Investigating the site of an ancient river delta, the Perseverance rover has collected some of the most important samples yet on its mission to determine if life ever existed on Mars, according to NASA scientists, CNN transmits.

A few of the recently collected samples include organic matter, indicating that Jezero Crater, which likely once held a lake and the delta that emptied into it, had potentially habitable environments 3.5 billion years ago.

“The rocks that we have been investigating on the delta have the highest concentration of organic matter that we have yet found on the mission,” said Ken Farley, Perseverance project scientist at the California Institute of Technology in Pasadena.

The rover’s mission, which began on the red planet 18 months ago, includes looking for signs of ancient microbial life. Perseverance is collecting rock samples that could have preserved these telltale biosignatures. Currently, the rover contains 12 rock samples.

A series of missions called Mars Sample Return will eventually take the collection back to Earth in the 2030s.