In February 2021, the newest Mars rover – Perseverance – touched down at Jezero Crater, interpreted to be the site of an ancient Martian lake. Modern Mars is dry and hostile. But billions of years ago, when the planet still had a magnetic field and thicker atmosphere, there was water on the surface. Multiple landers, rovers and orbiters exploring the Red Planet have all confirmed this. But did life ever evolve as well? Thus far, no fossils or positive proof of microbial life has been found.
Before committing millions (to billions) of dollars to high-stakes Mars missions, you need a reason and a justification for a landing site. High-resolution imagery from the Mars Reconnaissance Orbiter showed a fan-shaped feature interpreted to be a delta – a place where a river empties into a lake – at Jezero Crater. And where there’s evidence for past water, there’s potential for finding life, if any ever existed. Orbital imagery is extremely useful, but as any geologist will attest, ground-truth is needed to validate observations. Think about reading a book: the orbital imagery represents the cover of the book – you glean a little information about what the book may be, but you need more information; the ground-truth imagery and data is like reading through the pages of the book, where each page here is a layer of rock with its own story to tell.
Perseverance’s new imagery definitively proves that the fan-shaped feature is, indeed, a delta, and that it was formed by water. The delta has the best-preserved stratigraphy (layers of rock) visible on Mars thus far. Layers of older sediments on the bottom show sustained hydrologic activity in a persistent lake environment. But on top, where the younger layers are, there are boulder-bearing deposits. Some of the rocks are up to 5 feet wide! In order to transport such large boulders, you need high-energy events – like floods – to bring the rocks from a distance and then deposit them in the lake.
Floods on Mars could have originated from intense rainfall events, rapid snowmelt events, heating by volcanism or impact, or through progressive building of glaciers and glacial lakes that occasionally surged and flooded. Scientists hypothesize that the deposits they are finding in Jezero Crater could mark a transition in Martian history not seen elsewhere yet on the planet: when Mars went from a more Earth-like habitable environment to the desolate landscape we see today due to climate changes that caused the water cycle on the planet to also change.
It will be exciting to follow Perseverance’s explorations on the Red Planet as scientists devise sampling strategies to study more rock layers and deposits. Specifically, there are iron-magnesium bearing clays (called smectites) which have a high potential of preserving organic matter or potential biosignatures. Good rock candidates are being collected for an eventual Mars sample return mission, where scientists will then be able to study the rocks with higher precision in laboratory settings.