it has an atmosphere that is too thin to retain heat. “It’s so friggin’ cold,” said Ray Arvidson, MER deputy principal investigator of Washington University St. Louis. “The batteries just aren’t as efficient during winter, the shadows are pretty significant, and we have to spend more time recharging.”

From the sounds of it, Arvidson could have been talking about humans. But he was, of course, talking about the rover Opportunity. Despite the “friggin’ cold,” the rover, managed to brush and grind into two rock targets, as well as take dozens of images and forge on into winter, “moving, working, and testing the hypothesis that the red zone rocks are carrying the smectite signature detected by CRISM,” he said.