The rover took samples from the floor of the Jezero crater, which 3.7 billion years ago was a lake fed from a small river.
Watery Mars could have supported life billions of years ago, say scientists.
The rocks analysed and stored for return to Earth have been altered by water.
“These kinds of environments on Earth are places where life thrives,” said University of Florida astrobiologist Amy Williams, one of the long-term planners for the Perseverance mission.
“The goal of exploring the Jezero delta and crater is to look in these once-habitable environments for rocks that might contain evidence of ancient life.”
The rover is now surveying the river delta to collect additional rock samples for the Mars Sample Return mission.
This is a proposed mission to return Martian rock and dust samples to Earth in a small rocket brought to the planet surface in a lander.
“When that delta was deposited is one of the main objectives of our sample return programme, because that will quantify when the lake was present and when the environmental conditions were present that could possibly have been amenable to life,” said geochemist David Shuster, professor of earth and planetary science at the University of California, Berkeley.
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Perseverance landed at the bottom of the Jezero crater in February 2021.
Since then, scientists have been exploring the geological make-up of the crater floor using a suite of tools on board the rover.
The researchers discovered the crater floor had eroded more than they expected.
Erosion exposed a crater made up of rocks formed from lava and magma, known as igneous rocks.
Before the mission, geologists expected that the floor of the crater would be filled with either sediment or lava that had spilt on to the surface and cooled rapidly.
However, at two sites referred to as Seitah — the Navajo word for “amidst the sand” — the rocks appear to have formed underground and cooled slowly.
This suggests that whatever was covering them has eroded away over the past 2.5 to 3.5 billion years.
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“We have organisms on Earth that live in very similar kinds of rocks,” said Dr Williams.
“The aqueous alteration of the minerals has the potential to record biosignatures.”
Because the rock samples taken at the bottom of the crater likely predate the river delta, dating these rocks will provide important information about the age of the lake.
The findings are published in the Science journal.