Meridiani Planum, the home base of the Mars rover Opportunity, could be the location for a human landing. A recent paper advocated for a landing site somewhere in the vast plain, which Opportunity has explored for the past 13 years. The rover — and its twin, Spirit — landed on Mars in 2004 in search for signs of ancient water, which they've found in abundance.

Water flowed on Mars in the ancient past because its atmosphere was thicker. Over the years, erosion from the sun has thrown away some of the lighter particles, thinning the atmosphere and eventually making it too thin for water to flow on the surface. Atmospheric erosion is a focus of NASA's much newer MAVEN (Mars Atmosphere and Volatile Evolution) mission, which arrived at Mars in 2015.

Meanwhile, Opportunity is still working on the surface under a new two-year extended mission that began in 2016. The rover remains in good working order, although it has had some memory issues in the past couple of years that forced controllers to change how the rover collects and sends data to Earth.

Snake on a Plain

Image via NASA/JPL-Caltech/Cornell.

In 2011, Opportunity spotted signs of gypsum (a mineral deposited in water) in Meridiani Planum. The mineral vein is about 16 to 20 inches long, and about the width of a human thumb. There were several such veins around part of the rim of the Endeavour Crater, and it was the first time Opportunity had ever spotted this kind of feature.

"This tells a slam-dunk story that water flowed through underground fractures in the rock," Steve Squyres of Cornell University and principal investigator for Opportunity, said in a 2011 statement announcing the find. "This stuff is a fairly pure chemical deposit that formed in place right where we see it. That can't be said for other gypsum seen on Mars or for other water-related minerals Opportunity has found. It's not uncommon on Earth, but on Mars, it's the kind of thing that makes geologists jump out of their chairs."

The Waters of Mars

Image via NASA/JPL-Caltech/Cornell.

It took only six weeks for Opportunity to confirm that the area it was exploring — a tiny crater in which it had landed — had "soaking wet" conditions in the ancient past. One of the first firm pieces of evidence was a rock outcrop (shown above), which revealed the presence sulfates, as well as niches where crystals could grow.

"Liquid water once flowed through these rocks. It changed their texture, and it changed their chemistry," Squyres said in 2004. "We've been able to read the tell-tale clues the water left behind, giving us confidence in that conclusion."

Captain, Oh Captain

Image via NASA/JPL-Caltech/Cornell.

The above image shows El Capitan, one of the first rocks Martian rocks analyzed by Opportunity. It showed some signs of water, and with further probing around its landing area, Opportunity found abundant evidence beyond sulfates and crystals. One telltale sign was spheres that are about the size of peppercorns. Nicknamed "blueberries," these spheres were analyzed and shown to be made of hematite, which likely formed in the presence of groundwater.

Jarosite only forms in the presence of acidic water, NASA explained in a statement about the mission's findings. It was a discovery, however, that made its scientists "ecstatic." Acidic water is harsh, but certain microbes on Earth can thrive on it, the agency said.

Sideburns

Image via NASA/JPL-Caltech/Cornell/USGS

The image above shows a spectacular panorama from the early days of Opportunity rover. In it, Burns Cliff is seen through a composite of 46 different images and spanning more than 180 degrees. The rover's solar panels are seen in the bottom-right portion of the image.

The rock wall is roughly 33 feet high and is comprised of several layers of rock deposits, some of which were laid down by water and some which were formed by wind.

The cliff is named after the MIT geologist Roger Burns, who predicted jarosite would be found on Mars.

Gully Probing

Image via NASA/JPL-Caltech/Cornell/Arizona State Univ.

Even in its old age, Opportunity is still on the hunt for signs of water. A 2016 image, shown above, captures a Martian feature nicknamed Wharton Ridge.

"We are confident this is a fluid-carved gully, and that water was involved," said Squyres in a late 2016 statement. "Fluid-carved gullies on Mars have been seen from orbit since the 1970s, but none had been examined up close on the surface before. One of the three main objectives of our new mission extension is to investigate this gully. We hope to learn whether the fluid was a debris flow, with lots of rubble lubricated by water, or a flow with mostly water and less other material."