NASA made history when it landed rovers on Mars for the first time in 1976, but not in all the ways scientists had hoped. The rovers, Viking 1 and 2, had an ambitious mission: to discover life. But the pair of rovers turned up almost nothing, and the few positive results were controversial. This prompted scientists to shift their approach, instead investigating whether the Martian environment could support life, now or in the past. Today, four decades later, NASA scientists announced that Curiosity has found what Viking didn’t: organic molecules. This is not a certain detection of life. Organic molecules make up all known life, but they can also form in abiotic chemical reactions. Still, the discovery of any organics on Mars is an astrobiological breakthrough. Together with the other habitability clues scientists have amassed over the years, this opens up a new phase in astrobiology on Mars. “The next step,” says Jennifer Eigenbrode, a NASA astrobiologist on the Curiosity mission, “is to search for signs of life” again.

The Curiosity rover landed on Mars in 2012 with a detective’s mission: to find clues about whether or not life could exist on the Red Planet. On the top of scientists’ wish list was evidence of organic matter, which emerged in 2013. But the organic molecules identified by the car-sized rover then were too few and ambiguous. So mission scientists sent Curiosity on a four-mile journey to the base of Mount Sharp in hopes of finding more conclusive evidence buried in old lake sediments.

And that did the trick. Mission scientists announced today in a paper published in the journal Science that Curiosity discovered a whole catalogue of preserved organic matter in the first rock layers that the rover checked there.

“We have just satisfied a mission objective for Curiosity,” says Jennifer Eigenbrode, study lead author and a member of the Mars Science Laboratory mission team.

Is this the discovery of Martians? It could be. Organic molecules are the building blocks of all known forms of life. But chemical reactions that don’t involve life can also produce them. In this case, the scientists couldn’t tell how these organics were formed.

But finding a trove of organic molecules on Mars is a big breakthrough for astrobiology, as organic molecules could be food for microbes, even if it doesn’t represent life itself. And this discovery ushers in a new phase in the search for life on Mars.

“All sorts of big questions could be answered by finding life on Mars or by not finding life on Mars,” says David Weintraub, a professor of astronomy at Vanderbilt University in Nashville and author of the book “Life on Mars: What to Know Before We Go.” “We now have really good reasons to look a whole lot harder,” he says.

Courtesy of NASA Curiosity has detected organics embedded in the sediments of the 'Pahrump Hills' area of Gale Crater. The rover can be seen (center) in this image captured by NASA's Mars Reconnaissance Orbiter.

NASA actually isn’t looking for life on Mars right now. Sure, if a Martian strolled by a rover’s cameras scientists would see it, but the rover is not equipped to make an unambiguous detection of life.

That was the goal when NASA deployed Viking 1 and 2 in 1976, which were the first mechanical envoys to set foot on Mars. The rovers performed experiments designed to detect Martian life, but they largely found none. The one experimental result that wasn’t an outright “no” was controversial. Despite its aspirations, the Viking program never even found signs life on Mars. And NASA didn’t launch another mission to Mars for over a decade.

“It made us much more cautious,” says Alexandra Pontefract, an astrobiologist at the Massachusetts Institute of Technology in Cambridge. “We didn’t want to fund an incredibly expensive mission and come up short again.”

What went wrong? There were too many uncertainties, Dr. Eigenbrode says. NASA was only just setting foot on the Martian surface, so scientists didn’t know enough about the environment to be able to get a clear yes or no answer.

“Viking was this sort of shot in the dark,” says David Grinspoon, an astrobiologist with the Planetary Science Institute. “And then we went, ‘oops, not only did we not find it, but we don’t really know what we’re looking for if it’s not exactly like Earth.’ And maybe that was not the best way to go about it.”

Scientists realized that they had to take a step back and try a more cautious, methodical approach. Much as a detective figures out whodunnit by filling in all the details of a crime first, astrobiologists set about piecing together a picture of the Martian environment to figure out if the planet could even support life, now or in the past.

Building a case for habitability

Over the years, scientists have amassed a number of clues that can help answer the question of Mars’ habitability, including evidence of liquid water. And now they’ve added a catalogue of organic molecules to that list.

“If there are no organics, we can pretty much forget about there being life or ever having been life on Mars,” says Dr. Weintraub. Finding organics is critical.

The term “organic” means something different to a chemist than it does to a produce manager at a grocery store. In chemistry, nearly all molecules containing both carbon and hydrogen are organic compounds.

On Mars, organic molecules could have been produced by some form of either present or past lifeforms. But they could also be the result of abiotic chemical reactions on the surface of the planet. They could even have been transported from elsewhere in the solar system. Regardless of how these molecules originated, they are a key sign of habitability.

The case for past habitability on the Red Planet is bolstered by the discovery of preserved organic compounds revealed today, as they are as old as the around 3.5 billion year old sedimentary rocks in which they were found. But scientists revealed tantalizing hints about present-day Mars today, too. In another paper published simultaneously in the journal Science, they describe background levels of methane, also an organic, in Mars’s atmosphere that varies seasonally.

Methane gas can be a byproduct of life on Earth, but there are geological processes that can produce it, too. So like the organic molecules, it's not an unambiguous biosignature.

With future missions, scientists may be able to figure out whether or not the organic compounds are indeed biosignatures. “This gives us a lot of hope going forward in terms of organic detection on Mars,” Dr. Pontefract says.

Key details about the organics may have been obscured in Curiosity’s sample because the rover can only drill two inches into rock. The Martian surface is bombarded with radiation that can degrade organic compounds, explains Eigenbrode. But the planned ExoMars 2020 rover, part of a joint mission of the European Space Agency and the Russian space agency Roscosmos, will have a drill that can reach a depth of about 6.5 feet.

Furthermore, Pontefract says, ExoMars and NASA’s Mars 2020 mission will use tools that take a different approach to analyze organics. To look for organics, Curiosity drills about two inches into a rock, collects the dust created, then lights it on fire to break the samples down to their chemical components (which is a similar process to Viking’s). From those components, scientists work backward to figure out what molecules the rock was originally made of. The planned missions will use tools that can look for organic compounds directly in the rock.

The next chapter

This latest discovery of organics does more than open the door for more study of organic matter on Mars. It also signals the next step in our astrobiological investigation of Mars. That next step is to try to figure out if life is involved.

If Viking was phase one of our search for life on Mars, and the methodical quest for clues of habitability that followed was phase two, says Grinspoon, “this is the successful culmination of phase two.”

Previously, it was an open question whether signs of life might be preserved on the now-harsh Martian surface. This new discovery of old organics strengthens that possibility and offers new insights into how things preserve in Mars rock, Grinspoon says. “Maybe phase three would be to say, ‘okay, now we know there are preserved organics, let’s go for it and find the fossils.’ “

The next stage might not be just about past life, Pontefract says. “I think we’re moving toward extant [the opposite of extinct] life detection again.”

“I think part of the Mars community is frustrated with these incremental advances,” she says, so there’s a push to go look for life directly again.

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The methodical approach since Viking has set scientists up to look directly for signs of life, Eigenbrode says. And, she says, it gives us an idea where to look. For present life, some scientists say we should look below the surface, in soils or in caves, where there might be liquid water still flowing and organic compounds around. For extinct life, scientists have an idea where lakes and rivers used to flow and sedimentary rocks might have built up, preserving life that may have flourished around those Martian waters.

“The next chapter of Mars exploration is just getting going,” Eigenbrode says. “And Mars always surprises us.”