#phyllosilicate

If your first thought is that the target area in Jezero looks a lot like the Mississippi River birdsfoot delta shape, you’re exactly right. This area sure looks like a river delta – in fact, it is one of the best preserved delta shapes on Mars. It is known as the Western Delta, as it’s on the western side of Jezero Crater. The crater itself is found in Mars’s northern hemisphere, on the boundary between high ground in Mars’s southern hemisphere and the large, flatter lowlands in the northern hemisphere that scientists call the Isidis basin. Jezero also sits alongside a large volcanic structure called Syrtis Major and near a large fracture system called Nili Fossae that is thought to have formed from the stresses in the crust created by the growth of the volcano and the formation of the nearby crater. The colors on this image tell the story of the regional geology the rover will investigate. The yellow-orange units are rich in the minerals found in basaltic igneous rocks, like those erupted from volcanoes on Hawaii and Iceland. Much of the crater floor thus represents primary or near-primary igneous rock, and there are sand dunes visible in this image at the south that are made of these igneous minerals including mostly olivine. When the crater formed, it was first filled partially with igneous rocks, either impact melt from the crater forming or lava flows coming from the nearby volcanic complex. Eventually, there was enough water flowing through the system to erode some of these igneous minerals, sort them, and deposit them in the delta shape. The delta shape still has some of the reddish-tint associated with the igneous minerals, but it also has additional colors due to clay minerals. Clay minerals on Earth are found where there is substantial flowing water, and in this image clay-rich areas appear blue. So, the delta itself is a mixture of variable amounts of clay minerals, formed by alteration of igneous rocks by water, and reworked igneous minerals from the crater rim. Finally, Jezero contains some rocks we have almost never seen up close on Mars; carbonate minerals. Carbonates on Earth are found in a variety of settings, including lakes and oceans, and represent excellent places to find preserved fossil organic material because they can entomb and trap fossil remnants of life for billions of years. Carbonate rocks are one way to take carbon out of the atmosphere and lock it away, so growing carbonate minerals could be one factor in how Mars’s atmosphere lost its greenhouse effect and the planet froze. One carbonate-bearing outcrop was seen at Gusev crater by the rover Spirit, but she was unable to make it up to that outcrop before she was trapped, so it was never investigated up close. The carbonate rocks appear green in this image. They are found, as they are on Earth, near what was likely the shoreline of a lake that filled Jezero Crater. To make this geology, water entered Jezero, pooled there, began filling a lake, and sediments were dropped from that water flow to form the delta. The lake level reached high enough to burst out of Jezero Crater’s northern rim, allowing permanent flow through of water, unlike any other site we’ve visited on Mars – Gale Crater, where the Curiosity Rover landed, was an isolated lake, so water could not flow out through any other river. This is the area the Perseverance Rover is targeting for its landing. The goal is to land on the flat plains at the south, march across the delta and up to the crater rim, taking measurements and collecting and storing samples as it goes. This was a habitable environment, with water present long enough to shape the mineralogy of the shoreline and to grow a delta – at least tens of thousands of years. The hope is…if any rocks on Mars could preserve evidence of the existence of martian life, they should be in this photograph. -JBB Image source: https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA23239 Paper with mineralogical details: https://www.sciencedirect.com/science/article/pii/S0019103518306067#f0105 Read more: https://www.planetary.org/blogs/emily-lakdawalla/2018/jezero-landing-site-mars-2020-rover.html