Perseverance traces three separate water episodes through Jezero's igneous shoreline rock

Perseverance worked through roughly 265 metres of elevation in the Margin Unit ^[8]. Near the top, the rock is coarse-grained crystalline olivine, magnesium and iron, with almost no sign water ever reached it; grains that large mean slow cooling in a magma body underground, exposed only after the rock above it eroded away ^[9]. Down on the lakebed, the same olivine is fractured, with silica between the grains ^[10].

First, carbon-dioxide-rich groundwater got into those fractures and turned olivine into carbonate, and the carbonate ridges now stand above the softer rock wearing away around them ^[11]. The second event may have been the lake itself ^[12]. "Some of the Margin Unit rocks also contain silica," said Eleni Ravanis, a planetary scientist at the University of Hawaii at Manoa and a coauthor of the study. "Turning olivine into carbonate can leave silica behind, and we see more of that silica in rocks that sat below the water line." ^[13]

The third event left veins about 25 centimetres thick at one place in the eastern Margin Unit, containing calcium sulfate and fluorite ^[14]. Fluorite typically forms when hot water circulates through volcanic rock ^[15]. So the third episode points to hot water moving through volcanic rock.

The measurements come from SuperCam, which sits on the rover's mast, reads mineralogy from reflected light, and can fire a laser at rock up to 6.5 metres away, with the spectrum of the resulting plasma giving the chemistry ^[6]. More than 185 bedrock targets across the unit have been analysed this way ^[7]. Spread over 265 metres of elevation, that is at most about 1.4 metres of vertical section per target ^[22].

The team can put the three water interactions in order, but it cannot date them ^[16]. A sequence alone leaves open whether the episodes spanned a long stretch of Mars history or crowded into one wet period. The biological interest in these minerals rests on Earth chemistry: water reacting with olivine can release hydrogen that some microbes use as food, and the reaction leaves carbonate and silica, which lock in traces of past life ^[17].

Igneous rock is a good substrate for that work. Its mineral crystals preserve details of the precise moment they formed ^[21]. Before the rover arrived, the leading hypothesis, drawn from orbital observations, held that the carbonate seen from orbit formed through interaction with the crater lake ^[20]. "But now we know that this location became a sort of crossroads for aqueous systems," said Candice Bedford, a research scientist at Purdue University and the study's lead author ^[18]. She said the findings matter because "Jezero Crater sits inside one of the largest exposures of carbonate on Mars, so what we learn here reaches well beyond this crater" ^[19].