The search for life on Mars shouldn't focus exclusively on the distant past, some researchers say.
Four billion years ago, the Martian surface was apparently quite habitable, featuring rivers, lakes and even a deep ocean. Indeed, some astrobiologists view ancient Mars as an even better cradle for life than Earth was, and they suspect that life on our planet may have come here long ago aboard Mars rocks blasted into space by a powerful impact.
Things changed when Mars lost its global magnetic field. Charged particles streaming from the sun were then free to strip away the once-thick Martian atmosphere, and strip it they did. This process had transformed Mars into the cold, dry world we know today by about 3.7 billion years ago, observations by NASA's MAVEN orbiter suggest. (Earth still has its global magnetic field, explaining how our planet remains so livable.)
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But this turn of events doesn't necessarily mean that Mars is a dead planet today.
"If Mars had life 4 billion years ago, Mars still has life. Nothing has happened on Mars that would've wiped out life," said Michael Finney, co-founder of The Genome Partnership, a nonprofit organization that runs the Advances in Genome Biology and Technology conferences.
"So, if there were life on Mars, it may have moved around, it may have gone into hiding a bit, but it's probably still there," Finney said last month during a panel discussion at the Breakthrough Discuss conference at the University of California, Berkeley.
One of the most promising hiding places is the Martian underground. Though the Red Planet's surface has no liquid water these days — apart, possibly, from temporary flows on warm slopes now and again — there's likely lots of the wet stuff in buried aquifers. For example, observations by Europe's Mars Express orbiter suggest that a big lake may lurk beneath the Red Planet's south pole.
Earth's diverse residents advertise their presence in dramatic and obvious ways; an advanced alien civilization could probably figure out pretty quickly, just by scanning our atmosphere, that our planet is inhabited.
We don't see any such clear-cut evidence in the Martian air, but scientists have spotted some intriguing hints recently. For example, NASA's Curiosity rover has rolled through two plumes of methane inside the 96-mile-wide (154 kilometers) Gale Crater, which the six-wheeled robot has been exploring since its 2012 touchdown. The rover mission also determined that baseline methane concentrations in Gale's air go through cycles seasonally.
More than 90% of Earth's atmospheric methane is produced by microbes and other organisms, so it's possible the gas is a signature of modern Martian life.
But the jury is most definitely still out on that. Abiotic processes can generate methane, too; the reaction of hot water with certain types of rock is one example. And even if the Mars methane is biogenic, the creatures that created it could be long dead. Scientists think the Red Planet methane plumes leaked out from underground, and there's no telling how long the gas lay trapped down there before making its way to the surface.
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Looking for DNA
NASA's 2020 Mars rover, which is scheduled to launch next summer, will hunt for signs of long-dead Red Planet life. So will the European-Russian ExoMars rover, a mission that will lift off at about the same time.
But some researchers are pushing to expand the hunt to extant Martian life. One of them is molecular biologist Gary Ruvkun, who's based at Massachusetts General Hospital and Harvard Medical School.
Ruvkun is one of three principal investigators on the Search for Extra-Terrestrial Genomes (SETG) project, which is developing an instrument to detect past or present DNA- or RNA-based life on Mars and other alien worlds.
He was on the Breakthrough Discuss panel with Finney and several other researchers, and he also gave a talk at the conference laying out the case for putting the SETG instrument on future Mars rovers and other robotic explorers.
Part of that case centers on panspermia, the idea that life has spread widely throughout the solar system, and perhaps the galaxy, by either natural or artificial means. If life did indeed come to Earth from somewhere else, there's a good chance it once flourished on Mars as well, the thinking goes. The Red Planet could have been the source, or it may have been "seeded" as Earth was.
Ruvkun views panspermia as very likely; during his Breakthrough Discuss talk, he described himself as "a religious fanatic" about the idea. Ruvkun cited as supporting evidence the very early emergence of ATP synthase, the enzyme that makes the energy-storage molecule adenosine triphosphate.
ATP synthase goes all the way back to the base of the tree of life on Earth, meaning this intricate and complex molecule was up and running by about 4 billion years ago, Ruvkun said.
"It's not just that life kind of got up to kind of working," he said. "It's like it got to being super highly evolved very fast. That's why panspermia is so attractive."
If panspermia is indeed a thing, then any life-forms we find on Mars — or anywhere else in our solar system — will likely be related to us, Ruvkun and others have reasoned. That is, such organisms will use DNA or RNA as their genetic molecule. So, we should go hunt for this stuff.
"It seems really idiotic to not look for DNA on Mars," Ruvkun said during his talk. "It's an experiment that's worth doing, we would say."
Related: Ancient Mars Could Have Supported Life (Photos)
Not just Mars
Mars isn't the only place in our solar system where alien life might flourish today. Indeed, most astrobiologists would put the Red Planet down the list a bit, behind the Jupiter moon Europa and the Saturn satellites Enceladus and Titan.
Europa and Enceladus harbor deep oceans of salty liquid water beneath their icy shells. Titan is thought to have a buried water ocean as well, and it also sports lakes and seas of liquid hydrocarbons on its surface. (NASA is developing an ocean-characterizing Europa flyby mission that will launch in the early to mid-2020s. The agency also aims to send a life-hunting lander to the moon's surface in the near future. And a Titan mission is one of two finalists for a NASA "New Frontiers" launch in 2025, along with a comet sample-return project. We should learn which one NASA picks by the end of the year.)
Even hellish Venus, a climate-change cautionary tale for Earth, might still harbor some habitable redoubts, scientists say.
Like Mars, Venus once had plentiful surface water, but a runaway greenhouse effect baked the stuff away and left the planet with surface temperatures high enough to melt lead. However, conditions appear to be pretty clement about 30 miles (50 km) above the Venusian surface.
Penny Boston, director of the NASA Astrobiology Institute at the agency's Ames Research Center in California, said she thinks the chances of modern-day Venus life are low because of the "dewatering" of the planet.
Regardless, the possible existence of cloud-dwelling life on Venus "definitely needs to be interrogated," Boston said during the same Breakthrough Discuss panel discussion.
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Mike Wall's book about the search for alien life, "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), is out now. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.