NASA's Persistence Mars rover has now collected two rock samples, with indications that they were exposed to water long enough to fuel the case for ancient life on the Red Planet.
"Our first reefs appear to reveal a potentially habitable continuous environment," Ken Farley, the mission's project scientist, said in a statement Friday. "It's a big deal that the water was there for a long time."
The six-wheeled robot collected its first sample, called "Montdenier" on 6 September, and the second, "Montagnac," on 8 September from the same rock.
Both specimens, slightly wider than a pencil in diameter and about six centimeters long, are now stored in sealed tubes in the rover's interior.
The first attempt to collect a sample in early August failed because the rock was too crumbly to withstand of perseverance drill
The rover is operating in an area known as Jezero Crater just north of the equator and was home to a lake 3.5 billion years ago, when conditions on Mars were much warmer and wetter than they are today.
The rock that provided the first samples was found to be basaltic in composition and likely the product of lava flows.
Volcanic rocks contain crystalline minerals that are helpful in radiometric dating.
This in turn can help scientists build up a picture of the geological history of the region, such as when the crater formed, when the lake appeared and disappeared, and how the climate changed over time.
"An interesting thing also about these rocks is that they show signs of continued contact with groundwater," NASA Geologist Katie Stack Morgan told a press conference.
Scientists already knew that the crater was home to a lake, but could not rule out the possibility that it was a "flash in the pan" with floodwaters that filled the crater for 50 years.
Now they are more certain that groundwater was present for a very long time.
"If these rocks experienced water over a long period of time, there could be habitable places within these rocks that could have supported ancient microbial life," Stack Morgan said.
Salt minerals in rock cores may have trapped tiny bubbles of ancient Martian water.
"Salts are great minerals for preserving signs of ancient life on Earth, and we expect the same may be true for rocks on Mars," Stack Morgan said.
NASA is hoping to return the samples to Earth for in-depth laboratory analysis in the 2030s in a joint mission with the European Space Agency.