Washington, Aug 7 (IANS): NASA's Perseverance rover has grabbed one more sample from Mars, taking the total number of collection to 12.

The car-sized Perseverance drilled out and sealed up its 12th rock sample this week, mission team members announced via Twitter.

"Call them the 'dusty dozen'. I've now got 12 rock core samples on board, collected while #SamplingMars at these different sites around Jezero Crater," the team wrote.

It is the fourth such operation in less than a month.

In February 2021, Perseverance landed with NASA's tiny Ingenuity helicopter on the floor of the 28-mile-wide (45 km) Jezero Crater, which harboured a big lake and a river delta billions of years ago.

The six-wheeled rover is hunting for signs of ancient Mars life and bagging up dozens of samples for future return to the Earth, among other tasks.

According to the mission team's sample log, the first eight rock samples that Perseverance collected -- from September 2021 to March of this year -- were volcanic in origin, Space.com reported.

But the last four, which were gathered between July 7 and August 3 -- all come from sedimentary rocks.

As that change suggests, Perseverance is now exploring a remnant of the delta, examining layered rock formations that built up over time as silt fell out of Jezero's ancient river.

Such deposits are more likely than volcanic ones to harbour organic compounds and other possible signs of Mars life, if it ever existed, Perseverance officials said.

Perseverance carries a total of 43 sample tubes, 38 of which can be filled with Martian rock and dirt, the report said.

The other five are "witness tubes", which will help the mission team determine which materials, if any, in collected samples may be contaminants from Earth.

Perseverance has now sealed up 15 of those 43 tubes. Twelve contain rock cores, one holds Martian air, and the other two are witness tubes, th report said.

These tubes will be brought to Earth by a joint NASA-European Space Agency (ESA) campaign, as early as 2033.