When did water flow in Mars?

In its first year exploring Jezero Crater on Mars, the Perseverance rover collected rock samples that scientists anticipate will provide a long-awaited timeline for the planet’s geologic and water history.

They’ll just have to wait a decade to find out the answer, until the samples can be scooped up from the surface and returned to Earth for dating in 2033.

The scientists are nevertheless enthused by what they’ve discovered so far about the samples. These discoveries are outlined in a paper that will appear Aug. 25 in the journal Science, with more detailed analyses in a second Science paper and two other papers published online simultaneously in Science Advances.

Jezero Crater, just north of the Martian equator, was a target for NASA’s Mars 2020 Mission and its Perseverance rover because it contained what looked like a river delta that formed inside a lake bed and thus could potentially tell scientists about when water flowed on the planet’s surface. Rocks collected from the floor of the crater underlie the delta sediments, so their crystallization ages will provide an upper limit for the delta’s formation, according to geochemist David Shuster, professor of earth and planetary science at the University of California, Berkeley.

“When that delta was deposited is one of the main objectives of our sample return program, because that will quantify when the lake was present and when the environmental conditions were present that could possibly have been amenable to life,” said Shuster, who is a member of NASA’s science team for sample collection, one of three main authors of the Science paper that summarizes the work and co-author of two of the three other papers.

The two other lead authors of the summary Science paper are geochemist Kenneth Farley of Caltech, Perseverance’s project scientist, and Mars 2020 Deputy Project Scientist Katherine Stack Morgan of NASA’s Jet Propulsion Laboratory (JPL).

The main surprise is that the rocks collected from four sites on the floor of Jezero Crater are igneous cumulate rocks—that is, they were formed by the cooling of molten magma and are the best rocks for precise geochronology once the samples have returned to Earth. They also show evidence of having been altered by water.

From a sampling perspective, this is huge. The fact that we have evidence of aqueous alteration of igneous rocks—those are the ingredients that people are very excited about, with regard to understanding environmental conditions that could potentially have supported life at some point after these rocks were formed.

One great value of the igneous rocks we collected is that they will tell us about when the lake was present in Jezero. We know it was there more recently than the igneous crater floor rocks formed, Farley said. This will address some major questions: When was Mars’ climate conducive to lakes and rivers on the planet’s surface? And when did it change to the very cold and dry conditions we see today?”

Before the mission, geologists expected that the floor of the crater was filled with either sediment or lava, which is molten rock that spilled onto the surface and cooled rapidly. But at two sites referred to as Séítah—the Navajo word for “amidst the sand”—the rocks appear to have formed underground and cooled slowly. Evidently, whatever was covering them has eroded away over the past 2.5 to 3.5 billion years.

Science Daily