Jezero's Margin Unit proves to be igneous rock that water altered at least three times

The unaltered rock at the top of the unit formed inside a mass of magma deep underground, the team concluded, cooling slowly enough to grow large grains before erosion removed what lay above it ^[15]. That gives the study a baseline. The fractured olivine lower down ^[16] can be compared with olivine that water barely touched ^[14].

Position is a correlation, though. Alteration is heaviest near the old lakebed ^[16], but being close to a lake does not make the lake the cause. According to the release, each water episode changed the rock's chemistry and appearance in a different way ^[5].

Much of the evidence comes from SuperCam, on the rover's mast, which fires a laser at rock up to 6.5 meters away and reads its chemistry from the spectrum of the plasma the pulse creates ^[7]. Each shot measures a small spot from a distance. The rover has taken such readings on more than 185 bedrock targets across the Margin Unit ^[8].

"Before we arrived at the Margin Unit, the main hypothesis -- derived from orbital observations -- was that the carbonate seen from orbit formed from interaction with the lake that existed in Jezero Crater," said Candice Bedford, a research scientist at Purdue University and lead author of the paper in Communications Earth & Environment ^[9][12][6]. "But now we know that this location became a sort of crossroads for aqueous systems," she said ^[10].

Lake sediments were the hoped-for target because, on Earth, clay and silt deposits can preserve evidence of ancient microbial life ^[1][2]. Altered olivine is a different kind of candidate. On Earth, water reacting with olivine can release hydrogen that certain microbes use for energy, and the same reactions can make carbonate and silica, minerals capable of preserving traces of past microbial activity ^[17]. In my view that makes the heavily altered base of the unit, where silica now fills the gaps between broken grains, the more promising place to look ^[16]. The thing this doesn't tell you is whether any organism on Mars ever used that hydrogen. The link runs through Earth chemistry ^[17].

I think the harder change is in how the unit's minerals are interpreted. A carbonate or silica grain from the Margin Unit now has to be tied to one of several water episodes before it says anything about the lake ^[5][10]. The release does not say which of Perseverance's samples, if any, came from this unit. "The Margin Unit findings are important because Jezero Crater sits inside one of the largest exposures of carbonate on Mars, so what we learn here reaches well beyond this crater," Bedford said ^[11].