Ancient Martian Lake May Have Supported Life

December 9, 2013

Ancient Martian Lake May Have Supported Life

By KENNETH CHANG

About 3.5 billion years ago — around the time life is thought to have first arisen on Earth — Mars had a large freshwater lake that might well have been hospitable to life, scientists reported Monday. The lake lay in the same crater where NASA’s Mars rover Curiosity landed last year and has been exploring ever since. It lasted for hundreds or thousands of years, and possibly much longer.Whether any life ever appeared on Mars is not yet known, and Curiosity was not designed to answer that question. But the data coming back from the planet indicate that the possibility of life, at least in the ancient past, is at least plausible.

John P. Grotzinger, a professor of geology at the California Institute of Technology who is the project scientist for the Curiosity mission, said that if certain microbes like those on present-day Earth had plopped into that ancient Martian lake, they would most likely have found a pleasant place to call home.

“The environment would have existed long enough that they could have been sustained, prospered, grown, multiplied,” he said. “All the essential ingredients for life were present.

“Potentially the aqueous stream, lake, groundwater system could have existed for millions to tens of millions of years,” he added. “You could easily get a lake with the area of the Finger Lakes in upstate New York.”

The interpretation comes from detailed analysis of two mudstones drilled by Curiosity earlier this year. The structure, chemistry and mineralogy of the sedimentary rocks were not alien.

“The whole thing just seems extremely Earthlike,” Dr. Grotzinger said.

The scientists presented their latest findings at a meeting of the American Geophysical Union in San Francisco and in a set of six articles published in the journal Science.

The surface of Mars today is frigid and arid, bombarded by sterilizing radiation, but after it formed and cooled with the rest of the solar system about 4.5 billion years ago, it was initially a warmer and wetter place during its first billion years. Over the past decade, scientists have identified several sites on Mars that they think were once habitable.

In 2004, after NASA’s rover Opportunity discovered evidence that the Martian places it was traversing had once been soaking wet, Steven W. Squyres, the mission’s principal investigator, declared, “This is the kind of place that would have been suitable for life.”

But that location would have been an extremely challenging environment for life to take hold — very salty and highly acidic. Later, the scientists said the soils had been soaked not so much by water as by sulfuric acid.

NASA chose the 96-mile-wide Gale Crater as Curiosity’s landing site because readings from orbit identified the presence of clay minerals, which form in waters with a neutral pH. Curiosity’s instruments indeed detected clays in the two mudstones, named John Klein and Cumberland.

The clays appear to have formed at the lake bottom, not swept down from the walls of Gale Crater, strengthening the case that the lake water was not acidic.

Curiosity also measured carbon, hydrogen, oxygen, sulfur, nitrogen and phosphorus, elements that are critical for life on Earth, as well as iron and sulfur minerals that could have served as food for microbes.

“If there were microbial organisms around, I think they would have liked that environment,” said David T. Vaniman, a researcher at the Planetary Science Institute in Tucson and the lead author of a Science paper examining the mineralogy. On Earth, a class of such microbes known as chemolithoautotrophs live in caves, hydrothermal vents and the deep underground.

An impact, probably by an asteroid, excavated Gale Crater 3.6 billion to 3.8 billion years ago, and the John Klein and Cumberland mudstones formed out of sediments that subsequently accumulated in the crater. That is roughly the same age as rocks on Earth with the earliest signs of life.

“You can actually begin to line up in time what the Earth was doing and what Mars was doing,” Dr. Grotzinger said. “It’s kind of cool.”

The Gale Crater lake was also of the same era as the sulfuric-acid-soaked rocks that Opportunity found. That suggests that as Mars dried out, conditions in different regions varied widely. “Things have just gotten more complex than we thought,” Dr. Grotzinger said.

Curiously, even though the rocks formed in a lake, soluble elements like sodium and calcium had not been washed away. That suggests that the climate even then was cold and arid, just not as cold and arid as it is today — perhaps an ice-covered lake.

“What does it mean about the climate?” Dr. Vaniman said. “It’s something we’re all thinking about.”

What has not been found yet is solid evidence for the carbon molecules known as organics that could serve as the building blocks of life. Such molecules are not always preserved in stone and are destroyed by radiation.

By measuring the abundance of certain elements, a technique that has long been used to date Earth rocks, Kenneth A. Farley, a professor of geochemistry at Caltech, was able to estimate that the sediments eroded from rocks 4.2 billion years old, give or take 350 million years, and that the rocks had been exposed at the surface for about 80 million years.

Previously, planetary scientists estimated ages by counting craters — the older a surface, the greater the number of craters. Dr. Farley’s numbers fit with expectations for the Gale Crater rocks — “it’s a nice demonstration this method could work,” Dr. Farley said — and the dating technique could help locate rocks that have been exposed to radiation recently, raising the odds of finding organics, if they are present.

“That’s a big step forward for the exploration of life on Mars,” Dr. Grotzinger said. “We’re now exploring for that subset of environments can preserve organic carbon.”