As the two inlet streams spilled into Lake Jezero, they dropped loads of sediment, forming deltas, including the spectacular southern one that will be Mars 2020's target. How long did the lake and the streams that fed it last? That’s one of the major unanswered questions about Jezero. The delta deposit could have formed in as short a time as 20 years. But the crater is filled with material; if all that material beyond the delta is actually sedimentary, it might have taken 1 to 10 million years to fill. That’s a pretty wide spread, and makes a huge difference to whether life might once have thrived there.

After the stream activity petered out some time in the middle Hesperian age of Mars’ history, the delta started eroding away. What we see today isn’t what the delta looked like after the last stream flowed across it; Mars’ winds were already attacking it. Then the Syrtis Major volcano got going to the southwest, and its eruptions continued into the most recent, Amazonian era of Mars’ history. Lavas from Syrtis spilled into Jezero, covering its floor and lapping up on to the bottom of the delta, but not covering the delta completely. Based on crater counting, that volcanic filling probably happened around 3.5 billion years ago, still during the Hesperian era. After the lavas, the delta — made of material that is less resistant to erosion than lava is — continued to erode away from wind action, probably right down to the present day.

Throughout some or all of this time, groundwater could’ve been percolating through the rocks, altering original minerals to new ones. At present, there is a wide variety of minerals exposed within Jezero and just outside it. There are many kinds of carbonates and clay minerals, which typically form in wet environments, in addition to the lava-related minerals that are more common on Mars. Some of those minerals formed from groundwater action; some of them formed when the delta deposits became rock; and some mineral grains might be unchanged from when they were first eroded from the rocks way outside of Jezero, a long time ago. Disentangling which minerals formed where, and what that tells us about the history of the geology and climate at the site, is work for geologists.